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I was wondering why could superluminal communication send information to the past.

From wiki

Superluminal communication is believed to be impossible because, in a
Lorentz-invariant theory, it could be used to transmit information into    
the past

Can someone popularize this topic?

Let's say you have:

  • (planet X)
  • (planet Y)
  • ----> //Light emanation
  • .......> //Superluminal communication

The scenario is:

- ................................>
(planet X)                        (planet Y)
                      <-----------

I don't get why we would talk to planet Y in the past. We would talk to planet Y in the present even if light is on it's way to planet X. So my brain hurts.

I have an idea on how it would be possible to get two different light signals for the same cause but this would only mean that 1 of the signals would not represent the real state of the object being observed. I see no reason why an object could not have a different state than the one observed in his light signal.

Greg7000
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  • You might find this related: http://physics.stackexchange.com/questions/264750/how-can-i-send-a-message-to-myself-in-the-past?rq=1 –  Mar 06 '17 at 13:31
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    If communication is sub-$c$, then the arrival event occurs after the sending event in all inertial frames. If communication is faster than $c$, then we can find an inertial frame in which the arrival event precedes the leaving event. – BowlOfRed Mar 06 '17 at 18:08
  • The problem is that I don't understand "If communication is faster than c, then we can find an inertial frame in which the arrival event precedes the leaving event.". Maybe I don't understand inertial frame properly, but I still don't get where would be such a frame. – Greg7000 Mar 06 '17 at 18:29

1 Answers1

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this is confusing stuff and I've seen a lot of references made along the lines of 'we can find a frame in which information is sent back in time' without any further explanation. So I'll try to construct such a frame. I'm not sure I totally understand it myself but I'll have a go and hopefully someone can improve on it.

Say I'm on planet A and I measure planet B as 3 light years away. (Assume planet B is moving non relativistically compared with A which is usually a good assumption at least at intergalactic distances. )

Say a starship C, doing some velocity close to c passes my planet (planet A) at time t. As they pass by they send a message to planet B which travels oh so fast and arrives at t plus a little bit that we wont worry about.

By lorentz contraction the crew of C will measure the distance between A and B as smaller. Lets say they measure it as 2LY. The crew then travel on to planet B and it takes them about 2 years at a speed close to c. So when they get there 2 years has passed in their terms and their on board clock reads t+2years. However in the 'rest frame' of planet A and planet B 3 years have passed and the time is t+3 years.

So from the point of view of C the message they sent 2 years ago was received at B 3 years ago i.e. one year before they sent it.

A and B are in a different 'frames' to C because C is moving fast relative to them. In this big ole universe no-one can really be sure if they are at rest or not (Einstein again) so no-one can be sure who is A & B and who is C. From the point of view of C they are at rest and it is A and B who are moving.

user3473715
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  • I don't understand "By lorentz contraction the crew of C will measure the distance between A and B as smaller. Lets say they measure it as 2LY", I will go do some research. – Greg7000 Mar 07 '17 at 12:53
  • It's special a relativity thing. 'moving observers' measure distances as reduced by the 'gamma' factor which is 1/square root (1-v2/c2). – user3473715 Jul 15 '17 at 21:46