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Why does light travel with speed $3×10^8\,\text{m/s}$? and why not more?

Qmechanic
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Akshit Sharma
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  • This has been addressed in many other posts. Search of "maximum speed" or "speed of light" and take your pick. – Bill N Oct 12 '15 at 02:59
  • Also related: http://physics.stackexchange.com/q/205222/2451 – Qmechanic Oct 12 '15 at 06:49
  • @Qmechanic or anyone else: the duplicate is itself basically a duplicate: there are multiple copies (now at least 3) the same question with quite different answers (and lots of IMHO wrong answers). Can they all be somehow merged? – innisfree Oct 12 '15 at 07:15
  • @innisfree : I cannot promise a quick response if any to merge requests cf. various meta discussions. – Qmechanic Oct 12 '15 at 07:33
  • Generally speaking, please do not post duplicate answers. – Qmechanic Oct 12 '15 at 07:33
  • @q Sure, no worries. I will delete one of my identical answers. – innisfree Oct 12 '15 at 07:50

2 Answers2

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I will repeat what I have said elsewhere on another "why" question:

Physics is not mathematics. It is an observational discipline that uses mathematical formulations to fit observations and predict the behavior of new set ups.

Experimental observations can be fitted very accurately by using quantum mechanics and electrodynamics where the velocity of light c is constant. This constant has been measured and is an experimental fact.

The answer is it travels at that speed "because this is what we have observed/measured".

In general, physics does not answer "why" questions , only how from certain assumptions and using mathematical formulations one can describe physical systems.

anna v
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Let's take one of your comments:

So than this 3×10^8 os just an experimental observation , this mean its not right to say that nothing travel faster than speed of light .

The main experimental observation by, say, the Michelson-Morley experiment, or other experimental confirmations of special relativity is not the speed of light's numerical value, but that this particular speed is invariant: it is observed to be the same for all inertial observers. As I show here and in the linked answers, the form of the Lorentz transformation can be derived from very basic physical assumptions that have nothing to do with light. Such derivations show that either the World must be governed by Galilean relativity, or by Lorentz transformations between inertial frames, but they do not tell us what the value of the special parameter $c$ is. The do, however, foretell this parameter's invariance between inertial frames.

So, given the arguments I just alluded to, and given that experiments have shown the speed of light to have the invariance property, these experiments show that light happens to travel with this invariant speed $c$. One way of interpreting this experimental result is that light is mediated by a massless particle. Neutrinos were for many years thought to be massless and they are observed to travel at the same speed $c$ to within contemporary experimental uncertainty. They are only indirectly known to have a very small mass through the phenomenon of flavor oscillation. If the massless graviton of quantum gravity is shown to exist, it too must always move at the speed $c$ relative to any inertial observer.

But there is no explanation known as to why $c$ has this particular value. Everything I have said would be true if the invariant speed were $4\times10^8{\rm m\,s^{-1}}$ and light were observed to propagate at this speed.

Community
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Selene Routley
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  • What would be different if $c=4e8 m/s$? That's just a different definition of metres and seconds. – innisfree Oct 12 '15 at 06:51
  • @innisfree No what I meant was if the invariant speed were observed to be $4\times10^{8}{\rm m,s^{-1}}$ in current SI units. The crucial thing here is the invariant property of the speed, not its numerical value. – Selene Routley Oct 12 '15 at 06:53
  • Not sure I understand you - length and time actually have the same units. Your comments amount to: why isn't $c=4/3$ in natural units? We know why $c=1$ in natural units. it is because the photon is massless. – innisfree Oct 12 '15 at 06:57
  • @innisfree If you look, your last sentence is part of my answer. Of course $c=1$ in natural units, but there are still comparisons to be made between $c$ and other physical phenomena: for example its value as measured in terms of time and length units defined e.g. in terms of atomic transition periods and wavelengths (before the second was derived from the SI definition of $c$). Another way of looking at this, eloquently put by Jerry Schirmer, is that $c$ is very fast compared to everyday things because nuclear processes are greatly more energetic than chemical ones: one could imagine a ... – Selene Routley Oct 12 '15 at 08:03
  • @innisfree ... a universe wherein nuclear binding energies were only an order of magnitude greater than chemical bond energies, and then $c$ wouldn't have such a great numerical value compared to, e.g. speeds defined in terms of lengths comparable to our body length and times comparable to our heartbeats. – Selene Routley Oct 12 '15 at 08:04
  • Alright, then we agree that $c$ is fully explained, but there are other dimensionless constants governing speeds of everyday objects that we can't explain. Agreed. – innisfree Oct 12 '15 at 08:33
  • @innisfree Yes, I think we are on the same plane now! Actually I think I like your last comment better than my way of saying it. Many thanks – Selene Routley Oct 12 '15 at 09:06
  • don't mention it, now I understand your point of view now, I see we have been in agreement from the beginning. – innisfree Oct 12 '15 at 09:39