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Ok, I am majoring in physics (4th year) and I never understood this fundamental (kinda) question. Maybe I haven't explored it enough.

For example, why does it take 8min20sec for the light from the sun to get to us? I know the answer to this question on a 'surface' scale. The sun is 1AU away, c=3E8 m/s, and d=v/t to get approx 8 min 20 sec.

my question is on a deeper level.

Say you could "ride a photon" I KNOW THIS IS IMPOSSIBLE, but just say you could. Or a better question: what does a photon experience? The photon, as per my understanding, would leave the sun and (if it is on the right trajectory) hit the Earth instantaneously. A photon leaving Alpha Centauri would see the universe all at once, in a infinitesimal small unit of time (if directed out to space).

If a photon sees everything all at once, why do we perceive it to have a speed? I am sure this has something to do with frames of reference, special relatively, Lorentz transforms? but just seems strange. why is the speed of light finite to us... if it was infinite would this be problematic?

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Since 1983, the meter has been defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second, so that number is exact by definition. Of course that doesn't answer your question, but it's interesting. –  Keith Thompson Feb 13 '14 at 20:09
If you haven't read Feynmann's QED I'd really recommend it for more perspective on this. –  adrianmcmenamin Feb 14 '14 at 10:55
It is theorized that other multi-verses may have different constants than the ones we have in this universe. For example, the constant of gravity might be different, the speed of light might be different, etc. These universes would look entirely different than our universe. –  Scottie Jan 16 at 20:30

3 Answers 3

up vote 7 down vote accepted

Speed of light being finite is one of the fundamentals of our Universe.

If it were infinite, this would have a major implication in causality.

Besides, in non-quantum physics, light is just an electromagnetic wave. Eelectromagnetic field is described by Maxwell's equations, which predict that the speed $c$ of electromagnetic waves propagating through the vacuum depends on the dielectric permittivity $ε_0$ and the magnetic permeability $μ_0$ by the equation $c = {1\over\sqrt{ε_{0}μ_{0}}}$ so you can not have an infinite speed of light unless electric permittivity or magnetic permeability were zero, which in turn would cause all sorts of odd things to electromagnetical attraction (and thus, to matter existence beyond elemental particles).

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Ok, but why 300,000km/sec? I know km and sec are human inventions - but whatever your units of measure are - why that speed? what is so special about that number? and what about the other part of my question: if a photon experiences everything all at once - how can it be that it leaves the sun and hits earth in t=0 but to us t=8min20sec ? –  renegade Feb 13 '14 at 9:19
That speed (297000Km/s) comes from the formula I posted above: 1 over the square root of ε0μ0. About the photon experiencing everything at once, that is due to relativistic time dilation. For a photon, time is infinitely dilated so one instant extends infinitely long. –  Envite Feb 13 '14 at 9:30
oh ya.. that's the stuff. intellectual satisfaction. Thanks! I like that answer a lot. –  renegade Feb 13 '14 at 9:32
This answer is just a sophisticated version of "it is because it is" –  adrianmcmenamin Feb 14 '14 at 10:54
@Envite I think there needs to be more explanation of the formula c equals one over the square root of epsilon nought mu nought and how the speed of light relates to electric permittivity and magnetic permeability. –  called2voyage Feb 19 '14 at 15:33

Why is the speed of light 299,792,458 m/s, and not (for instance) 3,1 or 4,3 x 10^44 m/s?

The answer is that all those numbers are consequences of arbitrary choices of the units of measure. In fact there is nothing special about the number 299,792,458, so much so that one can correctly write that the speed of light is:

  • 1079252848.8 [km/h]
  • 0.3 [pc/year]
  • 1 [lightyear/year]

So much so, that in particle physics it is common to use the speed of light as the unit of measure of speed, so that it is effectively equal to unity. You would write that the speed of light is 1 [c], and that the speed of that particle was 0.99998 [c], while I typically get to work at a speed of 1. x 10^{-8} [c].

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In so-called "natural units" the speed of light is 1 (dimensionless) –  Envite Feb 14 '14 at 12:00
astabada, I think you missed the level of the question I was asking. I was asking whatever unit of measure you are using why is it that speed. For example if the meter and second stayed the same why is it 3E8 m/s and not 6E8m/s, Envite answered the quesiton nicely. Although, of course you can go on to ask why are electric and magnetic permeability the way they are...and even deeper question: why is any constant the value it is?!?! Is there a 'natural' optimization in the universe? or are these values related to the least amount of energy being used? –  renegade Feb 14 '14 at 12:05
@Envite I wanted to be clear, that is why I specified 1 [c]. I agree that if c = 1 the expression is redundant, much like writing "8 x 1" rather than just "8"... –  astabada Feb 17 '14 at 9:54
@AndrewH yes, there is a reason for which all these universal constants have the value they have. It is called the Anthropic Principle: If they had any other value, we will not be here to observe them. –  Envite Feb 17 '14 at 9:58
@astabada It seems you have a valid question. Please ask it as a Question on the site. It is pointless to discuss it via comments. –  Envite Feb 17 '14 at 10:24

There is no answer to your question as per Why? That assumes there is a purpose for light waves to travel at that speed. And there is not. The laws of physics are an accident of this universe. For what we know there could exist other universes where the speed of light is different, where all the laws of physics are different.

If you ask Why? in the sense of purpose then you are assuming the answer before you ask the question, you are assuming that there is a purpose, and as far as we know there is not. (Unless you believe in God, which I hope you don't)

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-1: Asking "why" is a request for reason, explanation, or purpose. Two of those three have a perfectly valid place in science, but this answer pretends the question is purely teleological, whereas the OP's clarifications make it clear it is not. –  Stan Liou Apr 23 '14 at 2:17
Then he should have used How instead of Why. Anyways I did answer his question asuming what he meant by Why was really a How. So I dont see reason for your negative vote. –  harogaston Apr 23 '14 at 2:25
"Why" is standard English usage for all of those, so no. –  Stan Liou Apr 23 '14 at 2:31
I don't see how. In any case, I would downvote Krauss for this answer as well. –  Stan Liou Apr 23 '14 at 3:38
Well apparently you will never get to the point that: ""I did answer his question assuming what he meant by Why was really a How. So I don't see reason for your negative vote." –  harogaston Apr 23 '14 at 3:46

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