String Theory

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Here, you can find out more here:

http://www.pbs.org/wgbh/nova/elegant/program.html

I'm familiar with some of the mathematics behind string theory, and behind elementary particle physics in general, so I'll chime in here.

It's a proposed "Theory of Everything", a physical theory that incorporates the existing fundamental theories -- gravity and the Standard Model.

Gravity is described by General Relativity, which has passed several experimental tests at varying degrees of precision, as described in The Confrontation between General Relativity and Experiment. Despite valiant efforts, it has been very difficult to construct a reasonable quantum-mechanical theory of gravity that has GR as its classical limit. String theory has the nice feature that it incorporates GR without much trouble.

Nongravitational physics is described by the Standard Model of particle physics, which features a veritable zoo of particles:

Spin 1/2:
Charge 2/3: Up, charm, top quarks (interacts with gluons)
Charge -1/3: Down, strange, bottom quarks (interacts with gluons)
Charge 0: Electron, muon, tau neutrinos
Charge -1: Electron, muon, tau

Spin 1:
Charge 0: Photon, Z
Charge 1: W
Charge 0: gluons (interact with quarks, each other)

The quantum of gravity is the graviton, with spin 2.

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But when has such complexity happened in the past? Several times. And when it's happened, scientists had recognized that there was some order before they found the underlying causes of that order.

* Chemical elements. In the eighteenth and nineteenth century, chemists discovered more and more of them, and in the mid nineteenth century, Dmitri Mendeleev worked out a pattern: the Periodic Table of Elements, complete with some predictions of elements. And some later-discovered elements fit those predictions very well.

But in the twentieth century, their properties were explained by quantum chemistry; working from a picture of atoms as electrons orbiting differently-charged nuclei.

* Nuclei. These were discovered to have masses that are approximate multiples of the proton's mass; many chemical elements were found to have nuclei with different masses (isotopes). Alongside protons were speculated to be "neutral protons", which turned out to be neutrons.

* Hadrons. Strongly-interacting particles. The proton and neutron were the first ones discovered, but particle physicists eventually found an enormous number of them, many of them very short-lived. The solution to this conundrum was that they are combinations of quarks, which have the curious quality of never appearing in isolation, always in combination with other quarks.

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So could something happen with the Standard Model? That's what Grand Unified Theories are all about; attempting to demonstrate that several elementary particles are different versions of the same particle. And string theory is the ulitmate such theory.

But how does one obtain such a zoo of particles from it? Especially particles with different spins? There is a theoretical proposal for relating particles with different spins: supersymmetry. In it, each particle is related to another particle with a spin different by 1/2. To date, no such sets of superpartners have been found, but superpartners of known particles may be found as some newer particle accelerators, like CERN's Large Hadron Collider, come online.

Even with supersymmetry, it is still difficult to construct a Grand Unified Theory where all the Standard Model's particles are superpartners of each other. And this is even true of supersymmetric gravity theories ("supergravity").

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Now to strings. The simplest sort is the bosonic string, which has only integer-spin modes. But that does not represent our Universe very well, so a theory of supersymmetric strings ("superstrings") has been developed. It has modes with half-odd spins as well as integer spins, modes which could conceivably match onto the elementary particles of our Universe. But do they?

A serious problem is that quantum-mechanical strings like to live in more than 4 space-time dimensions: 26 for bosonic strings and 10 for superstrings. The way out of this conundrum is to suppose that the six extra dimensions form a very tiny ball ("compactification"). The topology of this ball then determines what elementary particles we see at (relatively) low energies. So far, it's been possible to find a topology that produces a close approximation of the Standard Model, though I have not seen much success in predicting the Standard Model particles' masses.

Even with success in predicting the Standard Model, there is the question of why the Universe has the space-time topology that it does. Is there anything that constrains that?

Also, there are five possible superstring theories, but they are interrelated in various ways, and all five of them appear to be subsets of a still-obscure theory called M-theory.

And with these two puzzles I conclude this post.

Wow! Archimedes, thanks for the info. and the links. I read them all and understood little but it did give me a vague clue about many things; how our universe is made, how scientists discover these things, the realization that not every scientific principle can be easily tested but some theories you know are true because of their observable effects. (I am an infant in the scientific arena, but will not let this deter me)

This discussion thread made me realize that I know very little about science. I need to start on a basic level and begin to educate myself. If I don't even know these elementary principles, how am I to understand String Theory?

As I read these posts on String Theory, it is daunting but at the same time stirs a desire to learn more; a whetted appetite.

Any suggestions on the best, factual scientific magazines or journals that a beginner might understand? Even if it's for kids?

Bland-

try your local library first as they may have a lot on archive. I grew up reading Scientific American, Discover, and Smithsonian. That's a variety of science for laypeople, with Discover maybe being more mainstream? Though I haven't read any of those three recently but the Smithsonian so I'm out of touch.

There's a lot, I'd say it depends on what kind of science you're looking for but the library is a good place to search for magazines you may be interested in. Also, a bookstore that doesn't mind browsing.

A Short History of Nearly Everything by Bill Bryson covers a lot of ground.

Thanks, Hermit. My quest for understanding string theory will begin at the library. (wish I had more time to browse.) I'm not sure where to start. I guess I should get a high school text book; or on second thought, is most of this material at the college level? I only had science up to 10th grade Physics and the math was so intense I nearly failed it. (I hadn't had any trig. either and I've since learned that physics is difficult without some knowlege of trigonometry.) Of course, there's a big difference in becoming an "armchair" scientist and a working scientist and I only desire the former, realistically.

While we're at it, some more about the Standard Model of particle physics. It is predicted to have an additional particle, a "Higgs particle", with spin 0 and charge 0. This particle has the interesting property that its least energertic state is to have some nonzero value; the Universe is filled with constant nonzero Higgs. It interacts with other elementary particles, and its always presence gives them their masses.

But a Higgs particle apart from its nonzero ground state has yet to be seen; making one is at the limit of present particle accelerators' capabilities, though CERN's LHC should be able to make it -- if it exists.

Now to supersymmetry. The Minimal Supersymmetric Standard Model is the Standard Model with the smallest number of additional particles to make its supersymmetry work. It has some additional Higgs particles, two neutral and two charged (+1,-1) ones, and superpartners of all of these:

The spin-1/2 particles have superpartners with spin 0:
electron - selectron (scalar electron)
quark - squark (scalar quark)
etc.

The spin-0 and spin-1 particles have superpartners with spin 1/2:
photon - photino
W - wino
Z - zino
gluon - gluino
Higgs - higgsino

As a further complication, the photino, zino, and neutral higgsinos are expected to mix ("neutralinos"); the winos and charged higgsinos are also expected to mix ("charginos") -- their mass states are expected to be mixtures of these superpartner states.

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This mixture is much like the way that neutrinos' mass states are mixtures of their weak-interaction states and vice versa. When a neutrino is produced from an electron, it is actually a mixture of three different mass-state neutrinos, each with a different mass. But when one is produced from a muon, it is a different mixture of those three neutrinos. And one produced from a tau particle is yet another mixture.

Due to their different masses, these mass-state neutrinos oscillate at different rates as they travel, and they get out of their original phases, turning an original electron neutrino (say) into a mixture of electron, muon, and tau neutrinos.

This successfully accounts for why the solar neutrino flux is less than expected -- the Sun emits electron neutrinos, but they oscillate into muon and tau ones along the way to the Earth. Since the neutrino detectors can see only the electron-neutrino weak states, they thus see fewer neutrinos. This is because the neutrinos do not have enough energy to make muons or taus.

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And not only does the Standard Model have a lot of particles, it also has a lot of free parameters.

Masses of electrons and quarks: 9
(mu, tau counted as electrons)
Quark weak-decay mixing: 4
Higgs mass and ground-state value: 2
Coupling constants of strong, electromagnetic, and weak forces: 3

Giving 18 parameters

Neutrinos having mass add an additional 7: 3 mass and 4 mixing
Giving 25 parameters

The MSSM has even more parameters; a full-scale version has over 100 parameters, though many of them are expected to be related in various ways, bringing the total down to not many more than the Standard Model.

So can these parameters' values be predicted?

It doesn't seem that string theorists are there, at least not yet.