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I can't get it through my head!

  1. Mar 7, 2004 #1
    Relativity has been so hard for me to understand! One minute, it makes sense, and the next I am lost. I kind of understand how time can go slower based on the view it is taken from, but I can't understand how a person could actually not get older? That is, if the person is going really fast:

    "Suppose that a 25 year old mother leaves on a rocket trip. She travels at .99 c relative to the earth and is gone for 30 years measured by earthtime (=t). How old is the mother, in terms of physiological age, when she returns? How old is her daughter, who was 2 when the mother left?" So, the mother comes back and is 29.23 years old, and her 2 year old daughter is now 32 years old. It doesn't make sense...but it does...Of course, no one can go .99 c (99% the speed of light, right)

    I don't even know what I am going for anymore. I can't even find out the answer to the question I wrote above, for it all is just too confusing. But, at the moment I did it in class, it was okay.

    Sorry, I am just too lost to even figure out what I am trying to ask!
  2. jcsd
  3. Mar 7, 2004 #2


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    The mother has spent the time while 30 years passed on earth in an environment where time relative to earth ran slower.

    The key point is that there is no absolute time. There is your own (proper) time, which runs at the usual rate for you. And there is your time relative to somebody else, which depends on the speed between the two of you and is figured by the formula that you evidently know how to work.

    The mother really did only experience 4.23 years in her ship while 30 years were passing on earth.

    People can't yet travel at 99% of the speed of light but particles can, both cosmic rays and the particles sped up in accelerators. And they do indeed have slow time relative to the "stay-at-home" equipment that detects them.
  4. Mar 7, 2004 #3


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    Take heart!

    As selfAdjoint (may I call you 'hermitian'?) points out, accelerator experiments have demonstrated that time dilation is a real phenomenon. The CERN ring accelerated muons to nearly the speed of light in one of their experiments, and found that the decay lifetime of the muons was 30 times what it would have been for muons that are motionless relative to the laboratory frame. (Had they chosen a different ring energy, and hence muon speed, they would have found the lifetime ratio to be something other than 30, naturally.)
    Last edited: Mar 7, 2004
  5. Mar 7, 2004 #4
    If someone were to travel 99% the speed of light and come back 4 years older, I can't see why the people on earth aren't only 4 years older as well.

    Since you can't travel that fast, I suppose it doesn't really matter. Wow- Einstein thought up some pretty confusing stuff. Again, I understand, yet it doesn't make sense.

    Thank you for your replies!
    Last edited: Mar 8, 2004
  6. Mar 7, 2004 #5


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    SelfAdjoint is correct when he says time is not absolute. The reason we humans have a hard time with that, and didn't even realize it until about 1905, is that it was not essential to our survival on Earth during all those millions of years we were evolving. The fastest material bodies cavemen had to deal with were things like hurled stones, and maybe a spear or something. And objects like those obey Newtonian rules to an extremely good approximation. A spear's trajectory, for instance, as predicted using the rules of relativity is going to be within a hair's breadth of the trajectory using the rules of Newton. (And I am being way generous in using the width of a hair as my standard, by the way.) So when people in places like Britain started trying to quantify the laws of motion a few centuries ago, they quite naturally relied upon our own human instincts about dynamics and kinematics. It wasn't until the late 1800s, when physicists were delving into issues related to electromagnetism, that some subtle problems began to surface.
    Last edited: Mar 7, 2004
  7. Mar 7, 2004 #6
    It is not an intuitive thing - there are probably more pages written on the twin paradox than any other subject in physics - and even the relativists don't agree upon the explanation - Einstein addressed the problem in his first paper - and dodged it by implying that the frames of reference are not equivalent because the turn around twin undergoes acceleration - and this is commonly echoed by many including the experts on these boards - but this analysis is different from what one arrives it for a photon traveling to a distant location and being reflected - if relativity is correct then no time passes on the clock attached to the photon - it has not aged when it arrives at the distant location and it has not aged when it returns - but time on earth for the two events is measured by an earth clock that records the total time over and back - this is the proper time recorded on the earth clock but the proper time recorded by the photon clock is "0" Then there are the "Lorentz Ether" guys who claim that hi speed travel involves both an actual time dilation and an actual physical contraction relative to an at rest cosmic medium - and they arrive at the same result.

    Any way you look at it, the traveling entity does not enjoy any of the benefits of a longer life except he gets to see a lot more of the universe - but he will eat the same number of meals as the guy who stays behind and drink the same number of Gin and Tonics and put in the same number of hours sleeping. Well, he actually gets one other advantage - when he returns to earth those young girls that turned him down because he was a dork have by then grown old and hammered and he can have his day with their daughters.
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