# Quantum Physics / Quantum Mechanics

1. Mar 31, 2005

### LilSciWizGirl

Hey I'm Kristine & I was watching a show last night. It was called NOVA (It's about science), which this episode aired on channel: #50!
It was on Quantum Physics / Quantum Mechanics. Which was probably over my head, considering that I'm in the 10th grade & am currently taking an Earth Science Class. :shy: Well anyways I had a few questions, because I was a little hard for me to understand, and a little ing too! Well, I thought you help clarify them for me!

1.) How can string theory be science, if it can't be proven like any other theory is?
1a.) Is it science or is it just a philosophy?

2.) How can the Laws of the Large & the Laws of the small ( gravity & the other three forces) be combined together, if they can't all be one happy family?
2A.) What about when your trying to prove the theory of a black/dark hole?
Or the Big Bang theory?

I'm a little can you help un me, and please explain what's going on in as simple as terms as you can!

Thank You,
~*~LilSciWizGirl~*~

2. Apr 1, 2005

### Crosson

1) String theory falls under the category of "Theoretical Physics". Many predictions have been made by theoretical physics before any one had dreamed that it would be possible to do the experiment. This is the hope for string theory, although I myself do not believe in it.

1a) All physical theories are philosophy in the sense that they are not really true, they are merely rational ways of obtaining results. This understanding of the role of physical theory is demonstrated by Leonard Euler in the following quote:

"Although to penetrate into the intimate mysteries of nature
and thence to learn the true causes of phenomena is not allowed
to us, nevertheless it can happen that a certain fictive
hypothesis may suffice for explaining many phenomena."

2)
How can they combine together? For the forces to combine it would be necessary to see mass and charge as related properties.

It is possible to learn a lot about black holes and the big bang without a small scale theory of gravity.

3. Apr 1, 2005

### Mk

Was the show "The Elegant Universe," with Brian Greene?
Both episodes can be watched here.

1) String theory can be proven, well some of them, remember there's a lot of diffrent kinds. At Fermilab in Illinois, USA if they can catch a graviton leaving our brane, that would prove M-theory I think. The LHC (Large Hadron Collider) at CERN in Geneva, Switzerland, should be completed in 2006 and 2007, It will search for particles from the 11th dimension, sparticles, supersymmetrical particles, I'll get to that in a few paragraphs.

2.) If you aren't really interested in this, you read the first and can skip to the last paragraph.

The weak, strong, and electromagnetic forces are very different in our everyday world. However, big-bang energies, the forces should converge perfectly, this convergence takes place if we have what is known as a supersymmetric theory.

The string possesses some of the largest symmetries known to science, what's so cool about the symetries is that, they cancel out anomolies and divergences.

Anomolies are little tiny, or huge imperfections in math or a theory, that end up getting bigger and messing everything up. Like if there's a crack on the wing of a rocket, at big enough speeds and heats, that crack will get bigger and eventually the whole wing will come off, the rocket will sway, maybe try and recover, if someone smart who made it is controlling it, then smash into the ground.

Divergence is like on a graph, when a line splits up into two, and go in opposite directions, convergences is the opposite. Summations (http://en.wikipedia.org/wiki/Summation), are diffucult looking things that pop up a lot, like integrals, and they can converge, and diverge as well.

The strong force is based on three quarks (the messenger particle, the baseball, is called a gluon), which are labeled by giving them a fictious color, and the equations are wanted to work still, if you interchange the three diffrent colored quarks. Its called a SU(3) symmetry, when you shuffle the quarks around, the equations still work. It is beleived a SU(3) symmetry most accurately depicts descriptions of strong force interactions, a science known as Quantum Chromodynamics.

Like the last example we can throw in two leptons, like an electron and a neutrino (http://en.wikipedia.org/wiki/Neutrino The neutrino is a vary shy particle that has very small mass, and is only affected by gravity and the weak nuclear force - a light year of lead would be just enough to make an interaction with 50% of the neutrons that fly through it), which makes a SU(2) symmetry. You can add in light (photons), it has a U(1) symmetry, in which you can shuffle around polarizations of light). You can glue these three symmeties together, and it is written SU(3)xSU(2)xU(1). This new symmetry mixes the three quarks together, the two leptons together, and photons together (but not quarks with leptons). This new theory is called the Standard Model, you may have heard of it on NOVA. It is the most sucessful particle physics theory ever.

In string theory, these symmetries cancel out the divergences and anomalies of itself, your boyfriend's so hot, you don't notice the huge pimples on his face. Since symmetry is the most beautiful and powerful thing at your disposal, you might expect the ultmate theory of the universe must possess the most elegant and powerful symmetry, even if its falling apart or not. The most logical choice is symmetry that interchnges not just the quarks with themselves, and leptons with themselves and polarization, but all particles, with the math still being right. This describes.... supersymmetry, the biggest symmetry of all, the super-est.

If you look at all forces and particles of the universe, everything falls into two catogoris, bosons and fermions, depending on their quantum spin. Spin as a pretty complecated matter, I don't think I'll get into that. It is a property of subatomic particles that cannot be visualized, but is akin to the particles being a little top. The light has a spin of 1, the weak and strong nuclear forces have spin 1 also, gravity has spin 2. When a particle has a integer value spin (that's what the "spin-statistics theorem" says), its called a boson. Bosons were named after Satyendra Nath Bose. As I recall, Bose was in Einstein's crew, the 5th basic state of matter is also named after them, Bose-Einstein condensate.

Particles of matter (all the particles of forces are bosons, guage bosons to be precise) that are fermions, have fractional spin, such as 1/2, 3/2, or 5/2, fermions include electrons, neutrinos, and quarks.

In supersymmetry, all subatomic paricles have a partner, a superpartner, that is their opposite in spin - a fermion has a boson superpartner, and vice versa. Physicists use an "s-" prefix to denote that a particle is another's superpartner, like a selectron, squark, or slepton. The superpartners have an immense mass, we think, because they haven't been detected in current atom smashers. The LHC will be able to acheive energies of 14 TeV, much much more than ever acheived by any atom smasher. Since matter is but energy, the greater the energy the greater the mass that can be that results from the smashing of the projectile(s). If a sparticle can be produced in the lab, that would prove supersymmetry theory.

Although supersymmetry represents a powerful idea, there is absolutely no experimental evidence to support it. Though there is a tantializing piece of evidence that points the way towards supersymmetry.

We know the strengths of the quantum forces (all except gravity) are quite different. In fact, at low energies, the strong force is thirty times stronger than the weak, and a hundred times more powerful than the electromagnetic. Though at the instant of the big bang, it is suspected all three forces were equal in strength. Using a graph of what we know, and working backward to fill it in, physicists can calculate the strengths of the forces at the begining of time. By analyzing the Standard Model, physicists find that the three forces seem to converge in strength near the big bang. But they are not precisely equal. Through a blow up of where the strengths look like they converge, we see, that not all do, only the electromagnetic and weak (this is where the electroweak force comes in). When you add supersymmetry though, all three forces fit perfectly together, into the GUT force and are of equal strength, this is not direct proof, in mathematical form, but it does show that supersymmetry is consistant with known physics.

Sooo... at high enough energies, these forces do have equal strengths, thus we can call them one somewhat.

4. Apr 1, 2005

### Locrian

Hi Kristine,

I'll toss in a different position. I would disagree with MK; current string physics models make no testable predictions at all. Saying that they might "catch" a graviton is one thing, but actually determining what that will look like, when and how it will take place, and what you should measure is much more difficult. String physics has produced many interesting ideas, but it has had grave trouble bringing any of those "down to earth" enough to generate interesting predictions of what will be observed. The few things they claim they can predict... well, they can't agree on what they'll see. They've had awful problems coping with recent astronomical observations, such as the discovery of quintessence.

I don't mean that to sound overly harsh, but personally I find the way string physics is marketed to the public to be unacceptable. The caveats I listed above do not mean string physics is a failure or that funding should be stopped. However, you are correct that it is not currently a "theory" as the word is taught in school. It can be considered science for the time being because it builds on ideas that do have experimental verification, but to keep that status it will need to do something... someday. I find it ironic that in a discipline where we rely so much on evidence, that we would try and convince the public string physics is a scientific theory when there is no evidence.

It's worth noting that many of string physics most caustic critics work in other fields of theoretical physics which cannot be said to have produced significantly more. Then there are many of us who work in experimental physics, and simply have no use for decades of theoretical meanderings that have produced nothing we would consider useful.

You'll get other opinions here, from very knowledgable people, and some may disagree with me. I hope they will make their case, as the more voices that are heard on this, the better.