PDA

View Full Version : Gravitational dilation

Seiryuu
2008-Oct-07, 05:35 PM
As pointed out to me, my notion of gravitational time dilation seems to be wrong. Can anyone explain to me what's wrong with this quote?

GR predicts gravitational time dilation depending on the distance from a massive body. The effect is also confirmed. So the question is not whether time dilation happens, the question is why does it occur and how does it relate to gravitional fields? Why does a clock tick faster at higher altitude? GR answers that too: time passing at different rates in regions of gravitational potential.

alainprice
2008-Oct-07, 05:49 PM
Your clock always ticks 1 second per second.

In going from a low to high(or high to low) gravitational potential, we need to change reference frames and this shows up as dilations(time included).

GR is all about the following. Everything in your immediate vicinity is local and you need to realize stuff happens when you move from one place to another.

Seiryuu
2008-Oct-07, 06:00 PM
The local clock will tick 1 second per second, agreed. But when an observer from the ground compares it to a reference clock on the ground, it will appear to tick faster than the reference and similarly the observer at higher altitude will agree that the clocks on the ground are ticking slower compared to his.

So, not locally, but relatively speaking, the clock at higher potential "ticks faster". Am I wrong?

alainprice
2008-Oct-07, 06:09 PM
They tick faster because the signal you're measuring has gone from one frame to another.

If you gain altitude and compare your watch to that clock, they'll both elapse 1 second in 1 second.

It's all reference frames. It gets crazy when you consider dropping down into a gravity well and climbing back out. But again, this is not an inertial reference frame, the lost time is essentially meaningless.

Seiryuu
2008-Oct-07, 06:40 PM
They tick faster because the signal you're measuring has gone from one frame to another.

I see.

If you gain altitude and compare your watch to that clock, they'll both elapse 1 second in 1 second.

This does match my understanding of the concept though. However, if the change of frame has an impact on it's measurement, can it be that in different frames time passes faster or slower. And by time I do not mean the seconds themselves, but rather the "duration" of those seconds.

Is it possible in GR, that the seconds would become relative to eachother, in the sense that one second in one frame differs from one second in another? If so, changing frames would mean changing the duration so that one second is equally long in the same reference, no matter if you go from a high to low or a low to high potential and becomes stretched or shortened as you move inside or outside the gravity well.

It's all reference frames. It gets crazy when you consider dropping down into a gravity well and climbing back out. But again, this is not an inertial reference frame, the lost time is essentially meaningless.

No argue there either.

undidly
2008-Oct-08, 10:35 AM
I see.

This does match my understanding of the concept though. However, if the change of frame has an impact on it's measurement, can it be that in different frames time passes faster or slower. And by time I do not mean the seconds themselves, but rather the "duration" of those seconds.

Is it possible in GR, that the seconds would become relative to eachother, in the sense that one second in one frame differs from one second in another? If so, changing frames would mean changing the duration so that one second is equally long in the same reference, no matter if you go from a high to low or a low to high potential and becomes stretched or shortened as you move inside or outside the gravity well.

No argue there either.

Time does run more slowly in a G well.
Measure a tuning when you and the fork are far from Earth.
Move the fork to Earth (you stay where you are) and measure again.
The fork rings at a lower frequency.It really does,no trick of interpretation.

Why does this happen?.
It is because the mass has increased by being near other mass.
This is GR.
Time slows because all mass is increased ,even in your time measuring instruments and the molecules of your body.You age slowly but only if measured from outside the G well.If measured from in the well all is normal
as the measuring instrument (clock )also runs slower.

Cougar
2008-Oct-08, 02:48 PM
The local clock will tick 1 second per second, agreed.... the observer at higher altitude will agree that the clocks on the ground are ticking slower compared to his.

This is correct because you have specified who is observing what, i.e., you have specified both frames.

So, not locally, but relatively speaking, the clock at higher potential "ticks faster". Am I wrong?

According to whom? You've specified one frame, but not the other. As alain said, It's all reference frames.

thorkil2
2008-Oct-08, 08:48 PM
This is correct because you have specified who is observing what, i.e., you have specified both frames.

According to whom? You've specified one frame, but not the other. As alain said, It's all reference frames.

I think he had already specified the difference between surface frame (bottom of the well) and a frame farther from the surface.

Seiryuu
2008-Oct-08, 10:21 PM
That's what I meant ya, although I didn't mention it explicitly in that statement. My bad. ^^

Ken G
2008-Oct-08, 11:39 PM
The problem is, a reference frame is a local thing. There's really no such thing as a reference frame that includes two widely separated objects. Special relativity kind of pretended that was possible by specifying a string of observers who are all inertial and have no relative motion, since you can maintain that indefinitely. But you can't do that when there's gravity afoot, so in general relativity, a reference frame is an entirely local thing-- it's where an observer makes his/her measurements, and that can never include two widely spaced objects. So instead of global reference frames, you have local reference frames, and global coordinate systems. Trouble is, you can get all kinds of crazy sounding descriptions of what is happening if you simply choose a bizarre coordinate system. There's no "cure" for that, and no illness to cure-- it's just how it is in general relativity, one should not expect unique descriptions of why things are happening the way they are, you should only expect to get the same answer as to what did happen (i.e., what was the result of a local measurement). So in particular, whether or not time is "really" going more slowly, or if there is just less time occuring, or if it's going forward and backward and coming out less in the end, is all a question of the coordinates. What is not just a question of coordinates is, apparently, what can send a signal to what, what can receive a signal from what, and what can have no signal sent either way to what.

Seiryuu
2008-Oct-09, 12:33 AM
So in particular, whether or not time is "really" going more slowly, or if there is just less time occuring, or if it's going forward and backward and coming out less in the end, is all a question of the coordinates

Not quite as far as I'm concerned. There's a fundamental reason as to why this happens and I'm determined to find out.

Well... I actually already have a conceptual theory about it, hence why I asked this question in the first place... :p

Ken G
2008-Oct-09, 12:50 AM
Not quite as far as I'm concerned. There's a fundamental reason as to why this happens and I'm determined to find out.The "fundamental reason" in general relativity is spacetime curvature. But that's not a "reason", because saying time goes more slowly (or whatever other coordinatization is being used) is not actually any different from saying there is spacetime curvature. One does not explain the other, they are the same. Indeed, no theory of gravity can explain gravity, it is not the job of a theory of gravity. The job is to say what gravity does, not what it is. You need a deeper theory to say what gravity is, but then the deeper theory will merely involve other more other elments that will merely be described, not explained. All explanations originate in descriptions, there's nothing that is explanations all the way down.

Seiryuu
2008-Oct-09, 07:49 AM
The "fundamental reason" in general relativity is spacetime curvature. But that's not a "reason", because saying time goes more slowly (or whatever other coordinatization is being used) is not actually any different from saying there is spacetime curvature. One does not explain the other, they are the same.

So all I have to do to find the the reason, is finding out how gravity is caused by spacetime curvature and determine the nature of what spacetime curvature really means.
Pff, too easy! :p

Of course, this assumes that time dilation is caused by spacetime curvature in the same way gravity is.

Indeed, no theory of gravity can explain gravity, it is not the job of a theory of gravity. The job is to say what gravity does, not what it is.

Finding out what gravity is, requires finding out what spacetime curvature means or what causes it.

You need a deeper theory to say what gravity is, but then the deeper theory will merely involve other more other elments that will merely be described, not explained. All explanations originate in descriptions, there's nothing that is explanations all the way down.

The ideal deeper theory for me would be a "missing link" theory. If I can come up with one to explain what gravity and spacetime curvature are and my theory itself can be explained by more fundamental quantum mechanics, would that make it valid?

Ken G
2008-Oct-09, 08:38 PM
So all I have to do to find the the reason, is finding out how gravity is caused by spacetime curvature and determine the nature of what spacetime curvature really means.
Pff, too easy! :pYup, that's the trick. Or, you can go the route of particle physicists, that spacetime is not anything real and its curvature is just a mnemonic, gravity is really a field theory mediated by gravitons and we just haven't figured out yet how to do that.

Finding out what gravity is, requires finding out what spacetime curvature means or what causes it. Right, but that would be a theory of something different, something more fundamental, than gravity. At least we had the phenomenon of gravity to use to formulate general relativity-- what has spacetime curvature got to use?

The ideal deeper theory for me would be a "missing link" theory. If I can come up with one to explain what gravity and spacetime curvature are and my theory itself can be explained by more fundamental quantum mechanics, would that make it valid?I would say that any theory from which gravity and spacetime curvature emerge from a more fundamental form of quantum mechanics would indeed be a missing link. It would sound a lot like the modern "holy grail" of physics: grand unification.

cosmocrazy
2008-Oct-09, 09:28 PM
The problem is, a reference frame is a local thing. There's really no such thing as a reference frame that includes two widely separated objects. Special relativity kind of pretended that was possible by specifying a string of observers who are all inertial and have no relative motion, since you can maintain that indefinitely. But you can't do that when there's gravity afoot, so in general relativity, a reference frame is an entirely local thing-- it's where an observer makes his/her measurements, and that can never include two widely spaced objects. So instead of global reference frames, you have local reference frames, and global coordinate systems. Trouble is, you can get all kinds of crazy sounding descriptions of what is happening if you simply choose a bizarre coordinate system. There's no "cure" for that, and no illness to cure-- it's just how it is in general relativity, one should not expect unique descriptions of why things are happening the way they are, you should only expect to get the same answer as to what did happen (i.e., what was the result of a local measurement). So in particular, whether or not time is "really" going more slowly, or if there is just less time occurring, or if it's going forward and backward and coming out less in the end, is all a question of the coordinates. What is not just a question of coordinates is, apparently, what can send a signal to what, what can receive a signal from what, and what can have no signal sent either way to what.

When does a local reference frame become a global one? In other words at what distance would you consider 1 frame becoming 2?

Thanks David

Seiryuu
2008-Oct-09, 09:31 PM
Right, but that would be a theory of something different, something more fundamental, than gravity. At least we had the phenomenon of gravity to use to formulate general relativity-- what has spacetime curvature got to use?

The phenomenon of gravitational time dilation itself of course, which is what I'm getting at in the theory I'm building :)

alainprice
2008-Oct-09, 10:17 PM
When does a local reference frame become a global one? In other words at what distance would you consider 1 frame becoming 2?

Thanks David

Personally, I'd go with the planck length.

Any distance is a distance and therefore less and less local. Local = point.

Since the strength of these 'forces' is tiny for the spaces we tend to consider, a point can be blurred into a nice chunk of space. Our solar system can be viewed as local without any inconsistencies to gravity.

Ken G
2008-Oct-10, 12:16 AM
When does a local reference frame become a global one? In other words at what distance would you consider 1 frame becoming 2? That is indeed an important issue, but it pervades all of physics. Physics is expressed in terms of differential equations-- speed is an "infinitesmal change" in distance over an "infinitesmal change" in time. How small does "infinitesmal" have to be? The answer is, it all depends on how accurate you want your determination of the velocity to be. Same with "local" versus "global" reference frames-- you choose how local you want to take it, and the more local, the more accurate, but the more limited your scope. At some point, you run into fundamental limitations, like the size of your measuring apparatus.