Instantaneous Communication Between 2 Stationary Observers

1. Jul 14, 2011

consal

The "message" travels back in time, am I correct? Because event propagation is at the speed of light, but the message is instantaneous, so at the frame it arrives it has traveled back in time...

2. Jul 14, 2011

bcrowell

Staff Emeritus
Not sure what you mean by "at the frame it arrives." A single event such as the arrival of a message can be described in more then one frame.

A better way of putting it is this. If A could send an instantaneous message to B, then there would exist a frame of reference in which B received the message before A sent it.

3. Jul 14, 2011

consal

I have incoherent moments. But that's what I meant. Just wanted confirmation. Thanks!

4. Jul 14, 2011

abbott287

Its called quantum entanglement. :)

5. Jul 14, 2011

Staff: Mentor

Entanglement doesn't allow communication instantaneous or otherwise.

6. Jul 14, 2011

ghwellsjr

I think you missed Ben's point, at least I hope you missed it. I think Ben was trying to show you that since it is impossible for B to receive a message before A sent it, then A cannot send an instantaneous message to B. At least I hope that's what he meant.

Events don't propagate. In Special Relativity, an event is a coordinate in a Frame of Reference. It has a time value and three spacial values. You can track a message that goes from A to B at the speed of light in the Frame of Reference in which both A and B are stationary as a series of events starting with when the message left A at A's location and ending with another event when B receives the message at B's location. There is nothing instantaneous about this. If the spacing between A and B is 1 light-year in the frame where they are both stationary, then it will take 1 year in that same frame to make the trip.

You can then transform this scenario into any other Frame of Reference moving at any velocity with respect to the first FoR as long as that velocity is less than c. You can pick one where the time for the message to travel is as small as you want, but not zero or instantaneous. When you do, the distance between A and B will become almost zero, too, so the message still travels at the speed of light.

But whatever you do, you cannot use one FoR to specifiy the event of the transmission of the message at A and a different FoR to specify the event of the receipt of the message at B and subtract the values for time from those two events and draw any valid conclusion. That would be like saying that you got on an airplane in New York at 1PM and landed in Los Angeles at 2PM and concluding that your flight only took 1 hour. You have to use the same coordinate system for everything you are describing and calculating in order to get meaningful results and a FoR is nothing more than a coordinate system.

7. Jul 14, 2011

consal

No, I got that. I was not asking if instantaneous communication was possible. I was just wondering what the ramifications of it would be. I also received the impression that it might have been a "meaningless question" in the sense that there is no meaning for the word "instantaneous" in where special relativity is concerned, but I still wanted as close to an actual answer as possible, which brings up...

Okay, that was just me being stupid...

I understand this. I didn't miss the meaning of any response in this thread. The issue with this, though, is that my original question was admittedly "meaningless" in the sense that there is no such thing as instantaneous (I don't think so, at least).

So let's say A sends an "instantaneous" message to B at a certain time, and then travels to B. B tracks the time with a reasonable degree of accuracy accounting for discrepancies from A's traveling (dilation and all that). If A subtracted the time B has recorded, wouldn't the result be before A sent the message?

(Imagine I'm a journalist who cannot accept that there is no "instantaneous" and you have to somehow explain this to me satisfactorily so I can write something universally understandable on my column.)

8. Jul 14, 2011

ghwellsjr

OK, here's something a journalist can understand. An "instantaneous" message will cost you an "infinite" amount of money and your life, and you have to pay up before the message is sent.

9. Jul 14, 2011

Staff: Mentor

The problem here consal is that the type of question you are asking runs along the lines of "if the laws of physics didn't apply what would the laws of physics say about this situation?" There is no such thing as instantaneous communication. If it were possible you may get http://en.wikipedia.org/wiki/Tachyonic_antitelephone" [Broken] (which may answer your question) but its not so you wont.

Last edited by a moderator: May 5, 2017
10. Jul 14, 2011

Staff: Mentor

No it isn't. Quantum enganglement does not transmit information at any speed.

11. Jul 14, 2011

consal

I was under the impression that's the sort of stuff journalists ask all the time. And they get answers. Wrong answers, but answers anyways. D:

Well, fair enough. These are the physics forums, not the "dumbed-down physics" forums. The reason why I'm asking is because a friend asked me what would happen if a message is instantaneously transferred from A to B, and I didn't want to say, "There's no such thing." In this scenario, I felt like I should've played along a bit--and with my limited knowledge, I was thinking that perhaps the message would have been "sent back in time", in the sense that B was in possession of the message for a longer period of time than A had traveled since he'd sent it.

Last edited by a moderator: May 5, 2017
12. Jul 15, 2011

Staff: Mentor

Look up causality and faster than light. That well help you understand these things. Unfortunately like I said above, you cannot competently talk about impossible things and get a good answer.