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I am familiar with the limitations imposed by special relativity related to the existence of tachyonic observers. Still, since many experiments directed towards the detection of tachyons have been  conducted in the past, I think it is legitimate to consider tachyonic frames of reference.

For an observer very close (but under) the light speed, radiation from the universe would emanate from a single point in the direction of travel, and all radiation would be Doppler shifted to gamma ray wavelengths. 

In this direction,  this link was sent to me by Thomas Fritsch , related to another question. 

When we go into the FTL domain (for the tachyonic observer), that point will disappear (observables take imaginary values), and  the Doppler shifted radiation will have such high frequencies that a black hole will be formed. In other words, for a tachyonic observer, the whole universe will be a black hole (or a naked singularity?).

Related to the possibility that extremely high frequency radiation could create a black hole, check this link

This could be an explanation why  the existence of tachyons has never been  confirmed in experiments (or a way to design better experiments). 

For the status of these searches , check this link

Question. From the standpoint of a tachyonic frame of reference,  is the universe perceived as a black hole? 

Cristian Dumitrescu
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    The existence of an experiment to detect something which it couldn't detect is not a good reason to entertain the idea of the undetected entity. Moreover, the experiments were to detect tachyonic particles--not the legitimacy of tachyonic frames. The existence of tachyonic particles wouldn't have made the tachyonic frames legitimate. Just like the existence photons doesn't make the frame of a photon legitimate. –  May 26 '19 at 21:53
  • I don't consider this mainstream (it's practically a personal theory IMO), but if you want to clarify this more, I'd suggest starting with (a) in what sense the entire universe is a black hole and (b) why you think relatavistic Doppler shift in the Tachyon "domain" would result or equate to the universe being a black hole. – StephenG - Help Ukraine May 26 '19 at 21:58
  • Thank you @StephenG for your feedback. I am talking about perception, from a tachyonic perspective. You can ask a similar question from the other side of the barrier. In what sense the universe is a luminous point (source of gamma rays) for a subluminal observer (close to light speed)? In fact, the observer will perceive the universe as a black hole before light speed is even reached. – Cristian Dumitrescu May 26 '19 at 22:34
  • Thank you @FeynmansOutforGrumpyCat for your feedback. In fact, the observer will perceive the universe as a black hole even before the light speed barrier is reached. – Cristian Dumitrescu May 26 '19 at 22:38
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    No, that statement is deeply wrong. An observer going at 99.999999999% of the speed of light is physically no different than an observer at rest. That is the basic fact and principle of relativity. Moreover, existence of a blackhole is a topological fact. It cannot be changed/brought into existence by a change of reference frame or even a general coordinate transformation. –  May 26 '19 at 22:42
  • A "black hole" has a specific mean to do with a region of spacetime separated from another by an event horizon. You do not seem to be using a "black hole" in that common sense. If so the use of the "black holes" tag is not correct and it should be removed. You mean the effect of seeing the universe in front and behind in a smaller and smaller angle (like this, I think. – StephenG - Help Ukraine May 26 '19 at 22:43
  • Yes @StephenG , that 's the first link in my question. If that region is a source of radiation of increasing frequency, there will be a moment when the creation of a black hole (as perceived by this observer) is inevitable. You might be right though , it might be a naked singularity, I am not sure, but I am sure something has to happen. You cannot Doppler shift this point source radiation to infinity and expect nothing extreme to happen. That's not tenable. – Cristian Dumitrescu May 26 '19 at 22:54
  • You are right @FeynmansOutforGrumpyCat I don't see an easy solution here. But you cannot expect to Doppler shift the radiation (coming from a point source ) to infinity and expect nothing special will happen, that's not tenable. – Cristian Dumitrescu May 26 '19 at 22:59
  • Related to your previous comment @FeynmansOutforGrumpyCat "A black hole cannot be changed/brought into existence by a change of reference frame or even a general coordinate transformations". True for smooth transformations, not so much when singularities come I to play. – Cristian Dumitrescu May 26 '19 at 23:28
  • "Observers" travelling less than $c$, at $c$, and faster than $c$ are fundamentally different. This is a reflection of the tripartite division of spacetime due to the Lorentz symmetry. Observers going faster than $c$, if they make sense at all, see different physics (in fact, must see, because at the very least they must see slower-than-$c$ objects as moving faster than $c$!). There isn't any sensible way to make a "reference frame" that travels "faster than $c$" which has the same physics as observers traveling slower. – The_Sympathizer May 27 '19 at 11:35
  • (At $c$ is a singular case - good luck trying to define an observing framework that isn't "slant infinity".) The "principle of relativity" only applies within a domain. Also that means that if you had a magic device that could "jump" you to faster than light speed, it kills you. – The_Sympathizer May 27 '19 at 11:38

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It is a misunderstanding that SR would say that nothing can travel faster then c, so this question does have some merit (theoretically).

As per SR and GR (in vacuum, when measured locally):

  1. whatever travels slower then c (has rest mass) will always travel slower then c

  2. whatever travels at speed c, will always travel faster then c

  3. whatever travels faster then c, will always travel faster then c

Now what gives merit to the question is 3., it is not commonly known, and not really stated too much on this site though.

Now to understand what the universe would look like to an observer, we have to clarify:

  1. as per SR, anything with rest mass does have a reference frame

  2. as per SR, anything with no rest mass, does not have a reference frame

  3. as per SR, anything that travels faster then c, might or might not have rest mass (theoretically there is no model for this)

Since your question is about the observations of an observer traveling faster then c, this does not contradict with the fact that as per SR, massless particles, traveling at speed c, do not have a reference frame.

It does not make sense to talk about what the observations of an observer traveling at speed c (a photon) would be, because as per SR it does not have a reference frame.

This does not mean that anything traveling faster then c does not have a reference frame, so theoretically there is no contradiction with SR, though, experimentally, we cannot prove that anything could ever travel faster then c.

Let's disregard that, and say that we start with the frame of a neutrino, that is as close to the speed of light as possible.

Now a neutrino would see its own clock tick normally, but as soon as it compares its clock to the clocks in the universe at rest (relative to itself), it will see that those other clocks tick almost infinitely fast. The neutrino will see the whole 13.8 billion years (assuming it is flying around since then), as just a moment in time on its own clock. Like a fast forward of the whole movie called Universe, lasting 13.8 billion years, watched in a moment.

Now there is no sense in going even faster, but you get the idea, the photon would see (again, there is no reference frame of the photon) the whole 13.8 billion years in an infinitely small amount of time (assuming the photon is flying around since then), the photon could not even experience this 13.8 billion years, since for the photon, emission and absorption are in a spacetime distance of 0, that is, it is a lightlike path. The photon could not even watch this movie called Universe, it would be so short for it.

Now going even faster, theoretically, in your question, a tachyon. It would see the 13.8 billion years movie backwards (theoretically). This is not the same as traveling back in time. This is just that the frame of the tachyon (theoretically) would make the whole movie called Universe to be played in reverse. How fast it would play in the frame of the tachyon is just a matter of how much faster the tachyon is then c. But for a tachyon, that is still flying around since 13.8 billion years, this movie hasn't even started yet (in our frame at rest relative to the tachyon). It will start seeing the movie whenever it decays (or whenever the universe ends). Of course in the frame of the tachyon the movie has already started (playing backwards from the decay or end of the universe), that is why they say that (theoretically) the tachyon is foreseeing the future. That is not correct to say, but theoretically it is watching the universe backwards from ending to the Big Bang.

Árpád Szendrei
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  • All the reference frames allowed in SR are fully characterized by 3 boosts, 3 rotations, time-reversal transformation, parity transformation, and finally, 4 translations. No combination of them gives us a tachyonic frame. So, SR does forbid tachyonic frames. –  May 27 '19 at 08:14