## what does a tachyon observe?

So, I was thinking about tachyons, and I started wondering about what it would be like to be one. How would the universe look from a superluminal vantage point?

Tachyons (should they exist) travel faster than the speed of light. But according to the principle of relativity, a tachyon in a superluminal inertial frame must observe light travelling at speed c. However, if the tachyon is travelling away from a light source at a speed > c, then surely the light can never reach it? What about if it was travelling towards a light source at > c - how would the received light appear to the tachyon? What kind of time dilation/length contraction effects would be observable in the tachyon's frame?

Also, I've heard that tachyons travel backwards in time. Or rather, that there is always some subluminal observer that observes the tachyon travelling backwards in time. Does the reverse hold true: In the tachyon's frame of reference, does the observer appear to be moving backwards in time? Would a tachyon on a fly-by past the earth 'see' (hypothetically speaking) us going about our lives in reverse?

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Welcome to PF!

This is a cool question.

John Baez has a nice article on tachyons here: http://www.lightandmatter.com/cgi-bi...ivity#tachyons
Baez gives references to three older papers, which may also be helpful.

In your question, you talk about a tachyonic observer observing a beam of light. However, there are fundamental reasons (described in the Baez article) why tachyons can't be charged. (And one of the conclusions theorists seemed to agree on during the CERN neutrino fiasco was that tachyons also can't participate in the weak interaction.) So I don't think a tachyonic observer would be able to detect electromagnetic fields at all, except perhaps indirectly.

Baez also explains why the tachyonic telephone doesn't work. If a tachyonic physicist could observe our universe's non-FTL particles at event A, then travel to event B and "output" his memory of A back to us, then it would constitute a tachyonic telephone. Since this is impossible, it seems that either the tachyonic physicist cannot communicate with the non-FTL part of the universe, or else a tachyonic system of particles can't propagate at FTL velocities while maintaining any memory of its previous state. Since memory is required for consciousness, the latter possibility would imply that you can't make an FTL observer out of tachyons.

If tachyonic observers are possible, it's not obvious to me that such an observer would have to have a psychological or thermodynamic arrow of time that matched ours. Normally the second law of thermodynamics prevents two different systems from having opposite thermodynamic arrows of time. But this assumes that they interact. Tachyons can't interact with non-FTL particles through the electroweak interaction, so it's not clear to me that they would pick up an arrow of time from the low-entropy state of the early universe in the same way that non-FTL systems do.

If we ignore all these issues and try to use the Lorentz transformation to connect an FTL frame to a non-FTL frame, we get results that are imaginary numbers. We avoid letting tachyons have imaginary energy-momentum through assigning them an imaginary mass, but that doesn't help with the frame of reference. This suggests to me that it doesn't make sense to talk about their frame of reference, and such a result would seem pretty natural. It would be very similar to the idea that we can't have a frame of reference moving at exactly c, because the Lorentz transformation wouldn't be one-to-one, but there is a logical requirement that observers in different frames agree on whether or not two events are the same. (Either the arrow hit the target or it didn't. Either the pool balls collided or they didn't.)

 Quote by dynkindiagram However, if the tachyon is travelling away from a light source at a speed > c, then surely the light can never reach it?
You can draw a spacetime diagram in which a lightlike world-line intersects an FTL one. On the diagram, there is a region where we would describe the two world-lines as closing, and a region where we would describe them as separating. If we assume that FTL observers can exist, that they can somehow (indirectly) detect light rays, and that FTL observers agree with non-FTL ones on whether events are distinct, then the FTL observer will agree with us that there was an intersection between the two world-lines. The FTL observer, like us, will split the diagram into closing parts and receding parts, but I don't see any way to tell whether those labels would agree with ours or be flipped.

 Quote by dynkindiagram What about if it was travelling towards a light source at > c - how would the received light appear to the tachyon?
I suppose the FTL observer would have to describe the beam of light's (x,t) coordinates as imaginary numbers. That starts not making sense before we even get to the point of worrying about whether the beam's velocity appears to be c.

 Quote by dynkindiagram What kind of time dilation/length contraction effects would be observable in the tachyon's frame?
Formally, they'd be imaginary numbers. This suggests to me that FTL frames don't make sense.

 I am not convinced that tachyons do not exist. There was one experiment a long time ago where they created a water dome to try and detect them. The goal was to find an interaction between the electrons in the water being hit by them. But, knowing if they could be detected in this manner for sure was still unkown. And there is no way of knowing that tachyons where being generated near the location, if there should have even been any there at all even if they can exist. I think if the experiment was run again for a longer period of time that they could find them. It could also be possible that they escape into a higher dimension, since they end up taking this imaginary route or even a parallel universe. Or, it could mean that describing them may have to take a totally different approach to relativity. It could prove more difficult to set up because the distance the tachyon would travel would be greater than the distance the photon had traveled, then their is the question of does it even travel forward in time and distance at all? It could take tachyons being generated by some time in the future to even detect them. Current technology doesn't allow for particles to be sent faster than light, or some would say any future technology. But if some future technology did exist that could generate tachyons, it would most likely make them a lot easier to pick up.

## what does a tachyon observe?

I think the answer is simply that, since it doesn't exists a rest frame for a particle with $p^\mu p_\mu ≤ 0$, it doesn't make sense to ask what we would see in such a frame

Ilm

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 Quote by Ilmrak I think the answer is simply that, since it doesn't exists a rest frame for a particle with $p^\mu p_\mu ≤ 0$, it doesn't make sense to ask what we would see in such a frame Ilm
What is your reasoning for believing that there is no such frame? Is it different than what I gave in #2?

 Quote by bcrowell What is your reasoning for believing that there is no such frame? Is it different than what I gave in #2?
Well, the rest frame is defined as the one in which the particle has no spatial momentum, so for a space-like four-momentum there is not such a frame. No Lorentz transformation can take a space-like vector to a time-like vector (proof is quite trivial).

This obviously says nothing on the possibility of the existence of a tachyon.

Correct me if I'm wrong

edit: I don’t think using imaginary coordinates would make any difference here, even if they would make sense.

Ilm

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 Quote by Ilmrak Well, the rest frame is defined as the one in which the particle has no spatial momentum, so for a space-like four-momentum there is not such a frame. No Lorentz transformation can take a space-like vector to a time-like vector (proof is quite trivial).
I think it's clear that you can't use a Lorentz transformation to get from an FTL frame to a non-FTL frame. One way of seeing this is that you get imaginary coordinates. Another is your argument.

I don't think it necessarily implies directly that FTL frames don't exist, only that FTL and non-FTL frames could never be connected by the Lorentz transformation. However, this only leaves a couple of unsatisfactory options: (1) Maybe the FTL part of the universe doesn't interact with the non-FTL part, but then the FTL part would be undetectable, and its existence would be religion or philosophy, not science. (2) Maybe the two parts do interact, but Lorentz invariance is broken in some unspecified manner. This is pretty unsatisfactory, since the motivation for tachyons is to have FTL without violating Lorentz invariance.

 Quote by bcrowell I don't think it necessarily implies directly that FTL frames don't exist, only that FTL and non-FTL frames could never be connected by the Lorentz transformation. However, this only leaves a couple of unsatisfactory options: (1) Maybe the FTL part of the universe doesn't interact with the non-FTL part, but then the FTL part would be undetectable, and its existence would be religion or philosophy, not science. (2) Maybe the two parts do interact, but Lorentz invariance is broken in some unspecified manner. This is pretty unsatisfactory, since the motivation for tachyons is to have FTL without violating Lorentz invariance.
I'm failing in see why, if a tachyon would exist and would interact with non-tachyonic matter, option (2) would occur.

Could you please write the basic ideas for this?

Ilm

 Recognitions: Gold Member Science Advisor Staff Emeritus Geometrically, a tachyon in a universe with 3 spatial dimensions and 1 temporal dimension is the same thing as a tardyon in a universe with 1 spatial dimension and 3 temporal dimensions. I have no idea how physics would look with 1 spatial dimension and 3 temporal dimensions.

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 Quote by Ilmrak I'm failing in see why, if a tachyon would exist and would interact with non-tachyonic matter, option (2) would occur. Could you please write the basic ideas for this?
Suppose you're willing to propose some transformation that's not a Lorentz transformation that gets you back and forth between FTL frames and non-FTL frames. You can probably do this just fine, but it will violate Lorentz invariance, and without the constraint of Lorentz invariance it's probably non-unique.

 Quote by bcrowell Suppose you're willing to propose some transformation that's not a Lorentz transformation that gets you back and forth between FTL frames and non-FTL frames. You can probably do this just fine, but it will violate Lorentz invariance, and without the constraint of Lorentz invariance it's probably non-unique.
I agree.
What I don't understand is why, if tachyons would interact with non-tachyonic particles, would we need to introduce such a transformation as a symmetry of nature?

Ilm

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 Quote by Ilmrak I agree. What I don't understand is why, if tachyons would interact with non-tachyonic particles, would we need to introduce such a transformation as a symmetry of nature?
I don't think you need it just because they interact. You need it if they interact and you also want to define FTL frames of reference in which FTL observers can observe non-FTL particles. And I don't think this would mean introducing a symmetry, it would mean breaking a symmetry. It would break Lorentz invariance.

 Quote by Hurkyl Geometrically, a tachyon in a universe with 3 spatial dimensions and 1 temporal dimension is the same thing as a tardyon in a universe with 1 spatial dimension and 3 temporal dimensions.
Maybe I'm wrong, but this doesn't seem right to me. The metric has to be a symmetric matrix, and any transformation that carries out a change of frame has to be invertible. So Sylvester's law of inertia holds, and the signature of the metric is the same in both frames. All of this applies regardless of whether the transformation is a Lorentz transformation or something else. (Also, I don't think there is any meaningful way of distinguishing between 3+1 dimensions and 1+3. When we talk about a timelike dimension, we just mean the one whose contribution to the signature is the opposite of the others.)

 Quote by bcrowell I don't think you need it just because they interact. You need it if they interact and you also want to define FTL frames of reference in which FTL observers can observe non-FTL particles. And I don't think this would mean introducing a symmetry, it would mean breaking a symmetry. It would break Lorentz invariance.
Why should anyone want to replace Lorentz invariance with such a transformation?
Photons exist, interact with ordinary matter, but they have light-like four-momentum so no rest frame can be defined for them (preserving Lorentz invariance).
Still, no one is going to define a coordinate transformation such that a rest frame exist for photon to use instead of Lorentz transformations.
We can nonetheless observe photons ^^

(I think I misunderstood your point of view, maybe you simply tried to assume the OP question makes sense and then try to see what that would imply)

 Quote by bcrowell [...]Also, I don't think there is any meaningful way of distinguishing between 3+1 dimensions and 1+3. When we talk about a timelike dimension, we just mean the one whose contribution to the signature is the opposite of the others.
I think this is exactly what he did mean (or at least how I interpreted his words)

Ilm

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 Quote by Ilmrak Why should anyone want to replace Lorentz invariance with such a transformation?
The only reason would be if you wanted to define an FTL frame.

 Quote by Ilmrak Photons exist, interact with ordinary matter, but they have light-like four-momentum so no rest frame can be defined for them (preserving Lorentz invariance). Still, no one is going to define a coordinate transformation such that a rest frame exist for photon to use instead of Lorentz transformations. We can nonetheless observe photons ^^
The analogous question isn't whether we can observe photons, it's whether observers can exist who have a frame of reference moving at c.

The analogy breaks down somewhat because the mathematical misbehavior is qualitatively different in the two cases. When you put v=c in a Lorentz transformation, it's not one-to-one. When you put v>c in a Lorentz transformation, it gives imaginary results.

 Quote by bcrowell The only reason would be if you wanted to define an FTL frame.[...]
Ok, I did misunderstood you point of view, sorry. Now I see and absolutely agree with you

I think there are some point you are interpreting wrong.

Tachyons are defined to have four momenta $p^\mu p_\mu \equiv m^2< 0\,$* so if we assume $p^\nu \in ℝ$, then $|\vec p |^2 > E^2$, i.e. they travel faster then light.

I think the only reason in assuming they would interact weakly with ordinary matter is that they are never been observed.

The "negative direction" of time is considered in Standard Model (negative energy solution, or anti particle), but not related to FTL particle.
Actually every anti-particle could be interpreted as a particle going backwards in time, but they still have time-like four momentum, they don't travel FTL. From the point of view of an anti-particle (well defined because they have $m^2 >0$) we are all made of anti-matter ^^

*this means they are not massless, at least in the usual sense.

Ilm

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