ISF Logo   IS Forum
Forum Index Register Members List Events Mark Forums Read Help

Go Back   International Skeptics Forum » General Topics » Science, Mathematics, Medicine, and Technology
 


Welcome to the International Skeptics Forum, where we discuss skepticism, critical thinking, the paranormal and science in a friendly but lively way. You are currently viewing the forum as a guest, which means you are missing out on discussing matters that are of interest to you. Please consider registering so you can gain full use of the forum features and interact with other Members. Registration is simple, fast and free! Click here to register today.
Reply
Old 20th July 2010, 04:38 AM   #41
ponderingturtle
Orthogonal Vector
 
ponderingturtle's Avatar
 
Join Date: Jul 2006
Posts: 39,155
Originally Posted by arthwollipot View Post
If they are not found, then supersymmetry is not confirmed to be true. You can't prove something false by not finding evidence of it. But yeah, you're basically right, as I understand it.
That depends. If they hypothesis dictates that you will find them between energies X and Y, and you don't then the hypothesis is wrong. Absence of evidence is evidence of absence when the presence was specifically predicted.

For example look at the million dollar challenge. Does it disprove that psychics exist? No, but it can refute specific claims made by specific psychics. You can show that they can not divine the presence of something with any significant accuracy.

So if it really does reach the upper bounds of when we should start seeing particles like the selectron and we do not, then this is a positive refutation of the theory.
__________________
Sufficiently advanced Woo is indistinguishable from Parody
"There shall be no *poofing* in science" Paul C. Anagnostopoulos
Force ***** on reasons back" Ben Franklin
ponderingturtle is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 20th July 2010, 04:49 AM   #42
arthwollipot
Observer of Phenomena
 
arthwollipot's Avatar
 
Join Date: Feb 2005
Location: Location, Location
Posts: 55,878
Yes, fair enough. I agree.
__________________
Read my fantasy novel for free!
arthwollipot is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 20th July 2010, 05:36 AM   #43
Dancing David
Penultimate Amazing
 
Dancing David's Avatar
 
Join Date: Mar 2003
Posts: 38,493
Yet is those energies are speculative or ranged then it is very hard to say that an energy has been reached that would rule out a model.

When Gell-Mann did his work many of the energy levels of particles were already known, therefore his theoretical work on other particles and resonances was based upon at least some knowledge.

Much of the super symmetry models are not known as to the energies and masses, they are possible good guesses and therefore may not be reached at the LHC because they are guesses to some extent. They are decent ball park figures but some of those parks are quite large.

As Sol I, Cuddles and Tim T have explained , it is not cut and dried.

Ponderingturtle I may well have missed your source for saying that "the LHC was supposed to be a hard test of super symmetry", what is it?

Or what is the reasoning of the person who told you this.
__________________
I suspect you are a sandwich, metaphorically speaking. -Donn
And a shot rang out. Now Space is doing time... -Ben Burch
You built the toilet - don't complain when people crap in it. _Kid Eager
Never underestimate the power of the Random Number God. More of evolutionary history is His doing than people think. - Dinwar
Dancing David is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 20th July 2010, 06:01 AM   #44
Hellbound
Merchant of Doom
 
Join Date: Sep 2002
Location: Somewhere between the central U.S. and Hades
Posts: 11,202
Originally Posted by Tim Thompson View Post
One prediction is the presence of spatially compact extra dimensions (covered in some detail in Elegant Universe). Those dimensions can reveal themselves in high energy accelerator experiments if they are spatially very small, or in short range experiments on the inverse square law of gravity.
Just to clarify, these results (specifically wrt gravity) may show up if the "curled" dimensions are larger than currently expected (near the upper end of the range allowed in some of these theories). If the LHC was able to create a mini-black hole, that would set a size limit on the curled up dimensions and add support for some variations of string theory. (the reason being, briefly, that gravity "spreads" in all the dimensions, explaining it's weakness compared to the other three forces, which work primarily in the 4 dimensions we see. The "smaller" we get our high energy experiments, the more the gravity in the curled up dimensions would affect the results, as we get particles close enough together to include it's effects not just in our 4 dimenions, but also in however many curled up ones there are, too).

Greene's subsequent book, The Fabric of the Cosmos, goes into some detail on this aspect.

Just as another addendum, the early string theories from the 70s were dropped pretty quickly, and not revived until later when supersymmetry was added in (created superstring theories, which are the current area of interest...although mostly they're still just called string theories). Point being, that 40 years of research mentioned wasn't even 40 solid years, as it wasn't until the last couple decades that superstring theories really started getting a lot of looking into (IIRC...as always, I reserve the right to be wrong).
Hellbound is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 20th July 2010, 06:27 AM   #45
ponderingturtle
Orthogonal Vector
 
ponderingturtle's Avatar
 
Join Date: Jul 2006
Posts: 39,155
Originally Posted by Dancing David View Post
As Sol I, Cuddles and Tim T have explained , it is not cut and dried.

Ponderingturtle I may well have missed your source for saying that "the LHC was supposed to be a hard test of super symmetry", what is it?
A friend of mine, who has a phd in theoretical high energy physics. But there can be all kinds of ways to disprove a theory with a negative observation. Not seeing time dilation would kill relativity for example.
__________________
Sufficiently advanced Woo is indistinguishable from Parody
"There shall be no *poofing* in science" Paul C. Anagnostopoulos
Force ***** on reasons back" Ben Franklin
ponderingturtle is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 20th July 2010, 09:00 AM   #46
sol invictus
Philosopher
 
sol invictus's Avatar
 
Join Date: Oct 2007
Posts: 8,613
Originally Posted by ponderingturtle View Post
A friend of mine, who has a phd in theoretical high energy physics. But there can be all kinds of ways to disprove a theory with a negative observation. Not seeing time dilation would kill relativity for example.
Supersymmetry (susy) was first discovered in the context of string theory, but more recently has become a topic of major interest in particle physics phenomenology. The reason is that susy theories are immune from certain divergences that otherwise plague theories of particle physics. Specifically, quantum fluctuations in non-susy theories mean that certain quantities (the most famous are the cosmological constant and the mass of the Higgs particle) are expected to be very large, when in fact we know that they are not large.

So if you take a non-susy theory and supersymmetrize it - which essentially means you add a bunch of new particles with a particular mass spectrum and a particular set of couplings to each other - you can solve some of those problems. Specifically, you cannot solve the cosmological constant problem (because if susy were the solution some of those new particles would have to be very light, and we would have observed them already), but it might be the solution to the problem of the Higgs mass (which is called the hierarchy problem).

If supersymmetry is the solution to the hierarchy problem, the LHC will find it - or at least will find a bunch of new particles that could fit into a SUSY spectrum. That's probably what your friend meant.

All known consistent string theories are susy, but the energy required to observe the susy part of the spectrum isn't necessarily low enough that the LHC will see it. An observation of any kind of susy at LHC would be a major boost for string theory (but wouldn't prove it is correct), and by the same token the lack of such wouldn't falsify it (but would create a significant mystery for theoretical high energy physics generally, which is what solves the hierarchy problem?).
sol invictus is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 20th July 2010, 09:10 AM   #47
ponderingturtle
Orthogonal Vector
 
ponderingturtle's Avatar
 
Join Date: Jul 2006
Posts: 39,155
Originally Posted by sol invictus View Post
If supersymmetry is the solution to the hierarchy problem, the LHC will find it - or at least will find a bunch of new particles that could fit into a SUSY spectrum. That's probably what your friend meant.
Yea I sent an email to him and that is what he meant.

This is what he said to me

Quote:
SUSY by itself makes no predictions about the masses of the particles. In principle, the SUSY partners can be as heavy as they want to be. **IF** SUSY does what we want it to do however (solve the hierarchy and fine tuning puzzles), then their masses should be near a TeV. So I guess the answer to your question is "Yes. Sort of..."

But it's still not quite so simple as that. Remember that the LHC is in actuality a GLUON collider. So all of the hard collisions that will make the SUSY particles involve gluons. The gluons carry a lot of the energy/momentum of the proton, but not all of it. Their momentum distributions peak (in fact, blow up) as x --> 0, where x is the momentum fraction of the gluon in the proton. So for example, if a proton has an energy of 7 TeV (the goal), virtually all the gluons inside the proton will have energies less than 700 GeV (x=0.1), and in fact, the vast majority will have energies less than 70 GeV (x=0.01). So if you are out to make a SUSY particle, you will need close to 2 TeV of energy (assuming R-parity conservation which says you must pair produce), and even though it at first looks like that's not a problem ("14 TeV CM energy") it actually is very high and will be very hard to do.

From these arguments, and taking other issues into account such as detector efficiency, luminosity of the beam, etc, we are only going to be able to produce (and measure) particles will a mass less than about 3 TeV or so, over the 20 year lifetime of the machine. So that's right on the very edge of where SUSY would have to live if it solves the hierarchy problem.

Of course, if we don't see SUSY, that doesn't mean SUSY doesn't exist, but it does mean that it probably doesn't have anything to do with the hierarchy problem. Well, maybe that's too strong a statement, but my point is that another solution would have to be sought.

Notice, by the way, that the Higgs boson is expected to be around 120 GeV or so, and the LHC will produce those by the truckload!
__________________
Sufficiently advanced Woo is indistinguishable from Parody
"There shall be no *poofing* in science" Paul C. Anagnostopoulos
Force ***** on reasons back" Ben Franklin
ponderingturtle is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 20th July 2010, 09:44 AM   #48
Dancing David
Penultimate Amazing
 
Dancing David's Avatar
 
Join Date: Mar 2003
Posts: 38,493
Thanks PonderingTurtle!
__________________
I suspect you are a sandwich, metaphorically speaking. -Donn
And a shot rang out. Now Space is doing time... -Ben Burch
You built the toilet - don't complain when people crap in it. _Kid Eager
Never underestimate the power of the Random Number God. More of evolutionary history is His doing than people think. - Dinwar
Dancing David is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 20th July 2010, 09:58 PM   #49
Tim Thompson
Muse
 
Tim Thompson's Avatar
 
Join Date: Dec 2008
Posts: 969
Lightbulb String Theory: An Historical Narrative

I offer my understanding of a historical narrative for the development of string theory. I am not a specialist in string theory, nor in quantum field theory, nor in general relativity. But I have cobbled this together from my various sources and will try not to make too many mistakes (feel free to correct any errors you encounter).

Billiard balls scatter off of one another by simply bouncing, what physicists would normally call an elastic collision. Seen from a distance, one can pretend that quantum mechanical particles scatter off of each other in similar fashion. However, the resulting distribution of scattering angles suggests this is not the case, so to understand scattering quantum mechanically requires a closer look at the details of how one particle interacts with another. That leads to the idea of a resonance state. When two particles collide and bounce (i.e., scatter) off of each other, they actually merge together in a very short lived resonance state, and then go their merry way.

With that context in mind, the scattering of pions was a particular problem some years ago (and may still be for all I know) where theory & experiment simply did not agree. In 1968, Gabriele Veneziano tried to solve the problem with a radical notion - he replaced the single resonance state with a dual resonance state (Veneziano, 1968). I call this the moment of conception for string theory.

A scant two years later, in 1970, Yoichiro Nambu, Leonard Susskind (Susskind, 1970) & Holger Bech Neilsen, all acting independently, hit on the same idea. They thought of the dual resonance of Veneziano as the ends of a string, suggesting that fundamental particles might be properly described as tiny strings. This was the first time anyone had suggested deviating from the usual idea of representing fundamental particles as point like.

This marks the birth of what is now called bosonic string theory; it was able to describe the physics of bosons, at least qualitatively, but could not deal with fermions at all, and so was considered to be a not very useful idea. At this point string theory, such as it was, remained virtually unknown outside a small community of high energy particle physicists.

But it did not take long to overcome the boson problem. In 1971 Pierre Ramond (Ramond, 1971), and the team of John Schwarz (who is much more pleasant in person than his webpage picture implies) & Andrť Neveu (Neveu & Schwarz, 1971), independently discovered the idea of supersymmetry. This new particle symmetry allowed string theory to expand into the realm of fermions, and marks the birth of what is now properly called superstring theory (today the term string theory as we commonly use it actually refers to superstring theory). But string theory required the presence of a particle known to be impossible as a hadron by particle physicists (a massless spin-2 particle)

So far, string theory has remained a theory for quantum mechanical particles only, and still of no interest outside the community of particle physicists. That all changed in 1974, when the team of John Schwarz & Joel Scherk (Scherk & Schwarz, 1974), and Tamiaki Yoneya (Yoneya, 1974) acting independently, decided on a radical interpretation of the impossible particle. It turns out that massless and spin-2 describe the graviton particle required by any quantum mechanical theory of gravity, but which those searching for quantum mechanical theories of gravity had been unable to properly derive. Schwarz, Scherk and Yoneya were able to interpret the massless spin-2 particle as a graviton and use that interpretation to prove that the low energy limit of superstring theory is general relativity. This marks the transition of string theory from a narrow theory of particle physics to a fundamental theory of everything, a quantum mechanical version of Einstein's elusive unified field theory. To me, this is a point worth emphasizing: The low energy limit of superstring theory is general relativity. This means that if Einstein had never been, but string theory had been invented by someone, then general relativity could have been discovered by way of string theory.

In 1976 a parallel development to string theory, one of some significance, took place. The team of Daniel Freedman, Peter van Nieuwenhuizen and Sergio Ferrera (Freedman, van Nieuwenhuizen & Ferrera, 1976) and the team of Stanley Deser & Bruno Zumino (Deser & Zumino, 1976) combined general relativity with supersymmetry to invent supergravity. The idea is to work towards a quantum theory of gravity with general relativity as the starting point by adding supersymmetry to the mix. So supergravity is a direct competitor with string theory as the quantum theory of gravity.

At this point both string theory & supergravity are known to reduce to general relativity, an obvious requirement for any quantum theory of gravity. But it was not yet known how to prove that string theory was in fact entirely self consistent. In 1984 the team of Michael Green and John Schwarz successfully proved that string theory was entirely self consistent, but only if it were allowed 9 spatial dimensions, which makes it a 10-dimensional theory when 1 dimension of time is included (Green & Schwarz, 1984a; Green & Schwarz, 1984b). John Schwarz calls this the First Superstring Revolution. Allowing for the fact that he might be somewhat biased by his own role in this revolution, it is nevertheless true that string theory rather suddenly gained in popularity overnight.

Now string theory takes a few years on break. One string theory became 5 string theories, and people began to despair that string theory would continue to advance.

But after 11 years, in 1995, two big things happened. First, Joseph Polchinski showed that structures lurking around in the higher spatial dimensions of 10-dimensional string theory, called d-branes (short for Dirichlet membranes, and discovered by Polchinski and Horava a few years prior), could be identified with similar structures in 10-dimensional supergravity (Polchinski, 1995). Second, Edward Witten discovered the duality relationships which implied that all 5 string theories, and supergravity were in fact different aspects of one single underlying theory, which we now call M-theory, and which required the addition of one more spatial dimension, making string theory an 11-dimensional theory (Witten, 1995). Schwarz calls this the Second Superstring Revolution.

As far as I can tell, that basically brings us up to today. M-theory is the primary focus of string theory research. Theorists do not know the true equations of M-theory (or any other string theory), only the approximate forms of the equations. This is the major stumbling block. How does one predict, for instance, the true mass of a supersymmetric partner particle, if one does not know the true equations for the theory, as opposed to the approximate equations?

To the extent that my narrative does not suffer from major errors on my part, I think it reveals an important point overlooked by those who think we have done this long enough, without finding the true equations, and we should quit now. String theory advances in fits & starts, but advance it does. The historical narrative shows a steady advance, from the simple particle models of Veneziano and others, to the surprising and unexpected emergence of general relativity from string theory, to the surprising connection between supergravity and superstring theory. These are not inconsequential steps of progress in understanding string theory. I don't understand how it can make any sense to see this narrative, and respond by throwing hands up in despair and abandon string theory for greener pastures. Couple this history of consistent advance in knowledge with the fact that it took 228 years for Einstein to solve Newton's problem of general relativity, and I think string theorists are really on a roll here.

Obviously, nobody is in a position to declare string theory to be the true unifying theory of physics. But I think neither is anyone in a position to declare that it definitely is not.
__________________
The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell
Tim Thompson is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 20th July 2010, 11:05 PM   #50
marplots
Penultimate Amazing
 
Join Date: Feb 2006
Posts: 29,167
Excellent summary. Top drawer.
marplots is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 21st July 2010, 02:32 AM   #51
SusanB-M1
Incurable Optimist
 
SusanB-M1's Avatar
 
Join Date: Jul 2006
Posts: 3,417
Originally Posted by marplots View Post
Excellent summary. Top drawer.
Yes, agreed - a most interesting post. I can only look on as a spectator who greatly admires the skill and understanding of physicists.
__________________
I know that you believe that you understood what you think I said, but I am not sure you realize that what you heard is not what I meant.
SusanB-M1 is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 21st July 2010, 03:01 AM   #52
schrodingasdawg
Critical Thinker
 
Join Date: Jan 2010
Posts: 327
I also second this. The information in this thread both Tim Thompson, sol invuctus, and ponderingturtle is all very interesting, actually. Thanks all.
schrodingasdawg is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 21st July 2010, 05:55 AM   #53
Hellbound
Merchant of Doom
 
Join Date: Sep 2002
Location: Somewhere between the central U.S. and Hades
Posts: 11,202
Great job, Tim. I was working from my (obviously incorrect) memory. Thanks for filling in the gaps and correcting my mistakes
Hellbound is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 21st July 2010, 06:26 AM   #54
sol invictus
Philosopher
 
sol invictus's Avatar
 
Join Date: Oct 2007
Posts: 8,613
Excellent post, Tim. One comment:

Originally Posted by Tim Thompson View Post
As far as I can tell, that basically brings us up to today. M-theory is the primary focus of string theory research. Theorists do not know the true equations of M-theory (or any other string theory), only the approximate forms of the equations. This is the major stumbling block. How does one predict, for instance, the true mass of a supersymmetric partner particle, if one does not know the true equations for the theory, as opposed to the approximate equations?
I'd say there's been a third "revolution" since then, the AdS/CFT correspondence. I think over the last decade there's been considerably more work on that than on M-theory, and it has led to a lot of excitement across many areas of theoretical physics. Because of its enormous potential as a mathematical tool for solving otherwise intractable problems, even condensed matter theorists are working on aspects of it, as are nuclear physicists, particle physicists, and people that study quantum chromodynamics at strong coupling. That last includes the theory of pion scattering that you started your post with, bringing string theory full circle.

In brief, the AdS/CFT duality is an explicit example of "holography" - it asserts that string theory on a certain spacetime background is exactly equivalent to a field theory in one less dimension.

Since string theory is a theory of gravity, this has major implications even if string theory isn't the correct "theory of everything", because it means gravity is equivalent to a non-gravitational field theory. That may explain the otherwise mysterious laws of gravitational thermodyamics that emerged from the work of Hawking, Bekenstein, and others. So one can understand aspects of gravity by its field theory dual.

In the opposite direction it has applications to condensed matter and QCD because those are described by strongly coupled field theories, and one can apply the duality to relate them to classical gravity. So one might be able to understand phenomena like superconductivity and pion scattering by studying black holes in five dimensions.

It's truly remarkable.

Last edited by sol invictus; 21st July 2010 at 06:28 AM.
sol invictus is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 21st July 2010, 09:52 AM   #55
Tim Thompson
Muse
 
Tim Thompson's Avatar
 
Join Date: Dec 2008
Posts: 969
Lightbulb String Theory III

Originally Posted by sol invictus View Post
I'd say there's been a third "revolution" since then, the AdS/CFT correspondence. ... In brief, the AdS/CFT duality is an explicit example of "holography" - it asserts that string theory on a certain spacetime background is exactly equivalent to a field theory in one less dimension.
Ah, many thanks, this is something of great interest I was unaware of. I am particularly intrigued by the requirement of a string theory on a spacetime background. It certainly seems that background dependence in string theory had been considered a weakness, at least up to now. Carlo Rovelli, in his book Quantum Gravity (Cambridge University Press, 2004), asserts that quantum field theory discards the spacetime background altogether (e.g., "Thus, there is no background 'spacetime', forming the stage on which things move. There is no 'time' along which everything flows. The world in which we happen to live can be understood without using the notion of time." - page 31). He is not the only source indicating that quantum field theory (and string theory) can & should be background independent.

Now the AdS/CFT duality requires a string theory on a spacetime background, so this would appear to be indeed a "revolutionary" new direction to take things. But what about the equivalent field in one less dimension, is it background independent? If not, that would seem to be not just a departure from the norm for string theory, but a departure from the norm for quantum field theory in general.

At least that's the way it looks at first glance from where I sit. What are your thoughts on background independence and spacetime in quantum field theory?
__________________
The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell
Tim Thompson is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 21st July 2010, 10:01 AM   #56
steenkh
Philosopher
 
steenkh's Avatar
 
Join Date: Aug 2002
Location: Denmark
Posts: 5,089
In the June 2010 news note in Scientific American, it is claimed that
"Twistor" Theory Reignites the Latest Superstring Revolution
Quote:
String theorists, for their part, had a promising idea for the creation of space that they could not get to work. In 1997 they conjectured that particles zipping around in four dimensions can behave just like strings interacting in five dimensions. The new dimension materializes like a figure in a pop-up book. Yet this conjuring trick produced only a single dimension of highly warped space. Using twistor concepts, theorists have now shown how all the dimensions of ordinary space—and even time—can pop out.
Could anybody comment on whether the Twistor theory really is so big as claimed?
__________________
Steen

--
Jack of all trades - master of none!
steenkh is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 21st July 2010, 10:15 AM   #57
Wowbagger
The Infinitely Prolonged
 
Wowbagger's Avatar
 
Join Date: Feb 2006
Location: Westchester County, NY (when not in space)
Posts: 14,911
If string theory is on the way out, then what better idea is now on the way in?
__________________
WARNING: Phrases in this post may sound meaner than they were intended to be.

SkeptiCamp NYC: http://www.skepticampnyc.org/
An open conference on science and skepticism, where you could be a presenter!

By the way, my first name is NOT Bowerick!!!!
Wowbagger is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 21st July 2010, 10:23 AM   #58
Dancing David
Penultimate Amazing
 
Dancing David's Avatar
 
Join Date: Mar 2003
Posts: 38,493
Glue or stick theory?

Stick, string, glue, the three primal inventions.
__________________
I suspect you are a sandwich, metaphorically speaking. -Donn
And a shot rang out. Now Space is doing time... -Ben Burch
You built the toilet - don't complain when people crap in it. _Kid Eager
Never underestimate the power of the Random Number God. More of evolutionary history is His doing than people think. - Dinwar
Dancing David is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 21st July 2010, 11:41 AM   #59
Wowbagger
The Infinitely Prolonged
 
Wowbagger's Avatar
 
Join Date: Feb 2006
Location: Westchester County, NY (when not in space)
Posts: 14,911
I was hoping it would be Duct Tape Theory.
__________________
WARNING: Phrases in this post may sound meaner than they were intended to be.

SkeptiCamp NYC: http://www.skepticampnyc.org/
An open conference on science and skepticism, where you could be a presenter!

By the way, my first name is NOT Bowerick!!!!
Wowbagger is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 21st July 2010, 11:55 AM   #60
sol invictus
Philosopher
 
sol invictus's Avatar
 
Join Date: Oct 2007
Posts: 8,613
Originally Posted by Tim Thompson View Post
Ah, many thanks, this is something of great interest I was unaware of. I am particularly intrigued by the requirement of a string theory on a spacetime background. It certainly seems that background dependence in string theory had been considered a weakness, at least up to now. Carlo Rovelli, in his book Quantum Gravity (Cambridge University Press, 2004), asserts that quantum field theory discards the spacetime background altogether (e.g., "Thus, there is no background 'spacetime', forming the stage on which things move. There is no 'time' along which everything flows. The world in which we happen to live can be understood without using the notion of time." - page 31). He is not the only source indicating that quantum field theory (and string theory) can & should be background independent.

Now the AdS/CFT duality requires a string theory on a spacetime background, so this would appear to be indeed a "revolutionary" new direction to take things. But what about the equivalent field in one less dimension, is it background independent? If not, that would seem to be not just a departure from the norm for string theory, but a departure from the norm for quantum field theory in general.

At least that's the way it looks at first glance from where I sit. What are your thoughts on background independence and spacetime in quantum field theory?
Perhaps you've misunderstood Rovelli - I think that he must be talking about quantum gravity, not quantum field theory. "Background independence" usually refers to the idea that a theory of quantum gravity should be formulated in a way that is independent of any particular spacetime background. The reason is that in a theory of quantum gravity, the spacetime metric is one of the fields that can fluctuate. The theory itself should tell you what the possible backgrounds (i.e. solutions to the equations of motion) are - you shouldn't have to put one in by hand.

By contrast in quantum field theory the metric is fixed and rigid; one must simply specify it to define the theory. If you're familiar with Feynman's path integral, the issue is whether one integrates over all field configurations and all metrics (that's quantum gravity) or merely over all field configurations with fixed metric (that's quantum field theory).

As for AdS/CFT, it is believed that the CFT provides a non-perturbative definition (i.e. equivalent to a path integral) of string theory on AdS. As such, the CFT should "know" about the integral over metrics. How can that be, you may ask - isn't AdS a particular spacetime?

The answer to that question is not fully understood, but my understanding is as follows. When one integrates over metrics, not all configurations may contribute. Some might have infinite action, or might be excluded by a choice of boundary conditions (the same holds for path integrals over fields in quantum field theory). In the case of gravity/string theory on AdS, one can require as a boundary condition that the metric get close to the AdS metric at infinity, so that any metric without that property doesn't contribute to the path integral.

With that rule in place, the integral is only over asymptotically AdS metrics. Those asymptotics are part of the definition of the field theory (they define on which space the field theory path integral should be performed). But there are still multiple spacetimes with the same AdS asymptotics, and one can indeed see some evidence that bulk metrics sharing the same asymptotics are present (the so-called "Hawking-Page transition" is probably the clearest example).

As I said this issue remains poorly understood, so take this with a grain of salt (and as an explanation for why my response is so technical).
sol invictus is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 22nd July 2010, 05:40 AM   #61
arthwollipot
Observer of Phenomena
 
arthwollipot's Avatar
 
Join Date: Feb 2005
Location: Location, Location
Posts: 55,878
Okay, now we're way beyond my level of graspingness.
__________________
Read my fantasy novel for free!
arthwollipot is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 22nd July 2010, 10:07 AM   #62
Tim Thompson
Muse
 
Tim Thompson's Avatar
 
Join Date: Dec 2008
Posts: 969
Lightbulb String Theory IV

Originally Posted by sol invictus View Post
If you're familiar with Feynman's path integral, the issue is whether one integrates over all field configurations and all metrics (that's quantum gravity) or merely over all field configurations with fixed metric (that's quantum field theory).
That's the secret handshake that clarifies the key point. I didn't realize the real difference between gravity and quantum field theory.
Originally Posted by arthwollipot View Post
Okay, now we're way beyond my level of graspingness.
At some point this always happens to everyone, we just have different levels of graspingness. The closer we get to the frontiers of physics, the farther we get from things that can be readily explained to non-physicists. Popular level explanations, in order to avoid complication, usually imply that physicists actually know things that are in reality not necessarily well known.

String theory is as far out on the frontiers of physics as you are going to get. Most people who have opinions about whether or not string theory is worth pursuing are far short of the level of graspingness required to actually know what they are talking about. This is true even for people with a physics background, if they have not studied string theory in considerable detail. My own expertise is in physics, but mostly atmospheric physics. I have no direct experience with quantum field theory, quantum gravity or string theory. I am dealing with my own level of graspingness here and myself learning things as I go.

So welcome to the "limited level of graspingness" crowd.
__________________
The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell
Tim Thompson is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd February 2011, 09:04 AM   #63
Hercules Rockefeller
Woof!
 
Hercules Rockefeller's Avatar
 
Join Date: Jan 2011
Posts: 2,910
Clifford Johnson debated Smolin and Woit on his blog Asymptotia under Scenes: Storm In A Teacup. You have to google it (I can't post links yet).

I don't think you need to be a physicist to see that the critics of ST lost that argument.
Hercules Rockefeller is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd February 2011, 09:33 AM   #64
Tim Thompson
Muse
 
Tim Thompson's Avatar
 
Join Date: Dec 2008
Posts: 969
Lightbulb String Theory V

Originally Posted by Hercules Rockefeller View Post
Clifford Johnson debated Smolin and Woit on his blog Asymptotia under Scenes: Storm In A Teacup ...
Scenes:Storm In A Teacup
Clifford Johnson (Personal USC home page)
Clifford Johnson (Faculty USC home page)

I met Johnson at a talk he gave on string theory and had him autograph my copy of his book D-Branes (Cambridge Monographs on Mathematical Physics; Cambridge University Press, 2003). When I handed him the book he expressed some surprise that anyone actually bought it. It's a good book, I think, but you really have to be into that mathematical physics thing or it looks like a foreign language. I am not really surprised that Johnson would come out on top of such a debate, as I think I could probably out debate either Woit or Smolin on string theory and I know far less than any of them do about it on a technical level. But I understand the history and I think I understand the real weakness of the positions that Woit & Smolin take, and that weakness is philosophical more than technical, both of them being in the "give up and quit" camp for insufficient reasons.
__________________
The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell
Tim Thompson is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd February 2011, 10:47 AM   #65
BenBurch
Gatekeeper of The Left
 
BenBurch's Avatar
 
Join Date: Sep 2007
Location: The Universe 35.2 ms ahead of this one.
Posts: 37,531
My opinion is that we will probably need to focus on cosmic ray physics if we want to find a way to prove or disprove string theory. YES it takes a lot of time and a lot of data and a lot of processing to use cosmic rays to do physics, but we will never be able to make a machine to turn out particles with energies as high as that reached by some cosmic rays.
__________________
For what doth it profit a man, to fix one bug, but crash the system?
BenBurch is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd February 2011, 11:14 AM   #66
Dancing David
Penultimate Amazing
 
Dancing David's Avatar
 
Join Date: Mar 2003
Posts: 38,493
That should be easier with satelites than the old photo stacks on mountaintops.
__________________
I suspect you are a sandwich, metaphorically speaking. -Donn
And a shot rang out. Now Space is doing time... -Ben Burch
You built the toilet - don't complain when people crap in it. _Kid Eager
Never underestimate the power of the Random Number God. More of evolutionary history is His doing than people think. - Dinwar
Dancing David is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd February 2011, 02:42 PM   #67
BenBurch
Gatekeeper of The Left
 
BenBurch's Avatar
 
Join Date: Sep 2007
Location: The Universe 35.2 ms ahead of this one.
Posts: 37,531
Originally Posted by Dancing David View Post
That should be easier with satelites than the old photo stacks on mountaintops.
True that.

Though we have other terrestrial and airborne methods now;

http://en.wikipedia.org/wiki/High_Re...c_Ray_Detector

http://en.wikipedia.org/wiki/BESS
__________________
For what doth it profit a man, to fix one bug, but crash the system?
BenBurch is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd February 2011, 05:30 PM   #68
Beerina
Sarcastic Conqueror of Notions
 
Beerina's Avatar
 
Join Date: Mar 2004
Posts: 28,833
Originally Posted by Tim Thompson View Post
Ah, many thanks, this is something of great interest I was unaware of. I am particularly intrigued by the requirement of a string theory on a spacetime background. It certainly seems that background dependence in string theory had been considered a weakness, at least up to now. Carlo Rovelli, in his book Quantum Gravity (Cambridge University Press, 2004), asserts that quantum field theory discards the spacetime background altogether (e.g., "Thus, there is no background 'spacetime', forming the stage on which things move. There is no 'time' along which everything flows. The world in which we happen to live can be understood without using the notion of time." - page 31). He is not the only source indicating that quantum field theory (and string theory) can & should be background independent.

Now the AdS/CFT duality requires a string theory on a spacetime background, so this would appear to be indeed a "revolutionary" new direction to take things. But what about the equivalent field in one less dimension, is it background independent? If not, that would seem to be not just a departure from the norm for string theory, but a departure from the norm for quantum field theory in general.

At least that's the way it looks at first glance from where I sit. What are your thoughts on background independence and spacetime in quantum field theory?
I admit straight away I am not well-versed in physics at this level, so my objection might not actually be an objection.

However, time not really existing could be a problem from a computational point of view. We have examples of computational problems for which there is no known faster solution than trying all possibilities -- you basically have to run the computation itself, rather than using some clever shortcut to get the answer. One example might be using multiplication you learned in grade school to multiply 123 x 352, rather than adding up 123 352 times.

That such problems have no shortcut has not been proven, but it is believed true currently, IIRC. So...the universe, which can run such computers which can run such simulations "can't get there from here" if there is no time interacting. No stuff bouncing around, so to speak. Yet such states, in the real universe, actually exist. If there is no shortcut that exists, as we have reason to believe, how did such a state get here?

Alternatively, it suggests an equally amazing thing, that the universe is either finite in computational speed, yet superior to a Turing machine or there is something infinite in nature to the universe-as-computational-device. This is similar to the arguments about a human mind being able to solve problems a Turing machine cannot, with its identical implications for the universe.
__________________
"Great innovations should not be forced [by way of] slender majorities." - Thomas Jefferson

The government should nationalize it! Socialized, single-payer video game development and sales now! More, cheaper, better games, right? Right?

Last edited by Beerina; 2nd February 2011 at 05:33 PM.
Beerina is online now   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 3rd February 2011, 01:22 AM   #69
SusanB-M1
Incurable Optimist
 
SusanB-M1's Avatar
 
Join Date: Jul 2006
Posts: 3,417
Originally Posted by Hercules Rockefeller View Post
Clifford Johnson debated Smolin and Woit on his blog Asymptotia under Scenes: Storm In A Teacup. You have to google it (I can't post links yet).

I don't think you need to be a physicist to see that the critics of ST lost that argument.
Sounds interesting, thank you; I'll google that later.

On a flippant note - in Terry Pratchett's 'I Shall Wear Midnight', there is mention of 'the elasticated string theory'!
__________________
I know that you believe that you understood what you think I said, but I am not sure you realize that what you heard is not what I meant.
SusanB-M1 is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 5th February 2011, 06:23 PM   #70
Skamandros
Scholar
 
Join Date: Jan 2009
Posts: 69
The thoery of the string is based on the three methods of the stringss, only as three partickles fumdamental is based on the methods of quarks. The science of physics has discoveredd this like all contaning the three.
Skamandros is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Reply

International Skeptics Forum » General Topics » Science, Mathematics, Medicine, and Technology

Bookmarks

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -7. The time now is 06:31 AM.
Powered by vBulletin. Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
© 2014, TribeTech AB. All Rights Reserved.
This forum began as part of the James Randi Education Foundation (JREF). However, the forum now exists as
an independent entity with no affiliation with or endorsement by the JREF, including the section in reference to "JREF" topics.

Disclaimer: Messages posted in the Forum are solely the opinion of their authors.