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Kind of peculiar

  1. Jun 28, 2005 #1
    It's the old question of "if you're in a spaceship going near the speed of light, and you turn on your headlights...."

    Think about this: earth is spinning around the sun. The sun is spinning through the Milky Way. The Milky Way is moving away from other galaxies. That's a lot of moving around. Kinda like being in a fast-moving spaceship.

    If an experiment to measure the speed of light was conducted on Earth, wouldn't the results be skewed a little because of Earth's motion? i mean, if Earth were (just go with it) going near the speed of light, and you turn on a laser (which is moving along with the earth) and fire at a target (also moving, but in relation to the laser appears stationary) in order to measure the speed of light, it seems to me that your results would be a little wacky. Space and time would be warped...right?

    Now throw in the fact that Earth is moving around the sun, which is moving through a moving galaxy. Who knows how fast all the galaxies in the universe are moving, because nothing in the universe is completely stopped.

    Is any of this making sense? it seems to me that in order to get a very accurate speed of light, one would have to be completely stationary. But one isn't, is one?

    Any thoughts? or have i gone and confused myself again?
  2. jcsd
  3. Jun 28, 2005 #2


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    The Michelson-Morley experiment looked for variations in the measured speed of light due to the Earth's orbital velocity, just as you describe.

    It didn't find any such variation, no matter what the season, the speed of light was always found to be constant.

    This led to the idea that the speed of light was constant for all observers, regardless of their state of motion - the idea of special relativity.

    You might google for the MM experiment, google finds


    which looks like a pretty good summary. The next lecture gets into how relativity explains the null results of this experiment.
  4. Jun 30, 2005 #3
    If you want more info beyond the answer above, I suggest you go to a large bookstore and check out the laymen books on special relativity. There’s a lot of good, easy ones, like Relativity Visualized, and Understanding Einstein's Theories of Relativity : Man's New Perspective on the Cosmos.
  5. Jun 30, 2005 #4
    I am going to fix this sentence, in a way that is key to understanding special relativity (and resolving all your paradoxes):

    If the earth {according to person X} were going the speed of light, Space and time would be warped {according to person X}...right?

    Keep in mind that person X is someone who actually see's the earth moving near the SOL. We certaintly don't see that, from our point of view the earth is stationary so time and space are fine.
  6. Jun 30, 2005 #5


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    One of the foundational postulates of relativity is that all inertial (non-accelerating) observers measure the speed of light to be the same relative to themselves. In other words, if I see a light beam zoom past me at 186,000 miles/second, then I see you zoom by in your spaceship at 100,000 miles/second, that does not mean that you will measure the light beam to be moving at 86,000 miles/second relative to yourself as would be true in Newtonian physics...instead, you will also measure it to be moving away from you at 186,000 miles/second. This has to do with the fact that velocity is measured in terms of distance/time, but different observers' clocks tick at different rates and their rulers are different lengths, and also because of the fact that different observers have different definitions of what it means for two clocks at different locations to be "synchronized".
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