# How do we know tachyons don't exist?

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?

• Pick it up here 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 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
• 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 4:18
• 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 4:39

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.)

• 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