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Hi! I was wondering: From my understanding of physics and relativity, a person travelling very close to the speed of light would see the universe and history pass at an incredibly fast rate, and if one were to move as fast as a photon, time would essentially stop for them (from our perspective) and they would see the universe and history pass in an instant. But I was wondering, If from a photon's point of view all of time passes in an instant, then what would a tachyon see from it's perspective? I know tachyons are theoretical particles that travel faster than light, but from their perspective, would they see time pass in reverse or in some other weird form? Thanks for your help! Any math or graphs, equations, etc... that may support this idea that you guys can provide will also be greatly appreciated.
Question Date: 2010-01-22
Answer 1:

An observer moving close to the speed of light would see the rest of the universe having time moving slower, but the distances of everything would be contracted so that their "history" would proceed at a rapid rate anyway. The formulas are as follows:

Let Y = (1 - v2/c2)(1/2)

This expression shows up a lot in relativity, so just substitute it.
v = velocity of the object being observed relative to the observer,
and c is the speed of light.

Note that as v approaches c, Y goes to zero - this is the reason behind most of the counterintuitive things in relativity.

L = lY, where L is the length of an object in its own reference frame, and l is the length it has that the observer measures.

T = tY, where T is the time between two events in their own reference frame, and t is what the moving observer measures.

M = m/Y, where m is the rest mass of the object, and M is the mass measured by the relativistic observer.

p = Mv, where p is the momentum of an object (use M from the previous equation).

Now, tachyons:
If v > c, then Y above will be imaginary (you're taking the square root of a negative number). This means that a tachyon would have to have an imaginary rest mass, imaginary dimensions, and even an imaginary position in space and time. An imaginary particle would conversely see the "real" universe as imaginary. I find it difficult to imagine how adding an imaginary set of dimensions to the universe would be observable, so a being composed of tachyons would have similar problems observing us as we would then.

Of course, there are no data that indicate that tachyons exist at all, so anything we can say about them is pure speculation.

Answer 2:

Tachyons are indeed theoretical particles, so it can be very difficult to talk about what a tachyon would experience. The best we can tell, tachyons would see time passing by backwards, as you suspected. There isn't much wecan say other than that - although they exist theoretically, tachyons haven't been found in nature, and there's no indication that they actually exist in reality.

In fact, there are many things that would go awry in our world if tachyons as we think of them exist. If an object was allowed to travel faster than the speed of light, then it would be able to go back in time! This creates all sorts of problems, because if an object travels back in time, it could then interact with itself before it left to go back in time, creating all sorts of logical inconsistencies.

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