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I read on a Wikipedia article lately that tachyons are theoretical particles which always travel at or above the speed of light. That means that when one passes us, we see two images, a blue shifted one and a red shifted one.

Black holes on the other hand have gravity so strong that light gets pulled in. However, I haven't heard that black holes can exert a gravitational pull that can pull and object that is travelling greater than the speed of light.

So now, I am wondering that if it is possible to use/make tachyons and control those tachyons, could we send them into a black hole and use them like a "echolocation" thing, except the sensors would be on the other side of where the tachyons are first produced?

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    $\begingroup$ Tachyons probably don't exist. No serious theory has ever claimed that they're likely. Even Feinberg, who coined the name, was pretty sure they don't exist. And if they do exist, we have no idea how to make them. $\endgroup$ – PM 2Ring Feb 28 at 2:05
  • $\begingroup$ @PM2Ring, if it exists, is what I proposed possible? Can u answer? $\endgroup$ – Max0815 Feb 28 at 3:44
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    $\begingroup$ Black holes do not work this way : velocity is not the defining quality that decides if you can "escape". Spacetime is distorted and this is what prevents "escape". The closest answer to your question is in a Physics SE question and essentially boils down to "tachyons can't exist inside a black hole". $\endgroup$ – StephenG Feb 28 at 4:52
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    $\begingroup$ @StephenG Yeah, the speed of light meme is probably the most confusing thing about the way black holes are presented in popular science. It creates the impression that if you could go "faster", you could escape. Except there is no "faster" since speed of light is essentially infinite from the p.o.v. of the moving observer. And even if there was, it's not the speed that's the limiting factor, it's the closed-in spacetime within a black hole, the literal lack of an escape route, that prevents the escape. $\endgroup$ – Florin Andrei Feb 28 at 7:17
  • $\begingroup$ Let me begin by saying that the question @StephenG posts provides a good answer to this question by someone smarter than me. I would think, however, if tachyons did exist (they probably don't), and if we could use them to measure stuff, that is, they're not ghostly particles like neutrinos but they interact in a measurable way), then I see no reason why we couldn't, in theory, use tachyons to explore part of, but not the entire, inside of a black hole. But as the other answer indicates, when models for quantum theory are worked out, tachyons break too many rules, so there's that. $\endgroup$ – userLTK Feb 28 at 10:34
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Let's look at the validity of tachyons first. Travelling faster than light, as said by userLTK, would create time travel, which would create the famous paradox, the tachyonic antitelephone. The tachyonic antitelephone is a violation of causality, and thus most likely, tachyons would not exist.

Now, take a look at another assumption. Wikipedia is right almost all times, but sadly, it has some wrong information. Such is with tachyons. It gives us some contradictory statements of tachyons too. It is wrong to assume that tachyons always move faster than light. A tachyon, is a particle with an imaginary number as its rest mass. This, however, does not mean it can travel faster than the speed of light, nor that there's any conflict between their existence and the special theory of relativity.

The main idea here is that the typical intuition we have about particles -- them being billiard ball-like objects -- just fails in the quantum world. It turns out that the correct classical limit for quantum fields in many situations is classical fields rather than point particles, and so you must solve the field equations for a field with imaginary mass and see what happens rather than just naively assume the velocity will turn out to be faster than light.

Leandro M. wrote on physics stack exchange, that

a good way to think about tachyons is to imagine hanging several pendulums on a clothesline, one after the other. If you disturb one of them, some amount of force will be transmitted from one pendulum to the next and you'll see a traveling disturbance on the clothesline. You'll be able to identify a "speed of light" for this system (which will really be the speed of sound in the string). Now you can make a "tachyon" in this system by flipping all the pendulums upside down: they'll be in a very unstable position, but that's precisely what a tachyon represents. Nevertheless, there's absolutely no way that you could send a signal down the clothesline faster than the "speed of light" in the system, even with this instability.

This means that tachyons don't actually move faster than the speed of light, and your assumption is wrong.

Furthermore, StephenG wrote in a comment that tachyons essentially can't exist in a black hole.

user106422 wrote in the linked question on physics stack exchange that

in a nutshell, creating a tachyon itself would lead to some form of a spontaneous phase transition in which the tachyonic mode no longer exists. So from a rigourous QFT + GR setting, I don't think tachyons exist the way it is portrayed in simpler QM models. So no, tachyons will not escape the horizon simply because the theory will not allow for them to exist.

Thus, no, even if they exist, they don't really travel faster than the speed of light, and even if they do, that can't really "travel" through a black hole. So, it is not possible to use tachyons to explore a black hole.

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