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When I type tachyons into the search engine of the site I find a lot of open questions about tachyons but my question about tachyons was closed as off topic

If electrically charged tachyons exist, could multiple electrically charged tachyons be bound to each other?

The comment said that tachyons aren't considered mainstream physics but if that's the case why wouldn't other questions on tachyons be closed as off topic?

From what I understand special relativity does not forbid particles that move faster than light, but instead forbids particles that are not already moving faster than light from accelerating to a speed faster than light.

One reason I see given for tachyons being thought to not exist is that they can travel back in time and so create causal loops, but from what I understand the known laws of physics aren't known to forbid time travel or causal loops.

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The possibility that someone else might ask a question with the word "tachyon" in it that's on topic doesn't seem germane to the issue of whether your question is, on its own merits, on topic. (That's before considering that some open questions probably should have been closed too but slipped through for some reason or another.)

The comment by Chiral Anomaly on your question seems pretty descriptive. Your question does not allow for any answer within mainstream physics. The premise of your question is inconsistent. That's different than (drawing a few examples from the search that you suggested) questions about

  1. Why tachyons violate causality (uses the term but asks about them in context of why existing theory disfavors them)
  2. Various questions about the potential for tachyons to exist in unproven but plausible theories that have sensible limits to accepted theory
  3. Existence or non-existance of formal solutions to mainstream-theory equations with (real or perceived) FTL characteristics.
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Briefly speaking, asking about faster-than-light tachyonic particles is non-mainstream/primarily opinion-based and usually$^1$ off-topic (unless it's about experimental searches), while asking about tachyonic fields can be on-topic.

$^1$ No rule without exceptions, especially if the question is well-researched/well-documented, or illustrates in a particularly clear way related concepts, e.g, causality.

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    $\begingroup$ To be honest, I've never thought this was a good reason for closing questions about tachyons. Tachyonic particles have been studied by perfectly respectable physicists, such as Sudarshan, and numerous papers about them were published in the Physical Review in the 70s and 80s. The same goes for warp drive, which I think you've called non-mainstream on several occasions; research on them is regularly published in the Physical Review even today. $\endgroup$
    – knzhou
    Jul 18, 2021 at 23:05
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    $\begingroup$ I do agree that most such questions should be closed, but for a totally different reason: since they're usually inspired by sci-fi, they tend to be complicated, messy questions that are well beyond the reach of any existing work (such as "can I use tachyons to talk to my dead grandparents" or "how many dollars will a warp drive cost in the year 2200"). The proper close reason for such questions, I think, is "primarily opinion-based". It's the same reason questions about testing SUSY get closed: they're well-formed questions but we have no sensible objective answer. Anybody's opinion goes. $\endgroup$
    – knzhou
    Jul 18, 2021 at 23:07
  • $\begingroup$ I think that some justification should be given to the opinion that faster-than-light tachyonic particles is non-mainstream (see my answer arguments and @knzhou's comment above). $\endgroup$
    – GiorgioP-DoomsdayClockIsAt-90
    Jul 24, 2021 at 18:03
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Personally, I do not like tachyonic physics as a subject, but I'm afraid I have to disagree with Brick's and Qmechanic's point of view about the non-mainstreamness of the subject and this specific question for the following reasons:

  1. the subject "tachyons" is still nowadays present in serious papers published in reputable journals. A fast search on Web of Science returns more than 125 papers from 2015-01-01 with tachyons in the topic. Some of them are not directly related to faster than light particles, but some are and are papers by reputable authors in reputable journals;
  2. In the Meta question Is non-mainstream physics appropriate for this site? I read: a question that proposes a new concept or paradigm, but asks for evaluation of that concept within the framework of current (mainstream) physics is OK. In the case of the present question, I see a question that doesn't propose a new concept or paradigm but asks for evaluation of a concept present in mainstream literature within the framework of current (mainstream) physics. A non-opinion-based answer is in principle possible by using only mainstream physics or (if any) existing analysis published in mainstream journals.

Therefore, according to the public information about this site policy, I would say that the original question was in-topic, independently on subjective preferences about tachyonic physics.

I hope that further discussion on this issue will stick to the objective points I have just raised. I deliberately avoided any consideration based on my personal beliefs about tachyons.

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    $\begingroup$ I downvoted. The premise of the question - the existence of FTL electrically charged particles in a frame itself travelling FTL - is entirely non-physical. I could be wrong (it wouldn’t be the first time!) but I do not see how this can be answered by mainstream physics. The (selective) WebOfScience returns 95 papers in 6+ years with “tachyon” in the title, and (I didn’t scan all of the titles very closely) very few if any ($\epsilon\to 0$) have anything to do with the topic of the question that was closed. $\endgroup$
    – ZeroTheHero
    Jul 31, 2021 at 3:03
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    $\begingroup$ @ZeroTheHero if you have a proof that charged tachyons cannot exist according to mainstream physics, that would have been an interesting answer to a question about the consequences of mainstream physics on tachyons. Everything would be perfectly in-topic. If you do not like tachyons, I would be sympathetic with you, but from the point of view of this site policy, I remain with the opinion I have expressed. $\endgroup$
    – GiorgioP-DoomsdayClockIsAt-90
    Jul 31, 2021 at 10:04
  • $\begingroup$ Of course I do not have proof beyond stating that the laws of physics do not make sense for $v>c$ because of the singularity in the $\gamma$ factors. My friend: this is one where we have to agree to disagree. I do not believe “non-mainstream” is quantifiable. @dmckee nailed it IMO in this answer: physics.meta.stackexchange.com/a/10324/36194 and the question IMO is non-mainstream from the start. $\endgroup$
    – ZeroTheHero
    Jul 31, 2021 at 11:41
  • $\begingroup$ @ZeroTheHero at least points 2 and 4 of dmckee's answer apply to tachyon physics. $\endgroup$
    – GiorgioP-DoomsdayClockIsAt-90
    Jul 31, 2021 at 12:15
  • $\begingroup$ right but IMO not to the specific question that started the debate. Spurred by your observation on the number of papers with this title, I actually glanced at some of the published material on the topic: this is very far removed from what we’re talking about. @dmckee got it when he pointed that you know it when you see it. $\endgroup$
    – ZeroTheHero
    Jul 31, 2021 at 12:19
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    $\begingroup$ Don’t get me wrong: your opinion is well stated, but I don’t think there’s a clear line to be found in this matter. Thankfully it takes 5 votes to close such questions, hence there must be some sort of consensus, and the OP can always appeal (as they have done) in case of doubt. $\endgroup$
    – ZeroTheHero
    Jul 31, 2021 at 12:21
  • $\begingroup$ @ZeroTheHero The only instance, in which there is a singularity in the Lorentz Factor is when $v=c$, however in the case that $v>c$ the Lorentz Factor is defined even if it's an imaginary number. Note the Lorentz Factor is $$\gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$$ $\endgroup$
    – Anders Gustafson
    Aug 2, 2021 at 8:23
  • $\begingroup$ yes but an imaginary value of $\gamma$ doesn’t make much physical sense… $\endgroup$
    – ZeroTheHero
    Aug 2, 2021 at 12:00
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This might be too late (I didn't notice this Meta post until after several half-lives of viewing activity had already elapsed), but I'll clarify the intent of my original comment. Here's a copy of the comment:

Tachyons are not consistent with mainstream physics, so the only way to answer this "what if" question is to use a non-mainstream theory. What theory do you want to use?

Instead of casting a close-vote, I posted this comment as a request for clarification. I'm guessing that the question intended to use Maxwell's equations but with a charge-current four-vector $J^\mu$ that is nonzero only along a spacelike worldline, but that's still not enough information. In order to answer the question about bound states, we also need to specify the equations of motion of the charged matter (maybe the relativistic Lorentz force equation but using proper length in place of proper time?), and we need to define what "bound state" means when the particle world-lines are spacelike. Neither of those is self-evident, at least not to me.

Regarding whether or not the question should have been closed as off-topic: In my opinion, a question may be on-topic for this site even if it's about a toy model, which is what physicists call a simplified/modified model that isn't necessarily a good approximation to anything real but that is used to explore some specific mathematical feature(s) of other theories that are good approximations. The study of toy models makes important contributions to the progress of mainstream physics, so in hindsight, I probably should have worded my comment differently, like this:

Tachyons are not consistent with our current understanding of nature, so the only way to answer this "what if" question is to specify a toy model. What toy model do you want to use?

Regarding why other questions about tachyons are still open: maybe many of them shouldn't be. I don't know. I haven't even read most of them.

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  • $\begingroup$ I think that from your answer emerges the basic difficulty. As I stressed, I am not sympathetic with tachyon physics. The point I am trying to make is not about tachyons but about this site policy. If mainstream means (according to the Help Center) *present in serious papers published in reputable journals. *, I think that independently of personal tastes, the subject is in-topic. Otherwise, the statement of the site policy should be modified. $\endgroup$
    – GiorgioP-DoomsdayClockIsAt-90
    Aug 2, 2021 at 8:38
  • $\begingroup$ Well in order to get an equation of motion I would need to know if the electric force between the two electrically charged tachyons is non 0, but I'm not sure whether or not it would be 0 given that the electrically charged tachyons would be moving faster than the electric fields around the electrically charged tachyons could change. $\endgroup$
    – Anders Gustafson
    Aug 2, 2021 at 10:03

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