2

As I mentioned yesterday, Hollywood screenwriter working on a TV pilot about physics trying to get the details right.

What empirical evidence is there that tachyons do not exist? I understand that objects with mass cannot accelerate to (much less past) $c$. So anything capable of FTL travel would have to be massless or very strange. But is there any astronomical evidence that allows us to conclude that superluminal travel does not happen in nature?

Like is there some specific phenomenon we would expect to see in the sky if non-free, interacting tachyons existed, and we're not seeing it? Or is the objection entirely mathematical?

AccidentalFourierTransform
  • 53,248
  • 20
  • 131
  • 253
Murf
  • 31
  • 4
  • Pick it up here https://en.wikipedia.org/wiki/Time_travel#Backward_time_travel (remember truth is weirder than fiction.) – JMLCarter Sep 25 '17 at 22:24
  • Discarding physics is not going to lead to even approximately right. It just going to be be what Worldbuilding SE calls "handwavium". – StephenG - Help Ukraine Sep 25 '17 at 22:28
  • Thank you for that JML but the question is more about the astronomy than the theory. I'm curious if the planetary sensorium is sensitive enough to detect evidence of FTL travel in the universe, and what that evidence might look like. Apologies if this is the wrong place to ask that. – Murf Sep 25 '17 at 22:32
  • And Stephen, I'm fine with handwavium, but only highly enriched handwavium. :) – Murf Sep 25 '17 at 22:33
  • Physics SE is most definitely not OK with handwavium of any form. :-) – StephenG - Help Ukraine Sep 25 '17 at 23:16
  • "but let's assume mainstream physics is wrong. Hypothetically, how could it happen?" - using what assumptions? You've stipulated that we discard all that we know; what then should we consult in order to answer your question? – Alfred Centauri Sep 26 '17 at 02:31
  • 3
    Murf, consider joining the chat The h Bar and asking people there, it's more relaxed than in the SE. Before, though, you might search for "tachyons" here in PhysSE, which will return, among others, Do tachyons move faster than light? and especially Status of experimental searches for tachyons?. – stafusa Sep 27 '17 at 04:18
  • 4
    You know, I am usually one of the first to jump all over people for posting non-mainstream physics, but this one is a question about mainstream physics. It is perhaps a naive question, but the OP is quite clearly looking to understand mainstream physics, not to question or overthrow it. – WillO Sep 27 '17 at 04:39
  • Related meta post: https://physics.meta.stackexchange.com/q/10173/2451 – Qmechanic Sep 27 '17 at 08:42
  • Possible duplicates: https://physics.stackexchange.com/q/55869/2451 , https://physics.stackexchange.com/q/11320/2451 and links therein. – Qmechanic Sep 27 '17 at 08:44
  • 1
    @WillO In fairness to people who have commented earlier, the original question was quite different before editing. – StephenG - Help Ukraine Sep 27 '17 at 12:23
  • 1
    @StephenG : Point taken. I wish I had thought to look at the edit history before commenting. – WillO Sep 27 '17 at 16:45

3 Answers3

1

Like is there some specific phenomenon we would expect to see in the sky if non-free, interacting tachyons existed, and we're not seeing it? Or is the objection entirely mathematical?

The objection is from mainstream physics, and main stream physics is about mathematical models that fit observations and are predictive of future observations.

Laws of physics are the axioms necessary to pick up the mathematical functions relevant to measurements and observations.

A tachyon /ˈtæki.ɒn/ or tachyonic particle is a hypothetical particle that always moves faster than light. Most physicists believe that faster-than-light particles cannot exist because they are not consistent with the known laws of physics. If such particles did exist, they could be used to build a tachyonic antitelephone and send signals faster than light, which (according to special relativity) would lead to violations of causality.

Italics mine.

We have not observed or measured in our laboratory experimentally violations of causality, i.e. effects before cause, or communications from the future.

(If the mediums' communications , messages from the future, become accessible to laboratory experiments, maybe a drastic revision of the laws of physics will allow tachyonic particles in our list of observable particles.)

anna v
  • 233,453
  • So if I understand you correctly, the case against tachyons is entirely based on laboratory physics. And obviously it's not ridiculous to assume whatever laws govern physics labs on Earth work elsewhere. But is there any proof of that? Put another way, how do we know that causality and special relativity work in, say, the Andromeda galaxy too? That seems like a safe a assumption, but is it falsifiable? (Not trying to be a pest. Astronomical evidence for causality and special relativity is what I really need.) – Murf Sep 27 '17 at 16:41
  • The laws and postulates of physics are like axioms in mathematics, they define the model. We have extended the models validated on earth and the solar system to the whole universe. There has been no contradiction. There can be no proof of truth for laws and postulates. Physics theories are validated measurement by measurement, or falsified, and the model changes in the region of falsification. There has been no falsification in our model of the universe of the laws of physics used. – anna v Sep 27 '17 at 16:50
  • Example: newtonian mechanics is falsified at large masses and distances. General relativity fits that part of phase space . It is also falsified for large velocities, and special relativity fits that part of phase space. – anna v Sep 27 '17 at 16:50
  • That's really interesting. I feel like what you're saying is that causality - the simple idea that cause proceeds effect - has been experimentally established here on earth, but it's existence in the wider universe cannot really be proved. We accept it's universality largely as a matter of faith. (As the first 5 postulates of geometry are essentially a matter of faith.) Is that accurate? That's a little bit stunning to me if true. – Murf Sep 27 '17 at 19:39
  • Let me make a comparison. My background is in neurobiology. I know people here in LA who believe in ESP. When pressed, I can explain to them the reams and reams of anatomical data that prove that, say, sight or hearing or smell exist, and the complete lack of any such data supporting the existence of a sensory apparatus for ESP. – Murf Sep 27 '17 at 19:45
  • Granted, it's hard to blame people for not having replicated, say, the 2-slit experiment in the Andromeda galaxy. But I guess I just assumed the case against FTL travel would be a little more of a slam dunk. – Murf Sep 27 '17 at 19:48
  • Well , it starts as a matter of "faith" that the laws/ equations are the same through the universe, the fact that observations of the universe are fitted with our local theories is "validated", because we use patches that fit within the mainstream physics . We introduce dark mass for why trajectories of galaxies do no follow gravitational equations , and the observed expansion does not fit without dark energy. There are non-mainstream-physics theories for explaining these last with tachyons , physicists are conservative, and unless or until tachyons are seen in the lab it is no go. – anna v Sep 28 '17 at 04:24
  • for example https://cosmosmagazine.com/physics/can-faster-than-light-tachyons-explain-dark-matter-dark-energy-and-the-big-bang . I treat it as science fiction at the moment, as most physicists. – anna v Sep 28 '17 at 04:27
1

There have been many searches for tachyons in cosmic rays, $e^+e^-$ collisions, beta decays, neutrinos, and elsewhere, all with ultimately negative results. The Particle Data Group no longer lists tachyon limits, but earlier limits can be found on page 1811 of the 1994 Review of Particle Properties. For more up-to-date limits, see Robert Ehrlich's 2022 article "A Review of Searches for Evidence of Tachyons".

David Bailey
  • 11,172
0

You cannot (or at least, not without phenomenal effort, in the best of cases) prove a negative existence statement in science, i.e. find something that will tell you, with certainty, that some "whatever" does not exist. Not with tachyons, and not with Bigfoot, either.

The evidence that they don't exist is simply that in all attempts to find one, we never have. You may have heard that "absence of evidence is not evidence of absence" - but absence of evidence where we might expect to find evidence can indeed be evidence (not proof) of absence. Of course, we also don't know of all places where we might find evidence - so again, the strength of our "they don't exist" claim likewise has to be circumscribed there, too. Just like with Bigfoot - there's enough people in the woods that, since such are an environment where you'd expect to find them, the failure to observe bones likewise constitutes empirical evidence in the same way that it does not exist. But it is always - always possible whatever you're after exists in potentially any places you have not looked.

Theoretical arguments about causality are not disproofs - theory could be wrong. But the fact nobody has sent us a message from the future in any incident that passes scientific muster is, like above, also countermanding evidence. That said, there are ways tachyons could exist in which they would also be prevented from retrocausal action - one possibility is that absolute relativity is wrong instead, and there is a preferred rest frame (which might only be detectable using tachyons). Instead of considering them "disproofs", I'd actually consider them explanations for why that tachyons "don't exist" in the empirical sense mentioned above. There are many ways that our world could be put together that resolve these causality issues - but the easiest one is simply that the relevant retrocausal phenomena just don't exist to begin with.

The_Sympathizer
  • 20,136