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

Time Travel: Make me a believer

  1. Sep 28, 2010 #1
    This is not an attempt at humor or controversy. I spend my spare time trying to find truth. I have been trying to understand some of Physic's theoretical ideas, and to my chagrin, many of them seem preposterous. I hope there are people out there that have the time and patience to help me understand some of these theories. Time Travel Theory : Two people of the same age are on Earth. The year is 2000, and they are identical twins, twin A and twin B, at the age of 10. Twin A boards a spaceship and begins to travel near the speed of light, to a distant planet, many light years away. Twin B continues a normal life, remaining on earth. Fast forward to the year 2040, a time period of 40 years has passed. Twin B, still on earth is now 50 years of age. Twin A has just returned to Earth. It is generally considered (unless I am wrong. if so please correct me) that twin B has aged 40 years, but twin A has not, due to traveling near the speed of light. QUESTION: During the 40 year period, the Earth has circled the Sun 40 times, is Twin A not also 50 years of age? Thank You, Gregg.
     
  2. jcsd
  3. Sep 28, 2010 #2

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Twin A's own physical processes (aging, decay of carbon 14 in his/her body, etc) are independent of the earth's movement around the sun. A can see the earth revolve 40 times while experiencing much less than 40 years of time defined by any process local to twin A. The earth's revolutions are no more (or less) mysterious than find his/her twin much older.
     
  4. Sep 28, 2010 #3
    gctskippy,

    Which beam of light do you believe is faster, the beam emitted from a lamp post, or the headlight beam from a moving car?
     
  5. Sep 28, 2010 #4

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    While there are many real world examples showing this is real, one of my favorites for 'you better believe it' is that you can measure in the lab that a slow muon decays in about 2 microseconds. Without time dilation, a muon could not travel more than 600 meters without decaying. However, muons are produced in profusion by cosmic rays hitting air much higher than 600 meters (almost no cosmic rays make it to the bottom of the atmosphere without hitting nucleus) . Without time dilation, ground experiments should not need to shield against these muons. Instead, they reach the ground in huge numbers, making shielding against them crucial for many types of experiments.
     
  6. Sep 28, 2010 #5

    DaveC426913

    User Avatar
    Gold Member

    If Twin A pointed a telescope on his spaceship back toward Earth, he would not see the Earth revolve around the sun 40 times. He would see it moving very slowly, only aging a couple of years. The twins would actually be able to observe the time dilation of their counterparts.

    Also, even though it might be 20 light years to the distant star (thus, one would think, it would take 20 years to get there and 20 years to get home), as Twin B accelerates, the distance would get shorter. It might only take a few years to get there. thus, when he comes back, he hasn't had to drum his fingers, waiting for 40 years.

    It gets a lot more complicated than that but, in a nutshell, that'll get you to the idea that there is no "universal time".
     
  7. Sep 28, 2010 #6

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Well, I have to disagree with DaveC. What the traveling twin sees at what point in their journey depends on the mode of acceleration (e.g. burst of acceleration, coast, burst of acceleration to turn around, coast, burst of acceleration to stop; vs. continuous acceleration one direction then the other), however, no matter what, they everntually see the earth orbiting the sun 40 times, just as they see their twin 40 years older. In the coast path, they see the earth moving slow for extended periods, but eventually they must see all 40 revolutions.

    So it seems to me.
     
  8. Sep 28, 2010 #7

    PAllen

    User Avatar
    Science Advisor
    Gold Member

    Note, if the traveling twin does not eventually see 40 revolutions of the earth (despite aging only, e.g. 10 years), then you have the absurdity that on reconnecting with the earth twin, the earth twin has aged 40 years, but the solar system has not.
     
  9. Sep 28, 2010 #8
    These ideas (time dilation) seemed preposterous to physicists, too, at one time. Physicists were shocked, to say the least, to discover time dilation after hundreds of years of Newtonian physics that assumed time was a universal constant.

    Physicists didn't just decide to theorize time dilation, it was forced on them against their will by the reality of their ever more sensitive observations and measurements. One can only endure so many experiments contradicting their own beliefs before capitulating to reality.
     
  10. Sep 28, 2010 #9
    Huh? Of course he would, if he was looking the whole time, since 40 years passed on earth. Similarly, Twin A could just look at a clock on earth through a telescope and watch it advance 40 years.
     
    Last edited by a moderator: Sep 28, 2010
  11. Sep 28, 2010 #10

    DaveC426913

    User Avatar
    Gold Member

    Yes. I was dramatically simplifying. I was figuring the more complex aspects would take more explanation.
     
  12. Sep 28, 2010 #11

    DaveC426913

    User Avatar
    Gold Member

    Yes. :blush: That's what I get for dashing off a reply on my way out the door.
     
  13. Sep 28, 2010 #12

    Dale

    Staff: Mentor

  14. Sep 29, 2010 #13
    Thank you all for taking the time to reply. I realized that a "year" is just a way we humans measure time, but no matter where we are in the universe, would not the Earth still revolve around the sun? Would the following be correct: In "Earth Years" both of the Twins would be the same age, however, twin A would appear younger because his/her 's rate of decay has been slowed.
     
  15. Sep 29, 2010 #14
    Dr lots-o'watts A.) all the same speed.
     
  16. Sep 29, 2010 #15

    DaveC426913

    User Avatar
    Gold Member

    No. There is no such thing as universal time. That was a Newtonian concept. In GR, time really is dependent on your frame of reference. It is not simply a trick or aberration.

    In a nutshell: Twin B really is younger than Twin A.
     
  17. Sep 29, 2010 #16
    If you can accept that both beams of light travel away from the lamp post at the same speed, (the speed of light is independent from the speed of the emitter) then the rest of special relativity (including the twin paradox) follows from surprisingly simple math.

    (and then general relativity also follows, but with more difficult math)
     
  18. Sep 29, 2010 #17

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor

    New measurements show the twin paradox more or less directly.

    http://www.nist.gov/public_affairs/releases/aluminum-atomic-clock_092310.cfm

    There's more, I just quoted a brief section of the article in question to interest people in looking up the original if they are interested in the details. Though some of the details are not reported very well, the explanation of gravitational time dilation is not-quite-right, what one more or less expects from the popular media covering science nowadays.
     
  19. Oct 1, 2010 #18

    K^2

    User Avatar
    Science Advisor

    Yes, both twins would be able to verify that the Earth has made the same amount of turns around the Sun. However, according to twin A's time-keeping, Earth has been rotating around the Sun significantly faster than 1 revolution per "year" on average. ("Year" being measured by clock/calendar that twin A brought with him.) Coincidentally, according to twin A, the Sun has been significantly heavier to account for this decrease in orbital period.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook