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Hi,I just started reading

  1. Aug 4, 2011 #1
    I just started reading "The Elegant Universe" as an introduction to physics. Up to this point I thought I understood everything but now I get a to a part that doesn't make sense to me and was hoping someone could explain it.

    If you have the book you can follow along with what I am talking about... at the bottom of page 74 and beginning of page 75 he talks about George and Gracie floating in space, each with large digital clocks attached to them. He also states there is a line attached to a nearby space ship and then to the sun. As George pulls himself toward the sun Brian Greene states that his clock will go slower and slower the closer he gets to the sun. This seems contradictory to what has been said about time previously..... the faster an object is moving the slower time goes. Because Gracie is farther away from the sun (takes her longer to make one orbit around the sun) then she is moving faster than George and so Gracie's time clock should be moving slower than George's.

    If you do not have the book and don't understand what I am asking... basically I think what I am trying to understand is: How does gravity affect time?

    I'm am not sure what it is that I do not understand but if someone could explain it to me I would really appreciate it. Like I said, I am very new to this so please explain in very general easy to understand terms.

  2. jcsd
  3. Aug 4, 2011 #2
    Re: Confusion

    I don't know if it is possible to explain accurately without diving into the mathematics of GR. In a simplified way, the deeper you are in a gravitational potential well, the slower your clock ticks as observed by observers far away from any gravitational influence.
  4. Aug 4, 2011 #3


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    Re: Confusion

    You don't need General Relativity, just...
    Are you quite sure? Haven't you forgotten Kepler's 3rd law?
  5. Aug 4, 2011 #4
    Re: Confusion

    I just read the excerpt and in simple easy to understand terms: Greene didn't say they were orbiting the sun.

    Given that, you were right to be confused, great insight! As Greene mentions, the effect of gravitational time dilation in our solar system is small, and relative velocity may have had a larger impact.

    A decent real world analogy to objects experiencing both types of time dilation are satellites. I think for GPS satellites the dilation caused by relative velocity outpaces that of gravitational dilation.
  6. Aug 4, 2011 #5
    Re: Confusion

    Certainly have NOT forgotten Kepler's 3rd law.... I just stated in my original post that I am reading this book as in introduction to physics... I have never learned Kepler's 3rd law. Thanks for the input though.

    So... to reply to the others that posted.... Is he talking about just gravity and for simplicity's sake not talking about the orbit around the sun?

    I'm sorry... I still don't completely understand. The more gravitational force the slower time moves relative to an outside observer?
  7. Aug 4, 2011 #6


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    Gold Member

    Re: Confusion

    Yes. Time is passing a bit slower here at the bottom of our gravitational well than it is up on board the ISS.

    Here is a link about a guy that demonstrated this with nothing more than a super accurate clock, a camper van and a family vacation up into the mountains!


    Executive summary: After 2 days spent at an altitude of 5400 feet, his Cesium clocks has gained 22 nanoseconds.
    Last edited: Aug 4, 2011
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