# Your opinion on exceeding the speed of light means the ability to return to the past

by Sean Pan
Tags: opinion
 Mentor P: 12,014 So the graph uses a lorentz transformation correct? The tachyons appear to travel back in time because the lorentz transformation is set up as c as the speed limit? IE if 10c was the speed limit then the tachyons would not travel back in time? I'm just asking about the math aspect, not the real physical aspect.
P: 3,967
 Quote by Drakkith So the graph uses a lorentz transformation correct? The tachyons appear to travel back in time because the lorentz transformation is set up as c as the speed limit? IE if 10c was the speed limit then the tachyons would not travel back in time? I'm just asking about the math aspect, not the real physical aspect.
I think that is correct. If c is the speed limit and the speed of light was c/10 (in a given reference frame because the speed of light would no longer be invariant) then particles travelling at greater than the speed of light, but less than the maximum speed c, would not go back in time. Tachyons are generally described as particles that exceed the speed of light, but that is only accurate if we assume that the speed of light is exactly the maximum speed c (and that photons have no residual rest mass that is currently unmeasurable). If we define tachyons as particles that exceed the maximum possible speed limit c (the invariant speed), then all tachyons will appear to go backwards in time in certain reference frames.
 Mentor P: 12,014 Got it.
P: 255
 Quote by yuiop The prosecution claim that Cindy fired the first shot instantly killing poor Alex and then fired another shot into his dead body for good measure.
Looking at events E and F, although F seems to happen "first", it is the burn on Cindy's arm produced by the firing of the gun at E by the remains of Alex, so the effect precedes the cause.

Of course that's the point, the gun was fired by the corpse unless we insist that the timelike loop must be self-consistent.
P: 255
 Quote by Drakkith So the graph uses a lorentz transformation correct?
Correct, the transforms are built into the applet to calculate the coordinates of the lines and events as you move the slider.

 The tachyons appear to travel back in time because the lorentz transformation is set up as c as the speed limit? IE if 10c was the speed limit then the tachyons would not travel back in time? I'm just asking about the math aspect, not the real physical aspect.
The transforms are actually set up on the screen with c=1, if you draw a line and then move the slider, it limits at one horizontal unit per vertical unit. The word "tachyon" means going faster than that limiting value, there aren't two different quantities being represented either in the applet or the math or the real world physics.
Mentor
P: 12,014
 Quote by GeorgeDishman The transforms are actually set up on the screen with c=1, if you draw a line and then move the slider, it limits at one horizontal unit per vertical unit. The word "tachyon" means going faster than that limiting value, there aren't two different quantities being represented either in the applet or the math or the real world physics.
Ok, so if the lorentz transformation is correct and the limiting speed is c, then any hypothetical tachyon would go back in time in certain frames? Thus there cannot be faster than light travel?
Emeritus
PF Gold
P: 9,543
 Quote by Sean Pan Thanks for all of you above. When treating FTL particles, does Lorentz Boost still make sense? Anyone have studied that?
Yes, they do, but keep in mind that every Lorentz transformation is associated with a velocity of magnitude less than c. So there are no Lorentz transformations that boost to a frame in which a FTL particle is at rest.

There are some really counterintuitive things about FTL particles. In particular, if I measure a particle going to the right at 10c, then someone going to the right at 0.5c will measure the particle going faster than 10c, not slower.

 Quote by Drakkith Ok, so if the lorentz transformation is correct and the limiting speed is c, then any hypothetical tachyon would go back in time in certain frames?
Yes.

 Quote by Drakkith Thus there cannot be faster than light travel?
No. It's not obvious that this implies that tachyons don't exist. You need a more sophisticated argument that involves two tachyon transmitters (like the one I linked to in post #6), and even then the conclusion is just that either FTL particles don't exist, or they have some really counterintuitive properties. (Much more counterintuitive than the weird thing I mentioned in this post).
P: 1,162
 Quote by Fredrik There are some really counterintuitive things about FTL particles. In particular, if I measure a particle going to the right at 10c, then someone going to the right at 0.5c will measure the particle going faster than 10c, not slower.
Hi Fredrik
Teleportation came up in another similar thread.
Instantaneous translation between points. Absolute simultaneity of events at separate locations.

Assuming teleportation to a distant location in the same frame; How do you calculate the proper time at the point of arrival?? Given t=0 at point of departure.

BTW Congrats.
P: 3,967
 Quote by GeorgeDishman Looking at events E and F, although F seems to happen "first", it is the burn on Cindy's arm produced by the firing of the gun at E by the remains of Alex, so the effect precedes the cause. Of course that's the point, the gun was fired by the corpse unless we insist that the timelike loop must be self-consistent.
Well it is not clear that Cindy's arm would be burnt as we go forward in time again, because Alex, perhaps along with his tachyon gun, has been destroyed so he is not in a position to do anything. In this case it looks like "evil" Alex is killed in a single unprovoked attack by "nice" Cindy.

As mentioned in #18, the first time around the loop, Alex is alive between events E and H and the second time around he is dead between those same two events. This requirement for different histories probably requires the concept of parallel universes to make sense. Can we say FTL interaction not only implies travelling backward in time, but also implies parallel universes?
 P: 3,967 An energy paradox involving time travel. Let us say we have one very large uncharged lead battery and a small solar panel to charge the battery. We follow these steps: 1)We connect the solar panel to the uncharged battery. 2)We put a note on the uncharged battery saying "When this battery is charged please send it back to time 1" 3)Our future self obliges and sends the charged battery back in time. The first time a battery is sent back fully charged, we still have the initial uncharged battery. the second time a charged battery is sent back, we have the initial uncharged battery and two fully charged batteries. Repeat as often as required. We could in principle end up with more batteries than there is lead on the Earth to make them with and with a greater energy store than the total energy impinging on the Earth from the Sun between times 1 and 3.
P: 10
 Quote by GeorgeDishman Have we adequately clarified the connection between the two in your original question?
Thanks a lot and I have a clearer understand of this problem now. I actually knew that if there were FTL particles, causality would be violated. My opinion is that FTL observers can never actually traveling backwards. And thanks for all above that I now understand what other physicsts mean when they are saying 'returing to the past', it's just an illusion caused by the limited speed of information-carrier. But still, to an FTL particle, γ is no longer real, so I think lorentz boost can't be applied directly here. For example, how can time dilation effect being discussed here?
P: 255
 Quote by Drakkith Ok, so if the lorentz transformation is correct and the limiting speed is c, then any hypothetical tachyon would go back in time in certain frames? Thus there cannot be faster than light travel?
Exactly, FTL and time travel in this sense are synonymous. Whether that makes them impossible or not is another question.
P: 255
 Quote by yuiop Well it is not clear that Cindy's arm would be burnt as we go forward in time again, because Alex, perhaps along with his tachyon gun, has been destroyed so he is not in a position to do anything. In this case it looks like "evil" Alex is killed in a single unprovoked attack by "nice" Cindy. As mentioned in #18, the first time around the loop, Alex is alive between events E and H and the second time around he is dead between those same two events.
Hence he didn't shoot, Cindy didn't respond and the third time round Albert is alive. What you have built is the same as an electrical logic circuit with an inverter in a feedback loop, it creates an oscillator switching between two states.

 This requirement for different histories probably requires the concept of parallel universes to make sense.
Possibly, that's one way out in sci-fi stories. The other is "cosmic censure" which would require the history round the loop to be consistent, for example forcing a misfire of one of the guns or a non-fatal injury to Albert allowing him to fire (though he could die immediately thereafter).

If you think of the quantum wavefunction going round the loop and Young's Slits as an analogy, we could surmise that non-consistent outcomes lie in the dark bands of destructive interference when calculating their probabilities using the sum over histories method.

 Can we say FTL interaction not only implies travelling backward in time, but also implies parallel universes?
Personally, I think they are independent. Albert's shot has an infinite number of points on which it might graze Cindy's arm so alone generates an infinite number of universes and the same is true of every other particle interaction throughout the period. IMHO, any one universe either sees a paradox or a consistent outcome hence you need something more than parallel universes which acts in each. That said, it won't stop the sci-fi authors using it though ;-)
P: 255
 Quote by Sean Pan Thanks a lot and I have a clearer understand of this problem now. I actually knew that if there were FTL particles, causality would be violated. My opinion is that FTL observers can never actually traveling backwards.
That is an opinion most share as you can see from the replies.

 And thanks for all above that I now understand what other physicsts mean when they are saying 'returing to the past', it's just an illusion caused by the limited speed of information-carrier.
No, that is exactly the opposite of what everyone has said here! What Cindy or Albert can see is on a cone into the past at 45 degrees to the vertical from each event. They never see the tachyons coming but the shots still travel into the past in the frames of both. Albert is killed at event H before he even picks up his gun to start the fight at event E.

It sounds as though you are disregarding what has been said so that you can believe in FTL without dealing with its consequences.

 But still, to an FTL particle, γ is no longer real, so I think lorentz boost can't be applied directly here. For example, how can time dilation effect being discussed here?
The time dilation factor simply becomes negative. A Lorentz boost is nothing more than a geometric rotation so still applies perfectly to tachyonic particles. For example it would have applied to the reported FTL neutrinos last year (which turned out to be a faulty connection to a clock).
Emeritus
PF Gold
P: 9,543
 Quote by Austin0 Hi Fredrik Teleportation came up in another similar thread. Instantaneous translation between points. Absolute simultaneity of events at separate locations. Assuming teleportation to a distant location in the same frame; How do you calculate the proper time at the point of arrival?? Given t=0 at point of departure. BTW Congrats.
Hi Austin0.

I don't think I understand the question. Since you mentioned absolute simultaneity, you seem to be asking about instantaneous messages in Galilean spacetime. If there's absolute simultaneity, the question of "simultaneous in what coordinate system?" doesn't even arise. So if t=0 at the departure, then isn't t=0 at the arival too, by definition of "instantaneous".

"Proper time" is a property of a curve in Minkowski spacetime, not a point. The concept of proper time isn't needed in Galilean spacetime. (We would have to define it as the coordinate time difference between the endpoints).

The post I linked to in #6 deals with instantaneous messages in Minkowski spacetime.
PF Gold
P: 1,850
 Quote by Austin0 Teleportation came up in another similar thread. Instantaneous translation between points. Absolute simultaneity of events at separate locations. Assuming teleportation to a distant location in the same frame; How do you calculate the proper time at the point of arrival?? Given t=0 at point of departure.
Within the theory of relativity, if two events are simultaneous in some frame then the proper time between those events is an undefined concept.
P: 1,162
 Quote by DrGreg Within the theory of relativity, if two events are simultaneous in some frame then the proper time between those events is an undefined concept.
Hi DrGreg I was using the term proper time in it's common usage as simply meaning the clock reading at the location and time of arrival, not as a proper time interval. Sorry for my possible misuse.
Does this clarify my question??

Thanks
P: 1,162
 Quote by Fredrik Hi Austin0. I don't think I understand the question. Since you mentioned absolute simultaneity, you seem to be asking about instantaneous messages in Galilean spacetime. If there's absolute simultaneity, the question of "simultaneous in what coordinate system?" doesn't even arise. So if t=0 at the departure, then isn't t=0 at the arival too, by definition of "instantaneous". "Proper time" is a property of a curve in Minkowski spacetime, not a point. The concept of proper time isn't needed in Galilean spacetime. (We would have to define it as the coordinate time difference between the endpoints). The post I linked to in #6 deals with instantaneous messages in Minkowski spacetime.
Hi The question is purely in the context of SR It seems to me that the term proper time is frequently used to mean the observed reading on the local clock and not necessarily an interval. If this is technically incorrect then it's commonly incorrect and I will curb the habit.

So the absolutely instantaneous definition is applied to the phenomenon of teleportation.

So are you saying here that in a SR context the arrival time would be t=0 ??

Thanks

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