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Why is the light speed a limit? Why can't anything go faster than light? Not even a single atom?

Qmechanic
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ioanD
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2 Answers2

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There's no answer to this "Why". This is what we observe in nature. Nothing* can travel faster than light.

*Note: Denizens of the Quantum World like atom may not be agree with this. For example, information of quantum states of two entangled entities are shared between them faster than light. It has been tested with polarized light beams. Understand it this way: Suppose, two electrons are in Quantum Entanglement and both are in Quantum Superposition possessing up & down spin at the same time. You separate both electron to the opposite edge of the Galaxy and break the superposition of one electron. The other electron will know about it exactly at the same time faster than light. If one electron is ended up with Up Spin, Other one would start possessing Down spin instantly even if nothing's there to break the superposition.
Einstein called this faster than light Quantum interaction "Spooky action at a distance."

Earth is a Spoon
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  • information of quantum states of two entangled entities are shared between them faster than light. * This, of course, is 100% false .
    • – WillO Feb 14 '14 at 19:39
  • @WillO Einstein called it "Spooky action at a distance." Search this phrase. Or, see: http://www.livescience.com/27920-quantum-action-faster-than-light.html – Earth is a Spoon Feb 14 '14 at 21:01
  • @iani, Sachin Shekhar: "There's no answer to this "Why" [Why is light speed the limit?]. This is what we observe in nature" -- Wrong. "Speed" is a quantity to be measured, obtaining commensurate result values. They're not obtained by plainly "observing nature", but instead by applying a suitable measurement operator to (given) observational data. The answer is therefore: that's a consequence of the definition of how to measure "speed"; especially Einstein's (or at least Synge's) definition to measure "distance" between "ends" $A$, $B$ as "c/2 Pingduration_ABA" = "c/2 Pingduration_BAB". – user12262 Feb 14 '14 at 21:20
  • @SachinShekhar: I am well acquainted with quantum entanglement and so-called "spooky action at a distance". You most definitely cannot exploit this phenomenon to transmit information faster than light. – WillO Feb 14 '14 at 21:22
  • @user12262 What The heck... Go read meaning of Observation. And, you must know that Light Speed limit was found before the existence Relativistic Physics. It's not the consequence of measurement scale change. Maxwell's equation predict that there can't be standing electromagnetic wave. – Earth is a Spoon Feb 14 '14 at 22:17
  • @WillO Where did I say that I can transmit information faster than light? I am saying that entangled entities interact with each other faster than light. – Earth is a Spoon Feb 14 '14 at 22:20
  • @Sachin Shekhar: "[...] Maxwell's equation [...]" -- What shall we suppose might be the meaning of "$x$", for instance, in the expression "$\frac{d}{dx} J$" in Maxwell's equation (68)?; if such meaning were not based on the definition of "distance" as stated by Synge (and arguably already by Einstein). "Go read meaning of Observation." -- Go read about measurement, assessment of systematic uncertainties, commensurability of eigenvalues of self-adjoint measurement operators ... – user12262 Feb 14 '14 at 23:14
  • @user12262 Look... even if you use newtonian metrics, you'll learn that you can't accelerate something above speed of light and physics is wrong. It's property of nature. Relativistic Physics tries to adjust with that, not tried to create property of nature. Space and time scales have been adjusted with ultimate speed limit in mind. Einstein's postulates are followed from experimental data. – Earth is a Spoon Feb 15 '14 at 00:44
  • @user12262 As for Maxwell's equation, don't even think about arguing.. Standing Wave problem existed in 19th Century when only newtonian metrics were there. – Earth is a Spoon Feb 15 '14 at 00:46
  • @user12262 And, when you measure something, there's an observer (with reference frame) attached with it, or all measurements would be meaningless. Got something with the word observer which you see everywhere? All those things come under the domain of Observation. So, should I teach you English too? – Earth is a Spoon Feb 15 '14 at 00:51
  • @Sachin Shekhar: "even if you use newtonian metrics" -- A fair suggestion for advancing our discussion. So: how should we begin to obtain "newtonian metrics", for some particular trial, region, participants ?? (If not by: "first determine Einsteinian/Syngean metrics; then restrict to pairs with $1 >> \beta^2$ and regions of negligible curvature." ?) Recall Eddington's version of Newton's "primary law". "you'll learn that you can't accelerate something above [...]" -- How would you even begin to find out whether?. – user12262 Feb 15 '14 at 01:15
  • @Sachin Shekhar: "And, when you measure something," -- such as the "length" (or distance) "between" a sender and a receiver?, as part of the task of measuring "speed", right? "there's an observer (with reference frame) attached with it" -- Why only one?? There are already at least two distinct participants (typically called "ends") to reference systems as rudimentary as a stick. – user12262 Feb 15 '14 at 01:30
  • @user12262 See.. This isn't a philosophical discussion. Replace word Observation with whatever you want, the answer won't be changed. The ultimate speed limit is applied by nature, not by our theories. Our theories merely try to adjust with our practical results. – Earth is a Spoon Feb 15 '14 at 11:56
  • @user12262 And, you must know 19th century Physics had a trouble with speed of light when there was no Relativistic Physics. – Earth is a Spoon Feb 15 '14 at 11:57
  • @SachinShekhar: Your answer does say that entangled particles share information faster than light. To me the obvious meaning of that statement is that one particle transmits information to the other --- which is certainly not true. Perhaps you meant something else, but if so I can't figure out what you meant. If you and I are a million miles apart, stationary with respect to each other, and both consult our (synchronised) watches to discover that it's exactly noon, is that an instance of sharing information faster than light? – WillO Feb 15 '14 at 15:39
  • @WillO Your watch example is invalid. From my example.. 1. Other electron is out of superposition automatically at the same instant. 2. and, it achieves exactly opposite spin. How do you suppose this can happen without information exchange? Mode of this information exchange is unknown & Relativistic Physics gives up here because it wasn't designed for quantum world. – Earth is a Spoon Feb 15 '14 at 17:15
  • @SachinShekhar: Precisely what information do you suppose is exchanged? If I observe Electron 2 to be spin up, what do I instantaneously learn about Electron 1 that I didn't already know? – WillO Feb 15 '14 at 17:36
  • @WillO What do you want to say? Here, thing is not about learning. They are magically in synchronous state. And, for any type of state synchronization, state information is exchanged. Here, I borrowed state from Automata System. – Earth is a Spoon Feb 15 '14 at 18:49
  • @SachinShekhar: There are many mainstream interpretations of quantum mechanics, but not a single one of them invokes magic to explain entanglement. As for the rest, you insist that information is exchanged, but won't reveal exactly what information. Unless you explain what you mean, it will of course be quite impossible to pinpoint your mistake. – WillO Feb 15 '14 at 20:45
  • @WillO Ha.. Ha.. Ha.. Do you really think I am involving magic here? Even if yes, then I would refer you to Clark's third law. Anyways, back to the point: I have already mentioned that state information is exchanged. – Earth is a Spoon Feb 15 '14 at 22:39
  • @SachinShekhar: But state information is not exchanged. Initially the pair is in the state UD+DU. Now the first electron is observed to be (say) up and the state changes to UD. Nobody in the vicinity of the second electron can learn this any faster than it takes a light beam to travel between the electrons. All you can do is observe the second electron, in which case you will observe state D, which tells you nothing at all about the state of the first (unless, of course, you had some prior knowledge, but that's cheating). – WillO Feb 16 '14 at 00:23
  • Please continue this discussion in an new [chat] room. Long comment discussions on the main site will be removed. – Manishearth Feb 16 '14 at 11:35
  • @Manishearth Please, move this conversation to chat. I am not seeing that option. – Earth is a Spoon Feb 16 '14 at 12:00
  • @WillO There's no cheating if you put sensors to both side & later review that. It has been tested on Earth with sensors synchronized. – Earth is a Spoon Feb 16 '14 at 12:04
  • @SachinShekhar you can do it manually, or the link will appear when you try to add another comment. – Manishearth Feb 16 '14 at 12:06
  • @Manishearth That's the thing.. I am unable to see that link. – Earth is a Spoon Feb 16 '14 at 12:23
  • @SachinShekhar ... so do it manually. You can still create a room. – Manishearth Feb 16 '14 at 12:25
  • let us continue this discussion in chat – Earth is a Spoon Feb 16 '14 at 16:23