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Can someone explain curved space time?

  1. Feb 26, 2009 #1
    I have become very interested in physics and astronomy recently and have started dabbling.

    I have been reading about curved space time and its association with gravity. I've also been reading about time as it is related to travel at the speed of light.

    Can anyone explain the concept of curved space time using some type of simple analogy that might stick with me? Also, why is it that time slows as we near the speed of light? Are the two related?

    Thanks for being patient with someone who crunches numbers for a living!
  2. jcsd
  3. Feb 26, 2009 #2
    Curved space time and gravity is known as General Relativity. Search google for a picture of it. It's a simple concept when you see it. Basically, get a garbage bag and suspend it over the ground. You can hold it or make some crazy contraption, but it need to be horizontal or parallel to the ground. Take a ball and put it in the middle. This is a very simple representation of gravity. Roll another bouncy ball on the garbage bag and it will curve to the ball.

    In reference to speed vs. time, that's known as Time Dilation. It's part of Special Relativity. The faster you go, the slower time travels relative to someone not moving as fast. From your point of view, it wouldn't feel like time is moving slower at all.

    General Relativity and gravity also affect time. The more gravity, the slower time goes relative to someone under less gravity. Again, you wouldn't feel like time is running any slower. It all revolves around the fact that gravity affects space and time. I don't think this is related to Special Relativity, but I could be wrong.

    All of these concepts are difficult to explain. I'm sure someone else here could do a better job than I can.
  4. Feb 27, 2009 #3
    Thanks Zorn dawg,

    I've seen a similar example to what you told me, so I am ok with the concept of things like light curving in space to adjust to the mass/gravity of an object in space. I'm just having a hard time getting my head around time being different the faster you go or the less gravity you're exposed to.

    I saw an example where someone was trying to explain how theorectically time could curve and cause closed time loops or curve and go backward in time and I was like "uh, I don't think so" but then I read about curved space time and it almost seems as though some physicists agree, at least in part.
  5. Feb 28, 2009 #4
    I am just an interested amateur. My way of thinking about space-time is that matter displaces space-time. So the earth being matter displaces space- time around it: causing space- time to warp a little. This warping is gravity. Another way I look at it is that the earth is not pulling you to the surface but that the warping of space-time is pushing toward the surface. My thoughts may be way off on this and if so :someone please let me know.
  6. Feb 28, 2009 #5


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    Time, gravity and the smallest measure of distance are related.

    Time changes at differant rates in differant places.
    Time changes the quickest where there are no objects.
    Objects tend to slow down time in their general vicinity. That is time can not go as fast near objects. In addition, the smallest measure of distance (subatomic) expands as one moves closer to objects.

    Gravity is the response of objects to differances in the minimum distances-rate of time. That is objects save energy by moving towards areas of slower time (with accompanying larger minimum subatomic distances).

    In general:

    dS^2 = c^2*dT^2 + dD^2


    dS is differance in space time,
    c is the speed of light,
    dT is differance in time,
    dD is differance in distance.
  7. Feb 28, 2009 #6
    The basic idea is that space itself has a "shape", and the shape of the space depends on the distribution of matter and energy within it. So there are basically 2 parts here - 1) try to get a feel for what "curved space" means, and 2) learn how the presence of matter effects the shape of space.

    For a long time I was really stuck on concept 1): that space could even have a "shape" on its own. I could only think about it in terms of being embedded in a bigger space, which is a very limiting way to view it.

    To get a good intuitive feel, I would highly recommend the book "The Shape of Space" by mathematician and MacArthur fellow Jeffrey Weeks. It was written so that smart highschool students or interested laymen could understand it, and it has lots of pictures and analogies to explain things, but it is the real-deal - there are no "little white lies" or oversimplifications. Somehow by the end of the book it actually manages to build up to a very respectable level in terms of the mathematical concepts discussed.

    Anyways once you have got 1) figured out, then 2) should follow pretty easily.
  8. Mar 2, 2009 #7
    I did see an illustration this weekend that made sense. It was the ol' plastic sheet with a weighted ball in the middle illustration. That made sense to me in terms of how objects cause distortions in the "fabric" of space and how smaller objects are drawn to that larger object.

    The problem I'm still having is that there seems to be contradictions in the relativity of time as it has been described to me, but I know that's not the case.

    For example, I've seen that time would slow to basically a stop if one could travel at light speed and that time runs faster at someones head than at their feet when they are standing on earth because they are closer to the "mass" of earth at their feet. I guess I'm thinking about being out in space and traveling at light speed as being contradictory to the analogy of time moving faster at your feet, because if you are out in space it seems that time would be moving faster.

    Anyway, I'll try and find the book that maze suggested. I do love this stuff! Thanks for the help.
  9. Mar 2, 2009 #8
    In Einstein’s theory of special relativity he says that traveling at the speed of light can effect time
    In Einstein’s theory of general relativity he says that a gravitational field can effect time
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