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Foundation to the theory of relativity

  1. Oct 15, 2004 #1
    Hey, i don't know if this is the right spot to post this question, but I was reading "The Universe in a Nutshell" by Steven Hawkins. Got through the first few pages. I am just wondering what is "Einstein's postulate that the laws of nature should appear the same to all freeli moving observers was the foundation of the theory of realtivity, so called because it implied that only relative motion was important."

    This is very weird to me. Also, what did they mean by The speed of light always travel at the same speed relative to you or that object?
  2. jcsd
  3. Oct 15, 2004 #2


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    I'm not sure what you mean by "weird" but the statement is basically that the laws of physics are the same everywhere and there is no "preferred" frame of reference. The constancy of the speed of light is based on observations indicating that the motion of neither the observer nor the source have any effect on the speed of light. All attempts to demonstrate the contrary have failed and Einstein took it one step further. He adopted the constancy of the speed of light as a postulate from which Special Relativity and later General Relativity follow.
  4. Oct 16, 2004 #3

    "The constancy of the speed of light is based on observations indicating that the motion of neither the observer nor the source have any effect on the speed of light"

    can anyone go on detail about that?
  5. Oct 16, 2004 #4


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    I haven't read that particular book, but I'd be surprised if Hawking didn't explain it further. You may want to continue reading.

    But in a nutshell, if you have a guy walking on a moving train (the train is moving at 10m/s), to the guy walking, he thinks he's walking forward at 1m/s while a woman watching him from the platform measures him to be walking forward at 11m/s due to the forward motion of the train. If he throws a ball out the window at 10m/s (according to him), perpendicular to the train, she measures it at 14 m/s, traveling at an angle forward due to the train's motion. But if they carry sophisticated scientific equipment with them and fire lasers at each other, they will both always measure the laser light to be traveling at C.

    The stuff about the man's speed and the ball's speed seems to make sense. Its based on Newtonian physics and that's also the first postulate of SR (that the laws of the universe are the same for both the man and the woman). Einstein adds the second postulate: that the speed of light is constant.
    Last edited: Oct 16, 2004
  6. Oct 16, 2004 #5
    thanks, that made it clearer.

    Another question I have is: If someone was to travel at light speed, would their age remain constant? Such as the "Twin's Paradox". Anyone can detail?
    Last edited: Oct 16, 2004
  7. Oct 16, 2004 #6
    It's impossible for someone to travel at light speed, so it's really a pointless question. If you use c in Einstein's equations, then yes, the amount of time that an observer in another reference will observe to pass for the miraculous light speed traveler does come out to be 0. However, that same observer will see other things happening to you as your speed approaches c, namely that your mass will approach infinity. Therefore, you can never get that final push to travel at light speed since your mass (or more appropriately, relativistic mass, since it's another observer that notes the change) increases so you always need a little bit more to speed up. Since at c, your mass would hypothetically be infinite and therefore require an infinite amount of energy in order to reach light speed, it's impossible.
  8. Oct 16, 2004 #7
    The twin paradox is something quite different, and involves the question "Who aged less?" It's actually not really a true paradox, and has nothing to do with impossible light speed travel.
  9. Oct 16, 2004 #8
    cool, i'm trying to narrow tihngs down for me to understand. According to what you said,

    -Your mass will continuously increase while traveling the speed of light
    -If i really were able to travel at the speed of light, my age will remain constant?

    can someone confirm that?
  10. Oct 16, 2004 #9


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    Here's a better way of putting it.

    You can't travel at 'c', but you can approach it as a limit. You age less and less as you approach the speed of light, and in the limit (which you can't actually reach), you would not age at all, relative to a non-moving observer.

    Not that from your point of view, you don't experience anything odd, time seems to flow just as it always did. When you return from a long trip travelling at high velocities, though (this necessarily involves acceleration at some point), you'll find that the people you left behind aged more than you did - a lot more, as you reach higher and higher velocities.
  11. Oct 16, 2004 #10
    how fast of a trip would it take to notice the difference in aging?
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