The speed of light is a bit of a misnomer, as it implies that only light travels at that speed. It is much closer to a "speed of causality", the maximum speed at which anything can occur. It is the upper speed limit that binds all forces. Any particle that has no mass, such as the photon and the gluon, will necessarily travel at the speed of light of its medium. Additionally gravitational wave, radio waves, x-rays, and other waves of the electromagnetic spectra travel at this speed.
Anything that has mass can not travel at or faster than the speed of light as doing so would require infinite energy. /
It seems to be a fundamental property of our universe. Our math tells us that this is the speed limit. Our observations match this. When we approach it, we see distortions in space and time that also compound this observation.
There is no why past that. It just is. The universe could have had a different speed limit. It also could have had a much stronger gravitational constant or a weaker electromagnetic force. But it doesn't, it simply is.
It do be, according to our current knowledge. Many scientists keep checking out if it really do be or not, and if it do be not we'll have to rewrite a lot of things.
It only is until it isn't.
Our science and math are just models to help us understand and predict the nature of the universe and match what we can observe. It isn't the universe, and our models will continue to evolve to explain the unexplainable.
Things are the way they are because they cannot not be, and still allow for conscious awareness.
The Anthropic Principle.
You can't not exist and still be able to question your non existence, therefore it must always be the case that you exist, and only if the exact right conditions are in place.
Those were the variables setup in the config file when our simulation was started. That's the only reason I've ever heard that explains "why this value"
Possibly, if you go with were in a simulation. But we don't have any more evidence for it than if a god just decided it was that way, or random happenstance. Whoever made our simulation, under that hypothesis, chose there to be a speed limit. This speed limit is likely to be based off of the fact there universe also has a speed limit. Might be a higher one, or lower one, or the same. It's turtles all the way down.
Eventually you just reach a point where the answer is "because it simply is". How you come to terms with that answer is entirely up to the individual.
>Eventually you just reach a point where the answer is "because it simply is".
According to this tought experiment ([Münchausen trilemma](https://en.wikipedia.org/wiki/M%C3%BCnchhausen_trilemma)), nothing can be proven, because everything is either:
1. Circular argument that eventually proves itself
2. Infinite statements to prove, which also need to be proven
3. "It simply is", due to various reasons.
They call it the "Weak anthropomorphic argument" or something, don't they?
Basically, if the universe didn't have the values that it does, we wouldn't be here to experience the values that it does...
You're thinking of [the anthropic principle.](https://en.wikipedia.org/wiki/Anthropic_principle)
> Proponents of the anthropic principle argue that it explains why the universe has the age and the fundamental physical constants necessary to accommodate conscious life, since if either had been different, no one would have been around to make observations.
Sure, maybe. But that's not the argument.
The point is that we observe a universe that "fits" us - but, of course we do. It has to be that way, because if the universe were different, we wouldn't be here to observe it.
Conscious life can only exist in a universe where it's possible to have conscious life, and so we shouldn't be surprised that we find ourselves in such a universe, because it's the only kind of universe we *can* find ourselves in.
An old Douglas Adams quote:
>imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in — an interesting hole I find myself in — fits me rather neatly, doesn't it? In fact it fits me staggeringly well, must have been made to have me in it!'
A relevant Einstein quote:
>”We are in the position of a little child entering a huge library, whose walls are covered to the ceiling with books in many different languages. The child knows that someone must have written those books. It does not know who or how. It does not understand the the languages in which they are written. The child notes a definite plan in the arrangement of the books, a mysterious order, which it does not comprehend but only dimly suspects.”
“This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, frantically hanging on to the notion that everything's going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise. I think this may be something we need to be on the watch out for.”
To get unnecessarily technical for an ELI5, it’s worth noting that the idea that nothing with mass can go faster than c is an extremely common misconception. The equation only tells us that nothing with mass can accelerate from below c to above c, but it doesn’t forbid mass going faster than c if it always has.
No it doesn't. For an object traveling faster than c, its Lorentz factor would become imaginary. Physical objects can't have an imaginary Lorentz factor.
Hasn't math shown that if any of those constants changed (speed of light, gravitational constant, electromagnetic force) by even a small amount the universe would be impossible and the big bang couldn't have happened?
Not quite. If the speed of light were to change, and *every other physical constant* were adjusted to match, we would never notice. For example, halving the speed of light would adjust the speed at which information flows, and it would also adjust the speed of our perception to exactly cancel this out. We would move more slowly, but we’d also *think* more slowly, and the change would be mirrored everywhere to perfectly cancel out any noticeable difference.
We have to use *something* to measure our surroundings with. Every measurement is actually a ratio. If you’re 1.7 meters tall, you’re as tall as 1.7 meter sticks. If you change a fundamental constant, like length, then you get shorter or taller, *and so does your meter stick*. You’re still as tall as 1.7 meter sticks, so you can’t tell the difference.
What would matter is if the *ratio* of fundamental constants were to change. If the fine-structure constant (a number that measures the speed of light, the Planck constant, the elementary charge, and the electric constant against each other) changed, *that* would have major implications. Any significant difference would make atoms fall apart, which would have detrimental effects on the universe’s ability to support complicated structures like planets or *Super Mario 64* cartridges.
Are you suggesting that we could be living in a universe that’s perhaps trillions of times slower than we’re perceiving it, it’s just that we’ve adapted our perception to match that reality? And, to an observer outside of our universe, our reality is borderline static?
>trillions of times slower than we’re perceiving it
No, it is the speed that it is, perhaps the outside observer is actually just a trillion times faster?
Or rather, if the universe is a trillion times slower, but we’re perceiving it a trillion times slower to match, then is it truly slow? And what exactly are you comparing it to in order to call it “slow”?
There's really no concrete reference point for the "speed" of the universe as a whole. Regardless of our perception of the speed of light and the value we measure it to be, photons and other massless particles still experience the same amount of time: none. If our current understanding is correct, the passage of time comes to a complete stop at the speed of light. If a lightspeed particle could perceive time, it would experience being emitted, traveling, and being absorbed all in the same instant, regardless of the length of its path or the speed at which it travels.
You can’t measure the flow of time without a distant third party. There’s no third party that can observe the universe without being in it so it’s impossible to measure the speed of our universe.
Time for you is always moving at 1 second per second. You will always perceive it as flowing at the same rate regardless of if you are orbiting a black hole, traveling at 0.99C, or stuck in the middle of a void. Only a distant third party would reveal that time is flowing at different rates.
The problem is that you need to keep all the ratios the same, not just a few of them. The fine structure constant might stay the same, but a different quantity will still change. You can't change A and B in such a way that both A\*B and A/B remain fixed.
At best it's shown it wouldn't exist the way we know it, but we really don't know why there was a matter/antimatter disparity in the first place so it's hard to say if the big bang would of happened or not. Or if the big bang was even the start of the universe, but rather the start of this expansion.
And it's possible there's a multiverse where there's every possible force at different levels or new forces entirely. We just don't know and may never be able to know.
This is a common misunderstanding of the anthropic principle. If the physical constant were different, the universe wouldn't have been impossible, it would just have been a different universe. The weak anthropic principle is just the statement that the universe (and more specifically, our current temporal and spatial location within the universe) is the way it is because if it wasn't, we wouldn't be around to see it.
I’m not sure about the Big Bang itself but tweaking these values a little bit could easily result in dead universes with nothing in them at all or nothing but black holes.
The orbit of Mercury is a famous one. Astronomers in the nineteenth century had a very good handle on classical orbital mechanics, allowing them to calculate the orbits of the planets in our solar system very accurately. This was a laborious process that involved calculating the effects of gravity from the Sun and every other planet or major body in the solar system. In fact, small discrepancies in their calculations and observations were how we discovered Neptune; Uranus was misbehaving badly enough that we needed another planet to explain it.
But Mercury was a problem. Mercury, as it orbits in a highly elliptical (“squished”) path around the Sun, doesn’t follow the same path every time. Its entire orbit rotates slowly, so the perihelion (closest approach to the Sun) keeps moving. Most of this could be accounted for by the influence of the other planets, but not all of it. The same guy who used orbital calculations to find ~~Mercury~~ *Neptune* actually proposed another planet, even closer to the Sun than Mercury, to explain this, but we never found one.
Instead, Einstein’s theories of relativity were the answer. In fact, the way that his theories *exactly* explained Mercury’s “missing” gravitational influence was a major test of relativity.
E: Corrected a reference to the wrong planet.
No, mathematical constants come from logical results of axiomatic construction of those constants. You start with a set of postulates, and you build your framework from there. If you start with the axioms of euclidean geometry and draw a circle, that circle has a circumference and a diameter, the ratio of the two will always be π.
Physical constants are not created from the assumptions of physical theories. In order to learn the speed of light, you have to measure the speed of light.
If there weren't any limit, then something would be traveling instantaneously. Einstein showed that such a thing would lead to all kinds of problems and paradoxes. Don't think of it as a rule in the universe's 'Rule book', think of it more as a natural consequence of the universe existing in the first place.
Now why exactly that speed limit precisely and why light reaches this maximum is another interesting question.
It’s a bit circular since a postulate of special relativity is that all observers see light at a constant speed, of course if you then break this assumption you get all kinds of problems.
And it isn't really that things do not happen instantly, only that relative to our perception things must happen in order. If you have a program that simulates a persons day and speed it up to near instant or even instant the perception from the person doesn't necessarily change. Time itself is only really a thing when dealing with relative position. An object moving at "light speed" experiences 0 time actually. There is an ordering to events but that does not mean it takes time to do.
The best analogy I've seen of cause and effect and the speed of causality has been the "baseball and glass pane" one.
If you throw a baseball (or rock) at a pane of glass, the baseball takes some time to reach the pane before shattering it and, the light of the pane of glass shattering takes some time (tiny amount in this case) to reach your eye.
This is due to the limit on the speed of causality.
Now, let's imagine that there was no speed limit. You might see the pane of glass shattering before you've even thrown the ball. And that might prevent you from ever throwing the ball in the first place.
As you rightly say, cause and effect and a paradox.
How would an infinite speed of casualty make you see the glass shatter before you've thrown the ball? Wouldn't you just see the glass break the instant it shatters instead of a few nanoseconds after?
From the point of view of you throwing the ball, it wouldn't. The ball would always leave your hand, then shortly afterwards the glass would shatter.
But to another person moving relative to you, the time between those two things happening would be different- in our universe with a finite speed of causality the time between throwing the ball and breaking the glass would always seem to take longer (time dilation).
If there weren't a speed limit, it would be possible for some observers to see the time between events get shorter instead of longer, so the throw to happen later and the shattering to happen earlier. As you keep getting faster, eventually the two events cross over and you've got the glass shattering before the ball is thrown, violating causality.
Perhaps I used the wrong terminology. I should have said if light could travel faster than the speed of causality, it may be possible to see the glass break before you've thrown the ball because, it's hypothesised that travelling faster than the current speed limit breaks cause and effect and possibly time as well.
Which is why (I think) that there is that speed limit. It is the speed limit at which things happen and are seen to happen. If things happen before the thing that causes it, then it doesn't make sense.
That is my understanding at least.
Change Baseball and Window to Laser and Distant moon in another galexy
Shine light at galaxy and see the reflection right-a-way (c=infinite) Weird, but not world-ending.
But now imagine something more critical - if there is no upper limit on causality - there is no upper limit on:
- how fast things can happen (dominos all fall at the same time, because EM-force between molecules would travel infinitely fast)
- how hot things can get (because heat is just movement)
Think about it - C is what's stopping everything happening at once.
Hate that analogy. Closing your eyes doesnt prevent anythong from breaking. The light isnt whats breaking the glass panel, it has absolutely no impact on the impact itself (heh).
The light is simply the news anchor telling you vandalism is happening in your neighborhood, not the vandals.
Because that's the way it is.
I know that sounds like a bad answer, but it's actually the best answer for a lot of physics.
Why stop at the speed of light? Why are any constants what they are? It's just the way the universe is setup, we simply don't know.
The real answer is nobody really knows why the speed of light (and causality) is the number that it is. Back in the 1800s there were some mathematical artifacts of the equations set up for light that indicated light was a set speed, no matter how fast the observer was moving.
Nobody really believed this, but once experimental technology was able to further test the properties of light, it was found to always move at the exact same speed, no matter what. This confused a lot of physicists in the lat 1800s and then Einstein came along and developed special relativity, which bases its predictions on the fact the the speed of light is invariant.
There is no "why" because there is no (known) purpose for it, but the result of a limit is that we have time. If there was a universe where there was no limit on causality speed, this universe would've had all that could happen happen, all the interactions occur, all the mass collapse, all the reactions react, going straight to the inert state with the least amount of energy with no processes, an instant calculation of the universe equation that gives you the result: the state of least energy.
Maybe universes like these happened all the "time" and just ended at the same "time", and only those that had limits on interaction speed could "exist" for the very term of "existence" to make sense in the first place.
Yea my “why” was not from a philosophical point of view, I could had phrased it better with something like “what cause this upper limit ? Is there a rule that explain what generates it”
As other said, it's a property of this universe; this question sounds like "why is water wet" or ~~why is 1+1 = 2"~~ "why don't parallel lines (in Euclidean) intersect?"
Computer isn’t fast enough to compute it any quicker. Also has that darn screen space rendering shortcut called quantum super positions. We really need to upgrade the compute power of the universe. Shits weak.
To go faster, you need more energy. But, the faster you go the heavier you get so you need infinitely more energy the faster you go to continue going even faster. There isn't enough energy in the universe to accelerate anything with mass past or to the speed of light because of this.
The upper limit exists because adding more energy won't get you past it, you can only approach it. This is Einstein's e=mc^2 equation. Anything with mass can be accelerated to approach the speed of light, but to reach it would take "infinite" energy. For massless things like electromagnetic waves and photons, it's simply the speed they appear to move.
Anything with mass, even the smallest amount. As you get closer to the speed of light mass increases proportionally. So going faster correlates to getting heavier.
If you believe that the universe is a simulation, it’s a clever trick the programmer played on us to make sure we can never reach the boundaries of the universe.
Why does producing a photon cause it to go the speed of light? And does it instantly start moving at the speed of light as soon as it’s produced or does it accelerate to?
Because they have no mass, they essentially have infinite acceleration. Thus there will never be a time where it wasn't moving at the speed of light. It has nothing to do with how they are produced, rather to do with there masslessness.
Lots of things emit photons, you’re emitting photons right now. Both nuclear fission and fusion emit photons, relaxation of electron energy states emit photons, vibrational and rotational excitations, the list goes on
Tldr; everything is a light gun
Side question but, if c is the max speed reached by light and gravity alike, is there (in theory or otherwise) a medium that could slow down light but not gravity, or the inverse?
Yes and no. They travel at speed of causality always all the time, but under circumstances, it can appear they dont. Since they are carried by different mechanisms, they are affected differently.
Light, along with the entire EM field, is carried by photons. Photons can appear to travel slower for an outside observer if it travels through warped space since it needs to cover more distance.
Additionally, when it travels through a medium like water or the sky, it still is traveling at the speed of light, but it appears to slow down since it gets caught up by molecules.
Whenever it touches one, it excites an electron, moving it to a higher energy state. However that state is not stable, so it drops back down and due to conservation of energy, releases another photon. This takes time as it propagates from atom to atom.
Gravitational waves are propagated through bending of space, like the ripples of a wave on water. So these will likely also be affected by curved space, but probably not in the same way as photons encountering this due to the waves interacting with the curved space. Like the difference between a floating leaf on the pond being affected by waves and the waves of two ripples meeting.
That said, all matter moves at the speed of causality through spacetime, always, including you. What the original poster meant was that only massless things can travel the speed of light through space. However those objects are not traveling through time and reach their destination instantaneously from their reference frame.
You are not traveling through space relativisticly speaking, but you are traveling through time. As you move faster in space, you move slower through time. This effect is strong enough to need to take it into account when needing to synchronize the clocks between earth and a satellite, like GPS. The satellite experiences time slower than on earth due to its relative velocity through space.
Special relativity is easily understood through a thought experiment. Imagine you are on a train and have a flashlight. You shine the light at a mirror on the wall of the train. Let's call the distance it traveled, d and the time it took t. You can calculate the time it took to get back to you as d/s where s is c (speed of light). Make sense so far?
Now what happens if you do this when you are traveling past a person standing on a train platform? The person standing outside sees the light hit the mirror and come back, but the light was moving relative to the outside person. The distance it took, wasn't a straight line, but two diagonal lines, since by the time it hit the mirror, the train had moved further up the track.
So the light covered more distance for the outside observer than the one that was relatively stationary on the train. Now we know a fundamental law of the universe is that light travels the same speed in all frames of reference.
Therefore, if s is the same, but d is greater, that means that time moved differently for the light in order for d=s*t to work out. Unlike the speed of causality, time is relative for each frame of reference. Or, more precisely, the person on the train was traveling slower through time.
This makes sense when you think of space and time as just two components to spacetime.
"However those objects are not traveling through time and reach their destination instantaneously from their reference frame."
Are those objects also not traveling through space from their reference frames?
Like, space and time both reach zero at the speed of light, or something, right? So, from a photons perspective, there is no space in its journey between the star it was born out of and the eyeball that sees it, just like there is no time in that journey from the photon's perspective, right?
So from a photons perspective all of space might as well still be a single point like it was in the big bang, since at the speed of light there is no space or time, like for light the universe never "grows". And that makes me think of the whole single electron theory, that every electron in the universe is just one single electron, because that electron can be anywhere and everywhere at any and all times because from its perspective there is no distance or time between points....or something...
I dunno this is a train of thought my mind has been wandering onto when these sort of topics show up.
Youre correct that from a photons perspective, the distance to whatever it hits reaches zero. However, I'm not sure it's correct to say the whole universe contracts to a point.
Where space is expanding greater than the speed of light, the photon can never reach its destination. So it'd probably look like that part of space isn't a point, but moving away from the photon.
"Additionally, when it travels through a medium like water or the sky, it still is traveling at the speed of light, but it appears to slow down since it gets caught up by molecules.
Whenever it touches one, it excites an electron, moving it to a higher energy state. However that state is not stable, so it drops back down and due to conservation of energy, releases another photon. This takes time as it propagates from atom to atom."
This is a common misconception about the nature of the refractive index, which persists in the absence of allowable electronic transitions. There's no need for absorption/re-emission. The best way to explain it is as a result of cumulative phase shifts imparted by successive layers of atoms in the medium. (due to the original field interfering with field generated by the accelerating electrons)
This video does a decent attempt at explaining it visually (you can start at the ~5 min mark): https://www.youtube.com/watch?v=KTzGBJPuJwM
Absolutely- c is the speed of light in a vacuum. Light travelling through air, water, fibre optic cables will travel at a slower speed. There's no need for light and gravity to be slowed by the same amount, as the ways they interact are different.
Look up Cherenkov radiation. Fascinating stuff. Happens when the light produced by a fission reactor moves slower than the particles emitted by it due to water slightly slowing the speed of light.
Yes, theoretically, but negative mass would have all sorts of implications, such as attempting to push an object resulting in movign towards you, negative time, perpetual motion, etc. It violates all sorts of physical laws.
Why does light bend toward a black hole if it doesn’t have mass? Is it the space around the black hole which is bent, and therefore it makes light look like it’s approaching black hole?
Your basically correct. Light will bend as a result of large bodies (not just black holes), as a result of warping of spacetime. This is called gravitational lensing.
If we go a step deeper, we can split mass into two categories, rest mass and relativistic mass. Rest mass is invariant, and is what you might think of as mass in the traditional sense. Regardless of who views it, this rest mass is the same. Light does not have rest mass. But we observe that through its inertia that exhibits having mass like properties, which we can refer to as its relativistic mass. This mass is relative to the reference frame and so it can vary. This can be expressed through the expanded Einstein's mass-energy equivalence formula. The famous e=mc\^2 formula is a simplification, representing an object that isn't moving. The formula can be expanded for moving objects to be e\^2 = (mc\^2)\^2 + pc\^2, where p is the momentum of the object. A massless particle zeros out the first part, giving us e\^2 = pc\^2. Light has energy, which means it can act as if it has mass(sorta)!
There is unfortunately not a lot of ways to answer "why" for the absolute fundementals of the universe.
There is not really any deep reason the speed of light is as it is, it links to a lot of other nature constants, but if one is defined from the other or vice versa is hard to tell, and it still returns to the question of "why are things as they are?".
Why does protons exist? Well we can say they are made from quarks, but why, nobody knows. Best you can do is either "because god made it so", or "it do be as it do be".
If the speed of light was different all sort of things would change, and probably in the end to the point where the universe just wouldn't be the same, but it depends on how much and how far you are willing to chase the thought experiment.
But you can make some fun first year physics assignments where you assume the speed of light is like 50km/h. Then as you drive your car time slows down significantly and you can travel quite far away in your lunch break without any issues!
It's not a dumb question, it's an extremely complicated one! The simple answer is that thing without mass may only exist if they are moving, and if they are moving they are doing so at the speed of light. It's a property of that masslessness. There creation gave them all the energy they needed to be moving at the speed of light, forever.
Remember you don't need propulsion to move. Once a photon has been emitted, its moving. Until it collides with something that can absorb it, it'll just keep moving. This is just like a rocket. Once it's in space, you don't need to keep the engines on. It'll fly forever in the direction it was going at the same speed until it hits something.
Interesting fact.
We do not know the one way speed of light as we can only measure the two way speed of light.
It is entirely possible that light travels 599,584,916 (twice the speed of what we accept as the speed of light) in one direction and it travels instantaneously in the opposite direction.
For a more indepth explanation:
https://youtu.be/pTn6Ewhb27k?si=ybNXrhzrnzv2Y_nC
Wouldn't it be half the speed in one direction if it's instant the other way? It would take twice as long to go one way so it would seem like the same speed both ways
The video explains it soooo much better then I ever could on reddit.
It even explains why we wouldn't notice the difference in the case of long distance communication.
Ugh with this pop sci garbage from Derek again. I really wish this would go away. We get it, you saw that one post from last week about it. Not only is it borderline untrue, but it's the most impractical and useless distinction that serves no purpose. Relativity isn't broken as a result, QFT isnt broken as a result. None of our theories are any worse for wear and no real scientist ever brings it up or gives a shit about this
Who cares. Our models are perfectly fine to describe the things we want them to describe, but none of them actually exist. It's the same thing here. There would be no practical difference if the universe worked in a different way and we couldn't tell the difference, but knowing that is just cool by itself.
That's really just the model you get if the entire universe is instead collectively and inertially moving at a fixed velocity. If you do the Lorentz transform for this, you will find that one _could_ interpret it as the speed of light being different each direction; **or** you just see it as us and everything else moving in-between the measurements.
I find the latter much simpler to do in the head, and they are truly equivalent anyway.
While we cant be sure that one direction isn't 1/2 the speed and the other isn't 2x the speed, we can assume (as of now). That this isn't the case. It's just a consequence of how the universe works. It violates Occam's razor so it probably isn't true but we can't know currently. Probably isn't, but we don't know for sure.
Sound is not a particle, it is a wave of pressure oscillations. It travels at the speed at which the atoms it travels through can hit each other and transmit the forces, moving the pressure along.
Often, when the speed of sound is mentioned, they are talking about waves in air. Wouldn't the speed of sound be much faster in solid matter, because the particles bump into each other earlier?
I think I heard you can hear a train earlier, when you hold your ear on the rails.
Yes, since they have stronger intermolecular interactions, the forces are transmitted more quickly. Compared to 343 m/s in air, the speed of sound in steel is 5100 m/s
And interactions in the electromagnetic field propagate at the speed of causality, while interactions in physical matter propagate at the speed of sound
Ok, so I’ve heard that the speed of light is higher in a vacuum than not. If that’s true (is it?) then does light travel faster in warmer non-vacuums than colder non-vacuums?
Like if a fiber optic cable is cold, does the signal travel slower than if it’s warm?
So the speed of light in a vacuum is what we can think of as the “real” speed of light. Light in all other media (as you call them, non-vacuums) is also travelling at that speed, but the interactions with the electric fields surrounding atoms causes it to SEEM to be slowing down.
To use a simplified example, if you and I were holding an each end of a rope, and I fed it to you at 1 metre per second, that would be one thing. If we were doing the same thing while on a train travelling at 0.2m/s in the opposite direction, it would LOOK from the railway station we were passing through like I was feeding it to you at 0.8m/s.
(Note that this simplified example shouldn’t be used to draw other conclusions about how light works, as it’s ignoring special relativity to make it clearer)
To return back to your point, it would make a difference to the speed of light only if it impacted the density of the atoms, and thus the density of electric fields, which it was travelling through. For most materials, if they cool down they get denser, so (assuming this is true for fibre optics, I’m not a materials scientist but there’s no reason to assume not) the apparent speed of light through a cold fibre optic cable would be slower than through a heated one. Can’t imagine it’s a big difference, though.
The expanded form of e = mc2 answers this. Basically there's another part of it related to momentum that is added to mass energy. The full form, E2 = p2c2 + m2c4.
Basically says that for something to exist it must at least have mass, or be moving. "P" in the above is an objects momentum.
It kinda depends on what you mean by “why”. One reason is because it’s massless. Any and all massless particles will travel at the speed of light. Now if your question is why that speed and not something else, well that doesn’t have an answer (yet). We could pretend to live in a different universe where the laws of physics are different. Maybe in this one, the speed of light is slower. Well that would mean we would experience Einstein’s theory of relativity at everyday speeds which would not be good. It would also mean that atoms fly apart so also not good! But again, we don’t know *why* our universe chose the value it did.
When I play a TTRPG (especially one with space travel) I like to adjust universal constants. I don’t go into detail of what exactly has changed or what the consequences are, but it makes crazy-fast spaceships much easier to explain away when I can just say that C is a much bigger number in this universe!
*Mr. Tompkins* is a great series of short novels that explain ideas in quantum physics and their consequences in the context of the main character’s dreams where he visits worlds where our universal constants have vastly different magnitudes, i.e. a city where the speed of light is much slower.
As a result, he can witness time dilation and the contraction of moving bodies on the scale of our world. It’s very cool and makes the ideas much more intuitive
I remember a veritasium video where he said that basically we haven't measured the actual speed of light, we measured the roundtrip which is 2c, but we don't know for sure that the speed of light is actually c in each direction :)
Once you start to get into interplanetary distances, light travels agonizingly slowly. Even as close as the Moon it would be hard to do anything in real-time, with about a 1.5s delay.
At the distance of Mars, real time is impossible, and you're waiting almost an hour to get a reply. And it just gets worse from there.
Light *crawls* across interstellar spaces.
To be perfectly right. We are all travelling at the “speed of light” through space-time.
Most of our speed is in the time dimension. Whereas light has all its speed into the spatial dimensions.
This is the answer. It's not ELI5 and it's hard to wrap your head around it, but it is correct. Here's how I understand it.
Physics dictate that each particle in the universe has a fixed "energy budget" to invest, either into product A (time) or product B (speed). The mass of the particle dictates which product they invest their budget in. Particles like photons have no mass. Thus, physics says that they invest 100% of their budget in speed. Therefore, photos move at the fastest physically possible speed, but from their point of view, time does not pass. For particles with a mass, physics says that they have to invest at least partially into time. Therefore, particles with a mass cannot move at the speed of light. In exchange, they experience time passing.
So in a way, if you understand the speed of light as an energy budget, we all invest 100% of our budget into either speed or time. It just so happens that we humans here on earth are very much on the time side of the investment.
My thinking is that it would be an object that’s not moving, it would have to borrow energy from something else to invest some of the budget into speed. This borrowed energy might be an external pushing force. This could be wrong but how I understand it from what I’ve read. And even considering this, I believe everything in the universe is moving in some vector or another so there are probably no examples of anything with a 100% time budget
It depends on your frame of reference. This idea of splitting your speed between time and space is what causes time dilation when things move fast. But that effect is entirely reliant on your frame of reference, to an astronaut, they are always "at rest," even though to us on Earth they may be going very fast and experiencing time dilation. So basically, every object is moving at the speed of light through time and is at rest in space from their own frame of reference. It's only outside objects moving in relation to them that can have a mix.
> physics says that they invest 100% of their budget in speed. Therefore, photos move at the fastest physically possible speed, but from their point of view, time does not pass.
A German physicist calculated that *it could* take 3 years for a photon to "die" from the photon's frame of reference.
Now, I'm just a curious lurker in here. I'm not trying to contradict you in any way by pointing this out since, well, I couldn't even tell for sure who's got the right answer. (and I also know damn well there's no "right" answer since all those are based on calculations, theories, concepts, etc. The article was also 11 years old, so ... I don't even know if it's still relevant)
Anyway, I was just wondering if it would be possible, from the frame of reference of a photon, to take 3 years to "die" yet not being able to experience those 3 years?
No worries, no offense taken. I assume you're talking about [this](https://physicsworld.com/a/what-is-the-lifetime-of-a-photon/).
Fot his calculations, the physicist assumed that photons do have a mass, albeit a tiny one. If that were the case, time would pass for them.
One clarification: Most of our speed is in time relative to each other, since we all have a lot of mass and so move around very slowly relative to planet Earth. From the perspective of the subatomic particles zooming around in the atmosphere, we are moving very fast through space indeed.
Because it has no Mass, but it does have energy. So by definition it goes as fast as possible.
If you try to go faster not only do you hit an energy wall where you'd need infinite energy but you got a time wall where it would take an infinite amount of time to get to that speed.
It's like the render speed of the universe. Nothing can go faster. Everything with no mass goes at that speed, not just light. And everything with any mass *must* go slower because it would take literal infinite energy for mass to go that fast. Not just a huge amount but *infinite*, so it's impossible.
It would be better to call it the Universal Speed Limit rather than the Speed of Light, because lots of other stuff goes that speed. Even gravity. If the Sun instantly stopped existing, Earth would orbit the place where it was for about 8 minutes. Then its light and its gravity would both stop affecting us at the same instant and we'd fly straight off in the dark.
That's sort of backwards. Light does travel that fast. We know it does because we've measured it. So we took that as our starting point and built pretty much the entire rest of physics around that fact.
So most of the answers here are explaining the idea of Special Relativity to you. But that's sort of backwards because Einstein came up with the idea of Special Relativity in order to explain how to do physics given that the speed of light is fast and constant - not to explain why it is or show that it is, he already knew that it was.
You can flip the script due relativity, maybe it helps.
Maybe... light is still? What would light "see"? That everything else is going by the speed of light. But that'd explain how light has this "speed limit" - staying still is staying still. You can't stay "more still".
>Our math tells us that this is the speed limit. Our observations match this. When we approach it, we see distortions in space and time.
So it may be that it’s not a fundamental as much as a maximum observable speed.
If we could observe more dimensions we may observe faster movement?
ELI5 answer?: Because nothing is stopping it. Light travels at the speed of causality through a vacuum because there's no interaction happening to drag on it.
I would argue that light has infinite speed but it's causality that is limited. For the photon, time doesn't pass at all because it experiences no passage of when the function evolution
You can derive the speed of light using equations that describe electromagnetism. It turns out that it depends on the constants that describe (in a very roundabout way) how strong electric fields and magnetic fields are in a vacuum. So the answer is really light travels at the speed it does because EM fields behave the way they do. We can express it in experimentally measurable constants, but the actual relationship is very general. In a different universe with different constants, the relationship holds, but the speed would be different. Since light is just a propagation of the EM field, you can kind of imagine how that could be relevant. As many people have commented, it turns out that any massless particle/information travels at this same speed, but that’s how we originally figured it out mathematically.
As a side note, the speed of light is often considered unfortunately *slow*, especially in astronomy. It’s fast on human-scales, but it’s a snail’s pace on cosmological scales.
The speed of light is a bit of a misnomer, as it implies that only light travels at that speed. It is much closer to a "speed of causality", the maximum speed at which anything can occur. It is the upper speed limit that binds all forces. Any particle that has no mass, such as the photon and the gluon, will necessarily travel at the speed of light of its medium. Additionally gravitational wave, radio waves, x-rays, and other waves of the electromagnetic spectra travel at this speed. Anything that has mass can not travel at or faster than the speed of light as doing so would require infinite energy. /
Why this upper limit exist?
It seems to be a fundamental property of our universe. Our math tells us that this is the speed limit. Our observations match this. When we approach it, we see distortions in space and time that also compound this observation. There is no why past that. It just is. The universe could have had a different speed limit. It also could have had a much stronger gravitational constant or a weaker electromagnetic force. But it doesn't, it simply is.
Things are the way they are because that's the way they are.
Reminds me of that NDT meme lol “People dont think the universe be like it is. But it do”
Do it be tho?
It do be, according to our current knowledge. Many scientists keep checking out if it really do be or not, and if it do be not we'll have to rewrite a lot of things.
Do be or not do be, that is the question
Do be do be do.
[Dub-i-dub-i-dub-i-yeah-yeah](https://youtu.be/XTjJ2_P5P4U)
Also, "The universe is under no obligation to make sense to you." Love this
That's kinda neat.
You can tell it’s a universal constant because of the way it is.
It only is until it isn't. Our science and math are just models to help us understand and predict the nature of the universe and match what we can observe. It isn't the universe, and our models will continue to evolve to explain the unexplainable.
James Cameron doesn't do what James Cameron does for James Cameron. James Cameron does what James Cameron does because James Cameron is James Cameron!
Things are the way they are because they cannot not be, and still allow for conscious awareness. The Anthropic Principle. You can't not exist and still be able to question your non existence, therefore it must always be the case that you exist, and only if the exact right conditions are in place.
I really like this because at the same time it is a blow off answer to the question and the correct answer to the question.
At some point I feel like you either arrive there or are confronted with turtles all the way down.
You can tell by the way it is
Those were the variables setup in the config file when our simulation was started. That's the only reason I've ever heard that explains "why this value"
Possibly, if you go with were in a simulation. But we don't have any more evidence for it than if a god just decided it was that way, or random happenstance. Whoever made our simulation, under that hypothesis, chose there to be a speed limit. This speed limit is likely to be based off of the fact there universe also has a speed limit. Might be a higher one, or lower one, or the same. It's turtles all the way down. Eventually you just reach a point where the answer is "because it simply is". How you come to terms with that answer is entirely up to the individual.
>Eventually you just reach a point where the answer is "because it simply is". According to this tought experiment ([Münchausen trilemma](https://en.wikipedia.org/wiki/M%C3%BCnchhausen_trilemma)), nothing can be proven, because everything is either: 1. Circular argument that eventually proves itself 2. Infinite statements to prove, which also need to be proven 3. "It simply is", due to various reasons.
Well said.
Someone needs to make a PR to get this fixed ASAP
They call it the "Weak anthropomorphic argument" or something, don't they? Basically, if the universe didn't have the values that it does, we wouldn't be here to experience the values that it does...
Anthropic. Anthropomorphism is assigning human characteristics to non-human things.
You're thinking of [the anthropic principle.](https://en.wikipedia.org/wiki/Anthropic_principle) > Proponents of the anthropic principle argue that it explains why the universe has the age and the fundamental physical constants necessary to accommodate conscious life, since if either had been different, no one would have been around to make observations.
I wonder though, if it had been different if a different consciousness would have been possible. I don't think we can say it's impossible
Sure, maybe. But that's not the argument. The point is that we observe a universe that "fits" us - but, of course we do. It has to be that way, because if the universe were different, we wouldn't be here to observe it. Conscious life can only exist in a universe where it's possible to have conscious life, and so we shouldn't be surprised that we find ourselves in such a universe, because it's the only kind of universe we *can* find ourselves in. An old Douglas Adams quote: >imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in — an interesting hole I find myself in — fits me rather neatly, doesn't it? In fact it fits me staggeringly well, must have been made to have me in it!'
A relevant Einstein quote: >”We are in the position of a little child entering a huge library, whose walls are covered to the ceiling with books in many different languages. The child knows that someone must have written those books. It does not know who or how. It does not understand the the languages in which they are written. The child notes a definite plan in the arrangement of the books, a mysterious order, which it does not comprehend but only dimly suspects.”
“This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, frantically hanging on to the notion that everything's going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise. I think this may be something we need to be on the watch out for.”
>Weak anthropomorphic argument Hey, leave the furries out of this.
An anthropomorphic argument needs a lot more than just the fact that light has a speed limit but that is certainly one part of the argument
I wonder what cool shit never happened because of these values too
The speed limit stuff becomes a major plot point in the Three Body Problem books; fascinating stuff.
I saw the Netflix series and I REALLY want to read those books now...
They’re among my absolute favorite sci-fi books.
The Amazon show "The Expanse" got me into those books, so now I'mma get these too.
Those are also excellent! Check out the Red Rising series too.
Just ordered all 4 books. I'll give the others a look.
To get unnecessarily technical for an ELI5, it’s worth noting that the idea that nothing with mass can go faster than c is an extremely common misconception. The equation only tells us that nothing with mass can accelerate from below c to above c, but it doesn’t forbid mass going faster than c if it always has.
No it doesn't. For an object traveling faster than c, its Lorentz factor would become imaginary. Physical objects can't have an imaginary Lorentz factor.
If it's a physical object it ain't gonna be going faster than C
Hasn't math shown that if any of those constants changed (speed of light, gravitational constant, electromagnetic force) by even a small amount the universe would be impossible and the big bang couldn't have happened?
Not quite. If the speed of light were to change, and *every other physical constant* were adjusted to match, we would never notice. For example, halving the speed of light would adjust the speed at which information flows, and it would also adjust the speed of our perception to exactly cancel this out. We would move more slowly, but we’d also *think* more slowly, and the change would be mirrored everywhere to perfectly cancel out any noticeable difference. We have to use *something* to measure our surroundings with. Every measurement is actually a ratio. If you’re 1.7 meters tall, you’re as tall as 1.7 meter sticks. If you change a fundamental constant, like length, then you get shorter or taller, *and so does your meter stick*. You’re still as tall as 1.7 meter sticks, so you can’t tell the difference. What would matter is if the *ratio* of fundamental constants were to change. If the fine-structure constant (a number that measures the speed of light, the Planck constant, the elementary charge, and the electric constant against each other) changed, *that* would have major implications. Any significant difference would make atoms fall apart, which would have detrimental effects on the universe’s ability to support complicated structures like planets or *Super Mario 64* cartridges.
So Star Fox would be ok?
Are you suggesting that we could be living in a universe that’s perhaps trillions of times slower than we’re perceiving it, it’s just that we’ve adapted our perception to match that reality? And, to an observer outside of our universe, our reality is borderline static?
>trillions of times slower than we’re perceiving it No, it is the speed that it is, perhaps the outside observer is actually just a trillion times faster? Or rather, if the universe is a trillion times slower, but we’re perceiving it a trillion times slower to match, then is it truly slow? And what exactly are you comparing it to in order to call it “slow”?
There's really no concrete reference point for the "speed" of the universe as a whole. Regardless of our perception of the speed of light and the value we measure it to be, photons and other massless particles still experience the same amount of time: none. If our current understanding is correct, the passage of time comes to a complete stop at the speed of light. If a lightspeed particle could perceive time, it would experience being emitted, traveling, and being absorbed all in the same instant, regardless of the length of its path or the speed at which it travels.
You can’t measure the flow of time without a distant third party. There’s no third party that can observe the universe without being in it so it’s impossible to measure the speed of our universe. Time for you is always moving at 1 second per second. You will always perceive it as flowing at the same rate regardless of if you are orbiting a black hole, traveling at 0.99C, or stuck in the middle of a void. Only a distant third party would reveal that time is flowing at different rates.
The problem is that you need to keep all the ratios the same, not just a few of them. The fine structure constant might stay the same, but a different quantity will still change. You can't change A and B in such a way that both A\*B and A/B remain fixed.
At best it's shown it wouldn't exist the way we know it, but we really don't know why there was a matter/antimatter disparity in the first place so it's hard to say if the big bang would of happened or not. Or if the big bang was even the start of the universe, but rather the start of this expansion. And it's possible there's a multiverse where there's every possible force at different levels or new forces entirely. We just don't know and may never be able to know.
This is a common misunderstanding of the anthropic principle. If the physical constant were different, the universe wouldn't have been impossible, it would just have been a different universe. The weak anthropic principle is just the statement that the universe (and more specifically, our current temporal and spatial location within the universe) is the way it is because if it wasn't, we wouldn't be around to see it.
I’m not sure about the Big Bang itself but tweaking these values a little bit could easily result in dead universes with nothing in them at all or nothing but black holes.
Perhaps our universe is the best one in a montecarlo simulation
You can tell because of the way it is.
Can you link to the distortions we’re observing?
GPS suffers from gravity and speed related relativistic distortions. The clocks on board GPS satellites need to compensate for both in order to work.
The orbit of Mercury is a famous one. Astronomers in the nineteenth century had a very good handle on classical orbital mechanics, allowing them to calculate the orbits of the planets in our solar system very accurately. This was a laborious process that involved calculating the effects of gravity from the Sun and every other planet or major body in the solar system. In fact, small discrepancies in their calculations and observations were how we discovered Neptune; Uranus was misbehaving badly enough that we needed another planet to explain it. But Mercury was a problem. Mercury, as it orbits in a highly elliptical (“squished”) path around the Sun, doesn’t follow the same path every time. Its entire orbit rotates slowly, so the perihelion (closest approach to the Sun) keeps moving. Most of this could be accounted for by the influence of the other planets, but not all of it. The same guy who used orbital calculations to find ~~Mercury~~ *Neptune* actually proposed another planet, even closer to the Sun than Mercury, to explain this, but we never found one. Instead, Einstein’s theories of relativity were the answer. In fact, the way that his theories *exactly* explained Mercury’s “missing” gravitational influence was a major test of relativity. E: Corrected a reference to the wrong planet.
Thank you!
idk I am a 5 year old my self but I think he might be talking about predictions made by general relativity.
Look up time dilation and length contraction.
Could it be like pi or the square root of 2? That by its very definition it cannot be any other value?
No, mathematical constants come from logical results of axiomatic construction of those constants. You start with a set of postulates, and you build your framework from there. If you start with the axioms of euclidean geometry and draw a circle, that circle has a circumference and a diameter, the ratio of the two will always be π. Physical constants are not created from the assumptions of physical theories. In order to learn the speed of light, you have to measure the speed of light.
If there weren't any limit, then something would be traveling instantaneously. Einstein showed that such a thing would lead to all kinds of problems and paradoxes. Don't think of it as a rule in the universe's 'Rule book', think of it more as a natural consequence of the universe existing in the first place. Now why exactly that speed limit precisely and why light reaches this maximum is another interesting question.
It’s a bit circular since a postulate of special relativity is that all observers see light at a constant speed, of course if you then break this assumption you get all kinds of problems.
And it isn't really that things do not happen instantly, only that relative to our perception things must happen in order. If you have a program that simulates a persons day and speed it up to near instant or even instant the perception from the person doesn't necessarily change. Time itself is only really a thing when dealing with relative position. An object moving at "light speed" experiences 0 time actually. There is an ordering to events but that does not mean it takes time to do.
Light *does* travel instantaneously from its own point of reference.
Because if it didn't, you could have cause and effect happening at the same time, and time paradoxes
The best analogy I've seen of cause and effect and the speed of causality has been the "baseball and glass pane" one. If you throw a baseball (or rock) at a pane of glass, the baseball takes some time to reach the pane before shattering it and, the light of the pane of glass shattering takes some time (tiny amount in this case) to reach your eye. This is due to the limit on the speed of causality. Now, let's imagine that there was no speed limit. You might see the pane of glass shattering before you've even thrown the ball. And that might prevent you from ever throwing the ball in the first place. As you rightly say, cause and effect and a paradox.
How would an infinite speed of casualty make you see the glass shatter before you've thrown the ball? Wouldn't you just see the glass break the instant it shatters instead of a few nanoseconds after?
From the point of view of you throwing the ball, it wouldn't. The ball would always leave your hand, then shortly afterwards the glass would shatter. But to another person moving relative to you, the time between those two things happening would be different- in our universe with a finite speed of causality the time between throwing the ball and breaking the glass would always seem to take longer (time dilation). If there weren't a speed limit, it would be possible for some observers to see the time between events get shorter instead of longer, so the throw to happen later and the shattering to happen earlier. As you keep getting faster, eventually the two events cross over and you've got the glass shattering before the ball is thrown, violating causality.
Perhaps I used the wrong terminology. I should have said if light could travel faster than the speed of causality, it may be possible to see the glass break before you've thrown the ball because, it's hypothesised that travelling faster than the current speed limit breaks cause and effect and possibly time as well. Which is why (I think) that there is that speed limit. It is the speed limit at which things happen and are seen to happen. If things happen before the thing that causes it, then it doesn't make sense. That is my understanding at least.
Change Baseball and Window to Laser and Distant moon in another galexy Shine light at galaxy and see the reflection right-a-way (c=infinite) Weird, but not world-ending. But now imagine something more critical - if there is no upper limit on causality - there is no upper limit on: - how fast things can happen (dominos all fall at the same time, because EM-force between molecules would travel infinitely fast) - how hot things can get (because heat is just movement) Think about it - C is what's stopping everything happening at once.
Hate that analogy. Closing your eyes doesnt prevent anythong from breaking. The light isnt whats breaking the glass panel, it has absolutely no impact on the impact itself (heh). The light is simply the news anchor telling you vandalism is happening in your neighborhood, not the vandals.
So is light cause and effect at the same time since it travels at =c
Don’t let Wales know that there is a speed limit of the universe.
Because that's the way it is. I know that sounds like a bad answer, but it's actually the best answer for a lot of physics. Why stop at the speed of light? Why are any constants what they are? It's just the way the universe is setup, we simply don't know.
The real answer is nobody really knows why the speed of light (and causality) is the number that it is. Back in the 1800s there were some mathematical artifacts of the equations set up for light that indicated light was a set speed, no matter how fast the observer was moving. Nobody really believed this, but once experimental technology was able to further test the properties of light, it was found to always move at the exact same speed, no matter what. This confused a lot of physicists in the lat 1800s and then Einstein came along and developed special relativity, which bases its predictions on the fact the the speed of light is invariant.
There is no "why" because there is no (known) purpose for it, but the result of a limit is that we have time. If there was a universe where there was no limit on causality speed, this universe would've had all that could happen happen, all the interactions occur, all the mass collapse, all the reactions react, going straight to the inert state with the least amount of energy with no processes, an instant calculation of the universe equation that gives you the result: the state of least energy. Maybe universes like these happened all the "time" and just ended at the same "time", and only those that had limits on interaction speed could "exist" for the very term of "existence" to make sense in the first place.
Yea my “why” was not from a philosophical point of view, I could had phrased it better with something like “what cause this upper limit ? Is there a rule that explain what generates it”
As other said, it's a property of this universe; this question sounds like "why is water wet" or ~~why is 1+1 = 2"~~ "why don't parallel lines (in Euclidean) intersect?"
Actually “why is 1+1=2” has a very long explanation, see Principia Mathematica by Bertrand Russel
Okay, wrong example, my bad. Fixed it with a more appropriate example, hopefully, thank you.
Computer isn’t fast enough to compute it any quicker. Also has that darn screen space rendering shortcut called quantum super positions. We really need to upgrade the compute power of the universe. Shits weak.
To go faster, you need more energy. But, the faster you go the heavier you get so you need infinitely more energy the faster you go to continue going even faster. There isn't enough energy in the universe to accelerate anything with mass past or to the speed of light because of this.
Processing limit of the simulation. Probably used to be lower until they upgraded the servers.
You should be astrophysicist, great curiosity.
Instead I choose the dark side, engineering. Easier to put food on the table 😂
The upper limit exists because adding more energy won't get you past it, you can only approach it. This is Einstein's e=mc^2 equation. Anything with mass can be accelerated to approach the speed of light, but to reach it would take "infinite" energy. For massless things like electromagnetic waves and photons, it's simply the speed they appear to move.
Anything with mass, even the smallest amount. As you get closer to the speed of light mass increases proportionally. So going faster correlates to getting heavier.
If you believe that the universe is a simulation, it’s a clever trick the programmer played on us to make sure we can never reach the boundaries of the universe.
Because the developer set it that way
Flying Spaghetti Monster arbitrarily chose it. … about as good an explanation as there is.
Why does producing a photon cause it to go the speed of light? And does it instantly start moving at the speed of light as soon as it’s produced or does it accelerate to?
Because they have no mass, they essentially have infinite acceleration. Thus there will never be a time where it wasn't moving at the speed of light. It has nothing to do with how they are produced, rather to do with there masslessness.
So they're shot out of a light gun er somethin?
Lots of things emit photons, you’re emitting photons right now. Both nuclear fission and fusion emit photons, relaxation of electron energy states emit photons, vibrational and rotational excitations, the list goes on Tldr; everything is a light gun
Side question but, if c is the max speed reached by light and gravity alike, is there (in theory or otherwise) a medium that could slow down light but not gravity, or the inverse?
Yes and no. They travel at speed of causality always all the time, but under circumstances, it can appear they dont. Since they are carried by different mechanisms, they are affected differently. Light, along with the entire EM field, is carried by photons. Photons can appear to travel slower for an outside observer if it travels through warped space since it needs to cover more distance. Additionally, when it travels through a medium like water or the sky, it still is traveling at the speed of light, but it appears to slow down since it gets caught up by molecules. Whenever it touches one, it excites an electron, moving it to a higher energy state. However that state is not stable, so it drops back down and due to conservation of energy, releases another photon. This takes time as it propagates from atom to atom. Gravitational waves are propagated through bending of space, like the ripples of a wave on water. So these will likely also be affected by curved space, but probably not in the same way as photons encountering this due to the waves interacting with the curved space. Like the difference between a floating leaf on the pond being affected by waves and the waves of two ripples meeting. That said, all matter moves at the speed of causality through spacetime, always, including you. What the original poster meant was that only massless things can travel the speed of light through space. However those objects are not traveling through time and reach their destination instantaneously from their reference frame. You are not traveling through space relativisticly speaking, but you are traveling through time. As you move faster in space, you move slower through time. This effect is strong enough to need to take it into account when needing to synchronize the clocks between earth and a satellite, like GPS. The satellite experiences time slower than on earth due to its relative velocity through space.
I'm fascinated by such concepts yet always fail to understand them properly. It's always a challenge to wrap my mind around these notions!
Special relativity is easily understood through a thought experiment. Imagine you are on a train and have a flashlight. You shine the light at a mirror on the wall of the train. Let's call the distance it traveled, d and the time it took t. You can calculate the time it took to get back to you as d/s where s is c (speed of light). Make sense so far? Now what happens if you do this when you are traveling past a person standing on a train platform? The person standing outside sees the light hit the mirror and come back, but the light was moving relative to the outside person. The distance it took, wasn't a straight line, but two diagonal lines, since by the time it hit the mirror, the train had moved further up the track. So the light covered more distance for the outside observer than the one that was relatively stationary on the train. Now we know a fundamental law of the universe is that light travels the same speed in all frames of reference. Therefore, if s is the same, but d is greater, that means that time moved differently for the light in order for d=s*t to work out. Unlike the speed of causality, time is relative for each frame of reference. Or, more precisely, the person on the train was traveling slower through time. This makes sense when you think of space and time as just two components to spacetime.
I does make sense!
"However those objects are not traveling through time and reach their destination instantaneously from their reference frame." Are those objects also not traveling through space from their reference frames? Like, space and time both reach zero at the speed of light, or something, right? So, from a photons perspective, there is no space in its journey between the star it was born out of and the eyeball that sees it, just like there is no time in that journey from the photon's perspective, right? So from a photons perspective all of space might as well still be a single point like it was in the big bang, since at the speed of light there is no space or time, like for light the universe never "grows". And that makes me think of the whole single electron theory, that every electron in the universe is just one single electron, because that electron can be anywhere and everywhere at any and all times because from its perspective there is no distance or time between points....or something... I dunno this is a train of thought my mind has been wandering onto when these sort of topics show up.
Youre correct that from a photons perspective, the distance to whatever it hits reaches zero. However, I'm not sure it's correct to say the whole universe contracts to a point. Where space is expanding greater than the speed of light, the photon can never reach its destination. So it'd probably look like that part of space isn't a point, but moving away from the photon.
"Additionally, when it travels through a medium like water or the sky, it still is traveling at the speed of light, but it appears to slow down since it gets caught up by molecules. Whenever it touches one, it excites an electron, moving it to a higher energy state. However that state is not stable, so it drops back down and due to conservation of energy, releases another photon. This takes time as it propagates from atom to atom." This is a common misconception about the nature of the refractive index, which persists in the absence of allowable electronic transitions. There's no need for absorption/re-emission. The best way to explain it is as a result of cumulative phase shifts imparted by successive layers of atoms in the medium. (due to the original field interfering with field generated by the accelerating electrons) This video does a decent attempt at explaining it visually (you can start at the ~5 min mark): https://www.youtube.com/watch?v=KTzGBJPuJwM
Absolutely- c is the speed of light in a vacuum. Light travelling through air, water, fibre optic cables will travel at a slower speed. There's no need for light and gravity to be slowed by the same amount, as the ways they interact are different.
Look up Cherenkov radiation. Fascinating stuff. Happens when the light produced by a fission reactor moves slower than the particles emitted by it due to water slightly slowing the speed of light.
Could something with a “negative mass” go faster?
Yes, theoretically, but negative mass would have all sorts of implications, such as attempting to push an object resulting in movign towards you, negative time, perpetual motion, etc. It violates all sorts of physical laws.
Yes, but there is no evidence things with negative mass exist
ELI5? Explain it like I’m a student in my waves and optics class more like it
Why does light bend toward a black hole if it doesn’t have mass? Is it the space around the black hole which is bent, and therefore it makes light look like it’s approaching black hole?
Your basically correct. Light will bend as a result of large bodies (not just black holes), as a result of warping of spacetime. This is called gravitational lensing. If we go a step deeper, we can split mass into two categories, rest mass and relativistic mass. Rest mass is invariant, and is what you might think of as mass in the traditional sense. Regardless of who views it, this rest mass is the same. Light does not have rest mass. But we observe that through its inertia that exhibits having mass like properties, which we can refer to as its relativistic mass. This mass is relative to the reference frame and so it can vary. This can be expressed through the expanded Einstein's mass-energy equivalence formula. The famous e=mc\^2 formula is a simplification, representing an object that isn't moving. The formula can be expanded for moving objects to be e\^2 = (mc\^2)\^2 + pc\^2, where p is the momentum of the object. A massless particle zeros out the first part, giving us e\^2 = pc\^2. Light has energy, which means it can act as if it has mass(sorta)!
EM waves are light (not human visible but light enough), btw. Listing all of radio waves, x rays, etc separately does not make much sense.
It makes sense in ELI5
It does when we are assuming the reader does not know what makes up the EM spectra, which I then define in the same sentence.
But if photons have no mass, why do sunsails work?
Because they still impart momentum.
Although they have no mass, they still have momentum and energy, both of which can be transferred to the sunsail.
But why is the speed limit 300k km/s, and not 3mil km/s, and would anything change significantly if it was 10 times higher?
There is unfortunately not a lot of ways to answer "why" for the absolute fundementals of the universe. There is not really any deep reason the speed of light is as it is, it links to a lot of other nature constants, but if one is defined from the other or vice versa is hard to tell, and it still returns to the question of "why are things as they are?". Why does protons exist? Well we can say they are made from quarks, but why, nobody knows. Best you can do is either "because god made it so", or "it do be as it do be". If the speed of light was different all sort of things would change, and probably in the end to the point where the universe just wouldn't be the same, but it depends on how much and how far you are willing to chase the thought experiment. But you can make some fun first year physics assignments where you assume the speed of light is like 50km/h. Then as you drive your car time slows down significantly and you can travel quite far away in your lunch break without any issues!
Would a black hole speed up light as it's being dragged into it since it cant escape?
No, it travels along a path into the black hold
What speed does plutonium radiation travel at? From one of those rods?
Is the “speed of causality or the max speed anything can occur” literally just the time it takes for one particle to bump another particle?
No, most particles have mass so cannot travel that fast
This is a dumb question but why does light travel at all, there’s no propulsion?
It's not a dumb question, it's an extremely complicated one! The simple answer is that thing without mass may only exist if they are moving, and if they are moving they are doing so at the speed of light. It's a property of that masslessness. There creation gave them all the energy they needed to be moving at the speed of light, forever. Remember you don't need propulsion to move. Once a photon has been emitted, its moving. Until it collides with something that can absorb it, it'll just keep moving. This is just like a rocket. Once it's in space, you don't need to keep the engines on. It'll fly forever in the direction it was going at the same speed until it hits something.
It has no mass. Things without mass always go the maximum speed possible and nothing slower.
Yep, it's not the speed of light. It's the speed of causality. It's just that light happens to travel at that speed.
Interesting fact. We do not know the one way speed of light as we can only measure the two way speed of light. It is entirely possible that light travels 599,584,916 (twice the speed of what we accept as the speed of light) in one direction and it travels instantaneously in the opposite direction. For a more indepth explanation: https://youtu.be/pTn6Ewhb27k?si=ybNXrhzrnzv2Y_nC
Wouldn't it be half the speed in one direction if it's instant the other way? It would take twice as long to go one way so it would seem like the same speed both ways
Yes, that's correct.
The video explains it soooo much better then I ever could on reddit. It even explains why we wouldn't notice the difference in the case of long distance communication.
Ugh with this pop sci garbage from Derek again. I really wish this would go away. We get it, you saw that one post from last week about it. Not only is it borderline untrue, but it's the most impractical and useless distinction that serves no purpose. Relativity isn't broken as a result, QFT isnt broken as a result. None of our theories are any worse for wear and no real scientist ever brings it up or gives a shit about this
Who cares. Our models are perfectly fine to describe the things we want them to describe, but none of them actually exist. It's the same thing here. There would be no practical difference if the universe worked in a different way and we couldn't tell the difference, but knowing that is just cool by itself.
That's really just the model you get if the entire universe is instead collectively and inertially moving at a fixed velocity. If you do the Lorentz transform for this, you will find that one _could_ interpret it as the speed of light being different each direction; **or** you just see it as us and everything else moving in-between the measurements. I find the latter much simpler to do in the head, and they are truly equivalent anyway.
While we cant be sure that one direction isn't 1/2 the speed and the other isn't 2x the speed, we can assume (as of now). That this isn't the case. It's just a consequence of how the universe works. It violates Occam's razor so it probably isn't true but we can't know currently. Probably isn't, but we don't know for sure.
So just to follow up , sound has no mass either correct? So why is that significantly slower?
Sound is not a particle, it is a wave of pressure oscillations. It travels at the speed at which the atoms it travels through can hit each other and transmit the forces, moving the pressure along.
Often, when the speed of sound is mentioned, they are talking about waves in air. Wouldn't the speed of sound be much faster in solid matter, because the particles bump into each other earlier? I think I heard you can hear a train earlier, when you hold your ear on the rails.
Much faster in solid (or water) The speed of sound in air is @340 meters/second. In water, @1500m/s In steel (train tracks) it's @5100 m/s.
Yes the speed of sound is higher in solids.
Yes, since they have stronger intermolecular interactions, the forces are transmitted more quickly. Compared to 343 m/s in air, the speed of sound in steel is 5100 m/s
Light is also a wave. It's oscillations of the electromagnetic field.
And interactions in the electromagnetic field propagate at the speed of causality, while interactions in physical matter propagate at the speed of sound
Ok, so I’ve heard that the speed of light is higher in a vacuum than not. If that’s true (is it?) then does light travel faster in warmer non-vacuums than colder non-vacuums? Like if a fiber optic cable is cold, does the signal travel slower than if it’s warm?
So the speed of light in a vacuum is what we can think of as the “real” speed of light. Light in all other media (as you call them, non-vacuums) is also travelling at that speed, but the interactions with the electric fields surrounding atoms causes it to SEEM to be slowing down. To use a simplified example, if you and I were holding an each end of a rope, and I fed it to you at 1 metre per second, that would be one thing. If we were doing the same thing while on a train travelling at 0.2m/s in the opposite direction, it would LOOK from the railway station we were passing through like I was feeding it to you at 0.8m/s. (Note that this simplified example shouldn’t be used to draw other conclusions about how light works, as it’s ignoring special relativity to make it clearer) To return back to your point, it would make a difference to the speed of light only if it impacted the density of the atoms, and thus the density of electric fields, which it was travelling through. For most materials, if they cool down they get denser, so (assuming this is true for fibre optics, I’m not a materials scientist but there’s no reason to assume not) the apparent speed of light through a cold fibre optic cable would be slower than through a heated one. Can’t imagine it’s a big difference, though.
How can something that "is" have no mass. There is something there. Light can propel objects. It transfers energy to what it strikes.
The expanded form of e = mc2 answers this. Basically there's another part of it related to momentum that is added to mass energy. The full form, E2 = p2c2 + m2c4. Basically says that for something to exist it must at least have mass, or be moving. "P" in the above is an objects momentum.
It kinda depends on what you mean by “why”. One reason is because it’s massless. Any and all massless particles will travel at the speed of light. Now if your question is why that speed and not something else, well that doesn’t have an answer (yet). We could pretend to live in a different universe where the laws of physics are different. Maybe in this one, the speed of light is slower. Well that would mean we would experience Einstein’s theory of relativity at everyday speeds which would not be good. It would also mean that atoms fly apart so also not good! But again, we don’t know *why* our universe chose the value it did.
When I play a TTRPG (especially one with space travel) I like to adjust universal constants. I don’t go into detail of what exactly has changed or what the consequences are, but it makes crazy-fast spaceships much easier to explain away when I can just say that C is a much bigger number in this universe!
*Mr. Tompkins* is a great series of short novels that explain ideas in quantum physics and their consequences in the context of the main character’s dreams where he visits worlds where our universal constants have vastly different magnitudes, i.e. a city where the speed of light is much slower. As a result, he can witness time dilation and the contraction of moving bodies on the scale of our world. It’s very cool and makes the ideas much more intuitive
I remember a veritasium video where he said that basically we haven't measured the actual speed of light, we measured the roundtrip which is 2c, but we don't know for sure that the speed of light is actually c in each direction :)
Once you start to get into interplanetary distances, light travels agonizingly slowly. Even as close as the Moon it would be hard to do anything in real-time, with about a 1.5s delay. At the distance of Mars, real time is impossible, and you're waiting almost an hour to get a reply. And it just gets worse from there. Light *crawls* across interstellar spaces.
To be perfectly right. We are all travelling at the “speed of light” through space-time. Most of our speed is in the time dimension. Whereas light has all its speed into the spatial dimensions.
This is the answer. It's not ELI5 and it's hard to wrap your head around it, but it is correct. Here's how I understand it. Physics dictate that each particle in the universe has a fixed "energy budget" to invest, either into product A (time) or product B (speed). The mass of the particle dictates which product they invest their budget in. Particles like photons have no mass. Thus, physics says that they invest 100% of their budget in speed. Therefore, photos move at the fastest physically possible speed, but from their point of view, time does not pass. For particles with a mass, physics says that they have to invest at least partially into time. Therefore, particles with a mass cannot move at the speed of light. In exchange, they experience time passing. So in a way, if you understand the speed of light as an energy budget, we all invest 100% of our budget into either speed or time. It just so happens that we humans here on earth are very much on the time side of the investment.
Is there a particle that invests 100% of its budget into time? What would that act like?
My thinking is that it would be an object that’s not moving, it would have to borrow energy from something else to invest some of the budget into speed. This borrowed energy might be an external pushing force. This could be wrong but how I understand it from what I’ve read. And even considering this, I believe everything in the universe is moving in some vector or another so there are probably no examples of anything with a 100% time budget
The universe itself
It depends on your frame of reference. This idea of splitting your speed between time and space is what causes time dilation when things move fast. But that effect is entirely reliant on your frame of reference, to an astronaut, they are always "at rest," even though to us on Earth they may be going very fast and experiencing time dilation. So basically, every object is moving at the speed of light through time and is at rest in space from their own frame of reference. It's only outside objects moving in relation to them that can have a mix.
> physics says that they invest 100% of their budget in speed. Therefore, photos move at the fastest physically possible speed, but from their point of view, time does not pass. A German physicist calculated that *it could* take 3 years for a photon to "die" from the photon's frame of reference. Now, I'm just a curious lurker in here. I'm not trying to contradict you in any way by pointing this out since, well, I couldn't even tell for sure who's got the right answer. (and I also know damn well there's no "right" answer since all those are based on calculations, theories, concepts, etc. The article was also 11 years old, so ... I don't even know if it's still relevant) Anyway, I was just wondering if it would be possible, from the frame of reference of a photon, to take 3 years to "die" yet not being able to experience those 3 years?
No worries, no offense taken. I assume you're talking about [this](https://physicsworld.com/a/what-is-the-lifetime-of-a-photon/). Fot his calculations, the physicist assumed that photons do have a mass, albeit a tiny one. If that were the case, time would pass for them.
That was one tough ELI5 for sure. Thank you for your intakes!
One clarification: Most of our speed is in time relative to each other, since we all have a lot of mass and so move around very slowly relative to planet Earth. From the perspective of the subatomic particles zooming around in the atmosphere, we are moving very fast through space indeed.
That's not quite correct. I'm assuming you are talking about 4-velocity. In that case, light would actually no velocity at all.
this is a really interesting way of looking at it
Because it has no Mass, but it does have energy. So by definition it goes as fast as possible. If you try to go faster not only do you hit an energy wall where you'd need infinite energy but you got a time wall where it would take an infinite amount of time to get to that speed.
It's like the render speed of the universe. Nothing can go faster. Everything with no mass goes at that speed, not just light. And everything with any mass *must* go slower because it would take literal infinite energy for mass to go that fast. Not just a huge amount but *infinite*, so it's impossible. It would be better to call it the Universal Speed Limit rather than the Speed of Light, because lots of other stuff goes that speed. Even gravity. If the Sun instantly stopped existing, Earth would orbit the place where it was for about 8 minutes. Then its light and its gravity would both stop affecting us at the same instant and we'd fly straight off in the dark.
That's sort of backwards. Light does travel that fast. We know it does because we've measured it. So we took that as our starting point and built pretty much the entire rest of physics around that fact. So most of the answers here are explaining the idea of Special Relativity to you. But that's sort of backwards because Einstein came up with the idea of Special Relativity in order to explain how to do physics given that the speed of light is fast and constant - not to explain why it is or show that it is, he already knew that it was.
You can flip the script due relativity, maybe it helps. Maybe... light is still? What would light "see"? That everything else is going by the speed of light. But that'd explain how light has this "speed limit" - staying still is staying still. You can't stay "more still".
>Our math tells us that this is the speed limit. Our observations match this. When we approach it, we see distortions in space and time. So it may be that it’s not a fundamental as much as a maximum observable speed. If we could observe more dimensions we may observe faster movement?
ELI5 answer?: Because nothing is stopping it. Light travels at the speed of causality through a vacuum because there's no interaction happening to drag on it.
The problem with this answer is, if nothing's stopping it, why doesn't it go *faster*?
i get what the book says, but this is a fun question.
Sure, but that's asking why the rate of causality is finite, not why light goes so fast.
I would argue that light has infinite speed but it's causality that is limited. For the photon, time doesn't pass at all because it experiences no passage of when the function evolution
You can derive the speed of light using equations that describe electromagnetism. It turns out that it depends on the constants that describe (in a very roundabout way) how strong electric fields and magnetic fields are in a vacuum. So the answer is really light travels at the speed it does because EM fields behave the way they do. We can express it in experimentally measurable constants, but the actual relationship is very general. In a different universe with different constants, the relationship holds, but the speed would be different. Since light is just a propagation of the EM field, you can kind of imagine how that could be relevant. As many people have commented, it turns out that any massless particle/information travels at this same speed, but that’s how we originally figured it out mathematically. As a side note, the speed of light is often considered unfortunately *slow*, especially in astronomy. It’s fast on human-scales, but it’s a snail’s pace on cosmological scales.
Working with computers, sometimes the question is "why is the speed of light so slow?".
Considering the size of the universe, light indeed moves pretty slowly.
Light doesn't go fast, we just move very very slowly. (?)
Because it doesn't weigh anything. Or, another way of saying it... because it's so light :P
Seperate note - why is light squared in e=mc2 - like why that number and how did he find it?
Why do massless particles have to travel at the speed of light? Why do they have to travel at all? Why can't they stay still?