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#39839 - 08/30/11 09:33 PM Re: General relativity on large scales. [Re: Bill S.]
Orac Offline
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Registered: 05/20/11
Posts: 2140
Loc: Currently Illinois, USA
Originally Posted By: Bill S.
Could it not be equally true to say it is change brought about by physical processes, such as those designated by the laws of thermodynamics, and that time is simply an arbritrary measure of that change? This would seem particularly appropriate if we are dispensing with the concept of spacetime.


You could but thermodynamic processes can go both ways as can all classical physics so now you have a directionality issue ... that is I can reverse a thermodynamic or classic process so if I do that I can take time backwards.

Time has a arrow of direction ... why?

Originally Posted By: Bill S.

How can virtual particles be said to exist hey are if not in space?


Strict QM says they don't exist in space ... thats the point ... what you see is an illusion.

You "see" a rainbow but most people accept that it is an illussion bought about by certain conditions yet it has a definite visual shape and form and you can even photograph it.

From the photograph I could if I want measure it in many ways length, height, colour, intensity but what I am measuring is an illussion.

That it is appears to occopy a space and form in our speak but it actually represents a vastly different process to what it appears to us as. In some ways it is why rainbows have fascinated humans for a very long time.

We actaully do another version of the same distortion we get the students to walk up onto a platform and measure the height of a line from the platform floor to whatever accuracy they can. They dutifully do it and you ask them are you absolutely certain about your measurement there is no way you could be wrong. Some will realize they are about to be gotcha'ed most will say they are certain. At that point we cut the power to the platform which is actually a steel plate sitting on high frequency actuator.

See the human body can't sense the high frequency movement and you cant see it so what they think is the floor is actually the top of the stroke of the actuator. When you cut the power the actuator sinks back 2 inches (50mm). Everyone is actually out by that ammount in there measurement no matter how careful they were.

It's a very visual illustration that measuring something requires you to know what your references for the measurement are. Now think about a world with QM and measuring anything.


Edited by Orac (08/30/11 09:49 PM)
_________________________
I hate QM, always wrong, never believe me, sad and broken and now lost credibility and I cry wolf.

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#39846 - 08/31/11 10:36 AM Re: General relativity on large scales. [Re: Orac]
Bill Offline
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Registered: 12/30/10
Posts: 1425
Loc: Oklahoma, USA
Originally Posted By: Orac
The interesting thing about GR Bill is remember we have a spacetime loaf. So GR involves time but it's a static backdrop there is no evolution involved infact time is a quite static thing in GR. Thats one of the problems of visualizing it in the GR backdrop.

I have absolutely no chance of figuring out the math involved in GR, but I am aware that time is intimately involved in all GR calculations. Since time varies in all gravitational and accelerated frames I can't see that time is static (no evolution) in any GR calculations. In fact as things move in a GR field they are constantly changing their relationships to one another and therefore any change is accompanied by a time change.

And just as a quick real world example of how QM and GR are kind of separated, but both real, consider the Global Positioning System (GPS). The GPS satellites run almost completely on QM principles, from the solid state electronics in them to the atomic clocks they use to provide exact locations to receivers on the ground. But if you used only the QM principles the GPS wouldn't work. You have to include a GR gravitational correction to the clocks, because they are at a different gravitational potential from the ground stations and receivers.

Granted GR and QM don't work together in many areas, but neither one applies in all cases. QM works great in areas where gravity is predominant and GR works great in areas where other forces are predominant.

As far as I know the only place in which QM addresses gravity is that it does acknowledge a graviton as the carrier of the gravitational force. But there is, as far as I know, no good theoretical way to incorporate the graviton into the overall QM world.

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

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#39860 - 09/01/11 01:58 PM Re: General relativity on large scales. [Re: Bill]
Bill S. Offline
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Registered: 08/20/10
Posts: 2976
Loc: Essex, UK
Originally Posted By: Bill
I can't see that time is static (no evolution) in any GR calculations


Bill, you seem to be saying that static time rules out evolution. If you are standing still, and time is flowing past you, any changes you make constitute evolution in time. Because motion is relative; it must also be true that if you are moving through static time you must be able to observe no differences in the outcomes of your actions. Your evolution in time is the same in both cases. What is there in GR calculations that would make this untrue?
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#39861 - 09/01/11 02:02 PM Re: General relativity on large scales. [Re: Bill S.]
Bill S. Offline
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Registered: 08/20/10
Posts: 2976
Loc: Essex, UK
Originally Posted By: Orac
Time has a arrow of direction


Does this differ from saying "we have an arrow of direction through time"?
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There never was nothing.

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#39870 - 09/01/11 08:57 PM Re: General relativity on large scales. [Re: Bill S.]
Bill Offline
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Registered: 12/30/10
Posts: 1425
Loc: Oklahoma, USA
Originally Posted By: Bill S.
Originally Posted By: Bill
I can't see that time is static (no evolution) in any GR calculations


Bill, you seem to be saying that static time rules out evolution. If you are standing still, and time is flowing past you, any changes you make constitute evolution in time. Because motion is relative; it must also be true that if you are moving through static time you must be able to observe no differences in the outcomes of your actions. Your evolution in time is the same in both cases. What is there in GR calculations that would make this untrue?


I was referring to Orac's statement that time is a quite static thing in GR. I interpreted this to mean that GR calculations are made without regard to time. That would mean that time was not evolving, just passing. In GR time is a component of spacetime and therefore any changes in space also produce a change in time. As I understand it this is really the heart of GR, that there is no separation between time and space.

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

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#39877 - 09/02/11 04:37 AM Re: General relativity on large scales. [Re: Bill]
Orac Offline
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Registered: 05/20/11
Posts: 2140
Loc: Currently Illinois, USA
No Bill G what I am explaining is in GR time and space form spacetime they can't evolve they are a static spacetime loaf.

Lets put it in reference from translation from the horses mouth so to speak

Quote:

In the formalism of general relativity, we can distinguish different notions of time. In particular, we must distinguish the coordinate time t that appears as the argument of the field variable, for instance in guv(x, t), from the proper time s measured along a given world line.

The coordinate time t plays the same role as evolution parameter of the equations of motion as ordinary nonrelativistic time.

The equations of motion can be seen as second order evolution equations in t. However, the physical interpretation of t is very different from the interpretation of the variable with the same name in the non-relativistic theory.

While non-relativistic time is the observable quantity
measured (or approximated) by physical clocks, in general relativity clocks measure s along their worldline, not t.

The relativistic coordinate t is a freely chosen label with no direct physical interpretation. This is a consequence of the invariance under general changes of coordinates.

The solution of the equations is not in its dependence on t, but rather in what remains once the dependence on t (and x) has been factored away.



So under GR there are actually two sorts of time

1.) The one you see

non-relativistic time is the observable quantity measured (or approximated) by physical clocks, in general relativity clocks measure s along their worldline

2.) GR's version t


EXPLICITLY

In general relativity, there isn’t a preferred and observable quantity that plays the role of independent parameter of the evolution, as there is in non-relativistic mechanics. General relativity describes the relative evolution of observable quantities, not the evolution of quantities as functions of a preferred one

With general relativity we have understood that the Newtonian “big clock” ticking away the “true universal time” is not there.

As I said GR can't deal with evolution of time for there is not master clock in GR it actually explicity says so.

It's the same as it has no zero space reference frame.


Edited by Orac (09/02/11 04:50 AM)
_________________________
I hate QM, always wrong, never believe me, sad and broken and now lost credibility and I cry wolf.

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#39888 - 09/02/11 09:24 AM Re: General relativity on large scales. [Re: Orac]
Bill Offline
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Registered: 12/30/10
Posts: 1425
Loc: Oklahoma, USA
Originally Posted By: Orac
It's the same as it has no zero space reference frame.

I think there is a typo there and I'm not sure what the sentence means. What you meant to say is probably quite clear, but I got lost trying to parse that sentence. Could you correct it?

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

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#39889 - 09/02/11 09:59 AM Re: General relativity on large scales. [Re: Bill]
Orac Offline
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Registered: 05/20/11
Posts: 2140
Loc: Currently Illinois, USA
LOL yeah did I really write that .. must have been daydreaming

There is no non-inertial or zero frame for space which is the same as Newtonian absolute space frame.

Does that make sense now :-)

Wasn't really that important you go the important part in GR time is an abstract arbitrary thing. You can coincident it your local concept of time but GR doesn't really care about your time hence that sort of evolving time is not part of GR.

Getting back to the original problem now contrast that to QM, it needs a definite absolute version of time. Thats what leads to bells inequality and all the dramas.

Imagine entangle a particle take one of the partciles and accelerate it so GR/SR time changes all hell breaks loose for QM. Think of a particle near the event horizon of a black hole huge problem for QM.

The moment you accept QM stuff is real its not just a fancy mathematical description, time as seen between GR and QM definitions is a HUGE deal.

BTW if we got you to that point Bill here is the next obvious step. Entangle two partciles and we start changing the QM information of it at a known period. Now take one of the entangled particles and accelerate it very fast.
Now does the QM change occur simulataneously for both particles and if it does this means the accelerating particle can now act a reference clock to the original frame ... is it possible :-)


Edited by Orac (09/02/11 10:38 AM)
_________________________
I hate QM, always wrong, never believe me, sad and broken and now lost credibility and I cry wolf.

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#39891 - 09/02/11 02:24 PM Re: General relativity on large scales. [Re: Orac]
Bill Offline
Megastar

Registered: 12/30/10
Posts: 1425
Loc: Oklahoma, USA
Ah! That makes much more sense. Don't feel too bad about writing something without knowing what it is. I have done it many times. And trying to proof read your own writing is difficult.

I'm afraid that the quotes you give are rather beyond me. I have never taken any physics or math courses beyond the undergraduate courses I had in college over 30 years ago. What I do know is based more on descriptive texts than mathematical texts. Sometimes I can kind of follow along with more mathematical discussions if the descriptive text is good enough.

Now to the discussion. I agree that in GR there is no universal reference frame against which to measure anything. But in the observer's reference frame you can measure what is happening in all other reference frames around you. So as things move around space and time are both evolving, with respect to the observer. So in GR there is nothing that is static. That is why GR has been successful in predicting the evolution of the universe.

Part of the problem with the mismatch between QM and GR is that GR is actually concerned almost entirely with one force, gravity. QM of course has almost no interface with gravity, since it is so weak that at the energies at which particles interact it has no influence on the interactions.

I certainly agree that your example of accelerated bound states would produce some major problems. That is one of the areas where there can be big problems in a comparison between GR and QM. But as we look at the universe it is obvious that GR gives wonderful results, and as we look at particles QM gives wonderful results. Just trying to figure out how to make them mesh gives a lot of people head aches. So for the time being I accept that both of them are completely true, but don't try to figure out what the comparison is. I will wait for somebody a lot more highly trained to come up with some sort of answer.

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

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