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Author Topic:   String! Theory! What is it good for ?!?
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


(1)
Message 25 of 107 (535298)
11-14-2009 1:31 PM
Reply to: Message 20 by slevesque
11-13-2009 3:28 PM


Re: Popper pooped.
Karl Popper has made maybe the single most important contribution to science as a whole in the 20th century, which came in the form of his book ''The logic of scientific discovery''.
I think you'll find that Popper is not such a hero in the world of theoretical physics. And he never could quite handle the essence of quantum mechanics and the methodology behind the development of QCD and similar theories...
ABE: but this is for yet another thread
Edited by cavediver, : No reason given.

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


(1)
Message 26 of 107 (535299)
11-14-2009 1:42 PM
Reply to: Message 24 by Iblis
11-14-2009 6:49 AM


Re: Yes, that's science
Here's a snap of Robert Laughlin, another Nobel in Physics
Who gives a crap about what Laughlin thinks? His field is condensed matter physics. He's so lost that he thinks that he's qualified to theorise about black holes (and why they don't exist :rolleyes without understanding even the basics. He's a laughing stock to relativists.
Richard Feynmann
Now Dick is another matter. But he has always been about calculations. His development of QED was simply to get out the numbers he wanted. No disgrace, and he is still God to many of us - he and I share birthdays But his approach to quantum gravity is a different approach... (not least, that it is now from the other side!)
Edited by cavediver, : No reason given.

This message is a reply to:
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Replies to this message:
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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


(2)
Message 27 of 107 (535306)
11-14-2009 4:36 PM
Reply to: Message 19 by slevesque
11-13-2009 3:21 PM


Re: Yes, that's science
Because string theory has been viewed as unfalsifiable for this very reason: it can adapt to anything.
What is reality? - There are several of us who believe that our reality is just one infinitesimal part of a Grand Ensemble - an ensemble that makes the multiverse just our local backyard. This by necessity will involve a theory with an ability to produce an infinitude of possible realities. The fact that M-theory seems to be doing this is exactly what we expected. *If* this is how reality is built, then I guess you're suggesting that we should abandon research in the correct direction, and go play with some wrong theories that you will find more philosophically satisfying. No thanks. I don't give a shit about the philosophy. As a scientist, I'm trying to understand the nature of reality. If M-theory has something to do with it, great. If it doesn't, great.

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


(2)
Message 28 of 107 (535311)
11-14-2009 5:17 PM
Reply to: Message 10 by slevesque
11-13-2009 1:29 AM


Didn't string theory predict that the cosmological constant would be negative or null
Let me save you all the trouble and explain - you are essentially correct, but it is not the way we would look at it. We couldn't get a positive Cosmological Constant out of String Theory. You could say this is a prediction but we saw it as a major drawback - we couldn't generate cosmologically interesting solutions related to de-Sitter space, and then when we discovered that we were living in a de-Sitter-like space, there was concern. However, this provided the necessary motivation and inspiration to look for an answer. An answer was found, that opened the doors to the (infamous) String Landscape. This was in many ways what I had been waiting for and I was very excited. For all my time in Quantum Gravity I never expected to find a single obvious reduction of any grand theory into the Standard Model, a compactification that would give exactly what we see. The Standard Model is a mess, and whatever reduction gives rise to it will also be a mess, and essentially unpredictable.

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


(1)
Message 32 of 107 (535355)
11-15-2009 4:32 AM
Reply to: Message 30 by Iblis
11-14-2009 7:38 PM


Re: Yes, that's science
Maybe that sounds to me like exactly the sort of thing that would make one an expert on black holes.
That would be because...
...I don't have a good idea of what "condensed matter physics" is.
Simple really
Even particle physicists have little to no clue about black holes, unless they cross disciplines. String Theory is the intersting area where the two areas (particle physics and General Relativity) collide, and many practitioners on both sides had to play major catch up. I even had to explain some of the fundementals to Lenny Susskind, one of the grandfathers of String Theory.
Anyway, he's fun to read regardless
Yeah, always fun to read those acting well outside their own area of expertise
Laughlin notes that the argument may offend his peers, but that they have no valid criticism of his and his partner’s arguments. He insists their redefinition is correct.
Yeah, in the same way that we have no valid criticisms of creation science. What an arrogant twat.
Regarding Richard Feynman:
Did you know him personally?
No He died just before I arrived at Cambridge, and missed his last lecture there by months. But I know several of his peers and friends.
Edited by cavediver, : No reason given.
Edited by cavediver, : No reason given.

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


(2)
Message 33 of 107 (535359)
11-15-2009 5:19 AM
Reply to: Message 29 by Straggler
11-14-2009 7:37 PM


Re: Questions
M theory provides a sort of working ability to pertubitively (i.e. calculate in practical terms) various aspects of the standard model whilst also incorporating gravity.
Yes, it is the only scenario we have that incorporates gravity with other gauge and matter fields. Getting the precise Standard Model basket of fields was always problematic - finding the right compactification of the extra dimensions that would give us the world around us. And then with the Landscape, realising that we could just about get any mix of fields we liked. It's a bit like wanting to know why we have wood, stone, fire, water, and air, and being given atomic theory as an explanation:
quote:
huh, this is useless. This could predict all sorts of things. It doesn't tell us why we just have wood, stone, fire, water, and air
What is supersymmetry?
Or SuSy as we call her
Susy is the attempt to reconcile the two fundemental particle/field types: bosonic and fermionic. In the same way that a neutron and a proton are essentially the same thing (or an up and down quark - same thing really), just differentiated by electric charge. If we could switch off the electric charge, they would be identical. So each boson has a fermionic partner, which it would be identical to if the supersymmetric charge could be switched off and supersymmetry restored.
It would be a no-brainer if the observed bosons (photon, gluon, W+/-, Z0, graviton) were the supersymmetric partners of the observed fermions (electron, neutrino, quark) but the theory does not work that way. Each observed particle has a hitherto unobserved superpartner: the electron has the selectron boson, quark the squark boson, neutrino the sneutrino bososn, photon the photino fermion, gluon the gluino fermion, and the Wino, Zino, and gravitino fermions. All of these are much higher mass than the standard model particles by the necessity that we haven't seen them yet!
Why would we consider Susy? Because it simplifies everything massively. Much makes sense under Susy, that is so far inexplicable otherwise. If you assume Susy, then when you extrapolate up in energy, all of the Standard Model gauage forces converge on a single unification point. It has such an abundance of riches that if it's not part of reality then it damn well should have been and there better be a damn good reason why it's not!!!
Susy is what transfomed interesting but unphysical string theory into the beast that is Superstring Theory and now M-Theory. But long before that, Susy gave us the previous last best hope for Quantum Gravity: SuperGravity (SuGra).
What Susy does for QM/SR (what we call quantum field theory), it could also do for GR. And this gave us Sugra. What we didn't expect was that by making gravity supersymmetric, we would be forced to consider extra dimensions and extra fields, and suddenly we find ourselves in this higher dimensional world full of exotic fields that look as if they could compact down to the Standard Model and gravity, AND work as a renormalisable quantum theory!! This was IT!! This was the long sort after Qunatum Gravity and Theory of Everything, rolled into one. Hawking rather infamously and foolishly stated that the end of physics was in sight! This was 1980. But it didn't work... it was so close, but it just wouldn't work as a sensible quantum gravity, despite the low order calculations looking as if there was no way it could fail. Something was missing, but what was a complete mystery. And so Sugra faded into the background.
And then, low and behold, Superstring Theory turns up, and goes "hey, guess what? My low-energy sector IS Sugra!!!" Suddenly everyone who had been working on Sugra, and ignoring String Theory, goes mad. Here was possibly the missing framework whose absence caused Sugra to epically fail. And not only that, it is an explanative framework that tells us why we have Sugra in the first place, where-as Sugra just shrugs and says - this is how it is.
M-Theory is essentially the grand unification of the various Sugra theories and String Theory. For this alone, it is a grand work of mathematical physics, and its insights have revolutionised understanding in many areas of theoretical physics and mathematics. If it has NOTHING to do with reality (and again, if it doesn't, then there's some mighty good explaining to be done) then it will still have been very worthwhile.

This message is a reply to:
 Message 29 by Straggler, posted 11-14-2009 7:37 PM Straggler has replied

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


(3)
Message 36 of 107 (535362)
11-15-2009 5:56 AM
Reply to: Message 29 by Straggler
11-14-2009 7:37 PM


Re: Questions
Wow, I just thought that perhaps I'd better split my reply into two and submit what I had written just in case I lost it for some reason. Two seconds after hitting submit on that last post, I had a BOD!! Even had I copied the text to notepad, I would probably have lost it before managing to save... Or was hitting submit the cause of the BOD?
What are solitonic solutions and why are they important?
Good questions today
This is (as all of it is) a massive subject in of itself. Let's do some basics. The simplest example of a soliton is the following: take a long strip of paper, say 2cm wide and 2m long. Stretch it out and clamp it at each end. That's a zero energy state. Now, unclamp it, and introduce a single twist, and reclamp it. The strip now has a kink that you can play with. You can push the kink back and forth from one end to the other. It is a stable configuration of the paper, stablised by topology - you cannot untwist it! It is a soliton. Notice how like a particle it is! Want more? Let's go back to the untwisted strip. How can we add a soliton without undoing the clamp? Easy! Just grip a small length and twist it upside down. You have two twists - one at each end of the length you have gripped. Slide the two twists apart. You have two solitons! You can play with each one separately and slide them around. But beware, because actually one is a soliton and the other is an anti-soliton!! And guess what will happen if you allow them to collide...
A soliton is a particular configuration of something that can be configured, where the soliton is stabilised by something non-local: topology in this case. A famous example would be the magnetic monopole.
In a field theory, you can have fundemental excitations of the field (particles) and you can have extended solitons. There is a fundemental symmetry between the two, which is known as S-duality. This is very important in much of theoretical physics, including String Theory.

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


(3)
Message 37 of 107 (535363)
11-15-2009 6:06 AM
Reply to: Message 35 by Son Goku
11-15-2009 5:47 AM


Re: Yes, that's science
I'm sure you can appreciated that there would need to be serious thinking as to how one could go about testing it.
Of course, and there is. But there seems to be this bizarre notion going around that if a theory cannot (yet) be tested, it is wrong

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Replies to this message:
 Message 48 by Son Goku, posted 11-24-2009 8:23 AM cavediver has replied

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


(3)
Message 38 of 107 (535365)
11-15-2009 7:24 AM
Reply to: Message 5 by onifre
11-12-2009 8:47 PM


Also, I notice you quoted my "string predicts gravity" statement. While yes, like cavediver said, I couldn't explain that in any detail,
Sorry, I wasn't trying to be clever at your expense here - it was a dig at Iblis. What I meant was that it is highly non-trivial that String Theory predicts gravity, and asking a (interested and informed) layman to explain further is rather naive.
It goes back to what I said about Susy/Sugra and how String Theory was discovered to contain Supergravity in its low energy limit. To put this in context: you start with a simple two-dimensional field theory (that has little connection to anything), that remarkably you discover describes a pseudo-world of ten dimensions, and the simple two-dimensional equations (that bear no resemblance to reality) result in effective equations in the ten dimensions that are the equations of General Relativity and Supergravity!! It's like hunting for a ten dollar bill, that you thought you'd placed in a coat in a wardrobe, and in the process you find that the wardrobe leads you into the inner-vault of Fort Knox
Edited by cavediver, : No reason given.

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


Message 40 of 107 (535374)
11-15-2009 9:25 AM
Reply to: Message 39 by Iblis
11-15-2009 9:05 AM


Re: prediction again
Supergravity still came before superstring. Why are we certain that the string gurus weren't already aware of this math and specifically concocting their remarkably flexible theory to include it?
1) because I know personally just about everyone involved and I'm telling you how it happened
2) that would be like *claiming* that you didn't know that the wardrobe led to Fort Knox, when you knew all along 'cos you'd built it that way. IT'S STILL A FRIGGIN' DOORWAY TO FORT KNOX!!!
3) String Theory is not flexible in the slightest. It's entire early attraction was just how fixed it was, and how it contained no free parameters. There is no choice involved. String Theory has no choice but to predict Supergravity. All the bullshit spouted about flexibility lies below this level. The Landscape is as much a feature of the Supergravity.
we have heard more than one troll claim or imply that you "didn't know Dick."
Yep, they can claim away. Funny as hell from my perspective, but it does immediately reveal their own level of knowledge, if you were in any doubt before hand. I think Lucy the Ape was the last to point out I knew nothing about relativity

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


Message 45 of 107 (535569)
11-16-2009 7:12 PM
Reply to: Message 44 by slevesque
11-16-2009 4:42 PM


Re: Yes, that's science
Are you trying to justify string theory being unfalsifiable
Huh? I didn't even agree that string theory is unfalsifiable.
Because I mean, you were trying to defend that it can adapt to anything
Anything? That covers quite a bit.
it can produce an infinitue of possible realities
Does an infinitude of different numbers imply any number?
Why do you think it is necessary to understand the nature of reality ?
I don't. I try to understand it because it is what my brain seems to enjoy.
but will answer it on philosophical grounds
Apparently not
Same thing if I ask you why mathematics is only based on logical deductions of a set of axioms and not experimentation
Because that's its definition?
why I cannot posit a supernatural explanation for a natural phenomenon
Because people don't get better as a result, or don't manage to catch as many wooly mammoths, or go cold in the winter and die. Natural selection works wonders on stupid beliefs
All very important questions, all philosophical.
Apparently not
Edited by cavediver, : No reason given.
Edited by cavediver, : No reason given.

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


(1)
Message 47 of 107 (535813)
11-18-2009 5:00 AM
Reply to: Message 46 by Iblis
11-17-2009 8:01 PM


Re: Yes, that's science
Peter Woit has published 8 physics papers over the last 20 years in peer reviewed physics journals
Is that supposed to impress me
That is pathetic. As I said, he is a blogger who has found a niche that has gained him some recognition. And?
Smolin has an axe to grind. LQG is interesting and emerges from some very respectable work of Ashtekar, but has as many problems and issues with experimental verification as M-Theory, and is so far not as successful in giving low energy GR and QG results. But it is still excellent research material. Smolin feels that it is being ignored in favour of M-Theory. Throwing a fit and claiming that M-Theory is a complete waste and full of errors just makes us think he is a twat irrespective of the merits of LQG.

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


(1)
Message 52 of 107 (538372)
12-06-2009 6:11 AM
Reply to: Message 51 by Straggler
12-02-2009 6:28 PM


Answers, part 1
In terms of the conceptual model of vibrating strings what makes some particles (i.e fermions) obey the exclusion principle but others (i.e bosons) not?
Deep question! And perhaps biting off too much - you need to understand what makes particles fermions and bosons in the first place, before we start looking at string theory explanations.
Fermions are particles whose wavefunction changes sign on the swap of any two of the particles; e.g.
,
which immediately gives us the Pauli Exclusion Principle - if two identical fermions are located at the same point, then
and this can only be satisifed if . The lowest atomic orbital is allowed two electrons (1s2) because the electron can be distinguished by its spin - spin up or spin down. But you can only have one spin up electron in the shell, because the addition of a second will again give a zero wavefunction.
Boson wavefunctions are symmetric under exchange, so there is no such restriction -
.
This gives the possibility of
,
which gives rise to, amongst other things, Bose-Einstein condensates and lasers.
These are the only two sensible possibilities. I guess you could try to make a reality where the wavefunction halved on exchange,
,
but follow this through and you'll see it quickly falls apart.
Edited by cavediver, : No reason given.

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


Message 94 of 107 (538979)
12-12-2009 3:46 AM
Reply to: Message 48 by Son Goku
11-24-2009 8:23 AM


Re: Yes, that's science
Sorry SG, I was just reviewing this thread and realised I'd missed these posts of yours from last month:
However I find these "battlegrounds" concerning theories which have never been tested (String, Loop Quantum Gravity(LQG), e.t.c.) to be a bit silly. Let's say somebody has strong opinions against LQG, what does it matter? If the theory is correct, they will eventually been shown to be wrong. There were people who thought GR and QFT were a load of arse and to be fair we needed those people, they're part of the scientific method.
I understand that these theories sometimes meet with unfair criticism, but it shouldn't really be a divisive issue like it has become in some people's minds.
I couldn't agree more. Lubos is just as blinkered on the string/M side as Woit is on the anti-string side. There should be a blanket ban on all theo phys blogs - except for JB of course We all like JB, the inventor of the blog itself!
Also sometimes the criticism can be helpful, I know a few string theorists who were actually surprised to hear that there was no proof that String Theories caculations were finite beyond second order...
Yep, that's the danger of jumping into the middle of the field, as many have had to in the past ten years if they had any hope of getting to the 'coalface' research during their PhDs. The arguments for finiteness were always based on inference and hopefulness - just like in Sugra, and look where that got us. That said, as low energy sectors to M-Theory, the string theories should not necessarily be expected to be finite...

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


Message 95 of 107 (538981)
12-12-2009 3:58 AM
Reply to: Message 57 by Briterican
12-09-2009 2:51 PM


Re: CD's apotheosis is not yet complete...
Using quantum mechanics as an example, I think it is safe to say that there are some outlandish assumptions (or at least outlandish implications of some assumptions) inherent to quantum mechanics, and yet it (QM) is used as a tool to make extremely accurate predictions. This does not prove that it is an entirely complete and accurate model, but it seems fair to say that this demonstrates that it has to be right on some level.
Yes, I can gree with this - if by "outlandish" you simply mean "utterly contrary to common sense".
Assuming the above statement is an accurate assessment, can the same thing be said of string theory in any sense?
If not, what is it that makes string theory so worth pursuing?
Hmm, string/M-theory makes sense of several myserious aspects of theoretical physics, and provides a great framework for how everything hangs together - and on this basis, quite a few think that therefore it must be right on *some* level. But this is essentially the theoretical being used to make sense of the theoretical. QM is explaining some very real and very common observations, so is on a very different level.
Is it the potential to marry relativity and QM together?
Absolutely, though it goes much further in attempting to marry together everything: relaivity, QM, and all of particle physics. This is why it is a much greater ambition than LQG.
Edited by cavediver, : subtitle change

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