How do you manage an interstellar empire?

[Patrick Degan]

If you have no FTL travel, there is absolutely no way to sustain an interstellar empire. Even if you have physics-defying FTL communication, an empire would not be feasible. Imperial control depends absolutely upon how quickly you can rush troops from point-to-point to defend territory and enforce the imperial power upon the population.

But if your travel times are measured in decades or centuries, then only a lunatic would think he could set up an empire outside the boundaries of his own star system.

[RedImperator]

This is getting back into the 'Observer shapes reality' stuff that makes no sense to me, I think...

The basic principle here is that everything in the universe that conveys information travels at the speed of light. This includes photons, and gravity waves. This means that you can observe Alice composing her message, sending it, and then you can see Bob recieving the message. If you try to violate this, then, in other frames of reference, you can watch Bob recieve the message, and then watch Alice compose it. Explaining it thoroughly would involve complex integral equations and explaining the concept of light cones, which I don't feel like doing at the moment.

So in other words, if, say, Alpha Centauri blew up this instant, it wouldn't just be a matter of we won't know about it until 2007, it's a matter of, from our persepctive and the perspective of the universe in the immediate vicinity, the event didn't happen at all until 2007?

[kojikun]

Terwynn, your clearly an IDIOT. Just because it takes light x number of years to travel a distance doesnt mean that communicating with someone before they can SEE that happen is time travel!

Youre talking nonsense! When light arrives at the destination has nothing to do with when the event that created that light happened dont you understand that?

Thats like saying when you see lightning before you hear it, the light is travelling back in time because the sound hasnt arrived yet! The DELAY in one signal does not force the other signal to travel slow then that signal! just because the QE/T signal gets there before the light does doesnt mean theres been time travel involved, it simply means that the signal got there before the light.

[GrandMasterTerwynn]

Terwynn, your clearly an IDIOT. Just because it takes light x number of years to travel a distance doesnt mean that communicating with someone before they can SEE that happen is time travel!

Youre talking nonsense! When light arrives at the destination has nothing to do with when the event that created that light happened dont you understand that?

Thats like saying when you see lightning before you hear it, the light is travelling back in time because the sound hasnt arrived yet! The DELAY in one signal does not force the other signal to travel slow then that signal! just because the QE/T signal gets there before the light does doesnt mean theres been time travel involved, it simply means that the signal got there before the light.

That's not a valid example. Light travels faster than sound. What I am saying is that nothing can travel FTL. If you somehow transmitted an FTL signal from point A to point B, in the frame of reference encompassing the two points, causality isn't violated as our transmission clearly went from A to B. However, from another frame of reference, we notice that events in point B are being influenced by point A, even though their 'light cones' are too far apart for the information to have arrived in the proper way. (Each event generates a light cone. Anything that lies behind and inside the light cone occured in the past. Anything that lies in front and inside the cone is in the future.) As the light cone from A would intersect with the cone from B in the 'future' region, information coming from A to B FTL can be argued by our outside observer in her particular frame of reference to be time travel. And relativity says that's a no-no. It also violates causality.

And I did locate information about the Mozart's 40th Symphony experiment.

http://math.ucr.edu/home/baez/physics/R ... t/FTL.html

Apparently a group of physicists claims to have transmitted the symphony through an astonishing 11.4 cm barrier at an apparent speed of 4.7 c.

11. Quantum Tunnelling
Quantum Tunnelling is the quantum mechanical effect which permits a particle to escape through a barrier when it does not have enough energy to do so classically. You can do a calculation of the time it takes a particle to tunnel through. The answer you get can come out less than the time it takes light to cover the distance at speed c. Does this provide a means of FTL communication?
ref:T. E. Hartman, J. Appl. Phys. 33, 3427 (1962)

The answer must surely be "No!" otherwise our understanding of QED is very suspect. Yet a group of physicists have performed experiments which seem to suggest that FTL communication by quantum tunneling is possible. They claim to have transmitted Mozart's 40th Symphony through a barrier 11.4cm wide at a speed of 4.7c. Their interpretation is, of course, very controversial. Most physicists say this is a quantum effect where no information can actually be passed at FTL speeds because of the Heisenberg uncertainty principle. If the effect is real it is difficult to see why it should not be possible to transmit signals into the past by placing the apparatus in a fast moving frame of reference. (Editor's note: Emphasis mine.)

ref:
W. Heitmann and G. Nimtz, Phys Lett A196, 154 (1994);
A. Enders and G. Nimtz, Phys Rev E48, 632 (1993)

Terence Tao has pointed out that apparent FTL transmission of an audio signal over such a short distance is not very impressive. The signal takes less than 0.4ns to travel the 11.4cm at light speed, but it is quite easy to anticipate an audio signal ahead of time by up to 1000ns simply by extrapolating the signal waveform. Although this is not what is being done in the above experiments it does illustrate that they will have to use a much higher frequency random signal or transmit over much larger distances if they are to convincingly demonstrate FTL information transfer.

The likely conclusion is that there is no real FTL communication taking place and that the effect is another manifestation of the Heisenberg uncertainty principle.

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