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Thought experiment: Orbiting bodies close to C

  1. Aug 20, 2009 #1
    Hi all, I was bored a little while ago, and somehow started thinking about the Big bang and all that fun stuff. I once heard it said that some of the galaxies near the end of the universe are at close to light speed. Now I'm not here to dispute that (would be nice if it were true though) but another thought struck me.

    If you had, say, a sun in a solar system moving at close to c away from the center of the galaxy, the orbiting bodies in that system will probably at some point have to be moving in the same direction as it's sun. When you start changing acceleration (or deceleration) at close to c you need enormous amounts of energy to make the shift, something about mass changing at that speed. So wouldn't these orbiting bodies just lose their momentum or something and fall into their sun? What would happen?
  2. jcsd
  3. Aug 20, 2009 #2
    Hi there,

    Not really since the orbiting bodies have only small velocities compared to their star, like the Earth around the Sun being roughly 30km/s, not 30km/s + speed around galaxy. The system can be seen pretty much as a closed system.

  4. Aug 20, 2009 #3
    The is no beginning nor end to the universe in the sense you imply; however, objects at tremendous interstellar distances from an observer do appear to be receding at increasing velocities due to cosmological expansion...space between the observer and a distant body is

    lf expanding....Hubble discovered this experimentally. Earth would appear to be receding very fast from an observer, for example, 1,000 universes away to our east....

    The "end of the universe" for us (and for all other observers who also have their own cosmological horizon limits) is the cosmological horizon: we are limited in what we can observe because some light, if distant enough in origin, will never reach us...

    I don't think that happens within a typical galaxy. Massive objects move much more slowly, maybe 18,000 miles per hour for example, I think is an orbital speed of earth....relative to our sun. A sun in one solar system moving rather "slowly" in it's local orbit might well appear to be moving much more rapidly to a distant observer located in a far distant galaxy as described above.

    But I think your basic idea is correct: any massive body in a group of other bodies affects all others. If one suddenly explodes or collides and it's effective position changes, all the other bodies must also shift according to gravitational laws. For example, if something crashed into our moon, and blew it apart, things here on earth would be dramatically disrupted...perhaps massive tidal waves to start...
  5. Aug 20, 2009 #4


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    There is no such animal as absolute motion. So while the system is moving at near c relative to the center of the galaxy, it is not moving at all relative to itself. In other words, it is prefectly acceptable to describe the same scenario as the center of the galaxy moving away from the system at c. (there is no way to say which one is "really" moving.) So no, the planets wouldn't fall inot the star, because they wouldn't do so if the system was had no relative motion with respect to the center of the galaxy. Any relativistic effects are only measured by someone who has a relative motion wih respect to the system.
  6. Aug 20, 2009 #5
    Does this mean that theoretically, one could live on a solar system moving at close to c away from the center of the universe (blame it on the big bang) and they could live, work, live and play in such a system? And could they then theoretically accelerate objects away from our observational point at the center of the universe at a speed faster than c, but perfectly reasonable from their standpoint?

    I'm sorry, that was an error on my behalf. I meant the center of the universe.
  7. Aug 20, 2009 #6


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    There is no "center" to the Universe.

    But if you were in a star system traveling at near c relative to me, you could accelerate something up to near c relative to you. However, I would measure that same object as moving at less than c (but closer to c than you are moving.) relative to me. For example, if you are moving at .99c relative to me, and accelerate it to .99c relative to yourself from your standpoint, I would measure it as moving at.9999495c relative to me.
  8. Aug 21, 2009 #7
    That totally wrecks my ideas about the universe. How did you arrive at that number?
    But does this also mean that if I were in a ship travelling at .6c towards another ship travelling at .6c in the opposite direction, then each ship would still observe the other travelling at a speed less than, but close to c?

    And finally, is the speed of light merely a cap on the speeds we can observe, or is it a universal cap on all speeds, and if it is a universal cap, where is the reference point?
  9. Aug 21, 2009 #8


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    [tex] V_t = \frac{v_1+v_2}{\frac{1+v_1v_2}{c^2}}[/tex]
    Yes, at 0.882c.
    It is the limit for the relative speed between any two objects as measured by those objects. There is no absolute reference point to measure the speed of light from, because everyone measures light as moving at c relative to themselves.
  10. Aug 21, 2009 #9
    Thanks, that really cleared it up for me. And here I was thinking I'd found a loophole in relativity :tongue:
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