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Why speed of light is same in all reference frames? How classical mechanics and quantum mechanics approach this? Do they have two different answers to the question?

If no, how is Classical mechanics different from Quantum mechanics?

Qmechanic
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user291827
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  • Possible duplicate: https://physics.stackexchange.com/questions/2230/why-and-how-is-the-speed-of-light-in-vacuum-constant-i-e-independent-of-refer and dozens of links therein. I am $50-50$ as to if it should be closed as a duplicate because the OP invokes quantum mechanics (although without much justification) and the linked questions don't explicitly address this. –  Mar 14 '21 at 12:39

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Science usually addresses "why" questions (more formally known as teleological questions) by referring to a more fundamental underlying principle. "Why is the Earth round ?" - because this an energetically favourable shape for a body held together by its own gravity. "Why does salt dissolve in water ?" - because of the nature of the chemical bonds that hold its constituent elements together.

When it comes to relativity and quantum mechanics, we don't have more fundamental theories to refer to, so "why" questions applied to the principles of relativity and quantum mechanics tend not to have scientific answers.

The fact that the speed of light in a vacuum is the same for all observers is a fact that was discovered by experiment (see Michelson-Morley experiment) and has been empirically verified in many different ways since. We don't know why the universe works that way - we just know that it does.

The same applies to quantum mechanics. We know from multiple experiments that the universe obeys the laws of quantum mechanics. We don't know why that is - it is just the way things work in our universe.

The invariance of the speed of light has been incorporated into quantum mechanics, so we have a formulation of relativistic quantum mechanics that is consistent with special relativity. What we don't yet know is how to incorporate gravity into quantum mechanics to produce a theory of quantum gravity which combines both quantum mechanics and general relativity.

gandalf61
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Why speed of light is same in a reference frames?

This is an empirical fact and there is no other explanation other than nature is found to be like this. Classical mechanics assumed that there was no such maximum speed and that velocities added vectorially. It was only at the turn of the 20th C that it was realised that the speed of light was a universal constant and this arose from Maxwells equations for electromagnetism.

One could argue that it was the investigation of electromagnetism which led to Maxwells equations that demonstrated that the speed of light was a universal constant. However, this was indirect and one could only establish the validity of this experimentally, that is empirically. Historically speaking, this was the outcome of the Michelson-Morley experiment and which showed that the physics of Maxwell was at odds with that of Newton.

Mozibur Ullah
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    It may be misleading to say "classical mechanics" here because Maxwell's equations are entirely classical and relativistic classical mechanics are well-established/understood. – Richard Myers Mar 14 '21 at 08:17
  • @Richard Myers: But it is classical mechanics that demonstrated that there was no maximal speed whereas the classical physics of Maxwells theory showed that there was. – Mozibur Ullah Mar 14 '21 at 12:27
  • "Classical" in the sense of "old" proposed that there was no speed limit, sure. But "classical" physics in the modern parlance typically refers to any physics which is not quantum mechanical. The distinction you should really be making here is non-relativistic (Galilean) vs relativistic. Both are entirely classical in that the follow from the same formalism. They are simply different spacetime symmetry groups. – Richard Myers Mar 14 '21 at 23:31
  • @Richard Myers: There's quite a number of meanings for the term classical. I've seen it used in mathematics for standard or well accepted results. Also for 'old' as in the old quantum mechanics. I also tend to think of classical mechanics as Newtonian rather than Galilean, but then again I'm British rather than European and that might have something to do with it. I take it, that the term Galilean mechanics is used because it's namrd after the spacetime symmetry group, which I've only heard referred to as Galilean. – Mozibur Ullah Mar 14 '21 at 23:44
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If the speed of light were not universal- for example, if it were added to the velocity of an observer in a moving frame of reference- then it would be possible for a reference frame to exist in which an observer would see effects occurring before their causes in that other frame.

Fortunately, this is not how our universe functions. The universality of c provides us with a causal universe in which causes always precede their effects, and we get relativity and all of its consequences as a result.

Historically, in classical mechanics the speed of light was assumed to be infinite, and so questions like these do not even exist within that framework.

Quantum mechanics has nothing directly to say about relativity. Efforts to incorporate special relativity into quantum mechanics have succeeded, to great effect, but so far no one has found a way to cast quantum mechanics and general relativity within a common, unified framework.

niels nielsen
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