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Author Topic:   What is Time and Space
GDR
Member
Posts: 6202
From: Sidney, BC, Canada
Joined: 05-22-2005
Member Rating: 1.9


Message 76 of 204 (229844)
08-04-2005 5:55 PM
Reply to: Message 72 by Son Goku
08-04-2005 2:26 PM


son goku writes:
The best way to think of gravity is still as curved spacetime.
Although most of the effect of gravity come from the curvature of time rather than space.
No problem picturing that.
From reading Greene I have been thinking of gravity as a field similar to a magnetic field. Would you say that is accurate and is that consistent with what you are saying?
This message has been edited by GDR, 08-04-2005 03:47 PM

Everybody is entitled to my opinion.

This message is a reply to:
 Message 72 by Son Goku, posted 08-04-2005 2:26 PM Son Goku has not replied

  
cavediver
Member (Idle past 3644 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 77 of 204 (229863)
08-04-2005 6:40 PM
Reply to: Message 72 by Son Goku
08-04-2005 2:26 PM


Spoken like the great geometradynamicist I used to be (and still am on good days )
Sadly, too much time in quantum gravity has really persuaded me otherwise... at its most basic, given that the T in G=8piT has to be (T), it is very hard to see how G cannot be (G)... and belive me, I spent years trying to refuse this point.
But calling a graviton a particle is possibly where you think that gravitons are the domain of evil particle physicists (like Weinberg and his "no black hole" ideas). A graviton to me is simply the quantisation of the space-time... don't forget that a particle is merely a single root mode of the fourier transform of a quantum FIELD. QFT makes particle physics look like GR, not the other way round... despite what most particle physicists think and tell you!
[Edit to make the bra-kets appear... using () instead of the usual]
This message has been edited by cavediver, 08-04-2005 07:04 PM

This message is a reply to:
 Message 72 by Son Goku, posted 08-04-2005 2:26 PM Son Goku has replied

Replies to this message:
 Message 78 by GDR, posted 08-04-2005 6:46 PM cavediver has replied
 Message 81 by Son Goku, posted 08-04-2005 7:12 PM cavediver has replied

  
GDR
Member
Posts: 6202
From: Sidney, BC, Canada
Joined: 05-22-2005
Member Rating: 1.9


Message 78 of 204 (229867)
08-04-2005 6:46 PM
Reply to: Message 77 by cavediver
08-04-2005 6:40 PM


cavediver writes:
Spoken like the great geometradynamicist I used to be (and still am on good days )
Sadly, too much time in quantum gravity has really persuaded me otherwise... at its most basic, given that the T in G=8piT has to be , it is very hard to see how G cannot be ... and belive me, I spent years trying to refuse this point.
But calling a graviton a particle is possibly where you think that gravitons are the domain of evil particle physicists (like Weinberg and his "no black hole" ideas). A graviton to me is simply the quantisation of the space-time... don't forget that a particle is merely a single root mode of the fourier transform of a quantum FIELD. QFT makes particle physics look like GR, not the other way round... despite what most particle physicists think and tell you!
Is there, by any chance, a translator in the house?

Everybody is entitled to my opinion.

This message is a reply to:
 Message 77 by cavediver, posted 08-04-2005 6:40 PM cavediver has replied

Replies to this message:
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cavediver
Member (Idle past 3644 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 79 of 204 (229874)
08-04-2005 7:01 PM
Reply to: Message 69 by GDR
08-04-2005 1:56 PM


Re: interesting in this
In "The Fabric of the Cosmos" Greene says that if the moon were to disappear we would see the gravitational affects on the Earth immediately whereas we would have to wait for a second and a half to see the light disappear. (Pg 63) Is this wrong or am I misinterpretting it?
Does he really say that??? Are you sure? If he does, then yes, he is wrong. But then he is a particle physicist so I shouldn't expect anything more
Actually, though, what he is talking about is not possible (as in the moon disappearing) and he reveals his "ignorance" of GR by suggesting the scenario. Only magic or an act of God could remove the moon in total. No physical mechanism exists to reproduce the effect. You could disintegrate it, but all the mass would still be there. You could annihilate it with an anti-moon, and all the energy would still be there. It could be swallowed by a passing black hole or worm-hole, but again, its mass would continue to affect things, only now as part of the hole. The moon being there and then the moon not being there is comparing two different realities, and there is no super-metric to be able to define an "immediately" between those realities. And of course, as any student of SR will immediately say... "immedaitely in whose frame?" In one frame, the earth would react before the moon disappeared and in another, it would react after the moon disapperaed.

This message is a reply to:
 Message 69 by GDR, posted 08-04-2005 1:56 PM GDR has replied

Replies to this message:
 Message 82 by GDR, posted 08-04-2005 7:42 PM cavediver has replied
 Message 112 by madeofstarstuff, posted 08-12-2005 3:14 PM cavediver has replied

  
cavediver
Member (Idle past 3644 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 80 of 204 (229876)
08-04-2005 7:02 PM
Reply to: Message 78 by GDR
08-04-2005 6:46 PM


B*gg*r, my expectation bra-kets didn't come out properly... no wonder you were confused. I'll edit it, then see if it's any better...
No, thought not

This message is a reply to:
 Message 78 by GDR, posted 08-04-2005 6:46 PM GDR has not replied

  
Son Goku
Inactive Member


Message 81 of 204 (229882)
08-04-2005 7:12 PM
Reply to: Message 77 by cavediver
08-04-2005 6:40 PM


quote:
geometradynamicist
Haven't heard that word for a while.
You are, of course, correct. General Relativity is the classical limit of some grander theory.
I'm aware of what the graviton truely is, but I'm expressing the view of some that gravity mightn't be a quantum field, or that current QFT doesn't apply to it.
I haven't read up much on current Quantum Gravity (outside the preliminaries), but I think the original problem was that direct quantisation caused the theory to diverge wildely after the first loop order.
As it stands the quantum aspects of gravity (or if you ask some, the general relativistic aspects of QM) haven't been worked out, so for now I'm suggesting that the lay reader stick with General Relativity.
As a side note, are you from the String theory camp or the Loop Quantum Gravity camp?

This message is a reply to:
 Message 77 by cavediver, posted 08-04-2005 6:40 PM cavediver has replied

Replies to this message:
 Message 83 by cavediver, posted 08-04-2005 7:50 PM Son Goku has not replied

  
GDR
Member
Posts: 6202
From: Sidney, BC, Canada
Joined: 05-22-2005
Member Rating: 1.9


Message 82 of 204 (229887)
08-04-2005 7:42 PM
Reply to: Message 79 by cavediver
08-04-2005 7:01 PM


Re: interesting in this
I didn't mean to misrepresent what he said about the moon. He suggests what would happen if hostile aliens zapped the moon and beamed it clear across the galaxy.
Now I have express apologies to all. What I missed is that Greene was talking Newtonian physics and if I had checked further on I would have realized that GR changed all of that and that gravity does function at the speed of light.
Like so many other times in my life. All I had to do was say nothing.

This message is a reply to:
 Message 79 by cavediver, posted 08-04-2005 7:01 PM cavediver has replied

Replies to this message:
 Message 84 by cavediver, posted 08-04-2005 7:56 PM GDR has replied

  
cavediver
Member (Idle past 3644 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 83 of 204 (229891)
08-04-2005 7:50 PM
Reply to: Message 81 by Son Goku
08-04-2005 7:12 PM


I'm expressing the view of some that gravity mightn't be a quantum field, or that current QFT doesn't apply to it.
I'd certainly agree with the last part of that, as has been repeatedly demonstrated. But then it's not really surprising, given its nature. One way to think of it would be that at tree level, you'd expect pair creation of black holes, but of course the black holes themselves are intrinsically not describable at tree-level or at any order of a perturbative expansion.
As for gravity not being a quantum field, I would agree that it doen't appear as the other quantum fields. But then, if the other fields are generated Kaluza-Klein style, it makes more sense. Gravity is then THE quantum field, and the other fields appear from the dimensionally reduced quantum metric. Of coures, you need fermions in there as well, so this becomes supersymmetric. And this is our classic SUperGRAvity.
As it stands the quantum aspects of gravity (or if you ask some, the general relativistic aspects of QM) haven't been worked out, so for now I'm suggesting that the lay reader stick with General Relativity.
I totally agree... just look at GDR's reply to my post
As a side note, are you from the String theory camp or the Loop Quantum Gravity camp?
Well, like you I started as an astrophysicist. Then became a dyed-in-the-wool relativist. I got dragged into string theory, but as we were a relativity group, the approach was very geometrical, with a big de-emphasis on the particle side. But I kept a very open mind, and was very much into alegbraic approaches. That's where I got my cross over into the loop community, really through the sort of stuff John Baez now pushes. But these days, I'm more into the philosophy of it all. Which is why I'm a big fan of Penrose.

This message is a reply to:
 Message 81 by Son Goku, posted 08-04-2005 7:12 PM Son Goku has not replied

  
cavediver
Member (Idle past 3644 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 84 of 204 (229893)
08-04-2005 7:56 PM
Reply to: Message 82 by GDR
08-04-2005 7:42 PM


Re: interesting in this
Oh, I am pleased... he certainly didn't seem that screwed up 10yrs ago when I last spoke to him... but he's still a particle physicist

This message is a reply to:
 Message 82 by GDR, posted 08-04-2005 7:42 PM GDR has replied

Replies to this message:
 Message 85 by GDR, posted 08-04-2005 8:48 PM cavediver has replied
 Message 89 by GDR, posted 08-08-2005 1:57 PM cavediver has not replied

  
GDR
Member
Posts: 6202
From: Sidney, BC, Canada
Joined: 05-22-2005
Member Rating: 1.9


Message 85 of 204 (229898)
08-04-2005 8:48 PM
Reply to: Message 84 by cavediver
08-04-2005 7:56 PM


Re: interesting in this
cavediver writes:
given that the T in G=8piT has to be (T), it is very hard to see how G cannot be (G)... and belive me, I spent years trying to refuse this point.
Just so I can feel like one of the big boys for a couple of minutes could you tell me what the letters in that formula stand for. Also what is QFT.
Thanks
This message has been edited by GDR, 08-04-2005 08:07 PM
This message has been edited by GDR, 08-04-2005 08:09 PM

This message is a reply to:
 Message 84 by cavediver, posted 08-04-2005 7:56 PM cavediver has replied

Replies to this message:
 Message 86 by cavediver, posted 08-04-2005 9:25 PM GDR has replied

  
cavediver
Member (Idle past 3644 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 86 of 204 (229913)
08-04-2005 9:25 PM
Reply to: Message 85 by GDR
08-04-2005 8:48 PM


Re: interesting in this
I'm trying to go to bed, but ok...
G=8piT is Einstein's Equation. It is the core of GR. G is the Einstein Tensor no less, and T is the stress-energy tensor. Essentially , G is geometry (of space-time) and T is energy and matter in space-time.
But we know that energy and matter are quantised: so if T is quantised, then so must G ( as G=T), and thus the geometry is quantised.
QFT is quantum field theory. There are two real-world quantum field theories: Electro-Weak and QCD (quantum chromodynamics) and between them they explain three of our four "forces": em, weak nuclear and strong nuclear. A quantum field is very like our rubber sheet space-time- it's a sheet extending through-out spacce-time. A particle is the smallest sized bump you can have in the sheet. Particles can be created... an up-bump and a down-bump appear and move apart. And particles can be destroyed... an an up-bump and a down-bump come together and "annihilate". This is in great contrast to quantum mechanics, where partciles cannot be created nor destroyed. There is essentially one sheet for each "force" and additional sheets for each type of matter partcile (fermions). Gravitons are just bumps in the space-time rubber sheet.
The reason that Gravity is so different is that each of the non-gravitational sheets stretches over the space-time sheet. The shape of the total space is governed by the space-time sheet... the other sheets just follow the shape.
Bed
[edit to correct thousands of typos... but I'm tired and can't be bothered]
This message has been edited by cavediver, 08-04-2005 09:27 PM

This message is a reply to:
 Message 85 by GDR, posted 08-04-2005 8:48 PM GDR has replied

Replies to this message:
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randman 
Suspended Member (Idle past 4899 days)
Posts: 6367
Joined: 05-26-2005


Message 87 of 204 (229966)
08-04-2005 11:43 PM
Reply to: Message 75 by Son Goku
08-04-2005 5:51 PM


thanks
I have actually read that particular article, which is interesting as some others are in quantum physics that deal with time.
I am though a little surprised there is not more work on this, which I figured you or cavediver might know about.
For example, and tell me if you think this is unreasonable, but from the photon's persepective, everything would be present, no past, no present and no future, just space, right?
So if we were moving at that speed, we would see an object as a path of that object at all points on that path, at once. So we would be looking at it's past, present, and future all as one time, spread out in space.
Well, this is the part I want to get my brain around. This view of space-time has to be just as accurate as our perspective, correct? Just because the photon is not conscious should not make the perspective from the photon unreal, and if all is present, just space, from that perspective, then it seems likely this object, this space and filled trajectory of the object, could be affected by a wave-like effect on it, all at once, past, present, and future.
Or, an event at one point in the object could have an effect on the object as a whole, causing a vibration, and thus affecting the past, present, and future at the same time, or in a wave-like manner.
This message has been edited by randman, 08-04-2005 11:46 PM

This message is a reply to:
 Message 75 by Son Goku, posted 08-04-2005 5:51 PM Son Goku has replied

Replies to this message:
 Message 90 by Son Goku, posted 08-09-2005 2:42 PM randman has replied

  
GDR
Member
Posts: 6202
From: Sidney, BC, Canada
Joined: 05-22-2005
Member Rating: 1.9


Message 88 of 204 (230075)
08-05-2005 9:02 AM
Reply to: Message 86 by cavediver
08-04-2005 9:25 PM


Re: interesting in this
Thanks cavediver
In the last part of Greene's book he contrasts string theory and loop quantum gravity. He says that ST grew out of the particle physics tradition and that LQG grew out of a tradition tightly grounded in GR. He says'"string theorists start with the small (quantum theory) and move to embrace the large (gravity), while adherents of LQG start with the large (gravity) and move to embrace the small (quantum theory)".
After a lot of other detail he says the following.
Brian Greene writes:
If I were to hazard a guess on future developments, I'd imagine that the background-independent techniques developed by the loop quantum gravity community will be adapted to string theory, paving the way for a string formulation that is background independent. And that's the spark, I suspect, that will ignite a third superstring revolution in which, I'm optimistic, many of the remaining deep mysteries will be solved.
I find randman's comments of photons interesting as well. The idea of something that can be anywhere or everywhere at any time is intriguing. (Or at least it would be if I could properly comprehend it. )
This message has been edited by GDR, 08-05-2005 06:03 AM

Everybody is entitled to my opinion.

This message is a reply to:
 Message 86 by cavediver, posted 08-04-2005 9:25 PM cavediver has not replied

  
GDR
Member
Posts: 6202
From: Sidney, BC, Canada
Joined: 05-22-2005
Member Rating: 1.9


Message 89 of 204 (231033)
08-08-2005 1:57 PM
Reply to: Message 84 by cavediver
08-04-2005 7:56 PM


Re: interesting in this
Just came across this on. It is about the speed of gravity. I thought you might be interested.
What Is Gravity? | Space

This message is a reply to:
 Message 84 by cavediver, posted 08-04-2005 7:56 PM cavediver has not replied

  
Son Goku
Inactive Member


Message 90 of 204 (231456)
08-09-2005 2:42 PM
Reply to: Message 87 by randman
08-04-2005 11:43 PM


Re: thanks
quote:
Well, this is the part I want to get my brain around. This view of space-time has to be just as accurate as our perspective, correct?
Yes, absolutely. The only difficulty is that photons don't really have a rest frame. In a sense, the universe from their point of view is a little bit odd.
quote:
So if we were moving at that speed, we would see an object as a path of that object at all points on that path, at once. So we would be looking at it's past, present, and future all as one time, spread out in space.
Since a photon is moving at the speed of light, it sees its whole life as a single instant, however because of length contraction it also sees everything compressed to one point.
This is what I meant by their view of the world being unusual. They basically have no point of view, everything to them is crushed into a single moment, in a single spot.
So when we talk about a photon, we have to do it from the point of view of something else.

This message is a reply to:
 Message 87 by randman, posted 08-04-2005 11:43 PM randman has replied

Replies to this message:
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