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Does time depend on speed; or on velocity?

  1. Oct 11, 2015 #1
    Does the passage of time depend on speed only or on the direction as well?

    As far as I understand, the passage of time is different for a person approaching me and different for a person leaving me. So isn't velocity the cause of time? If not, why not?

    I am not an expert in Physics, I completed my college or high-school if you call it that way and am trying to pursue a career in Aviation Science, along with Physics as a passion.

    If possible, please do explain in layman's term, as I don't have enough understanding of advanced physics.

    PS - I don't believe in the concept of time. It's rather an illusion for me. As time is more of a philosophical study too, hence I wouldn't want to argue on this. Please just let me know whether space-time, aka, time is dependent on speed or velocity? If not on velocity, please explain why. A bit of maths is okay, I can understand them.
  2. jcsd
  3. Oct 11, 2015 #2


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    Locally, time passes identically for everything. Observations of an object that is moving in a frame of reference in which you are stationary will depend on speed, but other frames of reference will see that same object has having yet other passages of time. In other words, measurements of things that are moving in a frame are frame dependant.

    The AMOUNT of time that passes will depend on the world-line which is why the Twin Paradox looks like (but isn't) a paradox.
  4. Oct 11, 2015 #3
    I'm pretty sure that the time, as seen by you, runs with the same speed for someone approaching you, and for someone leaving you, or passing nearby (if they move at the same speed in each case, as seen by you).

    As for the questions about what is time, or what causes time to exist, these questions are rarely physical, and often philosophical. Philosophical questions can't really be answered by a physicist.
    If you have a good physical theory about time, let's hear it.
  5. Oct 11, 2015 #4


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    No, let's not. According to the site rules "We wish to discuss mainstream science.That means only topics that can be found in textbooks or that have been published in reputable journals." (Further details are in the rules.)
  6. Oct 11, 2015 #5
    That wasn't very clear to me. Do you mean it's dependent on speed then?
    (Apologies for my ignorance).
  7. Oct 11, 2015 #6
    I do have some theories and researches done (along with my elder brother - friend who is an electrical engineer), but as I don't have a perfect understanding of advanced mathematics, I can only explain them in words. However I don't intend doing that here, moreover, now I came to know it's not allowed either, fair enough!
  8. Oct 11, 2015 #7


    Staff: Mentor

    With respect to a given inertial frame three clocks which are all moving at v, one to the left, one to the right, and one in a circle, will all tick at the same rate.
  9. Oct 11, 2015 #8


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    The perception you have of the flow of time in an object that is moving relative to you depends on how fast it is moving relative to you, but something that is moving at one speed relative to you can be moving at a different speed relative to a 3rd observer and so that observer will not see the same flow of time in the object as you see, he will see a flow that depends on how fast it is moving relative to him.

    That makes it easy to see that time dilation has to be something that you observe in objects that are moving relative to you but NEVER something that you experience yourself, since you can be simultaneously assigned an infinite number of different time dilations from an infinite number of different reference frames.
  10. Oct 12, 2015 #9
    Thanks all, that helped :).
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