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Old September 20 2012, 05:31 AM   #46
gturner
Admiral
 
Location: Kentucky
Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

newtype_alpha wrote: View Post
gturner wrote: View Post
newtype_alpha wrote: View Post
No, relativistically the speed of light remains constant in all reference frames, even if you're FTL with respect to the receiver. Message transit time is the same either way (you might want to stop accelerating for a while, though, in case the warp field scatters your radio signals). The thing is, your signal would be redshifted by such an insane degree that it might be below the detection threshold of anything that might be able to receive and decode it. 10c is enough of a doppler shift to go from x-rays to ULF radio, and that would have certain complications for message integrity.
I think the redshift is easily calculated because the sender isn't undergoing time dilation.
He is from the receiver's point of view, and so is the receiver from the sender's point of view.

The sender traveling to Alpha Centauri sends 5 months (their perceived time) worth of data, which gets strung out from Earth to Alpha Centauri, so the end of their message arrives 4.2 years later, a factor of 10 (their velocity in C). So a 20 GHz signal arrives at a frequency of 2 GHz.
Which would, once again, have the appearance of time dilation for all the usual reasons: the signals are still arriving at the speed of light, but they are redshifted to the point that time appears to be running at 10% of its normal speed at the transmitting ship. What's interesting is, the ship would still APPEAR to be traveling at about or slightly less than the speed of light (if you were looking at it with a telescope, let's say) despite the fact that it suddenly returns to Earth ten months later with a bunch of photographs and rock samples from Alpha Centauri's dwarf planets; the crew of the ship would actually be able to look out into space and see a reflection of themselves, still in transit, nearly nearly seven years later. So not only the appearance of time dilation, but the appearance of time TRAVEL: the starship is seen arriving at Earth BEFORE it is seen arriving at Alpha Centauri.

Note that the time dilation effect is only observed from Earth's point of view and is again a consequence of the immense distances and the limitations of the speed of light. Time doesn't ACTUALLY dilate, it just seems that way because of the huge differences in their respective reference frames. Likewise, the starship traveling at FTL speeds will almost immediately begin to overtake its own radio transmissions and will eventually look back at Earth and see the planet the way it was years before they actually launched; warping back TO Earth, seems to speed up time in the same way.
I don't think we can call that time dilation, as it doesn't actually involve anything to do with time itself, just purely Newtonian tranmission speeds and velocities.

The same would hold true if we were talking about sound, where an airplane can fly so fast that the doppler shift is dramatic, and can even exceed the speed of sound so that the aircraft arrives before you hear it. Sound and a supersonic aircraft have all the same unusual properties as this warp ship.

If true time dilation were going on, the time on the ship would slow down, so that the crew would only perceive, for example, five days in transit instead of five months, and then get strung out over 4.x years, so their message would get redshifted by a factor of 300 instead of 10 (If I'm doing that right).
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