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Questions About Absolute/Relative Velocity and Time

  1. Dec 14, 2009 #1
    OK, so I know time and space are connected and that as you approach the speed of light that time slows down.

    They have shown this by synchronizing a clock on earth and a clock on a plane or even in space. In addition the GPS satellites actually use a different length of second relative to us.

    So, now I assume that if you brought your watch to mercury or mars.....the amount of time that passes on either will actually be different.

    Question #1- Since the earth is actually hurtling through space in our transit around the sun. The satellites rotating around the earth experience different time.....yet I assume that our speed through space is actually quite a bit larger than then speed of the satellites. Not to mention that since the satellites rotate around the earth they are travelling around the sun at the same relative speed because it would be clocked from the earth center. Why is time different on the satellites?

    Maybe I can answer my own question in that the orbit of the satellites aren't on the same plane as the earths orbit around sun so the velocities aren't relative? Or does it have to do with the fact that the satellites are both hurtling through more space.

    Question #2 - If time slows down when we are moving faster. Why can we not determine an absolute velocity? It seems that if experiments were conducted where you for example fired a clock in different directions and gauged the time slow down vs speed that is experienced from each direction you could at least determine earths 3 dimentional vector of velocity through the universe at that point in time. You could then repeat this experiment to constantly keep rediscovering this new vector.

    Question #3 - Time was originally invented on the basis of the sun in order to organize our society. We now seem to measure it by vibration of atoms or quartz or whatever. My question is. If we are Hurtling around our sun....and our sun is hurtling through the galaxy? How come our way of gauging time by quartz and atom vibrations doesn't change day to day? Our absolute velocity is constantly changing as the earths orbits the sun(sometimes the earth would be moving in same direction as galaxy and others not). Or is it actually happening but the speed of galaxy moving through space is overwhelming factor and therefore locks in our time perception for all intents and purposes??

    These things are hurting my brain:(
  2. jcsd
  3. Dec 14, 2009 #2
    Question #1: The time dilation for the satelites measured from the surface of the Earth is a combination of velocity time dilation (unless it it in a geocentric orbit around the equator) and gravitational time dilation. Time moves slower in a strong gravitational field.

    Question #2: By launching similar clocks at the same speed in different directions away from yourself, you would measure the exact same time dilation on every one, as if you are "standing still" with respect to space. It is a fundamental principle in relativity that you cannot distinguish between the situation "A travelling towards B" and "B travelling toward A".

    Question #3: You seem to be confusing how time dilation works. Any time dilation you measure in with respect to the from from which you make the measurement. As long as you keep yourself at rest with respect to a clock, it will exhibit no time dilation to you. However, any observer moving with respect to the clock will disagree with you. So if you are at location X, watch the Earth orbit the sun, time on Earth will seem to move slow to you. This will be a combination of velocity and gravitational time dilation.
  4. Dec 14, 2009 #3
    There is no such thing as "absolute velocity." You are making the mistake of thinking that there is some state of absolute rest, at which clocks all run the fastest. That is incorrect.

    It is sloppy and basically incorrect to say that "as you approach the speed of light that time slows down." Time always runs normally for you. Time dilation is something that happens to bodies that are moving---- moving with respect to *you*, not with respect to some absolute rest frame. You should say that clocks you observe to move run slow.

    In other words, If you are in a rocket traveling at nearly c, an observer on Earth will observe that your clock runs slow. But you on the rocket consider yourself to be at rest, and it is the Earth that is moving. Thus you observe your clock to run normally and the Earth's to run slow.

    #1. There is no such thing as absolute speed through space. Time dilation in special relativity is due only to the speed of a clock in your own reference frame. Add general relativity, and time dilation is also caused by gravity. Therefore, satellites have clocks that run at different rates because (a) they are moving with respect to us, and (b) they are in a different part of the gravitational field. Earth's motion around the Sun or the Sun's motion around the galaxy make basically no difference.

    #2. Time does not slow down when you are moving faster. You are always at rest in your own reference frame. The experiment you suggest will not work because time dilation is not determined with respect to a mythical absolute rest frame. It is determined with respect to you. Fire two clocks in different directions, and they will run slow by an amount determined only by their velocities as measured by you. Fire two clocks at the same speed (any directions) and their rates will always have slowed by identical amounts.

    #3. None of this makes any difference. There is no such thing as absolute speed through space. You are at rest in your reference frame. All your clocks always run normally. That is why it is called "Relativity," and not "Absoluteness."
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