faster-than-lightneutrinosparticle-physicsspecial-relativitytachyon
Earlier today, I saw this link on Facebook about neutrinos going faster than the speed of light, and of course, re-posted. Since then, a couple of my friends have gotten into a discussion about what this means (mostly about time-travel), but I don't really know what this really implies. This made me wonder…
What are the biggest and most immediate implications of this potential discovery?
Related: Superluminal neutrinos
The most obvious experimental signature of tachyons would be motion at speeds greater than $c$. Negative results were reported by Murthy and later in 1988 by Clay, who studied showers of particles created in the earth's atmosphere by cosmic rays, looking for precursor particles that arrived before the first gamma rays. One could also look for particles with spacelike energy-momentum vectors. Alvager and Erman, in a 1965 experiment, studied the radioactive decay of thulium-170, and found that no such particles were emitted at the level of 1 per 10,000 decays.
Some subatomic particles, such as dark matter and neutrinos, don't interact strongly with matter, and are therefore difficult to detect directly. It's possible that tachyons exist but don't interact strongly with matter, in which case they would not have been detectable in the experiments described above. In this scenario, it might still be possible to infer their existence indirectly through missing energy-momentum in nuclear reactions. This is how the neutrino was first discovered. An accelerator experiment by Baltay in 1970 searched for reactions in which the missing energy-momentum was spacelike, and found no such events. They put an upper limit of 1 in 1,000 on the probability of such reactions under their experimental conditions.
For a long time after the discovery of the neutrino, very little was known about its mass, so it was consistent with the experimental evidence to imagine that one or more species of neutrinos were tachyons, and Chodos et al. made such speculations in 1985. In a 2011 experiment at CERN, neutrinos were believed to have been seen moving at a speed slightly greater than $c$. The experiment turned out to be a mistake, but if it had been correct, then it would have proved that neutrinos were tachyons. An experiment called KATRIN, currently nearing the start of operation at Karlsruhe, will provide the first direct measurement of the mass of the neutrino, by measuring very precisely the missing energy-momentum in the decay of hydrogen-3.
References
Alvager and Kreisler, "Quest for Faster-Than-Light Particles," Phys. Rev. 171 (1968) 1357, doi:10.1103/PhysRev.171.1357, https://sci-hub.tw/10.1103/PhysRev.171.1357
Baltay, C., G. Feinberg, N. Yeh, and R. Linsker, 1970: Search for uncharged faster-than-light particles. Phys. Rev. D, 1, 759-770, doi:10.1103/PhysRevD.1.759, https://sci-hub.tw/10.1103/PhysRevD.1.759
Chodos and Kostelecky, "Nuclear Null Tests for Spacelike Neutrinos," https://arxiv.org/abs/hep-ph/9409404
Clay, A search for tachyons in cosmic ray showers, http://adsabs.harvard.edu/full/1988AuJPh..41...93C Australian Journal of Physics (ISSN 0004-9506), vol. 41, no. 1, 1988, p. 93-99.
Let me first say that I do not work in quantum foundations, really, so I might have a few misconceptions myself. I beg anyone to correct me, where I err and I will try to provide more references upon request.
After the question seems to have cleared up in chat, let me rewrite my answer:
You basically seem to ask: What if entanglement would allow superluminal communication in some way. And you propose a protocol. The protocol that you seem to advertise suggests active transfer of information. Since no experiment has ever seen anything like this, I guess we see this as a purely theoretical game.
Let's start with the obvious consequences: If we maintain our usual notions of special relativity, we could suddenly build a quantum cloner (in a way - yes I know there is criticism of this paper, but I don't see how this addresses the problem for us). This would leave us with a big problem, because the no-cloning principle is even more fundamental (in a sense) than superluminal communication, as it derives directly from the linearity of quantum mechanics itself. Already from here, there is no other possibility than to throw aside relativity and reconsider the classical notions of time and space. This is also the usual way I have seen superluminal communication being discussed in quantum information: As a way to show how this would lead to the usual violations of "no-cloning" or "Bell's telephone", etc.
This left aside, let's consider raw entanglement, which you actually want to talk about: Entanglement, as discussed in multiple posts, is nonclassical correlations. Now, when entanglement, as in your protocol, immediately sends information, it is no longer "just correlations", because it becomes active. And I don't see how to get around this "becoming active". At that moment, everything we understand about entanglement breaks apart - because it's no longer correlations, but it would be something completely different: an active link between particles. There are many posts here already describing how this is not possible (e.g. this one).
Now let's consider the protocol again, as the usual protocol involved in Bell's theorem. You claim that "Particle B transmits to particle A, by superluminal signals, the following information", which I interpret as "active transmission of information" - which would violate causality. You claim that this is transmission of information without violating causality and you claim (in chat) that:
All the quantum community researchers, distinguished professors, Nobel prize laureats, try to understand how the entanglement works.
While this is definitely an overstatement ("all" is just too strong), there are certainly many people trying to understand entanglement better. What they try to understand is which types of entanglement can be transformed into other types, how entanglement can provide help in certain protocols, etc. But hardly any one of them tries to understand what entanglement "is". The only answer you need to have in order to work with entanglement: entanglement is quantum-mechanically allowed and classically forbidden correlations. If you don't see
To answer your underlying question: "what is this sending of information with superluminal speed?". First of all, let's fix the terminology: "information", as it is usually defined, is something active: we can do something with it. This is something whose speed is limited by causality. Faster than light speed is nothing unusual: the phase velocity of a wave can be faster than light, even the group velocity of a wave packet can exceed the speed of light (anomalous dispersion case), but in these cases, the speed of information dispersion is still smaller than the speed of light. Since your "information" is something passive, it is no information.
This means that for special relativity, we do not (at all) have to care about something happening faster than light unless it is real information. So I think that your problem is not with superluminal information processing, but with the causal nonlocality of quantum mechanics and therefore, with the quantum state. I believe that you can get rid of the nonlocality by subscribing to an epistemic view instead of an ontic view, but this buys you other problems. This seems at the heart of the EPR-argument: you cannot have a fully ontological (realist) theory and complete locality. However, it is also something that most researchers I know don't care about, because it is too much philosophy for their taste. It is also not necessary, if one only wants a consistent theory to work with.
So I believe that your issue should be addressed in the debate on ontological and epistemological theories for quantum mechanics (see an overview about certain aspects of the discussion). Maybe you will find more satisfying answers there?
Best Answer
Before I answer, a couple caveats:
Anyway: if the discovery turns out to be real, the effect on theoretical physics will be huge, basically because it
has the potential to invalidate special relativityshows that special relativity is incomplete. That would have a "ripple effect" through the last century of progress in theoretical physics: almost every branch of theoretical physics for the past 70+ years uses relativity in one way or another, and many of the predictions that have emerged from those theories would have to be reexamined. (There are many other predictions based on relativity that we have directly tested, and those will continue to be perfectly valid regardless of what happens.)To be specific, one of the key predictions that emerges out of the special theory of relativity is that "ordinary" (real-mass) particles cannot reach or exceed the speed of light. This is not just an arbitrary rule like a speed limit on a highway, either. Relativity is fundamentally based on a mathematical model of how objects move, the Lorentz group. Basically, when you go from sitting still to moving, your viewpoint on the universe changes in a way specified by a Lorentz transformation, or "boost," which basically entails mixing time and space a little bit. (Time dilation and length contraction, if you're familiar with them) We have verified to high precision that this is actually true, i.e. that the observed consequences of changing your velocity do match what the Lorentz boost predicts. However, there is no Lorentz boost that takes an object from moving slower than light to moving faster than light. If we were to discover a particle moving faster than light, we have a type of motion that can't be described by a Lorentz boosts, which means we have to start looking for something else (other than relativity) to describe it.
Now, having said that, there are a few (more) caveats. First, even if the detection is real, we have to ask ourselves whether we've really found a real-mass particle. The alternative is that we might have a particle with an imaginary mass, a true tachyon, which is consistent with relativity. Tachyons are theoretically inconvenient, though (well, that's putting it mildly). The main objection is that if we can interact with tachyons, we could use them to send messages back in time: if a tachyon travels between point A and point B, it's not well-defined whether it started from point A and went to point B or it started from B and went to point A. The two situations can be transformed into each other by a Lorentz boost, which means that depending on how you're moving, you could see one or the other. (That's not the case for normal motion.) This idea has been investigated in the past, but I'm not sure whether anything useful came of it, and I have my doubts that this is the case, anyway.
If we haven't found a tachyon, then perhaps we just have to accept that relativity is incomplete. This is called "Lorentz violation" in the lingo. People have done some research on Lorentz-violating theories, but it's always been sort of a fringe topic; the main intention has been to show that it leads to inconsistencies, thereby "proving" that the universe has to be Lorentz-invariant. If we have discovered superluminal motion, though, people will start looking much more closely at those theories, which means there's going to be a lot of work for theoretical physicists in the years to come.