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New to Relativity:Need Answers

  1. Feb 23, 2005 #1
    New to Relativity:Need Answers!!

    Hello guys,

    It took me forever to become a member on here and get everything activated. Well I am new to SR and GR and I have more questions than answers. I read Michio Kaku's Book on Einstien. It was a really good book and I got some understanding of how Einstein became so great and why he had such a impact on the world of Physics. It seems that no matter how many times I read over the book and read websites trying to explain SR and GR I just don't get a few things. One thing that I have a hard time trying to understand is time dilation. I suck at math and don't understand any equations that someone throws at me. I really want to know why is it that time slows as you approach the speed of light without someone talking alot of stuff I don't understand. What is the physical explination that can explain time dilation.

    My other question is, what do you see if you were able to travel the speed of light. I remember in Kaku's book he talked about Einstein sitting in a cab and that he visualized what it would look like if the cab approached the speed of light. From what Einstein visualized thats when he had his big breakthrough with SR. Now I can't really understand how you still see something behind you, The Clock Tower that was mentioned in the book, when you are moving away from it at the speed of light.Also if you were an observer and you were watching someone speed off at the speed of light what would they look like, would they just disappear or would they become thin which is what i was lead to believe in the book.If someone can give me some easy explinations as to what I have asked or maybe some easy to read refrences on the net, note I have read alot of websites on the net about SR and GR and no luck on understanding, than maybe I can get a clear understanding about what it all means. I have a deep obsession to find out what it would be like to see all this happening.

    Thanks so much!

  2. jcsd
  3. Feb 23, 2005 #2


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    Imagine that one of your friends is going to set out on an interstellar spacecraft tomorrow, and is planning on going a significant fraction of the speed of light. You'd like to keep in touch with him, so you decide to erect a massive beacon on Earth -- a big lighthouse which flashes brilliantly once every second. Your friend will be able to look in his rear-view mirror and see the flashes.

    In turn, your friend puts a similar beacon on his spacecraft. It will flash once a second, and you'll be able to see its flashes as he flies away from you.

    When your friend is flying away from you, each of the flashes from his beacon has a longer and longer trip back to you, so the time between the flashes doesn't appear to be one second anymore. Instead, it seems like the flashes arrive only once every two seconds, or ten, or more, depending on just how fast he's going relative to you. The faster he's moving relative to you, the more widely separated in time his beacon's flashes will appear to you. All the while, your own beacon appears to flash normally, once per second.

    Your friend on his starship sees the same thing -- his own beacon flashes normally, but your beacon appears to be flashing slowly.

    That's time dilation.

    You are suffering from a very common misconception about relativity. People often say "time slows down when you go fast," but that just isn't true. It's a misconception. The truth is that time appears to slow down for an object that is travelling quickly with respect to you. In other words, time dilation doesn't exist in one person's frame of reference. No matter how fast you go, your own watch (or beacon) will appear to run just the same. Time doesn't slow down.

    On the other hand, when you look at someone who is moving at a high velocity with respect to you, it will appear as though that person's watch (or beacon) is running slowly.

    Time dilation requires two frames of reference, moving at high relative velocity. Each observer will measure the other's clocks running slowly.
    That's a nonsense question, because, in this universe, it's not physically possible. You can imagine if you'd like that photons experience neither time nor space -- the entire universe seems to be zero size, and it seems to take zero time to go anywhere.

    If you watched your friend's beacon as he flew away, accelerating to the speed of light, you'd see the flashes get further and further apart in time. If he reached the speed of light (which isn't actually possible), the flashes would stop -- you'd never see another flash. In fact, you'd never receive any more radio communications from him, or any other information. If he's travelling away from you at the speed of light, then his communications, which are also travelling at the speed of light, cannot ever "escape" from his spacecraft.

    - Warren
    Last edited: Feb 23, 2005
  4. Feb 23, 2005 #3
    I understand your response to a point. Now you say the beacon would emit a light back to me but the light would take, say 2 seconds instead of 1, and the faster you go the longer the light takes to reach you. Now you would see more that just the light beacon, you would see the light from the spacecraft right. If he was able to stick his hand outside the window and wave bye to me, you would also see the light from his hand and the whole ship around him frozen in time, right? Maybe I should of explained what I don't understand fully. With the twin paradox, explain why is it that one can return from a trip at the speed of light and when he returns the earth has aged more than he has. Einstein said that clocks beat differently depending on where you are in the universe. Well if I was at some distant star, say 200 light years away, and I wanted to sync my clock to earth time I could do this in my frame of reference and know that my clock is set at exactly the same time as on earth in their frame of reference.
  5. Feb 23, 2005 #4


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    Not "frozen in time," just slowed down, in the exact same way as the beacon.
    The only explanation I can tell you briefly is the technical one -- the moving twin's path has a shorter proper time than the stationary twin. For more meaty explanations, try this good site:


    Not unless you're considering gravity, with the general theory of relativity, is this true. In special relativity, which deals only with movement, clocks that are stationary with respect to each other appear to each other to run at the same rate. Clocks that are moving with respect to each other appear to each other to be running slowly.
    If you knew the exact distance and were exactly at rest with respect to the Earth, then yes, you could synchronize the clocks.

    - Warren
  6. Feb 23, 2005 #5


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    FWIW this should be in the GR subforum.
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