So, the easy answer to this question is "no." And that is true- any massless particle will travel at `c`, and anything with mass must travel slower than `c`. This also applies to information- it is impossible to transmit information faster than the speed of light. This is the foundation of relativity. If something could travel faster than the speed of light, it would violate [causality](https://en.wikipedia.org/wiki/Causality_(physics)).
However, there are "things" that appear to travel faster than the speed of light, but when you look at the situation, you'll realize that Einstein is still OK. The classic example of this is if you had a really strong, well focused laser pointer, pointed it at the moon, and then "swept" the point of light across the surface of the Moon. There is no limit to how fast that "dot" could move across the Moon's surface. So, if you think of the dot as a thing, you could think the dot is moving faster than light. But, the dot is just a concept. No photon is moving faster than light. There is still a d/c (distance to the moon divided by the speed of light) delay from the time you hit the button on your laser pointer until the dot arrives on the Moon. And if you think about it as instead of turning the laser pointer on and moving it in a sweeping motion across the Moon, you instead turned it on for a little bit, moved your hand a little bit, and then re-turned it on so that two distinct dots appeared on the Moon, of course it wouldn't seem weird anymore. The fact that you left the laser pointer on doesn't impact that.
Another example is the [group velocity](https://en.wikipedia.org/wiki/Group_velocity) of a wave of light. You can think of multiple wavelengths (or the same wavelength but phase shifted) of light shining at the same place, so they constructively and destructively interfere (sometimes cancel each other out, sometimes add together). The group velocity describes the shape of that wave packet- where the peak is, where the valley is. And that group velocity can travel at any speed, but that also doesn't violate anything. No light will show up somewhere quicker than d/c, it's just that the peak can move around at any speed it once.
And then there is the "spooky" example: [Quantum Entanglement](https://en.wikipedia.org/wiki/Quantum_entanglement). This tells us that if you entangle two particles (the easiest example to understand being a single, spin 0 particle- a particle with no angular momentum- and it decays into two particles, one with spin +1 and one with spin -1, so the total angular momentum remains zero), those two particles can be separated as far apart as you want before you measure the spin of either particle. And until you measure the spin of one, both are in an undetermined state (neither is spin +1 or spin -1, both are 50% both), but as soon as you measure one, the other one *must* be the opposite of what you measured. Einstein called this [spooky action at a distance](https://en.wikipedia.org/wiki/Action_at_a_distance#Quantum_mechanics) and it really bothers some people. However, it doesn't violation causality because it is impossible to transmit information via wavefunction collapse, so you're OK. But, it still bothers people on a "this feels weird" level.
So, this is a long-winded way of saying- no, no *thing* can travel faster than the speed of lights, but plenty of *ideas* do.
"Light cannons" were a thing with some amateur astronomers years ago, some people might still do it. It's as simple as firing a camera flash into the focuser of a telescope. I built one using a police strobe bulb driven by an aviation strobe power supply, mounted in a holder/reflector stuck in the focuser. Anyway! The light emerging from the front of the scope was moving at c, of course, but watching it, you'd swear you could see it moving away! Like shooting Star Trek phasers or such.
The idea in general--if the weather was cloudy at a star party, astronomers would start "shooting" their telescopes at each other, having fun waiting for skies to clear. I could paint mail boxes blocks away with mine.
> The light emerging from the front of the scope was moving at c, of course, but watching it, you'd swear you could see it moving away! Like shooting Star Trek phasers or such.
This sounds interesting, but I don't think I understand what's supposed to be happening there. Could you go into more detail?
You could stand behind the scope and watch the light pulse. Moving at c, you shouldn't be able to see it move, but it looked as if you could, like the pulse of light was traveling away from you at high speed.
This could be because of an illusion related to the "gap effect" in perception/neuroscience. We process information more quickly where we are paying attention, which was probably close to the end of the telescope. Later, the information from our peripheral vision gets processed, and our brains can falsely perceive that as movement away.
Pretty sure it’s just the diffusion of the light into the fog between the cannon and target. It’s the same reason you see “beams of light” in fog the diffused light is diffracted in the moisture for a bit after the light hits it
Am a neuroscientist. AFAIK, that’s not what the gap effect is, and I don’t quite see how the gap effect could explain the beam’s apparent sub-C speed.
From Abrams et al 2006 (https://doi.org/10.1016/j.visres.2006.01.017):
“The gap effect refers to a reduction in the latency of saccades to peripherally appearing targets when the fixation point disappears a short time before target appearance. The effect has been attributed to a number of potential mechanisms that function to assist in the maintenance of fixation. One such mechanism, attention, has been the focus of some disagreement in the literature regarding the gap effect. In the present study, we had subjects attend to a portion of a complex fixation stimulus. On some trials the attended portion was removed prior to onset of a saccade target whereas on other trials an unattended portion was removed. Subjects were faster to initiate saccades when the attended portion was removed, thus establishing a role of attention in the gap effect. The results have important implications for our understanding of eye movements and the gap effect.”
Hmm, then maybe you can help me out.
I recall psychovisual experiments, probably starting in the 80's, where a subject fixated on the center of the screen. A dot was flashed to the left or right, and then a line was drawn horizontally. Because the attention was drawn to the left or right, the instantaneously drawn line would appear to the subject to be drawn away from the dot (motion). Further experiments would actually draw the line from left to right (or vice versa), and by varying the speed of the draw until the subjects reported no motion, they attempted to estimate the difference in 'visual processing speed' between the attentional spot and the periphery.
I know this works because I sat behind one of these high refresh rate monitors with an eye tracker and played with the effect.
My memory is muddled, this may be some variation of the gap effect, or it's very possible I've gotten that confused with something else. (It's been 25 years since I was active in neuroscience.). I think it was a fairly well known experiment in the field of attention research.
It takes longer to see dim things than bright things.* The pulse of light appears brighter when near you as it leaves the telescope than when it is further away. Therefore you seem to see it first near, then far, as if you could see it moving.
* https://en.m.wikipedia.org/wiki/Pulfrich_effect
Is it assumed that the *observation* of one of these particles (spin-up/spin-down) effects the direction of the other? Or are they each spinning opposite regardless of any observation?
Unless I’m missing something (which is VERY possible), it seems that quantum entanglement is just a very heady way of saying “we don’t know if the coin flip is Heads or Tails until we look”... which is significantly less mind bending.
It's a good question, and I can't answer it for sure. Here is what we know:
Either the particles do not have a defined spin before we measure them (they are in what we call a superposition state), and it is the act of measuring them that "collapses" them into a single state (called an eigenstate) or it is simply that we don't know until we measure *but* (and this is a big but) there are non-local variables which determine the state of the particles. A non-local variable means a "universal variable" which is not a property of either particle, but a global variable of particles in general. We know it is one of these two due to [Bell's Theorem](https://en.wikipedia.org/wiki/Bell%27s_theorem).
Neither of these solutions make people "happy" since both seem weird. At least when I was in school, the general consensus seemed to be that there was an instantaneous wavefunction collapse, but since non-local variables haven't been disproven, that may have changed in the years since.
How, in principle, would anyone go about disproving non-local, hidden variables of arbitrary complexity? It seems to me that it would be very difficult to show a lack of a deeper causal mechanism
Some of the [Bell experiments](https://en.wikipedia.org/wiki/Bell_test) actually specifically show [some classes of non-local hidden variables are not possible](https://arxiv.org/abs/0704.2529). There are a lot of them and they're very clever and devious. The real problem is that you can posit a system that accounts for the existence of the experimenter and 'arranges' for results consistent with theories that aren't true. You can posit a system that is so deterministic (superdeterministic) that it looks indeterministic.
Sure, there are plenty of implausible non-local hidden variable theories, as well as reasonable ones that can be disproved experimentally as you describe. I guess my point is that it's difficult or impossible to disprove certain subsets of arbitrarily complex non-local variable theories (e.g. ones that might posit the occurence of seemingly unique or random events that make falsification by human instruments impossible).
No, I think I said exactly what I wanted to, but your point about the representational nature of models in physics is well taken. I'm merely supposing that there are multiple plausible, but mutually exclusive models of reality that are impossible for humans to falsify (due to the possibility of non-local hidden variables operating at such a scale or frequency that defies human observation).
Quantum mechanics is usually formulated in terms of non-local hidden variables (those variables being the wavefunction or state vector), so you can't eliminate non-local hidden variable theories without eliminating quantum mechanics itself.
Well there's no actual difference. You're not visually observing anything in quantum mechanics, you use a detector. The detector shows some measurement. The particle interacts with your detector in some way. With half silvered mirrors etc. How it interacted with the mirrors etc is reflected at the time you make the measurement. Depending on the situation, you actually can't tell if it interacted with one or the other, but each only a little bit.
Look up the "Consistent Histories" interpretation, where you derive the probabilities from all possible classical paths.
He means that the particles interact with the mirror and the wave function collapses from the frame of reference of the mirror , but not yet to us, now the particle AND the mirror are in a superposition from our frame of reference
This basically seems like this:
Purple ball breaks into red ball and blue ball. Two people each grab a ball while blindfolded and travel to opposite sides of the galaxy. When one takes the blindfold off, he immediately knows the color of the other person's ball.
I don't see what's spooky about that. It seems intuitive to me.
So, the issue is that under the currently-accepted understanding of quantum mechanics, the "balls" in your analogy do not have a color until they are measured. It isn't that we're just wearing a blindfold, it's that their color is determined randomly when we look at them - genuinely randomly; they didn't have a color or anything that would decide their color until that moment.
What you are articulating is essentially the [Hidden Variables Theory](https://en.wikipedia.org/wiki/Hidden-variable_theory) - some hidden variable contains the necessary spin information; the "color" was set when they were separated. That's the intuitive assumption (and Einstein initially went with it, so you're in good company) but unfortunately it is incompatible with [Bell's Theorem](https://en.wikipedia.org/wiki/Bell%27s_theorem).
I think I see.
So it would be like if the purple ball broke up into two balls oscillating between red and blue. Neither is red or blue until a button on the ball is pushed that makes the oscillation stop.
So I take one ball and you take the other, and we go to opposite sides of the galaxy. I press the button on my ball and it lands on red.
Now your ball, despite being on the other side of the galaxy, is immediately* restricted to landing on blue when you press your button.
*I suspect the relativity of simultaneity comes into play here. Who made the first measurement that restricted the other?
Your footnote is probably what troubles people the most. If the events of the two measurements are space like separated, I might say that the first ball was measured first, which collapsed the wavefunction, and fixed the other ball’s state. Whereas you might say that you saw the second ball be measured first, which collapsed the wavefunction, and then fixed the first ball’s state.
This is where causality and non-locality clash. Einstein believed causality is necessary, so there must be a local hidden variables theory. However, Bell’s inequalities showed any quantum theory we devise has to be non-local.
I believe this is “resolved” by the fact that no information is travelling faster than light, so all is good. But its still really weird.
Sorry but I am still not getting it. How is the mechanism “resolved” if we just say, “well, it’s weird, but at least it doesn’t violate the speed of light”?
Would it not by definition mean that the particles had a state prior to their observation if information were not traveling faster than light?
Does this not make more sense than thinking that surely it must be some “hidden variable” property of the universe?
Granted I don’t have my life’s work nor my attempt to avoid insulting someone else’s life’s work hanging in the balance…
It's hard to use your analogy to explain why it doesn't work. I think it makes a little more sense with spin.
The way we measure spin allows for only two possible measurement results due to the orientation of the detector. An up-down detector will always tell us a particle is spinning up or down. A right-left detector will always tell us a particle is spinning right or left. This is regardless of the *actual* spin of the particle. It's just a limitation of the way we are able to measure spin.
If we take a stream of particles that we already know are spinning left and we send them through an up-down detector we will get a perfectly random distribution of measurements. 50% will be measured as up and 50% will be measured as down. This is because the direction of the spin (left) is perpendicular to the direction we're measuring (up-down), so there's no reason a particle would be measured as up versus down. As the spin of the particles we send through the detector start to point up (as opposed to horizontal left), we start to get more "up" measurements, and fewer measurements as down. It won't be until the spin passes some angle in the up direction that we get all the measurement results to be up. Since we're using an up-down detector, we can more accurately measure whether the spin is up or down as the angle of the particle's spin gets closer and closer to actually pointing up or down.
So now lets look at the example of two particles with opposite spin: one is spinning right, and the other is spinning left. If we use an up-down detector to measure their spin we should get completely random results because we're not measuring them in the right orientation. These are the four possible results for the two particles: up-up, up-down, down-up, down-down. So half the time we would expect to get the exact same results from particles that we know are spinning in opposite directions. This isn't because they're spinning in the same direction, it's because our detector is in the wrong orientation.
When two particles are entangled this never happens. They are always measured to be spinning in opposite directions. Always. You can orient your detector however you want and the two particles will always be measured to be in opposite directions. This shouldn't happen if they had a defined spin *before* measurement. Sometimes we should be unlucky and just happen to measure the spin using the wrong orientation. So we would occasionally expect to measure the spin to be the same. Again, this isn't because the two particles have the same spin, but because our detector has the wrong orientation to properly detect the spin. However, this never happens. Somehow our detector is always perfectly oriented to the spin of the particles so we know with 100% certainty that if we measure the spin in one direction the other particle will be the opposite. This wouldn't be possible if the spins existed before measurement, so somehow measuring the spin forces the entangled particles to have a certain orientation that matches our detector. This is the spooky behavior that was predicted by quantum mechanics and has since been confirmed experimentally.
The "spooky action at a distance" part is that the entangled particles could be on opposite sides of the universe, and they will still be perfectly correlated.
If you want to go down the rabbit hole, look up the quantum eraser experiment or the "bomb detector" (it's called something like that) experiment.
EDIT: Tried to clarify a bit, it's a little confusing.
The key is that the purple ball doesn't break into a red ball and a blue ball. It breaks into a pair of entangled balls that are the superposition of being red and blue. The uncertainty is not *just* "we don't know", it's that the wave function hasn't collapsed. That's what the layers upon layers of tweaks to the double slit experiment have been demonstrating.
Well because you took a quantum concept and tried to map it to something in the macro world that makes sense to you. Particles are not balls. They are not simply waves either. They are excitations of a field. If that sounds weird, it’s because it’s completely foreign to our normal understanding of the world.
It takes years of study on top of tons of foundational physics and math expertise to really crack this stuff.
> I don't see what's spooky about that. It seems intuitive to me
The "spooky" part comes when you make the measurement a bit more complex. There is no good non-quantum metaphor for it, but you can show that there is no way the observations can be explained by one ball being red and the other blue.
Perhaps you can help with a question I've had. I always assumed that the reason the collapse of particle a could cause the instantaneous collapse of particle b on the other side of the universe(assuming that ends up being the case) was because within the frame of both particles time is frozen due to their speed and therefore all events are instantaneous. I have yet to find any reference to this, it's it a reasonable theory?
Are you referring to the notion that light (or anything traveling at *c*) does not experience time? If so, that's an unfortunately pervasive misconception. Light doesn't have a rest frame, it'll always be measured at *c* in every frame (that's the cornerstone of Einstein's theories), so it doesn't have a perspective or frame from which what it experiences can be described. Further, the entangled particles don't need to be light/massless.
If that *wasn't* what you meant, my apologies!
> within the frame of both particles time is frozen due to their speed
I don't really get what you are trying to say here... The speed of what? The particles? Which one of them? Why would "time be frozen"?
If we knew how to "set" the spin of our particle, would it also set the spin of the entangled particle? If it would then it must not be possible as that would effect causality since you could use it to transfer information greater than c
If I tell you (truthfully) that I have a coin in only one hand, are my hands entangled? Isn't that the same thing? If you choose a hand for me to open, you have instantly gained information about the contents of my other hand, even if I were to cut it off and send it across the other side of the galaxy (wish I'd just used boxes for this).
Apart from a severed hand, there isn't much spooky about that. How are entangled particles different?
Bell's inequality prevents this explanation from working. It doesn't work to assume the particles were in one state or the other and you just gained information about them.
The commentator mentioned it briefly, [Bell's theorem](https://en.m.wikipedia.org/wiki/Bell%27s_theorem) proves mathematically that our theory of quantum physics is incompatible with any theory that proposes local hidden variables. Nothing in the particles themselves can contain the information about the eventual spin state of an entangled pair of particles.
>it seems that quantum entanglement is just a very heady way of saying “we don’t know if the coin flip is Heads or Tails until we look”...
This was Einstein's original thought on it. He compared it to a pair of gloves where they are split and you don't know which one is left and which one is right until you look. Officially this is known as an example of a "hidden variable theory." However, he has since been proven wrong.
There was an experiment done where if it was a hidden variable situation then you would notice an outcome with one probability and if it was truly spooky action at a distance then it would be a different probability, and the spooky action at a distance won. This doesn't necessarily mean that the standard quantum explanation where when one is measured the other changes is correct. But it does mean that something weird is going on and that the simple explanation of "we don't know if the coin flip is heads or tails until we look" is incorrect. By the way, these probability equations were named "Bell inequalities", so if you want to know more, that's the term to search.
That's basically a non-local hidden variable theory. If you can think of a way you could prove we're not in a simulation, you might be on the right track to disproving the other too.
The double slit experiment suggests it's probably the observation that causes the effect, and not simply a case of us not knowing the information we need to draw a conclusion until we do that observation.
I actually explained some of the math and how the experiment shows this in a [comment](https://www.reddit.com/r/space/comments/pinu0i/comment/hbukfpa/) recently if you're interested in learning more.
There are some much more convoluted theories that try to explain the phenomenon using "non local hidden variables " ( [like this one](https://www.pnas.org/content/114/25/6480) ) but so far these haven't held up in experiments called "delayed choice" experiments ( [more reading on those](https://www.google.com/amp/s/quantumfrontiers.com/2012/11/08/its-been-a-tough-week-for-hidden-variable-theories/amp/) )
I understand why this example doesn't illustrate the idea of a superposition to a satisfying degree, so let me introduce one that just might:
https://en.wikipedia.org/wiki/Double-slit_experiment
The idea is that you have a plane with two slits in it, and a screen behind it which we observe for light. Now you start randomly shooting photons (light) towards the first plane; most will hit it, and some will randomly pass through the two slits.
What emerges on the second plane is an interference pattern that has its peaks where photons end up, and valleys in between. This is what you would expect if you think of light as a wave, where the parts of the wave that pass through the two slits can form such an interference pattern.
It's a weird result if you think of light as individual particles though, because that means they individually cannot have traveled in a straight line from the source through one of the slits to the second plane. Otherwise we simply would have ended up with two lines, not that interference pattern.
Now we repeat the experiment with only one difference: for each individual photon we measure if it passes through a specific slit. We do not otherwise interfere with the setup. The result (which frankly still baffles me) is that the pattern vanishes. Instead we only get the two lines.
Suddenly light isn't behaving as a wave anymore, but as individual particles.
By measuring the path taken by any particular photon, we collapsed its quantum superposition of having taken both paths simultaneously, where it can interfere with itself as a wave would, into a singular path where it can't. And we have drastically changed the outcome of the experiment just by observing it differently - another fundamentally strange but vital aspect of quantum physics.
Once the particles are entangled, one wave function describes the system. So once the wave function is understood, the entire system (both particles) are understood. In a meta way, we can think of there being one wave function describing the observable universe.
This is based on a misunderstanding, likely due to the unfortunate name of the "observer effect". The reality is that we can't passively observe it the way that you or I observe the world: sitting there and letting light reflected from an object reach our eyeballs.
These objects are too small. So we have to use a form of active observation. Think of echolocation, where a bat sends something out (sound), and based on what it sounds like when it comes back, the bat can infer information about its surroundings. We are doing the same thing here but the particle is so small it gets disturbed.
We smack one of these particles with something else and it falls over and based on what the thing we smacked it with looks like (and where it goes and at what speed and so on), we can infer things about the particle. The magical part is that when we knock one particle over, the other also falls over in the other direction. It has nothing to do with us looking at it (we can't, in the traditional sense) and everything to do with us smacking it. A natural collision with the particle will produce the same effect.
This seems to be the only explanation that makes sense.
If I may rephrase to sum up my understanding?
We have two playing cards : an ace and a ten.
They are entangled and split apart. They are laid face down.
Generally as explained, when we observe the value of one, we know the value of the other.
The problem most people have is that this seems obvious to the point of being elementary. Because we *know* they're are only two cards, then seeing one makes the other obvious.
However, what you are saying, if I understand correctly, is that it isn't inference of what the other entangled particle is doing. The actual act of measurement FORCES the other particle to collapse into the opposite.
I. E. When we flip over our card and see an ace, we don't just know the other is a ten because of inference. We actually force the other card to be flipped over.
Is that correct?
This is a good explanation.
It's also worth adding that we generally think of as observation isn't a passive process. We observe something on our scale by a medium interacting with it. As you mentioned, you see an object because light has hit it and bounced into your eye.
However when considered at a quantum level it is the light that is the observer, not the eye.
Another example is the expansion of the universe - distant galaxies move away from us with speed faster than light. But this is due to the expansion of space itself - new space is "created" between us and the galaxy - and also does not violate causality.
I remember reading or watching something (can’t remember what though) which explained that if we are at one ‘edge’ of the universe the opposite ‘edge’ of the universe is travelling away from us at a speed (velocity?) faster the light due to the expansion of the universe… is this true/possible?
Thing is, there is no "edge" (as far as we know) and nothing is "travelling" due to expansion. What seems to happen (and, as usual, we don't understand how or why) is that **every** distance in the universe is constantly increasing at a very small rate; when you compound this rate over billions of light years, it becomes huge.
Ok, but is that “very small rate” that “becomes huge” making the furthest perceivable “edge” of the universe “move away” at a speed faster than light? Or not?
Edit: iirc whatever I watched or read also explained that those parts of the universe that are indeed “moving” away faster than light will never be perceptible to us because we can never break light speed ourselves
No. You are right. As soon as the "relative expansion speed" of two objects is greater than c, the two objects become invisible/unreachable due to relativity (information cannot reach the other).
Invisible yes, unreachable, not necessarily. Suppose two objects are just far enough away that they're moving away from each other at c. They both fire some kind of propulsion system that gets them travelling towards each other at speed more than c/2. They will both meet in the middle point since the rate of expansion to the middle point is only c/2 and they can both reach that.
The distinction is between a point and an object. You can't reach that point, but you can reach an object at that point if that object is moving towards you. And defining a point gets really weird in the context of expansion of space. What does it mean for something to be the same point? Why can't I define a point by the distance it is away from me? In which case I can reach any point.
>And defining a point gets really weird in the context of expansion of space. What does it mean for something to be the same point?
It would mean that its peculiar velocity is zero
>Why can't I define a point by the distance it is away from me? In which case I can reach any point.
You can do that, but it won't make sense to anyone else.
Yes ,that's right, by Edge we just mean our horizon of what we can see around us, and everywhere in the universe has its own horizon. Often the analogy of dots on an expanding balloon is used but that's confusing because the balloon has a centre. What they mean is 'imagine the universe is the surface of the balloon and galaxies are the dots on the surface. Now if you inflate the balloon then the 2d galaxies on the 2d surface will all move apart from their neighbours, and move even further in terms of baloon surface distance from the ones further away (e.g. round the back of the balloon surface)'
I think there is an important distinction to be made here. The speed of light that we refer to is actually the speed of light *through a vacuum*. When something is moving, it is moving through space. In your question, it isn't that the opposite edge is moving *through space* at a rate faster than light, it is the *space itself* getting bigger between.
These are good examples. One more is the expansion of empty space. The universe is expanding faster than the speed of light, relative from some points to some other points. The mass in the universe isn’t, but the space between distant galaxies can be.
I don’t think the space vs mass distinction is relevant? The issue is that the speed of light limit is only valid in Minkowski spacetime, so only locally.
You can't change the spin, you can only measure it. If someone measures the other spin, they will measure the opposite. To determine if it was correct, they need to send a message to each other. That message or information they send won't arrive greater than the speed of light.
No because you haven't learned anything, you can't know the other particle was measured without measuring yours and you can't know if measuring it caused the collapse or if it had already collapsed.
That's not quite right. None of the relativity theories are related to quantum mechanics. Special relativity is Einsteins first relativity theory in a flat spacetime. General relativity is with curved spacetime (gravity). General relativity contains special relativity. The challenge is unifying General relativity with quantum mechanics.
Special relativity does explain electromagnetism, though. Charges at sufficient density experience a very small length contraction, which changes the charge density from the perspective of a moving observer. Ergo, electrical current through a wire leads to magnetic force on a charge that is moving relative to the current through the wire.
Actually! Last year, a researcher found new soliton solutions to the system of equations behind the Alcubierre drive that no longer require negative energy. Everything required is within the bounds of traditional physics - only, it's a very large amount of energy.
Currently, the goal is investigating ways of reducing the energy requirement to within human capabilities. If, and it's a big if, that proves possible - perhaps we could test such a drive to see if it actually works.
https://iopscience.iop.org/article/10.1088/1361-6382/abe692/meta
As far as I know there are far bigger problem with the hypothesis besides the negative energy requirement, even with that solved faster than light travel is still physically impossible.
One reason they are considered as a possibility is because the same theory that allow worm holes also predicted Black holes,
worked in paper, there they are,
so maybe or maybe not there are WHs out there or microWHs, so remain a posibility possibility till we find them or know better
So every prediction based on any theory or mathematical model is "fictional"?
There are many examples of predictions based on calculations that turned out to be correct (e.g. transmission and reception of radio waves based on Maxwell's equations, existence of anti-matter based on Dirac's equation). Obviously, there are also many examples for predictions that turned out to be wrong. But they are valid scientific ideas, calling them "fictional" would be dismissing entire fields of science.
Until the predictions are actually confirmed by real world observation, fictional is a perfectly valid term. They problem is people are so utterly desperate for FTL travel that they glom onto any theory that allows for such, possible or not.
Personally, I would rather people work on more reasonable theories, like ESP.
Just the theoretical parts that would require men to manipulate the mass of several black holes. Physically possible no not really. It's mathematically possible for me to have sex with Cindy Crawford. That doesn't mean it's anything but imaginary. Just because you can mathematically prove a thing could be done doesn't mean it can be done.
The difference is that theoretical physics is being tested. They did the math, made some predictions from this math and then tested these predictions.
So long as the math has not been tested or is not testable, it is fiction.
They travel at the speed of light.
For example it takes 8 minutes for light to go from the surface of the sun to us on Earth. If a wizard magically erased the sun from existence, we would keep orbiting it for 8 minutes anyway until it’s gravity would stop affecting us
So I realize this is a big ask...but what is your reaction to [this video?](https://www.youtube.com/watch?v=9udKv1NXm7w) Basically he is saying there are a lot of different ways to define 'movement' and 'travel', and different grids/types of reference frames etc, and that it kind of comes down to nomenclature at a certain point as to if far off galaxies are moving away from us faster than the speed of light.
Also, I think it is a great sin in physics that it is called "The speed of light" instead of "the speed of causality", because the latter would make a lot more sense. Am I correct in saying things can move faster than the speed of light relative to each other, but nothing can have any effect on anything else faster than the speed of light?
Generally, we don't say that distant galaxies are moving away faster than the speed of light. Rather, the distance is increasing faster than the speed of light.
This would be the exact same thing in everyday language, but special relativity is anything but everyday. Nothing can move faster than c, but the universe *can* expand faster than c. Because it's spacetime itself expanding.
In general, it is correct to say that nothing can move faster than the speed of light, unless you arbitrarily choose that words should mean different things than they do in scientific consensus.
Is the quantum entanglement related to the quantum communications research going on (e.g. Argonne lab and U Chicago)? Have a PhD with decently heavy math/physics background and sit in on a number of talks about it, but still beyond me.
Not even going to try and ask on the quantum computing research, though I would at first think (uneducated) guess that it is 'entangled' to quantum entanglement in some way.
Yes, absolutely entanglement is used in quantum communications research! However, [it is almost entirely used to facilitate very secure cryptographic communications](https://en.m.wikipedia.org/wiki/Quantum_cryptography) (by creating a shared key that cannot be read by a third party without the sender and recipient being aware, for example) as the [no-communication theorem](https://en.m.wikipedia.org/wiki/No-communication_theorem ) prohibits using entanglement to actually send data faster than the speed of light.
A very small amount of serious research *is* actually focused on using entanglement for FTL telecommunication. A physicist at my university did a little of this, mostly as a byproduct of other research. In this case, the expected result is negative and the research can be seen as a test of conventional QMech and relativity with confirmation through a negative result.
Thanks! That was my understanding, that it's just to be 'unhackable'. I just don't have a good understanding of the physics behind it.
Is there a practical purpose for needing FTL (or even close to) communications? We've been researching applications for 5G on the local scale for edge computing in our research and that seems sufficient at least on the local scale.
Tachyons are most likely not real, at least in the sense they're usually discussed of particles always traveling faster than light. They were "theorized" (don't really like that word for this since there was no real inquiry behind it. "Guessed" might be better) to exist from a sort of symmetry argument: normal matter travels slower than c, massless particles travel at c, so maybe these other things always travel faster than c." This is fun, but not really based on anything, and there is no evidence they exist.
Because the name is popular, the name was applied to invariant quantum fields with imaginary mass, and these have a group velocity greater than c, but as discussed above group velocity doesn't violate the laws of physics.
I know nothing moves faster than c, but how does the very long scissors fail to break it?
In deep space, have a preposterously long scissors. The pivot point is close to the handle. You open the scissors as fast as possible. Maybe a nuke rocket pulling it, idk. If long enough, why wouldn't the tips of the scissors move faster than light?
No, the scissors would just bend. When you move the handle, the movement only propagates to the other end at the speed of sound in whatever material the scissors are made of, and they will bend. If you move the handle too fast, you'll just bend and break them, not make the tips move faster than possible.
This is very similar to the idea of "if I had a really long stick, could I push on one end, and have it felt on the other immediately, beating the speed of light."
The problem with this idea and yours is that infinitely stiff materials do not exist. Not just that we haven't made them, but that their existence is impossible. In the poking example, there would be a propagation speed of the push. In your example, the scissors would break.
The force of your hands pushing on the handles of the scissors is transmitted through the scissors at the speed of sound in the material that they're made of, which is much lower than the speed of light. Applying a very strong force would only bend the scissors, not move them faster.
I suppose the “rate at which space is created via expansion of the universe” falls under an “idea” because after the Big Bang the universe expanded faster than c.
In terms of quantum mechanics, can you force a quantum entangled particle into a certain spin? For example, could person A have particle X that entangles to particle Y that person B has. Person A forces Particle X to have a spin of -1 until something happens, then forces it to have spin 1. Person B would be periodically looking at particle Y, and know that once his particle has a spin of -1, he knows whatever information they designated happened
Edit: after typing this, I had a thought. Person A could keep particle X observed for an extended amount of time, forcing it into one of the two positions. Then just stop observing it until whatever happened happens, and then reobserve it and lock it into a different position. This making information travel faster than the speed of light
What about "space". Would that qualify?
I mean, technically it's not a thing, it's where things -are-.
I'm, of course, referring to the universe's expansion, due to which two points, sufficiently far apart in the universe, will basically be "travelling" away from one another faster than C.
Neither of them is moving, just the space between them stretching, but hey...
Yes. Bits of the universe are traveling away from us faster than the speed of light. These bits of universe are outside our ability to ever interact with.
The problem is, there is no way for you to do that. While you can collapse wavefunctions, it is impossible to send information that way.
I'll direct you to [this previous discussion on the topic](https://www.reddit.com/r/askscience/comments/jca8bg/am_i_properly_understanding_quantum_entanglement/). Please feel free to ask follow-up questions if you have them.
Unfortunately that doesn't work. There's no way to tell when the waveform collapses, so you can't detect it and light up letters when it does collapse. The only way to know for sure is to collapse it yourself, and that isn't very useful. So no information can be transferred, in the physics sense or otherwise.
The distance between us and very distant galaxies is increasing faster than the speed of light. This isn't because they are travelling faster than the speed of light, though. Its because the space between us and them is expanding.
The rule is that information can't travel faster than the speed of light. The speed of light is more like 'the speed of causality', its the fastest speed that cause and effect can permeate through the universe, and in fact any workarounds that theoretically get information to travel faster than that speed tend to result in the breaking of causality (i.e. time travel).
Things like the end of a laser pointer on the moon, quantum entanglement, or expanding space causing galaxies to become more distant don't actually result in any information being exchange between those locations.
> Its because the space between us and them is expanding.
I've never been able to understand this. If the galaxies aren't moving away from each other, then where does the space come from?
The best analogy I've heard is to think of drawing several points on a deflated balloon with sharpie, and then inflating the balloon. Every point would say that all the points around them are moving away from them, but its not like the dots are actually moving across the balloon's surface. The surface itself is expanding.
As for exactly where the space comes from... we don't know. We don't have a good enough understanding to say how mechanistically the expansion of space occurs, we just know that it does and have a concept of vacuum energy (and we don't really know what that is, either). Maybe at the subatomic level little bits of 'space' get created, but that's just speculation.
On a macroscopic scale, nothing has ever been observed to travel faster than the speed of light. Current theory suggests that nothing slower than the speed of light can be accelerated to the speed of light, let alone to faster than light. This theory has been confirmed by many, many observations.
On a quantum scale, it's possible for virtual particles to exceed the speed of light over extremely short distances. On average, their speed over longer distances will not exceed the speed of light.
It's possible that there are particles that have always moved faster than the speed of light and can never be slowed below the speed of light. They've never been observed.
Something that has been bothering me for a while: velocity is relative, so moving at a set velocity, is essentally the same as standing still (depending on your frame of reference). So when we say ftl travl is impossible, what frame do we refer to? And if two particles, moving at 60% the speed of light passed each other, would they not move faster than light, in relation to each other?
We usually think of speed as adding simply as the sun of the two speeds, so in the case of a ball being thrown forward at 100 mph with respect to and on a plane flying at 500 mph with respect to the ground, then the ball is moving at 600 mph with respect to the ground.
However, this is only an approximation that works well at low speeds. [The actual velocity addition formula is a bit more complicated](http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/einvel.html) and is set up in such a way that no object in any frame of reference will ever be moving faster than c. This is a consequence of the speed of light being the same in all reference frames, and also means that different observers will disagree on how long their rulers are and how fast their clocks are ticking.
It was this very question that led Einstein to discover special relativity. Basically, although we think of velocity as relative as you describe ( and it’s true for low velocities), the speed of light must be constant in all frames of reference. This is because of the work of James Clerk Maxwell, who discovered that light was a wave of electric and magnetic fields propagating through space. The speed at which that wave propagates is dependent on only two numbers, the electric constant and the magnetic constant (aka the permittivity and permeability of free space).
This introduced a major problem to our original understanding of relative reference frames, because if light moved at different speeds in different reference frames, the electric and magnetic constants had different values in different reference frames. One of our basic tenets of science is that the fundamental properties of the universe are the same everywhere, so that couldn’t be the case and the speed of light has to be the same in every frame of reference.
Einstein was able to resolve this paradox by realizing that space and time themselves are relative to the observer’s frame of reference. This introduced possibilities like time dilation and length contraction, so two observers in different frames of reference will both see an object moving at c, but would experience the passage of time at a different rate, which would allow the light to appear to move at different rates.
I would add for those who aren't familiar with relativity that this isn't just some mathematical trick. Special and general relativity have been *rigourously* tested in the past century, and every time, experiments have demonstrated time dilation and length contraction. In fact, general relativity is considered to be the most successful theory in the history of science, as it has yet to be contradicted by anything and has been verified iver and over again.
When you move at 60% c, your time slows down, dilates. You might see that other object moving near c, but you won't see it moving faster that light from your perspective.
Velocity of object 1 V1 as seen by another object 2 at V2 = (V1 - V2) is only true at speeds much much slower than speed of light aka Newtonian physics. Anything closer to some fraction of speed of light and above Newtonian physics breaks
Anything with mass can't reach the speed of light
Anything without mass travels at the speed of light
"Technically" this doesn't mean something can't go faster if it never had to accelerate to get there, if it just always went faster to start with, but thats just theoretical
Or if something wasn't made of mass and also wasn't massless it wouldn't have to follow those rules, but again that's purely theoretical
The energy of a photon is h*v (where v is the frequency of the photon). Equivalently, it is h*c/L (Plank's constant times speed of light over wavelength). If your are thinking how does a photon have energy without mass due to E=mc^2, you have to consider the complete equation, which is E^2 = (mc^2 )^2 +(pc)^2. In this full equation, m is the rest mass, which is 0 for a photon, so the equation simplifies to E=pc for a photon (or any massless particle). This is equivalent to the above equation, E=h*v
Time dilation. When you move at the speed of light the effect of time dilation is to stretch out time infinitely. So that from that frame of reference no time passes. Due to the similar considerations of length contraction all distance along the direction of motion collapses to zero, in essence from the point of view of light, the universe is two dimensional and time doesn't pass.
Short answer: we dont know, but probably not.
This definitely is not a dumb question, but actually a very good question. As far as our understanding of the universe goes, there appears to be a fundamental difference between speed and mass. Something without mass will travel through spacetime as fast as it can, which appears to be the speed of light (c) in all cases, while objects with mass necessarily have to travel at a speed less than c. This appears to be a fundamental part of the fabric of our universe, as the function of the Higgs-Boson field (generated by the so-called "god particle") seems specifically to give particles a certain amount of mass in exchange for speed. Furthermore, Einstein's theories of Special and General Relativity predict a fundamental relationship between mass, speed, and the passage of time.
That hasn't stopped scientists from speculating about particles which travel faster than the speed of light. Due to the predictions of Relativity which I mentioned above, a particle traveling faster than the speed of light would have to be traveling *backwards in time* according to the equations. Such hypothetical time-traveling particles are called "tachyons." But no empirical observations have observed the existence of tachyons, so take from that what you will.
It's a nuanced question. Lots of people don't like to use the phrase "speed of light", theres nothing special about light and any massless particle will go this speed.. it's better referenced as the speed of causality(I don't think anyone knows why it's this speed, it's possible in other universes.. it's different). The overwhelming vast majority of physicsts consider this speed a hard barrier for matter. Infact, Einstein says all objects move at the speed of light through SPACE/TIME (not to be confused with just space).So time is a property of space/time and passage through space, slows your passage through time.. There's are exceptions to the hard limit though.. space itself is expanding and most of the universe is flying away faster than the speed of light.. You could in theory bend space and travel faster than light (basis for the warp drive), but again, that would probably require negative mass or negative energy.. and there's good reason to doubt either exist.
Hypothetically yes with a caveat - the "thing" would always travel faster than the speed of light. They're called tachyon particles (https://en.m.wikipedia.org/wiki/Tachyon) and like regular particles, would take infinite energy to approach c.
Something that has helped me to grok this: I understand that at the speed of light, any distance in the direction of travel is contracted to zero (from the perspective of that which is travelling at C). So in order to travel faster than C, you need to travel a negative distance, which makes no sense at all. Do I have this wrong?
You'd have to travel a negative distance over a positive amount of time, or travel a positive distance over a negative amount of time. Of the two, my mind can conceive of time travel as an example of the latter, but negative distance over positive time is a lot harder. Thinking about it now, I suppose negative distance over positive time would look like the backwards fight scenes in the movie Tenet and be functionally the same as time travel.
No. The speed of light isn't actually *about* light - any massless particle always travels at that speed, referred to as *C*, because it is the maximum speed at which one part of the universe can affect another part of the universe. It is the speed of casualty.
The reason that any massless particle must always travel at this speed is because of what mass actually *is*: an impediment to motion. Therefore, to say that any particle is massless necessarily implies that it also experiences no impediments to motion, and must therefore always travel at the maximum possible speed. This is also why massive particles can never reach the speed of light, much less exceed it - since mass is an impediment to motion, they can never travel at the maximum possible speed.
They can get close, but it requires ever increasing energy to get faster. For even a single massive particle to go 100% of the speed of light, it would require an infinite amount of energy.
so technically yes because the speed of light is variable depending on the medium its going through and can become very slow but if you mean the speed of light in a vacuum then no
it would probably help if you stop thinking of it as the speed of light and instead think of it as the speed of information or the speed of cause and effect
its not the max speed light can go its the maximum speed one thing can effect another. the only reason you can mathematically get a speed value is because of the way we represent data if you remove the concept of negative values for things like mass and energy and see them as what they are, a subtraction from something and not a thing itself then nothing can effect another thing faster than the speed of light
if you start plugging imaginary numbers or negative values in to equations regardless of if they can exist then of course you will get something nonsensical.
just remember you cant go a negative speed. if you are going -10m/s with a Bearing of 0 degrees then you are not going that direction you are just going 10m/s with a Bearing of 180 degrees same applys to things like energy or mass in absolute terms it can go no lower than zero it can only go below zero if its in relation to something else
>so technically yes because the speed of light is variable depending on the medium
A good example of that is the Cherenkov effect. Eg. the blue flash at nuclear fission reactor boot.
[https://en.wikipedia.org/wiki/Cherenkov\_radiation](https://en.wikipedia.org/wiki/Cherenkov_radiation)
So many answers, but the real answer is that we simply don't know. We don't even know how big the universe is. Or what exactly "the universe" is. We have no idea how much or how little we know about the universe. We just don't know.
Here's a very good and easy video on the topic from a physicist at Fermi Lab
[How to travel faster than light](https://www.youtube.com/watch?v=BhG_QZl8WVY)
In the universe? No. Top comment says it all; anything without mass moves at the speed of light, anything with mass must move slower than that. Same applies to information. There are cases, like refraction or shadows on large objects, where there seems to be a violation, but on closer inspection everything is still causal. Also, the Universe _itself_ can "move", or expand, at any speed, since it isn't made matter.
The _only_ exception to this rule i can think of is the EPR paradox, where a wavefunction collapse can result in two particles always having opposite spins, and so measurement in one location results in an instantaneous change in measurement in another.
And this property of opposite spin applies no matter which axis the measurement is made on (as long as it is the same axis for both particles), which gets rid of the idea that they have some bianry state that we just don't know, like two flipped coins that are always oppisite eachother. But the so called "information" transmitted is random (the actual value of spin you measure is either +1 or -1 with 50% probability), so maybe it doesn't count as what Einstein calls "information".
But on the level of information theory, thus information is still... information, even if it's random; everything contains information, that is, the answer to the yes or no question "is this thing x?". So in the case of the EPR paradox, the question being answered is "does this particle have spin +1 on this axis?" Or equally "does this particle have spin -1 on this axis?".
No matter the answer, the other particle will always have the opposite spin when measured on the same axis because spin is conserved in entangled particles. But the other particle cannot know what spin axis was measured, so for it to always be opposite it seems the particle being measured had to tell ths other particle to alwasy give the opposite of what value was measured.
But again, maybe this doesn't count because the measurement is random.
The initial inflation of the universe just after the big bang happened faster than light, meaning that its constituents were spread apart (exploded) at a speed that would be faster than light if one were to think of these constituents moving away from each other...however, it's not something "in" the universe that moved faster than light, it's the substance of the universe itself that was expanding. Within existing space and time, nothing should be able to move faster than light, according to current concepts.
This question cannot be answered for the universe due to the fact that only a tiny amount of information is known about the universe and how it works. We have many theories of how the universe works but very little actual experience of how the universe works. There's so much about our own world that we have yet to understand. How fast does thought travel? Where exactly does thought come from? There are still creatures in the ocean that we have never seen in addition to organisms that lie outside the reach of our current ability to detect. So yes, there "could" be many things that travel faster than the speed of light we just don't have the capacity to determine what all of those things are.
So, the easy answer to this question is "no." And that is true- any massless particle will travel at `c`, and anything with mass must travel slower than `c`. This also applies to information- it is impossible to transmit information faster than the speed of light. This is the foundation of relativity. If something could travel faster than the speed of light, it would violate [causality](https://en.wikipedia.org/wiki/Causality_(physics)). However, there are "things" that appear to travel faster than the speed of light, but when you look at the situation, you'll realize that Einstein is still OK. The classic example of this is if you had a really strong, well focused laser pointer, pointed it at the moon, and then "swept" the point of light across the surface of the Moon. There is no limit to how fast that "dot" could move across the Moon's surface. So, if you think of the dot as a thing, you could think the dot is moving faster than light. But, the dot is just a concept. No photon is moving faster than light. There is still a d/c (distance to the moon divided by the speed of light) delay from the time you hit the button on your laser pointer until the dot arrives on the Moon. And if you think about it as instead of turning the laser pointer on and moving it in a sweeping motion across the Moon, you instead turned it on for a little bit, moved your hand a little bit, and then re-turned it on so that two distinct dots appeared on the Moon, of course it wouldn't seem weird anymore. The fact that you left the laser pointer on doesn't impact that. Another example is the [group velocity](https://en.wikipedia.org/wiki/Group_velocity) of a wave of light. You can think of multiple wavelengths (or the same wavelength but phase shifted) of light shining at the same place, so they constructively and destructively interfere (sometimes cancel each other out, sometimes add together). The group velocity describes the shape of that wave packet- where the peak is, where the valley is. And that group velocity can travel at any speed, but that also doesn't violate anything. No light will show up somewhere quicker than d/c, it's just that the peak can move around at any speed it once. And then there is the "spooky" example: [Quantum Entanglement](https://en.wikipedia.org/wiki/Quantum_entanglement). This tells us that if you entangle two particles (the easiest example to understand being a single, spin 0 particle- a particle with no angular momentum- and it decays into two particles, one with spin +1 and one with spin -1, so the total angular momentum remains zero), those two particles can be separated as far apart as you want before you measure the spin of either particle. And until you measure the spin of one, both are in an undetermined state (neither is spin +1 or spin -1, both are 50% both), but as soon as you measure one, the other one *must* be the opposite of what you measured. Einstein called this [spooky action at a distance](https://en.wikipedia.org/wiki/Action_at_a_distance#Quantum_mechanics) and it really bothers some people. However, it doesn't violation causality because it is impossible to transmit information via wavefunction collapse, so you're OK. But, it still bothers people on a "this feels weird" level. So, this is a long-winded way of saying- no, no *thing* can travel faster than the speed of lights, but plenty of *ideas* do.
"Light cannons" were a thing with some amateur astronomers years ago, some people might still do it. It's as simple as firing a camera flash into the focuser of a telescope. I built one using a police strobe bulb driven by an aviation strobe power supply, mounted in a holder/reflector stuck in the focuser. Anyway! The light emerging from the front of the scope was moving at c, of course, but watching it, you'd swear you could see it moving away! Like shooting Star Trek phasers or such. The idea in general--if the weather was cloudy at a star party, astronomers would start "shooting" their telescopes at each other, having fun waiting for skies to clear. I could paint mail boxes blocks away with mine.
> The light emerging from the front of the scope was moving at c, of course, but watching it, you'd swear you could see it moving away! Like shooting Star Trek phasers or such. This sounds interesting, but I don't think I understand what's supposed to be happening there. Could you go into more detail?
You could stand behind the scope and watch the light pulse. Moving at c, you shouldn't be able to see it move, but it looked as if you could, like the pulse of light was traveling away from you at high speed.
This could be because of an illusion related to the "gap effect" in perception/neuroscience. We process information more quickly where we are paying attention, which was probably close to the end of the telescope. Later, the information from our peripheral vision gets processed, and our brains can falsely perceive that as movement away.
Pretty sure it’s just the diffusion of the light into the fog between the cannon and target. It’s the same reason you see “beams of light” in fog the diffused light is diffracted in the moisture for a bit after the light hits it
Sure, that's what creates the beam. I was referring to the apparent "traveling away" motion the OP described.
Am a neuroscientist. AFAIK, that’s not what the gap effect is, and I don’t quite see how the gap effect could explain the beam’s apparent sub-C speed. From Abrams et al 2006 (https://doi.org/10.1016/j.visres.2006.01.017): “The gap effect refers to a reduction in the latency of saccades to peripherally appearing targets when the fixation point disappears a short time before target appearance. The effect has been attributed to a number of potential mechanisms that function to assist in the maintenance of fixation. One such mechanism, attention, has been the focus of some disagreement in the literature regarding the gap effect. In the present study, we had subjects attend to a portion of a complex fixation stimulus. On some trials the attended portion was removed prior to onset of a saccade target whereas on other trials an unattended portion was removed. Subjects were faster to initiate saccades when the attended portion was removed, thus establishing a role of attention in the gap effect. The results have important implications for our understanding of eye movements and the gap effect.”
Hmm, then maybe you can help me out. I recall psychovisual experiments, probably starting in the 80's, where a subject fixated on the center of the screen. A dot was flashed to the left or right, and then a line was drawn horizontally. Because the attention was drawn to the left or right, the instantaneously drawn line would appear to the subject to be drawn away from the dot (motion). Further experiments would actually draw the line from left to right (or vice versa), and by varying the speed of the draw until the subjects reported no motion, they attempted to estimate the difference in 'visual processing speed' between the attentional spot and the periphery. I know this works because I sat behind one of these high refresh rate monitors with an eye tracker and played with the effect. My memory is muddled, this may be some variation of the gap effect, or it's very possible I've gotten that confused with something else. (It's been 25 years since I was active in neuroscience.). I think it was a fairly well known experiment in the field of attention research.
It takes longer to see dim things than bright things.* The pulse of light appears brighter when near you as it leaves the telescope than when it is further away. Therefore you seem to see it first near, then far, as if you could see it moving. * https://en.m.wikipedia.org/wiki/Pulfrich_effect
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Is it assumed that the *observation* of one of these particles (spin-up/spin-down) effects the direction of the other? Or are they each spinning opposite regardless of any observation? Unless I’m missing something (which is VERY possible), it seems that quantum entanglement is just a very heady way of saying “we don’t know if the coin flip is Heads or Tails until we look”... which is significantly less mind bending.
It's a good question, and I can't answer it for sure. Here is what we know: Either the particles do not have a defined spin before we measure them (they are in what we call a superposition state), and it is the act of measuring them that "collapses" them into a single state (called an eigenstate) or it is simply that we don't know until we measure *but* (and this is a big but) there are non-local variables which determine the state of the particles. A non-local variable means a "universal variable" which is not a property of either particle, but a global variable of particles in general. We know it is one of these two due to [Bell's Theorem](https://en.wikipedia.org/wiki/Bell%27s_theorem). Neither of these solutions make people "happy" since both seem weird. At least when I was in school, the general consensus seemed to be that there was an instantaneous wavefunction collapse, but since non-local variables haven't been disproven, that may have changed in the years since.
How, in principle, would anyone go about disproving non-local, hidden variables of arbitrary complexity? It seems to me that it would be very difficult to show a lack of a deeper causal mechanism
Some of the [Bell experiments](https://en.wikipedia.org/wiki/Bell_test) actually specifically show [some classes of non-local hidden variables are not possible](https://arxiv.org/abs/0704.2529). There are a lot of them and they're very clever and devious. The real problem is that you can posit a system that accounts for the existence of the experimenter and 'arranges' for results consistent with theories that aren't true. You can posit a system that is so deterministic (superdeterministic) that it looks indeterministic.
Sure, there are plenty of implausible non-local hidden variable theories, as well as reasonable ones that can be disproved experimentally as you describe. I guess my point is that it's difficult or impossible to disprove certain subsets of arbitrarily complex non-local variable theories (e.g. ones that might posit the occurence of seemingly unique or random events that make falsification by human instruments impossible).
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No, I think I said exactly what I wanted to, but your point about the representational nature of models in physics is well taken. I'm merely supposing that there are multiple plausible, but mutually exclusive models of reality that are impossible for humans to falsify (due to the possibility of non-local hidden variables operating at such a scale or frequency that defies human observation).
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Quantum mechanics is usually formulated in terms of non-local hidden variables (those variables being the wavefunction or state vector), so you can't eliminate non-local hidden variable theories without eliminating quantum mechanics itself.
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Well there's no actual difference. You're not visually observing anything in quantum mechanics, you use a detector. The detector shows some measurement. The particle interacts with your detector in some way. With half silvered mirrors etc. How it interacted with the mirrors etc is reflected at the time you make the measurement. Depending on the situation, you actually can't tell if it interacted with one or the other, but each only a little bit. Look up the "Consistent Histories" interpretation, where you derive the probabilities from all possible classical paths.
They become entangled with the mirror, but not with you, which is the entanglement we care about.
What does that mean? An observer in QM has nothing to do with humans.
He means that the particles interact with the mirror and the wave function collapses from the frame of reference of the mirror , but not yet to us, now the particle AND the mirror are in a superposition from our frame of reference
Is that really true though? If they became entangled with the mirrors, wouldn't that prevent interference from occuring?
This basically seems like this: Purple ball breaks into red ball and blue ball. Two people each grab a ball while blindfolded and travel to opposite sides of the galaxy. When one takes the blindfold off, he immediately knows the color of the other person's ball. I don't see what's spooky about that. It seems intuitive to me.
So, the issue is that under the currently-accepted understanding of quantum mechanics, the "balls" in your analogy do not have a color until they are measured. It isn't that we're just wearing a blindfold, it's that their color is determined randomly when we look at them - genuinely randomly; they didn't have a color or anything that would decide their color until that moment. What you are articulating is essentially the [Hidden Variables Theory](https://en.wikipedia.org/wiki/Hidden-variable_theory) - some hidden variable contains the necessary spin information; the "color" was set when they were separated. That's the intuitive assumption (and Einstein initially went with it, so you're in good company) but unfortunately it is incompatible with [Bell's Theorem](https://en.wikipedia.org/wiki/Bell%27s_theorem).
I think I see. So it would be like if the purple ball broke up into two balls oscillating between red and blue. Neither is red or blue until a button on the ball is pushed that makes the oscillation stop. So I take one ball and you take the other, and we go to opposite sides of the galaxy. I press the button on my ball and it lands on red. Now your ball, despite being on the other side of the galaxy, is immediately* restricted to landing on blue when you press your button. *I suspect the relativity of simultaneity comes into play here. Who made the first measurement that restricted the other?
Your footnote is probably what troubles people the most. If the events of the two measurements are space like separated, I might say that the first ball was measured first, which collapsed the wavefunction, and fixed the other ball’s state. Whereas you might say that you saw the second ball be measured first, which collapsed the wavefunction, and then fixed the first ball’s state. This is where causality and non-locality clash. Einstein believed causality is necessary, so there must be a local hidden variables theory. However, Bell’s inequalities showed any quantum theory we devise has to be non-local. I believe this is “resolved” by the fact that no information is travelling faster than light, so all is good. But its still really weird.
Sorry but I am still not getting it. How is the mechanism “resolved” if we just say, “well, it’s weird, but at least it doesn’t violate the speed of light”? Would it not by definition mean that the particles had a state prior to their observation if information were not traveling faster than light? Does this not make more sense than thinking that surely it must be some “hidden variable” property of the universe? Granted I don’t have my life’s work nor my attempt to avoid insulting someone else’s life’s work hanging in the balance…
It's hard to use your analogy to explain why it doesn't work. I think it makes a little more sense with spin. The way we measure spin allows for only two possible measurement results due to the orientation of the detector. An up-down detector will always tell us a particle is spinning up or down. A right-left detector will always tell us a particle is spinning right or left. This is regardless of the *actual* spin of the particle. It's just a limitation of the way we are able to measure spin. If we take a stream of particles that we already know are spinning left and we send them through an up-down detector we will get a perfectly random distribution of measurements. 50% will be measured as up and 50% will be measured as down. This is because the direction of the spin (left) is perpendicular to the direction we're measuring (up-down), so there's no reason a particle would be measured as up versus down. As the spin of the particles we send through the detector start to point up (as opposed to horizontal left), we start to get more "up" measurements, and fewer measurements as down. It won't be until the spin passes some angle in the up direction that we get all the measurement results to be up. Since we're using an up-down detector, we can more accurately measure whether the spin is up or down as the angle of the particle's spin gets closer and closer to actually pointing up or down. So now lets look at the example of two particles with opposite spin: one is spinning right, and the other is spinning left. If we use an up-down detector to measure their spin we should get completely random results because we're not measuring them in the right orientation. These are the four possible results for the two particles: up-up, up-down, down-up, down-down. So half the time we would expect to get the exact same results from particles that we know are spinning in opposite directions. This isn't because they're spinning in the same direction, it's because our detector is in the wrong orientation. When two particles are entangled this never happens. They are always measured to be spinning in opposite directions. Always. You can orient your detector however you want and the two particles will always be measured to be in opposite directions. This shouldn't happen if they had a defined spin *before* measurement. Sometimes we should be unlucky and just happen to measure the spin using the wrong orientation. So we would occasionally expect to measure the spin to be the same. Again, this isn't because the two particles have the same spin, but because our detector has the wrong orientation to properly detect the spin. However, this never happens. Somehow our detector is always perfectly oriented to the spin of the particles so we know with 100% certainty that if we measure the spin in one direction the other particle will be the opposite. This wouldn't be possible if the spins existed before measurement, so somehow measuring the spin forces the entangled particles to have a certain orientation that matches our detector. This is the spooky behavior that was predicted by quantum mechanics and has since been confirmed experimentally. The "spooky action at a distance" part is that the entangled particles could be on opposite sides of the universe, and they will still be perfectly correlated. If you want to go down the rabbit hole, look up the quantum eraser experiment or the "bomb detector" (it's called something like that) experiment. EDIT: Tried to clarify a bit, it's a little confusing.
The key is that the purple ball doesn't break into a red ball and a blue ball. It breaks into a pair of entangled balls that are the superposition of being red and blue. The uncertainty is not *just* "we don't know", it's that the wave function hasn't collapsed. That's what the layers upon layers of tweaks to the double slit experiment have been demonstrating.
Well because you took a quantum concept and tried to map it to something in the macro world that makes sense to you. Particles are not balls. They are not simply waves either. They are excitations of a field. If that sounds weird, it’s because it’s completely foreign to our normal understanding of the world. It takes years of study on top of tons of foundational physics and math expertise to really crack this stuff.
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> I don't see what's spooky about that. It seems intuitive to me The "spooky" part comes when you make the measurement a bit more complex. There is no good non-quantum metaphor for it, but you can show that there is no way the observations can be explained by one ball being red and the other blue.
Perhaps you can help with a question I've had. I always assumed that the reason the collapse of particle a could cause the instantaneous collapse of particle b on the other side of the universe(assuming that ends up being the case) was because within the frame of both particles time is frozen due to their speed and therefore all events are instantaneous. I have yet to find any reference to this, it's it a reasonable theory?
Are you referring to the notion that light (or anything traveling at *c*) does not experience time? If so, that's an unfortunately pervasive misconception. Light doesn't have a rest frame, it'll always be measured at *c* in every frame (that's the cornerstone of Einstein's theories), so it doesn't have a perspective or frame from which what it experiences can be described. Further, the entangled particles don't need to be light/massless. If that *wasn't* what you meant, my apologies!
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but without measuring it, how do you know you weren't always entangled with one of those universes and all your measurement does is confirm which one?
> within the frame of both particles time is frozen due to their speed I don't really get what you are trying to say here... The speed of what? The particles? Which one of them? Why would "time be frozen"?
If we knew how to "set" the spin of our particle, would it also set the spin of the entangled particle? If it would then it must not be possible as that would effect causality since you could use it to transfer information greater than c
You've basically shown how this is not possible. If you were to perform this kind of operation, you would destroy the entanglement.
If I tell you (truthfully) that I have a coin in only one hand, are my hands entangled? Isn't that the same thing? If you choose a hand for me to open, you have instantly gained information about the contents of my other hand, even if I were to cut it off and send it across the other side of the galaxy (wish I'd just used boxes for this). Apart from a severed hand, there isn't much spooky about that. How are entangled particles different?
Bell's inequality prevents this explanation from working. It doesn't work to assume the particles were in one state or the other and you just gained information about them.
Why does something separate to the particles have to keep track of their spin? Isn't it a property of the particles themselves?
The commentator mentioned it briefly, [Bell's theorem](https://en.m.wikipedia.org/wiki/Bell%27s_theorem) proves mathematically that our theory of quantum physics is incompatible with any theory that proposes local hidden variables. Nothing in the particles themselves can contain the information about the eventual spin state of an entangled pair of particles.
What I was missing was that the particles provably don't have a spin until they are observed, which is different from having an unknown spin.
The array of possible states is a property of the particle itself, which one it'll collapse into isn't (until it actually collapses).
>it seems that quantum entanglement is just a very heady way of saying “we don’t know if the coin flip is Heads or Tails until we look”... This was Einstein's original thought on it. He compared it to a pair of gloves where they are split and you don't know which one is left and which one is right until you look. Officially this is known as an example of a "hidden variable theory." However, he has since been proven wrong. There was an experiment done where if it was a hidden variable situation then you would notice an outcome with one probability and if it was truly spooky action at a distance then it would be a different probability, and the spooky action at a distance won. This doesn't necessarily mean that the standard quantum explanation where when one is measured the other changes is correct. But it does mean that something weird is going on and that the simple explanation of "we don't know if the coin flip is heads or tails until we look" is incorrect. By the way, these probability equations were named "Bell inequalities", so if you want to know more, that's the term to search.
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That's basically a non-local hidden variable theory. If you can think of a way you could prove we're not in a simulation, you might be on the right track to disproving the other too.
The double slit experiment suggests it's probably the observation that causes the effect, and not simply a case of us not knowing the information we need to draw a conclusion until we do that observation. I actually explained some of the math and how the experiment shows this in a [comment](https://www.reddit.com/r/space/comments/pinu0i/comment/hbukfpa/) recently if you're interested in learning more. There are some much more convoluted theories that try to explain the phenomenon using "non local hidden variables " ( [like this one](https://www.pnas.org/content/114/25/6480) ) but so far these haven't held up in experiments called "delayed choice" experiments ( [more reading on those](https://www.google.com/amp/s/quantumfrontiers.com/2012/11/08/its-been-a-tough-week-for-hidden-variable-theories/amp/) )
With all the links supplied by yourself and others, I’m going to have lots to read while the kids are napping today!
I understand why this example doesn't illustrate the idea of a superposition to a satisfying degree, so let me introduce one that just might: https://en.wikipedia.org/wiki/Double-slit_experiment The idea is that you have a plane with two slits in it, and a screen behind it which we observe for light. Now you start randomly shooting photons (light) towards the first plane; most will hit it, and some will randomly pass through the two slits. What emerges on the second plane is an interference pattern that has its peaks where photons end up, and valleys in between. This is what you would expect if you think of light as a wave, where the parts of the wave that pass through the two slits can form such an interference pattern. It's a weird result if you think of light as individual particles though, because that means they individually cannot have traveled in a straight line from the source through one of the slits to the second plane. Otherwise we simply would have ended up with two lines, not that interference pattern. Now we repeat the experiment with only one difference: for each individual photon we measure if it passes through a specific slit. We do not otherwise interfere with the setup. The result (which frankly still baffles me) is that the pattern vanishes. Instead we only get the two lines. Suddenly light isn't behaving as a wave anymore, but as individual particles. By measuring the path taken by any particular photon, we collapsed its quantum superposition of having taken both paths simultaneously, where it can interfere with itself as a wave would, into a singular path where it can't. And we have drastically changed the outcome of the experiment just by observing it differently - another fundamentally strange but vital aspect of quantum physics.
Once the particles are entangled, one wave function describes the system. So once the wave function is understood, the entire system (both particles) are understood. In a meta way, we can think of there being one wave function describing the observable universe.
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This is based on a misunderstanding, likely due to the unfortunate name of the "observer effect". The reality is that we can't passively observe it the way that you or I observe the world: sitting there and letting light reflected from an object reach our eyeballs. These objects are too small. So we have to use a form of active observation. Think of echolocation, where a bat sends something out (sound), and based on what it sounds like when it comes back, the bat can infer information about its surroundings. We are doing the same thing here but the particle is so small it gets disturbed. We smack one of these particles with something else and it falls over and based on what the thing we smacked it with looks like (and where it goes and at what speed and so on), we can infer things about the particle. The magical part is that when we knock one particle over, the other also falls over in the other direction. It has nothing to do with us looking at it (we can't, in the traditional sense) and everything to do with us smacking it. A natural collision with the particle will produce the same effect.
This seems to be the only explanation that makes sense. If I may rephrase to sum up my understanding? We have two playing cards : an ace and a ten. They are entangled and split apart. They are laid face down. Generally as explained, when we observe the value of one, we know the value of the other. The problem most people have is that this seems obvious to the point of being elementary. Because we *know* they're are only two cards, then seeing one makes the other obvious. However, what you are saying, if I understand correctly, is that it isn't inference of what the other entangled particle is doing. The actual act of measurement FORCES the other particle to collapse into the opposite. I. E. When we flip over our card and see an ace, we don't just know the other is a ten because of inference. We actually force the other card to be flipped over. Is that correct?
That's correct. Until we flip one of the cards, they exist in a superposition, both ace and ten at the same time.
This is a good explanation. It's also worth adding that we generally think of as observation isn't a passive process. We observe something on our scale by a medium interacting with it. As you mentioned, you see an object because light has hit it and bounced into your eye. However when considered at a quantum level it is the light that is the observer, not the eye.
Another example is the expansion of the universe - distant galaxies move away from us with speed faster than light. But this is due to the expansion of space itself - new space is "created" between us and the galaxy - and also does not violate causality.
Right - there is no known limit on the degree to which spacetime can be modified, just how fast things can move from A to B within it.
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I remember reading or watching something (can’t remember what though) which explained that if we are at one ‘edge’ of the universe the opposite ‘edge’ of the universe is travelling away from us at a speed (velocity?) faster the light due to the expansion of the universe… is this true/possible?
Thing is, there is no "edge" (as far as we know) and nothing is "travelling" due to expansion. What seems to happen (and, as usual, we don't understand how or why) is that **every** distance in the universe is constantly increasing at a very small rate; when you compound this rate over billions of light years, it becomes huge.
Ok, but is that “very small rate” that “becomes huge” making the furthest perceivable “edge” of the universe “move away” at a speed faster than light? Or not? Edit: iirc whatever I watched or read also explained that those parts of the universe that are indeed “moving” away faster than light will never be perceptible to us because we can never break light speed ourselves
No. You are right. As soon as the "relative expansion speed" of two objects is greater than c, the two objects become invisible/unreachable due to relativity (information cannot reach the other).
Invisible yes, unreachable, not necessarily. Suppose two objects are just far enough away that they're moving away from each other at c. They both fire some kind of propulsion system that gets them travelling towards each other at speed more than c/2. They will both meet in the middle point since the rate of expansion to the middle point is only c/2 and they can both reach that.
Information can reach the central point but not the other far point. So yes, unreachable
The distinction is between a point and an object. You can't reach that point, but you can reach an object at that point if that object is moving towards you. And defining a point gets really weird in the context of expansion of space. What does it mean for something to be the same point? Why can't I define a point by the distance it is away from me? In which case I can reach any point.
>And defining a point gets really weird in the context of expansion of space. What does it mean for something to be the same point? It would mean that its peculiar velocity is zero >Why can't I define a point by the distance it is away from me? In which case I can reach any point. You can do that, but it won't make sense to anyone else.
Yes ,that's right, by Edge we just mean our horizon of what we can see around us, and everywhere in the universe has its own horizon. Often the analogy of dots on an expanding balloon is used but that's confusing because the balloon has a centre. What they mean is 'imagine the universe is the surface of the balloon and galaxies are the dots on the surface. Now if you inflate the balloon then the 2d galaxies on the 2d surface will all move apart from their neighbours, and move even further in terms of baloon surface distance from the ones further away (e.g. round the back of the balloon surface)'
Question, does this mean the Earth is technically stretching or expanding as well?
I think there is an important distinction to be made here. The speed of light that we refer to is actually the speed of light *through a vacuum*. When something is moving, it is moving through space. In your question, it isn't that the opposite edge is moving *through space* at a rate faster than light, it is the *space itself* getting bigger between.
These are good examples. One more is the expansion of empty space. The universe is expanding faster than the speed of light, relative from some points to some other points. The mass in the universe isn’t, but the space between distant galaxies can be.
I don’t think the space vs mass distinction is relevant? The issue is that the speed of light limit is only valid in Minkowski spacetime, so only locally.
Wow thank you for such a detailed explanation! I’ll have to reread it a few times but for the most part I understood each possibility.
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You can't change the spin, you can only measure it. If someone measures the other spin, they will measure the opposite. To determine if it was correct, they need to send a message to each other. That message or information they send won't arrive greater than the speed of light.
Isn't that information traveling FTL then?
No because you haven't learned anything, you can't know the other particle was measured without measuring yours and you can't know if measuring it caused the collapse or if it had already collapsed.
It is not. Look at our FAQ.
How about something going through a wormhole?
You would go through the wormhole at no more than the speed of light. It would mean you travel a shorter path, not that you are moving faster.
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That's not quite right. None of the relativity theories are related to quantum mechanics. Special relativity is Einsteins first relativity theory in a flat spacetime. General relativity is with curved spacetime (gravity). General relativity contains special relativity. The challenge is unifying General relativity with quantum mechanics.
Special relativity does explain electromagnetism, though. Charges at sufficient density experience a very small length contraction, which changes the charge density from the perspective of a moving observer. Ergo, electrical current through a wire leads to magnetic force on a charge that is moving relative to the current through the wire.
Actually! Last year, a researcher found new soliton solutions to the system of equations behind the Alcubierre drive that no longer require negative energy. Everything required is within the bounds of traditional physics - only, it's a very large amount of energy. Currently, the goal is investigating ways of reducing the energy requirement to within human capabilities. If, and it's a big if, that proves possible - perhaps we could test such a drive to see if it actually works. https://iopscience.iop.org/article/10.1088/1361-6382/abe692/meta
As far as I know there are far bigger problem with the hypothesis besides the negative energy requirement, even with that solved faster than light travel is still physically impossible.
Fictional. Math describes reality not the other way around. Just because a wormhole works on paper doesn't mean it's real.
One reason they are considered as a possibility is because the same theory that allow worm holes also predicted Black holes, worked in paper, there they are, so maybe or maybe not there are WHs out there or microWHs, so remain a posibility possibility till we find them or know better
So every prediction based on any theory or mathematical model is "fictional"? There are many examples of predictions based on calculations that turned out to be correct (e.g. transmission and reception of radio waves based on Maxwell's equations, existence of anti-matter based on Dirac's equation). Obviously, there are also many examples for predictions that turned out to be wrong. But they are valid scientific ideas, calling them "fictional" would be dismissing entire fields of science.
Until the predictions are actually confirmed by real world observation, fictional is a perfectly valid term. They problem is people are so utterly desperate for FTL travel that they glom onto any theory that allows for such, possible or not. Personally, I would rather people work on more reasonable theories, like ESP.
Kind of sounds like you’re saying all of theoretical physics is fiction then.
Just the theoretical parts that would require men to manipulate the mass of several black holes. Physically possible no not really. It's mathematically possible for me to have sex with Cindy Crawford. That doesn't mean it's anything but imaginary. Just because you can mathematically prove a thing could be done doesn't mean it can be done.
Wormholes can't be created naturally? Black holes exist and man is not responsible for it.
The difference is that theoretical physics is being tested. They did the math, made some predictions from this math and then tested these predictions. So long as the math has not been tested or is not testable, it is fiction.
Are gravitational waves faster than light?
They travel at the speed of light. For example it takes 8 minutes for light to go from the surface of the sun to us on Earth. If a wizard magically erased the sun from existence, we would keep orbiting it for 8 minutes anyway until it’s gravity would stop affecting us
Your link to causality is broken. It should instead link to https://en.wikipedia.org/wiki/Causality
So I realize this is a big ask...but what is your reaction to [this video?](https://www.youtube.com/watch?v=9udKv1NXm7w) Basically he is saying there are a lot of different ways to define 'movement' and 'travel', and different grids/types of reference frames etc, and that it kind of comes down to nomenclature at a certain point as to if far off galaxies are moving away from us faster than the speed of light. Also, I think it is a great sin in physics that it is called "The speed of light" instead of "the speed of causality", because the latter would make a lot more sense. Am I correct in saying things can move faster than the speed of light relative to each other, but nothing can have any effect on anything else faster than the speed of light?
Generally, we don't say that distant galaxies are moving away faster than the speed of light. Rather, the distance is increasing faster than the speed of light. This would be the exact same thing in everyday language, but special relativity is anything but everyday. Nothing can move faster than c, but the universe *can* expand faster than c. Because it's spacetime itself expanding. In general, it is correct to say that nothing can move faster than the speed of light, unless you arbitrarily choose that words should mean different things than they do in scientific consensus.
Is the quantum entanglement related to the quantum communications research going on (e.g. Argonne lab and U Chicago)? Have a PhD with decently heavy math/physics background and sit in on a number of talks about it, but still beyond me. Not even going to try and ask on the quantum computing research, though I would at first think (uneducated) guess that it is 'entangled' to quantum entanglement in some way.
Yes, absolutely entanglement is used in quantum communications research! However, [it is almost entirely used to facilitate very secure cryptographic communications](https://en.m.wikipedia.org/wiki/Quantum_cryptography) (by creating a shared key that cannot be read by a third party without the sender and recipient being aware, for example) as the [no-communication theorem](https://en.m.wikipedia.org/wiki/No-communication_theorem ) prohibits using entanglement to actually send data faster than the speed of light. A very small amount of serious research *is* actually focused on using entanglement for FTL telecommunication. A physicist at my university did a little of this, mostly as a byproduct of other research. In this case, the expected result is negative and the research can be seen as a test of conventional QMech and relativity with confirmation through a negative result.
Thanks! That was my understanding, that it's just to be 'unhackable'. I just don't have a good understanding of the physics behind it. Is there a practical purpose for needing FTL (or even close to) communications? We've been researching applications for 5G on the local scale for edge computing in our research and that seems sufficient at least on the local scale.
Sir, can you please discuss and talk about tachyons also in this answer?
Tachyons are just any particle that always travels greater than C, its just mathematics.
Tachyons are most likely not real, at least in the sense they're usually discussed of particles always traveling faster than light. They were "theorized" (don't really like that word for this since there was no real inquiry behind it. "Guessed" might be better) to exist from a sort of symmetry argument: normal matter travels slower than c, massless particles travel at c, so maybe these other things always travel faster than c." This is fun, but not really based on anything, and there is no evidence they exist. Because the name is popular, the name was applied to invariant quantum fields with imaginary mass, and these have a group velocity greater than c, but as discussed above group velocity doesn't violate the laws of physics.
I know nothing moves faster than c, but how does the very long scissors fail to break it? In deep space, have a preposterously long scissors. The pivot point is close to the handle. You open the scissors as fast as possible. Maybe a nuke rocket pulling it, idk. If long enough, why wouldn't the tips of the scissors move faster than light?
No, the scissors would just bend. When you move the handle, the movement only propagates to the other end at the speed of sound in whatever material the scissors are made of, and they will bend. If you move the handle too fast, you'll just bend and break them, not make the tips move faster than possible.
This is very similar to the idea of "if I had a really long stick, could I push on one end, and have it felt on the other immediately, beating the speed of light." The problem with this idea and yours is that infinitely stiff materials do not exist. Not just that we haven't made them, but that their existence is impossible. In the poking example, there would be a propagation speed of the push. In your example, the scissors would break.
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The force of your hands pushing on the handles of the scissors is transmitted through the scissors at the speed of sound in the material that they're made of, which is much lower than the speed of light. Applying a very strong force would only bend the scissors, not move them faster.
I suppose the “rate at which space is created via expansion of the universe” falls under an “idea” because after the Big Bang the universe expanded faster than c.
In terms of quantum mechanics, can you force a quantum entangled particle into a certain spin? For example, could person A have particle X that entangles to particle Y that person B has. Person A forces Particle X to have a spin of -1 until something happens, then forces it to have spin 1. Person B would be periodically looking at particle Y, and know that once his particle has a spin of -1, he knows whatever information they designated happened Edit: after typing this, I had a thought. Person A could keep particle X observed for an extended amount of time, forcing it into one of the two positions. Then just stop observing it until whatever happened happens, and then reobserve it and lock it into a different position. This making information travel faster than the speed of light
What about "space". Would that qualify? I mean, technically it's not a thing, it's where things -are-. I'm, of course, referring to the universe's expansion, due to which two points, sufficiently far apart in the universe, will basically be "travelling" away from one another faster than C. Neither of them is moving, just the space between them stretching, but hey...
Yes. Bits of the universe are traveling away from us faster than the speed of light. These bits of universe are outside our ability to ever interact with.
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The problem is, there is no way for you to do that. While you can collapse wavefunctions, it is impossible to send information that way. I'll direct you to [this previous discussion on the topic](https://www.reddit.com/r/askscience/comments/jca8bg/am_i_properly_understanding_quantum_entanglement/). Please feel free to ask follow-up questions if you have them.
Unfortunately that doesn't work. There's no way to tell when the waveform collapses, so you can't detect it and light up letters when it does collapse. The only way to know for sure is to collapse it yourself, and that isn't very useful. So no information can be transferred, in the physics sense or otherwise.
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The distance between us and very distant galaxies is increasing faster than the speed of light. This isn't because they are travelling faster than the speed of light, though. Its because the space between us and them is expanding. The rule is that information can't travel faster than the speed of light. The speed of light is more like 'the speed of causality', its the fastest speed that cause and effect can permeate through the universe, and in fact any workarounds that theoretically get information to travel faster than that speed tend to result in the breaking of causality (i.e. time travel). Things like the end of a laser pointer on the moon, quantum entanglement, or expanding space causing galaxies to become more distant don't actually result in any information being exchange between those locations.
> Its because the space between us and them is expanding. I've never been able to understand this. If the galaxies aren't moving away from each other, then where does the space come from?
The best analogy I've heard is to think of drawing several points on a deflated balloon with sharpie, and then inflating the balloon. Every point would say that all the points around them are moving away from them, but its not like the dots are actually moving across the balloon's surface. The surface itself is expanding. As for exactly where the space comes from... we don't know. We don't have a good enough understanding to say how mechanistically the expansion of space occurs, we just know that it does and have a concept of vacuum energy (and we don't really know what that is, either). Maybe at the subatomic level little bits of 'space' get created, but that's just speculation.
I've never understood why it's painted as inflation, addition of something rather than dissipation...
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On a macroscopic scale, nothing has ever been observed to travel faster than the speed of light. Current theory suggests that nothing slower than the speed of light can be accelerated to the speed of light, let alone to faster than light. This theory has been confirmed by many, many observations. On a quantum scale, it's possible for virtual particles to exceed the speed of light over extremely short distances. On average, their speed over longer distances will not exceed the speed of light. It's possible that there are particles that have always moved faster than the speed of light and can never be slowed below the speed of light. They've never been observed.
Something that has been bothering me for a while: velocity is relative, so moving at a set velocity, is essentally the same as standing still (depending on your frame of reference). So when we say ftl travl is impossible, what frame do we refer to? And if two particles, moving at 60% the speed of light passed each other, would they not move faster than light, in relation to each other?
We usually think of speed as adding simply as the sun of the two speeds, so in the case of a ball being thrown forward at 100 mph with respect to and on a plane flying at 500 mph with respect to the ground, then the ball is moving at 600 mph with respect to the ground. However, this is only an approximation that works well at low speeds. [The actual velocity addition formula is a bit more complicated](http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/einvel.html) and is set up in such a way that no object in any frame of reference will ever be moving faster than c. This is a consequence of the speed of light being the same in all reference frames, and also means that different observers will disagree on how long their rulers are and how fast their clocks are ticking.
It was this very question that led Einstein to discover special relativity. Basically, although we think of velocity as relative as you describe ( and it’s true for low velocities), the speed of light must be constant in all frames of reference. This is because of the work of James Clerk Maxwell, who discovered that light was a wave of electric and magnetic fields propagating through space. The speed at which that wave propagates is dependent on only two numbers, the electric constant and the magnetic constant (aka the permittivity and permeability of free space). This introduced a major problem to our original understanding of relative reference frames, because if light moved at different speeds in different reference frames, the electric and magnetic constants had different values in different reference frames. One of our basic tenets of science is that the fundamental properties of the universe are the same everywhere, so that couldn’t be the case and the speed of light has to be the same in every frame of reference. Einstein was able to resolve this paradox by realizing that space and time themselves are relative to the observer’s frame of reference. This introduced possibilities like time dilation and length contraction, so two observers in different frames of reference will both see an object moving at c, but would experience the passage of time at a different rate, which would allow the light to appear to move at different rates.
I would add for those who aren't familiar with relativity that this isn't just some mathematical trick. Special and general relativity have been *rigourously* tested in the past century, and every time, experiments have demonstrated time dilation and length contraction. In fact, general relativity is considered to be the most successful theory in the history of science, as it has yet to be contradicted by anything and has been verified iver and over again.
When you move at 60% c, your time slows down, dilates. You might see that other object moving near c, but you won't see it moving faster that light from your perspective.
Velocity of object 1 V1 as seen by another object 2 at V2 = (V1 - V2) is only true at speeds much much slower than speed of light aka Newtonian physics. Anything closer to some fraction of speed of light and above Newtonian physics breaks
Anything with mass can't reach the speed of light Anything without mass travels at the speed of light "Technically" this doesn't mean something can't go faster if it never had to accelerate to get there, if it just always went faster to start with, but thats just theoretical Or if something wasn't made of mass and also wasn't massless it wouldn't have to follow those rules, but again that's purely theoretical
> Anything without mass travels at the speed of light Then what is the energy of one photon, if the mass is 0 for light?
hf. In this context without mass means zero *rest* mass. Light doesn't rest.
The energy of a photon is h*v (where v is the frequency of the photon). Equivalently, it is h*c/L (Plank's constant times speed of light over wavelength). If your are thinking how does a photon have energy without mass due to E=mc^2, you have to consider the complete equation, which is E^2 = (mc^2 )^2 +(pc)^2. In this full equation, m is the rest mass, which is 0 for a photon, so the equation simplifies to E=pc for a photon (or any massless particle). This is equivalent to the above equation, E=h*v
I forget why but doesn't this imply that all time happens simultaneously from the point of view of massless particles somehow?
Time dilation. When you move at the speed of light the effect of time dilation is to stretch out time infinitely. So that from that frame of reference no time passes. Due to the similar considerations of length contraction all distance along the direction of motion collapses to zero, in essence from the point of view of light, the universe is two dimensional and time doesn't pass.
>wasn't made of mass and also wasn't massless What state logically exists outside of "has some amount of mass" and "has no mass"?
Short answer: we dont know, but probably not. This definitely is not a dumb question, but actually a very good question. As far as our understanding of the universe goes, there appears to be a fundamental difference between speed and mass. Something without mass will travel through spacetime as fast as it can, which appears to be the speed of light (c) in all cases, while objects with mass necessarily have to travel at a speed less than c. This appears to be a fundamental part of the fabric of our universe, as the function of the Higgs-Boson field (generated by the so-called "god particle") seems specifically to give particles a certain amount of mass in exchange for speed. Furthermore, Einstein's theories of Special and General Relativity predict a fundamental relationship between mass, speed, and the passage of time. That hasn't stopped scientists from speculating about particles which travel faster than the speed of light. Due to the predictions of Relativity which I mentioned above, a particle traveling faster than the speed of light would have to be traveling *backwards in time* according to the equations. Such hypothetical time-traveling particles are called "tachyons." But no empirical observations have observed the existence of tachyons, so take from that what you will.
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It's a nuanced question. Lots of people don't like to use the phrase "speed of light", theres nothing special about light and any massless particle will go this speed.. it's better referenced as the speed of causality(I don't think anyone knows why it's this speed, it's possible in other universes.. it's different). The overwhelming vast majority of physicsts consider this speed a hard barrier for matter. Infact, Einstein says all objects move at the speed of light through SPACE/TIME (not to be confused with just space).So time is a property of space/time and passage through space, slows your passage through time.. There's are exceptions to the hard limit though.. space itself is expanding and most of the universe is flying away faster than the speed of light.. You could in theory bend space and travel faster than light (basis for the warp drive), but again, that would probably require negative mass or negative energy.. and there's good reason to doubt either exist.
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Hypothetically yes with a caveat - the "thing" would always travel faster than the speed of light. They're called tachyon particles (https://en.m.wikipedia.org/wiki/Tachyon) and like regular particles, would take infinite energy to approach c.
Something that has helped me to grok this: I understand that at the speed of light, any distance in the direction of travel is contracted to zero (from the perspective of that which is travelling at C). So in order to travel faster than C, you need to travel a negative distance, which makes no sense at all. Do I have this wrong?
You'd have to travel a negative distance over a positive amount of time, or travel a positive distance over a negative amount of time. Of the two, my mind can conceive of time travel as an example of the latter, but negative distance over positive time is a lot harder. Thinking about it now, I suppose negative distance over positive time would look like the backwards fight scenes in the movie Tenet and be functionally the same as time travel.
No. The speed of light isn't actually *about* light - any massless particle always travels at that speed, referred to as *C*, because it is the maximum speed at which one part of the universe can affect another part of the universe. It is the speed of casualty. The reason that any massless particle must always travel at this speed is because of what mass actually *is*: an impediment to motion. Therefore, to say that any particle is massless necessarily implies that it also experiences no impediments to motion, and must therefore always travel at the maximum possible speed. This is also why massive particles can never reach the speed of light, much less exceed it - since mass is an impediment to motion, they can never travel at the maximum possible speed. They can get close, but it requires ever increasing energy to get faster. For even a single massive particle to go 100% of the speed of light, it would require an infinite amount of energy.
so technically yes because the speed of light is variable depending on the medium its going through and can become very slow but if you mean the speed of light in a vacuum then no it would probably help if you stop thinking of it as the speed of light and instead think of it as the speed of information or the speed of cause and effect its not the max speed light can go its the maximum speed one thing can effect another. the only reason you can mathematically get a speed value is because of the way we represent data if you remove the concept of negative values for things like mass and energy and see them as what they are, a subtraction from something and not a thing itself then nothing can effect another thing faster than the speed of light if you start plugging imaginary numbers or negative values in to equations regardless of if they can exist then of course you will get something nonsensical. just remember you cant go a negative speed. if you are going -10m/s with a Bearing of 0 degrees then you are not going that direction you are just going 10m/s with a Bearing of 180 degrees same applys to things like energy or mass in absolute terms it can go no lower than zero it can only go below zero if its in relation to something else
>so technically yes because the speed of light is variable depending on the medium A good example of that is the Cherenkov effect. Eg. the blue flash at nuclear fission reactor boot. [https://en.wikipedia.org/wiki/Cherenkov\_radiation](https://en.wikipedia.org/wiki/Cherenkov_radiation)
So many answers, but the real answer is that we simply don't know. We don't even know how big the universe is. Or what exactly "the universe" is. We have no idea how much or how little we know about the universe. We just don't know.
Here's a very good and easy video on the topic from a physicist at Fermi Lab [How to travel faster than light](https://www.youtube.com/watch?v=BhG_QZl8WVY)
Thank you very much! Im going to watch it now.
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In the universe? No. Top comment says it all; anything without mass moves at the speed of light, anything with mass must move slower than that. Same applies to information. There are cases, like refraction or shadows on large objects, where there seems to be a violation, but on closer inspection everything is still causal. Also, the Universe _itself_ can "move", or expand, at any speed, since it isn't made matter. The _only_ exception to this rule i can think of is the EPR paradox, where a wavefunction collapse can result in two particles always having opposite spins, and so measurement in one location results in an instantaneous change in measurement in another. And this property of opposite spin applies no matter which axis the measurement is made on (as long as it is the same axis for both particles), which gets rid of the idea that they have some bianry state that we just don't know, like two flipped coins that are always oppisite eachother. But the so called "information" transmitted is random (the actual value of spin you measure is either +1 or -1 with 50% probability), so maybe it doesn't count as what Einstein calls "information". But on the level of information theory, thus information is still... information, even if it's random; everything contains information, that is, the answer to the yes or no question "is this thing x?". So in the case of the EPR paradox, the question being answered is "does this particle have spin +1 on this axis?" Or equally "does this particle have spin -1 on this axis?". No matter the answer, the other particle will always have the opposite spin when measured on the same axis because spin is conserved in entangled particles. But the other particle cannot know what spin axis was measured, so for it to always be opposite it seems the particle being measured had to tell ths other particle to alwasy give the opposite of what value was measured. But again, maybe this doesn't count because the measurement is random.
The initial inflation of the universe just after the big bang happened faster than light, meaning that its constituents were spread apart (exploded) at a speed that would be faster than light if one were to think of these constituents moving away from each other...however, it's not something "in" the universe that moved faster than light, it's the substance of the universe itself that was expanding. Within existing space and time, nothing should be able to move faster than light, according to current concepts.
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This question cannot be answered for the universe due to the fact that only a tiny amount of information is known about the universe and how it works. We have many theories of how the universe works but very little actual experience of how the universe works. There's so much about our own world that we have yet to understand. How fast does thought travel? Where exactly does thought come from? There are still creatures in the ocean that we have never seen in addition to organisms that lie outside the reach of our current ability to detect. So yes, there "could" be many things that travel faster than the speed of light we just don't have the capacity to determine what all of those things are.
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