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Thread: Question on the Theory of Relativity?

  1. #1 Question on the Theory of Relativity? 
    Forum Freshman LotusTiger's Avatar
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    I'm a novice to Science and have some questions on the theory of relativity concerning the twins idea. Where one twin leaves Earth and comes back and finds he's twin twenty years older then him. Is this becasue of the speed he travelled through space or gravity. Or something to do with both? I would be very happy if you could awnser these questions in particuler and what the theory of relativity is in general?


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    The solution to the twin paradox comes from special relativity. Gravity (general relativity) has no role.

    A simple way of looking at it. Go at (almost) light speed from point A to point B (which are one light year apart) and then return at the same speed. From the point of view of the twin at point A, the traveling twin took over two years to do it. On the other hand the traveler did the round trip almost instantly, since the distance from A to B was shortened because of his speed.


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    So, according to mathman, not very much time passes on the travelers clock, and since the earth is in motion with respect to the traveler even LESS time passes on earth. Therefore the traveler is entitled to conclude that he/she will return OLDER.

    I have seen this explanation posted many times by many sincere people in many forums. Im sorry but it doesnt work.
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    The twin who travels at near light speed between points A and B, in the frame of reference A/B , will still perceive time passage to be normal in his reference frame (at rest inside his spaceship). But in the A/B stationary frame, the moving twin's time is dilated according to special relativity so that at the end of the journey, when he comes to rest in the A/B frame, much less time will have elapsed for him as compared to his stationary twin.
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    For both twins, each sees the other going slower than himself. If they were to compare notes later, they'd see that each saw the other going at the same rate relative to themselves. (If one saw the other moving at half rate, the other would also see the first moving at half rate.)

    The difference is that the travelling twin turns around. That period of acceleration is not an inertial reference frame and is outside special relativity and is why there is an age difference when they get back together.
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    The twin in the space ship is in an inertial frame that is contained within the stationary twin's inertial frame even if he does not turn around and come back. Therefore there is a time dilation for the moving twin with respect to the stationary twin.
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    Inertial frames can't be contained within other inertial frames. That doesn't make any sense. And no, if he never came back, there'd be no meaningful way to compare how much they've aged. Their ages can only be compared when they're in the same place.

    Also, if A sped over to Alpha Centauri, then later B did the same, they'd be the same age when B arrived regardless of how fast the trips were (as long as they were the same) or how long it was between them.
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    Look up the definition of inertial frame. The frame itself cannot accelerate, ie a spaceship accelerating to speed is not an inertial frame, but a frame with the sun and alpha centaury in it, even though moving at constant velocity, is an inertial frame. And it remains an inertial frame if a spaceship accelerates from the sun to alpha centaury inside that frame.
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  10. #9 Re: Question on the Theory of Relativity? 
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    Quote Originally Posted by LotusTiger
    I'm a novice to Science and have some questions on the theory of relativity concerning the twins idea. Where one twin leaves Earth and comes back and finds he's twin twenty years older then him. Is this becasue of the speed he travelled through space or gravity. Or something to do with both? I would be very happy if you could awnser these questions in particuler and what the theory of relativity is in general?
    -It's like hibernation, the animals biological functions slow down, only in this case all processes slow down, the ships clock, computer, and the space twin himself, thus he is not aware of the slowing effect, only those who do not move with him.
    While each twin may conclude the other twins clock rate is different than his, for either outbound or inbound trip, the one who returns to the other will accumulate the least amount of time. The round trip forms a closed path and removes any symmetry between them. The one who returns would have counted more years/months/etc. for the other twin than for himself.

    Special Relativity concludes there is no universal time for all, but time is relative, depending on the observers speed.
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    MagiMasters comments are correct. And MigLs last post is also correct. The post that MagiMaster objected to is MigLs post that states,

    The twin in the space ship is in an inertial frame that is contained within the stationary twin's inertial frame even if he does not turn around and come back. Therefore there is a time dilation for the moving twin with respect to the stationary twin.

    And I also object to it. Since MigL seems to understand reference frames, I conclude MigL was just in a hurry and made a bad choice of words.

    For the record:
    The statement Inertial frames are contained within other inertial frames has no meaning in either Newtonian or Relativistic physics.

    I expect MigL meant:
    Inertial frames are defined from other inertial frames

    Using the word "stationary" is a bad choice of words too. But I do it sometimes.
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    does a object that has the same mass as a large star have the same effect on time if its smaller and more dense.
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    Quote Originally Posted by Joe246
    does a object that has the same mass as a large star have the same effect on time if its smaller and more dense.
    It does, at the same distance from the centre of mass. Not on the surface though.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    what do you mean by distance? does it need the same amount of space as the star?
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    Almost all that matters in questions of gravity in the neighborhood of a spherically symetric object is how far you are from its center of mass. At 1 AU, a black hole with a mass of 1 sun, a sun with a mass of 1 sun and a gas cloud (with radius of less than 1 AU) with mass of 1 sun will feel exactly the same, no matter who you are or what you're doing.

    (I say almost, because there are some very small effects that depend on the rotation speed of the object, which depends on its inertia, which depends on its geometry, but these effects are way below the level of importance for this question.)
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    My apologies. Words are not as precise as equations.
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  17. #16 Re: Question on the Theory of Relativity? 
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    Quote Originally Posted by LotusTiger
    I'm a novice to Science and have some questions on the theory of relativity concerning the twins idea. Where one twin leaves Earth and comes back and finds he's twin twenty years older then him. Is this becasue of the speed he travelled through space or gravity. Or something to do with both? I would be very happy if you could awnser these questions in particuler and what the theory of relativity is in general?



    theory of relativity states that if the speed of an object increases the time inside it goes too slow
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  18. #17  
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    Quote Originally Posted by mathman
    The solution to the twin paradox comes from special relativity. Gravity (general relativity) has no role.

    A simple way of looking at it. Go at (almost) light speed from point A to point B (which are one light year apart) and then return at the same speed. From the point of view of the twin at point A, the traveling twin took over two years to do it. On the other hand the traveler did the round trip almost instantly, since the distance from A to B was shortened because of his speed.
    Actually acceleration, and acceleration and gravity are equivalent (see the "equivalence principle" of general relativity) is the key to the resolution of the twin "paradox".

    General relativity provides the best resolution of the twin paradox, but there is a relatively simple explanation in terms of special relativity.

    Special relativity, by its very formulation, applies only in inertial reference frames. There is only one intertial reference frame in the twin paradox, and that is the fram of the non-traveling twin. The traveling twin feels acceleration when he takes off, when he turns around and when he stops to rejoin his twin. Therefore on cannon apply the equations that describe time dilation in the reference frame of the traveling twin by only in reference frame of the non-traveling twin. When one does this one sees that the traveling twin has aged less than the non-traveling twin.
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    I am not entirely sure whether this had been mentioned or not but this is how I perceive Special Relativity:

    Imagine this analogy:

    Two cars traveling in the same direction, one at 50mph, another at 70mph. The person in the 50mph car would view himself to be stationary whilst the latter car is travelling at 20mph. This is obvious so far but here is the principle applied to Special Relativity.

    No matter which way you perceive light (traveling in the same direction at thousands of mph or opposite it at thousands of mph), it still travels at light speed in vacuum (space).

    Essentially, it is fixed and constant.

    If the observer views no change then something else must flex or compensate for the lack of a change in speed.

    Speed is distance divided by time.

    The speed is fixed at approx. 300 millions metres per second. Therefore to prevent it from changing, distance and time flexes instead. Also, to prevent the pseed of light being exceeded, distance must shrink and time slowed to maintain the speed.

    As a result, the people on the rocket experiences time more slowly than those stationary.
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  20. #19 Re: Question on the Theory of Relativity? 
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    Quote Originally Posted by LotusTiger
    I'm a novice to Science and have some questions on the theory of relativity concerning the twins idea. Where one twin leaves Earth and comes back and finds he's twin twenty years older then him. Is this becasue of the speed he travelled through space or gravity. Or something to do with both?


    He is pushed here near the earth. Then he is pushed at the turning point.

    That must be the reason.


    Here's a law: when you are pushed around you freeze
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  21. #20  
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    Quote Originally Posted by Word-Weaver
    I am not entirely sure whether this had been mentioned or not but this is how I perceive Special Relativity:

    Imagine this analogy:

    Two cars traveling in the same direction, one at 50mph, another at 70mph. The person in the 50mph car would view himself to be stationary whilst the latter car is travelling at 20mph. This is obvious so far but here is the principle applied to Special Relativity.

    No matter which way you perceive light (traveling in the same direction at thousands of mph or opposite it at thousands of mph), it still travels at light speed in vacuum (space).

    Essentially, it is fixed and constant.

    If the observer views no change then something else must flex or compensate for the lack of a change in speed.

    Speed is distance divided by time.

    The speed is fixed at approx. 300 millions metres per second. Therefore to prevent it from changing, distance and time flexes instead. Also, to prevent the pseed of light being exceeded, distance must shrink and time slowed to maintain the speed.

    As a result, the people on the rocket experiences time more slowly than those stationary.
    If you pursue this line of reasoning a bit more rigorously you wiind up with the Lorentz transformation, and once you have that you have all of special relativity.
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  22. #21 Re: Question on the Theory of Relativity? 
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    Quote Originally Posted by jartsa


    He is pushed here near the earth. Then he is pushed at the turning point.

    That must be the reason.


    Here's a law: when you are pushed around you freeze


    To be very scientific:

    push energt / your energy = lost time / time
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  23. #22 Re: Question on the Theory of Relativity? 
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    We accelerate a spinning molecule in a particle accelerator. The spinning is slowed down.

    Spinning uranium atom splits so that parts fly along spinning axis. No slowing down of spinning in this case.
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  24. #23 Re: Question on the Theory of Relativity? 
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    Quote Originally Posted by jartsa
    We accelerate a spinning molecule in a particle accelerator. The spinning is slowed down.

    Spinning uranium atom splits so that parts fly along spinning axis. No slowing down of spinning in this case.


    That was very clear and simple.

    But what about a falling and spinning ball?

    Now there is speed time dilation, gravity time dilation and conservation of angular momentum.

    This is quite difficult 3 piece puzzle.

    The question was: "does the spinning of the ball slow down"?
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    No disrespect Dr.R. but if we disregard the acceleration/deceleration parts of the journey, looking at it from a frame of reference that contains both departure (earth?)
    and arrival (alpha centauri?) at rest, considering only the constant velocity , middle part of the journey, and if velocity is an appreciable fraction of c, we also see time passing slower inside the spaceship than on earth. to an outside observer, at rest with respect to source and destination, looking thruogh a telescope the clock on the wall of the spaceship will be running slow compared to his watch.
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  26. #25  
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    Quote Originally Posted by MigL
    No disrespect Dr.R. but if we disregard the acceleration/deceleration parts of the journey, looking at it from a frame of reference that contains both departure (earth?)
    and arrival (alpha centauri?) at rest, considering only the constant velocity , middle part of the journey, and if velocity is an appreciable fraction of c, we also see time passing slower inside the spaceship than on earth. to an outside observer, at rest with respect to source and destination, looking thruogh a telescope the clock on the wall of the spaceship will be running slow compared to his watch.
    This is exactly what I said. You are evaluating the problem in the reference frame of the Earth, which is the reference frame of the non-traveling twin and is an inertial reference frame in this problem.

    The so-called "paradox" arises from applying the Lorentz transformation, specifically time dilation, in the reference frame of the traveling twin to conclude that the traveling twin sees the non-traveling twin as aging less. That analysis is invalid because the reference frame of the traveling twin in not an inertial reference frame (he clearly accelerates and decelerates) and therefore the equations do not apply.

    It is probably worth noting that any reference frame in uniform motion with respect to an inertial reference frame is also inertial. But more importantly the converse is true. Once you have any single inertial reference frame, all inertial reference frames are in uniform motion with respect to that one. This completely characterizes inertial reference frames -- if one can establish the existence of any single inertial frame.
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  27. #26  
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    Quote Originally Posted by MigL
    No disrespect Dr.R. but if we disregard the acceleration/deceleration parts of the journey, looking at it from a frame of reference that contains both departure (earth?)
    and arrival (alpha centauri?) at rest, considering only the constant velocity , middle part of the journey, and if velocity is an appreciable fraction of c, we also see time passing slower inside the spaceship than on earth. to an outside observer, at rest with respect to source and destination, looking thruogh a telescope the clock on the wall of the spaceship will be running slow compared to his watch.
    Well, that will depend on where the telescope we are looking at the ship's clock through is located.

    For example, if the telescope is on Earth, you will see the clock run slow, if it is on Alpha C, you will see it run faster, although observers at both points will agree that less time will have accumulated on the ship clock.

    For example:

    Ship traveling at 0.99c. Ship clock as seen from Earth.

    The ship clock will be seen running at a rate of



    or 0.071 compared to his own.

    The time that passes on his clock while watching the ship will be 8.64 yrs (the time it takes for the ship to reach AC plus the time it takes for the light recording the ships arrival to reach Earth from AC.) During this time, he will see the ship clock run at 0.0709 the rate of his and accumulate 0.614 yrs in time. He then subtracts the 4.3 years it took the light to reach him from Alpha C to determine that when the ship clock read 0.612 yrs, his read 4.343 yrs


    Ship traveling at 0.99c. Ship clock as seen from Alpha Centauri.

    4.3 years after the ship leaves Earth, the light of its departure reaches Alpha C.

    At this point our observer will see the ship clock run at a rate of



    or 14.1 times his his own clock rate.

    The ship arrives 4.343 years after it left, so the observer will watch the Ship clock tick for 0.0434 yrs by his own clock, and accumulate 0.612 yrs in that time. Thus when the ship arrives, it will read 0.612 yrs and his own will read 4.343 yrs, agreeing with what the Earth observer saw.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    I misunderstood you Dr.R. I assumed you saidthe one twin had to make a round trip journey for the effect to be observed.
    Janus, I stated, during the midpoint of the journey, when the spaceship is neither accelerating or decelerating, but at constant velocity, looking at the spaceship clock with a telescope both observers will see time dilation, At any other point where there is acceleration or deceleration your pretty equations are useless anyway,
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  29. #28  
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    Quote Originally Posted by MigL
    I misunderstood you Dr.R. I assumed you saidthe one twin had to make a round trip journey for the effect to be observed.
    Janus, I stated, during the midpoint of the journey, when the spaceship is neither accelerating or decelerating, but at constant velocity, looking at the spaceship clock with a telescope both observers will see time dilation, At any other point where there is acceleration or deceleration your pretty equations are useless anyway,
    Actually the twin does have to make a round trip for the effect to be observed.

    This may not be obvious, but it is a result of general relativity, which tells us that time is a local concept and one can only compare world lines at a point of intersection in spacetime. In short there is no meaning to a comparison of "time here" and "time there".

    What I said is that the problem has to be evaluated in some single inertial reference frame in order to use special relativity. It does not matter what inertial reference frame is used, but the frame of the traveling twin is not an inertial frame and therefore one cannot use the usual equations of special relativity in that frame.
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