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# Jacques' question and JaKiri's answer

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in another thread Jacques (a new poster) asked

In relativity everything is relative.

The brother on earth Bob sees is twin Bill aging more slowly because Bill is going near the speed of light.

For Bill in the spaceship sees Bob on earth going away from him at near the speed of light so he see him aging more slowly.

That's the big paradox of relativity and I don't know it is resolved by the physicists.

and JaKiri who has been at SFN for a couple of years and posted around 2500 messages replied

That's not a paradox at all. The symmetry is broken because one has to accelerate to get back to the other.

IMHO Jakiri was right, and it didnt "take" in that thread. So I am going to amplify his point here: Acceleration is the key to resolving the apparent discrepancy between the clocks, but it was really too noisy in the other thread for anyone to discuss it

JaKiri and Swansont were saying the right things but they were getting drowned out.

So maybe JaKiri would like to explain this. Or someone else discuss it who knows what they are talking about.

It has to be able to happen that the traveling brother Bob whizzes past the earth and as he passes Bill (who is standing on earth) they both hold up their watches and they AGREE

and then Bob goes a lightyear or so and he decelerates and does a hairpin turn and reaccelerates and comes whizzing back

and as Bob AGAIN comes whizzing past the earth (on his return trip) they again hold up their watches and they must both agree that more time has elapsed for Bill, who stayed on earth, than for Bob, who traveled.

there can be no paradox at this point. They must both be able to glance at each other and Bob must see that Bill has aged more, is more wrinkly and more bald etc, and Bill must see that Bob has aged less and is more youthful looking.

Now somebody should explain how this can be and why the whole thing hinges on the fact of ACCELERATION, which is not "relative" in the sense that they both can acknowledge that it was Bob that did it. Steady motion is relative, but acceleration (or its gravity equivalent) is absolute. If you are accelerating (or in the equivalent gravity field), then you KNOW IT by how it feels. And if you arent, then you know that by how it feels. And everybody observing you can agree about this. It is an absolute fact.

So as long as Bob and Bill are just in steady constant motion relative to each other then as long as they can see each other's clocks Bob will say that Bill's clock is running slow. And viceversa Bill will say that Bob's clock is running slow. So you THINK you are going to eventually run into a paradox where they cant agree on who has aged more!

But you only have to check when Bob gets back! And by that time everything will be clear.

Because will Bob is decelerating and doing his hairpin turn and reaccelerating----out there at Alpha Centauri or wherever----it will be as if

HE IS AT THE BOTTOM OF A VERY DEEP GRAVITY WELL. that is the "equivalence principle"

And if you are at the bottom of a deep gravity well and I am at the top looking over the edge and down at you, then you and I can BOTH AGREE that your clock is running slower. There is no "relativism" about this. That is what general relativity tells us.

And the GPS position system would not work if this were not true because they have atomic clocks up there in the satellites and they CORRECT for the clock being higher up in the gravity well and for you and me being lower down in the earth gravity well. So the atoms in the satellite atomic clock actually do their vibrations faster and the atoms in our bodies down here do their vibrations slower and everybody can agree on this. they look fast to us down here, and we look slow to them up there. It's absolute. And that is a 1915 Gen Rel effect. And they are programmed to correct for it. And if they werent accurate they wouldnt use GPS to navigate aircraft and do landings and takeoffs and stuff.

Welcome to jacques because he's a new poster. I hope he stays around because he asked one or two astute questions in his first two posts!

I hope he didnt get discouraged by the confused response.

This is really a case where Swansont or Jakiri should take over

and do the honors on the twin paradox, or refer to some past SFN thread that does it. I dont usually get into those topics myself.

I think the best thing that has been said at SFN lately is YT new sig:

Dont wrestle with a pig because the pig just enjoys it.

You get these totally confident crackpots who tell you all about

"relativity" and dont know diddlysquat.

Some of them even construct websites purporting to explain it.

dont argue with them because they will never realize what they'r missing

and they will just enjoy sounding off.

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To be fair, it isn't clear whether the website he was linking to was his; the english wasn't utterly different, the information content was similar to his posts, but it still could have been coincidence; the latter point has a fairly obvious explanation, and the former can easily happen.

Of course, I suspect it to be true, purely by his continual reference to it, but there we go.

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Martin, I cant really comment on the Physic part of your post, Im still a learner

as for the sig tho, its a quote (that I was given full permission to use) by Swansont, he also doesnt know where it came from

glad you like it tho, I did

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To be fair' date=' it isn't clear whether the website he was linking to was his; the english wasn't utterly different, the information content was similar to his posts, but it still could have been coincidence; the latter point has a fairly obvious explanation, and the former can easily happen.

Of course, I suspect it to be true, purely by his continual reference to it, but there we go.[/quote']

the person who wrote the essay at the website signs himself

"KJS (the uncredentialed)" and IIRC says he was born 14 Feb, 1951.

Maybe he is sore because he doesnt have a teaching credential and thinks

he understands relativity better than people who do. It could be a tough case.

someone who is that much loudmouth and confident they are right at the age of 54 is not going to get cured any time soon, I worry.

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Martin' date=' I cant really comment on the Physic part of your post, Im still a learner

as for the sig tho, its a quote (that I was given full permission to use) by Swansont, he also doesnt know where it came from

glad you like it tho, I did [/quote']

whoa! so Swansont is involved with that one too!

On Usenet "sci.physics.research" there used to be a poster named Maddy from Fermilab or UChicago who was very nice and I remember his sig:

he said

"If you find yourself arguing with a fool, it could be that he is doing the same."

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Thanks Martin for your reply

I see here it is almost a chat by looking at the post time.

So you are telling that the paradox is solved because you have a prefered reference frame ?

What if the spaceship accelerate at exactly 1 G ?

Bob and Bill will feel the same acceleration, so what is the prefered frame.

Do you have some internet references or other post, about the solution to this paradox?

Thanks

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)))

Indeed!

also a good Sig, normaly I detest Cliche` type material, and "wise old sayings", but I must confess that OCCASIONALY! you DO come across one or two that are just perfect

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BTW here is another good question from Jacques, it was post #11 on the "measuring time" thread.

It did not get answered, i guess because of noise

Space is the absence of ANYthing, and does not constitute a framework, a grid, a backdrop..

Then what is the space curvature used to explain gravity?

http://www.scienceforums.net/forums/showthread.php?p=121095#post121095

what is it that can be bent by the influence of concentrations of energy

what is it that curves, and has a changeable geometry

is it the gravitational field itself

if space is nothing (as some people assume) then how can it have a shape

hello Jacques , YT and others. I just wanted to keep track of this other question

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Thanks Martin for your reply

I see here it is almost a chat by looking at the post time.

So you are telling that the paradox is solved because you have a prefered reference frame ?

What if the spaceship accelerate at exactly 1 G ?

Bob and Bill will feel the same acceleration' date=' so what is the prefered frame.

Do you have some internet references or other post, about the solution to this paradox?

Thanks[/quote']

Jacques I am not the local authority about these things. We must wait until someone else weighs in who customarily handles this kind of thing.

but I will tell you my personal opinion.

the world is full of preferred frames. the expansion of the universe, and the CMB defines one----the FRW metric that all cosmologists use has a built in preferred frame. Everybody knows that Gen Rel does not enjoy Poincare symmetry.

only Special Rel has poincare symmetry. cosmologists call the expansion of the universe the "Hubble Flow" and they often use frames that are "at rest with respect to the Hubble Flow"

basically that just means that the CMB is the same temperature in all directions----it has the "dipole anisotropy" taken out.

the sun's absolute motion wrt CMB is, IIRC, about 350 km/second in the direction of Leo------the COBE people determined this in the mid 1990s.

So the CMB has a hotspot in Leo and a coldspot 180 degrees from Leo----some millikelvins

and if you factor that out then you can get a rest frame and the CMB does not have a hotspot.

Anyway it is only in a FLAT UNEXPANDING UNIVERSE that there is poincare symmetry and special rel works. that is, no gravity bending space, complete flatness. It is the MINKOWSKI space approximation to reality. just an approximation.

Special rel is a very useful approx, because space IS approximately flat, because gravity is comparatively weak.

but it is ONLY an approx. so you cant take 1905 special rel completely serious.

this is not controversial

this what I have said is only a preamble to answering your question

NOW I WILL GET TO THE QUESTION

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Thanks Martin for your reply

I see here it is almost a chat by looking at the post time.

So you are telling that the paradox is solved because you have a prefered reference frame ?

What if the spaceship accelerate at exactly 1 G ?

Bob and Bill will feel the same acceleration' date=' so what is the prefered frame.

Do you have some internet references or other post, about the solution to this paradox?

Thanks[/quote']

the important thing to notice is that the earth field is not UNIFORM

it peters out as you get away from the earth

so yes Bill stands on earth in 1 gee

but it is an inverse square field that is only 1 gee where he stands

so this is a trivial insignificant thing compared with Bob

because listen (and this is my personal opinion)

Bob is uniform decelerating for a whole lightyear!!!

he has to slow down so he can do his hairpin turn!

As he is turning around, at the distant flyby destination

HE IS LIKE AT THE BOTTOM OF A more than ONE LIGHTYEAR DEEP UNIFORMLY ONE GEE FIELD GRAVITY WELL

so he is way mondo deep down and stayathome Bill is way mondo high up

and as Bob is turning the ship around he looks back at Bill thru his telescope and he sees Bill aging very rapidly and walking around like Charlie

chaplin in the speeded-up old silentmovies.

or maybe he does not see it exactly like this but in his mind he has a model of Bill based on science knowledge and he says "now Bill is aging very rapidly because he is at the top of the deep well"

So then, when Bob finally gets back and goes whizzing past earth and is able to check things, it all checks out! because Bill does show the effects of all that passage of time which Bob knew happened during the time he was decelerating and turning around and accelerating-----at the distant place he went to see.

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HE IS LIKE AT THE BOTTOM OF A more than ONE LIGHTYEAR DEEP UNIFORMLY ONE GEE FIELD GRAVITY WELL

The difference of clock rate of the GPS and the surface clock is consequence of the difference of the gravity field. Yes the difference is caused by an inverse square field so there is a relation with distance. But the depth of the gravity well or more exactly the difference of depth, is the difference of the acceleration.

The difference between Bob and Bill is 0 so the gravity field depth is zero.

Do you think that make sense ?

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ACCELERATION, which is not "relative" in the sense that they both can acknowledge that it was Bob that did it. Steady motion is relative, but acceleration (or its gravity equivalent) is absolute. If you are accelerating (or in the equivalent gravity field), then you KNOW IT by how it feels. And if you arent, then you know that by how it feels. And everybody observing you can agree about this. It is an absolute fact.

I'm still not sure how acceleration can not be an absolute. I could accelerate (in space) from a particle at 3ms-2, therefore the seperation would widen at 3ms-2 and so the particle could in effect be accelerating from me at 3ms-2. The only reason i feel g when i accelerate in a car is because of the acceleration of me with repect to the back of the seat. "feel" is only our measure of something else. That something else could be doing as much as we do. I assume acceleration is a nice thought to pick on because if velocity is constant the time change would be inverse to the acceleration. i.e. when we accelerate time slows down. The statement also doesn't quantify (for me) why the return journey does not reverse the "aging" phenomena.

And if you are at the bottom of a deep gravity well and I am at the top looking over the edge and down at you, then you and I can BOTH AGREE that your clock is running slower. There is no "relativism" about this. That is what general relativity tells us.

Ok, this is another confusion I am having. My understanding of relatvity is largely based on what it says on the tin, i.e. relativity...how i observe something relative to someone else/another frame of reference. But from the way others have spoken in threads, relativity is about a physical phenomena where clocks are literaly slowed down. Can we confirm if relativity is an apparent difference in measurement between two parties, or a literal/physical effect on the object/measuring instruments themselves. For example, if I tied a bit of string to the needle of a tachometer and pulled it, that is a physical effect on the measurement. If two parties read the needle at different angles, they would be different due to parallax, what I call an apparent measurement.

And the GPS position system would not work if this were not true because they have atomic clocks up there in the satellites and they CORRECT for the clock being higher up in the gravity well and for you and me being lower down in the earth gravity well. So the atoms in the satellite atomic clock actually do their vibrations faster.

Agreed, there is a difference in reference to time dilation formulae. But what is the basis of these formulae. Is it because of a literal effect on the atoms in the clocks, or is it an apparent difference due propagation between an event happening and it being observed. If it is literal (physical) effect, doesn't this then indicate that this is a physical property and not that time itself is changed.

What I am relating to, in particular with gravitational dilation, is how can we assess the integrity of the elasticity within the atom due to changes in the gravational field, heat, vibration, etc. Is this not physical effects put on to the atoms not time.

Quite simply, is relativity apparent (i.e. the way we measure) or physical (i.e. the way the thing being measured actually changes)?

Finally, on the point of pigs. I hate big heads myself but if you sit back sometimes they spur a moment of thought. And don't forget people used to think the universe revolved around the earth, the earth was flat, that when you tied someone to a chair and held them underwater they'd drown if they were a witch. Who really knows what's right and what's wrong????

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Quite simply, is relativity apparent (i.e. the way we measure) or physical (i.e. the way the thing being measured actually changes)?

That's a question I been asking myself for a long time too. I don't have an answer

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I'm still not sure how acceleration can not be an absolute. I could accelerate (in space) from a particle at 3ms-2, therefore the seperation would widen at 3ms-2 and so the particle could in effect be accelerating from me at 3ms-2.

There would be a force acting on you. It would be fairly trivial to arrive at that conclusion because, even if you're not in contact with anything, you're not a rigid body.

Can we confirm if relativity is an apparent difference in measurement between two parties, or a literal/physical effect on the object/measuring instruments themselves.

Sort of both. Constant velocity interactions are the former, acceleratory thingies are the latter. It changes from one to the other, depending on what you're talking about.

Agreed, there is a difference in reference to time dilation formulae. But what is the basis of these formulae. Is it because of a literal effect on the atoms in the clocks, or is it an apparent difference due propagation between an event happening and it being observed.

The basis of this formula is that the speed of light is constant for all observers; time dilation, and all the other lorenz effects, are upshots of this. It's a literal, physical effect of the inertial frame.

If it is literal (physical) effect, doesn't this then indicate that this is a physical property and not that time itself is changed.

Scientifically, this statement is meaningless; the only way we can say that time exists is through our experiences of it, measuring it. The only way we can test time dilation is through measuring it. Hence, we cannot say that time exists independent of our measurings, and to say that there is a universal 'clock' is unscientific, and an untestable hypothesis.

Finally, on the point of pigs. I hate big heads myself but if you sit back sometimes they spur a moment of thought. And don't forget people used to think the universe revolved around the earth, the earth was flat, that when you tied someone to a chair and held them underwater they'd drown if they were a witch. Who really knows what's right and what's wrong????

This is a science forum. Science. It is perfectly scientific to say that the earth is flat, if all the evidence you have gathered suggests that it is flat. You cannot say things, from supposed authority, without evidence. Furthermore, since we are discussing an abstracted part of science ('Einstein's Relativity'), you can be absolutely correct or incorrect about it.

Oh, and re: witches, the way the drowning test was that if you survived you were a witch.

And then you were killed.

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Constant velocity interactions are the former (mesurement differences)

If it was a measurement difference then when Bill come back, Bob will see Bill's clock going faster. Lorentz transform doesn't distinguish between motion away from the observer and motion toward the observer: the v term is squared v^2 .

I am still puzeled....

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If it was a measurement difference then when Bill come back' date=' Bob will see Bill's clock going faster. Lorentz transform doesn't distinguish between motion away from the observer and motion toward the observer: the v term is squared v^2 .

I am still puzeled....[/quote']

That's not a constant velocity interaction, because the velocities aren't constant.

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Do you mean that Lorentz transform apply only to constant velocity ?

Suppose Bill going away at constant speed we will see his clock going slower. We can use Lorentz transform to calculate how much slower we will read. (It is a guess )

Suppose Bill coming back at constant velocity after is turning back. We will see his clock runing faster than our clock (mesurement effect). But the Lorentz transform will not give the correct answer.

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Do you mean that Lorentz transform apply only to constant velocity ?

Suppose Bill going away at constant speed we will see his clock going slower. We can use Lorentz transform to calculate how much slower we will read. (It is a guess )

Suppose Bill coming back at constant velocity after is turning back. We will see his clock runing faster than our clock (mesurement effect). But the Lorentz transform will not give the correct answer.

No, you're misunderstanding.

In constant velocity interactions (ie one where are the parties are going at a constant velocity), all Lorenz Transformations are SUBJECTIVE. They only apply to each individual rest frame, and are different for every one. That's what I meant by a measurement difference; because all rest frames are equally valid, there's a difference in measurement that's reflected by no physical change over all the rest frames, unlike Bill, undergoing acceleration to come back to earth.

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It seems that this discussion really gets to the heart of the relativity debate. Velocity can only be defined wrt to something else. That is, we cannot describe an absolute velocity (according to relativity). With respect to what is acceleration defined?

If velocity wrt something else is truly relative, and can lead to the "observed" conclusion that clock A is running faster than clock B from A's point of view, and vice versa from B's point of view, what is it that allows acceleration to make that "observed" clock rate difference a reality?

If velocity is relative, how can changing the relative rate of movement between two bodies (acceleration) be an absolute? What makes acceleration real, while it's resulting velocity is simply relative.

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If velocity is relative, how can changing the relative rate of movement between two bodies (acceleration) be an absolute? What makes acceleration real, while it's resulting velocity is simply relative.

To accelerate, you need a force. SR does surprisingly little to forces.

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I agree with pretty much everything that Martin (and JaKiri) said. I only have one clarifying comment:

The twin paradox is normally formulated assuming Bob is at rest in a frame with no gravity - so he is feeling no force, while Bill feels the acceleration 'force' when he turns round. If Bob were on Earth the whole time and Bill maintained an acceleration/decelaration of 1g the entire time of his travel, their clocks at the end would show the same time - they would have aged the same amount because one could not tell their frames apart from the forces they feel.

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they would have aged the same amount because one could not tell their frames apart from the forces they feel

That make sens to me!

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I bow to Severian on this, thanks for checking in!

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I just wanted add (similar to what I think Severian is pointing to) that the "paradox" could happen in different gravity situations too. If one twin stayed on Earth, and the other were instantly beamed to a neutron star (and could survive there), stayed there for a year, and then was instantly beamed back to Earth, the twins would notice the neutron traveler had aged less, and that his watch was behind the Earth twin's watch.

Relativity can be understood better if we look at the history of the universe. What we call "time" is simply our observation of the universe changing at a certain rate. That change is toward disorganization, so time is really the rate of entropic change. The universe started out tightly compacted, it is now flying apart. The balance between this density and expansion is moderated by gravity, which is also reproduced when one accelerates. What gravity and acceleration do, in a sense, is create anti-expansion and is therefore anti-entropic. It slows everything down, sends it back toward its originating direction of density. So the rate of entropy of clocks, body aging, and everything else in a frame of reference are affected.

A question I asked at another science site once was if the traveling twin, Bill, would notice in any way. Let's say Bill spends 35 years on Earth, takes off in his space ship, and observes his clock register the passing of five years. When he gets back to Earth, he finds his twin has aged 15 years. Would Bill feel like those were the longest five years he'd ever experienced? Remember Bill had had 35 years in Earth's frame of reference. He would have that at least to compare to his space ship frame of reference.

All the physicalist science types insisted there would be absolutely no way to tell in terms of measurement. I agreed. But what I wanted to know was if Bill might feel it. Really I was posing a question about the nature of consciousness. If consciousness does emerge from the physical processes of the brain, then Bill probably wouldn't notice. But if consciousness is not physical but, for instance, some quality associated with the brain, then it might have its own internal frame of "feeling" reference which might sense it had lived through an awfully long five years.

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• 4 weeks later...
I agree with pretty much everything that Martin (and JaKiri) said. I only have one clarifying comment:

The twin paradox is normally formulated assuming Bob is at rest in a frame with no gravity - so he is feeling no force' date=' while Bill feels the acceleration 'force' when he turns round. If Bob were on Earth the whole time and Bill maintained an acceleration/decelaration of 1g the entire time of his travel, their clocks at the end would show the same time - they would have aged the same amount because one could not tell their frames apart from the forces they feel.[/quote']

Is this correct?

Bob is in a constant situation. Let's say you doubled the duration of the test (Bob's time) he would be twice as old. Bill would be closer to light speed (Bob's perspective) through the "middle half" of the longer test than in the first and last quarters (the "other half" which is identical to the original test) of the longer test. So for Bill the "middle half" would be of less duration than the "other half" whereas for Bob it would be the same duration.

Bob would think the test doubled where Bill would think it increased somewhat but not doubled.

Thoughts?

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