The Double-Slit Experiment

This is the experiment that lays the groundwork for understanding the bizarre behavior of quantum mechanics. Be prepared to question the results, because they challenge our every-day perception of reality. But countless repeated experiments have confirmed them, and quantum mechanics now appears to explain them.

The experiment was first performed by Thomas Young in 1800, but has been repeated countless times since then. The basic setup, which is usually modified is as follows: A slit is made in an opaque sheet. This makes only one thin light source. Another sheet is set up behind the original, this with two slits. What would you expect the pattern on a final sheet to be? Two vertical columns, probably. This is not the case. In fact, there are actually several bands of light. This in itself is not remarkable. The same pattern is created with waves: when a high wave and trough combine, they cancel each other out. So this seems to prove that light is a wave. Recent advances have shown that light can be either. When a photon gun, which fires individual electrons, is aimed at the sheet, the results require quantum mechanics to interpret.

The photon gun is slowed to fire one photon at a time. What would you expect? Again, probably two individual columns. This would be the case if the experiment was a machine gun shooting through a brick wall. Instead, the same pattern as a wave of light is created. How can a single particle create the same pattern as waves? The answer is made with the Copenhagen Interpretation, now the standard of quantum theory. In essence, the interpretation states that the photon is a wave function that collapses when observed. (A photon is both a particle and a wave). This does not merely applie to light. Electrons and other elementary particles observe the same rules. Another theory is called the Multiverse. It basically states that each photon forms a part of a myriad of photons that condense when observed. The other photons are vanquished to an infinte amount of other universes.

If you can understand the basic theory behind the double slit experiment, then you are well prepared for the other mysteries of the quantum world.

Quantum Optics

The major diference between quantum optics and classical optics is the description of light. Classical physicists debated whether light was a wave or a particle for decades. Finally, after evidence from Christian Huygens, classical physicists decided that light was a wave. Quantum optics, however, treats light as if it is both a wave and a particle. It relies heavily on lasers.

Heisenberg's Uncertainty Principle

The principle that lays the framework for all of quantum theory is Heisenberg's Uncertianty Principle. In its essence, the principle states that the position and momentum of a particle cannot ever be known exactly. They are inversely proportional. Many people, even physicists misunderstand this. It is not that we cannot determine the position of, say, an electron because our measurements are not precise enough, but that the elictron itself does not know its position and momentum. Remember, momentum refers to both speed and direction. The effects of this simple equation are incredible.

The Uncertainty principle opposes Newtonian physics. Newton, who developed the basic theory of gravity in Principia, thought that if we had a powerful enough computer we could determine the position and momentum of every particle and predict the future. But with the Uncertainty Principle, it is impossible to predict the future.

Schrödinger's Cat

Perhaps one of the most famous anecdotes of quantum mechanics involves Schrödinger's Cat. Although many people have heard of the topic before, few actually realize what the experiment is and what its effects are.

Suppose that a cat is placed into a magical type of box. This box disallows any type of communication between inside and outside once it is closed. Now place in the box a small portion of radioactive substance, a Geiger counter (detects radioactive particles), and a beaker full of poisonous gas. The radioactive material is precisely enough to trigger detection 50% of the time. If the radioactive substance is detected by the counter, the beaker breaks, killing the cat. Otherwise, the cat remains fine. Once the allotted time has passed, the box is opened up and the cat is either dead or alive. But before that, the quantum state of the cat is dead and alive.

Quarks

The most elementary particle known is a quark. Quarks are unbelievably small. If you know how large a proton is, then you can imagine something less than 1/3 as small. If you don't know how small a proton is, just imagine tens of thousands of them in the period at the end of this sentence. There are six kinds of quarks, each with its own properties and weights they are: "Up, down," "top, bottom," "strange and charmed". Notice the quotation marks. Each set of quotation marks denotes a "generation" of quarks.

As you probably know, quarks compose hadrons: large subatomic particles, including neutrons and protons. Protons are made of two up quarks and one down quark (2/3 + 2/3 - 1/3 = 1); neutrons consist of two downs and one up (Just add up the charges, and you'll find that neutrons are neutral). Other hadrons have varying compositions. This means that up and down quarks are both fairly common. The rarest quark is the top quark, which was discovered in 1995. It is extremely heavy and took years to merely get the data. After it was acquired, physicists and computers had to analyze the reams of resulting information for more than a year.

The origin of the humorous name "quark" is even more interesting. In 1964, Murray Gell-Mann and George Zweig independently deiscovered the quark and Gell-Mann decided on the term quark out of a poem that said "three quarks for Muster Mark." Since then, admittedly humorous names have been heaped upon quarks, ranging from "strange" to "charmed." Above all, these names and the fact that they are the smallest identified piece of matter have made quarks famous.