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Light speed and time.

  1. Dec 7, 2006 #1
    Something I was wondering, since time slows down as we approach the speed of light. Say a person in a ship travailing close to the speed of light only ages by one hour when the rest of the earth ages by 100 years, does that mean the ship only uses one hours worth of fuel? But travels 100 years relative to earth?

  2. jcsd
  3. Dec 7, 2006 #2
    if it's not accelerating it won't use any fuel
  4. Dec 7, 2006 #3
    According to special theory of relativity, when speed limits to speed of light, energy (kinetic energy) limits to infinity. because mass limits to infinity.

    m2 = m1 / SQRT(1 - (v/c)^2)

    *when we approach the speed of light, necessary amount of energy is too much !!
  5. Dec 7, 2006 #4


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    The short answer to your question is yes. It is true that the ship, along with everything and everyone in it, will experience a mere one hour passing.

    You can also see a mind-boggling implication of this: that star that was 100+ light years away when you were on Earth, is now much, MUCH closer (this is the length contraction component to relativity), which is why it can be reached with a mere hour of fuel expended.


    As others are pointing out, it's not that easy. Reaching speeds that manifest that amount of time dilation would burn through an unimaginable amount of fuel - even if it could be done - which it can't. The journey specified requires starting out at Earth and achieviing infinite acceleration to .99999+ c (such that there is no ramp up component to your speed) and then reversing at the other end so that you spend your entire 100ly-in-1h journey at .99999+c. Your passengers and equipment will be vapourized.

    As you make your experiment more realistic, you will see that the journey must become a long rampup to ~c and then a long ramp down to 0. The time dilation is still significant, but the fuel expended becomes nigh-unimaginably large.
    Last edited: Dec 7, 2006
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