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Relativity doubts

  1. May 25, 2003 #1
    I'm not sure that i'm going to understand relativity someday. Well, I can comprehend that each frame of reference has its own time. But for me time is not something physical, I believe that time is simply the velocity with which physical phenomena occurs. If i am travelling with a clock, then the hands of the clock will rotate more slow than if I am at rest.
    Then I imagine two points A and B, and a beam of photons that made his way crossing first point A and then point B. I am at certain distance, for example point C. When a photon pass through point A, I click my chronometer, and stop the chronometer when the photon pass through point B. Perfect, 5 seconds for example.
    Now, imagine that I'm going to repeat the experiment. I'm in point C.
    But when the photon cross through point A,I click the chronometer and then I start a little trip, and return at point B just in the moment that the photon pass through point B, and I stop the chronometer. During the trip, the chronometer must have been going slowest than in rest, then now I've measured another time for the voyage of the photon from A to B I've measured 2 differents velocities for the light!
    I'm misconcepting something? I would appreciate any help.
    Last edited: May 25, 2003
  2. jcsd
  3. May 25, 2003 #2


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    Wrong. If you're travelling in a comoving frame with the clock, then you will never notice anything funny going at all. The clock will operate, in every way, normally.
    You can't do this. When you're at point C, you can't say exactly what's happening at what time at point A or B. You'd have to involve the period of time it took the message to travel to you from those points.
    This is also false. The length of the spacetime interval (think of it as a distance in space-time) between any two events is constant, and all observers will always agree on that length.

    You're still getting caught up in the idea that "going fast makes time slow down," which is a butchered statement.

    - Warren
  4. May 25, 2003 #3


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    Greetings meteor !

    You are correct, time is the velocity of
    physical proccesses. In fact, special relaitvity
    is based on two basic premisses:
    1. The laws of nature are the same in any
    frame of reference (for everyone).
    2. The speed of light c, is a constant.

    I suggest you try these links:

    Live long and prosper.
  5. May 25, 2003 #4
    yes,I will not notice nothing strange, because my neurons, will go slower than commonly, and I will think that all is going OK. But this
    doesn't change that INDEED, the chronometer is going more slow than in rest, so I still believe that I will measure two different times.
    Last edited: May 25, 2003
  6. May 25, 2003 #5


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    This is incorrect. I'm not going to say it again.

    - Warren
  7. May 25, 2003 #6
    I don't understand general relativity also.
    I mean I get it, but what good is it? I mean if were the complete truth then why are many people searching for a quantum gravity theory that unifies all the forces and shows relationship?

    I asked my brother(a musician), "if you were travelling at the speed of light next to someone else travelling at the speed of light what would you see?".
    He replied, "well if you were moving at the speed of sound next to someone moving at the speed of sound would you hear them?".

    I posted my take on GR in topic "time is what what?" by Tesla, maybe it'll give you another way of viewing it Meteor maybe not. To me GR explains how time and gravity relate very well but not how gravity works, not it's relationship to the other forces.
    Last edited by a moderator: May 27, 2003
  8. May 26, 2003 #7


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    It provides correct predictions to a lot of experiments to which Newtonian gravity does not.
    It most certainly is not "the complete truth." At the very least, it is incompatible with quantum mechanics. Eventually a theory will have to be created that unifies the two in a physically verifiable way. This would be called the 'grand unified' theory. While we're making rapid progress, this theory is still beyond us.

    - Warren
  9. May 26, 2003 #8
    Which is the physical explanation for the FitzGerald-Lorentz contraction? That is, why the dimensions of an object reduces at high velocities?
    The same for the mass, why the mass of an object augment with the velocity?
  10. May 26, 2003 #9


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    You essentially just asked me to teach you the whole of relativity theory, which I do not have the time to do. I suggest you buy any of the great number of good introductory books on the subject, or browse some of the many good websites.

    - Warren
  11. May 26, 2003 #10
    no no.....
    you will not measure the two different time
    because you still go with your reference frame
    you slow down, and the clock also
    you and clock are in the same reference frame
    the two different time will occur when the other people try to measure your time when you are moving but he are not
  12. May 26, 2003 #11
    Technically, your neurons don't slow down if your scenario where possible.

    Relativity describes a universe where everything is relative. So for you, time flows 'normally' and you only appear to be moving in slow motion to someone else; that doesn't mean you are in your perspective. Comprehendo?
  13. May 27, 2003 #12
    No, I don't comprehend. I think that if somebody see me moving in slow motion, is because i'm really moving in slow motion (and my neurons too). But well, i will try to learn more about relativity and i will tell you.
  14. May 27, 2003 #13


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    You see a tree in the forest -- a great tall one. Say you're half a mile from the tree -- it looks rather small.

    Say your friend Sam is right at the base of the tree. To him, the tree looks extremely large.

    How is it possible that the tree looks small to you and large to him? After all, it's the same tree -- right?

    - Warren
  15. May 27, 2003 #14
    Originally posted by jammieg
    I mean I get it, but what good is it?

    Well if GR/SR wasn't correct or we did not understand it, GPS systems that we use for almost all navigation would not operate correctly. Planes and boats would end up 1000's of miles from were they're suppose to be. Physics Today May 2002 had a very good article on GPS system and corrections for relativity.

  16. May 27, 2003 #15
    Thanks for correcting me on that, surely there are lots of uses I don't have a clue about. GR is what I feel leaves too many unaswered questions to be the whole truth.
    Last edited by a moderator: May 27, 2003
  17. May 27, 2003 #16
    If you're going to use your own definition of time, rather than what time is, then you surely won't understand relativity.

    No, if you travel with a clock, it will not matter weather you're moving or still.

    The time that the photon passes A for the photon is not the same time that the photon passes A from any other position at all whatsoever in the entire universe, including your point at C.

    I would question where you ever read about reletivity, as pretty much everything you said was incorrect. I hope you didn't have a teacher this poor at making you understand? I would recommend you pick up a reletivity book!
  18. May 27, 2003 #17
    GR is what I feel leaves too many unaswered questions to be the whole truth.

    Hardly any theory will explain any and everything. It explains what we know now and maybe what we will discover in the future, if not it will be modified or replaced all together.

  19. May 27, 2003 #18

    When a scientist so adamantly proclaims a claim to fact, she (or he) means that it is a fact NOW, and will work with any data involved. But it is not always a fact.
  20. May 27, 2003 #19
    I have read some books. For example, in "The universe in a nutshell",S.Hawking, page 6, puts that the clocks slow down when they are not at rest. This is what i'm saying!
    And what happen with the twin paradox? One twin is in earth, and the other returns to the earth after a trip for the universe. When return to the earth, the traveller twin is younger that its earthbound twin, but this is because he has been travelling at high velocity and then the biological processes in his body has been occurring at slow rapidity, I think
  21. May 27, 2003 #20
    Then I suppose you're seriously misunderstanding the concept.

    The twins don't age any faster or slower at all reletive to themselves.

    It's just reletive to eachother that things change.

    What is your age? Perhaps you're reading a bit early!

    You must remember the difference between you to yourself, and you to anything other than yourself.

    You're just missing the concept of reletivity.

    An event appears different from wherever in the universe you percieve the event, no two points view it the same way.
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