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What is the theory of relativity?

  1. Dec 16, 2006 #1
    I'm trying to understand the theory of relativity can someone tell me (in English!) if i'm close to correct.

    The theory of relativity says that the closer you get to the speed of light the faster time goes (i think someone said it was impossible to get to the speed of light)

    e.g two people got to somewhere really far away
    person A is at 0.99 the speed of light and person B isn't

    the journey for person B takes 100 years but for person A only a short time has passed.

    Does that make any sense?
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  3. Dec 16, 2006 #2


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  4. Dec 16, 2006 #3
    so in english, what is the theory of relativity?
  5. Dec 16, 2006 #4


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    In plain English, Special Relativity says:

    1. The laws of the universe in a non-accelerating reference frame are the same everywhere.
    2. The speed of light is constant.

    But that doesn't really tell you how time dilation works...
  6. Dec 16, 2006 #5
    I've think i've got the idea of time dilation, time is percieved as constant for everyone but time could be going for faster for someone than me but that person would still think that time is passing normally.

    so am i right about the idea of time sppeding up for me as i got closer to the speed of light?
    Last edited: Dec 16, 2006
  7. Dec 16, 2006 #6


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    In relativity there is no objective notion of speed, you can only talk about your speed relative to some other observer. If I measure you to be moving close to the speed of light in my rest frame, then I will measure your clock to be ticking slower than mine. But as long as both of us are moving inertially (constant speed and direction, no acceleration) the situation is symmetrical--you measure me to be moving close to the speed of light in your rest frame, and you measure my clock to be ticking slower than your own. If one of us accelerates to catch up with the other, though, this symmetry is broken, and whichever one accelerated will be found to have aged less between the time we first passed each other and the time we reunited later. You might want to read some information on the twin paradox to learn more about this.
  8. Dec 16, 2006 #7


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    No, your (received) time period would be less than that measure by a 'stationary observer', hence, time would 'slow down' for you. I suggest you re-read the link which Zappers gave you and apply the Lorentz transformations (or more particularly the time dilation formula) to your situation.

    Edit: JM got there before me :smile:
  9. Dec 16, 2006 #8


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    You've got the time dliation backwards.

    The short part that you are looking for, wrt time dilation, is that if you travelled at close to the speed of light for a long time, time outside your spaceship would appear to march by faster. When you arrive at your destination, decades or centuires may have passed everywhere but aboard your ship.

    You'll need to read up to understand it better, it's not simple.
  10. Dec 16, 2006 #9


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    That statement is somewhat questionable--it depends what you mean by "time outside your spaceship would appear to march by faster". If you're travelling from Earth to Alpha Centauri, then in the rest frame of your ship, clocks on Earth and Alpha Centauri are ticking slower, not faster, since after all in your rest frame it is they that are moving at relativistic speeds while you are at rest. However, in your frame the clock at Alpha Centauri is considerably ahead of the one on Earth (that's the relativity of simultaneity--the two clocks are synchronized in the rest frame of the Earth and Alpha Centauri), so that explains why the difference of (time at Alpha Centauri's clock when you arrive) - (time on Earth's clock when you depart) will be greater than the time it took you to get from Earth to Alpha Centauri according to your own clocks.

    On the other hand, if you're not talking about what's happening in the ship-observer's frame, but rather what the ship-observer sees using light-signals, then due to the Doppler shift, you'll see clocks at Alpha Centauri ticking faster than your own as you move towards it, while you'll see clocks at Earth ticking slower than your own as you move away from it (slowed down by a greater factor than just the time dilation factor). The issue of what you see vs. what you measure in your rest frame was discussed at length on this thread.
    Last edited: Dec 16, 2006
  11. Dec 16, 2006 #10


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    It's best not to think in terms of the speed that you are traveling at, because (a) it depends on what you're measuring the speed with respect to, and (b) in your own reference frame, you yourself are always stationary!

    It's better to think in terms of the speed(s) that other objects are traveling at, in your reference frame, when analyzing those objects' behavior in your reference frame.

    The quickest way I can think of to summarize time dilation using that point of view, is: "A clock that is moving with respect to you, runs slower (in your reference frame) than a clock that is stationary with respect to you. The faster it moves, the slower it runs. But in the reference frame of another observer who is moving along with the clock, it always runs at its normal rate."
    Last edited: Dec 16, 2006
  12. Dec 16, 2006 #11


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    There are several different ways of describing relativity. The simplest treatment that I'm aware of is "Al's relativistic adventure" at http://www.onestick.com/relativity/ (this is the winning entry to Pirelli's relativity challenge, the challenge being to explain relativity as simply as possible).
    Last edited: Dec 16, 2006
  13. Dec 17, 2006 #12

    Suppose I consider the famous law E = m c^2.
    Is this law violated when observed from an accelerated reference frame ?
    (I think so, but I 'd like to have this confirmed)
  14. Dec 17, 2006 #13
    accelerated E=mcc

    I think not if in
    E=m0cc/(1-VV/cc)^1/2 you replace the instantaneous velocity which depends on the accelerated motion you consider (hyperbolic). Probably that answer will generate better answers. We find a simillar situation in the case of the Doppler Effect with accelerating observer and stationary source of light.
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  15. Dec 17, 2006 #14
    Actually there is no such thing as an accelerated reference frame.
    Furthermore the speed of light for an accelerated observer is no longer c. In the extreme an accelerated observer can even travel in such a way that a light beam following him can never catch up with him.
  16. Dec 17, 2006 #15
    accelerated reference frame exists?

    As far as I know an accelerated reference frame and what takes place inside it is known in special relativity. E.A.Desloge and I.Philpott, "Uniformly accelerated reference frames in special relativity," Am.J.Phys. 55 (3) 252-261 (1987)We can consider the problem in the reference frame considered as stationary and relative to which the body of rest mass m performs the hyperbolic motion its velocity never reaching c and in which light propagates with c.
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  17. Dec 17, 2006 #16
    Actually you are wrong (Bernhard has already given you another very good reference). See another one http://arxiv.org/PS_cache/physics/pdf/0509/0509161.pdf [Broken]
    Last edited by a moderator: May 2, 2017
  18. Dec 17, 2006 #17
    Really, so for instance an observer who has a proper acceleration of 1G for say 10 seconds can be represented by one single frame? How?

    By the way I am not quite sure what the relevance is to the referenced document it seems to deal with the twin paradox. :confused:
    Last edited by a moderator: May 2, 2017
  19. Dec 17, 2006 #18
    A sequence of frames. Try reading the second reference I gave above.
  20. Dec 17, 2006 #19
    And each single frame is everywhere orthonormal?
  21. Dec 17, 2006 #20
    I agree that the speed of light for an accelerated observer is no longer c. I'm not sure about the non-existence of an accelerated reference frame. Suppose one replaces "reference frame" by "observer" (since I guess that a frame is used as a tool for an observer), then one can certainly talk of an accelerated observer. Nothing prevents an accelerated observer to make observations of the things around him, just like an observer in constant motion. On the other hand, I'm not sure of the actual rigourous mathematical definition of reference frame, so I could be wrong :confused:. Some help of the experts would be useful here.
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