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Special Relativity and Distance/Space

  1. Dec 17, 2009 #1
    I'm currently reading Brian Greene's The Fabric of the Cosmos. I have a question about Special Relativity. I understand that time slows down when there's relative motion. I wanted to ask something about distance/space. When time slows down, does that mean that, e.g. the speed of a car, slows down too? I'm new to studying physics. I'm sure this question can be easily answered. Thanks in advance!
  2. jcsd
  3. Dec 17, 2009 #2


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    Actually that's a tricky question. And not really well defined, although don't blame yourself ;-) If you explicitly describe a specific physical situation, we can tell you what you would see in it, but there isn't really anything to say about e.g. the speed of a car in general.
  4. Dec 17, 2009 #3


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    To say that "a car slows down" or to talk about the speed of a car at all, without saying in what frame it is measured is meaningless. That's the first lesson of "relativity".
  5. Dec 17, 2009 #4
    In reference to a person who's stationary. I want to understand how distance is relative.
  6. Dec 17, 2009 #5
    I am really starting to dislike this phrase "time slows down." I started out perfectly fine with it, but it certainly seems to engender a large amount of confusion. The phraseology makes it seem as though speed is absolute. I'm starting to get irritated by Brian Greene as well.

    It would be better to phrase it like "clocks run slow when they are measured to be in motion." When a clock is moving, relative to you the observer, then you will observe that it runs slower than your own clock. Your question is asking if time dilation makes really fast things start going really slow as they get faster. Hopefully, saying it like that makes the answer obviously no.

    There is no such thing as "stationary" in an absolute sense. Whether a body is stationary or not depends on your frame of reference.
  7. Dec 17, 2009 #6
    That does answer my question. Thank you! Could someone give me a simple example of how distance is relative when moving relative to one another?
  8. Dec 17, 2009 #7
    You observe that moving meter sticks (again, moving relative to you) are shorter than your own meter sticks, in the direction of their motion.

    For instance, take the twin paradigm in which a spacecraft travels from the Earth to Vega at nearly c. The earth observer observes the clocks on board to run too slowly, and the ship to be shorter than the pilot does. So when he reaches Vega, his clock has only ticked a few seconds (let's say) even though in the Earth's reference frame, the trip took ten years.

    On the other hand, the ship pilot considers himself to be at rest and the Earth and Vega to be in motion. His observations are that the Earth's clocks are running slower than his own. Nevertheless, he explains the fact that he arrives at Vega in only a few seconds by the fact that the distance between Earth and Vega is very small, much smaller than the Earth observer's measurement of 10 light years.
  9. Dec 22, 2009 #8
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