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Is SR real ?

  1. Oct 11, 2006 #1
    Is SR "real"?

    What I mean to say is that as SR predicts time dilation, do the biological processes also slow down?
    Suppose A and B are two twin brothers.one of them(A) goes for a space tour in a spaceship with a velocity comparable with c, while the other(B)remains on earth. Clearly, for B the time will run slowly in A's spaceship, while for A it will be normal in his frame. Further suppose that A returns to earth after t years(according to earth frame). Again suppose that A does not trim his hair while on spaceship and his hair has grown a bit during tour.The same applies to B. Now what I want to ask that if we compare the lengths of both brothers' hair ,whose length will be greater and why?

    This question I was discussing with my friend and he says that biological processes are not affected in SR both lengths will be same. I argued that time dilation takes place in everything including biological processes and therefore length of A's hair will be smaller. However I too was confused therefore sought to help here.Hoping some comments.

  2. jcsd
  3. Oct 11, 2006 #2

    Doc Al

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    You are correct; your friend is wrong. Time dilation affects all kinds of "clocks", biological or otherwise.

    In your own frame, time always appears to run normally. According to your clocks, it takes the same time for your hair to grow 6 inches no matter how fast you move. If someone else (moving with respect to you) says your clocks run slowly, they must also say your hair grows slowly!
    Last edited: Oct 11, 2006
  4. Oct 11, 2006 #3
    Thanks Doc. One more question: does SR predict the concept of time machines.If yes, can we go back and forth in time? My intution says that we may be able to go in future but not in past. What is you comment?

  5. Oct 11, 2006 #4

    Doc Al

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    SR does predict (after a fashion) that you can travel to the future (faster than you are now): Just take a high-speed round-trip rocketship journey to anywhere you like. When you return to Earth, you will find that more time has passed for the folks you left behind than has passed for you in your rocket--so you have gone to the future. But I'm afraid it's a one-way trip.
  6. Oct 11, 2006 #5


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    An early test of relativity was to compare the average lifetimes of high speed elementary particles with the average lifetimes of the same elementary particles stationary with respect to the lab. The average lifetimes of high speed particles were found to be extended in keeping with the Lorentz tranform calculations.
  7. Oct 11, 2006 #6
    In a sense, the special theory states we can travel to the future. What this really means is that one can "cheat time". The faster you go, the slower your rate of time compared to those you left behind. Let's say your twin brother on earth ages 10 years, but you have only aged 10 days by the time you return. At no point has anyone stepped thru a door into the future. You exist in space & time the entire time your twin brother exists in space & time. Only difference, your rate of time was slower while his was faster, over the same interval. You aged slower. As Doc_AL stated, there's no going back though. Cheating time cannot cause others to age slower then you, nor can it send you backwards in time.

    Last edited: Oct 11, 2006
  8. Oct 12, 2006 #7
    As Doc Al said, you are right and your friend is wrong. Each observer A and B is at rest in an inertial frame. Therefore if you set up a light clock in that frame then it can serve as a clock to measure all other processes that occur in that frame including biological clocks which determine the rates of bilogical functions. Nothing has changed in this frame, nothing. So a light clock can be used to synchronize any other clock which you choose to use as a time piece. The same for the inertial observer B. He too is in a state of rest where he can also set up a light clock which can be used to synchronize other clocks and then be used to measure the rates of biological functions. Since each observer can observer the others light clock and it is readily shown here


    that A measures Bs light clock to be running slower than his. B measures As light clock to be running slow compared to his too. The process of one observer turning around provides a break in the symetry of the situation and when all the t's are crossed and the i's dotted it is seen that B is older in every way that a person can be older than A.

  9. Oct 12, 2006 #8
    What I noticed that everyone says about "rate of time ".Exactly what does this signify?When we talk about rate of something we usually quote it in terms of derivative of time or space.How can "rate of time" be expressed in mathematical notation?
  10. Oct 12, 2006 #9
    We can go to past if we travel around the geometry of space time.
  11. Oct 12, 2006 #10
    by ... dT/dt, or dt/dT.

    No one can tell the rate at which their own time goes by by observation of your own clock alone. They can only tell how fast it goes relative to another's rate of time. You compare your clock vs his, or maybe you compare some steady atomic activity of known constant rate between your frame & his.

  12. Oct 12, 2006 #11
    I think you're talking about Gödel's Universe, right?
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