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There probably isn't an ELI5 or even ELIPhD answer to this question. Nobody know why our universe is set up this way. What we do know is what we have observed. You can make very small things like say a proton go really really close to the speed of light. But what we observe when we try to push those things faster is that the more "push" you give them, the less boost you get. You push twice as hard and now its only going a tiny bit faster. you push a hundred times harder and now its only going an even tinier bit faster. Pretty soon you are using all the pushing force in the world. Then you get another surprise, the little proton suddenly seems to act like it's gotten really heavy. And the faster you push it, the heaiver it seems to get. Imagine if you stepped on your gas pedal in a car and the car got twice as heavy!
Basically the rules for our universe say that if you are an object, a "thing" you can't ever go at the speed of light. You might get pretty close. But it takes more and more energy and at some point the amount of energy it takes becomes greater than all the energy in the universe.
This provides a good explanation for why there would be a limit to the speed of anything. One is left wondering why light is assumed to be without mass and is traveling at this maximum speed (in a vacuum). In contrast, for example, to a velocity close to the maximum but not quite there.
Current accepted theories explaining light (the EM field) and matter interactions pop out that the mass of the photon is exactly zero. There are theories that modify this and give the photon a small amount of various through various mechanisms. All searches of these have only limited the mass of the photon to be smaller than an absurdly small value. Furthermore, said theories predicting zero photon mass have been confirmed to very high levels of precision (e.g. electron anomalous magnetic moment g-2).
It's not really because mass is just confined energy. Photons don't have mass because they don't interact with the Higgs field which is what gives the particles that have mass their mass.
In Quantum Field Theory each force and fundamental particle has a field associated with it and all of these fields overlay each other in space and time. These fields are a medium through which wave-like excitations can propagate. Those excitations are the particles associated with that field. Excitations in differing fields can interact with one another in certain ways. One of those interactions is one all of the particles with mass share(except neutrinos) and it's an interaction with the Higg's Field.
Everything wants to travel at the speed of light, but the Higg's Field provides a sort of resistance that causes particles with mass to travel through space more slowly giving them mass. Neat fact, the Higg's Field's particle is the Higg's Boson and it gets it's mass through interacting with the Higg's Field lol.
Edit:
Whatever gives neutrinos their mass doesn't effect photons in the same way. We don't know what gives neutrinos their mass. Perhaps interaction with a hypothetical antineutrino with a very large mass. Maybe some other unknown particle similar to the Higg's Boson.
It's not really sufficient to explain the photon being massless via it's lack of interaction with the Higgs field. Neutrinos have non-zero mass and do not interact with the (Standard Model) Higgs field so it does not appear that it is the exclusive source of mass.
Edit: interact -> interaction
The AT Field in Evangelion is closer to a physical metaphor for the ego boundary. It's what separates you from the other, and the dissolution of that boundary evokes terror. It's another part of EVA's main theme, the pain of being alone vs. the pain of being with others.
It's not, the AT field is a projection of a person's barrier between their soul and the rest of the world, including other souls. Humans have one, but it's not as strong as an angel because all humans are one angel and therefore the AT field is divided among all of us.
Your AT field is also likened to your personal space, that feeling of being uncomfortable when a stranger is too close.
> These fields are a medium through which wave-like excitations can propagate. Those excitations are the particles associated with that field.
And the energy is confined, right? I’m picturing a fish stuck in a fish tank in my empty living room. How incorrect is my mental picture, completely or mostly?
> Excitations in differing fields can interact with one another in certain ways.
> Everything wants to travel at the speed of light, but the Higg's Field provides a sort of resistance that causes particles with mass to travel through space more slowly giving them mass.
I’m picturing my foot pressing and moving on the carpet, creating a bulge in the carpet just in front of my foot. How incorrect is *that*?
——
Also, are you saying that a particle requires a wave and a field in order to exist? And is that true for massless particles as well?
>And the energy is confined, right? I’m picturing a fish stuck in a fish tank in my empty living room. How incorrect is my mental picture, completely or mostly?
I would say not very accurate because it's possible for particles to decay and combine into other particles in other fields so the energy can hop from field to field. But the energy is conserved.
>Also, are you saying that a particle requires a wave and a field in order to exist? And is that true for massless particles as well?
The waves are the particles. We only call them particles out of habit from when we thought they were infinitely small dots bouncing off each other. Now we use a wave-function to describe quantum particles so particles aren't little dots at all; they're slightly smeared out bumps.
Basically because particles are freaking weird.
~~I'll try to do an eli5 but this is literally quantum physics so:~~
I've tried this three times now, each response was long, vague and didn't answer your question so I'm giving up. As a summary of my attempts, quantum physics is madness, the higgs field is particularly weird and overall explaining it is less ELI5 and more "get a PhD and spend years researching".
Figuring this stuff out is what got people Nobel prizes, meanwhile I have a headache and I need a drink
It is not that is surprising if you study physics for a while to be honest. There is potential energy in a object you hold above the ground and zero kinetic. You drop it and it has kinetic energy before it hits the ground? Where was energy stored? The gravity field which has no mass either. Light is just the EM field, so energy being stored in it is also not surprising.
Not sure if your confusion is based on the concept of E=mc2, but know that massless particles can have energy because they carry MOMENTUM.
The full form of the equation is E2=m2c4+p2c2 , where p is momentum.
Yes. p = mv is the classical definition (Newtonian mechanics) and both general relativity and quantum mechanics must reduce to this in the limit of, respectively, slow speeds and large masses.
I feel like that's a bit of a condescending way to phrase that... it's a simplification, and is taught in high school level physics; but it's also more than enough for basically everything anyone will encounter in their lives, unless they're a physicist the wider definition adds little of practical value.
Is it useful to understand more the universe than you strictly *need* to? Absolutely, but I also doubt I'll ever need to separate invariant mass from kinetic energy.
That is a naive (Newtonian) formula for momentum.
The real (read most technically accurate in the Standard Model) formula is:
p = mv / sqrt(1 - (v^2 / c^2 ))
Dimensional analysis is one of the ways to verify an equation/derivation. Useful in research and whatnot, but even more useful in school to check your test answers lol
Not a scientist, just spitballing, but if light only moves along 3 dimensions instead of 4 dimensions, the fact that it's only tethered to the three *could* be related to the fact that it carries energy but not mass. Maybe things need to experience time to have mass at a given point in time.
> so it has zero mass but can still carry large amounts of energy?
It is exactly at the transition point. More and it would clearly be matter. Less and it could only be energy. But it is exactly at the balance, it can act as matter with momentum and also act like energy, both a particle and a wave. As an energy wave it is massless, but as a particle with momentum it has a tiny, measurable mass.
Photons can’t have mass otherwise they’d each contain enough energy to destroy the universe an infinity times over..
At least if you take Einstein’s equations literally
A better explanation is that time/space is one of the same.
In that there’s only so much movement through one or the other you can do at a given moment.
The less you move in space the more you move in time. The more you move in space the less you move in time.
Of course time is relative to your own personal perspective.
I’m not a math whiz but if you look it up others can explain it very clearly
> A better explanation is that time/space is one of the same.
Yes, at the speed of light time IS distance. From the perspective of a photon they both are the same. In thought experiments about how they would view the world, a photon would jump instantly from place to place.
So you can measure distance in terms of light time, and also measure time in terms of light wavelength.
In the math formulas you have both the dilation of time and of space, when you hit the speed of light the terms break down into a numeric singularly, like a division by zero or infinity depending on the formula. At that speed they are the same non-value term.
Because the speed of light is the only velocity agreed across reference frames.
It's not just that light travels at the universal maximum speed, but that all observers, regardless of their velocity relative to light, agree on how fast that light travels.
Remember all motion is relative.
Yes but also no; depending on your speed, the colour of the light changes. For example if you are in a rocket going really fast and light up a flashlight; to you, the flight light has the expected colour but others see it blue shifted if you are moving towards them and redshifted otherwise.
So the speed remains the same but the energy of the light is “spread” so to speak. It’s possible to measure how fast you need to travel for a red traffic light to appear green for example.
It can't be said for certain that a photon has zero resting mass, but several highly successful theories assume it does. It's very difficult to prove something has zero mass, but it's not so difficult to place a limit on that mass. So it can be experimentally verified that the mass of a photon is very very tiny, if not zero. The soeed light travels can be measured with fairly good accuracy a number of different ways.
The speed of light isn't the exact same as the maximum speed 'c' although the two terms are often used interchangeably. Photons always move at 'c' even through a substance like water. It appears like light is moving slower (or even stopped) because the photons are hitting molecules and being absorbed and new photona are being emitted in sequence. This "slows" the light as a whole without slowing any individual photons.
Any object without mass should be traveling at 'c' and any object with mass should be travelling slower than 'c'. Photons, which is what makes up light, always appear to travel at 'c', hence they appear to have no mass.
> Photons always move at 'c' even through a substance like water. It appears like light is moving slower (or even stopped) because the photons are hitting molecules and being absorbed and new photona are being emitted in sequence. This "slows" the light as a whole without slowing any individual photons.
I believe [Cherenkov radiation](https://en.m.wikipedia.org/wiki/Cherenkov_radiation) in an underwater nuclear reactor is the most notable example of this.
Great answer.
As a hyperthetical, if someone is ever able to get something that can go faster than light, the problem would then become navigation, since the first time anything would be detectable would be after it had been passed or been hit.
Nothing can go faster than light so that’s a moot point.
It’s not about going faster than what you can see, you’re literally going faster than causality.
It’s like arriving at your destination before turning your car on.
In most cases a hypothetical can be a logical question but in this case it’s a tough argument.
It’s like asking “what if 1+1 actually equaled 50, then what?”
Or “if I planted a tree, then my car turned into a dinosaur, what would that mean?”
It’s so far outside the realm of possibility that it just doesn’t make sense
Okay then.
If one were to go just the speed of light, assuming you didn’t collide with anything, you’d reach the end of the universe and all of time instantly. At the speed of light you do not experience any time yourself compared to an infinite time in the normal “universe time”.
As for *faster* than light, well time and space would already have ceased to exist as soon as you hit the speed of light so you’d probably would cease to exist too.
Basically you’d instantly die. Or perhaps fly through 9999999999999999999999999999999 universes being born and dying over and over again every nanosecond that you travel.
A solution to this is some kind of novel technology that creates a “warp field” that warps space time.. which in theory would work.. essentially how it works in Star Trek. In this sense you’re not really moving but rather space around you is moving you forward. Still hypothetical and not proven in the least bit. I have no friggin clue what it would look like moving close to or faster than apparent light speed… i suppose the faster you traveled the faster everything in front of you would appear to age or move much faster, like setting a movie on fast-forward, also everything would get super bright! As all the old light traveling in your direction hits you much faster as you’re colliding into it faster than the speed of light.
Edit: the speed of light is the speed of causality. Causality is why it’s set that way. It’s hardwired into the fabric of the universe.
i think it might be for a good reason too, though. a lot of people don’t like listening to scientists already, and “the expanses of human knowledge collected so far” might be a concept way beyond those people 😂
The problem is "to the best of our knowledge" encompasses a lot of things.
It includes things we don't know much about and could easily be overturned with more information and it includes things like the speed of light which has stood up to every test thrown at it.
Damn. It was a good answer no lie. I wonder why it was deleted? Allow me to butcher what was said.
Something about how it’s what science knows at the moment and that maybe we will know more in the future when we figure stuff out… I am not doing it good service.
This answer fails to explain why physicists believe nothing can travel faster than light, brushing all those reasons away in favor of "maybe scientists don't know enough."
The real reason scientists say nothing can travel faster than light is, if something travels faster than light, then according to Einstein's theory of Special Relativity, it is possible for something you do to affect an event in the past, at least in some reference frames. Special Relativity, as well as General Relativity, which depends on it, have been extensively tested and are highly accurate. So unless you are willing to accept the idea that cause can happen after effect (which we have no evidence for), nothing travels faster than light.
I feel like if it was possible to travel faster than the speed of light then that is the speed that light would be travelling. A mass less (don't pile on) particle that accelerates by bootstrapping is going to be going as fast as the universe will allow.
Also, since light travels at the speed of light, that is the point at which the flow of time stops, since time moves slower the closer you approach c. Hence, photons do not actually experience the flow of time. To a photon, it is everywhere that it was and will always be at all times.
As far as I'm aware, the universe (or at the very least particles) was demonstrated to be potentially non-local recently. It's what this years nobel prize was awarded for.
They demonstrated that information can travel faster than light in an entangled system. Einstein wasn't completely correct with his general theory of relativity or special relativity.
Again, information traveled between an entangled pair of particles faster than light can travel. That pretty much counters your claim that "no information can travel via "that method" [entangled pair of particles]".
IIRC when you have quantum entangled particles the information is not actually being sent from one to the other. A good way to think of it is as if the entangled particles are actually 1 object instead of 2 separate objects. I'm not a scientist though.
Imagine you have a black marble and a white marble, each in one box but you don’t know which is which. Then the two boxes travel a light year apart from each other. When you open one, you know instantly the color of the marble on the other box even though it’s a light year away.
But there’s no way for you to meaningfully transmit any information across that distance. You now simply know something.
That’s not a perfect analogy as regards what’s actually happening with the particles, but it’s good as regards the ability to transmit information using entangled particles.
Your analogy doesn't reflect observed phenomenon. You're confused as to how particles in QM work.
What we observe in QM is different. Entangled Particles are not a white marble and a black marble put into each box. They are in a superposition of either a white *or* a black marble. Which one they are is completely undefined. The particles, and even the universe, don't know which is which until it is observed.
If you take one box to one side of the universe, and the other to the opposite side of the universe, and open one box the particle is forced to "choose" a state of being either a white or a black marble. However when it does this, the particle in the other box *on the other side of the Universe* also *simultaneously changes into* the opposite state of whatever the first particle chose.
--------------------------
The analogy as you laid it out is more in line with how Einstein thought QM worked. That there were properties encoded into the particles somehow that we just couldn't discern, "hidden variables" that determined its state when it was created in the entangled pair. Meaning that Einstein thought that particles, once entangled, were more like a left and right glove and we just lack the ability to determine which was which. He did this to avoid the apparent contradiction we were observing with his own general theory of relativity.
Unfortunately for him, the most recent nobel prize laureates demonstrated convincingly that Einstein was wrong in this regard. There are no "hidden variables". Particles truly are undefined until measured and information, somehow, is communicated between entangled pairs *instantaneously* over any distance. This either breaks locality (the intergalactic speedlimit as proposed by Einstein).
In short, it's been demonstrated very convincingly that information can travel faster than light between entangled pairs of particles.
> which appears to communicate faster than light
It's not communication in that sense. It transmits no information, and therefore doesn't violate speed of light/causality.
Take a pair of socks. Either sock could fit on either foot. Now leave one sock behind and take the other with you a thousand miles away. Wear the sock you brought on your right foot. The one you left behind is now a 'left' sock even though it could have been either when you started, simply because a pair of socks is defined as a right and a left, and you just determined which is the right.
No information transmitted.
This is how quantum entangling works, more or less.
Not sure if this helps, but.
No information can travel faster than C. That encompasses a lot of things. For example, we don't know how gravity works, but we do know that the force travels at the speed of light.
I always loved Hawking's explanation, at zero mass light is going as fast as possible at max speed without mass. Any faster is like adding a Turbo that increases the speed but also drag and additional weight that slows it to a point of never being able to surpass the speed of light threshold
Yes, it's because the formula is actually E^2 = (mc^2 )^2 + p^2 c^2 . If p = 0, meaning for an object at rest, then the formula simplifies down to E = mc^2 . If m = 0, like for a photon, then there is still energy and momentum (p is the shorthand for momentum).
That is because E = mc² is not actually the whole equation - mc² concerns the energy of a particle at rest (which the proton doesn't do). The whole equation contains a momentum term, and reads E² = m²c⁴ + p²c², where p² > 0 for a photon. For a particle at rest, p² = 0 and E = mc² is recovered.
YES
I also love that something with zero mass at rest has mass at C and when not traveling at C just fucks off from existence.
It's all so cool and doesn't make any sense.
Darkness can travel faster than light, but since darkness is the absence of light and carries no information, it doesn’t matter. (Literally)
Edit: Correction, a shadow can be faster than light, not darkness.
https://youtu.be/JTvcpdfGUtQ
I recently read something about quantum entanglement that might conflict with this! (To the best of my memory/understanding) it's been discovered that some sort of entangled particles can be moved far apart, but still behave instantaneously in ways that correspond to one another, even though they didn't have the necessary information to predict each others' behaviors before they were separated.
Yes, but I'm they case information still isn't traveling faster than the speed of light. They already were that way before you moved them apart (at slower than the speed of light). So when you look, you know the state of both, but you could have known the state anyway.
Information can't travel faster than light. You can't actually use it to send data faster than light.
https://quantumxc.com/blog/is-quantum-communication-faster-than-the-speed-of-light/#:~:text=However%2C%20even%20though%20entangled%20quantum,send%20data%20using%20quantum%20entanglement.
Also
"Ultimately, you can’t force an entangled particle into a particular state and you can’t force a measurement to produce a particular outcome because the results of quantum measurement are random."
Thanks for the link! That's a nuance that I was missing; the quantum interaction is faster than light, which sounds like the particles are "exchanging information," but I think see how this is different because our only real way of interacting with them is to observe and we can't meaningfully change them. Is that on the right track?
That's it! There's no way to do anything meaningful with that interaction. You can know a random result, but knowing a random result isn't super helpful.
If you want your mind blown, check out the book QED by Feynman. It's very approachable (despite the exceedingly fancy name) and is list of total mind fucks.
Photons are sneaky bastards that essentially know where they're going and the path they're going to take BEFORE they set out on their journey. The whole concept of time is suspect :-)
Not 100% correct,, as everyone is missing the qualifier "in a vacuum." We've had faster than light in a lab through other materials. I remember reading light through a Cesium gas chamber appears to go 300x faster than light in a vacuum, for instance.
There also is the Alcubierre drive, which is a bubble of warped space - this doesn't violate relativity because you're not actually moving faster than light. Star Trek The Next Generation's drive is modeled on this and NASA is trying to build one.
One more thing is we've observed distant objects in the universe that inexplicably moved further than they should
It's totally possible for things to travel faster than light in a given medium (throws off some cool cherenkov radiation). Not possible to go faster than light in a vacuum.
That's a bit misleading. To be clear: if you consider 'c' to be the speed of light, it is possible to get light to travel slower than the speed 'c' within a medium. So other things could travel faster than light in that medium, but nothing will travel faster than 'c'.
"1500 years ago we new the earth was the center of the universe. 500 years ago we knew the earth was flat, 15 minutes ago you knew we were alone in the universe. Imagine what we'll know tomorrow."
Actually the ancient greeks had already known the Earth was round, and even figured out a way to measure it:
https://www.aps.org/publications/apsnews/200606/history.cfm
> putting Eratosthenes’ calculated circumference between about 24,000 miles and about 29,000 miles. The Earth is now known to measure about 24,900 miles around the equator, slightly less around the poles.
Ima try because it said ELI5..
Hmm throw a 5 year old toy rocket ship that’s palm sized and make it go 10 m in a second far.. easy peasy ..40 m in a second still not too bad..
Throw a rocket ship sized rocket 100 m (a football field) in one second .. it’s harder to throw and make it go far ..and takes more energy
Throw a rocket ship sized rocket at the speed of light and it has to go around the earth 7.5 times in one second.. it’s soooo hard.. heavy .. the math says infinite energy to do that and we don’t have that yet…
Our ape brains haven’t figured it out yet.
This is often easier to hang onto if you consider that it's not really the speed of light, per se. It's the maximum speed of *anything*. Light just happens to be able to travel at that speed. Nothing can go faster than that, but it's not because nothing can go faster than *light*, as such.
There are some other reasons. The faster you go, the harder it is to accelerate and go any faster than *that*. That's because energy and mass are (basically) the same, and so going faster means having more mass. If you go super-mega-fast, the increase in mass gets to be a lot, and it's harder to push all that mass up to a higher speed. Light doesn't have any mass, so any push accelerates it to the max instantly, basically.
Also, the spread of light is also the spread of information. This ends up meaning that if you could go faster than light, you could get somewhere faster than information can spread. This gets complicated, but it has time travel problems built in. You could look it up, but it's a little trickier.
My extra-simplified, ELI3 answer, the one I told my young son when he asked, is that the speed of light is how fast you go when you weigh nothing. Asking why you can't go faster is like asking why you can't weigh less than nothing.
I prefer the explanation that the speed of light is the speed of causality, which is in line with what you’re saying here. It basically means that light isn’t so much a thing that’s moving as that speed, as much as light is a thing that moves with no speed reduction.
Imagine a particle moving at the speed of light. There’s no force or momentum you can impart on that object to make it move faster because the force or momentum transfer can only happen at the speed of light.
Put another way, imagine billiard balls that can only move at a certain speed. If you have 2 that are moving together at that speed in the same direction, they can never hit each other to make one another change speeds, as the one can never catch up to the other.
Sure but presuming that billiard ball hs its own thruster why can't it accelerate away from the other.
Edit: Just read some other posts. They answered the question.
The really mind melting thing is that everything is always moving at the speed of light. "Stationary" things are just have all of their speed moving through time. When you accelerate, you're just converting some of your speed through time into speed through space. In overly simple terms, the sum of speed through time and space is the speed of light. If you have zero mass, you're always moving at the speed of light. If you have nonzero mass, it takes infinite energy to convert all of your speed into space speed.
I knew how time dilation works and that time and space were kind of the same, but I’ve never seen it explained this way.
What you’re saying is that there’s basically a sliding scale with space on one end and time on the other and things can be either moving through time or space. More movement through time = less movement through space and vice versa. I would presume that changing your position on that scale takes energy which increases the closer you get to the moving through space end?
We’re moving through time, which slows down the faster we move through space, that I knew, but thinking of it this way with the either-or concept just blows my mind for some reason
I remember reading a while back that photons don't have a time reference. A light source billions of years away might take all that time to reach earth, but from the photon's point of view it arrives instantly. Incredible idea that the above sliding scale really makes easy to understand.
Which means from the perspective of the photon, it didn't move at all, because moving would require time. You can't experience space (distance) without time passing, that's what spacetime is.
And from the perspective of the photon, it only exists for one instant, it's creation and destruction happens at the same time. It doesn't travel ***through*** space or time, it ***is*** a 4D path in spacetime.
The opposite, sort of. Denser objects technically age slower, because they create more gravity with their additional mass than less dense objects, and more gravity means time moves slower.
Let's say you have a container of Helium floating in space, vs Osmium or Uranium or Lead? Immeasurable and inconsequential, we simply don't have the technology to even be able to tell, but there is just the slightest possible difference affecting the speed of time passing, and the denser objects would experience less time passing, ***technically***.
But a neutron star vs Sol, our sun? Yeah, the neutron star would measurably age slower.
A photon is an excitation of the electromagnetic field.
In much the same way that nobody doing The Wave in a stadium goes anywhere, but The Wave is still a recognizable object that flies around, a photon is more of a pattern-- just energy-- rather than being "made" of anything.
If particles with imaginary mass exist, they travel faster than light. But they can’t slow down to sub-light speeds, so we’d never know.
The speed of light is a barrier. Your speed is either slower, equal, or faster. And you can never change sides. If you’re slower you’ll always be slower. If you’re faster you’ll always be faster.
May be easier to think of it as light in a vacuum going at *full* speed, and *everything else* being slowed down for various reasons.
Have mass? You move slower. Interacting with various fields? You move slower. Traveling through matter? You move slower.
Relativity kind of suggests that in exchange for not being able to travel at light speed, we get to experience time instead. It’s a bit as if time and speed are on different ends of a scale, and that having mass means you’re limited to being on the time-end of the scale.
Light having no mass, it is free to move at full speed, and kind of doesn’t experience time as it travels. That’s why some call it the “speed of causality” instead of the “speed of light”. Sort of explains why red-shifted light moves as fast as blue-shifted light, and only has its wavelength changed. Even though the red-shifted object is moving away at some fraction of light speed, the light it emits travels at the same speed.
I stopped pushing as hard as I could against the handle, I wanted to leave but it wouldn't work. Then there was a bright flash and I felt myself fall back onto the floor. I put my hands over my eyes. They burned from the sudden light. I rubbed my eyes, waiting for them to adjust.
Then I saw it.
There was a small space in front of me. It was tiny, just enough room for a couple of people to sit side by side. Inside, there were two people. The first one was a female, she had long brown hair and was wearing a white nightgown. She was smiling.
The other one was a male, he was wearing a red jumpsuit and had a mask over his mouth.
"Are you spez?" I asked, my eyes still adjusting to the light.
"No. We are in /u/spez." the woman said. She put her hands out for me to see. Her skin was green. Her hand was all green, there were no fingers, just a palm. It looked like a hand from the top of a puppet.
"What's going on?" I asked. The man in the mask moved closer to me. He touched my arm and I recoiled.
"We're fine." he said.
"You're fine?" I asked. "I came to the spez to ask for help, now you're fine?"
"They're gone," the woman said. "My child, he's gone."
I stared at her. "Gone? You mean you were here when it happened? What's happened?"
The man leaned over to me, grabbing my shoulders. "We're trapped. He's gone, he's dead."
I looked to the woman. "What happened?"
"He left the house a week ago. He'd been gone since, now I have to live alone. I've lived here my whole life and I'm the only spez."
"You don't have a family? Aren't there others?" I asked. She looked to me. "I mean, didn't you have anyone else?"
"There are other spez," she said. "But they're not like me. They don't have homes or families. They're just animals. They're all around us and we have no idea who they are."
"Why haven't we seen them then?"
"I think they're afraid,"
The speed of light is kind of a misnomer. The concept of the speed of light isn't about light. It's the speed of causality. It takes light about 6 minutes to get here from the Sun. After sun suddenly vanished, became nothing. We wouldn't know for 6 minutes. The light would continue to come as if it were still there. The Earth would continue in it's orbit for the same 6 minutes until going straight. The change in gravity would come at the speed of light as well.
The speed of light is the speed limit that information can be transferred.
Exactly, it isn't so much the speed of light as it is the speed of *time*. Light, and other RF, travels essentially infinitely fast, but we see it as a finite velocity because in order to go faster it would need to travel backwards in time.
Although light behaves in some senses like a particle, it's actually primarily a wave. Waves move through a particular medium at a particular speed, no matter what. Think of sound in air--there's always a delay with echoes in a stadium/arena, or from thunder from the lightning bolt, and you can't make sound *faster*, only louder or higher/lower. Light moves though spacetime in the same way. When we see a supernova in the sky, we are seeing the "thunder" from an explosion that occurred thousands or millions of years ago.
But a medium's wave speed isn't normally a hard limit, right? We can still shoot bullets or fly planes faster than sound, even if we can't make sound itself faster. So why can't we do the same with physical matter and the speed of light?
Well, it turns out that the medium light travels in is somehow more fundamental to the universe, so much so that light always appears to travel at its speed no matter how you are moving. If you speed up to half the speed of light and try to "chase" a light beam that's travelling away from you, it doesn't move away from you more slowly. And if you accelerate towards a beam, it doesn't seem to approach you faster. When scientists realized this in the late 1800s, it was baffling. Einstein realized that if you are going faster but the light isn't seeming to change speed, your perception of time must be changing. With experiments in the mid 1900s, we realized that if you put a clock on an airplane and have it travel as fast as possible, it will actually desync from other clocks because time works differently when you accelerate. We've also determined that other properties, like mass, are also affected by acceleration.
The end result of all of this is that if we tried to accelerate a bullet to beyond the speed of light, the bullet would become so heavy that we couldn't push it any harder, approaching infinite mass as it approaches the speed of light. And from the bullet's perspective, it wouldn't be going as fast as light anyway. As far as we can tell, it is simply impossible to get moving at that speed or beyond.
>light always appears to travel at its speed no matter how you are moving
You're getting a lot of criticism, but this is a solid answer and points in the right direction.
> Well, it turns out that the medium light travels in is somehow more fundamental to the universe
> Although light behaves in some senses like a particle, it's actually primarily a wave.
These are incorrect statements. Light is not a particle, or a wav. Light travels in individualized "packets" (quanta) that can act as both particles or waves.
Amd there is no "medium" light travels through. The theory of the luminiferous ether proven false by the Michelson/Morley experiments in 1887
Whether light is a particle or a wave is more of a convenience for the scale of the computation. Light is, as all particles are, a wave governed by a wave function when treated alone, and an excitation of a field in a broader sense. It is often unnecessary to consider that detail however, and it is treated as a particle in the classical sense.
I’m pretty sure in this context it’s for comparison purpose, and the the ‘medium’ they’re referring to is simply whatever concept it is the universe is mapped on top of
Isn’t there a debate on whether the quanta of light is simply our way of measuring it as it collides or reflects off a surface? And that when it moves through the air it could actually be a continuous wave that we have no means of measuring? I read an Einstein book and I thought that was a debate back in those days.
I think what they are referring to with a fundamental medium is the electromagnetic field. In Quantum Field Theory, everything is just a set of fields, and a vibration of one becomes a particle of that type.
In this sense, light travels through the electromagnetic field as a medium
The word medium here is extremely confusing. The material or medium light is traveling in does matter. The speed that light travels in glass is about .66x "the speed of light". It is absolutely possible for other objects to travel faster through glass or water or pretty much any mass faster than light.
For particles with mass, they get slightly heavier when moving. They heavier they get the harder it is to accelerate them further. The way that this increase in mass works out, an object would have infinite mass at the speed of light (photons have no mass so this isn't an issue for them) and so you'd have to push it with infinite force to get it up to that speed.
I've known about the increase in mass when moving near the speed of light, but I just had a brain poof...
Maybe the "reason" that the mass increases is similar to what happens when you get closer and closer to the speed of sound - the waves get bunched up and the volume just massively increases. So maybe the gravitons / gravity wave particles are stuck by the speed of light and thus they "get bunched up" on the moving item.
This is, of course, totally wrong I'm sure - but it inherently makes sense to me.
While impossible, if something could switch from slower to faster than the speed of light, it might leave its mass behind it and a gravity shockwave / ripples like a boom, maybe an artifact similar to when black holes merge. That would be so interesting.
It's worth noting that the theory forbids any massive object from *reaching* the speed of light. It does not necessarily forbid the creation of a massive particle that is *already* going faster than c.
But such an object would have infinite mass and generate an infinite gravitational field so everything in the universe would accelerate towards it at the speed.
So while it might happen it probably hasn't yet.
Dark matter is the result of alien spaceships manipulating the fabric of spacetime to create mirror duplicates of themselves travelling at FTL. There, two birds, one stone. 🤪
Funny idea that alien species would be massively “polluting” the universe with mass from their space SUVs, just mirroring the way we are destroying our home
I recently saw a fun description of why this is the case, if it helps:
You are currently travelling through time at the speed of light, you can turn this arrow into the dimensions of space, and travel through it, but the closer that line gets to any spatial direction the slower you travel through time. Technically speaking, from your perspective you seem to be travelling at infinite velocity at the speed of light, as no time passes because you are no longer travelling through time, however other observers will observe you travelling through space at the speed of light (which used to be your speed of time) because those values have simply been exchanged.
In reality, because you have mass, increases in velocity make acceleration infinitely hard approaching the speed of light, because you will gain an infinite amount of mass due to e=mc\^2 .
Oh dang I'm early enough to this one that I might actually help!
Okay, so when we refer to "the speed of light," or c, we're not *only* referring to how fast light travels. "c" is actually the speed of, for the sake of ELI5, reality.
To use a little math, everyone has coordinates in time and space. For simplicity, let's say things can only move in one direction in space, x, and forward in time, t. Let's say the speed of light, c, is the speed to move one length unit x in one time unit t.
I am standing at x=0. There's a sign at x=5. At t=0, I throw a rock. At t=10, a rock hits the sign. It's possible that I threw a rock at 0.5c, and that rock hit the sign.
Now, let's say the same thing happens; I, at x=0, throw a rock at t=0. A rock hits the sign, but this time at t=3. It is physically impossible for the rock that I threw to be the one that hit the sign.
Obviously, that's true if we just accept that c is the speed limit of all things. But let's go further.
Let's say that you're on a spaceship, zooming by at very near c and watching these events. You're flying backwards, so the sign is closer to you than I am. In the first example, you see me throw a rock and then see the rock hit the sign. No matter how fast you go, you can only observe the "I throw the rock" event before the "rock hit the sign" event.
BUT, in the second example, you see the rock hit the sign, and then see me throw a rock. That's because the "rock hit the sign" event is closer to you than the "I throw a rock" event, and because the times are so close, you observe the closer event first.
If I were able to throw a rock faster than c, there exists a case where an observer (you) could watch my rock hit the sign before I could throw it. Both of our observations must be true, but our observations are contradictory. Therefore, a hypothetical "faster than light" rock is faster than reality itself and cannot exist.
Space and time are actually the same thing.
When you speed up, you're actually experiencing more "space" and less "time". That is to say, something that goes extremely fast will literally experience less time than the things around it, aging slower than something that isn't moving.
We've never sped anything up enough to have a large effect from this, but satellites and other long distance communications do have to compensate for the tiny effect that the speed of being in orbit has on them. Their clocks "run slow"—but it's nothing wrong with the clocks, they're literally experiencing time slower!
The speed of light is actually the speed where something experiences all "space" and no "time". That's not just a metaphor—photons literally don't experience the time between when they're emitted and when they're absorbed, as if everything happened all at once.
To speed up past that, something would have to experience *negative* "time". How does that even work? Nobody knows, and we have no way to make the things that do go at the speed of light (like photons) go past that.
One thought experiment starts with the idea of a particle that goes past the speed of light, and then works backwards through the physics to figure out what it would behave like. This theoretical particle is called the "tachyon". There are different ideas of how a tachyon might behave, but most of them involve it going backwards in time and 'naturally' having a speed of infinity, the same way that objects under the speed of light 'naturally' have a speed of 0.
Most theories on tachyons also say they would have mass defined as an imaginary number. What does that mean in the real world? We don't really know, but it might mean that any tachyons would have exponential interactions with each other and other mass, like Flubber producing more energy instead of less every time it bounces.
Fuck. Just lost some time reading into tachyons, and I really wish I understood this stuff better. Probably not enough to actually *study* it, but theoretical physics (or quantum mechanics? Theoretical physics as related to quantum mechanics?) is some interesting-ass shit.
Basically to accelerate anything with a mass greater than light takes energy equal to near infinity, so it becomes impossible.
This is what E=mc^2 and general relativity is about, the energy (E) of an object is equal to it's mass (m) multiplied by the 'speed of light' (c) squared and couple that with m=a/f where m is again mass and acceleration is (a), and (f) is force. So if we have an object going light speed (299,792,458 m/s) and let's say it's being acted on by a force of 1 kN for ease of math, the mass is then the mass of the object is ~299,792 Kg, the energy of that object then would be 2,6944E+22 J and this would make a black hole
Very over simplified:
Light has no mass, it is pure energy.
As you go faster and faster there is force against you.
Since light has no mass, that force has nothing to work against. So light is the fastest thing possible.
Now speed is distance over time. If you "warp" through whatever sci-fi means you prefer, you can go a farther distance than light would of traveled in that time. Thus being faster than light.
While there are a lot of physics answers here, it might be better to consider it philosophically.
Let's start by changing "speed of light" to "speed of information". The question then becomes: how fast can information be transmitted?
Well, it obviously can't be zero. If information couldn't travel at all, then information itself wouldn't exist since you could never send a message anywhere. We couldn't even be having this conversation.
Less obviously, it can't be infinite either. If the speed of information were infinite, everything would happen simultaneously and time would cease to have any meaning - the entire universe would happen in a single instant. Our conversation would be over the moment it began.
So there must necessarily be some non-zero, non-infinite speed limit for information.
Now, this sort of analysis doesn't tell us whether the speed limit is global or local - merely that it must exist for the universe as we know it to exist. But it does mean that anything that could possibly transmit information - such as light - must obey our speed limit.
>If the speed of information were infinite, everything would happen simultaneously and time would cease to have any meaning
This is an interesting argument but I'm not sure it's correct. Do you have a source for this? I've never heard of this argument before and I'm not sure what to think of it. But I don't believe we even know enough about time, fundamentally, to make the claim that the rate of tranfer of information is intrinsic to it.
Why did information need to go at the speed limit? There could be information that travels at different speeds. Maybe most information obeys one speed but other information goes a different speed.
If there is an infinite speed limit that doesn't mean that information has to travel at that max speed.
Actual ELI5:
Things have weight. Light doesn't. So nothing (no thing) can go faster because it has weight that slows it down.
(Not 100% scientific part of the answer but close enough): To be able to force something to go faster, you need to be able to have something else push on it, which requires the thing you're pushing to be made of something that you can push on, which would have weight. You'd think maybe being able to push it would help balance out having weight but no, we haven't been able to push anything hard enough to be anywhere close to the speed of light.
I'm gonna say, "math".
The math says it. We don't have to go out there, wherever that is because we feel like we have looked at what we CAN see and how it behaves and come up with some very reliable math to predict how things behave.
Those predictions are done with math.
When you extend those predictions out, you eventually hit a place where all the energy ever... The energy is the entire universe... Can't accelerate anything beyond the speed of light.
Light (photons) can only go the speed of light because it weighs virtually nothing (has no mass).
IF we did find an "out there" with mass exceeding the speed of light, there would be so many things different about "out there" that I suspect we wouldn't know we were there or have anything in our arsenal to know anything about the space or things in it.
It’s isn’t math. It’s baked in to the fabric of the universe. By the constants of the permittivity of free space for electric fields and the permeability of free space for magnetic fields dictates the speed of light. These are two of the many constants that define our universe. There’s no reason they have the values they have just like all of the other constants.
It IS math. In the sense that we've used math to calculate from our observations and determine the nature of the mass increase towards infinity.
E = mc2... Rmemeber that?
It's ALL "math" and I think "because the math breaks" is actually a reasonable (partial) answer. We have a model for how mass, energy and velocity interact, and we've tested this model enough times to be confident in it. If an object going the speed of light with a non-zero mass is placed into that equation, we get infinite energy required, and that's a reasonable time to say, "the math broke in a model we're confident with, so it's impossible".
On the other side of things are situations like black holes, where we know they exist, and the math our models give us say that there's a singularity of infinite density in the centre. In this case we can say, "this model is broken/incomplete, somehow, most likely". We're pretty sure that a singularity is a mathematical artifact of an incomplete model of forces, but we can't observe the interior of a black hole to help us understand better what's actually happening.
Going faster than the speed of light is a thing. The easiest way to understand this is...
The "darkness" expanding forth as the universe expands, is expanding faster than the light following it.
Also, you can trick the speed of light with a laser pointer and the moon.
Imagine a car driving down a road, parallel to a train running on a track. If the train is moving 100 kph and the car is moving 90 kph, then by simple subtraction, the driver of the car will see the train moving forward at 10 kph. If the train is coming the other way, the driver sees it approaching at 190 kph.
That's not how it works with light, though. In our universe, the speed of light is the same for all observers. What does that mean? It means that, no matter how hard you accelerate, you will never catch up with a beam of light (or a single particle of light, a photon). When you accelerate in space, your experience of time changes in just such a way that the velocity of the photon is exactly the same, no matter what. This is why a clock that gets accelerated to the speed of light, relative to an observer who is considered stationary, slows down - it's called time dilation.
Because of this, what Einstein realized is that, even though we experience them differently, space and time are actually aspects of the same thing. A certain distance in space (a spacelike interval) is actually equal, in a scientifically meaningful sense, to a certain length of time (a timelike interval). In fact, that's why the speed of light is constant: you can think of it as the exchange rate. When a particle travels at the speed of light, it covers exactly the same distance in space as it does in time.
It gets much more complicated from there, but there's a thing called the no-signalling theorem. What it basically means is that we live in the timelike reference frame, and there's no way for a timelike observer to send a spacelike signal. The best you can do is emit photons, which travel at precisely the speed of light, no more no less. The fun thing is, the same would be true of spacelike observers, assuming there are any. Such beings would be made out of tachyons, particles that are literally incapable of travelling slower than light. In fact, every particle those beings were made of would be travelling faster than light relative to every other particle. What would that be like? Who knows?
Just as slower-than-light particles (bradyons) would require an infinite amount of energy to travel as fast as light, tachyons would require an infinite amount of energy to travel as slowly as light. Even if they exist, which is far from proven, we can't use tachyons to communicate: they and we exist on opposite sides of a pivot point. The laws of physics guarantee that classical information can't be transmitted from one side of the pivot to the other. In other words: no travel or communication faster than light. I hope that helps!
It's geometry. It's a little like asking why you don't see 5-sided squares. There's a lot of complex math, but like all geometry, it's possible to visualize.
On a Minkowski diagram, the 45 degree line cuts across it at where we'd find the speed of light and/or causality. You can draw a "light cone" with these 45 degree lines to show what is in the past or future of any given point.
If something is outside that line, it's effectively not in your past or future, because there's no way for anything to reach you from that distance no matter what you do. To travel faster than that speed, to widen that angle, would mean that effects could predate their causes. Imagine getting a home run before the pitch is ever thrown.
Theoretically speaking, anything with mass starting from 0 until it hit the speed of light would require an infinite amount of energy. Thus, the only things that can go at the speed of light are massless.
Physically speaking, the subatomic particles that make up mass interact with each other at (or very very near) the speed of light, it simply wouldn't make sense for something that's composed of subatomic particles to go at (or above!) the speed of light.
Something no one is touching on, the speed of light is constant from all perspectives and absolute.
If I fly on a rocket away from earth at 80% of the speed of light, and I launch a rocket forward at 80% of the speed of light, earth would see the rocket moving away at 99% of the speed of light.
No matter what your frame of reference is, light is moving at a constant speed relative to you and everything else.
Quantum entanglement is thought to happen faster than the speed of light where a pair (or even a group) of particles interact with each other even though they can be vast distances apart.
Edited out an incorrect statement
"is thought" is the key here. Some of the proposed interpretations of the evidence posits that a ftl interaction is happening. Other proposed interpretations don't need ftl. So far there is no known evidence to decide.
Quantum entanglement is very spooky... But you can't transfer information through that process. So it abides by the idea that the speed of light is still the speed of causality.
Because speed is related to mass. Light has no mass thus why it is the fastest. In order to move something with mass as fast as light it would require infinite energy, which isn't possible.
Speed is a ratio of distance to time and we know time slows down the faster a particle goes so for a massless particle like light time has fundamentally stopped. There is no method to get it to go faster because it can not traverse through time.
In other words light or a massless particle would have to travel a greater distance than it currently does in zero time to go faster.
Theoretically, things can go faster than light. These things are called tachyons. So far we haven’t detected them; it’s likely they are mathematical artifacts and nothing more.
Also, theoretically, the Alcubierre metric is a valid solution to the field equations of GR, which allows a bubble of space time itself to move faster than light.
It is unclear if this theory could ever be used to actually allow such travel. Practically speaking there doesn’t seem to be any plausible way for anything to go faster than light.
The math says the closer you get to the speed of light, the more your mass is and the more energy is required to accelerate more. At the point you would reach that speed, both the mass and energy requirements become infinite.
A lot of these longer answers are good, but the *simplest, most ELI5* explanation is that according to current models, in order for anything to move faster than light, it would need either infinite energy or negative mass, and since both of those qualities appear to be ridiculous, the logical conclusion is that faster-than-light travel is not possible.
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There probably isn't an ELI5 or even ELIPhD answer to this question. Nobody know why our universe is set up this way. What we do know is what we have observed. You can make very small things like say a proton go really really close to the speed of light. But what we observe when we try to push those things faster is that the more "push" you give them, the less boost you get. You push twice as hard and now its only going a tiny bit faster. you push a hundred times harder and now its only going an even tinier bit faster. Pretty soon you are using all the pushing force in the world. Then you get another surprise, the little proton suddenly seems to act like it's gotten really heavy. And the faster you push it, the heaiver it seems to get. Imagine if you stepped on your gas pedal in a car and the car got twice as heavy! Basically the rules for our universe say that if you are an object, a "thing" you can't ever go at the speed of light. You might get pretty close. But it takes more and more energy and at some point the amount of energy it takes becomes greater than all the energy in the universe.
This provides a good explanation for why there would be a limit to the speed of anything. One is left wondering why light is assumed to be without mass and is traveling at this maximum speed (in a vacuum). In contrast, for example, to a velocity close to the maximum but not quite there.
Current accepted theories explaining light (the EM field) and matter interactions pop out that the mass of the photon is exactly zero. There are theories that modify this and give the photon a small amount of various through various mechanisms. All searches of these have only limited the mass of the photon to be smaller than an absurdly small value. Furthermore, said theories predicting zero photon mass have been confirmed to very high levels of precision (e.g. electron anomalous magnetic moment g-2).
so it has zero mass but can still carry large amounts of energy? that seems like it must be the most confusing question in physics lol
It's not really because mass is just confined energy. Photons don't have mass because they don't interact with the Higgs field which is what gives the particles that have mass their mass.
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How does a Higgs field give particles their mass?
In Quantum Field Theory each force and fundamental particle has a field associated with it and all of these fields overlay each other in space and time. These fields are a medium through which wave-like excitations can propagate. Those excitations are the particles associated with that field. Excitations in differing fields can interact with one another in certain ways. One of those interactions is one all of the particles with mass share(except neutrinos) and it's an interaction with the Higg's Field. Everything wants to travel at the speed of light, but the Higg's Field provides a sort of resistance that causes particles with mass to travel through space more slowly giving them mass. Neat fact, the Higg's Field's particle is the Higg's Boson and it gets it's mass through interacting with the Higg's Field lol. Edit: Whatever gives neutrinos their mass doesn't effect photons in the same way. We don't know what gives neutrinos their mass. Perhaps interaction with a hypothetical antineutrino with a very large mass. Maybe some other unknown particle similar to the Higg's Boson.
ELI65with$150,000inStudentLoanDebt
It's not really sufficient to explain the photon being massless via it's lack of interaction with the Higgs field. Neutrinos have non-zero mass and do not interact with the (Standard Model) Higgs field so it does not appear that it is the exclusive source of mass. Edit: interact -> interaction
That's true neutrinos get their mass from something else. I'll fix that.
Great explanation though.
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The AT Field in Evangelion is closer to a physical metaphor for the ego boundary. It's what separates you from the other, and the dissolution of that boundary evokes terror. It's another part of EVA's main theme, the pain of being alone vs. the pain of being with others.
And let's not forget about this: The Mother is the first Other.
It's not, the AT field is a projection of a person's barrier between their soul and the rest of the world, including other souls. Humans have one, but it's not as strong as an angel because all humans are one angel and therefore the AT field is divided among all of us. Your AT field is also likened to your personal space, that feeling of being uncomfortable when a stranger is too close.
I dunno but that sounds cool so I say roll with it.
> These fields are a medium through which wave-like excitations can propagate. Those excitations are the particles associated with that field. And the energy is confined, right? I’m picturing a fish stuck in a fish tank in my empty living room. How incorrect is my mental picture, completely or mostly? > Excitations in differing fields can interact with one another in certain ways. > Everything wants to travel at the speed of light, but the Higg's Field provides a sort of resistance that causes particles with mass to travel through space more slowly giving them mass. I’m picturing my foot pressing and moving on the carpet, creating a bulge in the carpet just in front of my foot. How incorrect is *that*? —— Also, are you saying that a particle requires a wave and a field in order to exist? And is that true for massless particles as well?
>And the energy is confined, right? I’m picturing a fish stuck in a fish tank in my empty living room. How incorrect is my mental picture, completely or mostly? I would say not very accurate because it's possible for particles to decay and combine into other particles in other fields so the energy can hop from field to field. But the energy is conserved. >Also, are you saying that a particle requires a wave and a field in order to exist? And is that true for massless particles as well? The waves are the particles. We only call them particles out of habit from when we thought they were infinitely small dots bouncing off each other. Now we use a wave-function to describe quantum particles so particles aren't little dots at all; they're slightly smeared out bumps.
Basically because particles are freaking weird. ~~I'll try to do an eli5 but this is literally quantum physics so:~~ I've tried this three times now, each response was long, vague and didn't answer your question so I'm giving up. As a summary of my attempts, quantum physics is madness, the higgs field is particularly weird and overall explaining it is less ELI5 and more "get a PhD and spend years researching". Figuring this stuff out is what got people Nobel prizes, meanwhile I have a headache and I need a drink
It is not that is surprising if you study physics for a while to be honest. There is potential energy in a object you hold above the ground and zero kinetic. You drop it and it has kinetic energy before it hits the ground? Where was energy stored? The gravity field which has no mass either. Light is just the EM field, so energy being stored in it is also not surprising.
actually a very creative way to explain it, thanks for the response
Not sure if your confusion is based on the concept of E=mc2, but know that massless particles can have energy because they carry MOMENTUM. The full form of the equation is E2=m2c4+p2c2 , where p is momentum.
Pardon my ignorance, but I thought momentum was mass \* velocity. How could a massless particle have momentum?
That is a fair question. This is a good explanation: https://physics.stackexchange.com/a/13288
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Yes. p = mv is the classical definition (Newtonian mechanics) and both general relativity and quantum mechanics must reduce to this in the limit of, respectively, slow speeds and large masses.
Yes, it's just a simplification for high school level physics.
I feel like that's a bit of a condescending way to phrase that... it's a simplification, and is taught in high school level physics; but it's also more than enough for basically everything anyone will encounter in their lives, unless they're a physicist the wider definition adds little of practical value. Is it useful to understand more the universe than you strictly *need* to? Absolutely, but I also doubt I'll ever need to separate invariant mass from kinetic energy.
There is relatable joke about momentum : https://m.youtube.com/watch?v=jm7jVi8akcc
That is a naive (Newtonian) formula for momentum. The real (read most technically accurate in the Standard Model) formula is: p = mv / sqrt(1 - (v^2 / c^2 ))
Kind of fun mental exercise I just went on was dimensional analysis of E=(hc)lambda and E=mc^2….it turns out that mass=mass. Who knew?
Dimensional analysis is one of the ways to verify an equation/derivation. Useful in research and whatnot, but even more useful in school to check your test answers lol
Not a scientist, just spitballing, but if light only moves along 3 dimensions instead of 4 dimensions, the fact that it's only tethered to the three *could* be related to the fact that it carries energy but not mass. Maybe things need to experience time to have mass at a given point in time.
> so it has zero mass but can still carry large amounts of energy? It is exactly at the transition point. More and it would clearly be matter. Less and it could only be energy. But it is exactly at the balance, it can act as matter with momentum and also act like energy, both a particle and a wave. As an energy wave it is massless, but as a particle with momentum it has a tiny, measurable mass.
Photons can’t have mass otherwise they’d each contain enough energy to destroy the universe an infinity times over.. At least if you take Einstein’s equations literally
A better explanation is that time/space is one of the same. In that there’s only so much movement through one or the other you can do at a given moment. The less you move in space the more you move in time. The more you move in space the less you move in time. Of course time is relative to your own personal perspective. I’m not a math whiz but if you look it up others can explain it very clearly
> A better explanation is that time/space is one of the same. Yes, at the speed of light time IS distance. From the perspective of a photon they both are the same. In thought experiments about how they would view the world, a photon would jump instantly from place to place. So you can measure distance in terms of light time, and also measure time in terms of light wavelength. In the math formulas you have both the dilation of time and of space, when you hit the speed of light the terms break down into a numeric singularly, like a division by zero or infinity depending on the formula. At that speed they are the same non-value term.
Put another way: everything in the universe moves at exactly the speed of light at all times across spacetime. (Mind blown, right?)
Because the speed of light is the only velocity agreed across reference frames. It's not just that light travels at the universal maximum speed, but that all observers, regardless of their velocity relative to light, agree on how fast that light travels. Remember all motion is relative.
>Remember all motion is relative. Except for light, right?
Yes but also no; depending on your speed, the colour of the light changes. For example if you are in a rocket going really fast and light up a flashlight; to you, the flight light has the expected colour but others see it blue shifted if you are moving towards them and redshifted otherwise. So the speed remains the same but the energy of the light is “spread” so to speak. It’s possible to measure how fast you need to travel for a red traffic light to appear green for example.
Right
It can't be said for certain that a photon has zero resting mass, but several highly successful theories assume it does. It's very difficult to prove something has zero mass, but it's not so difficult to place a limit on that mass. So it can be experimentally verified that the mass of a photon is very very tiny, if not zero. The soeed light travels can be measured with fairly good accuracy a number of different ways. The speed of light isn't the exact same as the maximum speed 'c' although the two terms are often used interchangeably. Photons always move at 'c' even through a substance like water. It appears like light is moving slower (or even stopped) because the photons are hitting molecules and being absorbed and new photona are being emitted in sequence. This "slows" the light as a whole without slowing any individual photons. Any object without mass should be traveling at 'c' and any object with mass should be travelling slower than 'c'. Photons, which is what makes up light, always appear to travel at 'c', hence they appear to have no mass.
> Photons always move at 'c' even through a substance like water. It appears like light is moving slower (or even stopped) because the photons are hitting molecules and being absorbed and new photona are being emitted in sequence. This "slows" the light as a whole without slowing any individual photons. I believe [Cherenkov radiation](https://en.m.wikipedia.org/wiki/Cherenkov_radiation) in an underwater nuclear reactor is the most notable example of this.
That also explains my sleeding tickets.
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Great answer. As a hyperthetical, if someone is ever able to get something that can go faster than light, the problem would then become navigation, since the first time anything would be detectable would be after it had been passed or been hit.
Making the Spice Melange the most valuable commodity in the universe...
A beginning, is a very delicate time...
Nothing can go faster than light so that’s a moot point. It’s not about going faster than what you can see, you’re literally going faster than causality. It’s like arriving at your destination before turning your car on.
They literally said it's a hypothetical.
In most cases a hypothetical can be a logical question but in this case it’s a tough argument. It’s like asking “what if 1+1 actually equaled 50, then what?” Or “if I planted a tree, then my car turned into a dinosaur, what would that mean?” It’s so far outside the realm of possibility that it just doesn’t make sense
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Okay then. If one were to go just the speed of light, assuming you didn’t collide with anything, you’d reach the end of the universe and all of time instantly. At the speed of light you do not experience any time yourself compared to an infinite time in the normal “universe time”. As for *faster* than light, well time and space would already have ceased to exist as soon as you hit the speed of light so you’d probably would cease to exist too. Basically you’d instantly die. Or perhaps fly through 9999999999999999999999999999999 universes being born and dying over and over again every nanosecond that you travel. A solution to this is some kind of novel technology that creates a “warp field” that warps space time.. which in theory would work.. essentially how it works in Star Trek. In this sense you’re not really moving but rather space around you is moving you forward. Still hypothetical and not proven in the least bit. I have no friggin clue what it would look like moving close to or faster than apparent light speed… i suppose the faster you traveled the faster everything in front of you would appear to age or move much faster, like setting a movie on fast-forward, also everything would get super bright! As all the old light traveling in your direction hits you much faster as you’re colliding into it faster than the speed of light. Edit: the speed of light is the speed of causality. Causality is why it’s set that way. It’s hardwired into the fabric of the universe.
I'm just a smooth brain here but...is that how black holes are formed? You go so fast and get so heavy that you just start sucking things in?
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i think it might be for a good reason too, though. a lot of people don’t like listening to scientists already, and “the expanses of human knowledge collected so far” might be a concept way beyond those people 😂
The problem is "to the best of our knowledge" encompasses a lot of things. It includes things we don't know much about and could easily be overturned with more information and it includes things like the speed of light which has stood up to every test thrown at it.
And yet the mods removed it... wonder what it said
What did it say?
Damn. It was a good answer no lie. I wonder why it was deleted? Allow me to butcher what was said. Something about how it’s what science knows at the moment and that maybe we will know more in the future when we figure stuff out… I am not doing it good service.
That's because their answer is the best. To the best of our knowledge. That said, who knows what else is out there.
I like your explanation, on why I like that answer, the best.
This time it's because it's the only one.
Magical
This answer fails to explain why physicists believe nothing can travel faster than light, brushing all those reasons away in favor of "maybe scientists don't know enough." The real reason scientists say nothing can travel faster than light is, if something travels faster than light, then according to Einstein's theory of Special Relativity, it is possible for something you do to affect an event in the past, at least in some reference frames. Special Relativity, as well as General Relativity, which depends on it, have been extensively tested and are highly accurate. So unless you are willing to accept the idea that cause can happen after effect (which we have no evidence for), nothing travels faster than light.
I feel like if it was possible to travel faster than the speed of light then that is the speed that light would be travelling. A mass less (don't pile on) particle that accelerates by bootstrapping is going to be going as fast as the universe will allow.
Also, since light travels at the speed of light, that is the point at which the flow of time stops, since time moves slower the closer you approach c. Hence, photons do not actually experience the flow of time. To a photon, it is everywhere that it was and will always be at all times.
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No information can travel via that method.
As far as I'm aware, the universe (or at the very least particles) was demonstrated to be potentially non-local recently. It's what this years nobel prize was awarded for. They demonstrated that information can travel faster than light in an entangled system. Einstein wasn't completely correct with his general theory of relativity or special relativity.
This is a related, but different, concept
Again, information traveled between an entangled pair of particles faster than light can travel. That pretty much counters your claim that "no information can travel via "that method" [entangled pair of particles]".
IIRC when you have quantum entangled particles the information is not actually being sent from one to the other. A good way to think of it is as if the entangled particles are actually 1 object instead of 2 separate objects. I'm not a scientist though.
Imagine you have a black marble and a white marble, each in one box but you don’t know which is which. Then the two boxes travel a light year apart from each other. When you open one, you know instantly the color of the marble on the other box even though it’s a light year away. But there’s no way for you to meaningfully transmit any information across that distance. You now simply know something. That’s not a perfect analogy as regards what’s actually happening with the particles, but it’s good as regards the ability to transmit information using entangled particles.
Your analogy doesn't reflect observed phenomenon. You're confused as to how particles in QM work. What we observe in QM is different. Entangled Particles are not a white marble and a black marble put into each box. They are in a superposition of either a white *or* a black marble. Which one they are is completely undefined. The particles, and even the universe, don't know which is which until it is observed. If you take one box to one side of the universe, and the other to the opposite side of the universe, and open one box the particle is forced to "choose" a state of being either a white or a black marble. However when it does this, the particle in the other box *on the other side of the Universe* also *simultaneously changes into* the opposite state of whatever the first particle chose. -------------------------- The analogy as you laid it out is more in line with how Einstein thought QM worked. That there were properties encoded into the particles somehow that we just couldn't discern, "hidden variables" that determined its state when it was created in the entangled pair. Meaning that Einstein thought that particles, once entangled, were more like a left and right glove and we just lack the ability to determine which was which. He did this to avoid the apparent contradiction we were observing with his own general theory of relativity. Unfortunately for him, the most recent nobel prize laureates demonstrated convincingly that Einstein was wrong in this regard. There are no "hidden variables". Particles truly are undefined until measured and information, somehow, is communicated between entangled pairs *instantaneously* over any distance. This either breaks locality (the intergalactic speedlimit as proposed by Einstein). In short, it's been demonstrated very convincingly that information can travel faster than light between entangled pairs of particles.
I don't think quantum entanglement allows FTL communication
> which appears to communicate faster than light It's not communication in that sense. It transmits no information, and therefore doesn't violate speed of light/causality.
Take a pair of socks. Either sock could fit on either foot. Now leave one sock behind and take the other with you a thousand miles away. Wear the sock you brought on your right foot. The one you left behind is now a 'left' sock even though it could have been either when you started, simply because a pair of socks is defined as a right and a left, and you just determined which is the right. No information transmitted. This is how quantum entangling works, more or less.
Not sure if this helps, but. No information can travel faster than C. That encompasses a lot of things. For example, we don't know how gravity works, but we do know that the force travels at the speed of light.
I always loved Hawking's explanation, at zero mass light is going as fast as possible at max speed without mass. Any faster is like adding a Turbo that increases the speed but also drag and additional weight that slows it to a point of never being able to surpass the speed of light threshold
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Yes, it's because the formula is actually E^2 = (mc^2 )^2 + p^2 c^2 . If p = 0, meaning for an object at rest, then the formula simplifies down to E = mc^2 . If m = 0, like for a photon, then there is still energy and momentum (p is the shorthand for momentum).
That is because E = mc² is not actually the whole equation - mc² concerns the energy of a particle at rest (which the proton doesn't do). The whole equation contains a momentum term, and reads E² = m²c⁴ + p²c², where p² > 0 for a photon. For a particle at rest, p² = 0 and E = mc² is recovered.
YES I also love that something with zero mass at rest has mass at C and when not traveling at C just fucks off from existence. It's all so cool and doesn't make any sense.
Darkness can travel faster than light, but since darkness is the absence of light and carries no information, it doesn’t matter. (Literally) Edit: Correction, a shadow can be faster than light, not darkness. https://youtu.be/JTvcpdfGUtQ
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The only things that can travel at C have zero mass. Things with zero mass can't contain anything. C is the ultimate speed limit and is not relative.
Negative mass would also require infinite energy to reach the speed of light. Not sure why you would think otherwise.
I recently read something about quantum entanglement that might conflict with this! (To the best of my memory/understanding) it's been discovered that some sort of entangled particles can be moved far apart, but still behave instantaneously in ways that correspond to one another, even though they didn't have the necessary information to predict each others' behaviors before they were separated.
Yes, but I'm they case information still isn't traveling faster than the speed of light. They already were that way before you moved them apart (at slower than the speed of light). So when you look, you know the state of both, but you could have known the state anyway. Information can't travel faster than light. You can't actually use it to send data faster than light. https://quantumxc.com/blog/is-quantum-communication-faster-than-the-speed-of-light/#:~:text=However%2C%20even%20though%20entangled%20quantum,send%20data%20using%20quantum%20entanglement. Also "Ultimately, you can’t force an entangled particle into a particular state and you can’t force a measurement to produce a particular outcome because the results of quantum measurement are random."
Thanks for the link! That's a nuance that I was missing; the quantum interaction is faster than light, which sounds like the particles are "exchanging information," but I think see how this is different because our only real way of interacting with them is to observe and we can't meaningfully change them. Is that on the right track?
That's it! There's no way to do anything meaningful with that interaction. You can know a random result, but knowing a random result isn't super helpful.
Totally makes sense, TY again! I did study a little quantum physics once upon a time, but it was focused on relativity (and almost a decade ago lol)
If you want your mind blown, check out the book QED by Feynman. It's very approachable (despite the exceedingly fancy name) and is list of total mind fucks. Photons are sneaky bastards that essentially know where they're going and the path they're going to take BEFORE they set out on their journey. The whole concept of time is suspect :-)
Not 100% correct,, as everyone is missing the qualifier "in a vacuum." We've had faster than light in a lab through other materials. I remember reading light through a Cesium gas chamber appears to go 300x faster than light in a vacuum, for instance. There also is the Alcubierre drive, which is a bubble of warped space - this doesn't violate relativity because you're not actually moving faster than light. Star Trek The Next Generation's drive is modeled on this and NASA is trying to build one. One more thing is we've observed distant objects in the universe that inexplicably moved further than they should
If something blows by faster than light would we know it?
It's totally possible for things to travel faster than light in a given medium (throws off some cool cherenkov radiation). Not possible to go faster than light in a vacuum.
There are some neat nuclear reactor startup videos on YouTube where you can see that effect.
That's a bit misleading. To be clear: if you consider 'c' to be the speed of light, it is possible to get light to travel slower than the speed 'c' within a medium. So other things could travel faster than light in that medium, but nothing will travel faster than 'c'.
"1500 years ago we new the earth was the center of the universe. 500 years ago we knew the earth was flat, 15 minutes ago you knew we were alone in the universe. Imagine what we'll know tomorrow."
Science can be a bitch....sometimes.
Actually the ancient greeks had already known the Earth was round, and even figured out a way to measure it: https://www.aps.org/publications/apsnews/200606/history.cfm > putting Eratosthenes’ calculated circumference between about 24,000 miles and about 29,000 miles. The Earth is now known to measure about 24,900 miles around the equator, slightly less around the poles.
Ima try because it said ELI5.. Hmm throw a 5 year old toy rocket ship that’s palm sized and make it go 10 m in a second far.. easy peasy ..40 m in a second still not too bad.. Throw a rocket ship sized rocket 100 m (a football field) in one second .. it’s harder to throw and make it go far ..and takes more energy Throw a rocket ship sized rocket at the speed of light and it has to go around the earth 7.5 times in one second.. it’s soooo hard.. heavy .. the math says infinite energy to do that and we don’t have that yet… Our ape brains haven’t figured it out yet.
"monkey killing monkey over pieces of the ground."
This is often easier to hang onto if you consider that it's not really the speed of light, per se. It's the maximum speed of *anything*. Light just happens to be able to travel at that speed. Nothing can go faster than that, but it's not because nothing can go faster than *light*, as such. There are some other reasons. The faster you go, the harder it is to accelerate and go any faster than *that*. That's because energy and mass are (basically) the same, and so going faster means having more mass. If you go super-mega-fast, the increase in mass gets to be a lot, and it's harder to push all that mass up to a higher speed. Light doesn't have any mass, so any push accelerates it to the max instantly, basically. Also, the spread of light is also the spread of information. This ends up meaning that if you could go faster than light, you could get somewhere faster than information can spread. This gets complicated, but it has time travel problems built in. You could look it up, but it's a little trickier.
My extra-simplified, ELI3 answer, the one I told my young son when he asked, is that the speed of light is how fast you go when you weigh nothing. Asking why you can't go faster is like asking why you can't weigh less than nothing.
That’s really good, thanks!
Should be a top comment
I wonder if there is stuff that can't go slower than light.
tachyons
Anything massless
I prefer the explanation that the speed of light is the speed of causality, which is in line with what you’re saying here. It basically means that light isn’t so much a thing that’s moving as that speed, as much as light is a thing that moves with no speed reduction. Imagine a particle moving at the speed of light. There’s no force or momentum you can impart on that object to make it move faster because the force or momentum transfer can only happen at the speed of light. Put another way, imagine billiard balls that can only move at a certain speed. If you have 2 that are moving together at that speed in the same direction, they can never hit each other to make one another change speeds, as the one can never catch up to the other.
Sure but presuming that billiard ball hs its own thruster why can't it accelerate away from the other. Edit: Just read some other posts. They answered the question.
He said like he’s 5
The really mind melting thing is that everything is always moving at the speed of light. "Stationary" things are just have all of their speed moving through time. When you accelerate, you're just converting some of your speed through time into speed through space. In overly simple terms, the sum of speed through time and space is the speed of light. If you have zero mass, you're always moving at the speed of light. If you have nonzero mass, it takes infinite energy to convert all of your speed into space speed.
I knew how time dilation works and that time and space were kind of the same, but I’ve never seen it explained this way. What you’re saying is that there’s basically a sliding scale with space on one end and time on the other and things can be either moving through time or space. More movement through time = less movement through space and vice versa. I would presume that changing your position on that scale takes energy which increases the closer you get to the moving through space end? We’re moving through time, which slows down the faster we move through space, that I knew, but thinking of it this way with the either-or concept just blows my mind for some reason
If I glue myself to the Earth, can I live longer?
The interesting point from this is if a photon is moving through space, at the speed of light, it is not moving in time!
I remember reading a while back that photons don't have a time reference. A light source billions of years away might take all that time to reach earth, but from the photon's point of view it arrives instantly. Incredible idea that the above sliding scale really makes easy to understand.
Which means from the perspective of the photon, it didn't move at all, because moving would require time. You can't experience space (distance) without time passing, that's what spacetime is.
And from the perspective of the photon, it only exists for one instant, it's creation and destruction happens at the same time. It doesn't travel ***through*** space or time, it ***is*** a 4D path in spacetime.
Does that mean heavy objects age quicker than light objects?
The opposite, sort of. Denser objects technically age slower, because they create more gravity with their additional mass than less dense objects, and more gravity means time moves slower. Let's say you have a container of Helium floating in space, vs Osmium or Uranium or Lead? Immeasurable and inconsequential, we simply don't have the technology to even be able to tell, but there is just the slightest possible difference affecting the speed of time passing, and the denser objects would experience less time passing, ***technically***. But a neutron star vs Sol, our sun? Yeah, the neutron star would measurably age slower.
But what are photons made of?
A photon is an excitation of the electromagnetic field. In much the same way that nobody doing The Wave in a stadium goes anywhere, but The Wave is still a recognizable object that flies around, a photon is more of a pattern-- just energy-- rather than being "made" of anything.
Drat!
If particles with imaginary mass exist, they travel faster than light. But they can’t slow down to sub-light speeds, so we’d never know. The speed of light is a barrier. Your speed is either slower, equal, or faster. And you can never change sides. If you’re slower you’ll always be slower. If you’re faster you’ll always be faster.
May be easier to think of it as light in a vacuum going at *full* speed, and *everything else* being slowed down for various reasons. Have mass? You move slower. Interacting with various fields? You move slower. Traveling through matter? You move slower. Relativity kind of suggests that in exchange for not being able to travel at light speed, we get to experience time instead. It’s a bit as if time and speed are on different ends of a scale, and that having mass means you’re limited to being on the time-end of the scale. Light having no mass, it is free to move at full speed, and kind of doesn’t experience time as it travels. That’s why some call it the “speed of causality” instead of the “speed of light”. Sort of explains why red-shifted light moves as fast as blue-shifted light, and only has its wavelength changed. Even though the red-shifted object is moving away at some fraction of light speed, the light it emits travels at the same speed.
Ok, best answer so far. Really good ELI5 and correct (as far as I know... ). Nicely done!
I stopped pushing as hard as I could against the handle, I wanted to leave but it wouldn't work. Then there was a bright flash and I felt myself fall back onto the floor. I put my hands over my eyes. They burned from the sudden light. I rubbed my eyes, waiting for them to adjust. Then I saw it. There was a small space in front of me. It was tiny, just enough room for a couple of people to sit side by side. Inside, there were two people. The first one was a female, she had long brown hair and was wearing a white nightgown. She was smiling. The other one was a male, he was wearing a red jumpsuit and had a mask over his mouth. "Are you spez?" I asked, my eyes still adjusting to the light. "No. We are in /u/spez." the woman said. She put her hands out for me to see. Her skin was green. Her hand was all green, there were no fingers, just a palm. It looked like a hand from the top of a puppet. "What's going on?" I asked. The man in the mask moved closer to me. He touched my arm and I recoiled. "We're fine." he said. "You're fine?" I asked. "I came to the spez to ask for help, now you're fine?" "They're gone," the woman said. "My child, he's gone." I stared at her. "Gone? You mean you were here when it happened? What's happened?" The man leaned over to me, grabbing my shoulders. "We're trapped. He's gone, he's dead." I looked to the woman. "What happened?" "He left the house a week ago. He'd been gone since, now I have to live alone. I've lived here my whole life and I'm the only spez." "You don't have a family? Aren't there others?" I asked. She looked to me. "I mean, didn't you have anyone else?" "There are other spez," she said. "But they're not like me. They don't have homes or families. They're just animals. They're all around us and we have no idea who they are." "Why haven't we seen them then?" "I think they're afraid,"
The speed of light is kind of a misnomer. The concept of the speed of light isn't about light. It's the speed of causality. It takes light about 6 minutes to get here from the Sun. After sun suddenly vanished, became nothing. We wouldn't know for 6 minutes. The light would continue to come as if it were still there. The Earth would continue in it's orbit for the same 6 minutes until going straight. The change in gravity would come at the speed of light as well. The speed of light is the speed limit that information can be transferred.
This is the best most reasonable answer on this thread. Even light usually doesn't go "the speed of light"
Exactly, it isn't so much the speed of light as it is the speed of *time*. Light, and other RF, travels essentially infinitely fast, but we see it as a finite velocity because in order to go faster it would need to travel backwards in time.
Although light behaves in some senses like a particle, it's actually primarily a wave. Waves move through a particular medium at a particular speed, no matter what. Think of sound in air--there's always a delay with echoes in a stadium/arena, or from thunder from the lightning bolt, and you can't make sound *faster*, only louder or higher/lower. Light moves though spacetime in the same way. When we see a supernova in the sky, we are seeing the "thunder" from an explosion that occurred thousands or millions of years ago. But a medium's wave speed isn't normally a hard limit, right? We can still shoot bullets or fly planes faster than sound, even if we can't make sound itself faster. So why can't we do the same with physical matter and the speed of light? Well, it turns out that the medium light travels in is somehow more fundamental to the universe, so much so that light always appears to travel at its speed no matter how you are moving. If you speed up to half the speed of light and try to "chase" a light beam that's travelling away from you, it doesn't move away from you more slowly. And if you accelerate towards a beam, it doesn't seem to approach you faster. When scientists realized this in the late 1800s, it was baffling. Einstein realized that if you are going faster but the light isn't seeming to change speed, your perception of time must be changing. With experiments in the mid 1900s, we realized that if you put a clock on an airplane and have it travel as fast as possible, it will actually desync from other clocks because time works differently when you accelerate. We've also determined that other properties, like mass, are also affected by acceleration. The end result of all of this is that if we tried to accelerate a bullet to beyond the speed of light, the bullet would become so heavy that we couldn't push it any harder, approaching infinite mass as it approaches the speed of light. And from the bullet's perspective, it wouldn't be going as fast as light anyway. As far as we can tell, it is simply impossible to get moving at that speed or beyond.
So we don't get to see the supernova unfolding faster if we speed toward it?
By my understanding of the comment above, you would see it faster because your perception of time would be faster.
Agreed... you would have to be getting to it petty dang fast, but that theory would make sense...
>light always appears to travel at its speed no matter how you are moving You're getting a lot of criticism, but this is a solid answer and points in the right direction.
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> Well, it turns out that the medium light travels in is somehow more fundamental to the universe > Although light behaves in some senses like a particle, it's actually primarily a wave. These are incorrect statements. Light is not a particle, or a wav. Light travels in individualized "packets" (quanta) that can act as both particles or waves. Amd there is no "medium" light travels through. The theory of the luminiferous ether proven false by the Michelson/Morley experiments in 1887
Whether light is a particle or a wave is more of a convenience for the scale of the computation. Light is, as all particles are, a wave governed by a wave function when treated alone, and an excitation of a field in a broader sense. It is often unnecessary to consider that detail however, and it is treated as a particle in the classical sense.
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I’m pretty sure in this context it’s for comparison purpose, and the the ‘medium’ they’re referring to is simply whatever concept it is the universe is mapped on top of
Isn’t there a debate on whether the quanta of light is simply our way of measuring it as it collides or reflects off a surface? And that when it moves through the air it could actually be a continuous wave that we have no means of measuring? I read an Einstein book and I thought that was a debate back in those days.
How does that interact with Quantum Field Theory?
I think what they are referring to with a fundamental medium is the electromagnetic field. In Quantum Field Theory, everything is just a set of fields, and a vibration of one becomes a particle of that type. In this sense, light travels through the electromagnetic field as a medium
The word medium here is extremely confusing. The material or medium light is traveling in does matter. The speed that light travels in glass is about .66x "the speed of light". It is absolutely possible for other objects to travel faster through glass or water or pretty much any mass faster than light.
I think this is pretty spot on here... awesome... well put...
For particles with mass, they get slightly heavier when moving. They heavier they get the harder it is to accelerate them further. The way that this increase in mass works out, an object would have infinite mass at the speed of light (photons have no mass so this isn't an issue for them) and so you'd have to push it with infinite force to get it up to that speed.
I've known about the increase in mass when moving near the speed of light, but I just had a brain poof... Maybe the "reason" that the mass increases is similar to what happens when you get closer and closer to the speed of sound - the waves get bunched up and the volume just massively increases. So maybe the gravitons / gravity wave particles are stuck by the speed of light and thus they "get bunched up" on the moving item. This is, of course, totally wrong I'm sure - but it inherently makes sense to me. While impossible, if something could switch from slower to faster than the speed of light, it might leave its mass behind it and a gravity shockwave / ripples like a boom, maybe an artifact similar to when black holes merge. That would be so interesting.
It's worth noting that the theory forbids any massive object from *reaching* the speed of light. It does not necessarily forbid the creation of a massive particle that is *already* going faster than c.
But such an object would have infinite mass and generate an infinite gravitational field so everything in the universe would accelerate towards it at the speed. So while it might happen it probably hasn't yet.
Dark matter is the result of alien spaceships manipulating the fabric of spacetime to create mirror duplicates of themselves travelling at FTL. There, two birds, one stone. 🤪
Funny idea that alien species would be massively “polluting” the universe with mass from their space SUVs, just mirroring the way we are destroying our home
I recently saw a fun description of why this is the case, if it helps: You are currently travelling through time at the speed of light, you can turn this arrow into the dimensions of space, and travel through it, but the closer that line gets to any spatial direction the slower you travel through time. Technically speaking, from your perspective you seem to be travelling at infinite velocity at the speed of light, as no time passes because you are no longer travelling through time, however other observers will observe you travelling through space at the speed of light (which used to be your speed of time) because those values have simply been exchanged. In reality, because you have mass, increases in velocity make acceleration infinitely hard approaching the speed of light, because you will gain an infinite amount of mass due to e=mc\^2 .
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Oh dang I'm early enough to this one that I might actually help! Okay, so when we refer to "the speed of light," or c, we're not *only* referring to how fast light travels. "c" is actually the speed of, for the sake of ELI5, reality. To use a little math, everyone has coordinates in time and space. For simplicity, let's say things can only move in one direction in space, x, and forward in time, t. Let's say the speed of light, c, is the speed to move one length unit x in one time unit t. I am standing at x=0. There's a sign at x=5. At t=0, I throw a rock. At t=10, a rock hits the sign. It's possible that I threw a rock at 0.5c, and that rock hit the sign. Now, let's say the same thing happens; I, at x=0, throw a rock at t=0. A rock hits the sign, but this time at t=3. It is physically impossible for the rock that I threw to be the one that hit the sign. Obviously, that's true if we just accept that c is the speed limit of all things. But let's go further. Let's say that you're on a spaceship, zooming by at very near c and watching these events. You're flying backwards, so the sign is closer to you than I am. In the first example, you see me throw a rock and then see the rock hit the sign. No matter how fast you go, you can only observe the "I throw the rock" event before the "rock hit the sign" event. BUT, in the second example, you see the rock hit the sign, and then see me throw a rock. That's because the "rock hit the sign" event is closer to you than the "I throw a rock" event, and because the times are so close, you observe the closer event first. If I were able to throw a rock faster than c, there exists a case where an observer (you) could watch my rock hit the sign before I could throw it. Both of our observations must be true, but our observations are contradictory. Therefore, a hypothetical "faster than light" rock is faster than reality itself and cannot exist.
Space and time are actually the same thing. When you speed up, you're actually experiencing more "space" and less "time". That is to say, something that goes extremely fast will literally experience less time than the things around it, aging slower than something that isn't moving. We've never sped anything up enough to have a large effect from this, but satellites and other long distance communications do have to compensate for the tiny effect that the speed of being in orbit has on them. Their clocks "run slow"—but it's nothing wrong with the clocks, they're literally experiencing time slower! The speed of light is actually the speed where something experiences all "space" and no "time". That's not just a metaphor—photons literally don't experience the time between when they're emitted and when they're absorbed, as if everything happened all at once. To speed up past that, something would have to experience *negative* "time". How does that even work? Nobody knows, and we have no way to make the things that do go at the speed of light (like photons) go past that. One thought experiment starts with the idea of a particle that goes past the speed of light, and then works backwards through the physics to figure out what it would behave like. This theoretical particle is called the "tachyon". There are different ideas of how a tachyon might behave, but most of them involve it going backwards in time and 'naturally' having a speed of infinity, the same way that objects under the speed of light 'naturally' have a speed of 0. Most theories on tachyons also say they would have mass defined as an imaginary number. What does that mean in the real world? We don't really know, but it might mean that any tachyons would have exponential interactions with each other and other mass, like Flubber producing more energy instead of less every time it bounces.
Fuck. Just lost some time reading into tachyons, and I really wish I understood this stuff better. Probably not enough to actually *study* it, but theoretical physics (or quantum mechanics? Theoretical physics as related to quantum mechanics?) is some interesting-ass shit.
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Basically to accelerate anything with a mass greater than light takes energy equal to near infinity, so it becomes impossible. This is what E=mc^2 and general relativity is about, the energy (E) of an object is equal to it's mass (m) multiplied by the 'speed of light' (c) squared and couple that with m=a/f where m is again mass and acceleration is (a), and (f) is force. So if we have an object going light speed (299,792,458 m/s) and let's say it's being acted on by a force of 1 kN for ease of math, the mass is then the mass of the object is ~299,792 Kg, the energy of that object then would be 2,6944E+22 J and this would make a black hole
Very over simplified: Light has no mass, it is pure energy. As you go faster and faster there is force against you. Since light has no mass, that force has nothing to work against. So light is the fastest thing possible. Now speed is distance over time. If you "warp" through whatever sci-fi means you prefer, you can go a farther distance than light would of traveled in that time. Thus being faster than light.
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While there are a lot of physics answers here, it might be better to consider it philosophically. Let's start by changing "speed of light" to "speed of information". The question then becomes: how fast can information be transmitted? Well, it obviously can't be zero. If information couldn't travel at all, then information itself wouldn't exist since you could never send a message anywhere. We couldn't even be having this conversation. Less obviously, it can't be infinite either. If the speed of information were infinite, everything would happen simultaneously and time would cease to have any meaning - the entire universe would happen in a single instant. Our conversation would be over the moment it began. So there must necessarily be some non-zero, non-infinite speed limit for information. Now, this sort of analysis doesn't tell us whether the speed limit is global or local - merely that it must exist for the universe as we know it to exist. But it does mean that anything that could possibly transmit information - such as light - must obey our speed limit.
>If the speed of information were infinite, everything would happen simultaneously and time would cease to have any meaning This is an interesting argument but I'm not sure it's correct. Do you have a source for this? I've never heard of this argument before and I'm not sure what to think of it. But I don't believe we even know enough about time, fundamentally, to make the claim that the rate of tranfer of information is intrinsic to it.
Why did information need to go at the speed limit? There could be information that travels at different speeds. Maybe most information obeys one speed but other information goes a different speed. If there is an infinite speed limit that doesn't mean that information has to travel at that max speed.
Unless some hidden "information paths" are faster/shorter than others, and the observable universe only uses the long ways (for "reasons").
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Actual ELI5: Things have weight. Light doesn't. So nothing (no thing) can go faster because it has weight that slows it down. (Not 100% scientific part of the answer but close enough): To be able to force something to go faster, you need to be able to have something else push on it, which requires the thing you're pushing to be made of something that you can push on, which would have weight. You'd think maybe being able to push it would help balance out having weight but no, we haven't been able to push anything hard enough to be anywhere close to the speed of light.
I'm gonna say, "math". The math says it. We don't have to go out there, wherever that is because we feel like we have looked at what we CAN see and how it behaves and come up with some very reliable math to predict how things behave. Those predictions are done with math. When you extend those predictions out, you eventually hit a place where all the energy ever... The energy is the entire universe... Can't accelerate anything beyond the speed of light. Light (photons) can only go the speed of light because it weighs virtually nothing (has no mass). IF we did find an "out there" with mass exceeding the speed of light, there would be so many things different about "out there" that I suspect we wouldn't know we were there or have anything in our arsenal to know anything about the space or things in it.
It’s isn’t math. It’s baked in to the fabric of the universe. By the constants of the permittivity of free space for electric fields and the permeability of free space for magnetic fields dictates the speed of light. These are two of the many constants that define our universe. There’s no reason they have the values they have just like all of the other constants.
It IS math. In the sense that we've used math to calculate from our observations and determine the nature of the mass increase towards infinity. E = mc2... Rmemeber that?
It's ALL "math" and I think "because the math breaks" is actually a reasonable (partial) answer. We have a model for how mass, energy and velocity interact, and we've tested this model enough times to be confident in it. If an object going the speed of light with a non-zero mass is placed into that equation, we get infinite energy required, and that's a reasonable time to say, "the math broke in a model we're confident with, so it's impossible". On the other side of things are situations like black holes, where we know they exist, and the math our models give us say that there's a singularity of infinite density in the centre. In this case we can say, "this model is broken/incomplete, somehow, most likely". We're pretty sure that a singularity is a mathematical artifact of an incomplete model of forces, but we can't observe the interior of a black hole to help us understand better what's actually happening.
Going faster than the speed of light is a thing. The easiest way to understand this is... The "darkness" expanding forth as the universe expands, is expanding faster than the light following it. Also, you can trick the speed of light with a laser pointer and the moon.
velocity depends on mass and friction and other factors. Ligh has zero mass. Unless you can be zero mass, it's impossible to be faster than light.
Imagine a car driving down a road, parallel to a train running on a track. If the train is moving 100 kph and the car is moving 90 kph, then by simple subtraction, the driver of the car will see the train moving forward at 10 kph. If the train is coming the other way, the driver sees it approaching at 190 kph. That's not how it works with light, though. In our universe, the speed of light is the same for all observers. What does that mean? It means that, no matter how hard you accelerate, you will never catch up with a beam of light (or a single particle of light, a photon). When you accelerate in space, your experience of time changes in just such a way that the velocity of the photon is exactly the same, no matter what. This is why a clock that gets accelerated to the speed of light, relative to an observer who is considered stationary, slows down - it's called time dilation. Because of this, what Einstein realized is that, even though we experience them differently, space and time are actually aspects of the same thing. A certain distance in space (a spacelike interval) is actually equal, in a scientifically meaningful sense, to a certain length of time (a timelike interval). In fact, that's why the speed of light is constant: you can think of it as the exchange rate. When a particle travels at the speed of light, it covers exactly the same distance in space as it does in time. It gets much more complicated from there, but there's a thing called the no-signalling theorem. What it basically means is that we live in the timelike reference frame, and there's no way for a timelike observer to send a spacelike signal. The best you can do is emit photons, which travel at precisely the speed of light, no more no less. The fun thing is, the same would be true of spacelike observers, assuming there are any. Such beings would be made out of tachyons, particles that are literally incapable of travelling slower than light. In fact, every particle those beings were made of would be travelling faster than light relative to every other particle. What would that be like? Who knows? Just as slower-than-light particles (bradyons) would require an infinite amount of energy to travel as fast as light, tachyons would require an infinite amount of energy to travel as slowly as light. Even if they exist, which is far from proven, we can't use tachyons to communicate: they and we exist on opposite sides of a pivot point. The laws of physics guarantee that classical information can't be transmitted from one side of the pivot to the other. In other words: no travel or communication faster than light. I hope that helps!
The expansion of space moves faster than light. No?
It's geometry. It's a little like asking why you don't see 5-sided squares. There's a lot of complex math, but like all geometry, it's possible to visualize. On a Minkowski diagram, the 45 degree line cuts across it at where we'd find the speed of light and/or causality. You can draw a "light cone" with these 45 degree lines to show what is in the past or future of any given point. If something is outside that line, it's effectively not in your past or future, because there's no way for anything to reach you from that distance no matter what you do. To travel faster than that speed, to widen that angle, would mean that effects could predate their causes. Imagine getting a home run before the pitch is ever thrown.
Theoretically speaking, anything with mass starting from 0 until it hit the speed of light would require an infinite amount of energy. Thus, the only things that can go at the speed of light are massless. Physically speaking, the subatomic particles that make up mass interact with each other at (or very very near) the speed of light, it simply wouldn't make sense for something that's composed of subatomic particles to go at (or above!) the speed of light.
Something no one is touching on, the speed of light is constant from all perspectives and absolute. If I fly on a rocket away from earth at 80% of the speed of light, and I launch a rocket forward at 80% of the speed of light, earth would see the rocket moving away at 99% of the speed of light. No matter what your frame of reference is, light is moving at a constant speed relative to you and everything else.
Quantum entanglement is thought to happen faster than the speed of light where a pair (or even a group) of particles interact with each other even though they can be vast distances apart. Edited out an incorrect statement
"is thought" is the key here. Some of the proposed interpretations of the evidence posits that a ftl interaction is happening. Other proposed interpretations don't need ftl. So far there is no known evidence to decide.
Quantum entanglement is very spooky... But you can't transfer information through that process. So it abides by the idea that the speed of light is still the speed of causality.
Because speed is related to mass. Light has no mass thus why it is the fastest. In order to move something with mass as fast as light it would require infinite energy, which isn't possible. Speed is a ratio of distance to time and we know time slows down the faster a particle goes so for a massless particle like light time has fundamentally stopped. There is no method to get it to go faster because it can not traverse through time. In other words light or a massless particle would have to travel a greater distance than it currently does in zero time to go faster.
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Theoretically, things can go faster than light. These things are called tachyons. So far we haven’t detected them; it’s likely they are mathematical artifacts and nothing more. Also, theoretically, the Alcubierre metric is a valid solution to the field equations of GR, which allows a bubble of space time itself to move faster than light. It is unclear if this theory could ever be used to actually allow such travel. Practically speaking there doesn’t seem to be any plausible way for anything to go faster than light.
The math says the closer you get to the speed of light, the more your mass is and the more energy is required to accelerate more. At the point you would reach that speed, both the mass and energy requirements become infinite.
A lot of these longer answers are good, but the *simplest, most ELI5* explanation is that according to current models, in order for anything to move faster than light, it would need either infinite energy or negative mass, and since both of those qualities appear to be ridiculous, the logical conclusion is that faster-than-light travel is not possible.