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Author Topic:   GRAVITY PROBLEMS -- off topic from {Falsifying a young Universe}
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 160 of 205 (255507)
10-29-2005 5:17 PM
Reply to: Message 159 by Son Goku
10-24-2005 6:53 PM


Re: long way to go
Sorry, a little late on these...
Your personal opinion of String Theory and Loop Quantum Gravity?
I don't know, what day is it?
To be honest, I'll side with string. I don't really believe we can understand gravity without involving everything else. In other words, QG is also our TOE. Loop tries to quantise gravity in the absence of anything else. I'm not sure that is the right approach. I think the presence of the other gauge and matter fields is critical.
Think just how incredible is the Einstein-Hilbert action. Imagine something just as elegant that gives rise to all fields... this was the goal of SUGRA and dimensional reduction. String gives us an avenue into this.
That said, it may not matter. The "fundemental" analogue of the EH action may be over some other space, more "real" than our 3+1 or 10+1 "target space". But that still wouldn't explain the reason behind why Langrangian mechanics works (classical and quantum)! That's my BIG question.
Do you believe we should put more attention into trying to find the electronuclear/G.U.T. force?
Yes, but not that's for the particle guys. Quadruple the theorists' budget and you may have dented the experimentalists' coffee budget We need to find clues to glue-balls, SUSY, X bosons, Higgs, etc. The standard model does need firming up. This should all point towards GU.
Opinions on how we'll do in the next few years when it comes to finding exact solutions of GR and QCD?
Exact of GR is no big deal... finding useful ones is a different matter! I think we've probably got all the easy stuff. QCD is pretty much impossible at our energy levels. More lattice work until we can get the proton mass within say 0.1% would be nice.
I don't think focussing on GR or QCD is going to help with the deeper questions.

This message is a reply to:
 Message 159 by Son Goku, posted 10-24-2005 6:53 PM Son Goku has not replied

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


Message 162 of 205 (255589)
10-30-2005 4:49 AM
Reply to: Message 161 by Son Goku
10-29-2005 9:24 PM


I'd gotten a bit too used to just thinking of the Lagrangian as a tool.
That's certainly the image you get classically... but it still doesn't explain why it works. Why minimise? And what do you minimise? Hilbert wrote down the EH action because it was the simplest thing you could write down. But you could also use R_ab R^ab or R_abcd R^abcd etc, etc. But GR is just R.
But as Feynman realised, QM is simply allowing all trajectories, not just those minimising the action. Now the action weights the contribution of each trajectory. This is where things get really Platonistic! If all elements of the Lagrangian contribute, then the Lagrangian itself must in some sense exist. It is no longer a tool/method to reach the classical solution... every part of it represents a facit of reality.
so I like it when "wierd" things turn up in certain solutions, like Closed Timelike Curves, e.t.c.
CTCs are great Some of my first work was on wormholes and wormhole "billiards". If pushed though, I have to admit that CTCs are probably restricted to pure solutions. I can't really believe of a scenario where a CTC can be stable to matter perturbations. Unless you can guarentee there will be no trapped trajectories, a single electron will destroy the whole thing.
This though brings to light the glaring problem with GR. It is purely undynamical 4d, where as we are blatently dynamical 3d. Until we understand the reason behind "time evolution", we are going to struggle. It's funny, becasue "everyone" harps on about the role of conciousness in QM, but that's largely irrelevant. The question is what role does conciousness have in the 3d / 4d dichotomy. This is the real "problem of time" as I see it. Barbour is one guy who spends a lot of time on this - did you read the thread on him a while back? That may be where we are forced to go... an evolution through a moduli space, with time emerging from this evolution. We already see hints of this behaviour in soliton/monopole dynamics.

This message is a reply to:
 Message 161 by Son Goku, posted 10-29-2005 9:24 PM Son Goku has replied

Replies to this message:
 Message 163 by Son Goku, posted 11-02-2005 1:48 PM cavediver has replied

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


Message 164 of 205 (256271)
11-02-2005 2:15 PM
Reply to: Message 163 by Son Goku
11-02-2005 1:48 PM


In short big things are more comfortable with (4), smaller things with (3+1).
And that in short is the QM/GR problem. They have totally different mindsets.
I brought this up with a few philosophers I know and they were shocked that physics contained such an interesting idea.
Yeah, it's funny how ignorant the world is of this stuff... you certainly see that here on this site, with good levels of intellectual ability and understanding, yet a "shocking" lack of knowledge of this area at any real depth.
To be honest, I think popular science gets in the way far too much, presenting a very fake picture. I have seen actual papers written by good physicists on why QCD will prevent the formation of "classical" black holes, and I had demolished their argument before the end of their opening paragraph.
You get the impression that because everyone has heard of Einstein, GR, and black holes, that everyone has at least some understanding. This is completely false This subject is just as opaque to everyone as every other field: evolutionary biology, geology, etc, etc. This field just has more popular books and science fiction films!
This message has been edited by cavediver, 11-02-2005 02:15 PM

This message is a reply to:
 Message 163 by Son Goku, posted 11-02-2005 1:48 PM Son Goku has not replied

Replies to this message:
 Message 165 by madeofstarstuff, posted 11-23-2005 10:53 AM cavediver has replied

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


Message 167 of 205 (263208)
11-26-2005 7:56 AM
Reply to: Message 165 by madeofstarstuff
11-23-2005 10:53 AM


Re: Understanding
I'd echo everything SG said. Go for it. I knew SG would pip in here with Schutz's book, and I do like it. However, some of my better 17-18 yr old students have had success with D'Inverno... it is much more readable as he puts in the effort to put you at ease. He then hits you hard In his intro I think he mentions how he learned GR at 16, which I'm never sure helps that much!
Check it out here
It's more expensive than Schutz but I really think it is worth it. If you are serious about this, I would definitely recommend getting two books to compare and contrast as you move through the theory.
For SG: I remember you saying you liked Schutz becasue of the mathematical emphasis. Have you tried a real mathemtical text like de Felice and Clark (Relativity on Curved Manifolds) or Stewart's (my own GR lecturer) Advanced General Relativity?

This message is a reply to:
 Message 165 by madeofstarstuff, posted 11-23-2005 10:53 AM madeofstarstuff has replied

Replies to this message:
 Message 168 by RAZD, posted 11-26-2005 9:18 AM cavediver has replied
 Message 172 by Son Goku, posted 11-26-2005 11:13 AM cavediver has replied
 Message 177 by madeofstarstuff, posted 12-29-2005 2:22 AM cavediver has replied

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


Message 169 of 205 (263219)
11-26-2005 9:29 AM
Reply to: Message 168 by RAZD
11-26-2005 9:18 AM


Re: Understanding
I'm not sure what you're saying...
Massive amounts of maths education and natural ability enable you to understand the basics of GR? I'd totally agree.
But I'm not sure it's a helpful comment for those that don't feel that they have quite that much mathematics nor quite that level of natural ability, who may still want to understand the basics of GR
The problem is more where you go from there. And staying with it.
Of course. There are many good physicists with good solid undergrad/grad training in GR who are not equipped to explain or even understand the deeper GR/black hole/QG/TOE concepts. I've seen them try and utterly fail to teach just a graduate GR class. You have to live and breath this stuff to get anything close to familiarity.

This message is a reply to:
 Message 168 by RAZD, posted 11-26-2005 9:18 AM RAZD has replied

Replies to this message:
 Message 170 by RAZD, posted 11-26-2005 10:24 AM cavediver has replied

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


Message 171 of 205 (263246)
11-26-2005 10:35 AM
Reply to: Message 170 by RAZD
11-26-2005 10:24 AM


Re: Understanding
I try to deal with facts and truths... I tend not to pull punches. GR is a common 3rd year undergard level course. Anyone with enough determination and sufficient ability can get that far. But what I am talking about is as far removed from that course as that course is from basic calculus.

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


Message 173 of 205 (263268)
11-26-2005 11:19 AM
Reply to: Message 172 by Son Goku
11-26-2005 11:13 AM


Re: Understanding
And if you ever do QM at least three.
never truer words
I never read Stewart's Advanced General Relativity, in fact I've only ever skimmed his Elementary General Relativity.
I never looked at his elementary book... I still have nightmares from the P3 exam he set us!
As for Felice and Clark I thought it was like the end of Wheeler Misner and Thorne, except more serious and little more fully developed mathematically.
For instance the Spinor and NP sections were a good read in MTW, but you learned more in F&C.
Yep, I'd agree with that.
Landau and Lifshitz: "Classical theory of Fields"
This is a science thread so you are not supposed to refer to religious texts The holiest book around...

This message is a reply to:
 Message 172 by Son Goku, posted 11-26-2005 11:13 AM Son Goku has not replied

Replies to this message:
 Message 174 by madeofstarstuff, posted 11-29-2005 1:13 AM cavediver has not replied

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


Message 178 of 205 (273854)
12-29-2005 1:20 PM
Reply to: Message 177 by madeofstarstuff
12-29-2005 2:22 AM


Re: Understanding
Great stuff! Stick with it, and if you want to really understand this stuff, do not let a sentence or equation go by without understanding it. It slows you down considerably, but it pays massive dividends. The more you skip and intend to come back to, the more vague your understanding becomes until it will all be over your head. Use the two books to good effect, and don't forget Wikipedia and John Baez's site. You may just find an odd sentence worded the correct way for understanding to dawn. And believe me, when that understanding starts to dawn, you'll appreciate why relativists don't waste money on LSD If you're really stuck, just drop a note here and I will lend a hand.

This message is a reply to:
 Message 177 by madeofstarstuff, posted 12-29-2005 2:22 AM madeofstarstuff has replied

Replies to this message:
 Message 179 by SuperNintendo Chalmers, posted 12-29-2005 3:28 PM cavediver has not replied
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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 194 of 205 (313529)
05-19-2006 12:26 PM
Reply to: Message 193 by Son Goku
05-19-2006 12:13 PM


Re: 6 Microns?
Great paper, but reminds me why I hate experimental physics

This message is a reply to:
 Message 193 by Son Goku, posted 05-19-2006 12:13 PM Son Goku has not replied

Replies to this message:
 Message 196 by fallacycop, posted 05-19-2006 12:38 PM cavediver has replied

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


Message 197 of 205 (313539)
05-19-2006 12:45 PM
Reply to: Message 195 by fallacycop
05-19-2006 12:35 PM


Re: 6 Microns?
hep-exp, hep-ph... it's all the same.
There is only one true hep... TH!!!!
(not fogetting GR-QC of course)

This message is a reply to:
 Message 195 by fallacycop, posted 05-19-2006 12:35 PM fallacycop has replied

Replies to this message:
 Message 199 by fallacycop, posted 05-19-2006 12:51 PM cavediver has replied

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


Message 198 of 205 (313541)
05-19-2006 12:49 PM
Reply to: Message 196 by fallacycop
05-19-2006 12:38 PM


Re: entanglement
We can do but I'm not sure what's left to do. I was thinking about going through the maths. We could look at some quantum computation but it's not really my forte.
Actually, Mermin has another intersting point that I may try to present, if I get time, which touches on the more philosophical aspects of QM.
And I guess we could look at some particle-statistics weirdness.

This message is a reply to:
 Message 196 by fallacycop, posted 05-19-2006 12:38 PM fallacycop has replied

Replies to this message:
 Message 200 by fallacycop, posted 05-19-2006 12:54 PM cavediver has not replied

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


Message 201 of 205 (313549)
05-19-2006 1:11 PM
Reply to: Message 199 by fallacycop
05-19-2006 12:51 PM


Re: HEP
Ahh, but to publish in hep-ph suggests that you are making testable predictions... that's just not quite esoteric enough for me
Funny thinking about pre-lanl/post-lanl these days... I only just caught the pre-lanl, and it seems a dim distant memory now! It was probably the biggest revolution of the century in physics research, when you think of what the pre-print delay used to be and the (lack of) speed of response.

This message is a reply to:
 Message 199 by fallacycop, posted 05-19-2006 12:51 PM fallacycop has not replied

  
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