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Thread: Time\Light relationship???

  1. #1 Time\Light relationship??? 
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    Just want to start this off by saying I have very little knowledge in this subject so this question might be easy to answer.. still its been on my mind for a while now. My understanding of time is that its measured by us in earth rotations, (days = 1, hours = 1\24th) but still passes at the same speed everywhere, no matter how its measured.

    I came across the twins paradox in "the universe in a nutshell" by Stephen Hawking and it makes absolutely no sense to me.
    The paradox says that if Fred Weasley went on a space journey at almost the speed of light while George remained on earth; when Fred returned he would be younger than his twin, because time will have been "travelling slower for him as seen by the earthborn twin"

    I understand why travelling away from earth at a fast speed might make less days pass (The earth would appear to be turning slower due to light taking longer to reach the spaceship.) However, wouldn't the journey back towards earth make up for that due to light reaching the ship faster? And how would this cause someone to age slower? Just because it takes longer for someone to see the earth spin in the distance doesn't mean any less time has actually passed, does it?

    Thanks friendly science people


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  3. #2  
    Moderator Moderator Janus's Avatar
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    What is involved here is the Theory of Relativity, which basically states that time and space measurements are not absolute and are relative. Basically, people in motion with respect to each other will not measure the time or distance between events as the same.

    The effect you describe as seeing the Earth clock running slow as you travel away is not a part of this theory. This effect is known as Doppler shift. The difference in time between Fred and George is what is left over after you account for this effect.

    You can also consider things using "Relativistic Doppler Shift". This what you see combining Relativity and Doppler shift.

    When you do this, you find that the time Fred sees the Earth Clock run fast, is not exactly canceled out by the time he sees it running slow.

    For example: Fred travels for a year at a speed where he sees George's clock run 1/2 as fast as his while his is going away. Thus after one year he sees George's clock advance 6 mo. He turns around and heads back at the same speed, he now see George's clock running twice as fast as his and sees it advance 2 years during his 1 year return trip. He has now seen George's clock advance a total of 2 years 6 mo. while his clock ticked off 2 years.

    George sees things some what differently. He sees Fred's clock run slow also, Up to the time he sees Fred turn around. But remember, Fred turned around when his clock read one year, so George has to wait until he sees Fred's clock read 1 year before he sees him turn around. This takes 2 yrs by George's clock. He then sees Fred turn around. But now Fred is chasing his own light and arrives only 6 mo. after George sees him turn around. During which time he sees Fred's clock run twice as fast and accumulate 1 year. Thus Fred Returns when George's own clock reads 2 years 6 mo. while he sees Fred's clock tick off just 2 years.

    If you want a more detailed discussion about the Theory of Relativity, check out the Special Relativity Primer at the top of this section:
    Special Relativity Primer


    "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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  4. #3  
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    The intuitive idea, that time is a constant, is not true. This is not the only place where the intuitive idea is not correct but it is one of the big ones.
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    I hope my rambling thoughts help more than confuse.. Time or the passage of it is not a human construct.. Only the rate at which we measure it is a man made tool. Motion of mass takes time.. and stationary is very rare. If it exists at all. The near to light speed velocities present all sorts of issues of timing. There are rules however.. Every action requires a period of time.. You can not arrive before you leave.
    Have some fun with the question and ask whatever you want.. but just remember that some of humanities greatest minds have immersed themselves in this subject.. and still find argument of fact.
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  6. #5  
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    Quote Originally Posted by 9white5
    Just want to start this off by saying I have very little knowledge in this subject so this question might be easy to answer.. still its been on my mind for a while now. My understanding of time is that its measured by us in earth rotations, (days = 1, hours = 1\24th) but still passes at the same speed everywhere, no matter how its measured.
    First off let me say welcome to the forum!

    That is not how time is defined. You're thinking about how the units of time are defined. In physics time is undefined. See http://users.wfu.edu/brehme/time.htm

    The unit of time is the second. It's currently defined as it was in 1967 by the Thirteenth General Conference on Weights and Measures as the SI unit as the second as follows
    t
    he duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom
    Quote Originally Posted by 9white5
    I came across the twins paradox in "the universe in a nutshell" by Stephen Hawking and it makes absolutely no sense to me.
    The paradox says that if Fred Weasley went on a space journey at almost the speed of light while George remained on earth; when Fred returned he would be younger than his twin, because time will have been "travelling slower for him as seen by the earthborn twin"
    That's correct.

    Quote Originally Posted by 9white5
    I understand why travelling away from earth at a fast speed might make less days pass (The earth would appear to be turning slower due to light taking longer to reach the spaceship.)
    No. That's not way. That'd be a mere illusion, not an observed fact. It has to do with the effects of special relativity. For a derivation of time dilation see
    Light Clock
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  7. #6  
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    Quote Originally Posted by physicist View Post
    No. That's not way. That'd be a mere illusion, not an observed fact. It has to do with the effects of special relativity. For a derivation of time dilation see
    Light Clock
    The conclusion of your linked website thread :

    "Hence moving clocks run slower than the same clock at rest. Since there is nothing inherent in the clock that we relied on in the derivation above this implies that it is time itself that is running slower!!"

    is incorrect. There are multiple mistakes:

    1. First off, all clocks run at the same rate, 1s per second. Otherwise, they will not qualify as clocks. So "moving clocks run slower" is incorrect
    2. "moving clocks run slower than the same clock at rest" is false since we know that "movement" and "rest" are all relative. All clocks run at the same rate (see point 1)
    3. "time runs slower" is a misconception based on the misconceptions exhibited at points 1 and 2. The correct statement is that elapsed proper time as a function of coordinate time can be expressed as .

    One more thing, it is bad form to keep referencing your own website. Especially when it contains basic misconceptions.
    Last edited by Howard Roark; September 26th, 2014 at 06:29 PM.
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  8. #7  
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    Quote Originally Posted by Howard Roark View Post
    Quote Originally Posted by physicist View Post
    No. That's not way. That'd be a mere illusion, not an observed fact. It has to do with the effects of special relativity. For a derivation of time dilation see
    Light Clock
    The conclusion of your linked website thread :

    "Hence moving clocks run slower than the same clock at rest. Since there is nothing inherent in the clock that we relied on in the derivation above this implies that it is time itself that is running slower!!"

    is incorrect. There are multiple mistakes:

    1. First off, all clocks run at the same rate, 1s per second. Otherwise, they will not qualify as clocks. So "moving clocks run slower" is incorrect
    2. "moving clocks run slower than the same clock at rest" is false since we know that "movement" and "rest" are all relative. All clocks run at the same rate (see point 1)
    3. "time runs slower" is a misconception based on the misconceptions exhibited at points 1 and 2. The correct statement is that elapsed proper time as a function of coordinate time can be expressed as .

    One more thing, it is bad form to keep referencing your own website. Especially when it contains basic misconceptions.
    I have three clocks at home which get out of synch after about a month.
    If you replaced 'all' with 'some', you would have a true statement.


    The "light clock" link referenced by Physicist, shows the comparison of a moving clock M vs a local clock L. The moving clock runs slower than the local clock, as observed from L. Additionally L runs slower than M as observed from M (reciprocity/symmetry of SR).


    'Proper' or local time is perceived as constant as a result of time dilation and length contraction. The owner of the clock slows to the same extent as his clock.
    The returning twin and his clock would have aged less only if he and his clock actually accumulated less time (or lost more time) than his twin.
    The theory exists on paper, and corresponds to real world phenomena, so you have to remember which mode you are using.
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  9. #8  
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    Physicist:
    "Hence moving clocks run slower than the same clock at rest. Since there is nothing inherent in the clock that we relied on in the derivation above this implies that it is time itself that is running slower!!"

    The light path is determined by the design and motion of the clock. That is why it's a function of v/c, with v equal to clock speed.
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  10. #9  
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    Quote Originally Posted by phyti View Post
    Quote Originally Posted by Howard Roark View Post
    Quote Originally Posted by physicist View Post
    No. That's not way. That'd be a mere illusion, not an observed fact. It has to do with the effects of special relativity. For a derivation of time dilation see
    Light Clock
    The conclusion of your linked website thread :

    "Hence moving clocks run slower than the same clock at rest. Since there is nothing inherent in the clock that we relied on in the derivation above this implies that it is time itself that is running slower!!"

    is incorrect. There are multiple mistakes:

    1. First off, all clocks run at the same rate, 1s per second. Otherwise, they will not qualify as clocks. So "moving clocks run slower" is incorrect
    2. "moving clocks run slower than the same clock at rest" is false since we know that "movement" and "rest" are all relative. All clocks run at the same rate (see point 1)
    3. "time runs slower" is a misconception based on the misconceptions exhibited at points 1 and 2. The correct statement is that elapsed proper time as a function of coordinate time can be expressed as .

    One more thing, it is bad form to keep referencing your own website. Especially when it contains basic misconceptions.
    I have three clocks at home which get out of synch after about a month.
    If you replaced 'all' with 'some', you would have a true statement.


    The "light clock" link referenced by Physicist, shows the comparison of a moving clock M vs a local clock L. The moving clock runs slower than the local clock, as observed from L. Additionally L runs slower than M as observed from M (reciprocity/symmetry of SR).
    Yes, yours is the correct way of stating it:
    -M runs slower than L as observed by an observer comoving with L.
    -L runs slower than M as observed by an observer comoving with M

    "Physicist", not so much, this is why I corrected it.

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  11. #10  
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    Quote Originally Posted by 9white5 View Post
    Just want to start this off by saying I have very little knowledge in this subject so this question might be easy to answer.. still its been on my mind for a while now. My understanding of time is that its measured by us in earth rotations, (days = 1, hours = 1\24th) but still passes at the same speed everywhere, no matter how its measured.

    I came across the twins paradox in "the universe in a nutshell" by Stephen Hawking and it makes absolutely no sense to me.
    The paradox says that if Fred Weasley went on a space journey at almost the speed of light while George remained on earth; when Fred returned he would be younger than his twin, because time will have been "travelling slower for him as seen by the earthborn twin"

    I understand why travelling away from earth at a fast speed might make less days pass (The earth would appear to be turning slower due to light taking longer to reach the spaceship.) However, wouldn't the journey back towards earth make up for that due to light reaching the ship faster? And how would this cause someone to age slower? Just because it takes longer for someone to see the earth spin in the distance doesn't mean any less time has actually passed, does it?

    Thanks friendly science people
    Can some one please explain the bolded to me? How is it possible for one twin to be biologically younger than the other based on Hawkings Theory? Have I misinterpreted the theory or is he indicating that their biological process of aging is being affected by their time differences?
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  12. #11  
    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by ski View Post
    Quote Originally Posted by 9white5 View Post
    Just want to start this off by saying I have very little knowledge in this subject so this question might be easy to answer.. still its been on my mind for a while now. My understanding of time is that its measured by us in earth rotations, (days = 1, hours = 1\24th) but still passes at the same speed everywhere, no matter how its measured.

    I came across the twins paradox in "the universe in a nutshell" by Stephen Hawking and it makes absolutely no sense to me.
    The paradox says that if Fred Weasley went on a space journey at almost the speed of light while George remained on earth; when Fred returned he would be younger than his twin, because time will have been "travelling slower for him as seen by the earthborn twin"

    I understand why travelling away from earth at a fast speed might make less days pass (The earth would appear to be turning slower due to light taking longer to reach the spaceship.) However, wouldn't the journey back towards earth make up for that due to light reaching the ship faster? And how would this cause someone to age slower? Just because it takes longer for someone to see the earth spin in the distance doesn't mean any less time has actually passed, does it?

    Thanks friendly science people
    Can some one please explain the bolded to me? How is it possible for one twin to be biologically younger than the other based on Hawkings Theory? Have I misinterpreted the theory or is he indicating that their biological process of aging is being affected by their time differences?
    It isn't Hawking's theory, he is talking about Einstein's theory of Relativity. To put it simply, time and space are not absolute but are relative and depends on who is measuring it. In the twin's case the traveling twin measures the time(and even the distance) for the trip is less than what his stay at home brother measures. This includes all means of measuring time by the brothers, including biological processes like aging.
    "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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  13. #12  
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    Quote Originally Posted by ski View Post
    Quote Originally Posted by 9white5 View Post
    Just want to start this off by saying I have very little knowledge in this subject so this question might be easy to answer.. still its been on my mind for a while now. My understanding of time is that its measured by us in earth rotations, (days = 1, hours = 1\24th) but still passes at the same speed everywhere, no matter how its measured.

    I came across the twins paradox in "the universe in a nutshell" by Stephen Hawking and it makes absolutely no sense to me.
    The paradox says that if Fred Weasley went on a space journey at almost the speed of light while George remained on earth; when Fred returned he would be younger than his twin, because time will have been "travelling slower for him as seen by the earthborn twin"

    I understand why travelling away from earth at a fast speed might make less days pass (The earth would appear to be turning slower due to light taking longer to reach the spaceship.) However, wouldn't the journey back towards earth make up for that due to light reaching the ship faster? And how would this cause someone to age slower? Just because it takes longer for someone to see the earth spin in the distance doesn't mean any less time has actually passed, does it?

    Thanks friendly science people
    Can some one please explain the bolded to me? How is it possible for one twin to be biologically younger than the other based on Hawkings Theory? Have I misinterpreted the theory or is he indicating that their biological process of aging is being affected by their time differences?
    Janus explained the theoretical part, here is one of the many experimental confirmations. The GPS is the most spectacular confirmation of the effect.
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  14. #13  
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by ski View Post
    Quote Originally Posted by 9white5 View Post
    Just want to start this off by saying I have very little knowledge in this subject so this question might be easy to answer.. still its been on my mind for a while now. My understanding of time is that its measured by us in earth rotations, (days = 1, hours = 1\24th) but still passes at the same speed everywhere, no matter how its measured.

    I came across the twins paradox in "the universe in a nutshell" by Stephen Hawking and it makes absolutely no sense to me.
    The paradox says that if Fred Weasley went on a space journey at almost the speed of light while George remained on earth; when Fred returned he would be younger than his twin, because time will have been "travelling slower for him as seen by the earthborn twin"

    I understand why travelling away from earth at a fast speed might make less days pass (The earth would appear to be turning slower due to light taking longer to reach the spaceship.) However, wouldn't the journey back towards earth make up for that due to light reaching the ship faster? And how would this cause someone to age slower? Just because it takes longer for someone to see the earth spin in the distance doesn't mean any less time has actually passed, does it?

    Thanks friendly science people
    Can some one please explain the bolded to me? How is it possible for one twin to be biologically younger than the other based on Hawkings Theory? Have I misinterpreted the theory or is he indicating that their biological process of aging is being affected by their time differences?
    It isn't Hawking's theory, he is talking about Einstein's theory of Relativity. To put it simply, time and space are not absolute but are relative and depends on who is measuring it. In the twin's case the traveling twin measures the time(and even the distance) for the trip is less than what his stay at home brother measures. This includes all means of measuring time by the brothers, including biological processes like aging.
    Thank you for the clarification Janus. This seems to support the possibility of time travel.
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