# Tachyons travel backward in time?

by CJames
Tags: backward, tachyons, time, travel
P: 8,470
 Quote by peter0302 Ok I finally figured out how to say what I mean mathematically. Nothing goes backwards in time because for every "t" coordinate, there can be one and only one observed value of "x,y,z".
What does that have to do with "moving backwards in time" or "moving forwards in time"? If I hang a string from my ceiling and then use an x-y-z grid to plot spatial coordinates of things in my room, it may be that for ever "z" coordinate, there is only one observed value of "x,y"...does that tell me whether the string is "ascending from the floor to the ceiling" or "descending from the ceiling to the floor"?
P: 4,214
 Quote by peter0302 Nothing goes backwards in time because for every "t" coordinate, there can be one and only one observed value of "x,y,z".
This is not preventing anything from going back in time. It just prevents from changing the direction in time. Move forward and then back in time doesn't work.

See my thoughts about distinguishing objects moving in different directions in time:
 P: 869 A.T. is right. There may be no true distinction between "forwards" and "backwards" but the point is that everything is either moving in the same direction in time or not. JesseM, you're clearly going to argue with me regardless of what I say, but my point has been made.
P: 8,470
 Quote by peter0302 A.T. is right. There may be no true distinction between "forwards" and "backwards" but the point is that everything is either moving in the same direction in time or not.
If can't even define "moving" then the statement "everything is either moving in the same direction in time or not" makes no sense.
 Quote by peter0302 JesseM, you're clearly going to argue with me regardless of what I say, but my point has been made.
I'm only going to argue with statements that seem to depend on poorly-thought out intuitions and fuzzily-defined verbal terms, if you can translate your statements into something that has a clear meaning in terms of mathematical theories of physics I won't have a problem with it. For example, the notion of "closed timelike curves" in relativity is certainly a well-defined one, if that's all you mean by "moving backwards in time" I have no objection, but in this case you should stop claiming that antiparticles move back in time, because their worldlines are not closed timelike curves.
P: 869
 I'm only going to argue with statements that seem to depend on poorly-thought out intuitions and fuzzily-defined verbal terms, if you can translate your statements into something that has a clear meaning in terms of mathematical theories of physics
I just did. Going in a single direction in time means that for all observers, for any value of t there is exactly one (x,y,z) where a particle will be observed. If antiparticles moved "backwards in time" then this statement would not be true. That is my definition, very well defined mathematically.

And I am not claiming anything. I'm trying to have a conversation that's getting derailed because you are claiming ignorance of virtually every word in the English language.
P: 8,470
 Quote by peter0302 I just did. Going in a single direction in time means that for all observers, for any value of t there is exactly one (x,y,z) where a particle will be observed. If antiparticles moved "backwards in time" then this statement would not be true. That is my definition, very well defined mathematically.
This isn't well-defined unless you give a physical procedure for determining if an antiparticle and a particle which exist at different positions at a single time are "really" the same particle or two different particles.
 Quote by peter0302 And I am not claiming anything. I'm trying to have a conversation that's getting derailed because you are claiming ignorance of virtually every word in the English language.
English is not the language of physics--ordinary-language statements about physics are only meaningful if there is a transparent way to translate them into precise mathematical ones.
 P: 1,060 Good morning Peter0302. Just to clarify things for me, i take it that for a particle to be at a certain spacetime coordinate (event) more than once it must "travel back in time". If we were to alllow the possibility of a particle's ability to "travel back in time" surely it would have to be one and the same particle to satisfy this requirement of being at the same spacetime coordinate more than once. If it were an anti particle "travelling backwards in time" it would not be the SAME particle at the same spacetime coordinate more than once. Matheinste.
P: 869
 Quote by matheinste Good morning Peter0302. Just to clarify things for me, i take it that for a particle to be at a certain spacetime coordinate (event) more than once it must "travel back in time". If we were to alllow the possibility of a particle's ability to "travel back in time" surely it would have to be one and the same particle to satisfy this requirement of being at the same spacetime coordinate more than once. If it were an anti particle "travelling backwards in time" it would not be the SAME particle at the same spacetime coordinate more than once. Matheinste.
I don't know if that's true. Perhaps when a particle and antiparticle seem to collide, in fact the "antiparticle" is the original particle knocked backwards in time.

Another way to tell if antiparticles move forward in time or not would be if they were gravitationally repelled by normal matter. Anyone know if this has been tested or can be tested?
P: 8,470
 Quote by peter0302 I don't know if that's true. Perhaps when a particle and antiparticle seem to collide, in fact the "antiparticle" is the original particle knocked backwards in time.
But unless these two hypotheses lead to different predictions, this is not a question that can be addressed by physics, any more than the question of which "interpretation" of quantum mechanics is correct.
 Quote by peter0302 Another way to tell if antiparticles move forward in time or not would be if they were gravitationally repelled by normal matter. Anyone know if this has been tested or can be tested?
What theoretical argument leads you to believe that something "moving backward in time" would be gravitationally repelled by normal matter? In any case, see here and here for some info on the gravitational properties of antimatter.
 P: 6 Time dilation and tachyon speed are two different things. That negative time may in reality just mean that tachyon gets from start to finish faster than information. And that is a general problem in theory of relativity i think. It's not well defined in terms when do we think about the problem as physical matter and when in terms of information that we receive. When they sort that out, everything will be much clearer. I think someday in future it will come as a surprise to scientists that relativity can be explained with Mach's principle and Doppler effect.
P: 8,470
 Quote by Xantos Time dilation and tachyon speed are two different things. That negative time may in reality just mean that tachyon gets from start to finish faster than information.
The idea that tachyons could be used to communicate information backwards in time has absolutely nothing to do with the time dilation equation (tachyons don't have their own reference frame at all), it's just a product of the relativity of simultaneity (which implies that if two events have a spacelike separation, then different frames must disagree on the order of the two events) and the fact that the laws of physics are supposed to work the same way in every inertial frame.
P: 6
 Quote by JesseM The idea that tachyons could be used to communicate information backwards in time has absolutely nothing to do with the time dilation equation (tachyons don't have their own reference frame at all), it's just a product of the relativity of simultaneity (which implies that if two events have a spacelike separation, then different frames must disagree on the order of the two events) and the fact that the laws of physics are supposed to work the same way in every inertial frame.
STL travel; C ; FTL travel

When you're approaching C you're making information so dense that it radiates in terms of Cerenkov radiation and traveling as fast as infromation about yourself relative to the observer towards which you're flying. So it would appear to the observer somehow like this:

Let's assume that traveler and the observer are so far apart that relativistic delay of information is observed.
Let's say that you slowly accelerate to 0.5c and then suddenly to 1c. What observer on the other side would see is a very small ship instantaniously followed by your physical self. It is very likely that he would also pick up on sensors a lot of Cerenkov radiation.

Now let's assume that we developed technology that allows us to go FTL. You accelerate to 0.5c, then to 1c (a lot of gravitational disturbance occurs at this point) and then you engage FTL drive at that point you have negative time (as seen in the equation) - you're traveling faster than the information about yourself which means you will be at the finish faster than your information (that is not backwards in time). What the observer on the finish line would see is this: you suddenly appear from nowhere followed by your own image.
P: 8,470
 Quote by Xantos Let's say that you slowly accelerate to 0.5c and then suddenly to 1c.
Impossible according to relativity, even if you allow tachyons. Accelerating from slower-than-c to c would require infinite energy; decelerating a tachyon down to c would be equally impossible.
 Quote by Xantos Now let's assume that we developed technology that allows us to go FTL. You accelerate to 0.5c, then to 1c (a lot of gravitational disturbance occurs at this point) and then you engage FTL drive at that point you have negative time (as seen in the equation)
What equation are you talking about? Again, the time dilation equation simply cannot be used for ftl velocities in relativity, because a faster-than-light reference frame would violate the postulate that the laws of physics should be the same in all frames (and if you try plugging in a v>c into the time dilation equation, you get an imaginary number, not a negative one).
 Quote by Xantos you're traveling faster than the information about yourself which means you will be at the finish faster than your information (that is not backwards in time). What the observer on the finish line would see is this: you suddenly appear from nowhere followed by your own image.
This is true, but it has nothing to do with the relativity of simultaneity or the claim that tachyons could be use to send information backwards in time. Do you understand that the question of when events happen in a given reference frame is totally different from the question of when observers see signals from events--that if I see the light from an event 10 light years away in 2010 (as measured by my clocks and rulers), and then see the light from an event 20 light years away in 2020, this means that the events actually happened simultaneously in my frame? If not, you need to actually learn the basics of SR before you jump to conclusions about what physicists are saying.
P: 6
 Quote by JesseM Impossible according to relativity, even if you allow tachyons. Accelerating from slower-than-c to c would require infinite energy; decelerating a tachyon down to c would be equally impossible.
Something similar was said when they were testing supersonic flight. And guess what, they broke the barrier. Now it's time to break the next barrier. With FTL I think we'll have to develope technology that allows masking the mass (or gravity). When you acomplish that, you need very little energy to travel FTL.

 Quote by JesseM What equation are you talking about? Again, the time dilation equation simply cannot be used for ftl velocities in relativity, because a faster-than-light reference frame would violate the postulate that the laws of physics should be the same in all frames (and if you try plugging in a v>c into the time dilation equation, you get an imaginary number, not a negative one).
I was referring to the equation on the first page and it was almost 3 in the morning. And even with this mistake that I made, I'm 99% sure that those equations mean exactly that - information gets to the finish line behind physical object. So it would appear that event happened before it even started. And that's not sending info into the past.

 Quote by JesseM This is true, but it has nothing to do with the relativity of simultaneity or the claim that tachyons could be use to send information backwards in time.
You cannot send information backwards in time. Impossibility, even with tachyons.

 Quote by JesseM Do you understand that the question of when events happen in a given reference frame is totally different from the question of when observers see signals from events--that if I see the light from an event 10 light years away in 2010 (as measured by my clocks and rulers), and then see the light from an event 20 light years away in 2020, this means that the events actually happened simultaneously in my frame? If not, you need to actually learn the basics of SR before you jump to conclusions about what physicists are saying.
Yes, that is perfectly undestandable and logical because information spreads with a constant C. Those two events happened at the same time during 2000.
P: 8,470
 Quote by Xantos Something similar was said when they were testing supersonic flight. And guess what, they broke the barrier.
No it wasn't--no one said that fundamental laws of physics made it impossible, or that it would take an infinite amount of energy.
 Quote by Xantos I was referring to the equation on the first page and it was almost 3 in the morning. And even with this mistake that I made, I'm 99% sure that those equations mean exactly that - information gets to the finish line behind physical object. So it would appear that event happened before it even started. And that's not sending info into the past.
Well, you're completely wrong. The concept of when things happen in relativity is based on local measurements, so information delays are irrelevant. For example, I could set up a rod at rest in my reference frame and that's 9 light-seconds long in this frame, and on each end attach a clock which is also at rest, and with the two clocks being synchronized in my frame. Now suppose a clock moves past the rod at 0.6c to the right in my frame, and suppose we have two photographers, each standing around in the immediate vicinity of each end of the rod, who each take a picture of the moving clock at the moment it passes the clock attached to their end of the rod (since the pictures are taken right next to this event--a local measurement--light speed delays are negligible). If the first picture shows the clock attached to the left end of the rod reading t=0 seconds and the moving clock also reading $$\tau = 0$$ seconds, then since the two clocks are synchronized in my frame, the second picture must show the clock on the right end of the rod reading t=15 seconds when the moving clock passes it (since the rod is 9 light-seconds long and the moving clock moves at 0.6c, and 9/0.6 = 15). However, this second picture will only show the moving clock reading $$\tau = 12$$ seconds as it passes the clock at the right end of the rod. So you can see that in my frame, the moving clock is slowed down, and this can be observed with local measurements where there is no issue with delays between when the events happen and when I see them.
 Quote by Xantos You cannot send information backwards in time. Impossibility, even with tachyons.
If it's possible to send signals faster than light and relativity is correct that the laws of physics work the same way in every inertial frame, then it's just a logical consequence of this must be possible to send information backwards in time. Of course you could reject the notion of FTL signals, or reject relativity's claim that the laws of physics are the same in all inertial frames. But you simply can't accept both of these and reject sending information backwards in time.

Do you understand what is meant by "the relativity of simultaneity"? Do you understand that if two events happen at different spatial coordinates but the same time-coordinate in one frame, then they happen at different time-coordinates in other frames? Do you understand that this means that for any exchange of FTL signals, different frames must disagree on whether the event of the signal being received happened at an earlier time-coordinate or a later time-coordinate than the event of it being sent?
 Quote by JesseM Do you understand that the question of when events happen in a given reference frame is totally different from the question of when observers see signals from events--that if I see the light from an event 10 light years away in 2010 (as measured by my clocks and rulers), and then see the light from an event 20 light years away in 2020, this means that the events actually happened simultaneously in my frame? If not, you need to actually learn the basics of SR before you jump to conclusions about what physicists are saying.
 Quote by Xantos Yes, that is perfectly undestandable and logical because information spreads with a constant C. Those two events happened at the same time during 2000.
So then you understand that in this example, the time coordinate I assign to the two events--t=2000 in both cases--already factors out the lags in when I saw the events due to the speed of light. The same would be true if I assigned coordinates to events using local measurements by people at different positions in space carrying clocks which were synchronized in my frame (note that because of the relativity of simultaneity, different frames disagree on whether two clocks at different locations are 'synchronized' or not). And time dilation is purely a function of the time coordinates I assign to different ticks of a moving clock, not how fast I see a clock ticking...in my example above, if clock moving at 0.6c reads $$\tau = 0$$ seconds at time-coordinate t=0 seconds in my frame, then at time-coordinate t=15 seconds in my frame, the moving clock reads only $$\tau = 12$$ seconds. So, it's ticking at 0.8 the normal rate in my frame, as predicted by the time dilation equation $$\Delta t = \frac{\Delta \tau}{\sqrt{1 - v^2/c^2}}$$
P: 1,544
 Quote by Xantos So it would appear that event happened before it even started. And that's not sending info into the past. You cannot send information backwards in time. Impossibility, even with tachyons.
I think I understand what you are driving at, but you are stating part of your case poorly.

Example; assume you can detect and use the information from a tachyon passing by at twice the speed of light in our reference frame and know that it came from a Gamma Burst Event some 10 light years away.

That would signal us that 5 years ago a GBE occurred and we won’t see the light form it for 5 more years. Such an early warning signal, not only does not send information into the past, it does not mean “it would appear that event happened before it even started”.
It would only signal us 5 years in advance of receiving a message with normal light tell us about a GBE that happened 10 years ago. Even with light we know the information is 10 years old, receiving the information 5 years early does not make it appear the event happened before it started.

Also from standard Doppler effects already useful within SR we would know tackyons traveling at such a speed in our reference frame view would not be seen to travel at that same speed any other reference frame. Every frame would see a different speed, meaning no one speed as every frame should see a variety of different speeds for them, so measuring their speed would also be required to get usful info.

So I don’t see where SR would require tachyons cause even the appearance of “backwards time”. However, understanding that does not improve the chance that such a thing traveling faster than light or gravity might exist anywhere but in our imaginations.
P: 8,470
 Quote by RandallB So I don’t see where SR would require tachyons cause even the appearance of “backwards time”.
It's because of the relativity of simultaneity in different frames; if the tachyon signal moves FTL in our frame, but the event of it being emitted still happens before the event of it being received, there must be some other frame where the event of it being received happened before it was emitted. For example, suppose in my frame the tachyon is moving at 10c in the +x direction, so if it's emitted at position x=0 light-seconds at time coordinate t=0 seconds, then at time coordinate t=10 seconds it will have reached the position x=100 light-seconds in my frame. Now consider the frame of an observer who is moving at 0.6c relative to me in the +x direction of my coordinate system, using his own coordinate system x' and t', with his x'-axis parallel to my x-axis, and the zero position on each of our coordinate systems coinciding at a coordinate time of t = t' = 0. In this case, our coordinates will be related by the Lorentz transformation:

$$x' = \gamma * (x - vt)$$
$$t' = \gamma * (t - vx/c^2)$$
$$\gamma = \frac{1}{\sqrt{1 - v^2/c^2}}$$

And here with v=0.6c, $$\gamma$$ is 1.25. If we plug in the coordinates x=0, t=0 in my frame (the event of the tachyon emission), the Lorentz transformation tells us that in the other observer's frame this event happened at x'=0, t'=0. But if we plug in coordinates x=100, t=10 (the event of the tachyon being received), in this observer's coordinate system this happens at the coordinates:

x' = 1.25 * (100 - 0.6*10) = 1.25 * 94 = 117.5
t' = 1.25 * (10 - 0.6*100) = 1.25 * -50 = -62.5

So you can see that in this coordinate system, the tachyon was received a full 62.5 seconds before it was emitted. It's just part of the way the Lorentz transform works that if you pick two events with a spacelike separation (like two events on the worldline of a tachyon), you'll always be able to find a pair of frames which disagree about which of the two events happened earlier than the other, as well as some frame where the events happened simultaneously.

P: 1,544
 Quote by JesseM It's because of the relativity of simultaneity in different frames; if the tachyon signal moves FTL in our frame, but the event of it being emitted still happens before the event of it being received, ….
Nonsense
That is a gross misunderstanding of SR simultaneity!
First you establish a preferred frame with t=10 at x= 6, 10, 100 all happening simultaneously.
When you calc t’= -62.5 at x=100(x’= 117.5) for one of those events; no problem with that.
But then you assume that event is simultaneous with the x’=0 starting point at t’= -62.5 (near x= -47) only because the t’ times are the same!
Not only does “relativity of simultaneity” not say those events are simultaneous, SR specifically states you cannot consider them as simultaneous events.

Orthodox SR does not even acknowledge the t= 0 times at x = 6, 10 and 100 as being simultaneous! And also says a preferred frame should not be assumed.
I understand Astrophysics does use a preferred reference frame based on CMBR. I have no real problem with that. But what will never work is using two different frames as preferred at the same time as you are doing here. Once you define one preferred frame all other frames must be considered inaccurate for establishing causality, you cannot have two preferred frames.

Your thought experiment will need to reroute the tachyon back into local proximity with the starting point in one or both frames, and attempt to show it returning before it started. And that is not going to happen in any thought experiment that follows the SR rules rationally.

 Again, check out the page with the minkowski diagrams illustrating such a situation that I linked to near the beginning of this thread.
Another poor reference for describing SR events, there are lots of them;
“ …superluminal transmission. Alice transmits from event P and the signal is instantaneously received by Bob at event Q. Alice and Bob are at rest relative to each other. ”
FTL and instantaneous are not the same thing, NO where in SR does it expect FTL to mean instantaneous. It is a bad starting assumption that renders the entire example pointless and false.

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