# Faster than light and time travel

## Main Question or Discussion Point

Why does faster than light travel or propagation mean time travel backwards to violate causality?

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DrGreg
Gold Member

I see,so if i were in a spaceship and were to faster than light,i would travel backwards in time to the past?

HallsofIvy
Homework Helper
That is one possible interpretation of the Lorenz time contraction formula where, if it were possible for v to be larger than c, then t becomes an imaginary number. A more reasonable interpretation would be that this is yet another reason why one can't go faster than c.

Why does faster than light travel or propagation mean time travel backwards to violate causality?
Because of the relativity of simultaneity. If you are traveling faster than light then there are frames of reference where you arrive before you start and that's a violation of causality.

Because of the relativity of simultaneity. If you are traveling faster than light then there are frames of reference where you arrive before you start and that's a violation of causality.
is there anything is physics that travels simultaneity?

That is one possible interpretation of the Lorenz time contraction formula where, if it were possible for v to be larger than c, then t becomes an imaginary number. A more reasonable interpretation would be that this is yet another reason why one can't go faster than c.
how come entanglement happens faster than light?

how come entanglement happens faster than light?
Because nothing travels and no information is transmitted.

jtbell
Mentor
is there anything is physics that travels simultaneity?
What do you mean by "travels simultaneity?"

Because nothing travels and no information is transmitted.
Quantum entanglement means, basically:

If 1=A, 2=B.
If 1=B, 2=A.

But what happens is that, when 1 is made to equal A, then 2 equals B.

Or does changing the value of A result in decoherence? I think so, I would just like a confirmation.

What do you mean by "travels simultaneity?"
what i meant is are there things or situation in physics that travel simultainisly?

Because nothing travels and no information is transmitted.
ok let me make something clear sorry i didnt,Faster than light travel has to cope with time travel because in one frame it will be seen that the traveler traveled backwards in time,does entanglement deal with faster than light time travel backwards,meanung will some one see entanglement happen backwards in time?

I've read that if group velocity exceeds speed of light, t2- t1 to some reference frame would be reversed to t1- t2, which means to some reference frame there is violation of causality. According to the principle of relativity, no frame is preferred. Therefore this argument is valid. But I forgot how to derive that...

what about lijun wang's expierament done in 2000,does that deal with time travel?

If I was the one about to go faster than c, I would be less concerned about time and causality, much more concerned about length contraction...

Length to "0", then
Through "0" into what?
Negative length?
Imaginary length?
Hyper-real length?

Or length expansion beyond c...

Even if within my frame I measured nothing unusual, how much of "me" continues to function after that transition through true zero length?

Maybe this suggests why nothing should pass through c from either the slower or faster side?

also i forgot to add,can something travel backwards in time and not violate causality?

cause cannot preceed event so to answer your question the answer is no any backward movement in time can only violate causality

also i forgot to add,can something travel backwards in time and not violate causality?
Well, you should look at the definition of causality.

"Causality is the relationship between an event (the cause) and a second event (the effect), where the second event is understood as a consequence of the first."

based on this, if you travel backward, what would happen is that you have travelled back before you start travelling. So I think this is the violation

I know that group velocity can exceed the speed of light,but does it travel backwards in time since it travels faster than light?

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ok,i was doing some research and it seems that the group velocity can exceed the speed of light and travel backwards in time it just can't send information,is this correct? can something travel faster than light without it traveling backwards in time in any frame?

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Thus far nothing has been shown to break the laws of causality as the arguments have always been the no information travels faster than c, with the experiments that have shown that light can appear to travel faster than c giving effect before cause. I've tried numerous times to get a clear cut answer to exactly what they term as no information so I also am curious concerning the answer to your post. The way I've been thinking of FTL effects is similar to GR's observer descriptions. Say in the the case of no time ina Blackhole. To the observer it appears that there is no time, but to matter and energy at that point time behaves normally. This is how I think they mean by backwards in time. That to the observer its effects happen prior to cause, but to the lightbeam its still moving normally. I'm fairly certain thats not quite correct and that there is more to it.
I would be interested in a better understanding of the backward in time description used by Physicists in that regard.. Also recongnizing that due to causality many top notch physicists feel that time travel to a time prior to cause is impossible.

Would it be possible to use quantum entanglement to communicate at any usable distance?

Would it be possible to use quantum entanglement to communicate at any usable distance?
In a similar manner to how two computers talk to each other then yes, Provided they can get the entanglement stable enough to be practical. It could be a usable wireless communication without the use of light or radio. There was a recent article concerning a diode that they fashioned that can generate several million entangled particles per second. I can't recall the exact amount, However certain types of radiation tends to break the entanglement down rapidly, Again its been a few months since I read that article so cannot name the particulars on it. To the best of my knowledge the maximum distance I've heard they achieved is 100 km.

In a similar manner to how two computers talk to each other then yes, Provided they can get the entanglement stable enough to be practical. It could be a usable wireless communication without the use of light or radio. There was a recent article concerning a diode that they fashioned that can generate several million entangled particles per second. I can't recall the exact amount, However certain types of radiation tends to break the entanglement down rapidly, Again its been a few months since I read that article so cannot name the particulars on it. To the best of my knowledge the maximum distance I've heard they achieved is 100 km.
People might want to use up-down spins to correspond to 1s and 0s on one end, and because the opposites will occur on the other, for 1s and 0s to be reversed so as to decode the original message. However, I'm pretty sure that once you try to measure the state of the atom, proton, whatever, it gets manipulated, so whatever you happen to send through quantum entanglement will be corrupted and nothing usable can be actually read.

DaveC426913
Gold Member
People might want to use up-down spins to correspond to 1s and 0s on one end, and because the opposites will occur on the other, for 1s and 0s to be reversed so as to decode the original message. However, I'm pretty sure that once you try to measure the state of the atom, proton, whatever, it gets manipulated, so whatever you happen to send through quantum entanglement will be corrupted and nothing usable can be actually read.
No, the problem is, that what you read about the state of the entanglement at the receiving end does not tell you anything - unless and until you communicate with the source - which you can only do at luminal or subluminal speeds.