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Explain Relativity Please

  1. Oct 19, 2007 #1
    Could some one please try to explain relativity to me? How can time be slowed down?
  2. jcsd
  3. Oct 19, 2007 #2


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    No-one's going to be able to "explain relativity" to you in an internet forum. Do you have any specific question? And, no, time is not "slowed down" anywhere in relativity theory.
  4. Oct 19, 2007 #3
    A simple clock consisting of two mirrors A and B, between which a photon is bouncing. The separation of the mirrors is L, and the clock ticks once each time it hits a given mirror. In the frame where the clock is at rest, the photon traces out a path of length 2L and the period of the clock is 2L divided by the speed of light. From the frame of reference of a moving observer (diagram at lower right), the photon traces out a longer, angled, path. The second postulate of special relativity states that the speed of light is constant in all frames, which implies a lengthening of the period of this clock from the moving observer's perspective. That is to say, in a frame moving relative to the clock, the clock appears to be running slower. could you explain this to me. in a simpler manner
  5. Oct 19, 2007 #4
    Arguments like this are unnecessarily strong. "Time is not slowed down" is right assuming that the original claim "time slows down" meant something that was wrong, but the "time slows down" can mean something that is right also.
  6. Oct 19, 2007 #5
    in what instance is it right
  7. Oct 19, 2007 #6


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    In order to compare the rate at which clocks tick, you have to define some notion of simultaneity.

    In relativity, simultaneity is relative, which confuses a lot of people. This means that A can think that B clock is slow, while B also thinks that A clock is slow.
  8. Oct 19, 2007 #7
    Let me describe an experiment done at the cyclotron at Princeton in the 1950's. Muons were accelerated to near the speed of light by the cyclotron and their rate of decay while in the cyclotron measured in the laboratory. Sure enough the rate slowed greatly for the moving muons. Since nothing else known changes the rate of decay of a muon, it was deduced the reduced rate of decay as measured in the laboratory was due, in fact, to their high rate of speed in the cyclotron. The speed of the muons was known by the experimentors; after all, they had caused to motion. The experimentors then calculated the slowing was quantatively in accord with the predictions of Special Relativity.

    What caused this result? For reasons not well understood all observers, irrespective of their motion with respect to each other, measure exactly the same physics in their own laboratories. This means when one physicist moving with respect to another observers the other doing an experiment, he doesn't get the same result as if he had done the experiment in his own frame. Some people say they understand why this is, but others find it troublesome since in other systems where objects move with respect to a medium, disturbances caused by objects take place with respect to the medium. In Special Relativity they don't. To say that Special Relativity is true because it treats time like a coordinate and is invariant under rotations of its coordinates is to say it's true because it's true.

    Since no one esle can answer your question, please feel free to find the answer and be remembered forever. Be forewarned, however, it may be hard to get the work published even if its dead on. Be prepared to receive your Nobel in heaven. Best wishes.
  9. Oct 19, 2007 #8


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    Well, consider the twin paradox scenario--if I leave Earth at relativistic velocities and return, my clock will have elapsed less time than my twin on Earth, and I will have aged less as well. In this case I think it's not unreasonable to use the shorthand "time ran slower for me" to describe this situation.
  10. Oct 19, 2007 #9


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    I think it is better to first focus on the ticking-events rather than on the clock-rate.
    One can say with more clarity and to the point:
    the time-interval between the two successive ticks of the inertially moving clock
    is longer than the time-interval between two successive ticks on my identically-constructed inertial clock.
    Discussions of rates or simultaneity or whatever can follow next.
  11. Oct 19, 2007 #10


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    One of the big problems I see is that people start out with a preconceived notion of absolute time. I think you may be right about your explanation being more direct, but I think to have a chance of succeeding it needs to address this unfortunately common preconception.

    What do you think of:

    One can then go on further to explain that space is also affected in a similar manner, and that there is an invariant space-time interval, or perhaps in some other direction.

    I'm also not sure about how to handle the "perceived vs derived" issue, that comes up a lot too. The best word I could come up with was "assign".

    What do you think? I still have the feeling that people will try and fit this into an "absolute time" framework, without spending even more words on the topic. But I'm not sure how to avoid that. Also, I'm not sure how clear it is from the wording of the post that the "ticks" should be regarded as events (points in space-time),probably the concept of events as points in space-time and "interval" as a measure needs to be further explained as well. More words, and probably some diagrams, before one can expect comprehension :-(.
    Last edited: Oct 19, 2007
  12. Oct 19, 2007 #11
    Sure. Its too much to do in a post or a thread so for questions on things like relativity etc. I started creating a web site for each instance of a problem that I needed to do some math and for which pictures help. It eventually grew into a many paged website. The site for special relativity is here


    I tried to organize it such that you start with #1 and go to #2 etc. Its still a work in process so order may be out of seqquence further down.

    Time dilation is easily demonstrated if you start with the postulates of relativity and use a light clock and then you can derive the expression for time slowing down. E.g. see start with #2, "Light Clock"

    The site for general relativity is here


    Please ask anything you need to in order to understand my explanation as well as anything that you think of as a result of reading them.

    Good Luck

  13. Oct 19, 2007 #12


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    My suggestion is very direct because you can directly compute the spacetime coordinates of the next tick (back at the local mirror)... using ordinary kinematics. At this point, there's no need to discuss simultaneity explicitly or other observers... except that one tick has elapsed for the moving clock. (Soon after one should invoke the principle of relativity.)

    So, get that part established first.
    Then, if you want, proceed to address the other preconceived notions of absolute time.
    So, I'm not disregarding them... they can be addressed... but later.

    Personally, I would prefer such a statement later... after my direct statement.
    I think if too many unfamiliar things (e.g. warnings) are raised at the beginning, your target audience will be lost.

    I would use "assign"... [derived from a radar measurement].

    The notion of an event is [or should be] familiar from ordinary distance-vs-time diagrams... as one finds in the kinematics chapter of an introductory textbook. But for some reason, "events" and "spacetime" are regarded as some kind of strange construction. Certainly, when one does a boost on the situation, there's some complications... just as there would be when doing a Galilean-boost. However, at this stage, I'm just referring to drawing a distance-vs-time graph of the relevant events.... nothing more.
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