What is the basis of the twin paradox and general relativity?

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Offline saspinski

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I have found the question below in an old thread about the twin paradox. I could not get an answer at that thread or from other sources. The problem for me is: if the situation is totally simmetric, both twins return at the same age. But isn't it against SR theory?
I assume that all acelerations and desacelarations are the same for both the traveller twins, so GR should not explain any difference.


Things get interestinger when you have triplets, one stays on the ground, one goes off in a spacecraft to the celestial north, the other off to the celestial south, and both later return. What are their relative ages, and why?
« Last Edit: 20/06/2016 07:56:48 by chris »

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Offline Toffo

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Re: Twin paradox again...
« Reply #1 on: 20/06/2016 05:24:26 »
The problem for me is: if the situation is totally simmetric, both twins return at the same age. But isn't it against SR theory?


You mean this chronological sequence of assessments uttered by one traveling twin seems kind of impossible:
 
1: Me and the other guy are the same age
2: The other guy is time dilated, I am normal
3: The other guy is time dilated, I am normal
4: The other guy is time dilated, I am normal
5: Me and the other guy are the same age


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Offline jeffreyH

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Two twins on earth measure the expansion of the universe and determine the redshift of light for selected galaxies. Then one twin goes on a space flight at close to the speed of light to a distant planet. Both twins carry on measurements of their selected galaxies. While the earthbound twin gets the predicted results the journeying twin finds the galaxies redshifted directly behind and blue shifted in front. From this he can determine he is in motion. Therefore both twins can determine who will be time dilated and age more slowly. Thus when they meet up again they won't be surprised at the age difference. No paradox.
« Last Edit: 20/06/2016 08:30:53 by jeffreyH »

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Offline hamdani yusuf

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Two twins on earth measure the expansion of the universe and determine the redshift of light for selected galaxies. Then one twin goes on a space flight at close to the speed of light to a distant planet. Both twins carry on measurements of their selected galaxies. While the earthbound twin gets the predicted results the journeying twin finds the galaxies redshifted directly behind and blue shifted in front. From this he can determine he is in motion. Therefore both twins can determine who will be time dilated and age more slowly. Thus when they meet up again they won't be surprised at the age difference. No paradox.
Let's replace the earth with another planet moving at constant high speed relative to the earth, while other conditions stay the same.
Is your logic still applicable in this situation?

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Offline Alan McDougall

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Although this is a bit off topic it is still about relativity.

What if the two twins were approaching each other at 90%c  how would their relative time "Zones" be effected relative to the perspective of each other?

Or

If the were separating from each other at 90%c how would their relative time "Zones" be effected relative to the perceptive of each other?

Alan
The Truth remains the Truth regardless of our beliefs or opinions the Truth is always the Truth even if we know it or do not know it (The Truth remains the Truth)

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Offline jeffreyH

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Two twins on earth measure the expansion of the universe and determine the redshift of light for selected galaxies. Then one twin goes on a space flight at close to the speed of light to a distant planet. Both twins carry on measurements of their selected galaxies. While the earthbound twin gets the predicted results the journeying twin finds the galaxies redshifted directly behind and blue shifted in front. From this he can determine he is in motion. Therefore both twins can determine who will be time dilated and age more slowly. Thus when they meet up again they won't be surprised at the age difference. No paradox.
Let's replace the earth with another planet moving at constant high speed relative to the earth, while other conditions stay the same.
Is your logic still applicable in this situation?

Then that may be the twin planet paradox. It is an entirely different situation since the time dilation due to motion is not the only consideration. The time dilation due to the gravitational fields of the planets also have to be taken into consideration.

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Offline saspinski

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Re: Twin paradox again...
« Reply #6 on: 20/06/2016 22:29:12 »

1: Me and the other guy are the same age
2: The other guy is time dilated, I am normal
3: The other guy is time dilated, I am normal
4: The other guy is time dilated, I am normal
5: Me and the other guy are the same age

It is really very strange.
But on the other hand, if
1) both leave earth to opposite directions,
2) acelerate until some relativistic speed, and remain some some time at that speed,
3) acelerate to the opposite direction until get the previous speed, but now direction home.
4) brake to meet again on earth.

If A is now younger than B, B is also younger than A, so they must have the same age.

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Offline Toffo

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A human being can easily tell apart a ticking clock from a stopped clock, right? A clock that is ticking has some moving parts that are aging slowly because of the ticking motion.

If a human being itself is a moving part of a clock, the human being can be aware of that fact, then said human being says: "I am a human being that is time dilated because of motion".

A goldfish is quite different: Its memory does not last long enough for it to be aware of either it's own motion or any paradoxes.

So both human beings and goldfish are living in a paradox free world.



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Offline hamdani yusuf

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Two twins on earth measure the expansion of the universe and determine the redshift of light for selected galaxies. Then one twin goes on a space flight at close to the speed of light to a distant planet. Both twins carry on measurements of their selected galaxies. While the earthbound twin gets the predicted results the journeying twin finds the galaxies redshifted directly behind and blue shifted in front. From this he can determine he is in motion. Therefore both twins can determine who will be time dilated and age more slowly. Thus when they meet up again they won't be surprised at the age difference. No paradox.
Let's replace the earth with another planet moving at constant high speed relative to the earth, while other conditions stay the same.
Is your logic still applicable in this situation?

Then that may be the twin planet paradox. It is an entirely different situation since the time dilation due to motion is not the only consideration. The time dilation due to the gravitational fields of the planets also have to be taken into consideration.
What If the other planet is identical in size and density with earth?

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Offline jeffreyH

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Unlike the case of the twin paradox the masses involved are not negligible. For an earth like planet travelling at relativistic speed you have gravitational radiation to take into account. Frequency shift also becomes more complex, involving both the motion of the planet and the state of its gravitational field. The latter will be affected by its motion due to radiation of gravitational waves.

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Offline saspinski

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Anybody knows how relativity theory explains the twin (or better the triplet) paradox, as stated in my initial post?

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Offline jeffreyH

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There is no fixed background in relativity against which to measure things. This means that any observer in a non accelerating inertial frame of reference will judge other to be in motion relative to themselves. They assume another object is moving away at a constant speed. An observer on the other object will in turn see the first observer moving away at a constant speed. Who is correct? Relativity says both since all things are relative. The reason I suggested the fixed stars as a stand in for a fixed background is that they are to all intents and purposes fixed. It even uses the relative redshift of expansion to prove the point. However this would only be an assumption in relativity as the fixed stars could in fact be moving in a preferred direction with respect to something exterior to the observable universe.

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Offline Toffo

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Re: Twin paradox again...
« Reply #12 on: 26/06/2016 13:15:26 »


You mean this chronological sequence of assessments uttered by one traveling twin seems kind of impossible:
 
1: Me and the other guy are the same age
2: The other guy is time dilated, I am normal
3: The other guy is time dilated, I am normal
4: The other guy is time dilated, I am normal
5: Me and the other guy are the same age

At most one of sentences 2,3,4 is true.
So at least two of those sentences is wrong.
Accelerating person is allowed to say "I am normal" at any time.
Accelerating person is not allowed to say "I am normal" every time.
If an accelerating person says "I am normal" two times, he contradicts himself.









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Offline hamdani yusuf

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There is no fixed background in relativity against which to measure things. This means that any observer in a non accelerating inertial frame of reference will judge other to be in motion relative to themselves. They assume another object is moving away at a constant speed. An observer on the other object will in turn see the first observer moving away at a constant speed. Who is correct? Relativity says both since all things are relative. The reason I suggested the fixed stars as a stand in for a fixed background is that they are to all intents and purposes fixed. It even uses the relative redshift of expansion to prove the point. However this would only be an assumption in relativity as the fixed stars could in fact be moving in a preferred direction with respect to something exterior to the observable universe.
Since the accepted result of this thought experiment is that traveling twin would be younger than staying twin, it means that traveling twin's observation is invalid. Is it invalid because of the accelerations?

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Offline jeffreyH

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There is no fixed background in relativity against which to measure things. This means that any observer in a non accelerating inertial frame of reference will judge other to be in motion relative to themselves. They assume another object is moving away at a constant speed. An observer on the other object will in turn see the first observer moving away at a constant speed. Who is correct? Relativity says both since all things are relative. The reason I suggested the fixed stars as a stand in for a fixed background is that they are to all intents and purposes fixed. It even uses the relative redshift of expansion to prove the point. However this would only be an assumption in relativity as the fixed stars could in fact be moving in a preferred direction with respect to something exterior to the observable universe.
Since the accepted result of this thought experiment is that traveling twin would be younger than staying twin, it means that traveling twin's observation is invalid. Is it invalid because of the accelerations?

In order to start the journey the travelling twin has to accelerate with respect to the stay at home twin. The traveller is therefore already in a non inertial frame of reference with respect to his twin on the ground. If the earth were in a very extended elliptical orbit it could itself be considered to be in a non inertial frame of reference with respect to the sun. etc etc. So yes the accelerations are the cause but more importantly it is knowing about them. If the travelling twin were to make no attempt to determine his rate of motion with respect to a known source, the fixed stars, then he has no way to determine his rate of acceleration with respect to his earthbound twin. Accurate information is always important in any experiment. Otherwise people can easily cry foul and state an error in a theory that really doesn't stand up to close scrutiny if viewed in light of the correct assumptions.


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Offline jeffreyH

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Take two clocks synchronised at a position on earth directly away from the sun on the dark side of the earth. Let one remain stationary for 24 hours while the other travels at a speed that keeps it directly away from the sun on the dark side of the earth. When they meet again 24 hours later will the clocks differ and by how much?

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Offline saspinski

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Since the accepted result of this thought experiment is that traveling twin would be younger than staying twin, it means that traveling twin's observation is invalid. Is it invalid because of the accelerations?

That is why the paradox is stronger if we make both twins to travel, but to opposite directions. If they suffer all the acelerations and desacelerations necessary to a meet in earth, after some years,  it seems reasonably, due to the simmetry of the situation, that they will have the same age.

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Offline Thebox

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Since the accepted result of this thought experiment is that traveling twin would be younger than staying twin, it means that traveling twin's observation is invalid. Is it invalid because of the accelerations?

That is why the paradox is stronger if we make both twins to travel, but to opposite directions. If they suffer all the acelerations and desacelerations necessary to a meet in earth, after some years,  it seems reasonably, due to the simmetry of the situation, that they will have the same age.


Take two twins at birth and separate the twins, place one twin in a constant inertial reference frame and the second twin in constant motion.
At birth we attach one Caesium clock and one digital stop watch with a lifetime battery to each of the twins.
50 years later the twins meet up in a lab to compare clocks, both digital clocks show an equal amount of time for both twins to the exact nano second, the Caesium clocks both show a difference in time.


They conclude they are both the same age  and not the same age at the same time, so both agree that one of the methods to keep time is broken. I wonder which one that could possibly be?

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Offline Bill S

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Offline Thebox

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http://home.earthlink.net/~owl232/twinparadox.pdf

This is worth a read.

''According to STR, different intertial reference frames (RFs) can disagree on the time that
elapses between two events, as well as on the length of an object. These discrepancies are
described by the Lorentz Transformation equations:''


No they can't disagree on length of an object or disagree on the elapsed time between events,


They can agree the visual length angle of light is different to give an illusion of a different length object and agree that their measurements of time are different and their clocks are not synchronised constants.
« Last Edit: 28/06/2016 23:34:25 by Thebox »

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Offline saspinski

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http://home.earthlink.net/~owl232/twinparadox.pdf

This is worth a read.

It is well written, avoiding answers like "the traveller twin knows that he accelerates" or "it is a problem for GR because there is at least one change of speed direction".

But he could find a define answer because one twin remains on earth surface, always an inertial reference frame. The other has two inertial reference frame, one when he is going away and other when return.  Choosing any of that three RF, the traveller twin returns younger, applying the Lorenz tranformations for time.

But if both are traveller twins, each one going away from earth to opposite directions, and then returning, that asymmetry disappears. A supposed third twin (or triplet) staying on earth, could agree that the traveller ones came back younger, but not one of the travellers regarding his (also travelling) brother.

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Offline saspinski

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In the morning of the next day after my last post, I thought of a solution:

Based on the article of the link (http://home.earthlink.net/~owl232/twinparadox.pdf), the calculations have to be made for just one inertial reference frame for all the jorney. Any of them are acceptable.

When one of the triplets travels out from earth, and starts monitoring his brother ship going to the opposite direction, he can use his own RF, so he is not moving. The other ship is travelling fast, so his brother is aging slowly for him.

After reaching the return point, he changes direction and come back. Now, he must keep using the same RF, (his ship while it was travelling from earth to the return point). By the way, that is now precisely the RF of his brother, because he was travelling to the opposite direction and are returning to earth.

So, his brother is not moving now, and he is travelling fast. So he is aging quickly than his brother.

After meeting on earth, both will have the same age, and as shown in the link, younger than the third brother, that remained home.


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Offline PhysBang

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Two twins on earth measure the expansion of the universe and determine the redshift of light for selected galaxies. Then one twin goes on a space flight at close to the speed of light to a distant planet. Both twins carry on measurements of their selected galaxies. While the earthbound twin gets the predicted results the journeying twin finds the galaxies redshifted directly behind and blue shifted in front. From this he can determine he is in motion. Therefore both twins can determine who will be time dilated and age more slowly. Thus when they meet up again they won't be surprised at the age difference. No paradox.
This doesn't work because the Earth is in motion relative to the CMB average, but the twin scenario still works.

The "answer" to the is non-paradox is that the situation is not symmetric. The one twin does not have one inertial reference frame; this is required for the twin to turn around and come back.

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Offline PhysBang

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http://home.earthlink.net/~owl232/twinparadox.pdf

This is worth a read.

''According to STR, different intertial reference frames (RFs) can disagree on the time that
elapses between two events, as well as on the length of an object. These discrepancies are
described by the Lorentz Transformation equations:''


No they can't disagree on length of an object or disagree on the elapsed time between events,


They can agree the visual length angle of light is different to give an illusion of a different length object and agree that their measurements of time are different and their clocks are not synchronised constants.
You are 100% wrong.

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Offline Alan McDougall

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http://home.earthlink.net/~owl232/twinparadox.pdf

This is worth a read.

''According to STR, different intertial reference frames (RFs) can disagree on the time that
elapses between two events, as well as on the length of an object. These discrepancies are
described by the Lorentz Transformation equations:''


No they can't disagree on length of an object or disagree on the elapsed time between events,


They can agree the visual length angle of light is different to give an illusion of a different length object and agree that their measurements of time are different and their clocks are not synchronised constants.
You are 100% wrong.

All theories are 100% wrong unless they are 100% correct. 

No, you are wrong, present information may say I am wrong by their subjective thinking, but my objective thinking says that they are wrong, so who's thinking is correct?

Answer = neither of us, because it is both subjective to our thoughts,

The objective reality which you call the subjective reality, is different to the subjective reality you call the objective reality. It is not me who has it backwards according to definition.


Subjective - influenced by the mind or personal feelings /  belief


Objective - uninfluenced by personal feeling or belief,   (that which I use)

It is a misnomer  to call it a paradox because in reality both twins in their respective time frames are moving into the future.
The Truth remains the Truth regardless of our beliefs or opinions the Truth is always the Truth even if we know it or do not know it (The Truth remains the Truth)

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Offline granpa

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I have found the question below in an old thread about the twin paradox. I could not get an answer at that thread or from other sources. The problem for me is: if the situation is totally simmetric, both twins return at the same age. But isn't it against SR theory?
I assume that all acelerations and desacelarations are the same for both the traveller twins, so GR should not explain any difference.


Things get interestinger when you have triplets, one stays on the ground, one goes off in a spacecraft to the celestial north, the other off to the celestial south, and both later return. What are their relative ages, and why?


https://commons.wikimedia.org/wiki/File:Twin_paradox.png




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Offline hamdani yusuf

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I have found the question below in an old thread about the twin paradox. I could not get an answer at that thread or from other sources. The problem for me is: if the situation is totally simmetric, both twins return at the same age. But isn't it against SR theory?
I assume that all acelerations and desacelarations are the same for both the traveller twins, so GR should not explain any difference.


Things get interestinger when you have triplets, one stays on the ground, one goes off in a spacecraft to the celestial north, the other off to the celestial south, and both later return. What are their relative ages, and why?


https://commons.wikimedia.org/wiki/File:Twin_paradox.png


how can someone in the rocket see the clocks run backward without moving faster than light?
photons arranged to show prior to 12:00 have already pass the rocket, so to see them again, the rocket must catch them up faster than photon's speed.

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Offline granpa

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Draw A Spacetime diagram. Let the x-axis a space and the y-axis be time. Draw clocks on the x-axis at time equals zero.

Then draw a diagonal line representing 1 simultaneous moment from the point of view of the rocket.



How to draw a spacetime diagram:
https://www.physicsforums.com/threads/how-to-draw-a-spacetime-diagram.314080/

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Offline hamdani yusuf

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Draw A Spacetime diagram. Let the x-axis a space and the y-axis be time. Draw clocks on the x-axis at time equals zero.

Then draw a diagonal line representing 1 simultaneous moment from the point of view of the rocket.



How to draw a spacetime diagram:
https://www.physicsforums.com/threads/how-to-draw-a-spacetime-diagram.314080/
IMO, a clock can be said to run backward if at one instance it shows a particular time reading, and then later on, the same clock shows the reading of time prior to that instance. For example, at start a clock shows 12:00, but later on, it shows 11:59.
Your spacetime diagram doesn't show this case.

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Offline granpa

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When the rocket is stationary then 1 simultaneous moment now is represented by the x-axis. When they work it starts moving then the line representing one one moment now becomes a dog and a lion. The faster the Rock and moves them or Tilted the line is. You can obviously see that Clark's further away will seem to be more out of sync. Just consider the case of a park that is millions or billions of light-years away from the rocket. Consider how far out of sync it must become.

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Offline IAMREALITY

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When the rocket is stationary then 1 simultaneous moment now is represented by the x-axis. When they work it starts moving then the line representing one one moment now becomes a dog and a lion. The faster the Rock and moves them or Tilted the line is. You can obviously see that Clark's further away will seem to be more out of sync. Just consider the case of a park that is millions or billions of light-years away from the rocket. Consider how far out of sync it must become.

Not a clue what you just tried to say.

What I do know, is that the theory above is wrong.  Under no circumstance in physics would the clock on the left suddenly be 11:59 unless almost 24 hours has elapsed and it's showing the time for the next day.  But no, a clock that once read 12:00 am will under no circumstances tick backwards to 11:59, no matter what you do.  Period.

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Offline granpa

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I didn't say it would take backwards. I said from the point of view of the rocket it would seem to be.

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Offline IAMREALITY

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I didn't say it would take backwards. I said from the point of view of the rocket it would seem to be.

Wouldn't that mean they would see 11:59?  That's what your diagram shows.  but if they left when it was synchronized at 12, it's an impossibility for it to then show 11:59, which would in fact be ticking backwards.

The point of view of the rocket could never see any of the clocks show anything prior to 12 am, if they started at 12 am.

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Offline granpa

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 What would a o'clock at infinity say from the Rockets point of view? What would a o'clock at negative infinity say?

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Offline IAMREALITY

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What would a o'clock at infinity say from the Rockets point of view? What would a o'clock at negative infinity say?
It would say "warning: fatally flawed logic ahead".

Stick to what we're talking about.  Which is a bunch of clocks starting at 12 am and how it's impossible for the rocket to ever see a time prior to 12 am on them, regardless of your diagram saying otherwise. 

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Offline granpa

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So you don't believe what a simple space-time diagram is telling you?

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Offline IAMREALITY

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So you don't believe what a simple space-time diagram is telling you?

You mean the patently false diagram?  No.

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Offline PhysBang

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What would a o'clock at infinity say from the Rockets point of view? What would a o'clock at negative infinity say?
It would say "warning: fatally flawed logic ahead".

Stick to what we're talking about.  Which is a bunch of clocks starting at 12 am and how it's impossible for the rocket to ever see a time prior to 12 am on them, regardless of your diagram saying otherwise.
You are confusing what one way of creating a system of coordinates might assign to the time of an event with what another way of creating a system of coordinates might assign to an event with what any particular clock might say at a particular event. Not every clock is synchronized.

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Offline IAMREALITY

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What would a o'clock at infinity say from the Rockets point of view? What would a o'clock at negative infinity say?
It would say "warning: fatally flawed logic ahead".

Stick to what we're talking about.  Which is a bunch of clocks starting at 12 am and how it's impossible for the rocket to ever see a time prior to 12 am on them, regardless of your diagram saying otherwise.
You are confusing what one way of creating a system of coordinates might assign to the time of an event with what another way of creating a system of coordinates might assign to an event with what any particular clock might say at a particular event. Not every clock is synchronized.

No idea what on earth you just tried to say. 

What I know, is if the clocks displayed 12am when the rocket left, no event on the rocket would allow them to see 1157,as the diagram shows. 

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Offline granpa

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Look at the space time diagram that I told you to draw. The x axis represents the line clocks.
Now look at the line representing one simultaneous moment from the point of view of the rocket. It's the diagonal line. Wherever that diagonal line goes below the x axis then the Clarks there will sing from the point of view of the rocket to be earlier. The only way no clock could be hurt earlier would be for the day now I can never get below the x-axis. That's obviously impossible.

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Offline hamdani yusuf

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When the rocket is stationary then 1 simultaneous moment now is represented by the x-axis. When they work it starts moving then the line representing one one moment now becomes a dog and a lion. The faster the Rock and moves them or Tilted the line is. You can obviously see that Clark's further away will seem to be more out of sync. Just consider the case of a park that is millions or billions of light-years away from the rocket. Consider how far out of sync it must become.
if a clock is billions of light years away to the left, then it will not look synchronized with the clock right next to the rocket even when it is not moving.

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Offline granpa

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Which is why it has to be calculated not observed

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Offline IAMREALITY

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Which is why it has to be calculated not observed
Yet your graphic above mentions nothing about calculations, seems to infer observation, and specifically shows a time on the clock face that was to be observed. If you were specifically talking about merely what might be calculated, then you should have just said so.

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Offline jeffreyH

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I must admit skipping most of the discussion but need to address a point made in a post by granpa. No clock will ever appear to run backwards unless the rocket can outrun photons coming from the clock. The clock will appear to slow down and fall behind others but it will NEVER go backwards. It matters not what metric is used the rocket will never exceed the speed of light.

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Offline granpa

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Relativity is much more than just how things appear

Why would you think I was talking about the appearance of the clocks?
« Last Edit: 06/07/2016 20:16:18 by granpa »

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Offline IAMREALITY

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Relativity is much more than just how things appear

Why would you think I was talking about the appearance of the clocks?

Are you for real?  Maybe because in your graphic, you specifically start off with the declaration of how "they appear"?  And the main part of the graphic is specifically how they appear?

You specifically address how 'they appear'.  You further specifically call attention to appearance by showing very specific graphics of their appearance.  You furthermore state from their "point of view", which also is therefore referencing appearance (point of view = what they would see from where they are).  What isn't found anywhere in the details of your explanation within that graphic itself though, is anything specifically mentioning Relativity (I'm not talking about the title of the graphic nor the end line conclusion.  But your actual theory and explanation within the graphic you put forth.  And it was that of which I was in disagreement.  And it is why the graphic is quite simply wrong.  Cause nothing will make the clock go backwards.)

So my advice to you would be to start the graphic over from scratch.  Cause it does a horrible job at making your case.

« Last Edit: 06/07/2016 21:44:45 by IAMREALITY »

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Offline granpa

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I don't believe anybody could be that stupid. Why are you harassing me?

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Offline Alan McDougall

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There is no paradox both twins are both advancing in time in the only possible direction into their relative futures, albeit at different rates, depending on their mass, gravity field, or acceleration away or towards each other.

What we do know about the "Flow" of time in our universe is that it is linear like an arrow from the past into the present and then into the future. We are all on the ride into the future, but at different rates, because time is not a fundamental constant and varies fro place to place, indeed time flows slightly faster at your feet which are nearer to the gravity field of the earth, than your head.

Thus; if you could hypothetically stand still for a few billion Earth years, the result would be that your feet would have become much younger than your head .
 
Alan
« Last Edit: 06/07/2016 21:02:52 by Alan McDougall »
The Truth remains the Truth regardless of our beliefs or opinions the Truth is always the Truth even if we know it or do not know it (The Truth remains the Truth)

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Offline Alan McDougall

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I don't believe anybody could be that stupid. Why are you harassing me?

Report him to the moderator he likes to insult people such as calling you "Dude"

Don't let him get away with it, he persisted in insulting me in an exact manner he is doing to you!

From one Granpa to another

Great to make your acquaintance!

Warm regards from a like thinker

Alan McDougall

« Last Edit: 06/07/2016 21:06:16 by Alan McDougall »
The Truth remains the Truth regardless of our beliefs or opinions the Truth is always the Truth even if we know it or do not know it (The Truth remains the Truth)