String! Theory! What is it good for ?!?

Richard Feynman is a hero to most who have ever chosen to study physics in recent times. But as a key proponent of physics as a predictive and ultimately testable discipline Feynman himself said: I was "raised" on Feynman as an undergraduate. But I was also raised on string theory as the most likely "theory of everything" (in the sense that those teaching me were working on it even if what they were teaching me was well below that level).So the attitude of current string theorists to RPF and his very "practical predictions are all that count" mentality are very interesting to me.

That would be because... 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. Yeah, always fun to read those acting well outside their own area of expertise Yeah, in the same way that we have no valid criticisms of creation science. What an arrogant twat.Regarding Richard Feynman: 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.

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:

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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

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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.

I'll try to answer this.
Back in the 1970s people were very interested in using symmetries to figure out what theories might describe nature. Symmetries mean that some quantity in the theory is conserved, so I will talk about them that way. Symmetries come in two forms, internal and external.
Internal symmetries mean that things like electric charge, color (American spelling intentional) for quarks are conserved. Internal properties of the particle.
External symmetries mean things like momentum and energy are conserved. They relate to things external to the particle, like space and time.I should also mention that there are two kinds of particles, bosons which can exist in the same state as each other at the same time (they can pile up in one place) and fermions which will not exist in the same state as each other (they can't pile up in one place).In the 1970s physicists wondered if you could find an enormous symmetry containing both internal and external symmetries. Two physicists, Coleman and Mandula, showed that this was impossible assuming the algebra in quantum field theory obeyed certain conditions. Later people showed it didn't need to satisfy such conditions and a bigger symmetry was possible, Supersymmetry.
Supersymmetry basically says the theory is exactly the same when you turn fermions into bosons and vice versa. Basically the probability for any collection of particles to turn into any other collection of particles is exactly the same if I switched the type of particle around. Example:
Probability for two bosons and one fermion to turn into three fermions
equals
Probability for two fermions and one boson to turn into three bosons.Finally the Haag—Lopuszanski—Sohnius theorem was proved which shows that Supersymmetry is the biggest symmetry you can have if you want the theory to be a quantum field theory. Any bigger and it would have to be something else.

Although I agree that if reality is like this, then the correct theory would have to model it, I'm sure you can appreciated that there would need to be serious thinking as to how one could go about testing it.

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? 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.

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

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.

Yep, you beat me to it. I'm going to go ahead and scribe a chunk of my imagined posts anyway, for continuity's sake So what we are really saying then is that superstring predicts supergravity? Oh, never mind then, that's fine. Cancel this thread No seriously, yes that's a much more compelling story than thinking it's similar to saying "L. Ron Hubbard predicted that in 1066 William the Conqueror would permanently pidginize the English language." But can we walk through the fundamental differences between this and the parody FSM prediction I posted?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?This may sound needlessly paranoid, and, well, it is. But I once read an awesomely wrong variable-c and recent-expansion non-theory from some creative-scientist that asserted that one of the people involved in Inflation had a dad that was a radio engineer, and therefore he could have "heard" the CMB and kept it a secret and conspired with his godless sodomite buddies around the world to bury an implication of it in their big lie which could then be conveniently "discovered" later.There's something suspicious to the non-thinker about "predicting" things that are already known. Let's beat them to death. I'm sorry to hear that. I only ask because we have heard more than one troll claim or imply that you "didn't know Dick." And I was hoping to put the lie to that No way! Anything that helps us to understand pseudo-science, the quasi-theology of falsifiability, and its relation to quantum physics and relativity belongs right here where we can bang at it! You just like making people start threads for you, don't you? It's your way of pwning them Anyway, back to the point ... It's really more like hunting for a ten-dollar bill, not finding it, going ahead and taking that tedious low-paying job you had hoped to avoid in that exotic restaraunt you can't stand, and eventually getting tipped ten dollars by someone who also helps you get your dream-job that you really wanted at the LHC.Edited by Iblis, : spling

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. 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

No worries, cavediver. I knew it was toward Iblis.I'm glad you have taken the time to explain it further. I knew when I said it I didn't know the depth to which it dealt with gravity, I'm actually glad Iblis brought it to our attention and got a conversation started on it. That is an awesome analogy! - Oni

This is actually the post I have gotten the biggest kick out of so far in this thread Yeah! That's the spirit! Screw "wanting" the answer to be something and "trying to prove" it, let's find out what the real answer really is and DAMN THE TORPEDOES !!!

This may be the wrong thread for this, but I was wondering if the expansion of space since the Big Bang could really be described a space "unfolding" from another dimension, and at some point, space will have unfolded all the way and stop expanding...or am I taking analogies too literally and applying them to concepts for which they were never meant to be in the same room with?

I don't think I understand your post, but here it goes:Are you trying to justify string theory being unfalsifiable ?Because I mean, if that is the case, then the issue is settled. I haven't taken a stance on string-theory as of right now, but from what I see there are some problems with the debate in itself. Because proponents of it keep saying how awesome it is etc. and keep making doubters look stupid, and objectors even more stupid. (primarily saying ''you oppose it because you don't understand it'' implying therefore every single objector doesn't understand it,)I never proposed abandoning research on a philosophical basis (as you seem to by applying this stance on me, one that I never took), but what I don't understand is, if truly it is unfalsifiable, then why do M-Theory proponents act this way ? Doesn't it's unfalsifiability provide a legitimate reason to doubt it ? Here you seem to imply that it is falsifiable, making all my previous comment unecessary.Because I mean, you were trying to defend that it can adapt to anything, stating that this was the case because it can produce an infinitue of possible realities, which was expected because of the previous belief in some sort of a multiverse. Then this last sentence does becomes a contradiction, since if it can adapt to anything, then you will never be able to know if it is part of the nature of reality.And of course, you may consciously don't give a shit about reality, but I can say that incounsciously you value it. Because if I ask you: Why do you think it is necessary to understand the nature of reality ? You will recognize it as an important question, but will answer it on philosophical grounds (because it is a philosophical question)Same thing if I ask you why mathematics is only based on logical deductions of a set of axioms and not experimentation, or why I cannot posit a supernatural explanation for a natural phenomenon, etc. etc.All very important questions, all philosophical. All will be answered with a philosophical answer. The modern scientist has lost this notion mainly because of the seperation of domains in our culture.

Huh? I didn't even agree that string theory is unfalsifiable. Anything? That covers quite a bit. Does an infinitude of different numbers imply any number? I don't. I try to understand it because it is what my brain seems to enjoy. Apparently not Because that's its definition? 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 Apparently not Edited by cavediver, : No reason given.Edited by cavediver, : No reason given.