danalwyn

Because

eye_of_a_cat asked, I tried to write something about string theory. But then I realized that I don't actually know anything about string theory, so I rambled on about Physics. This will teach people to ever ask me anything ever again. Ha!

One thing that laypeople do not realize about Physics is how segmented it has become as a field, possibly second only to biology. A neurological researcher and an observer of salmon mating habits can both be considered biologists, but their work is probably pretty much incomprehensible to each other. Similarly, I cannot answer eye_of_a_cat's query because I simply do not understand anything about string theory, which is about what I know of semiconductor physics or quantum optics. Even though string theory is right next door, phenomenologically speaking, their work is completely incomprehensible to me, and vice versa.

All Gaul may be divided into three parts, but Physics is divided into two, and then into two again, and finally into N where N is a number that increases exponentially with your advancement in the field. Literature is one field, which can be divided at the High School level into Western and non-Western literature, and again in undergraduate work into British Literature and African Literature and so on until you arrive at the perfectly logical state of having entire magazines devoted to Early Victorian Literature Featuring Women In Giant Skirts. Physics takes this system to inherently outrageous levels, we have entire fields where not only do I not understand what they study, I don't even understand the name (usually anything involving phonon interactions and crap like that). Physics probably has it worse than literature because literature has underlying themes and tools that span the entire field, whereas the language in Physics is completely mind-boggling.

For instance, the first line from the abstract of the most recent CDF publication:

“We present results of a search for anomalous production of events containing a charged lepton (l, either e or mu) and a photon (gamma), both with high transverse momentum, accompanied by additional signatures, X, including missing transverse energy (missing E(t)) and additional leptons and photons.”

That's it. The simplest and easiest to understand line in the abstract, which also happens to be essentially the opening to the piece. Now chances are, you don't fully understand all of this, which is understandable, because you're not in the field. The scary thing is, if I showed it to my friends who do semiconductor physics or biophysics, they will understand more words, but they will not understand why this is a well-motivated search, any of the experimental techniques, or the significance of the outcome at any but the most base level. I don't understand half of it, and I am in the field. And not just the field, but the subfield as well.

So I can't say a damn thing about string theory, which, even as physics goes is far out there, but I will try to motivate its existence.

To do that, I'm going to have to explain about physics again.

There are two types of Physicists, experimentalists and theorists. Institutionally there is a rivalry between those two groups; theorists come up with ideas, and experimentalists see if they work. Both sides see themselves as doing the hard work. Both sides tend to think that they should get more credit. I'm an experimentalist, so I'm biased in that direction.

There are no sting theory experimentalists.

To start with, all Physics is divided in two parts. There's Classical Mechanics, and Quantum Mechanics. Classical Mechanics is for big things, Quantum for small ones. The difference between them, unfortunately, is arbitrarily small.

Classical Mechanics:

Generally, everything that can make you say “Ouch” is included in this category, of which there are several subcategories. This is the field that studies balls running down slopes, zapping your friends with high voltage, pushing them off of buildings, and shooting them with lasers. Classical mechanics lives in the domain of Newton's deterministic universe, heat flow, fluids, bumper cars, and everything else that are large enough to smack into you live here. As this is the older of the two fields, a lot of the basic theory has been fleshed out. Research has moved to the more remote fields, biophysics, plasma physics, and fluid dynamics.

Classical Mechanics is also home to one of the two “Theories of Everything”, General Relativity. First postulated by Einstein, General Relativity is like Special Relativity (the one you were probably taught) only less so. It is often portrayed on television with pictures of warped spacetime and planets falling down gravity wells, which is nothing what it is like in real life. In reality it is a long series of equations that may or may not be completely random. In any case General Relativity, which claims to describe the way that space and time interact with matter and energy (and it actually might do so for all I ever understood of it). GR is explained completely inside the classic text Gravitation by Misner, Thorne and Wheeler, a book that is so massive in and of itself that if you put two copies together on a bookshelf you stand a reasonable risk of creating your own black hole.

Quantum Mechanics:

On the other side of the spectrum lies Quantum Mechanics, the study of the very small. At very small scales, you have to deal with particles. Particles are extremely tiny objects (usually ideal particles are considered to be smaller than atomic size) that may or may not be waves, depending upon your point of view. The problem is that at small scales you cannot tell where anything is; statements like “Particle X is at Y” create trouble. Uncertainty takes over.

Every particle can be described by a set of observables, such as position, momentum, etc. These are all numbers that can be measured to define the particle. The bad news is that when you measure one number you interfere with measuring another number, and vice versa. As a result, the best you can get is usually a probability function, which tells you were something might be, if it's not somewhere else. It's all very confusing, but it works. Quantum Mechanics governs the behavior of semiconductors, high-energy lasers, waveguides, nuclear weapons, and other such implements of modern technology. It is the most revolutionary theory in the history of physics simply because it, along with GR, tear the entire framework of what we think we know about the universe apart. It is though, not the end of the story.

Quantum Field Theory

The problem with quantum mechanics is that is assumes that things are constant. Three particles come in, interact, and then the same three particles leave again. This is usually common, after all, your atoms don't change that much, but it doesn't work at high energies. And it doesn't answer a fundamental question, where do all these particles come from? Quantum Field Theory essentially solves that problem by pushing it off into the distance. In QFT, your universe is no longer stable, once you hit a certain energy limit you can actually create particles. The easiest way to think of this is to realize that Einstein said that mass and energy are essentially the same thing. At the human scale they are very different, one is big and sits there, the other blows up. But at the quantum scale, matter is energy that's taking a breather (actually it has to do with the mysterious Higgs, but I'm not going to try and explain that). So if you have enough energy lying around, you can create particles. An electron and a positron (the electron's evil anti-matter brother) can collide, explode, and create two different particles from that energy.

What's really weird is the existence of what we call virtual particles. Virtual particles can appear whenever two particles interact. When two electrons get close together, they repel. When this happens, you can visualize it as the two particles exchanging a tiny packet of energy and momentum, a packet that looks suspiciously like a particle. Normally these particles are entirely virtual, they disappear and are never seen. Give them enough energy though and they can actually become real. This allows us to measure them.

Combined with the theory of Quantum Electrodynamics, the age of particle physics eventually led to the most important discovery of all:

The Standard Model

The Standard Model is the most accurate and complete theory in the entirety of physics, and perhaps even in the entirety of science. Components have been tested past ten decimal points of accuracy, a level that most comparable sweeping theories of biology and chemistry cannot compete with yet. It is a conglomeration of theory that predicted the nature and type of many of the particles that exist today. It is the foundation of modern physics, the key to our understanding of the universe, and the pinnacle of our experimental method. All this would be perfect if it were not for one minor flaw.

It is completely, absolutely, awfully, and totally wrong. Horribly, horribly wrong.

Everyone knows that the Standard Model is wrong. Nobel Prize winners, physicists, administrators, graduate students, researchers, and social scientists all know it. Even the homeless couple down the street know that the Standard Model is wrong, and they ask me about it whenever I go by. But so far, nobody has been able to prove it. Every prediction made by the Standard Model is more or less correct. We know that it's wrong, but every time we go and test it, the Standard Model just stands in the corner, radiating smugness, knowing that we will simply confirm its correctness until we finally go ballistic. Physicists spend their entire lives trying to disprove the standard model, which often just lands them in the madhouse before they hit thirty.

The Standard Model is the name we give to the theory that describes the particles that make up the world. Currently the Standard Model contains eighteen particles, one of them special. There are six leptons (the family that includes the electron), six quarks (which make up most of the matter we can interact with) and five bosons (particles that carry forces. The photon is the most famous). Quantum Electrodynamics derived the existence of three of the bosons from basic principles of the universe, but got the masses totally wrong (it predicted massless bosons instead of big honkin' ones). To solve this they invented the Higgs boson, which gives everything mass. Since their predictions turned out to be correct, we assume that a Higgs mechanism of some sort exists.

But the Standard Model is essentially 42. If this is the answer, then it's time to look back and figure out what the question was. For instance, the standard model includes six quarks, split into three generations. This is because there are three "colors" that govern Quantum Chromodynamics. Why are there three? The Standard Model has no answer. Random chance perhaps? Why are there also three generations of leptons? Maybe three is a popular number. Why the anti-matter/matter imbalance? Who knows? Why such the disparity between forces? Because. The Standard Model is good at making testable predictions, but it is horribly bad at explaining them. There is no "why" in the Standard Model. It's like having a theory that says that the sky is blue. No explanation, just stating that the sky is blue.

Even worse, the Standard Model has too many constants. There are essentially only two constants in all of Classical Mechanics, the speed of light and the universal gravitational constant. The Standard Model has something on the order of thirty. That's far too many knobs for a satisfying theory. Every other theory gets progressively simpler as you get closer to the "truth". Why does the Standard Model go the other way? It assumes that each of these constants are just preset in nature, but are they really just chosen randomly, or is there some underlying order? The Standard Model has no explanation because it is essentially a catalogue. It is inherently limited from looking deeper.

Of course, physicsts have been looking for an answer for a long time. One key is the Higgs boson, the last particle in the Standard Model to be discovered, and the strangest. The original predictions of the Higgs boson gave it impossible masses, so the physicsts essentially fudged the result into a more understandable regime. The question is how right they were. There are three competing theories that can explain the unusual situation of the Higgs, the supersymmetric zoo with its dozens of new particles, the technicolor theory, and the extra large dimensions theories. All of these will be the subject of searches at the LHC, but they are on the verge of provable. For instance, the easiest way to test for supersymmetry is to look for supersymmetric particles. But the theorists have been unable to pin down exactly what mass ranges to look for these particles in. The fear is that if we can't find them where they tell us to look for them, they'll just raise the bar, tell us to look elsewhere, and we won't have proved a damn thing. Needless to say, this has raised many accusations (although it's not their fault, they know damn well that they can't tell us where these suckers are). The other theories are just as hard to test for.

But this doesn't get rid of the Standard Model's most perplexing problem. Namely that of the strangest force of all: gravity. Gravity is the weakest of the four fundamental forces, but it is also the most dominant one in our universe. It is described almost completely by General Relativity, but GR and the Standard Model cannot coincide in the same universe. They are simply too different. To unify these two, theorists developed string theory.

This is where I get out of my league. String theory operates close to the GUT scale (relatively speaking), the scale at which all known physics breaks down. It describes the unification of all forces at that level, and explains why gravity operates differently at lower energies. It is, in itself, full and complete, a theory of everything. It has only one problem, it is essentially creationism in physics.

Because string theory only manifests at such high energy, there is no way to test it. No phenomenon yet observed in the universe operates at the GUT scale. You could drop nuclear weapons on the moon all day, and you wouldn't even come close. A typical answer to being asked how to test string theory can start out with "Well, if you had a particle accelerator as large as the universe...". It is also, at this stage, an infinitely flexible theory. There is no way to disprove string theory, because it can be adapted to almost any discovery. It is the ultimate in fuzzy thinking, and even though it is incredibly mathematically rigorous, there are many who doubt that it actually constitutes a scientific theory. After all, since we won't be able to test until the theorists are all dead, how can we be sure that they aren't just making stuff up?

String theory is the ultimate frontier of modern particle physics, but it is also the most frustrating. For all the effort poured into it, none of the results they come up with will be testable within the next century, barring some unforseen miracle. It is the ultimate theory of everything, yet it predicts nothing.

And given how much time it takes to disprove the Standard Model, I don't think I need something new to fight with.