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Relativity query - communication with an object at light speed

  1. Sep 9, 2010 #1
    Hello, I'm new here and not too bright so go easy please..

    My question is something that has bugged me for years;

    The setup -

    A light speed (or nearly light speed) craft is orbiting earth or at least circum-navigating it at a distance and at speeds close to the SOL.

    A cable runs from the craft to earth as it travels, allowing the crew to communicate with the groundcrew (forget about the logistics of this if you would).
    As the crew in the craft are experiencing a 'slower time' than that of the ground crew on earth, how would they perceive a 2 way conversation (audio or a/v)?

    Would there be any difference in audio or visual signals (say the crew on board the craft would experience an hour conversation in 1 minute and vice versa) or would there be no difference (like a standard phonecall or Skype conversation on earth) and if this is the case, why?
    I can grasp relativity (I think) in basic hypotheses but this one has always confuddled me.

    Oh wise ones, please put an end to my confusion :)
    lloyd

    *sorry, just realised I stuck this in QM like an idiot, feel free to move, sorry :/
     
    Last edited: Sep 9, 2010
  2. jcsd
  3. Sep 9, 2010 #2

    bcrowell

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    Relativity puts fundamental limits on the strengths of materials. As an example, you might think you could lift material out through a black hole's event horizon by using a bucket on a rope, but you can't, because you can't have a rope strong enough -- not even in theory. I think the same issue would apply here.
     
  4. Sep 9, 2010 #3
    Forgetting the cable for a minute, you're still dealing with the same issue, which then becomes a simple question of what the Gamma is for the EM signals being exchanged.

    If the cable is an essential part, then bcrowell is right, end of story, also nothing with "rest" mass can reach lightspeed, so it has to be some fraction of c.

    Edit... even if you had magically strong material, length contraction relative to motion, and shear stresses would mean it would have to be super-flexible. One way or another, rope, or "beams" of light or radio signals... you have warping.
     
    Last edited: Sep 9, 2010
  5. Sep 9, 2010 #4

    DaveC426913

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    What does the cable have to do with anything? The signal still has to travel through the cable, and a signal can only travel at the speed of light, cable or no, so why not reform your question without it?
     
  6. Sep 10, 2010 #5
    Hey all, thanks for the reply. The main point of my question was really wondering about the logistics of communication between a stationary object on earth and an object travelling near the SOL and how each crew would see and hear the transmissions of the other (as their perceptions of time would be very different due to the speed the crew on the craft are travelling).

    DaveC- I see what you're saying. If the data transfer was cableless and photonic and the craft travelling at only just sub-luminal speeds would this be more possible? That way the signal would be faster than the craft i think.

    Even if the setup I am describing does not work, is the principle of a video call between the 2 crews possible and if it were to take place, what would it be like for each crew?
    cheers
     
  7. Sep 10, 2010 #6
    This is an easy question, again, the percentage of c that you're traveling at determines gamma. To put it simply, lets say that you're traveling fast enough that the difference in clocks between the ship and the sender is 4 hours, then you'll have a round-trip delay of... 8 hours. Go faster, and make it 800 years, and you'll receive the message long after the sender dies, and you'll be long dead before your return signal makes it back.

    This all assumes you could intercept and interpret a signal at those speeds, but that's a needless complication to the point you're making.
     
  8. Sep 10, 2010 #7

    DaveC426913

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    nis, you're misunderstanding. Reread the OP. The craft is not separated by distance, it is in a nape-of-Earth trajectory. i.e. latency due to distance is not a factor, the only factor to consider is time dilation discrepancies between frames of reference.
     
  9. Sep 10, 2010 #8
    Ooooooh. Oh. Oh hell. Well, I feel like an idiot! Thanks for stopping me before my swallowed by entire leg. *wince*
     
  10. Sep 10, 2010 #9

    DaveC426913

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    You know, it is an interesting question.

    Let's say they're travelling at c=.98, so that gamma is 5.
    (http://www.1728.com/reltivty.htm - bottom of page)

    Aboard the craft, they should observe Earth as moving very slowly, which means the spaceships's one minute transmission might be received on Earth in the space of a mere 10 seconds (and commensurately blueshifted).

    On the other hand, on Earth, should observe those aboard the craft as the ones moving very slowly. Their transmission should take 5 miutes to complete (and be commensurately red-shifted).

    What's confounding this is that the craft is not in an inertial frame of reference. I don't know what the result is.
     
  11. Sep 10, 2010 #10
    If you could "see" the shifting of the transmission at precisely halfway from ship-earth or earth-ship, would there be a net shift? Maybe this is something that cancels out to some degree when you work out the math.
     
  12. Sep 10, 2010 #11

    DaveC426913

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    No. The signal does not change from source to destination. The signal's frequency is entirely a product of the frame of reference from which it was emitted.
     
  13. Sep 10, 2010 #12

    bcrowell

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    I think the whole thing is really just a transformation from Minkowski coordinates to a rotating frame of reference. This is the same example that Einstein originally described in "The foundation of the general theory of relativity" as motivation for GR's description of gravity in terms of curved spacetime. The observer in the spaceship experiences an outward gravitational field, and says that clocks up "above" him on Earth's equator experience both a gravitational speed-up and a kinematic slow-down. In addition to this there are longitudinal Doppler shifts of signals. If they're carrying on a conversation, each will see the other as alternately too fast and too slow, as the rotation goes through one full cycle.
     
  14. Sep 10, 2010 #13
    Damn, this really is a weird scenario... it's shaking me up to the point where I'm thinking like a classicist. I'm going to take a break from offering bad notions here, and do some research on this.
     
  15. Sep 10, 2010 #14

    DaveC426913

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    Ahh. That's how it's resolved...

    Now, that assumes the spaceship is rotating around a point at a distance, such that its Doppler shift is changing.

    What if the spaceship rotates directly around the Earthbound observer, so that distance does not change over time? i.e. No Doppler shift?

    I surmise that time dilation will perfectly cancel out, leaving them in a discussion that is undistorted,
     
  16. Sep 10, 2010 #15
    Are you saying I was right for all of the wrong reasons? That's just bizarre... and this question is WAY out of my pay grade. I'll be reading and asking questions here now, and not commenting. :blushing:
     
  17. Sep 10, 2010 #16

    DaveC426913

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    Well, I don't read your answer as right at all:

    You seemed to be suggesting that the signal would undergo a shift in frequency/duration while in-transit between emitter and receiver.
     
  18. Sep 10, 2010 #17
    Not the totality of my answer, just the portion about it being an issue where in the math, everything cancels out. The frequency shift part was just wrong, and the former portion, even if the result is right, is right through a series of misconceptions and being wrong too. I'm talking about the final conclusion, and reflecting on how weird that was, believe me there is no back slapping going on with me right now.
     
  19. Sep 10, 2010 #18

    DaveC426913

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    It's definitely weird for me too. I've never encountered this particular thought experiment. I wonder if others have.
     
  20. Sep 10, 2010 #19
    I'd love to know, but it's safe to say this is one hell of a good post from a newcomer (the OP)!
     
  21. Sep 10, 2010 #20
    There is one practical reason that you might need a cable. Relativistic speeds are much greater than natural orbital speeds of the Earth, so a cable might be required to act as a tether to keep the "orbiting" object at constant distance from the Earth.

    Another factor that no one seems to be taking into account is that gravitational time dilation is more significant than velocity related time dilation for a natural orbit and clocks on the orbiting object are actually ticking faster than clocks on the Earth (for a natural orbit). The OP has to make it clear if he wants this analysed in a purely SR context, ignoring gravitational time dilation. However, if as Dave seems to be suggesting, the "orbiting object" is at the surface of Earth (surface skimming) then it can be considered an SR scenario for practical purposes, but if the Earth observer is also on the surface of the Earth then we would have to consider the purely classical Doppler effect due to the orbiting object going towards or away from the surface observer and I an not convinced that this is what the OP is looking for. Maybe he can confirm that? In his original post he said:
    and "at a distance" suggest to me that he did not intend a a surface skimming orbit and was more interested in the transverse effects rather than the classical longitudinal line-of-sight Doppler effects

    To make it a purely SR scenario, take two objects tethered to each other by a long cable so they orbit around a mutual central point. Place an observer at the center of the cable to represent the Earth observer. Now time dilation is just velocity related.

    The next factor you have to take into account is that Newtonian calculations do not predict a transverse Doppler shift, while relativity does, due to time dilation. In the SR scenario, clocks on the orbiting objects tick slower than the central clock and electromagnetic messages from the orbiting object will sound like a tape recording that is played back slower with voices sounding deeper. Messages received by the orbiting objects from the central observer will sound like a tape recording that has been sped up and voices will have a higher pitch. There is no cancelling out.

    To consider the effects on communications due to gravitational time dilation, consider a high tower on a non-rotating planet. The result is the opposite of the SR scenario. Clocks at the base of the tower (nearer the center) are ticking slower than clocks at the top of the tower. Messages coming from the base will sound like they have been slowed down when the observers at the top listen to them, while messages from the top will sound like they have been sped up when the observers at the base listen to them. Again, no mutual cancelling out.

    In either scenario, if the observers were look at the senders through a telescope they would see the lips of the senders moving faster or slower like a video recording that has has been sped up or slowed down, in sync with the sped up or slowed down messages.

    In short, messages will be distorted, except in one special circumstance where the orbital velocity is artificially arranged so that the time dilation due to velocity is deliberately designed to cancel out gravitational time dilation. This special velocity will be much greater than the natural orbital velocity and will have to be maintained by a restraining tether or have inward facing thrusters continually providing the required centripetal force which would be difficult to maintain for any period of time.
     
    Last edited: Sep 11, 2010
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