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Hypothetical tachyons (not the kind suggesting field instability in superstring theory) are thought of as superluminal particles that are always traveling faster than light, in this case suggesting that they're traveling backward in time.

Suppose they can be created as a byproduct of some event in spacetime (say, the firing of a McGuffinite weapon that discharges its photonic payload at c).

Would it be possible for the victim to detect the tachyon (edit) before the weapon fires? That is, 1) event happens relative to victim 2) victim detects event because he is hit by photons from event but 1.5) tachyons created by event travel faster than light / backwards in time towards victim, effectively before event occurs.

Am I missing some logical step here? Could we preserve causality by saying that there is not enough time for the victim to act because the tachyon doesn't contain any information about the event itself? (edit)

Edit: To be more clear my phrasing "detect the event" and the headline of this question suggests I'm interested in creating some means of FTL communication. I'm not. Just trying to see if the ordering of "events" is sensible. It's fine if the victim can't know anything about the event itself. (Or even that an event occurred at all.) The goal from a storytelling POV would be for the victim to be momentarily worried that something is about to happen because they noticed these tachyons.

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    $\begingroup$ Re your edit: you are missing a major point here. something about the event is communication. Knowing it happened counts as FTL communication. The dramatic usage you need specifically requires FTL causality. $\endgroup$ – JDługosz Jun 11 '16 at 10:13
  • $\begingroup$ But what if the detection of the tachyon does not tell you anything about the event? That is, here is a tachyon zipping past you. It doesn't convey anything about the event that spawned it, but would it get to you before that event that spawned it? $\endgroup$ – alkah3st Jun 11 '16 at 10:14
  • $\begingroup$ In your latest edit, you contradict the question. "victim to detect the event" but "not know an event occured at all". What's the point? $\endgroup$ – JDługosz Jun 11 '16 at 10:22
  • $\begingroup$ The point (from a storytelling POV) would be momentary panic/tension $\endgroup$ – alkah3st Jun 11 '16 at 10:24
  • $\begingroup$ I know the point as originally stated from a story-telling pov. I mean that if you don't know that anything happened (in your postscript) you no longer have that. As for the question, "would it get to you before that event that spawned it?" the answer is No: locallized disturbances in the tachyon field are subluminal. $\endgroup$ – JDługosz Jun 11 '16 at 10:31
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What you want to achieve is a situation where there are tachyon precursors to some kind of what is up until now an unspecified disaster or catastrophe.

Taking a cue from this article about tachyons it is both safe and plausible to assume the tachyon precursor is a non-local event. For example, this might mean that a tachyon precursor happens and is detected all over the galaxy. Now, in this case, everybody, human or alien, with a tachyon detector will know something bad is going to happen -- somewhere in the galaxy.

Perhaps they even know when it is going to happen, but nobody knows where it will be. It is also possible they don't even know when the bad thing will happen. This is fiction and as an author you are allowed some poetic licence. They might even know the magnitude of the disaster.

"That tachyon precursor they detected last week, that was big enough to destroy a planet."

"As long as it's not our planet I don't care," he replied.

"But it could be," she said gravely.

This suggests there are three variables you can play around with depending whether the tachyon detector has enough information to know about (1) when the event will happen, (2) the magnitude of the event, and (3) none of the above. For this scenario to work there will never any information about where the event will happen. This must be a fixed point in the tachyon precursor.

By making it a galaxy wide detection allows everybody in the galaxy to be worried about an unspecified calamity coming somebody's way. Somewhere and somewhen in the galaxy and at ill-defined level.

The assumption has been made implicitly that the tachyon precursors are caused by an advanced type of weapon. Perhaps a typical piece of ancient alien weapons technology that can only be used once. That would add an element of surprise and uncertainty. This is unlike situations where weapons that generate tachyon precursors as a side-effect for their use are standard issue. On the other hand, this might not be the result of enemy action it could be a natural phenomenon.

Imagine regions of spacetime that undergo phase changes and emitting massive pulses of gamma radiation, high-energy particles, and the tachyon precursor. Some kind like an 'earthquake' happening in spacetime instead of a planet's mantle. There might be small pockets of false vacuum left over from the Big Bang. When these go pop tachyons are also emitted.

This does assume that the pockets of false vacuum aren't capable of destroying the universe. Although that might be one of the additional fears, that there could be a pocket of false vacuum powerful enough to take out the entire universe.

This answer sketches out some possibilities about a range of scenarios where tachyon precursors might be in play and how they might affect your characters. use this as a starting point to design the most effective scenario in your story. Good luck and have fun with it.

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Yes, you are missing something. See this post for summary and links.

Tachyons as described still won't allow FTL communication! This was known from the initial inventing of the idea, and is an example of not reading past the headline.

So no, “...localized tachyon disturbances are subluminal...”. The signal created using tachyons would not travel backward in time, and would arrive at the same time as the photon burst.

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  • $\begingroup$ I'm not trying to use them for FTL communication. Only asking if their detection would precede an event in the scenario I laid out. $\endgroup$ – alkah3st Jun 11 '16 at 9:57
  • $\begingroup$ Kind of looking for a "oh f--" we detected a tachyon precuror, then BAM unavoidable event happens, for dramatic tension. $\endgroup$ – alkah3st Jun 11 '16 at 10:00
  • $\begingroup$ (That is, perhaps no information is encoded locally about the event: the tachyon is just created along with the event, so detecting the tachyon wouldn't let you know about the event, just that something has already happened and you're about to be in trouble?) $\endgroup$ – alkah3st Jun 11 '16 at 10:04
  • $\begingroup$ "Only"... er, that would be communication. If you can't understand that, just imagine firing in morse code to tell someone (outside my light cone) when to sell short, not just when to duck. $\endgroup$ – JDługosz Jun 11 '16 at 10:08
  • $\begingroup$ Do you mean that detecting the tachyon (supposing it's created as a byproduct of some event) is the same as detecting photons emitted by the event? $\endgroup$ – alkah3st Jun 11 '16 at 10:10

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