Faster than light and time travel

In summary: What this analogy would suggest for FTL is that something could theoretically travel faster than light, without violating causality as long as it's not sending information.This is a difficult question. I'm not sure what you're asking.
  • #1
byron178
157
0
Why does faster than light travel or propagation mean time travel backwards to violate causality?
 
Physics news on Phys.org
  • #3
DrGreg said:

I see,so if i were in a spaceship and were to faster than light,i would travel backwards in time to the past?
 
  • #4
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.
 
  • #5
byron178 said:
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.
 
  • #6
DrStupid said:
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?
 
  • #7
HallsofIvy said:
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?
 
  • #8
byron178 said:
how come entanglement happens faster than light?

Because nothing travels and no information is transmitted.
 
  • #9
byron178 said:
is there anything is physics that travels simultaneity?

What do you mean by "travels simultaneity?"
 
  • #10
DrStupid said:
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.
 
  • #11
jtbell said:
What do you mean by "travels simultaneity?"

what i meant is are there things or situation in physics that travel simultainisly?
 
  • #12
DrStupid said:
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?
 
  • #13
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...
 
  • #14
what about lijun wang's expierament done in 2000,does that deal with time travel?
 
  • #15
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?
 
  • #16
also i forgot to add,can something travel backwards in time and not violate causality?
 
  • #17
cause cannot preceed event so to answer your question the answer is no any backward movement in time can only violate causality
 
  • #18
byron178 said:
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 traveled back before you start travelling. So I think this is the violation
 
  • #19
I know that group velocity can exceed the speed of light,but does it travel backwards in time since it travels faster than light?
 
Last edited:
  • #20
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?
 
Last edited:
  • #21
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 that's 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.
 
  • #22
Would it be possible to use quantum entanglement to communicate at any usable distance?
 
  • #23
easyrider said:
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.
 
  • #24
Mordred said:
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.
 
  • #25
xeryx35 said:
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.
 
  • #26
DaveC426913 said:
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.

Okay, let me take this hypothetical scenario. Interstellar travelers from Earth are going to Alpha Centauri, and with them they take a quantum-entanglement Internet link to Earth, so they can communicate in a reasonable amount of time (assume they have no FTL technology, that is way too speculative for this era and forum).

For our purposes, let's assume that information is sent from Earth to Alpha Centauri with a binary system, which at Alpha Centauri is "flipped over" so to speak in order to cancel out the flipping which happens with quantum entanglement. So 0s on Earth are converted to 1s in entanglement which are converted to their true form of 0 by the computer. The computer was programmed to do the conversions.

So does this mean that the programmer who wrote the program to "make sense of "the entanglement results, and the fact that the skills to use a computer to "make sense of" the data were taught to the crew, was the necessary subluminal communication component? How does it work, I don't really have a physics background so I don't understand it too well.
 
  • #27
All you can do with entangled particles is read what state they are in. As soon as you try to write a new state, the particles are no longer entangled.
 
  • #28
If the spacetime continuum is an unviolated principle of nature, then faster than light travel requires backwards time travel. A violation of SR would be a violation of the one or more assumptions about spacetime, as presently defined. If the neutrinos are verified to travel faster (in a local frame) than light speed observed in a vacuum (in a local frame), then given that the scientists at CERN have measured a positive velocity in their experiments, then discovery of FTL travel would be directly incompatible with the existence of a space-time continuum, as presently defined.
 
  • #29
kmarinas86 said:
If the spacetime continuum is an unviolated principle of nature, then faster than light travel requires backwards time travel.

It requires backwards time travel in some frames of reference but not in all frames of reference.

kmarinas86 said:
given that the scientists at CERN have measured a positive velocity in their experiments, then discovery of FTL travel would be directly incompatible with the existence of a space-time continuum, as presently defined.

That the scientists at CERN have measured a positive velocity in their own rest frame wouldn't mean that the velocity is positive in every frame of reference.
 
  • #30
DrStupid said:
It requires backwards time travel in some frames of reference but not in all frames of reference.

That the scientists at CERN have measured a positive velocity in their own rest frame wouldn't mean that the velocity is positive in every frame of reference.

What do you call a velocity that is "positive" when going from point A to point B and "negative" when going from point B to point A? (Hint: It is not velocity as measured by an observer relative to that observer.)

And the focus here anyway is the velocity of neutrinos measured from the laboratory frame in which point A and point B are fixed coordinates. This is what actually matters. The scientists at CERN didn't discover that the neutrinos traveled at 60 m/s now did they?

And how exactly does one travel such that otherwise backward time travel is not observed as backwards? Wouldn't such a traveler be traveling backwards in time too?
 
  • #31
kmarinas86 said:
What do you call a velocity [...]?

The derivation of the position with respect to the time.

kmarinas86 said:
And the focus here anyway is the velocity of neutrinos measured from the laboratory frame in which point A and point B are fixed coordinates.

If you want to have "backwards time travel" you will have to change the frame of reference.
 
  • #32
DrStupid said:
The derivation of the position with respect to the time.

Not that I disagree with that, but you obviously misunderstood what I meant by positive and negative. There are three spatial dimensions and one time dimension. So let's guess what I meant by positive and negative.

That's right, the line crossing point A and point B in the experiment determines which direction is positive and which one is negative.

DrStupid said:
If you want to have "backwards time travel" you will have to change the frame of reference.

I'm not talking about comparing frames of reference. We are discussing velocity respect to the laboratory frame, not acceleration with respect to an arbitrary observer. This is where we address the measured speed of neutrinos, which is still being debated.

Perhaps I've got this wrong, and the tachyons (or other alleged faster-than-light particles) would simply be aging in reverse (time reversal limited to the "internals" of the particle, if any) without having to arrive at the detector before being fired from the accelerator?
 
Last edited:
  • #33
DrGreg said:
All you can do with entangled particles is read what state they are in. As soon as you try to write a new state, the particles are no longer entangled.

Yes, that is what I was trying to say. Manipulating the atoms would end the entanglement and make interstellar Internet impossible.

On the upside, if any FTL technology for spacecraft is discovered (I will not go into too much detail, trying to avoid over-speculation) postal services will thrive.
 
  • #34
kmarinas86 said:
I'm not talking about comparing frames of reference. We are discussing velocity respect to the laboratory frame

Than your statement "faster than light travel requires backwards time travel" is wrong.
 
  • #35
xeryx35 said:
So does this mean that the programmer who wrote the program to "make sense of "the entanglement results, and the fact that the skills to use a computer to "make sense of" the data were taught to the crew, was the necessary subluminal communication component? How does it work, I don't really have a physics background so I don't understand it too well.

You generate a stream of entangled particles. For simplicity's sake we'll say the information in them is 1s and 0s. Here's the first 3 pairs:
p1a+p1b, p2a+p2b, p3a+p3b
You do not yet know which of p1a+p1b is a 1 and which is a zero.

Now, you send one of the pair off to A. Centauri and keep one for yourself.

You observe your p1a and see it is a 1. You now now that A. Centauri just received a 0.
You observe all your particles and see that they are 1,0,1. You now know that A. Centauri has received the sequence 0,1,0.

The particles at A.Centauri did not have a defined sequence at all until you onbserved yourse, at which piont the aprticvles at A. Centauri somehow instantly became 0,1,0. That is the spooky part that happened instnatly across 4 light years.

But what use is the sequence of 010 to you or to A.Centauri? The sequence of 1's and 0's occurred before you could observe them, so it carries no information that you could encode in it.
 
<h2>What is faster than light?</h2><p>Faster than light (FTL) refers to any object or particle that travels faster than the speed of light, which is approximately 299,792,458 meters per second. According to Einstein's theory of relativity, nothing can travel faster than the speed of light in a vacuum.</p><h2>Is it possible for anything to travel faster than light?</h2><p>Currently, there is no scientific evidence or technology that supports the idea of FTL travel. According to the laws of physics, it is not possible for any object or information to travel faster than the speed of light. However, some theories and experiments suggest that it may be possible to manipulate space-time to achieve FTL travel, but this is still a subject of ongoing research and debate.</p><h2>What is time travel?</h2><p>Time travel is the concept of moving between different points in time, either forwards or backwards. It is a popular topic in science fiction and has been explored in various theories and experiments in physics. However, the possibility of actual time travel is still a subject of debate among scientists.</p><h2>Is time travel possible?</h2><p>The concept of time travel is still a subject of ongoing research and debate in the scientific community. Some theories and experiments suggest that time travel may be possible through the manipulation of space-time, but there is currently no conclusive evidence to support this idea. More research and experimentation is needed to fully understand the concept of time travel.</p><h2>What are the potential consequences of time travel?</h2><p>If time travel were to become possible, it could have significant consequences on our understanding of the universe and the laws of physics. It could also have major implications for the concept of causality and the potential for paradoxes. Additionally, the ethical and moral implications of time travel would need to be carefully considered. </p>

What is faster than light?

Faster than light (FTL) refers to any object or particle that travels faster than the speed of light, which is approximately 299,792,458 meters per second. According to Einstein's theory of relativity, nothing can travel faster than the speed of light in a vacuum.

Is it possible for anything to travel faster than light?

Currently, there is no scientific evidence or technology that supports the idea of FTL travel. According to the laws of physics, it is not possible for any object or information to travel faster than the speed of light. However, some theories and experiments suggest that it may be possible to manipulate space-time to achieve FTL travel, but this is still a subject of ongoing research and debate.

What is time travel?

Time travel is the concept of moving between different points in time, either forwards or backwards. It is a popular topic in science fiction and has been explored in various theories and experiments in physics. However, the possibility of actual time travel is still a subject of debate among scientists.

Is time travel possible?

The concept of time travel is still a subject of ongoing research and debate in the scientific community. Some theories and experiments suggest that time travel may be possible through the manipulation of space-time, but there is currently no conclusive evidence to support this idea. More research and experimentation is needed to fully understand the concept of time travel.

What are the potential consequences of time travel?

If time travel were to become possible, it could have significant consequences on our understanding of the universe and the laws of physics. It could also have major implications for the concept of causality and the potential for paradoxes. Additionally, the ethical and moral implications of time travel would need to be carefully considered.

Similar threads

Replies
6
Views
1K
  • Special and General Relativity
Replies
9
Views
1K
  • Special and General Relativity
Replies
21
Views
2K
  • Special and General Relativity
Replies
8
Views
957
  • Special and General Relativity
Replies
19
Views
1K
  • Special and General Relativity
Replies
19
Views
2K
  • Special and General Relativity
Replies
2
Views
875
  • Special and General Relativity
Replies
10
Views
2K
  • Special and General Relativity
Replies
1
Views
602
  • Special and General Relativity
2
Replies
40
Views
2K
Back
Top