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Time Dilation - Relative to Light Speed or Reference Point - I'm Confused?

  1. Nov 16, 2011 #1
    Hi,
    I've been listening to a Professor Jim Al-Khalil's scipod on time travel. I think this guy is bloody excellent by the way.

    http://reader.mac.com/mobile/v1/http%3A%2F%2Fwww.jimal-khalili.com%2Fpodcast%2Fjim.rss [Broken]


    Anyway,
    He said to an observer on the earth a clock on a rocket (traveling close to light speed) slows down. And yet to an observer on the rocket the clock on the earth slows down (because relatively the earth is speeding away from the rocket).

    But from what he says it's only the person on the rocket that experiences the REAL effect on time slowing down?


    So,
    If your rocket was zooming around the earth in a circular path (close to the surface). And both yourself on the rocket and an observer on the earth could see each others clocks.

    As the rocket slowed down prior to landing, would the observer on the rocket see the clock on the EARTH speed up and catch up with, and then overtake the clock on the rocket? Surely it MUST.

    I think also he assuming that for observational purposes the speed of light is instantaneous, otherwise you have to factor in all sorts of variables
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Nov 16, 2011 #2
    :) "As the rocket slowed down prior to landing, would the observer on the rocket see the clock on the EARTH speed up and catch up with, and then overtake the clock on the rocket?"

    Why would it overtake it?

    When the rocket slows down to a standstill relative Earth, it will be at rest relative Earth. The only effects left to differ the clock on Earth from the clock in the Spaceship will be gravitational, and that is a form of motion too according to the equivalence principle.

    What it means is that the Earth could be seen as 'constantly uniformly accelerating' at one Gravity. So even though being 'at rest' relative Earth the spaceship still will be further out of the 'gravity well' (and so 'motion') that Earth defines. This means that the clocks still should differ, with Earth's clock being slightly 'slower' according to the spaceship, due to Earth being further down in the 'gravitational potential'.

    Ahh, yes. I see how you thought there.
     
  4. Nov 16, 2011 #3
    But it really makes no sense discussing it. It would make sense if I could assume that I could get more done by relative motion. But I won't. According to my own clock nothing ever change for me, no matter how fast I move relative something else. So going close to light does not allow me more 'time' to do anything, even if Earth finds me being in 'suspended animation' relative it.

    There is no way for you to 'speed up' or 'slow down' that local time.
     
  5. Nov 16, 2011 #4
    Thankyou for your reply but it's a bit beyond me!

    I just thought that if the clock on the rocket runs slower (real time?) than the clock on the earth.

    But to the astronaut on the rocket, the clock on earth runs slower.

    Then at sometime the hands on the clock on the earth have to catch up with, and then overtake the hands on the rocket clock?

    Because the astronaut on the rocket (near light speed) is the only one who experiences time running slower? But Im probably confused
     
  6. Nov 16, 2011 #5
    He* you're not the only one :)

    'Time' is a local effect, your watch keeps a 'same time' no matter where you go, or how fast. As soon as you compare a 'frame of reference' relative your clock, you automatically define it from your 'local time' finding that other 'frame' to go 'slower' or 'faster' right?

    But if you go to that 'frame', and measure that 'time' by your own 'clock', there will be no difference. Your 'clock' will not 'tick slower/faster' so the definition of 'time' being different can only exist relative 'frames of reference'. But it's also real, as proven by the twin experiment, and as by NIST using very sensitive clocks on earth, showing them to differ due to gravity at as short distances as half a meter.

    If 'time' really would give me more 'time' to do things by being in a lower gravity, then we should have noticed it (astronauts etc). But we don't.

    Although the twin experiment allow one twin to be younger when reunited, all things he did was to him done in a normal manner, and locally he got no more 'done' by that motion than if he had stayed at home. The years that passed for him went in the usual manner according to his clock, and the same goes for the twin staying at home.
    =

    So yeah, it's confusing :)
     
  7. Nov 16, 2011 #6

    phinds

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    Dickey boy, it's not nice to post a link that can ONLY be viewed on an iPhone.
     
  8. Nov 16, 2011 #7
    Sorry didn't realise, although I'm not using an iPhone

    For you: http://www.jimal-khalili.com/podcasts

    But I don't think it's rocket science (pun intended) for anyone interested to google the guy.
     
  9. Nov 16, 2011 #8

    ghwellsjr

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    Normally, when we discuss the reciprocal effect of time dilation, we are ignoring the additional effects from gravity which, if we were actually doing the experiment described, we would have to take into account, but since this is a thought experiment, I'm going to assume that the professor was only focusing on the time dilation due to speed differences.

    OK, look at this comment:
    He said to an observer on the earth a clock on a rocket (traveling close to light speed) slows down. And yet to an observer on the rocket the clock on the earth slows down (because relatively the earth is speeding away from the rocket).​
    This is true when the rocket is traveling in a straight line away from earth and not in orbit. This would be a situation where each sees the others clock as running slower than their own and by the same amount. But you have to realize that there are two effects going on here. One is the actual time dilation and the other is the delay caused by the finite speed of light in transmitting the image of the clock over an ever increasing distance. The combination of these two effects is called Relativistic Doppler.

    However, if the rocket turns around and heads back toward earth, then the observer on the rocket will immediately see the clock on the earth speed up because now the delay in transmitting the image is getting smaller due to the ever decreasing distance. However, the observer on the earth will not immediately see the image of the rocket's clock speed up because he has to wait for the image of the turn-around to be transmitted to him. The net result is that when they finally get back together, the rocket clock will have less time on it. The reason is that the earth observer sees the rocket clock running slow for way more than half of the trip but the rocket observer sees the earth clock running slow for exactly half the trip.

    So now we have explained this comment:
    But from what he says it's only the person on the rocket that experiences the REAL effect on time slowing down?​

    OK, now the professor is going to discuss a different situation where the rocket is in orbit around the earth:
    So,
    If your rocket was zooming around the earth in a circular path (close to the surface). And both yourself on the rocket and an observer on the earth could see each others clocks.​
    And we have to assume that the earth is transparent so they can each see the others clock.

    Now we have a situation where the rocket is continually changing its direction although its speed is constant. Every time it is going away from the observer on the earth, the observer on the rocket will see the earth clock running slow and every time he is going toward the observer on the earth, he will see the earth clock running faster, so it appears half going faster and half going slower. But the guy on the earth will not see the same thing. He will see the clock on the rocket going slower for more than half the time and going faster for less than half the time. The net result is that every time the rocket completes an orbit and flies over the observer on the earth, the time on the rocket clock will be less than the time on the earth clock.

    So the rocket finally comes in for a landing:
    As the rocket slowed down prior to landing, would the observer on the rocket see the clock on the EARTH speed up and catch up with, and then overtake the clock on the rocket? Surely it MUST.​
    Yes, this is true, but it's no different than what he has been seeing on each orbit. On each orbit, he sees the earth clock speeding up and gaining more time than it had on the previous orbit. Of course, the guy on the earth is also seeing the rocket clock speeding up but since he doesn't see it speeding up until near the end of the orbit, it doesn't pass the accumulated time on his own clock.

    And, as to your last comment:
    I think also he assuming that for observational purposes the speed of light is instantaneous, otherwise you have to factor in all sorts of variables ​
    No, the speed of light is never instantaneous. If it were, we wouldn't have to worry about relativity. It's the finite speed of light that causes the differences in images of that each observer sees. But there is also the real effect of time dilation which can be reciprocal for two observers while they are separated but when they come back together, ends up with the traveler's clock having accumulated less time.
     
  10. Nov 17, 2011 #9
    @George

    Thankyou for detailed answer. I probably muddied the waters (for myself at least) by overcomplicating it.

    What I can't get my head round (but it's my problem) is that both objects experience the SAME time dilation and yet when they meet up later, only ONE has?


    Two objects in free space

    One traveling away from the other at close to light speed

    Both experience the SAME time dilation effect
    (To an observer on each object, time on the other object has slowed down)

    And yet if they were to meet at a later date in the future and compare clocks.

    ONE clock (the one traveling at close to light speed) would show less time passed than the other clock (traveling slowly relative to light speed).

    This is what confuses me???
     
  11. Nov 17, 2011 #10
    Back Again!!!

    I've re-read and re-read all the answers several times and I think at last the penny has dropped! (English phrase).

    It is hard (for me a least) to get my head round, but I'm pretty sure I get it now.

    Thanks Y'all
     
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