The Merits of String Theory

[JacobEvans]

From what I've read, string theory, if proven true, would effectively replace the standard model. I've also read that it is almost untestable, and based entirely on theoretical physics.

Despite my glaring lack of in depth knowledge of both the Standard Model, and String Theory, I find this to be an interesting topic for discussion. Who here believes in the validity of string theory, and perhaps someone could better explain how it would make the Standard Model more obsolete.

 

[Squagnut]

The horrible thing about String Theory, and more particularly M-Theory, is that it's pretty much untestable.

The great thing about it is that at the moment we have two wonderful theories to describe the universe, relativity for big things, quantum for small things. They both explain their respective phenomena with a dazzling degree of accuracy, but they're incompatible. This is a big problem. What String Theory does is provide a mathematical framework which marries the two up.

I'm not a great mathematician so it's hard for me to comment much, but going on what folk say, I accept the validity of M-Theory, but that's not to say that I believe it's true. But then I don't believe that the theory of evolution is true - it's just the best candidate we have to explain the phenomena it describes.

If someone good at sums could go through the numbers of String Theory here, with as much of an explanation as possible, I'd love it.

 

[Einstein's_Advocate]

It's not like string theory was just pulled out of the airy blue sky from a theorist who was snorting crack cocaine and tossing hatchets to his fellow theoreticians and said "Hey, what if the universe were made out of strings?" and then his friend, while getting a leg hacked off, replied "I dunno". It never worked like that. String theory is accepted as a good, viable model by very intelligent theorists because it fits the data AND explains QM and GR. The experimentalists still need to devise a viable way to test string theory before it can become a serious rival to the standard model --but this doesn't mean that it's utter shit. And by the same token, theorists who protest that string theory is the greatest thing ever and must be true are, also, idiots.

String theory and M-theory, guys, has one good feature: It actually explains everything. And stating that GR and QM are better, like many ST opponents do, because, separately, they explain most everything is ridiculous and anti-scientific. The universe does not operate on two logically and mathematically incompatible laws, and to say otherwise is either ignorance or stupidity. Science can only feasibly work if it is entirely logically compatible, otherwise it's time to turn in the lab coats and devise a new way to generate knowledge. And it is wholly irrelevant if QM and GR explain a lot of stuff; we already know that. Newtonian mechanics and E&M both explained a lot of phenomena, separately. But they were (and GR and QM are) invariably wrong, internally inconsistent with each other, and needed to be normalized with a new theory. This is a scientific fact if ever there were one and it holds true with QM and GR.

So it's a tricky situation, and I don't really support either of the two extremes.

 

[blinddesign]

It's obviously true. The FSM made the universe in His image. Isn't that the undeniable truth? The only reason you can't test it is because he fiddles with the equipment with His noodly appendages, rendering it impossible to "prove" Him and thus render faith pointless.
You should all quit trying to test Him!

 

[Pulsar]

There's actually great news from the string guys: string theory is able to explain high-temperature superconductivity.

From sciencedaily:

Physical Reality Of String Theory Shown In Quantum-critical State Of Electrons


String theory has come under fire in recent years. Promises have been made that have not been lived up to. Leiden theoretical physicists have now for the first time used string theory to describe a physical phenomenon -- the quantum-critical state of electrons leading to high-temperature superconductivity. Their discovery has been reported recently in the journal Science.

Electrons can form a special kind of state, a so-called quantum critical state, that plays a role in high-temperature superconductivity. Superconductivity at high temperatures has long been a 'hot issue' in physics. In superconductivity, discovered by Heike Kamerlingh Onnes in Leiden, electrons can zoom through a material without meeting any resistance. In the first instance, this only seemed possible at very low temperatures close to absolute zero, but more and more examples are coming up where it also occurs at higher temperatures. So far, nobody has managed to explain high temperature superconductivity.

Jan Zaanen makes no attempt to hide his enthusiasm. Together with Mihailo Cubrovic and Koenraad Schalm, he has successfully managed to shed light on a previously unexplained natural phenomenon using the mathematics of string theory.

This is the first time that a calculation based on string theory has been published in Science, even though the theory is widely known. "There have always been a lot of expectations surrounding string theory," Zaanen explains, having himself studied the theory to satisfy his own curiosity. "String theory is often seen as a child of Einstein that aims to devise a revolutionary and comprehensive theory, a kind of 'theory of everything.' Ten years ago, researchers even said: 'Give us two weeks and we'll be able to tell you where the big bang came from.' The problem of string theory was that, in spite of its excellent maths, it was never able to make a concrete link with the physical reality -- the world around us."

But now, Zaanen, together with his colleagues Cubrovic and Schalm, are trying to change this situation, by applying string theory to a phenomenon that physicists, including Zaanen, have for the past fifteen years been unable to explain: the quantum-critical state of electrons. This special state occurs in a material just before it becomes superconductive at high temperature. Zaanen describes the quantum-critical state as a 'quantum soup', whereby the electrons form a collective independent of distances, where the electrons exhibit the same behaviour at small quantum mechanical scale or at macroscopic human scale.

Because of Zaanen's interest in string theory, he and string theorist Koenraad Schalm soon became acquainted after Schalm's arrival in Leiden. Zaanen had an unsolved problem and Schalm was an expert in the field of string theory. Their common interest brought them together, and they decided to work jointly on the research. They used the aspect of string theory known as AdS/CFT correspondence. This allows situations in a large relativistic world to be translated into a description at minuscule quantum physics level. This correspondence bridges the gap between these two different worlds. By applying the correspondence to the situation where a black hole vibrates when an electron falls into it, they arrived at the description of electrons that move in and out of a quantum-critical state.

After days and nights of hard grind, it was a puzzle that fitted. "We hadn't expected it to work so well," says a delighted Zaanen. "The maths was a perfect fit; it was superb. When we saw the calculations, at first we could hardly believe it, but it was right." Gateway to moreAlthough the mystery of high temperature superconductivity isn't fully resolved, the findings do show that major problems in physics can be addressed using string theory. And this is just the start, Zaanen believes. "AdS/CFT correspondence now explains things that colleagues who have been beavering away for ages were unable to resolve, in spite of their enormous efforts. There are a lot of things that can be done with it. We don't fully understand it yet, but I see it as a gateway to much more."

This is really great stuff. Not only is this the first experimental confirmation of string theory, it is not some exotic particle accelerator phenomenon, but something very practical and very important for future technologies. Maybe it deserves the term 'theory' after all

 

[TheJilvin]

I remember being introduced to physics in 1999 with Brian Greene's book, "The Elegant Universe". I kept up at layman level physics until the 2004 release of "The Fabric of the Cosmos", which I also bought. But then I immediately after started to teach myself calculus, then multivariable calculus, and I got serious about truly mathematical physics when I obtained Arfken and Weber's mathematical methods book. I partially read both "Gravitation" (a giant textbook on GR) and a huge Quantum Mechanics textbook , learned of the true nature of the GR-QM discrepancy in mathematical terms and found out it was more problematic than previously conceived.

I started to trail off from string theory, especially when Penrose's book "The Road to Reality" gave it a highly skeptical, almost dismissing, passing mention, coupled with Lee Smolin's strong reaction to the development of string theory as watering down rigorous science, I became highly skeptical of it myself and began personally to call it the "String Hypothesis".

Early this summer, however, the aforementioned data came up. And not only did the (incredible) math of string theory conform exactly to fixing a problem in QM, but it solved the CENTRAL PROBLEM of quantum condensed matter physics, with macroscopically observable results. This, along with further mathematical and scientific confirmation of the phenomenon Schalm, Cubrovic, and Zaanen discovered; along perhaps with some results in particle accelerators predicted by string theory, make it worthy of the term "String Theory" again.

If the math is as correct as they say it is in the paper, then this will be one of the most famous problems ever solved in the history of physics. Comparable to Max Planck's realization that only discrete amounts of energy are emitted from a blackbody in 1900 (he modified the Raleigh-Jeans law of classical mechanic's prediction for the energy density of a blackbody emission spectrum by replacing an integration technique with a summation technique, because the previous law predicted the energy to shoot up to infinity very quickly.), or the Michelson-Morley interferometer experiment in 1887 ( a failure to detect the luminiferous aether as a medium for electromagnetic waves resulting in the invariance of time and perceived spacial length resulting in the proposal of the Lorentz-Fitzgerald contraction in 1889 and the mathematical development of it by Lorentz and Poincare; culminating in Einstein's 1905 thesis on the electrodynamics of moving bodies, resulting in Minkowskian space-time in 1907); or the detection of light bending by a gravitational field in 1919, confirming general relativity; or even Newton's answer to the simple question "Why does the moon orbit the earth?" by triangulating the distance of the moon, and predicting its gravitational acceleration and confirming the universality of gravity by extension.

However, the real question on my mind is: Is it really a theory of everything!? Even if it turns out to be true, there always seems to be another problem cropping up after physicists reach a point where they begin to say "Well, physics is pretty much done!", then another huge problem comes up and sparks 120 more years of research that radically changes everybody's notion of everything. *sigh*, such is the way of science.

 

[Dumbfounded]

There's actually great news from the string guys: string theory is able to explain high-temperature superconductivity. [...] This is really great stuff. Not only is this the first experimental confirmation of string theory, it is not some exotic particle accelerator phenomenon, but something very practical and very important for future technologies. Maybe it deserves the term 'theory' after all

"String theory" didn't explain anything, and the result is in no-way a confirmation of string theory; they have, as best I can gather, just used a mathematical tool called "Anti-deSitter / Conformal Field Theory correspondance", sometimes called the "holographic principle", which was developed by string theorists to solve another unrelated problem. If they had worked out the answer using tensors would you count it as proof of special relativity?

Also, that you can use X to expain Y is not confirmation of X. Confirmation would happen if the model successfully predicted something about HTSCs that we didn't already know.

(Edit: Hmm, New Scientist apparently covered this last month.)

 

[Deleted member 619]

Untestable? Neil Turok et al disagree.

Colliding Branes In Heterotic M-theory by Jean-Luc Jehners, Paul McFadden and Neil Turok, arXiv.org (12 February 2007) [Download from here]

Generating Ekpyrotic Curvature Perturbations Before The Big Bang by Jean-Luc Lehners, Paul McFadden, Neil Turok & Paul J. Steinhardt, arXiv.org, 19th February 2007 [Download from here]

So, given that M-Theory has given us a prediction that will manifest in the observable universe, namely gravitational waves, that can be tested by the LHC when it gets up to full power, I would contend that it is indeed testable. Whether or not those tests yield the predicted results is another matter. At the moment, it's untested, but not untestable.

We live in interesting times.

 

[Dumbfounded]

M-Theory has given us a prediction that will manifest in the observable universe, namely gravitational waves, that can be tested by the LHC when it gets up to full power

For gravitational waves to interact with the LHC they would have to resonate with the frequency of the cycling particles in the storage rings. That way the squash-stretch of the GWs as they pass would have a cumulative effect on the particles' path. I've never read anything to suggest they are even looking for such an effect, and I doubt that the LHC would be more sensitive a GW detector than dedicated experiments like LIGO.

Are you sure you aren't thinking of supersymetric partners? Certain models predict a particular spectrum of GWs from the presence of supersymetry, so should supersymetric particles be detected by LHC, and potentially they could be, then that's a long way towards confirming such models (Softpedia).

 

[JacobEvans]

Well all I can say is DAMNITYAY.
It's cool and exciting, but I've barely begun to comprehend CLASSICAL mechanics and now this comes along! :lol:

 

[Deleted member 619]

For gravitational waves to interact with the LHC they would have to resonate with the frequency of the cycling particles in the storage rings. That way the squash-stretch of the GWs as they pass would have a cumulative effect on the particles' path. I've never read anything to suggest they are even looking for such an effect, and I doubt that the LHC would be more sensitive a GW detector than dedicated experiments like LIGO.

Are you sure you aren't thinking of supersymetric partners? Certain models predict a particular spectrum of GWs from the presence of supersymetry, so should supersymetric particles be detected by LHC, and potentially they could be, then that's a long way towards confirming such models (Softpedia).

This is not part of the remit of the LHC as yet, but the experiments are being devised as we speak, I am given to understand. The testability is outlined in the papers presented in my last post.

 

[Skribb]

It's not like string theory was just pulled out of the airy blue sky from a theorist who was snorting crack cocaine and tossing hatchets to his fellow theoreticians and said "Hey, what if the universe were made out of strings?" and then his friend, while getting a leg hacked off, replied "I dunno". It never worked like that. String theory is accepted as a good, viable model by very intelligent theorists because it fits the data AND explains QM and GR.

No, in fact that is precisely the problem. There is no data for it to fit yet. As many have pointed out, it is very difficult to test. Part of the problem with your statement is that it ignores the point that string theory was constructed in large part in an attempt to fit the the pre-existing theories of quantum mechanics and general relativity. We know with absurd amounts of accuracy that QM and GR are right in their respective fields and an inability for string theory to reproduce those results and to make that extra step of unifying the two would be considered a failure on the theories part.

The experimentalists still need to devise a viable way to test string theory before it can become a serious rival to the standard model --but this doesn't mean that it's utter shit. And by the same token, theorists who protest that string theory is the greatest thing ever and must be true are, also, idiots.

I don't think anybody is suggesting it is utter shit. But it absolutely is not physics yet, just mathematics. Pretty math, but math none the less.

String theory and M-theory, guys, has one good feature: It actually explains everything. And stating that GR and QM are better, like many ST opponents do, because, separately, they explain most everything is ridiculous and anti-scientific. The universe does not operate on two logically and mathematically incompatible laws, and to say otherwise is either ignorance or stupidity. Science can only feasibly work if it is entirely logically compatible, otherwise it's time to turn in the lab coats and devise a new way to generate knowledge.

This is just flat out untrue and short sighted. There is no universal law that says that there is some unifying principle to the universe let alone that it is possible to write it down in any sense. In fact, I would argue that a more fundamental understanding of QM would make it understandable on a certain level that the universe would need 2 different theories. After all, any time you get any interactions on these Planck levels, you necessarily get a collapse of the wave function. This would suggest that two distinct scenarios may exist, collapse or no collapse of the wave function. While I agree that a unifying principle is desirable, I don't agree that there necessarily must be a gradation in function from quantum to relativity, especially considering how discontinuous and discrete most things tend to be in our universe.

And it is wholly irrelevant if QM and GR explain a lot of stuff; we already know that. Newtonian mechanics and E&M both explained a lot of phenomena, separately. But they were (and GR and QM are) invariably wrong, internally inconsistent with each other, and needed to be normalized with a new theory. This is a scientific fact if ever there were one and it holds true with QM and GR.

This is complete crap. It is incorrect to think of Newtonian mechanics and E&M as "wrong" and/or "internally inconsistent". Both of the theories are very accurate subsets of their more advanced theories. Your arguments are even bigger forms of crap in relation to GR and QM. Precisely the struggle we are going through is that the two theories are not internally inconsistent but are externally (i.e. with respect to the other predominant theory) inconsistent. GR and QM are fundamentally different but because we insist that there can be only one law of physics applicable to all scales and scenarios, we seek a way to resolve the two.

 

[Deleted member 619]

^^^ I approve of this post.

 

[Dumbfounded]

This is not part of the remit of the LHC as yet, but the experiments are being devised as we speak, I am given to understand. The testability is outlined in the papers presented in my last post.

Yes, I read them. Gravitational waves are an expected consequence of GR, so their detection alone would prove nothing about M-theory. What particular prediction about GWs have these papers made? Also, though it is certainly feasible to detect GWs by monitoring the paths of particles round a storage ring, what advantage is there to doing so over the laser interferometry methods used by LIGO or GEO?

 

[Deleted member 619]

Hmm. You appear not to have read from page 16 of the second paper, which deals with differing predictions in spectral shift and curvature perturbations.

As for advantages between LHC and laser interferometry methods, I made no comment. It has been shown that such detections are possible in particle accelerators, through the interaction of GWs with particles in storage rings. I never made any comment on which was the better method, only that this work was posited to be within the remit of the LHC.

Here's a paper from CERN on the topic:

http://209.85.229.132/search?q=cache:ENQjmkQnVCcJ:cdsweb.cern.ch/record/355151/files/ep-98-063.ps.gz+LHC+gravitational+waves&cd=3&hl=en&ct=clnk&gl=uk&client=firefox-a

 

[Dumbfounded]

Er, that concerns scale invariance (or the lack of it) in pertubations of the cosmic microwave background. If there was no spectral tilt, ns = 1, then the anisotropy of the CMB would be scale invariant, i.e. the Harrison-Zeldovich spectrum. You can also not use direct measurement of gravitational waves to determine their power spectrum against scale unless you have also determined their point of origin, so would not be able to determine the spectral tilt.

The 1998 CERN paper seems to be the only time this particular technique in detecting GWs has ever been raised, I certainly can't find anything on CERN's own site indicating that they're putting the idea into practice. Mind you, I'll happily withdraw one caveat, as the paper makes clear, there is no reason that the GWs have to be "in tune" with the cycling particles in the ring to be detected. I imagined looking for the cumulative effect of many successive waves when of course their extreme wavelengths mean that even individual GWs would have a detectable effect on the fast cycling particles in the ring. Even so, the paper's own calculations for bandwidth and sensitivity would put CERN far behind GW projects already in operation.

BTW, the best fit of the data from WMAP is P = k^0.97, i.e. ns = 0.97. IIRC, that would be a "red-shifted" spectral tilt, not the blue-shifted one (>1) predicted by the paper.

 

[Deleted member 619]

You raise some interesting points, and further avenues of research for me. Cheers.