Let us say that an astronaut has left the earth at very close to the speed of light.

Would we or he notice a large difference (time dilation) in radio communications between earth and the speeding vessel?

or more interesting if we were communicating via video, what would be the effect which would be noticed from both sides?

Well, if the astronaut were travelling that fast there would be a definite time dilation effect. The longer he travels the more delay you would get. I would imagine (without doing any math right now) that at some point it would become impossible to communicate, since we'd be receiving his messages many years from now.

The signal he is receiving will be tremendously red-shifted (the relativistic red shift formula is pretty easy to derive from the classical red shift plus time dilation)--99%c gives about a factor of about 14 (if the signal was transmitted with a frequency of 14 GigaHertz, he'd pick it up at 1 GigaHertz). He'd have to tape record the message and play it back at 14X speed in order to understand it.

Earth would have the same problem with communications from the spacecraft (its signal would be red-shifted, and slowed, by the same amount).

daver wrote:
Why would he have to play it back at 14X???

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My guess is that not only the carrier, but also the data carried would be shifted...

Though I am not so sure about the time dillation effects, as I understand it, the earth would move at 0.99c from the craft's point of reference, after all, so for the astronaut it is the earth who is time dillated. The acceleration would cause dillation though...

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If the data and the carrier were shifted by the same amount, then there wouldn't be a need to play it at the original frequency, would there???

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Hmmm.. No, I don't think you would use a recorder. You would likely lock on to the carrier and so find out the shifting, then use Doppler compensation like the ones used in satellite communication, just with a wider compensation range... I am not certain how it works, but my guess is that it would probably employ heterodyne frequency mixing and/or some signal prosessor algorithm...

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If we were in the 50's and the transmission was analog radio, you'd have to record and play back at a higher speed. Obviously with digital data that's not an issue.

Hey, there is a good way that allows you to figure out some of this yourself.

Have you ever noticed when the President is speaking, his speech is delayed by a second or two on some of the channels? This is caused by some of the networks using more satellite up and down links, so the signal travels farther. Both the advanced and retarded versions of the speech are “live”, but one or two channels has the “live speech” delayed by a second or so, because of the extra travel time of the EM signal up and down to and from the various satellites, which are about 23,500 miles up in space.

Ok, now, imagine the speech being shown with the least number of up and down links. That will be the first one you see. Now imagine that satellite going out into space at a fast speed, so that the signal travel time gets longer and longer. So what would you see? First, the engineers at the TV network would have to re-tune their transmitters and receivers to make up for the Doppler effect of the frequency change, and next they would have to invent some type of special “time-base corrector” device to convert the 30 fps down to fewer fps (frames per second). So, you would see and hear the President’s speech in slow motion.

This would be due to the classical Doppler effects, and “relativity” would not be involved. Any “relativity” effects cause by the moving satellite electronics could be compensated for, so that the satellite effectively acts like a turn-around mirror. In fact, we could use a turn-around mirror or just a big dish in space, to reflect the signal back, so we would not have to deal with the “electrodynamical relativity” aspects of the signal at all.

Of course we can’t move a satellite or a dish that fast, but we do move spacecraft transmitters fast enough so that the radio signals being received from the spacecraft are slightly redshifted in frequency and time delayed.

Interestingly enough, if you recall the movie Contact in the opening sequence, travelling at the speed of c (or faster), the further from Earth, the further back in time the radio broadcasts occurred in relation to the object travelling at c. However, I was wondering if the radio would be heard backwards if there were some way to maintain constant contact with a radio broadcast . . .

-Kevin

Well I had, but I had always assumed that was the period of time for his 'speech' to come in via the hidden earpiece......

(Sorry gotta go. The guys in the black suits are banging down the door again).

:wink:

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[quote="AGN Fuel"]I thought it was in case he had a sudden wardrobe malfunction.

[quote="Bob"]

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If the signal was analogue, then I believe you'd be alright if you speed up the playback of the signal. If the signal was digital, I'm not sure what would happen.

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Doesn't really matter what transmission mode is used - you still need to record and play back at a higher speed. Both carrier frequency and the actual message being transmitted would be slowed down, as determined by the relativistic Doppler shift.

If the ship is racing away at 99% lightspeed, the message would be received at a frequency of 1/14 the original, and be slowed down to 1/14 the original speed.

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Your radio would only pick up the bits at 1/14th the rate the were sent out (your 56k modem becomes a 4k modem). Presumably your receiver designers were smart enough to compensate for this. But instead of taking five minutes to download your quicktime movie, you're now well over an hour.

What's the fastest a modem could theoretically be then?

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to workout possible bandwidth you do:

bandwidth = maximum frequency X data bits

The main reason why a your modem is only capable of 56k is that its all thats possible on the narrow band of frequencies available on a voice phone line. Modem can only use the frequencies that are available to a voice conversation (Why? because of "companding", a mechanism to good signal to noise ratios)

But ADSL can use the available frequencies outside this range, thus more available ranges of maxium frequencies for higher data transfer. THis is why you need a line filter if you have ADSL, so you don't hear the modem screeching on a phone call....

sorry for the lack of detail, but i got a tooth ache and its 4:30am in the morning.

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