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This question already has an answer here:

I was wondering what could be a possible reason/reasons because of which we cannot break the speed of light barrier. I was reading this where they stated that, Quantum Action Is 10,000 Times Faster Than Light.
I am not from a physics background, but if quantum particles interact faster than speed of light, why dont they go back in time?
(i had read it that if we can travel faster than the speed of light, we go back in time)

so my two questions are

  1. Why cant we break the speed barrier, is there a reason, or its just a fact that we have to accept?
  2. if quantum particles move faster than light, then why dont they travel back in time?

PS: i have read the possible duplicates, but i did not understand the possible reason for not being able to break the speed barrier.

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marked as duplicate by Robin Ekman, ACuriousMind quantum-mechanics Sep 20 '16 at 15:22

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ A lot of confusion seems to come from people interpreting $c$ as the speed of light, while it really is the speed of causality and light just happens to travel as fast as it can. $\endgroup$ – PlasmaHH Sep 20 '16 at 13:20
  • $\begingroup$ Surely this question is a duplicate. $\endgroup$ – Robin Ekman Sep 20 '16 at 14:10
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    $\begingroup$ Possible duplicate of Do we know why there is a speed limit in our universe? $\endgroup$ – Robin Ekman Sep 20 '16 at 14:11
  • $\begingroup$ Velocity is not linear, rapidity is. When you strap yourself to a Booster Rocket of Infinite Fuel +1 and light it up, you will accelerate towards infinite rapidity after infinite time. To go faster than the velocity of light would be to wait longer than infinity in order to go with more than infinite rapidity. $\endgroup$ – Fax Sep 20 '16 at 14:12
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    $\begingroup$ Just saying that you didn't understand the duplicate does not mean it's not a duplicate. Please ask a more specific question what you don't understand instead. $\endgroup$ – ACuriousMind Sep 20 '16 at 15:23
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  1. The article you linked is about a false alarm that was caused by some error in data processing. The neutrinos didn't go faster than light. The popular media of course blew this out of proportion, as usual.

  2. It's not really like a speed limit/barrier. It's the geometry of space. It's not like a highway sign - it's more like the fact that you can't go more vertical than 90 degrees, there simply isn't such a thing.

  3. Information/matter can't travel faster than light. But events that are not causally related (one didn't cause the other or vice-versa) can of course happen with arbitrary timing.

  4. In quantum entanglement, collapse of a wavefunction on one side appears to put the entangled partner instantly into a different state, but it can be shown that this can't transmit information or matter. It's a bit more complicated than that, but roughly speaking it's more like discovering late something about the state that was decided at an earlier time.

  5. "Faster than light" might appear to happen if we falsely assume flatness of space. "hyperspace" shortcuts (curved space-time on large cosmological scale (even wormholes, if they exist), or tiny space-time "foam") could make it appear that something went faster than light until you realize you oversimplified things.

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    $\begingroup$ To the defense of the neutrino scientists in point (1), they did not claim the neutrinos went faster than the speed of light; they published it stating essentially "something is wrong because it appears faster than light, please help us find our mistake". $\endgroup$ – gerrit Sep 20 '16 at 13:18
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    $\begingroup$ @gerrit yes, and then the mainstream media completely twisted the result, as usual :) $\endgroup$ – orion Sep 20 '16 at 14:05
  • $\begingroup$ Could you refocus 1 away from "premature publication" and towards "mainstream media misreporting"? They published something useful and interesting and appropriate at an appropriate time. $\endgroup$ – Yakk Sep 20 '16 at 15:14
  • $\begingroup$ so is it like, the fastest speed anything in the cosmos can achieve is the speed of light ? $\endgroup$ – Bhagyesh Chaudhari Sep 22 '16 at 5:30
  • $\begingroup$ @BhagyeshChaudhari that's correct. $\endgroup$ – orion Sep 26 '16 at 20:23
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I will answer in short.

The forces themselves propagate at c. So, say an object is moving at speed v. To speed it up, we have to apply the force that acts faster than v. When speeds approach c, the forces become ineffective in the direction of speed. The forces can work in other directions, for example, the object can be slowed down. Think about trying to push a car with your hands, when it is already moving at 300 miles/hr. You can not, because, you can not move your hand faster than the car. Same way, the forces act through space and nothing in space can be faster than c. This is the reason nothing can move faster than c. Because objects can move at any speed up to c, therefore c is a speed limit for objects.

The next question can be why forces can not act faster than c? That takes us to next part.

For light and waves and fields, it is not a limit, it is a property (of space). A wave speed depends upon the medium. Space being medium for EM waves, defines their speed. It is a property because the waves can travel only at c, no less, no more. Same way, speed of forces is also a property of space.

The speed limit comes out of relativity theory which is fully proven, and secure theory.

Your points about quantum particles pertain to a part of quantum mechanics that is not yet proven and is highly in-secure (and therefore defensive). This part is entanglement/spooky. Most of QM is very successful and makes common sense. That includes uncertainty principle, computing averages, wave particle duality ... pretty much everything except some of the nonsense like spooky and going back in time. These parts are mathematical speculations and most likely do not pertain to reality. So, they do not mean any FTL either.

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  • $\begingroup$ How would this work if I push an object? Can it not move faster than my arms if I put enough strength into it? $\endgroup$ – Yousend Sep 20 '16 at 12:02
  • $\begingroup$ With a push from your arms, at the time the object loses contact with your arms two things are true: 1) The object stops accelerating, and 2) the object is moving exactly the same speed as your arm at the point the object loses contact. Still trying to wrap my brain around reactive acceleration and speed of the matter you send in the opposite direction. $\endgroup$ – BenPen Sep 20 '16 at 14:22
  • $\begingroup$ The weird thing to me is that we talk about space as a "medium" but aren't willing to make an analogy with other mediums in physics. In fluids and gasses (really compressible and uncompressible fluids) we can travel faster than the waves travel in the medium, it just causes a bow shock. There are signs that space is "compressible" by masses, so it's not a space solid... Just an oddity to me. $\endgroup$ – BenPen Sep 20 '16 at 14:40
  • $\begingroup$ "Nothing can move faster than c because c is the fastest speed they can move" is a tautology. $\endgroup$ – Andrew Lewis Sep 20 '16 at 14:52
  • $\begingroup$ @BenPen: you can travel FTW in some media, because, we use same forces for speeding up and the forces act faster than those wave speeds. In case of space, the forces are not FTW. i.e. the force you use for compression itself is limited in speed. $\endgroup$ – kpv Sep 20 '16 at 14:54
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There is no faster than light communication or motion in quantum mechanics. This is a misunderstanding that is common even among physicists.

In classical physics, a system can be described by a set of numbers whose values can all be measured using a single instance of that system. There is a mathematical result called Bell's theorem saying that no local theory can reproduce the predictions of quantum mechanics using classical physics. Quantum mechanics is not classical physics and so it is not surprising that they give rise to different predictions.

In quantum mechanics, a system is characterised by the values of observables where those values are represented by mathematical objects called Hermitian matrices. To describe how information is transferred between quantum systems you have to describe the ways in which the observables of one system depend on those of another. In general, an observable does not represent just a single valued measurable quantity changing over time. Rather, it represents a more complex structure that involves multiple different versions of that quantity interfering with one another. And if there are going to be multiple versions of each system, then any given system has to carry information about how a particular version of that system will interact with a particular version of another system. In general, you can't get that sort of information by measuring just one system and for that reason it is called locally inaccessible information. An explanation of how locally inaccessible information gives rise to EPR correlations, teleportation etc by entirely local interactions is given here:

http://arxiv.org/abs/quant-ph/9906007.

See also

http://arxiv.org/abs/1109.6223.

For popular treatments see "The Fabric of Reality" and "The Beginning of Infinity" by David Deutsch.

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  • $\begingroup$ Dear alanf. It is usually frown upon to directly copy-paste identical answers. (The problem is if everybody start to copy-paste identical answers en mass.) In general in such situations, please consider one of the following options: (i) Delete one of your answers. (ii) Flag for duplicate posts and delete one of your answers. (iii) If you think the two posts are not duplicates, then personalize each answer to address the two different specific questions. $\endgroup$ – Qmechanic Sep 28 '16 at 7:59
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1) The amount of mass an object has = how resistant to change in momentum that object is. The more mass, the more energy required to change momentum. Since photons are massless, they have no resistance to changes in momentum. Photons are essentially the speed that energy propagates through the universe. As for why it's c, that's just how the universe happens to work out. There's no law that something can't move faster, but if we consider the physical world to be the duality of energy/mass, then nothing physical can go faster than light.

2) You'll hear a lot of different explanations for why the crazy stuff in QM happens the way it does. I'm one of those people that thinks we don't know enough to speak in terms of definitives.

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