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Where does time travel fastest? And a moving galaxy/light speed question.

  1. Jan 12, 2008 #1
    Hello, I’ve been browsing these forums for a while now; I like physics and I get really interested in all sorts of physics stuff. My main interest is space because I find it really fascinating.

    I have two questions that I’ve been curious about for a while now;

    The first question is about time dilation, I believe that I have a good grasp on certain aspects of it; I have been interested in how time takes place slower on moving objects and how time moves slower with a greater gravitational potential (being near to earth)

    I understand that we observe that the passage of time moves slower nearer to the event horizon of a black hole because of time dilation; then is there, somewhere in the universe a place with absolutely no gravity or movement where time takes place a lot quicker? So if we were to take a clock, and place it where there is no gravitational effect at all, would it tick quicker compared to clocks on earth?

    I also understand that our galaxy moves tremendously fast through the universe, if the clock was be slowed down to complete stop (no longer moving with our galaxy), would that cause the clock to tick faster? Even though it would seem that the clock is moving and we are staying still, but that wouldn’t be the case, because we move through the universe and it would be the clock that is not moving.

    My second question is a little more simple; if all the galaxies are moving at near to the speed of light away from each other and away from the universe centre, would that mean within one of those galaxies you would not be able to send a light beam from point “A” to point “B” at the speed of light (relative to the galaxies inhabitants) with point “A” being in the centre of the galaxy and point “B” being near to the edge of that galaxy (the edge leading the galaxy away from the centre of the universe)

    Because if you sent the light beam from “A” it would not be able to travel as fast as light in a vacuum because of the speed that galaxy is already moving at, otherwise the light beam would have moved quicker than light speed because the galaxy is already at almost light speed.

    (Sorry about my terrible English.)

    Thank you very much

    Joseph G
  2. jcsd
  3. Jan 12, 2008 #2
    Joseph G,

    I'll try to answer just your second question:
    I think you have two misunderstandings here, although they are somewhat unrelated. First, it is not really correct to speak of the "center of the Universe," as such a point is not defined (that I know of). I wonder if you say this because you imagine that all the objects in the Universe must be moving away from a single point, because the distances between them are all increasing(?). This is not the case. Imagine a 2-D universe on the surface of a sphere that is growing in radius. All the points on the sphere are moving farther apart, but they do not move from a single point. If you can extend this picture to our 3-D spatial Universe, that's a closer picture of what the expansion of the Universe means.

    As for the speed of transmission of light between bodies moving at near the speed of light themselves - no, you're making the common mistake of applying Galilean relativity rather than Einstein's Special Relativity. You could ask the same question about a beam of light going from one of end of a train moving at near the speed of light to the other end. The answer is that all observers will still see light traveling at the same speed (c), regardless of their motion relative to the source of the light. That fact is what leads to such things as time dilation and length contraction.

    So, for the case of observers in the galaxy moving at close to light speed, they would see the light cross their galaxy in a very normal way, since as far as they are concerned, their galaxy is at rest. To an observer who sees the galaxy moving by at close to light speed, he will see the light travel from one edge of a very much length-contracted galaxy to the other, and in the time it takes to do that, the galaxy will have moved some distance, too. The total distance he sees it move is then the distance across the shortened galaxy plus the distance the galaxy moved in that time, but he will still see light travel at speed c.

    By the way, your English is excellent. If you had not apologized, I would never have guessed that English was not your first language.
    Last edited: Jan 12, 2008
  4. Jan 13, 2008 #3
    This makes sense, however, would the galaxies still not be moving away from an original starting point, like in the balloon example (before it was blown up)? Or not?

    That’s a really hard concept for me to get to grips with, how comes the galaxy is length contracted?
    Thank you for your answer, I only wish I could understand this science a little more.

    English is actually my first language, I think it’s so bad though, I apologised in case anyone thought that I rushed writing the questions and wasn’t more willing to put more time and effort into them, which I did.

    Thank you very much for your reply, I think I just need to get to grips with the science behind this.

    Joseph G
  5. Jan 13, 2008 #4
    No, that's an easy mistake to make, though. In the 2-D example, it's important to keep in mind that we're talking only about the 2-D surface. Inhabitants of that universe don't know about the third dimension in which you can talk about the center of the sphere. This might be unconvincing, since it seems that we need the 3rd dimension just to talk about the 2-D surface being curved. In fact, it was a very important breakthrough in mathematics when Gauss discovered that the curvature of a surface could be defined without making any reference to a higher-dimensional space in which the curved space is embedded. What that means is that we could talk about our 3 spatial dimensions (or even our 4 dimensions of space-time) being a "closed" surface (actually, a "manifold", when you talk in general) like the surface of a sphere. The analogy of the "center of the sphere" in that case would be in a space of one higher dimension, but we are not aware of any such thing.

    Another way people suggest to think about this is to imagine a rectangular grid, where all the distances between the corners are growing at the same rate (imagine zooming in on a checkerboard with your camera lens). You'll see the whole thing expanding, but there is no "central point" from which everything is clearly receding. Compare that to the paths taken by debris from an explosion, where all the paths clearly point to a central location.
    That's a result of Special Relativity. I recommend that you go and read up on this if it interests you. One of the appealing things about this theory is that it does not require math more advanced than algebra, so it's approachable by non-experts. That also means that there are a lot of people out there who think they understand it, so you have to be careful of what you read! :wink:
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