Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about time dilation

  1. Jan 15, 2015 #1
    Please forgive my ignorance in STO, but I don't understand something about the following thought experiment.
    Spaceship flies from Earth with large speed. We observe the clock inside the spaceship from Earth with strong telescope.
    We see the clock showing 12:00. When we see on the clock 12:05 (in telescope, from Earth), the ship will be further from Earth, so it will take to the light more time to reach us, so on Earth more than 5 minutes will pass. From this I deduce, that for observer on Earth the time on the spaceship goes faster, than on Earth. But if the spaceship approaches the Earth, then we well see 12:05 in less than 5 minutes, and it means the clock on the ship goes slower, than on Earth.

    From STO we know, that clock always goes slower in a moving system (for external observer).

    Where is the mistake in my thought experiment?

    Thanks in advance.
  2. jcsd
  3. Jan 15, 2015 #2


    User Avatar
    Science Advisor

    I think you have faster and slower the wrong way round there.

    What you are describing is the Doppler effect. It would happen in a Newtonian universe too. It's the cause of the pitch change of a siren when a police car passes you.

    However, in relativity, once you have corrected for the speed of motion of the source, you find that you still have the clocks running slow. That is "time dilation" at work, and happens whether the spaceship is coming towards you or going away.

    There is nothing wrong with your reasoning. You will see clocks ticking slower on ships going away from you and faster on those coming towards you. But correct for the varying travel times of the light, and in a relativistic universe you will always find that the other fellow's clocks are running slow.
  4. Jan 15, 2015 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Note that the time delay caused by the finite speed of light when making observations is NOT the basis of time dilation. When thinking about these problems you must factor out any time lag. One way to do this is to think about a Reference Frame, rather than a single observer. A Reference Frame can consist of many observers (all at rest with respect to each other) with synchonised clocks and who will agree about length measurements.

    In your experiment, you could consider two observers at rest with respect to the Earth, who observe the ships clock as it moves past them. In this reference frame, the ship may take 1s, say, to travel from one observer to the next. Each observer notes the time on the ship's clock. The second observer will find that the ship's clock has moved forward by less than 1s, and conclude that time is running slower on the ship than in the "at rest" reference frame.
  5. Jan 15, 2015 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    The effect you're talking about is called the relativistic Doppler shift. If a clock is moving toward you, you will see it running faster, because the delay for light to travel from the clock to you keeps getting shorter. If a clock is moving away from you, you will see it running slower. Neither of these effects are relativistic time dilation. Relativistic time dilation is the computed rate of the moving clock after taking into account the travel time for light.
  6. Jan 15, 2015 #5
    OK, I agree, that in your experiment they will see the clock slowed down. But what is wrong with my experiment?
    For example, particles coming from Sun to Earth have longer lifetime, as their time slowed down. Say, particle approaches us and not passes by, and the particle has a clock and we observe it on Earth - how do you show the time slowes down in this case?
  7. Jan 15, 2015 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    If you only have one observer, then he/she must take into account the time lag. The clock reading you see as a clock moves towards you very quickly is partly explained by the movement of the clock coming towards you (speeding up the reading) and partly explained by time dilation (always slowing down).

    I would strongly recommend trying to understand time dilation without the time lag first. Once you understand the time dilation between reference frames, you can learn about the relativistic Doppler effect.

    One thing you might like to look up is the Transverse Doppler effect. This is a Doppler effect of motion at right-angles to your observation and is caused purely by time dilation.
  8. Jan 15, 2015 #7


    Staff: Mentor

    Typically this is done simply counting the number of particles at two different distances. Unstable particles carry a clock with them, their half life. It is an inaccurate clock, so we have to average over a very large number, but over a large number we can compare the "at rest" decay rate clock to the "moving" decay rate clock. We find that more particles survive the trip then you would expect from the "at rest" rate. The exact amount more is as predicted by their decay clock ticking slow in SR.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook