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Some questions,espacielly about time

  1. Apr 23, 2009 #1
    Hello PF,
    I have some questions which I cant get an answer, and everything I think is based on intuition and not on knowledge.
    These are not HomeWork questions, I don't even study relativity in school.
    1) Michelson and Morally experiment showed that there is no ether right??
    so relative to what is the speed of light measured? or It's absolute speed? What is absolute speed?? so basically there is no medium that the light travels in it? it just travels in nothing ?
    2)Time deletion:
    How does the height affect the time? If I understood correctly if lets say You're on the top of a mountain , and some one is waiting for You down there , the clock down there will tick more slowly than the upper one , but I was thinking, as You go up your linear speed increases (v=wR,w=const R>>>increases) so the clock is moving(higher velocity) faster -> should tick slower? what am I missing??
    3) if some 1 can just point out what is the main difference between special and general relativity i would be very thankful(just main difference).

    Thank You in advanced!

    Dweirdo.

    P.S
    Any web source for reading more would be appreciated although I have most of them i think, so just name it so I know what is recommended.
    thanks.
     
    Last edited: Apr 23, 2009
  2. jcsd
  3. Apr 23, 2009 #2

    HallsofIvy

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    Not exactly. It showed that "ether" is an unnecessary concept. If it does "exist" it has no useful properties and cannot be detected by any experiment.[/quote]

    The speed of anything must be measured relevant to some specific frame of reference. One of the basic tenets of relativity, from experimentation, is that the speed of light is the same in any frame of reference.

    So now you are talking about general relativity, not special relativity? It is not so much "height" as "gravitational force". Yes, increased linear speed (assuming the body is rotating) will cause a "higher" clock to go slower than a "lower clock" but that effect is proportional to height while gravitational force is proportional to [itex]1/r^2[/itex] so assuming a rotating body of constant density the change due to gravity is greater.

     
  4. Apr 23, 2009 #3
    Oh, I see , now it makes more sense to me.
    Thanks.
    and something about the twin paradox:
    My velocity is relative to my twin and it is 0.8C(YOU CANT KNOW WHO MOVES,I JUST SEE HIM MOVE,lets assume they both move) now after 5 years(mine) we both stop, and we sens SMS to each other to check hwo old is the other twin, how can You determine who is younger?
    btw "proportional to LaTeX Code: 1/r^2 " so doesn't that mean that speed has greater effect?
     
  5. Apr 23, 2009 #4

    NWH

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    Special Relativity talks about objects in motion (to put it lightly) and their comparison to objects in other frames of reference. General Relativty talks more about the gravitational effects that mass has on Space-Time and the other objects/bodies which exist within it.
     
  6. Apr 24, 2009 #5
    Oh,Is there something that is right in theory of "Special..." but in "General..." It's different?
    just for example: "Special..."says that moving body has a blue hair and "General..." says that moving body in gravitational bluh bluh has red hair... I mean do they have differences on a specific subject?

    Thank You.
     
  7. Apr 24, 2009 #6

    Hootenanny

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    Roughly speaking, SR can be thought of as a correction of Newtonian Mechanics without gravitation. And GR can be thought of as incorporating both SR and gravitation.
     
  8. Apr 24, 2009 #7
    Ahh, great.
    Thank You!
    the twin question is still open :} some 1 knows the answer for it?
     
  9. Apr 24, 2009 #8

    Ich

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    There are 2.5 undefined statements in your sentence. I could resolve 1.5 of them by guessing and give you an answer that depends on the last one, but that would be of no use.
    Instead, I'll ask some questions.

    Right, so
    makes no sense unless you specify a third observer relative to which both are moving.
    Again, "to stop" is not defined unless you mean "they come to rest relative to said third observer". If so, you have to give the velocities as measured by this observer, because they define what you mean by "stop". If not, those velocities are rather irrelevant, but you still have to define what you mean by "stop".
    Ok, you stop after 5 years of your time. When does your twin stop?
    -at the same time, as measured by you
    -at the same time, as measured by the third observer .That's different, because "at the same time" (or "now", as you stated it) has different meanings if you talk about events that do not happen at the same place. See "relativity of simultaneity".
     
  10. Apr 24, 2009 #9
    Well for sure there are some holes in what I've said.
    When I said assuming both are moving they know they are, cause they planned it, then both stop, they feel weird things in the ship, so they know they stopped.
    as for "Ok, you stop after 5 years of your time. When does your twin stop?
    -at the same time, as measured by you
    -at the same time, as measured by the third observer .That's different, because "at the same time" (or "now", as you stated it) has different meanings if you talk about events that do not happen at the same place. See "relativity of simultaneity"."
    I see where You going to, so let's say the move from very far towards each other( with different velocities, and the mil second they reach each other their super spaceship stops instantly, who aged more? You can't know IMO, unless You know the velocity of each one of them,but this velocity is relative to what? relative to 1 planet my speed is greater than the other one but relative to another plant it's the exact opposite...what am I missing??
     
  11. Apr 24, 2009 #10

    Ich

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    I planned to be a millionaire. But, contrary to the twins, I know that i am not.
    Formulate your setup without reference to absolute velocity.

    Ok, when did your twin start moving?
    -at the same time, as measured by you
    -at the same time, as measured by the third observer .That's different, because "at the same time" (or "now", as you stated it) has different meanings if you talk about events that do not happen at the same place. See "relativity of simultaneity".
    And how did he get there?

    You can't find a workaround. If you could, relativity would not work.
     
  12. Apr 24, 2009 #11
    "Ok, when did your twin start moving?
    -at the same time, as measured by you
    -at the same time, as measured by the third observer .That's different, because "at the same time" (or "now", as you stated it) has different meanings if you talk about events that do not happen at the same place. See "relativity of simultaneity"."
    how about we send instant telepathic message,we are very close twins,then we start :D
    "And how did he get there?"
    easy, he walked on foot , took him 20000billion years,but we are aliens we live 299999 billion years.

    Well my point is that the thing are not so clear, I'll read up on "relativity of simultaneity".
    maybe It's not the books-It's me.
    well thanks ;)
     
  13. Apr 25, 2009 #12
    i think the difference between A and B is that one who leave and accelerate in a spaceship will feel the acceleration and one who stay doesn't feel the acceleration, even if both see each other accelerating.
    isn't this the difference??????
     
  14. Apr 25, 2009 #13
    It's not the case here,
    They both accelerate in my problem.
    the answer i think about is just 2 stupid,It must not be the right solution.
    so does anybody know how to solve it?
     
    Last edited: Apr 25, 2009
  15. Apr 25, 2009 #14

    Mentz114

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    The answer is proper length. This is the length of a wordline between two events measured using the Lorentzian metric where

    proper length^2 = (time)^2-(distance)^2/c^2

    notice that time and space have opposite signs. If this quantity is added up for each twin's journey through space-time it gives the elapsed time on their clocks.

    If their journeys had the same proper length, their clocks would show the same elapsed time when they met on returning.
     
  16. Apr 25, 2009 #15
    I see, Interesting!
    "same proper length"
    just so it will be clear for me, length of what?

    thank You
     
  17. Apr 25, 2009 #16

    Mentz114

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    It's the length of the journey through space-time. You can write it in seconds or metres whichever suits you ( c is the conversion factor ). If you write it in seconds, it's the elapsed time on the comoving clock.
     
  18. Apr 25, 2009 #17
    Oh great.
    thanks!
     
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