Wikipedia:Reference desk/Archives/Science/2024 April 7

= April 7 =

Can light move in a quantum vacuum at the maximal velocity C?
If it cannot, then why doesn't our article speed of light indicate that the concept of "speed of light in vacuum" is only a theoretical limit and does not exist as a real velocity? 147.235.221.47 (talk) 20:04, 7 April 2024 (UTC)


 * Propagation through a state, including a quantum vacuum state, is nonsensical. It's like asking "how fast can you run through the number 2?" or "how fast can a car drive through the color yellow?" It does propagate in a vacuum at the velocity of c, obeying all other concerns. --OuroborosCobra (talk) 20:34, 7 April 2024 (UTC)
 * Despite the link, I didn't refer to a quantum vacuum state, but rather to a vacuum with the lowest possible energy. This vacuum contains no matter; Yet it may contain other aspects of quantum vacuum, e.g. photons, virtual particles, electromagnetic fields, dark energy, and the like.
 * So my question is: When measuring the speed of light that moves in such a vacuum, is the result expected to be the maximal velocity C? In other words, does the refractive index in such a vacuum equal exactly 1? More generally and more importantly: Is there any real medium/vacuum whose refractive index equals exactly 1? 147.235.221.47 (talk) 20:49, 7 April 2024 (UTC)
 * "Quantum vacuum" is short for "quantum vacuum state". It is a description in terms of quantum theory of the current understanding of a vacuum when viewed as a quantum system. The vacuum-viewed-as-a-quantum-system is not a different thing than the vacuum of "empty" space, but a description of the same thing in terms of a different theoretical framework. --Lambiam 08:21, 8 April 2024 (UTC)
 * Your remark is correct, and that's why I have already clarified, that despite the link I've supplied, I didn't refer to a quantum vacuum state, but rather to a vacuum with the lowest possible energy.
 * Anyway, I suspect I didn't receive a direct answer to my original question, so let me be more specific now:
 * Let V be a vacuum, containing no matter, yet it may contain other aspects of quantum vacuum, e.g. photons, virtual particles, electromagnetic fields, dark energy, and the like. So, when measuring the speed of light that moves in the vacuum V, is the result expected to be the maximal velocity C? In other words, does the refractive index of the vacuum V equal exactly 1?
 * Additionally, is the vacuum V real or theoretical only? 147.235.221.47 (talk) 10:25, 8 April 2024 (UTC)
 * I think the answer is you're right, light would be slowed down by a tiny amount by the virtual particles. Really tiny. I wonder if one could make it go slightly faster by sending it between two plates as in the Casimir effect. I'm pretty certain if one cold measure that it would make Nature :-) NadVolum (talk) 12:02, 8 April 2024 (UTC)
 * In other words, the refractive index can never be exactly 1 in the real world (rather than the theoretical one), right? In other words, light can never reach the maximal velocity C in the real world, as far as measured velocities are concerned, right? 147.235.221.47 (talk) 18:52, 8 April 2024 (UTC)
 * Vacuum as defined is invariant under Lorentz transformations. It will look exactly the same in any frame of reference. Therefore the speed of light will be the same in any reference frame as well. If this is the case, it must be equal to $$c$$. If the vacuum were not invariant, then some privileged frame of reference would exist, i.e. there would be aether. Ruslik_ Zero 19:54, 8 April 2024 (UTC)
 * Everyone having basic knowledge in mechancis knows that any particle moving at the maximal velocity C moves at this velocity in any reference frame, so this was not what I was asking about. Actually my question is another one: It's about whether light moves at the maximal velocity C in a theoretical vacuum only, i.e. only in a vacuum which contains no energy and actually nothing, or also in a vacuum which indeed contains no matter but still contains other aspects of quantum vacuum, e.g. photons, virtual particles, electromagnetic fields, dark energy, and the like.
 * So, do you claim that light moves at the maximal velocity C, also in a vacuum which indeed contains no matter but still contains some low energy, including photons, virtual particles, electromagnetic fields, dark energy, and the like? 147.235.221.47 (talk) 21:30, 8 April 2024 (UTC)
 * It seems Wikipedia has an article about this! Scharnhorst effect. Not that it says much. NadVolum (talk) 22:39, 8 April 2024 (UTC)
 * Oh, quite interesting. Thank you. So, any answer to my question is still speculative, just as the Scharnhorst effect is still "hypothetical". 147.235.221.47 (talk) 23:44, 8 April 2024 (UTC)
 * It is well known that in a metallic waveguide the phase speed of waves exceeds c. No problem here. The phase speed of X-ray radiation in matter exceeds the speed of light as well. Again no problem here. Ruslik_ Zero 20:59, 9 April 2024 (UTC)
 * But the Scharnhorst effect refers to the front speed, whereas the measured value of the front speed - of light moving in vacuum - is considered to be the maximal measured value of any front speed of light (because only the front of waves carries information, whereas no information can be transmiited faster than the maximal speed possible in nature) . Anyway, you are right, no problem here, although the Scharnhorst effect refers to the front speed. The reason for there being no problem is found in the last sentence of the article Scharnhorst effect. 147.235.221.47 (talk) 06:33, 10 April 2024 (UTC)
 * The article does not say anything about the "front", only about the index of refraction. Ruslik_ Zero 20:53, 10 April 2024 (UTC)
 * If the article had referred to the phase speed only, the last sentence of the article would have been needless. 147.235.221.47 (talk) 20:58, 10 April 2024 (UTC)
 * I only want to make clear that there is just one vacuum with all virtual particles etc and it is invariant under any reference frame transformation meaning that the speed of light is invariant as well. You basically asking whether the luminiferous aether exists and whether the physical vacuum can be considered as such. The answer is no. Ruslik_ Zero 20:56, 9 April 2024 (UTC)
 * "I only want to make clear that there is just one vacuum with all virtual particles etc and it is invariant under any reference frame".
 * "With all virtual particles etc"? Have you got any source for this? Maybe this "just one vacuum" must contain no energy at all? 147.235.221.47 (talk) 06:37, 10 April 2024 (UTC)
 * I admit that most of the details of the above long discussion are over my head, but to the extent that I understand the issues, surely "a vacuum that contains no energy at all" is not something that can exist in our Universe? {The poster formerly known as 87.81.230.195} 151.227.145.123 (talk) 20:32, 11 April 2024 (UTC)
 * Yes, it is, and this is exactly the origin of my question: Since (as you correctly say) "a vacuum that contains no energy at all" is not something that can exist in our Universe, so the only question remaining is whether the maximal velocity [upper limit of all velocities that are] possible in our Universe is the speed of light moving in a vacuum that contains no energy.
 * In my view, this is really an important question, because If the answer it positive, i.e. if the maximal velocity [upper limit of all velocities that are] possible in our Universe is the speed of light moving - in a vacuum that contains no energy i.e. a vacuum that cannot exist in our Universe - then also the maximal velocity [upper limit of all velocities that are] possible in our Universe is not something that can exist in our Universe (although all velocities that can exist in our Universe must be slower than this upper limit which does not exist in our Universe, just as all natural numbers must be smaller than "infinity" which actually does not exist among natural numbers). 147.235.221.47 (talk) 22:08, 11 April 2024 (UTC)
 * By the usual meaning of the word possible, the maximal velocity possible in our Universe can exist in our Universe. (In the sense used in "possible world", there is no upper limit.) In any case, c refers to the speed of light in the kind of vacuum that can exist, that is, a vacuum with vacuum energy. --Lambiam 13:34, 12 April 2024 (UTC)
 * As for what "possible" means, you're right, so due to your first remark, I've just struck out the words "maximal velocity" in my previous contribution, and replaced them by the words "upper limit of all velocities that are" [which I've put in brackets].
 * As for your last remark, do you think you've got any sources for it? Actually, my original question has been about whether the well known constant c refers to a vacuum with vacuum energy - in which case c can exist as an actual velocity in our universe, or c refers to the absolute vacuum containing no energy at all - in which case c is only an upper limit of all velocities that are possible in our Universe - but c itself cannot exist as an actual velocity in the universe. 147.235.221.47 (talk) 10:34, 14 April 2024 (UTC)