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Local relativity

  1. Apr 13, 2008 #1
    I am not a physics major but have always been perplexed by the apparent paradox of two issues:

    1. If we use the Hubble telescope to view items 10B+ years in the past yet the universe is expanding, what was the actual physical location of these bodies at the time the light was produced? Wouldn't they have been much closer to begin with? e.g. the light seems to be coming from distances much larger then the universe would have been at thei time of the photon emission.

    2. I'm familiar with the concept of local time compressing as a body accellerates to provide for constant perception of c. e.g. an accelerated body's local time slows to prevent its light from overtaking light from a "stationary" source.

    However, let's say we try this experiment local to the Earth. The Earth is in motion around the sun, the sun in motion around the galaxy, the galaxy towards the Great Attractor, what if the "accelerated body" is actually *decelerating* from this relative motion -- wouldn't its local time potentially speed up as it's the perceived "stationary" bodies which are actually traveling at a high rate of speed?

    e.g. If we were to fire off rockets in 14 different directions each 45 degrees from another in 3d space, and then measured local time, couldn't we then detect in which direction we're actually heading in as the rocket whose time slows the most would point towards greater velocity vs. the rocket whose clock slows (or speeds up) points to "true" stationary space?
  2. jcsd
  3. Apr 13, 2008 #2


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    Staff: Mentor

    Yes. And much further than 10B years today.
    Since there is no universal rest frame, all that experiment would tell you is that the earth is stationary wrt those 14 rockets.

    And the way you put it doesn't sound quite right: You never notice any variation in your own "local time".
  4. Apr 13, 2008 #3
    For example one rocket stays in earth orebit and the other fires a roct and speeds away but what if the case is that the two rockets are already in motion and by firing a rocket is actually slowing in relation to a larger reference point? e.g. the rockets are moving at 60,000km/s when local orbital velocity and planetary motion are factored in and then one fires its rocets in the opposite direction to this motion. Wouldn't then their motion be "slower" in relation to galactic space then the rocket which didn't fire its rocket in the planetary frame?
  5. Apr 13, 2008 #4


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    Staff: Mentor

    What "larger reference point"? You can place an observer anywhere in the universe you wish. But there is no basis for belieivng any observer is any more special than any other.
    Those motions only matter if you place an observer in one of those other frames. As long as the reference point is an earth-based clock, it is earth that is stationary and the rockets moving wrt earth.
  6. Apr 13, 2008 #5


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    lohphat ,
    Just want to emphasize what has been pointed out - there is no "true stationary space".
    All motion is relative. Take two cars driving in parallel at 60kph with respect to the road. You could say they are stationary wrt each other, but moving relative to someone standing by the road.
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