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Confused about Special Theory of Relativity?

  1. Nov 10, 2012 #1
    Hi, I have a question (or maybe more) about special relativity. I'm reading The Fabric of the Cosmos by Brian Greene right now, and what I got from one of the sections is confusing. I might be understanding it wrong, or maybe what he's saying is wrong, or maybe it's just right. So this is what I'm confused about:
    Greene says that if we run away from light, in relation to us, it will stay at the same velocity. This is because velocity is just distance/time, and if we measure time slower while we're going faster, we will measure light as going faster. Everything will even out, and we will measure light as going at the same speed. Well, if this is true, then wouldn't this be true for all things coming towards us? Wouldn't everyday objects maintain a constant speed (in relation to you) as well? Sorry if I just got all of this completely wrong, I'm relatively new to physics.
    On a sort of unrelated note, why is our four-velocity "c"?
    Thanks
     
  2. jcsd
  3. Nov 10, 2012 #2

    Simon Bridge

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    Well, since that is a popular science book, it is probably a mixture of all of the above ;)
    ... time dilation and length contraction happen to keep the speed of light constant. Other things are going slower - so you see times and lengths, associated with them, adjusted by a different amount. The amount of the adjustment depends on the relative speed.
    It isn't. It is the magnitude of the 4-velocity which is a constant. We usually choose the constant to be 1 and measure distances in terms of the speed of light ... so if time is in seconds, then distance is light-seconds, and |V|=1.
    That happens because time dilation and length contraction are complimentary terms.
    I take it the book does not have a lot of math in it?
     
  4. Nov 10, 2012 #3
    Oh, thanks a ton! That really clears things up. And I should have been more clear with the part about our four-velocity; I did mean magnitude. Why is our four-velocity MAGNITUDE the speed of light?
    And no, the book has no math in it :S
     
  5. Nov 10, 2012 #4

    Simon Bridge

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    Neato. Note: JIC...
    That happens because time dilation and length contraction are complimentary terms. It just a way of keeping track of them.
    It is really hard to describe this stuff without math.
     
  6. Nov 10, 2012 #5
    Yeah, I know, but Greene usually does a decent job of it. Do you know how I can move forward faster in physics? I'm 15 years old and in my first year of physics (not with calculus) in high school. We're studying Newton's Laws right now, and before that studied motion. I read a ton of popular science books, and I'm teaching myself calculus. Are there any books, resources you can recommend? Are the Feynman lectures on physics any good?
     
  7. Nov 10, 2012 #6

    micromass

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    Any good?? They're practically the best!! But don't use them as a primary textbook though, they're not made for that. Use them as a secondary resource.
     
  8. Nov 10, 2012 #7
    What should I use as a primary source? (considering I want to go a lot faster than my class, which is going painfully slowly)
     
  9. Nov 10, 2012 #8

    micromass

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    Perhaps you should ask this in a new thread in the science book forum: https://www.physicsforums.com/forumdisplay.php?f=21
     
  10. Nov 10, 2012 #9
    Thanks, I will :)
     
  11. Nov 11, 2012 #10
    I am decades older than you, and my methods of educating myself are exactly what you already use. Keep it up. You'll find that school is a great place to get knowledge in a systematic, step by step manner, and that when you supplement it with self taught subjects, you'll learn at a prodigious rate.

    BTW, the Feynman lectures are excellent. Additionally, many notable colleges and universities have video lectures and other resources. Check out MIT and Harvard. I've used their resources, and they are truly excellent.
     
  12. Nov 11, 2012 #11
    Check out the curriculum at some good universities. They often have required reading lists which include their main textbook on the subject you are interested in. If/when you find one book is used commonly, that is a decent indication that it is a good textbook.

    Buy it used on eBay for short money, or go to your local college bookstore and pay a little more for a used copy.

    And if you can't decide which of a couple alternatives is better, ask here. This place has lots of good people who are happy to help.
     
  13. Nov 12, 2012 #12
    just an FYI, if you have more than a "layman" interest in this stuff, I suggest not reading much, or at least not thinking much about popular physics literature.

    Terminology is important, and at 15 you still have remarkable abilities to "absorb" new concepts / terminology. May as well start off on the right foot.

    To word this differently, there is literature out there that is more respectful of physics terminology & concepts & is not biased to any particular "school of thought". imo Greene does not meet those expectations of mine.

    So yea, always be conscientious of who the author is.
     
  14. Nov 12, 2012 #13

    Simon Bridge

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    There are more than a few questions here arising from Greene's pop-sci books ... thinking about it, basic college texts should be accessible to a motivated 15yo. You've almost got the tools at that age and, with the internet, you can back-fill the gaps. It would take a bit of determination though.

    Something like cosmology 101 materials should be fine.
     
  15. Nov 14, 2012 #14
    Sorry to bother you again, but I just thought of this; how does Greene's logic (which I explained in my original post) make sense if you're moving TOWARDS light? Wouldn't it be a lot faster than if you were standing still?
     
  16. Nov 14, 2012 #15

    Simon Bridge

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    Your question actually does not make sense in terms of relativity:
    for instance: "standing still" has no meaning. With respect to what?

    If you rephrased your question so it made sense, then the answer would be apparent.

    We already know that Greene's logic is flawed - there is no point exploring it further than that. As written, the argument given for an object moving away from the observer does not apply to one moving towards the observer. Move on to real physics.
     
  17. Nov 14, 2012 #16
    Yeah, you're right. I'll finish up my homework on slants and return those Greene books to the library...
     
  18. Nov 15, 2012 #17
    As you summarized it, he seems to have omitted the very important issue of synchronizing distant clocks. See the following discussion:
    https://www.physicsforums.com/showthread.php?t=641102
    In particular posts #3, #10, #12+#15+#38, #18, #46.

    Then, when you understand the main points (if you are not a genius then that should take some time), you could next try to follow my calculation example in post #50. :tongue2:
     
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