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Idiot trying to grasp relativity with a couple questions

  1. Dec 3, 2008 #1
    I am no scientist, nor did I stay at a Holiday Inn Express last night. I have been trying to understand this theory for a while, but it seems that my questions never get answered in what I read. Either that, or I am just too stupid to get it.

    As an example, I have been reading "The Dogschool of Mathematics presents: The Dummies' Guide to Special Relativity" and I am doing just fine right up until they talk about the light clock in chapter 6. From the image I can clearly see how the light moved a greater distance from the initial frame of reference's point of view and therefor made a single tick take a longer amount of time than it would have from within it's own frame. Got cha there.

    Where that fails to register in my brain is if you take an electronic clock and do the same frame by frame analysis are you saying that the electrons in the electronic circuits are taking longer to flip the transistors which causes the clock to run slower? If so, are you not making the assumption that the electrons are moving parallel to the line instead of perpendicular? Would not the fact that all electrons move from one terminal of a battery back to the other terminal of the battery in a loop cause it to balance out?

    Basically what I can not grasp is that if two clocks are keeping time accurately, the speed of travel of one relative to the other seems irrelevant.

    I realize this has probably been asked a million times, so I welcome links or suggested reading that will clearly explain this to my limited mind. Thank you in advance.

  2. jcsd
  3. Dec 3, 2008 #2


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    Hi Allan,

    Imagine that person A has both an electronic clock and a light clock with him, ticking at the same rate. Now if person A starts moving relative to person B, we know that the light clock will appear to tick slower. But what about the electronic clock? If the relative ticking rate of the two clocks are different when A is moving, A could tell that he was moving. This is not allowed by the principle of relativity, which says that A should not be able to tell whether hes moving uniformly or not, so the two clocks must tick at the same rate. Therefore the electronic clock must also slow down according to B.
  4. Dec 3, 2008 #3
    I think that when we speak about the light clock, it is better to consider that at each of the two mirrors we find an electronic clock all in the inertial reference frame I'. Considering it from the I inertial reference frame we should consider three electronic clocks located at the point where from the light signal starts, at the point where it arrives on the upper mirror and at the point where it arrives after reflection respectivelly. Pythagoras' theorem will do the job to derive the formula that accounts for the time dilation relativistic effect.
  5. Dec 3, 2008 #4
    OK, I must be really stupid.....

    I see the work of the light clock because as the light travels up, it is also traveling to the side, that creates an angle which is a longer distance than if it stayed stationary and traveled up and bounced back. Got that.

    When you put an electronic device there, there may or may not be an angle depending on the flow of electrons through the circuits of the clock. Example:

    If the circuit travels up and down, then the result would be obvious just like the light clock (now this is not to say I agree with it, but I understand what is being said). However if the circuit in the clock is parallel to the motion of the frames, then there is no angle, and Pythagoras' theorem does not apply.

    See, the angle idea I get, what I can not fathom is how the discussions always assume a perpendicular travel of the electrons in a clock. What happens if most of the circuitry in the clock is parallel to the plane of travel?

    Now once we get that figured out, how about this for a stupid question:

    If you assume we tell time using the light clock, it becomes obvious why our measurement of time slows down. From what I have read, they say this applies to a biological system as well. While I certainly see the application as it applies to say, breathing, where the breath of air would take longer to breath as it has to go further, wouldn't the strain of pumping the blood further, being harder to breath, etc, actually make the body age faster? Not to mention slow your reaction time as the neurons had further to go.

  6. Dec 3, 2008 #5


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  7. Dec 4, 2008 #6


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    It doesn't matter how the other clock works, for the purpose of this argument. This argument assumes as a starting point that the principle of relativity is valid. According to the principle of relativity, there is no way that we can detect that our laboratory is moving at constant velocity, by doing experiments that are confined inside the laboratory, that is, we don't "look outside." This includes comparing the ticking rates of different kinds of clocks (mechanical, electronic, heartbeat, light-beam-and-mirrors) inside the laboratory, or in any other inertial reference frame.

    We single out the "light-clock" as part of the analysis only because it is easy to analyze by assuming that light always travels at the same speed regardless of whether we're observing it from inside the laboratory or from the outside, and regardless of the direction that the light travels.

    The two assumptions stated above cannot be justified by purely logical means. They can only be verified by experiment: either by direct observation or by testing predictions of the theory that is logically derived from them.
  8. Dec 4, 2008 #7
    Here's the simple answer, Allan. Relativity is all about how light and other electromagnetic radiation behave when traversing empty space, so the functioning of a Light Clock is indeed a special case without great parallels to other devices. It serves as an apt example.

    You wanted a link. http://placido.u21.0web-hosting.com/kavs/kjs/briefingetr.html" [Broken]
    Last edited by a moderator: May 3, 2017
  9. Dec 4, 2008 #8


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    No, that's wrong, relativity is about all laws of physics--the first postulate of SR is that the laws of physics will look the same in every inertial reference frame (this is what jtbell meant by the 'principle of relativity'). One physical way of thinking about this is that if two experimenters are in windowless ships moving inertially at different speeds relative to one another (or relative to some external landmark like the Earth) in deep space, any experiments they both do inside the ship will have the same result, so neither one can tell their speed relative to anything external without looking outside the ship. So, imagine each experimenter has a light clock along with some other type of clock sitting next to it--if one experimenter sees her light clock ticking at the same rate as her other clock next to it, this must be true for the second experimenter's two clocks as well.
  10. Dec 4, 2008 #9
    That's right. He's right, Allan, about everything he said, except where he implied that my statement was wrong.
  11. Dec 4, 2008 #10


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    I took your statement "the functioning of a Light Clock is indeed a special case without great parallels to other devices" to mean you were saying that one couldn't generalize conclusions about time dilation in a light clock to other types of clocks. Even if that's not what you meant, then your statement was likely to confuse Allan, since clearly his line of questioning was about whether or not the analysis of time dilation on the light clock could be generalized to other clocks, so the most obvious interpretation would be that you were answering his question in the negative.
  12. Dec 4, 2008 #11
    Righto, good chap! Later you realized I was truly addressing his concern with the assertion, "it serves as an apt example" -- the quintessential example perhaps. And then you graciously provided a bridge to the needed generalization
  13. Dec 4, 2008 #12


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    But given your previous statement "Relativity is all about how light and other electromagnetic radiation behave when traversing empty space, so the functioning of a Light Clock is indeed a special case without great parallels to other devices", I thought you meant it was an apt example of how "light and other electromagnetic radiation" behaves, "without great parallels" to non-electromagnetic devices. Anyway, you've clarified your meaning so it's a moot point.
  14. Dec 4, 2008 #13


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    I rather think you clarified his meaning for him :rolleyes:
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