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Breaking laws

  1. Jun 15, 2010 #1
    How come we say perpetual motion is not possible according to the laws of thermodynamics. Yet our universe is expanding faster and faster. How can we say nothing is faster than the speed on light but the universe expanding is? Keep it simple please.
     
  2. jcsd
  3. Jun 15, 2010 #2
    The universe is not expanding faster than C. if it were, light from distant galaxies would never reach us because the space between them would grow faster than the light could cross it.

    As for perpetual motion, It is not the same as a force. We do not know why the universe is expanding faster and faster, but it seems to be due to some accumulating acceleration. We do not say an orbiting satellite is perpetual motion, yet it perpetually moves. Be careful with the definitions.
     
  4. Jun 15, 2010 #3
    I think what bin is asking is that if it is expanding at any rate, wouldn't it eventually exceed the speed of light far enough away. The answer is no, relative to any observer in the universe nothing is moving faster than the speed of light.

    Speed is distance over time, but take the time of any relative object and you get

    [tex] t' = \frac{t}{1-\frac{v^2}{c^2}} [/tex]

    So

    [tex]\frac{d}{t'}[/tex]

    will never exceed c where c is the speed of light.
     
  5. Jun 15, 2010 #4
    Back in one of my astro classes my professor tried to explain that what's happening is that the objects aren't 'moving' away faster than the speed of light, but instead, the distance between the objects is getting longer! kind of like drawing two dots on a balloon and filling it up, they aren't moving with respect to the balloon, but are becoming farther apart due to its expanding nature. now, obviously this balloon example is wholly crude, but the point is that it's not that the objects are moving, but instead, the way we perceive distances have changed and are changing with respect to time. i hope this helps.
     
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