# Near-light speed and time

2022 Award
Can you give an example of faster than light signaling that will create reference frame in which the signal arrived before it was sent ?
You are headin away from me at any speed. I send a signal that reaches you instantaneously in my frame. You reply with a signal that reaches me instantaneously in your frame, which arrives before I sent my signal.

You can do it with non-instantaneous communication too, but the circumstances where things go acausal are a bit more restrictive and I'd need to do some maths.

You could Google for "tachyonic anti-telephone".

Steeve Leaf
You are headin away from me at any speed. I send a signal that reaches you instantaneously in my frame. You reply with a signal that reaches me instantaneously in your frame, which arrives before I sent my signal.

You can do it with non-instantaneous communication too, but the circumstances where things go acausal are a bit more restrictive and I'd need to do some maths.

You could Google for "tachyonic anti-telephone".
What is heading away from me at any speed, got to do with it, is it just appear to be because you know (assume in this case) that FTL is not possible ?.

nitsuj
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2022 Award
Can you give an example of faster than light signaling that will create reference frame in which the signal arrived before it was sent ?

In a single reference frame, FTL signals do not do anything strange. They simply get from one place to the other faster than light.

To derive a problem with FTL signals, you need to consider the relationship between two reference frames moving with respect to each other. And, you need to assume as a postulate of relativity (or an experimentally proven fact) that the speed of a light signal is the same for both frames. It's when you put this together with a FTL signal between the two that you get a problem.

Mentor
What is heading away from me at any speed, got to do with it, is it just appear to be because you know (assume in this case) that FTL is not possible ?.
As Ibix said... Google for "tachyonic anti-telephone".

2022 Award
What is heading away from me at any speed, got to do with it
Because that's the condition for the simultaneity conventions of your frame and mine to clash in a way that is paradoxical if faster than light communication is possible within relativity.

Steeve Leaf
I don't see any observer that will say that the reply was received before the message was send.
Math must be interesting .

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Gold Member
2022 Award
I don't see any observer that will say that the reply was received before the message was send.
Math must be interesting .

You should read post #73. You need two frames of reference and a Lorentz Transformation between them. Otherwise, there isn't going to be a problem in anyone frame.

Steeve Leaf
2807 Ok.
It was mention as " any speed " , in order for Lorentz Transformation to play a role they have to move apart close to the speed of light, then time dilation might cause the reply to be received before... but I THINK it will work only with instantaneous info transfer .

Am I 'm even getting closer

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Homework Helper
Gold Member
2022 Award
2807 Ok.
In was mention as any speed , in order for Lorentz Transformation to play a role they have to move apart close to the speed of light,than time dilation will cause the reply to be received before... but I THINK it will work only with instantaneous info transfer .

Am I 'm even close

FTL causing a problem is not that easy to demonstrate since you first need to really understand how events (time and place) in one frame relate to another frame. It's a thing called the relativity of simultaneity (not time dilation) that is really the issue.

If you have an absolute notion of time (as in classical physics), then FTL is not a problem at all. But, time is not absolute, simultaneity is relative and when you have a FTL signal, that causes a problem.

Using an instantaneous signal is easier than using a signal at, say, twice the speed of light; but both can lead to a problem.

Did you find anything on line?

Steeve Leaf
To use the tachyonic antitelephone, the user must aim it at the position in space occupied by the Earth in 1930. The user then must dial the telephone number a2807a and the device will transmit a beam of modulated tachyons back into time. When the beam arrives in 1930 the telephone will ring at the University of Berlin and none other than Albert Einstein will answer to help the caller with his or her physics questions.
http://www.physics.buffalo.edu/ubexpo/posters/tachyonic.pdf
You see that you got the answer before the question.
You absolutely right the relativity of simultaneity was recommended by the Prof but I thought I can skip it, I guess I was wrong ( again ).

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nitsuj
To derive a problem with FTL signals, you need to consider the relationship between two reference frames moving with respect to each other. And, you need to assume as a postulate of relativity (or an experimentally proven fact) that the speed of a light signal is the same for both frames. It's when you put this together with a FTL signal between the two that you get a problem.

Maybe oxymoron is a better term? One hand says you can do instantaneous physics and the other says you're limited to c.

Mentor
Maybe oxymoron is a better term? One hand says you can do instantaneous physics and the other says you're limited to c.
It's more a proof by contradiction, because the argument that causal effects cannot propagate faster than ##c## is that if you assume it is possible, contradictions appear.

Mentor
Dale, Ibix and PeroK
2022 Award
It was mention as " any speed " , in order for Lorentz Transformation to play a role they have to move apart close to the speed of light
No. The only thing moving faster does is let you detect the effects more easily. For the case where you pass me at time zero and I send you a tachyon pulse at time Tlater, the "reply" arrives ##v^2T/c^2## seconds before the original signal. Either T needs to be really huge or v needs to be very close to c for this kind of gap to be detectable. But it's there for any speed.

Faster than light communication is impossible in relativity, but it is a general truth that relativistic effects are present at all speeds, just undetectably small for every day speeds.

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Maybe oxymoron is a better term? One hand says you can do instantaneous physics and the other says you're limited to c.
An oxymoron is a linguistic device, where you combine two words that ought not to go together. Perhaps "thought experiment" is a relevant example.

Dale
nitsuj
An oxymoron is a linguistic device, where you combine two words that ought not to go together. Perhaps "thought experiment" is a relevant example.

? in other words speed is limited and unlimited...an oxymoron.

Homework Helper
? in other words speed is limited and unlimited...an oxymoron.
A claim that speed is both limited and unlimited would be a contradiction, not an oxymoron. But no such claim has been advanced in this thread.

Mentor
You absolutely right the relativity of simultaneity was recommended by the Prof but I thought I can skip it, I guess I was wrong ( again ).
The relativity of simultaneity is the single most difficult concept in relativity. Students struggle more with that than anything else.

Steeve Leaf
Maybe oxymoron is a better term? One hand says you can do instantaneous physics and the other says you're limited to c.
http://www.theculture.org/rich/sharpblue/archives/000089.html
If we need others point of view I recommend the above link.
I'm ready for the first lesson about relativity. ( I Think ).

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2022 Award
Steeve Leaf
Arkalius
Can you give an example of faster than light signaling that will create reference frame in which the signal arrived before it was sent ?

Keep in mind here the numbers are just arbitrary, but the effect they're pointing to is real. The concept of "now" is relative. There is no universal instant that everyone shares.

This effect can still happen with non-instantaneous signals that travel faster than light, but this puts a lower bound on the speed at which the target of the signal must be traveling away from you for it to happen. The faster the signal, the less this lower bound is. That bound basically uses the velocity addition formula for doubling the velocity. So the lower bound for velocity ##v## a frame must be traveling away from you for a signal that moves at faster than light velocity ##k## such that it could repeat that signal back to you so that it arrives before it was sent is given by ##v = \frac {2k} {1 + k^2}##

So, if you could send signals at twice the speed of light, the target has to be moving away from you faster than 0.8c to send it back to you before you sent it. If you can send it at 10x the speed of light, the target only has to be moving away faster than about 0.198c. You'll see that as ##k## approaches infinity (instant communication), ##v## approaches 0.

How much further in the past the return signal will go depends both on how much faster than the lower bound the target is moving and how far from you they are.

Dale
Steeve Leaf
Decent article. The comments might be subtitled "fifty ways to miss the point", though.
Some of the comments are from Rich the writer himself and some others are interesting and informative I didn't go through all of them( did you ? ) so maybe fifty is the right figure , ☺

Steeve Leaf

Keep in mind here the numbers are just arbitrary, but the effect they're pointing to is real. The concept of "now" is relative. There is no universal instant that everyone shares.

This effect can still happen with non-instantaneous signals that travel faster than light, but this puts a lower bound on the speed at which the target of the signal must be traveling away from you for it to happen. The faster the signal, the less this lower bound is. That bound basically uses the velocity addition formula for doubling the velocity. So the lower bound for velocity ##v## a frame must be traveling away from you for a signal that moves at faster than light velocity ##k## such that it could repeat that signal back to you so that it arrives before it was sent is given by ##v = \frac {2k} {1 + k^2}##

So, if you could send signals at twice the speed of light, the target has to be moving away from you faster than 0.8c to send it back to you before you sent it. If you can send it at 10x the speed of light, the target only has to be moving away faster than about 0.198c. You'll see that as ##k## approaches infinity (instant communication), ##v## approaches 0.

How much further in the past the return signal will go depends both on how much faster than the lower bound the target is moving and how far from you they are.
I don't have to be there physically do I ? We can replace me with a device that just return the information back to us at Earth, don't tell nobody about it and make a lot of money. We tried to get patent on it but every time at appears that it already patented .
I think that the problem here is first to receive the signal of FTL radiation/interaction than work on not breaking causality if it is in danger. Suddenly causality becomes a FORCE a fifth one.

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