The speed of light is a fundamental universal constant that marks the maximum speed at which energy and information can propagate. Its value is $299792458\frac{\mathrm{m}}{\mathrm{s}}$.
Questions tagged [speed-of-light]
2915 questions
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How does gravity escape a black hole?
My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. I assume that the gravitational attraction caused by a black hole…
Nogwater
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Do we know why there is a speed limit in our universe?
This question is about why we have a universal speed limit (the speed of light in vacuum). Is there a more fundamental law that tells us why this is?
I'm not asking why the speed limit is equal to $c$ and not something else, but why there is a limit…
TheQuantumMan
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Is it possible for information to be transmitted faster than light by using a rigid pole?
Is it possible for information (like 1 and 0s) to be transmitted faster than light?
For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them starts pulling and pushing on his/her end.
The…
Jonathan.
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If all motion is relative, how does light have a finite speed?
I've often heard that Einstein shattered the notion of absolute motion (i.e. all things move relative to one another) and that he established the speed of light as being absolute. That sounds paradoxical to me; I cannot understand how the two…
CircleSquared
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How did Einstein know the speed of light was constant?
I often hear the story of how Einstein came up to the conclusion that time would slow down the faster you move, because the speed of light has to remain the same.
My question is, how did Einstein know that measuring the speed of light wouldn't be…
Pablo Fernandez
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What really causes light/photons to appear slower in media?
I know that if we solve the Maxwell equation, we will end up with the phase velocity of light being related to the permeability and the permittivity of the material. But this is not what I'm interested in - I want to go deeper than that. We know…
Emitabsorb
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What is the mechanism behind the slowdown of light/photons in a transparent medium?
So light travels slower in glass (for example) than in a vacuum. What causes light to slow down? Or: How does it slow down? If light passes through the medium, is it not essentially traveling in the "vacuum" between the atoms?
Henry
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Why is light bent but not accelerated?
Light is bent near a mass (for example when passing close to the sun as demonstrated in the famous sun eclipse of 1919). I interpret this as an effect of gravity on the light.
However, it seems (to me, at least) that light is not accelerated when it…
René Nyffenegger
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If a mass moves close to the speed of light, does it turn into a black hole?
I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever since.
From the show transcript:
Arunus Gidgowdusk…
shopsinc
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Why is there no absolute maximum temperature?
If temperature makes particles vibrate faster, and movement is limited by the speed of light, then I would assume that temperature must be limited as well.
Why is there no limit?
serg
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If the speed of light is constant, why can't it escape a black hole?
When speed is the path traveled in a given time and the path is constant, as it is for $c$, why can't light escape a black hole?
It may take a long time to happen but shouldn't there be some light escaping every so often?
I'm guessing that because…
Zaibis
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10 answers
Would time freeze if you could travel at the speed of light?
I read with interest about Einstein's Theory of Relativity and his proposition about the speed of light being the universal speed limit.
So, if I were to travel in a spacecraft at (practically) the speed of light, would I freeze and stop moving?…
64
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7 answers
If I run along the aisle of a bus traveling at (almost) the speed of light, can I travel faster than the speed of light?
Let's say I fire a bus through space at (almost) the speed of light in vacuum. If I'm inside the bus (sitting on the back seat) and I run up the aisle of the bus toward the front, does that mean I'm traveling faster than the speed of light?…
ed209
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In substances where light is slowed down, is its momentum lessened and if so, how does it speed up again when emerging from that substance?
I think in some media, light can be significantly slowed down; but even if only slightly, where would the momentum go when the light slows down and where does it get the extra momentum when it leaves that medium? An example is a water.
releseabe
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Why can't the light from a candle light the whole of a dark room?
If you light a candle it starts emitting photons, right? And photons travel at the speed of light. But how come they can't light the whole of a dark room? It means that the photons are not reaching the dark corners of the room, but that is…
ansergeyg
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