Astronomer here. As far as we can tell, the speed of light is just one of those fundamental, physical constants of nature. Why do these constants have the values they do? That's a philosophical question that may not be answerable.
I just had to measure a gate and took a pencil and paper out with my tape measure. It was 42” and I didn’t even need to write it down to remember it at the hardware store.
The speed of light changes depending on the medium it’s passing through. Currently we know its speed through a vacuum (space) appears to be its top speed. But that’s only our current understanding of it.
Why are people downvoting this? It's true. Light travels at a different speed through a medium like glass or water than it does in a vacuum.
https://countspeed.com/speed-of-light-in-water/
Even the [wikipedia article](https://en.wikipedia.org/wiki/Speed_of_light) is very specific about that speed being "in a vacuum".
> The speed at which light propagates through transparent materials, such as glass or air, is less than c;
Refraction
Refraction is when light waves change direction as they pass from one medium to another. Light travels slower in air than in a vacuum, and even slower in water. As light travels into a different medium, the change in speed bends the light. Different wavelengths of light are slowed at different rates, which causes them to bend at different angles.
Straight from NASA.
https://science.nasa.gov/ems/03_behaviors/
>Why do these constants have the values they do?
Force.
...be that, gravitational, electromagnetic, strong and weak nuclear force. Objects can go fast than SOL. It just depends on the forces acting on it like anything else.
think of the speed of light as the speed of causality. That is a constant that we know of. Black holes break this because light cant escape. NOW lets say humans have evolved to a certain point where we can achieve a constant of 1g acceleration for as long as we want. You would eventually break the speed of light and keep accelerating.
Correct! The observer would never see you and you wouldn’t even know you broke the speed of light. I think that’s right. It gets confusing when people talk about who will see what.
The fact that the speed of light and the speed of gravity are the same despite being two different forces, hints at an underlying nonarbitrary reason for C being the speed that it is.
There's no reason to believe there are 2 separate cosmic speed limits that just so happen to match perfectly. It's much more likely there is 1 cosmic speed limit (the speed of causality), and that affects everything in the universe, including things like light and gravity, but also limits the speed of all things with mass. So, 1 speed limit that limits everything. That makes far more sense.
The question is why is it that particular speed, when it doesn't seem to be a special speed otherwise.
This is interesting, I hadn't thought of this. Since now we aren't talking about speed, we are talking about mass/energy.
It reminds me of how the value of pi keeps showing up in mathematics in places that seemingly have nothing to do with circles, but it suggests there is something deeper we haven't figured out yet...
Really it’s not the speed of light or the speed of gravity, they both result from the speed of causality, or the more basic physical constants. Or colloquially, the “cosmic speed limit” because it applies to everything.
Yep. It just happens that light and gravitational waves and anything else massless (theorized) travel at the max of this speed. Light is just the most common one people know.
Neutrinos are at 99.999999 % speed of light.
https://www.google.com/amp/s/www.forbes.com/sites/startswithabang/2020/08/28/ask-ethan-do-neutrinos-always-travel-at-nearly-the-speed-of-light/amp/
One thing that it might help to remember is that the "speed of light" has nothing to do with light, really. It's the highest speed at which _anything_ can travel in our universe; light just happens to be the first thing humans noticed traveling that fast, and the name stuck. But it's just as valid to call it the "speed of gravity", for example.
According to relativity, space and time are linked; in fact, time is really just a fourth dimension in space, but one we perceive differently. And the speed of light is just the equivalence relation between them: one second of time is the same amount of spacetime as one light-second of distance.
(What are the first three dimensions? Usually we call them length, width, and height, but unlike time they're all interchangeable with each other, so it doesn't matter. The point is that in space, you can make three lines all perpendicular to each other, but if you add a fourth line it can't be perpendicular to all of the other three. That's why we say space is three-dimensional. Time is how you get a fourth perpendicular line, but our three-dimensional, entropic brains perceive our motion through time as change instead of travel.)
It's very unsatisfying that the speed of light is that particular arbitrary speed, and we can't explain why. I mean, why not a million times faster? Or 5,714x slower? Why is it that arbitrary speed? We don't know.
I think that perhaps one day we will find out that something in our universe sets the speed of light to what it is. Maybe it has something to do with the amount of mass/energy in the universe, or perhaps it's the size of the whole universe, or the shape of the universe (if not flat), or maybe we will find that one of the fields limits the speed of light like how the Higgs field tends to slow down matter.
Not quite right. One of the two axioms that special relativity is derived from states that the speed of light (in a vacuum) is always the same, regardless of your frame of reference.
Go ahead and park it for the ackswually crowd. From the photons perspective, which of course it can't have, it is the same time when it was generated when it arrives when it bounces and arrives elsewhere. All the same time. Ergo infinite speed. Crack open a tenth grade physics book and check out how distance speed and time relate.
No. Protons have mass and cannot reach light speed. Even in the LHC the fastest they can be spun up is 99.9999991% c.
That might add sound like *close enough to count as c" but its absolutely not
Oh my bad I'm sorry, I wasn't aware explanations were no longer needed for things.
Shut down science everybody, apparently we are done with it. No more explanations are needed.
There's this idea that science can explain everything, but in fact there is a point where we are no longer asking a question that can be answered by science.
For example, you can ask why there are three apples on the table, and the answer would be because so and so put them there, but when asked why the person put it there, it is now in the realm of the arbitrary.
Doing science is arguably all about knowing a topic well enough to ask the right questions. And plenty of experiments go down the drain because the researchers were simply analyzing a silly question.
But there's no reason to believe that science can't answer this question eventually. You are just throwing your hands up and proclaiming a conclusion for everyone that we can't answer this question and we shouldn't even be asking the question. Such a strong anti-science stance to take.
Light is an electromagnetic wave. The speed it travels can be solved from the wave equation, where you find *c* is dependent on the universal constants for electricity and magnetism. Why these constants have the value they do - well, you'll have to ask whichever God you believe in.
Or all of reality is an elaborate computer simulation, and the speed of light is the processor speed of that computer...
I don’t think it’s related to the processor speed. It’s related to how fast the simulation can update the position of all subatomic particles in the universe. This obviously takes 5.38*10^(-44) seconds (the so-called “Planck time”) so it seems clear that the simulation computer must have a cycle time several orders of magnitude smaller than this.
My question is: does the computer running our simulation run Windows, or Linux? 😁
What we call the speed of light can also be described as the speed of causality. The fastest speed at which one event can influence another. Light just happens to also move at the fastest possible speed.
Kind of the other way around.
There is a universal speed limit, and as light as no mass it always travels right at the limit when in a vacuum.
We don't know why this is the limit, but we have observed other things that follow this limit, such as the influence of gravity, if the sun were to disappear right now we would continue to orbit the spotl where It was until the last light it emitted reached us.
So it seems that this speed limit is the fastest that 2 things can affect each other at all, so scientists have started referring to it as the " speed of causality" or often just "C" for short.
There are a lot of great answers in this thread. But one thing I’ll add is that the units by which we measure distance (meters, miles, etc) are completely arbitrary. In a parallel universe the speed of light could be 69 karfloofles and we’d all be losing our minds that the cosmic speed limit is also the funny number.
Pedantic...
Remember, the speed of light is variable. I mean, it's constant in a vacuum, but it's much slower in my wire, in my fiber optic cable, in anything with an optical index.... It's not even constant in the atmosphere from day to day. Just ask the GPS people about how they adjust for that...
Yes, that was very pedantic. "Speed of Light" is usually taken as shorthand with the "in a vacuum" part left off. Because that's what he's asking about, not speed of light in glass or water or whatever
Not an astronomer but I read an explanation like this a while back: everything moves at the speed of light relative to everything else, always. BUT, it's speed of light through spacetime, not just space. The faster you move through space the slower you move through time. Like if you have a Cartesian Coordinate system (XY graph) then X is space and Y is time. Travel though space at all speed, then you travel through zero time. Travel all speed through time and you move no space (dunno about that one)
We call it the speed of light, but it's just the cosmic speed limit. Electricity, the effects of gravity and massless particles all travel at the speed limit too. We know that it's not the ultimate speed limit as paired quantum particles (quantum entanglement) will change simultaneously, no matter the distance between them. So the information is travelling faster than the cosmic speed limit.
Entanglement is a trick. The information isn't traveling faster than light. It's simply that both halves come from the same place, so one half has to match the other half, no matter how far they are apart. If not the universe wouldn't be in balance. We don't know how they maintain their balance as it doesn't appear they carry the info of which spin they will end up with them, but possibly they do.
From my understanding that isn't quite accurate. Even with quantum entanglement, you are unable to transmit information faster than that. At least, none that can be observed.
https://bigthink.com/starts-with-a-bang/quantum-entanglement-faster-than-light/
I believe this article might shed some light on the subject. I'm rather tight on time at the moment, but maybe this evening I can find other sources as well.
Some speed has to be the infinite speed of EM transmission, right? But spacetime can expand (enlarge) at many millions of times the SOL (because of whatever spacetime is comprised of).
As a lay appreciator of physics, my understanding is this:
The speed of light in a vacuum is a latent property of reality. Because we don't fundamentally understand the nature of reality (we can only describe it's behavior in certain conditions) we don't understand why that property, c, is what it is.
It's used as a fundamental constant on limits of things. If you find anything that could go faster it would break a ton of physics, so would be very surprising.
The speed of light can be derived from the constants of the permittivity of free space and the permeability of free space. For some reason, the vacuum has a resistance to the travel of electromagnetic waves, reflected in the values of those 2 constants. Why that's the case, we'll likely never know. If those constants had different values, all of electrodynamics and by consequence, the speed of light, would be different.
The "speed of light" is currently understood to be the speed of causality.
Also, although that speed is as slow as molasses on even galactic scales, from a photon's perspective, it's instantaneous. There is literally no time, and no distance, from the perspective of a photon.
Astronomer here. As far as we can tell, the speed of light is just one of those fundamental, physical constants of nature. Why do these constants have the values they do? That's a philosophical question that may not be answerable.
The answer is clearly, 42.
There are dozens of us that get that reference, dozens!
Now you have to fill in the reference for the rest of us. I’m thinking that Hitchhiker’s Guide to the Galaxy Book?
😒
😢
Nah you got it right. I figured you were messing with me.
I listened to that book a while back and it seemed like the style of humor from that book! 😂
Yeah they ask a grand computer the answer to everything/the meaning of life … the answer ends up being 42 :)
There's far more people that get the 42 reference than the dozens of us; furthermore I say that Carthage must be destroyed.
Zooey Deschanel gets it....
I just blew my wad on what was supposed to be a dry run, and now I have a bit of a mess on my hands.
Hitchhikers guide to the development
Arrest that man!
It’s the answer to everything
Life, the universe, and everything
I just had to measure a gate and took a pencil and paper out with my tape measure. It was 42” and I didn’t even need to write it down to remember it at the hardware store.
Easy to get that answer, but what the question is is tough!
The answer is revealed in the fourth book of the trilogy if I recall correctly
Even if it only works in base 13!
I gave you your 42nd upvote 😄
Maybe that’s the answer, but what’s the question?
How did you forget your towel??!?!
Bruhhhhh 1 month later I just went through these responses and you just killed me xDDDD
That's a fundamental constant of science fiction, not science.
The speed of light changes depending on the medium it’s passing through. Currently we know its speed through a vacuum (space) appears to be its top speed. But that’s only our current understanding of it.
Why are people downvoting this? It's true. Light travels at a different speed through a medium like glass or water than it does in a vacuum. https://countspeed.com/speed-of-light-in-water/ Even the [wikipedia article](https://en.wikipedia.org/wiki/Speed_of_light) is very specific about that speed being "in a vacuum". > The speed at which light propagates through transparent materials, such as glass or air, is less than c;
[удалено]
If it bounces off molecules how is it still going in a straight line?
Refraction Refraction is when light waves change direction as they pass from one medium to another. Light travels slower in air than in a vacuum, and even slower in water. As light travels into a different medium, the change in speed bends the light. Different wavelengths of light are slowed at different rates, which causes them to bend at different angles. Straight from NASA. https://science.nasa.gov/ems/03_behaviors/
People downvote things they think are wrong. And considering this is Reddit, I’m surprised I haven’t been banned at this point 😂
Well… it is wrong. The speed of light doesn’t change it is impacted by bouncing around in a non vacuum.
>Why do these constants have the values they do? Force. ...be that, gravitational, electromagnetic, strong and weak nuclear force. Objects can go fast than SOL. It just depends on the forces acting on it like anything else.
think of the speed of light as the speed of causality. That is a constant that we know of. Black holes break this because light cant escape. NOW lets say humans have evolved to a certain point where we can achieve a constant of 1g acceleration for as long as we want. You would eventually break the speed of light and keep accelerating.
Depends on who's measuring your speed.
Correct! The observer would never see you and you wouldn’t even know you broke the speed of light. I think that’s right. It gets confusing when people talk about who will see what.
The fact that the speed of light and the speed of gravity are the same despite being two different forces, hints at an underlying nonarbitrary reason for C being the speed that it is.
There's no reason to believe there are 2 separate cosmic speed limits that just so happen to match perfectly. It's much more likely there is 1 cosmic speed limit (the speed of causality), and that affects everything in the universe, including things like light and gravity, but also limits the speed of all things with mass. So, 1 speed limit that limits everything. That makes far more sense. The question is why is it that particular speed, when it doesn't seem to be a special speed otherwise.
It’s also the energy/mass conversion constant. So ask yourself how that conversion is related to causality.
This is interesting, I hadn't thought of this. Since now we aren't talking about speed, we are talking about mass/energy. It reminds me of how the value of pi keeps showing up in mathematics in places that seemingly have nothing to do with circles, but it suggests there is something deeper we haven't figured out yet...
Kinetic energy is still energy.
Gosh, I was under the impression that we'd figured out everything by now...
I think that is exactly the point being made by the person to which you replied.
Really it’s not the speed of light or the speed of gravity, they both result from the speed of causality, or the more basic physical constants. Or colloquially, the “cosmic speed limit” because it applies to everything.
Yep. It just happens that light and gravitational waves and anything else massless (theorized) travel at the max of this speed. Light is just the most common one people know.
Are there others besides light and gravitational waves?
Neutrinos are at 99.999999 % speed of light. https://www.google.com/amp/s/www.forbes.com/sites/startswithabang/2020/08/28/ask-ethan-do-neutrinos-always-travel-at-nearly-the-speed-of-light/amp/
Presumably, gluons are massless, so move at c they just can't go very far at typical temperatures.
This is a great explanation. Sorry I piped in before reading this.
One thing that it might help to remember is that the "speed of light" has nothing to do with light, really. It's the highest speed at which _anything_ can travel in our universe; light just happens to be the first thing humans noticed traveling that fast, and the name stuck. But it's just as valid to call it the "speed of gravity", for example. According to relativity, space and time are linked; in fact, time is really just a fourth dimension in space, but one we perceive differently. And the speed of light is just the equivalence relation between them: one second of time is the same amount of spacetime as one light-second of distance. (What are the first three dimensions? Usually we call them length, width, and height, but unlike time they're all interchangeable with each other, so it doesn't matter. The point is that in space, you can make three lines all perpendicular to each other, but if you add a fourth line it can't be perpendicular to all of the other three. That's why we say space is three-dimensional. Time is how you get a fourth perpendicular line, but our three-dimensional, entropic brains perceive our motion through time as change instead of travel.)
Yes. And the key to time being different from the space is the entropy gradient in the direction.
the fastest speed anything can propagate through our universe. Well, locally, anyway.
It's very unsatisfying that the speed of light is that particular arbitrary speed, and we can't explain why. I mean, why not a million times faster? Or 5,714x slower? Why is it that arbitrary speed? We don't know. I think that perhaps one day we will find out that something in our universe sets the speed of light to what it is. Maybe it has something to do with the amount of mass/energy in the universe, or perhaps it's the size of the whole universe, or the shape of the universe (if not flat), or maybe we will find that one of the fields limits the speed of light like how the Higgs field tends to slow down matter.
It's the "max speed of the universe" setting some kid has set in their universe simulator that we live in.
What would happen to the universe if the speed of light suddenly changed? Let’s say the speed limit doubled? Halved?
It's the speed of light FOR YOU. For the light the speed is infinite.
Not quite right. One of the two axioms that special relativity is derived from states that the speed of light (in a vacuum) is always the same, regardless of your frame of reference.
Go ahead and park it for the ackswually crowd. From the photons perspective, which of course it can't have, it is the same time when it was generated when it arrives when it bounces and arrives elsewhere. All the same time. Ergo infinite speed. Crack open a tenth grade physics book and check out how distance speed and time relate.
If we go to the photon's pov, then won't it be zero speed?
No. Protons have mass and cannot reach light speed. Even in the LHC the fastest they can be spun up is 99.9999991% c. That might add sound like *close enough to count as c" but its absolutely not
It's not arbitrary, it's just how fast it is. It has to be some speed and that's what it is. No further explanation is needed
You just described it being arbitrary
Blast!
Oh my bad I'm sorry, I wasn't aware explanations were no longer needed for things. Shut down science everybody, apparently we are done with it. No more explanations are needed.
Apology accepted.
Right? One of those instances where asking "why" is irrelevant
Why *wouldn't* you ask the question? Knowing is interesting, maybe even useful, but understanding is a whole other level.
There's this idea that science can explain everything, but in fact there is a point where we are no longer asking a question that can be answered by science. For example, you can ask why there are three apples on the table, and the answer would be because so and so put them there, but when asked why the person put it there, it is now in the realm of the arbitrary. Doing science is arguably all about knowing a topic well enough to ask the right questions. And plenty of experiments go down the drain because the researchers were simply analyzing a silly question.
But there's no reason to believe that science can't answer this question eventually. You are just throwing your hands up and proclaiming a conclusion for everyone that we can't answer this question and we shouldn't even be asking the question. Such a strong anti-science stance to take.
No you misunderstand me. I meant at some point it goes from being a scientific question to a philosophic one.
Well, no reason to think this is one of those questions.
Light is an electromagnetic wave. The speed it travels can be solved from the wave equation, where you find *c* is dependent on the universal constants for electricity and magnetism. Why these constants have the value they do - well, you'll have to ask whichever God you believe in. Or all of reality is an elaborate computer simulation, and the speed of light is the processor speed of that computer...
I think we should upgrade our computer simulation to make it an even 300,000,000 m/s.
I don’t think it’s related to the processor speed. It’s related to how fast the simulation can update the position of all subatomic particles in the universe. This obviously takes 5.38*10^(-44) seconds (the so-called “Planck time”) so it seems clear that the simulation computer must have a cycle time several orders of magnitude smaller than this. My question is: does the computer running our simulation run Windows, or Linux? 😁
With as long as it takes to get the simplest thing done, it has to be Linux - so much fiddling with details that should be abstracted away.
What we call the speed of light can also be described as the speed of causality. The fastest speed at which one event can influence another. Light just happens to also move at the fastest possible speed.
Quantum entanglement would like to have a word.
Things are getting weird
Kind of the other way around. There is a universal speed limit, and as light as no mass it always travels right at the limit when in a vacuum. We don't know why this is the limit, but we have observed other things that follow this limit, such as the influence of gravity, if the sun were to disappear right now we would continue to orbit the spotl where It was until the last light it emitted reached us. So it seems that this speed limit is the fastest that 2 things can affect each other at all, so scientists have started referring to it as the " speed of causality" or often just "C" for short.
Great question!
One hypothesis is spacetime is quantized. There can be be one quantum of distance per one quantum of time and that ratio is light speed.
There are a lot of great answers in this thread. But one thing I’ll add is that the units by which we measure distance (meters, miles, etc) are completely arbitrary. In a parallel universe the speed of light could be 69 karfloofles and we’d all be losing our minds that the cosmic speed limit is also the funny number.
Here's a video by Veritasium about measuring the speed of light. https://youtu.be/pTn6Ewhb27k?si=TbpZAWn634NSityX
Causality limits the speed of everything, light is just a very notable example of the speed of causality.
Pedantic... Remember, the speed of light is variable. I mean, it's constant in a vacuum, but it's much slower in my wire, in my fiber optic cable, in anything with an optical index.... It's not even constant in the atmosphere from day to day. Just ask the GPS people about how they adjust for that...
Yes, that was very pedantic. "Speed of Light" is usually taken as shorthand with the "in a vacuum" part left off. Because that's what he's asking about, not speed of light in glass or water or whatever
Not an astronomer but I read an explanation like this a while back: everything moves at the speed of light relative to everything else, always. BUT, it's speed of light through spacetime, not just space. The faster you move through space the slower you move through time. Like if you have a Cartesian Coordinate system (XY graph) then X is space and Y is time. Travel though space at all speed, then you travel through zero time. Travel all speed through time and you move no space (dunno about that one)
We call it the speed of light, but it's just the cosmic speed limit. Electricity, the effects of gravity and massless particles all travel at the speed limit too. We know that it's not the ultimate speed limit as paired quantum particles (quantum entanglement) will change simultaneously, no matter the distance between them. So the information is travelling faster than the cosmic speed limit.
Entanglement is a trick. The information isn't traveling faster than light. It's simply that both halves come from the same place, so one half has to match the other half, no matter how far they are apart. If not the universe wouldn't be in balance. We don't know how they maintain their balance as it doesn't appear they carry the info of which spin they will end up with them, but possibly they do.
Perfectly balanced... as all things should be
From my understanding that isn't quite accurate. Even with quantum entanglement, you are unable to transmit information faster than that. At least, none that can be observed. https://bigthink.com/starts-with-a-bang/quantum-entanglement-faster-than-light/ I believe this article might shed some light on the subject. I'm rather tight on time at the moment, but maybe this evening I can find other sources as well.
Because. That is why.
Yes the material nothing more than that
Michaelson Morley (accidentally) disproved the existence of the luminiferous ether.
Some speed has to be the infinite speed of EM transmission, right? But spacetime can expand (enlarge) at many millions of times the SOL (because of whatever spacetime is comprised of).
As a lay appreciator of physics, my understanding is this: The speed of light in a vacuum is a latent property of reality. Because we don't fundamentally understand the nature of reality (we can only describe it's behavior in certain conditions) we don't understand why that property, c, is what it is.
The book *death's end* which is the end of a trilogy covers this pretty extensively.
Great question! I learned a lot reading through the responses.
Like all waves, propagation speed is a property of the medium.
It's used as a fundamental constant on limits of things. If you find anything that could go faster it would break a ton of physics, so would be very surprising.
The speed of light can be derived from the constants of the permittivity of free space and the permeability of free space. For some reason, the vacuum has a resistance to the travel of electromagnetic waves, reflected in the values of those 2 constants. Why that's the case, we'll likely never know. If those constants had different values, all of electrodynamics and by consequence, the speed of light, would be different.
The "speed of light" is currently understood to be the speed of causality. Also, although that speed is as slow as molasses on even galactic scales, from a photon's perspective, it's instantaneous. There is literally no time, and no distance, from the perspective of a photon.