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Author Topic:   The Twins Paradox and the speed of light
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 91 of 230 (534181)
11-05-2009 2:48 PM
Reply to: Message 90 by Trae
11-05-2009 11:28 AM


Re: Twin Paradox Revisited Sans Rocket-ships.
Acceleration isn't limited to spaceships and visitation takes time, so wouldn't the twins paradox still be in play since those objects (earth and the alien home-world) are under the influence of different bodies in different gravitational fields?
If you're asking if gravitation and cosmological expansion can create a 'twins paradox' effect, then the answer is yes. We see this here on Earth with just the GPS system, where the satellites are aging differently to us owing to Earth's differential gravitation.
You get a much more pronounced effect around compact bodies such as neutron stars and black holes, and you can use these to fire yourself far into the future.
If aliens from galaxy A (a galaxy not in our local group) somehow managed to arrive on Earth instantaneously, and if galaxy A and galaxy B are hurdling away from each other, it seems to me that time would pass at a different rate on the alien planet than on Earth.
This is a bit confused. 'somehow managed to arrive on Earth instantaneously' means very little unless you can specify details. I think I understand what you are getting at - one moment the alien has a time direction aligned with that of his own galaxy, and the next moment he is soemwhere where the time direction is very different - does this affect him in any way? The trouble is, the detail is all in the mechanism of your 'simultaneous travel'. You cannot postulate magic and then hypothesise how science behaves in the presence of this magic. To do this, you need to understand the science of the magic, and then it is science, not magic

This message is a reply to:
 Message 90 by Trae, posted 11-05-2009 11:28 AM Trae has replied

Replies to this message:
 Message 92 by Huntard, posted 11-05-2009 3:15 PM cavediver has replied
 Message 94 by Trae, posted 11-06-2009 11:56 PM cavediver has replied

  
Huntard
Member (Idle past 2294 days)
Posts: 2870
From: Limburg, The Netherlands
Joined: 09-02-2008


Message 92 of 230 (534186)
11-05-2009 3:15 PM
Reply to: Message 91 by cavediver
11-05-2009 2:48 PM


Re: Twin Paradox Revisited Sans Rocket-ships.
This topic sparked my interest, let's see if I can make more sense of it for myself (being mostly educated by popular sceince programs, which are undoubtedly too simplistic)
cavediver writes:
The trouble is, the detail is all in the mechanism of your 'simultaneous travel'.
Let's say they went through a wormhole. How does this effect their experience of time? If they go back though the wormhole, will they have aged any differently from the ones that stayed behind?

I hunt for the truth
I am the one Orgasmatron, the outstretched grasping hand
My image is of agony, my servants rape the land
Obsequious and arrogant, clandestine and vain
Two thousand years of misery, of torture in my name
Hypocrisy made paramount, paranoia the law
My name is called religion, sadistic, sacred whore.
-Lyrics by Lemmy Kilmister of Motorhead

This message is a reply to:
 Message 91 by cavediver, posted 11-05-2009 2:48 PM cavediver has replied

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 93 of 230 (534197)
11-05-2009 4:58 PM
Reply to: Message 92 by Huntard
11-05-2009 3:15 PM


Re: Twin Paradox Revisited Sans Rocket-ships.
If they go back though the wormhole, will they have aged any differently from the ones that stayed behind?
Ok, a wormhole we can deal with - BUT the space-time curvature of the wormhole will have a far more massive effect upon our time reckoning than the cosmological effect. And more importantly, bringing the wormhole into a situation where it does connect two distant areas at the same 'time' is already involving quite serious time-travel (being able to break the causal structure of space-time is time-travel) and in that case, quite frankly, who gives a damn about piddling little effects such as the twins paradox

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Trae
Member (Idle past 4306 days)
Posts: 442
From: Fremont, CA, USA
Joined: 06-18-2004


Message 94 of 230 (534342)
11-06-2009 11:56 PM
Reply to: Message 91 by cavediver
11-05-2009 2:48 PM


Re: Twin Paradox Revisited Sans Rocket-ships.
Thanks Cavediver,
Let's consider the issue of 'alien visitors', but we'll use Earthlings visiting a planet far, far, away.
The first reason for being vague about the method of 'travel' is that I suspect that 'getting there is only part of the problem' while being actually here and away from the alien home world is part of the problem which gets ignored. I may certainly be completely mistaken on this idea, but the aspect of the problem I'd like to explore, is time dilation as relates to being a world far away from one's home world.
The second reason for being vague is that aliens being able to bypass time dilation isn't my own argument, but that of others (often circular and vague). In this one aspect of this problem I don't wish to explore the science (or non-science) of wormholes/stargates/astral-projection/tesseracts/FTL/probability-drives/etectera, but instead, explore just what actually happens to the visiting alien.
Still, let's set a 'techno-magic' solution and go from there. I'll pick 'stargates'. Further, for this discussion, we don't know how stargates work (they don't use wormholes, we can call it the Copperfield/Penn & Teller/Carrot-top Uncertainty Principle Bridge) we can only say that on exiting the 'stargate' somehow very little time has passed. Let's say under an minute. How would we know? Something along the lines of comparing current measurements against previous measurements and the departure time, visitation time, and return trip vs. time absent from the point of departure.
My assumption is in this thought-experiment (let's not call it an example) presumes the 'stargate' can account for all time dilation effects from traveling, but only from traveling and wouldn't compensate for being on a world far away from Earth.
So let me try spout some garbage here and see if you can massage it into shape.
quote:
Starting Point:
Even were aliens somehow able to compensate for or bypass time dilation effects to reach Earth, there are still time dilation effects apart from those traveling here which would needing to be addressed. Gravitation and cosmological expansion between where we are in the cosmos and where the aliens originate also creates time dilation.
I'd like to hold off on the degree of those effects until you're in agreement that we have a workable starting point.

This message is a reply to:
 Message 91 by cavediver, posted 11-05-2009 2:48 PM cavediver has replied

Replies to this message:
 Message 95 by cavediver, posted 11-07-2009 7:00 AM Trae has replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 95 of 230 (534361)
11-07-2009 7:00 AM
Reply to: Message 94 by Trae
11-06-2009 11:56 PM


Re: Twin Paradox Revisited Sans Rocket-ships.
The first reason for being vague about the method of 'travel' is that I suspect that 'getting there is only part of the problem' while being actually here and away from the alien home world is part of the problem which gets ignored.
No, you cannot split the issue like this. The Twins Paradox and associated gravitational and cosmological time-dilation issues are all inextricably linked to the method of travel. Time-dilation itself is a factor of Lorentz Transformations (and the finite speed of light), but you are not performing a Lorentz Transformation with your 'instantaneous' travel, and you're completely circumventing the finite speed of light.
You may be thinking that teleporting from galaxy A, with time direction TA, to galaxy B, with time direction TB has the traveller arriving somewhere where time moves differently to his own. But I have no idea how to explain what this situation means because the situation is an impossibility. It's very similar to the question - well, what would happen if we could go faster than light? The question itself is sufficiently ill-defined that there is no possible answer, without replacing the question with one that is actually reasonably well-defined. In your case, you have to specify the mechanism of travel to be able to make any sense of your question.
I guess a similar question would be - if I teleported from here in the UK to Fremont, would I arrive leaning over at an angle? (because of the difference in normal vectors to the Earth's surface) Again, it depends upon the technical spec of the teleport device...
ABE: just getting back to the other reason this is all immaterial, if you can teleport instantaneously, this can be used as a time-machine by time-dilating one end of the teleport device. This is how we turn a wormhole into a time-machine. So you can stargate to somewhere else, spend a year there, then stargate back and simply pick at what time you'd like to return: a moment after you left, the same time as now, or anything else you choose. You can't postulate FTL teleportation and ignore the time-machine implications. Breaking the speed of light by whatever method has consequences...
Edited by cavediver, : No reason given.

This message is a reply to:
 Message 94 by Trae, posted 11-06-2009 11:56 PM Trae has replied

Replies to this message:
 Message 96 by Trae, posted 11-08-2009 1:57 PM cavediver has replied

  
Trae
Member (Idle past 4306 days)
Posts: 442
From: Fremont, CA, USA
Joined: 06-18-2004


Message 96 of 230 (534459)
11-08-2009 1:57 PM
Reply to: Message 95 by cavediver
11-07-2009 7:00 AM


Back to the basics
Let's try this another way. Forget twins, space travel, aliens, and time-travel.
We build a space station orbiting the Earth. A child is born on the Earth and another on this space station at the exact moment. Due to the child in the space station under less gravitation it ages and experiences time at a slightly lesser rare. Am I correct?

This message is a reply to:
 Message 95 by cavediver, posted 11-07-2009 7:00 AM cavediver has replied

Replies to this message:
 Message 97 by cavediver, posted 11-08-2009 4:17 PM Trae has replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 97 of 230 (534478)
11-08-2009 4:17 PM
Reply to: Message 96 by Trae
11-08-2009 1:57 PM


Re: Back to the basics
it ages and experiences time at a slightly lesser rare. Am I correct?
Not quite - it's the twin on Earth that ages more slowly than the twin on the space-station. There will also be a relative-velocity time dilation which will work to reduce the difference.

This message is a reply to:
 Message 96 by Trae, posted 11-08-2009 1:57 PM Trae has replied

Replies to this message:
 Message 98 by Trae, posted 11-09-2009 6:09 PM cavediver has replied

  
Trae
Member (Idle past 4306 days)
Posts: 442
From: Fremont, CA, USA
Joined: 06-18-2004


Message 98 of 230 (534596)
11-09-2009 6:09 PM
Reply to: Message 97 by cavediver
11-08-2009 4:17 PM


Re: Back to the basics
quote:
Not quite - it's the twin on Earth that ages more slowly than the twin on the space-station. There will also be a relative-velocity time dilation which will work to reduce the difference.
Ah, right. I said it backwards. Okay let's shift back to aliens, but this time aliens who have no ability to time or space travel.
As one point of reference we have Earth a one-G world on the other point we have the aliens who we'll also assume for now exist on a one-G world.
My understanding here is that given both planets are one-G worlds that only acceleration would cause any significant time dilation, is this correct?
Under what variables, if ever, would the time dilation effect be significantly great enough in this two planet scenario? Would we have worlds in our own galaxy which would experience time very much faster than we ourselves experience? Do we ever get scenarios along the line of a day is equal to a year or more?

This message is a reply to:
 Message 97 by cavediver, posted 11-08-2009 4:17 PM cavediver has replied

Replies to this message:
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lyx2no
Member (Idle past 4715 days)
Posts: 1277
From: A vast, undifferentiated plane.
Joined: 02-28-2008


Message 99 of 230 (534615)
11-09-2009 8:20 PM
Reply to: Message 98 by Trae
11-09-2009 6:09 PM


Re: Back to the basics
Under what variables, if ever, would the time dilation effect be significantly great enough in this two planet scenario? Would we have worlds in our own galaxy which would experience time very much faster than we ourselves experience? Do we ever get scenarios along the line of a day is equal to a year or more?
One would be hard put to locate a planet where the time is exactly the same as ones own. Even another Earthly home is not exactly the same. I live at 18 feet above sea level; Zeus lives at 9,577 feet above sea level. I live 42.45N; Zeus at 40N. I live on 325 feet of soggy glacial till over granite basement; Zeus lives on a few thousand feet of carbonates over basaltic basement. Each makes an absolutely miniscule difference, but none-the-less there.
The difference only become significant at the extremes of velocity and mass density. Do you have a reasonably different circumstance from our own that an alien would be living under? A planet orbiting its super-dense sun in a matter of minutes is not a likely home. Nor would a 100,000g planet.
However, in a galaxy far, far away enough your aliens would be travailing away from us fast enough to make any time dilation you care to name.

It's not the man that knows the most that has the most to say.
Anon

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 Message 98 by Trae, posted 11-09-2009 6:09 PM Trae has seen this message but not replied

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 Message 100 by Iblis, posted 11-09-2009 9:24 PM lyx2no has replied

  
Iblis
Member (Idle past 3895 days)
Posts: 663
Joined: 11-17-2005


Message 100 of 230 (534624)
11-09-2009 9:24 PM
Reply to: Message 99 by lyx2no
11-09-2009 8:20 PM


Re: Back to the basics
However, in a galaxy far, far away enough your aliens would be travailing away from us fast enough to make any time dilation you care to name.
Would they? I don't believe that's correct.
To begin with, the standard argument might be that it is the acceleration that creates the time-dilation we expect in bodies moving at speeds relative to one another. But expansion isn't acceleration in a classical sense, there isn't a force pushing on either of the planets making them move faster and faster. What is happening is that the space between them is expanding at a constant rate. This causes bodies at greater distances from one another to recede faster and faster.
The usual prop is to take a yardstick and double it in size so its now 6 feet long. Marks that were at 1 inch and 2 feet 11 inches (2 feet 10 inches apart) are now at 2 inches and 5 feet 10 inches (5 feet 8 inches apart). But marks that were at 1 foot 5 inches and 1 foot 7 inches are now only 2 inches further apart. Thus, the outer marks are "moving much faster" than the inner ones.
Do it again a couple times, and now the outer marks are "moving faster and faster and faster" at inordinate rates, while the inner marks, though also "increasing in speed", are being more reasonable about it. Do it every second, get the length up past 30,000 km or so and the outer marks are going "faster than light". Yes? But they aren't really, as far as they are concerned, they are just sitting there right where they always have. Hard to see where real time-dilation, as opposed to red-shifting, would come from in this case.
But, let's skip that for now. Let's say expansion of space does accelerate them somehow. Fine. If so, it accelerates both planets equally. This would be similar to a twins problem where the guys both get into spaceships, rocket off on identical trips in opposite directions, stop "at the same time" and turn around and zoom back, to meet on earth again; and are very surprised after hearing all this jargon about warp-drive messing up your family chronology, to discover that they are actually still the same age!
What you are looking for is something more like maybe, a galaxy or star or whatever traveling at real speeds relative to us. Say some yellow sun was shootng past us at 0.7 the speed of light, yeah? Due to some sort of big explosion a few billion years back. And it had a planet orbiting it that was pretty much identical to earth. But because it had really accelerated to attain its speed, it would experience real time-dilation. So then when you "teleport" there everything will be happening much slower than it would here on earth, and be much more massive. This would probably kill you, imagine a bunch of very slow but massive air molecules sort of perforating you.
See the difference?

This message is a reply to:
 Message 99 by lyx2no, posted 11-09-2009 8:20 PM lyx2no has replied

Replies to this message:
 Message 101 by lyx2no, posted 11-09-2009 11:09 PM Iblis has replied

  
lyx2no
Member (Idle past 4715 days)
Posts: 1277
From: A vast, undifferentiated plane.
Joined: 02-28-2008


Message 101 of 230 (534633)
11-09-2009 11:09 PM
Reply to: Message 100 by Iblis
11-09-2009 9:24 PM


Re: Back to the basics
To begin with would come from in this case.
In our 3D view of the Universe we still see the expansion as motion; hence, the red shift.
But, let's skip that for now. Let's say expansion of space does accelerate them somehow. Fine. If so, it accelerates both planets equally. This would be similar to a twins problem where the guys both get into spaceships, rocket off on identical trips in opposite directions, stop "at the same time" and turn around and zoom back, to meet on earth again; and are very surprised after hearing all this jargon about warp-drive messing up your family chronology, to discover that they are actually still the same age!
Both see the others times a dilated. It's when one of them gets into a ship and accelerates into others reference frame that both agree to who's lagging. The traveler will be the younger and see that it is his world that is slow; only to get back in his ship with his sister world friend to travel back to his home where where he'll find he and his friend were slow and his world was fast, while the friend would see and agree that his world was indeed slow.
So then when you "teleport" there everything will be happening much slower than it would here on earth, and be much more massive. This would probably kill you, imagine a bunch of very slow but massive air molecules sort of perforating you.
To truly be considered tele-ported "onto" the other world one would have to, sort of, change reference frames or you'll be splattered against that other world at 0.7c. That, too, would probably kill you. But if you're in their frame you get their time and mass being normal. No poky, massive O2. This is the bit where cavediver wondered if he'd end up in a westward listing head-stand if he tele-ported to Fremont.

It's not the man that knows the most that has the most to say.
Anon

This message is a reply to:
 Message 100 by Iblis, posted 11-09-2009 9:24 PM Iblis has replied

Replies to this message:
 Message 102 by Iblis, posted 11-10-2009 12:20 AM lyx2no has replied

  
Iblis
Member (Idle past 3895 days)
Posts: 663
Joined: 11-17-2005


Message 102 of 230 (534638)
11-10-2009 12:20 AM
Reply to: Message 101 by lyx2no
11-09-2009 11:09 PM


Re: Back to the basics
This is the bit where cavediver wondered if he'd end up in a westward listing head-stand if he tele-ported to Fremont.
Yep I was thinking of that a lot while I wrote the post.
or you'll be splattered against that other world at 0.7c.
Sure, absolutely. Perforated by unusually massive air molecules moving at a low speed relative to one another, but 0.7c relative to you. And noticeably foreshortened along the lines of acceleration. And probably a couple other things I am neglecting to remember now too.
Or sure, if we compound our magic with even more magic and "change our frame of reference too" then I suppose the only difference one would note related to time-dilation would be when one traveled back and forth? Very similar to the way it would work when actually traveling, without the magic nonsense at all, except discarding the extra acceleration and consequent dilation involved in moving.
only to get back in his ship with his sister world friend
You realize you are answering the main version of the twins problem from this thread rather than my variation that you are quoting, right? My twins undergo the same amount of acceleration and deceleration and consequent Lorentz time-dilation. They both travel, in opposite directions, and when they get back, they are the same age again after all.
Yes, when they observed each other during the voyage apart each would see the other as slowed down. This is Doppler time-dilation. An observer on the planet of origin would observe both of them as even more slowed down. This is a combination of Doppler and Lorentz. The trip back would be even more interesting. Even though both would be slowed down relative to the planet by Lorentz time-dilation, they would observe each other as being incredibly sped up! This is Doppler again, blue-shift this time. An observer on the planet would observe them as sped up also, but not as much, because he would also be observing the Lorentz dilation. Then when they meet, all that stuff will have canceled out altogether, and they will be the same age. Their children may have had kids who had kids already on the point of origin already, sure. That's the basic twins problem again, only with two travelers. But our two travelers undergo identical time-dilation, both the part that cancels out on return and is effectively an optical illusion, Doppler; and also the part that doesn't undo itself on the return trip, acceleration relative to the frame of reference regardless of direction, Lorentz.
See the difference now?
see the expansion as motion
Seeing something doesn't make it "real".
Percy and I baited cavediver for hours in one of Jar's threads to get me this understanding, I'm pretty attached to it.
Message 142
cavediver writes:
It's not really a coincidence. There is a real part - the length of the space-time path, known as the Proper Time of the asttronaut - and there are distorted observations of the path. As the astronaut and observer come back to rest wrt each other back on Earth, there are no more distortions.
...
The observer really has no clue what is going on until the astronaut stops accelerating (at Centauri or back on Earth) The combination of Doppler and Lorentz severely screws up observations.

This message is a reply to:
 Message 101 by lyx2no, posted 11-09-2009 11:09 PM lyx2no has replied

Replies to this message:
 Message 104 by lyx2no, posted 11-11-2009 6:58 AM Iblis has not replied

  
tis---strange
Junior Member (Idle past 5244 days)
Posts: 14
From: Oslo, Norway
Joined: 11-11-2009


Message 103 of 230 (534801)
11-11-2009 5:56 AM


Why it is called the Twin PARADOX
Hi,
The thing is, it is not possible to explain the twin paradox using only special relativity. In special relativity (which is valid in inertial systems only, or in english, non-accelerated systems only) the space-time intervall is constant (as stated before). The whole point of special relativity is that we cannot find out who is moving and who is not, meaning that we could look at the problem from the position of the traveling twin(clock) and find out that the other twin (on earth) is younger, thous we can not use SR to find our solution without ending in a paradoxal state.
However, the one twin is in a spaceship which is accelerated to get too the speed it needs, and decelerated to turn when it gets to the moon. As we all know, when we are low in a gravitational field, the clocks run slower than clocks higher in a grav-field.
(a simple example: the light coming in from the roof of your room has a slightly lower frequency than the light on the floor. We do not want the light to gain energy on the way to the floor, this would be in conflict with energy conservation. We can however solve this dilemma by using that the time goes slower on your floor, therfore the frequencys are the same.)
In the system of the traveler, he experiences a strong gravitational field/acceleration (the main assumption of GR is that these are physical equal) when he turns while arriving on the moon. This field has upwards direction towards earth, in the travelers system, earth is very high in the gravitational field the traveler experiences, therfore time goes slower for him then for earth. The two other accelerations (landing and starting) are negelicable, because the two twin are virtually in the same place (meaning exactly as high in the gravitational field)
THAT is the reason that the earthbound twin is older than the flying twin.

"Hey, hvor hen du er i verden...
Det er deilig slve i skyggen!"
-Dumdum Boys

Replies to this message:
 Message 105 by cavediver, posted 11-11-2009 7:29 AM tis---strange has replied

  
lyx2no
Member (Idle past 4715 days)
Posts: 1277
From: A vast, undifferentiated plane.
Joined: 02-28-2008


Message 104 of 230 (534810)
11-11-2009 6:58 AM
Reply to: Message 102 by Iblis
11-10-2009 12:20 AM


Re: Back to the basics
Or sure, if we compound our magic with even more magic and "change our frame of reference too"
When one buys a plane ticket it is not entirely unreasonable to assume that that plane have landing gear.
Trae's aliens and teleporter are window dressing to his question. cutting to the chase: were it possible to observe two clocks separated by half a universe would they keep the same time? Not bloody likely.

It's not the man that knows the most that has the most to say.
Anon

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 Message 102 by Iblis, posted 11-10-2009 12:20 AM Iblis has not replied

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 Message 106 by cavediver, posted 11-11-2009 7:54 AM lyx2no has replied

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 105 of 230 (534816)
11-11-2009 7:29 AM
Reply to: Message 103 by tis---strange
11-11-2009 5:56 AM


Re: Why it is called the Twin PARADOX
The thing is, it is not possible to explain the twin paradox using only special relativity.
Welcome to EvC
Though it's not a good start because you're completely wrong
In special relativity (which is valid in inertial systems only, or in english, non-accelerated systems only)
Strange, because I have been performing SR calculations in accelerated frames for over twenty years. I think you need to get a proper book on Special Relativity (Rindler's Essential Relativity is a good old-school undergrad-accessible textbook), and stop depending on garbage you read on the internet - present company excluded of course

This message is a reply to:
 Message 103 by tis---strange, posted 11-11-2009 5:56 AM tis---strange has replied

Replies to this message:
 Message 108 by tis---strange, posted 11-11-2009 11:02 AM cavediver has replied

  
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