Quantum Physics

When I was applying to colleges, one of the schools I was applying to (namely, the University of Chicago) had a rather odd question: Discuss the possibility of multiple worlds, from the point of view any discipline you desire (literature, philosophy, science, etc.). Naturally, I chose to discuss parallel universes from a scientific perspective. For some reason, though, I decided that to properly discuss the many-worlds interpretation of quantum physics, I would need to explain quantum physics itself from the ground up. What you see is the explanation that followed.

Quantum physics arose out of a simple enough question asked by scientists in the 18th century: Is light a particle or a wave? Newton was certain that light was made up of a stream of particles, but later scientists believed that it was a wave. Early in the 19th century, a physicist named Thomas Young devised an experiment to prove once and for all whether light was a particle or a wave. He took a metal sheet, and cut two slits into it. Then, he shined a light source at the two slits, and observed what pattern formed on the photograph plate behind them. If light were a particle, then there should be two stripes: the light which passed through the left slit would go straight forward and continue to form one strip, and the light from the right slit would form a second strip.

However, if light were a wave, a more complex pattern would emerge. When the light reaches the first slit, it will emerge in outgoing circular waves. These waves will be made up of peaks and troughs. When the light goes through the second slit, it will form a second set of circular waves. When these waves interact with one another, they form an interference pattern: where two peaks or two troughs meet, the form an especially high peak or especially deep trough; when a peak meets a trough, they cancel one another out and leave nothing. So, if light were a wave, the pattern on the photographic plate would be a series of light and dark stripes, corresponding to the peaks and troughs of the interference pattern.

Anyway, when Young conducted this experiment, he found that light appeared to be a wave. This view remained unchanged until Einstein, who discovered the photoelectric effect, which (for reasons which need not be discussed here) proved that light must be made up of particles. So how does one account for this particle-wave duality? In 1926, German physicist Max Born, with the help of some colleagues, came up with a solution: light travels in "probability waves." Wherever the peaks were represented where the photon was "likely" to be; the troughs were where the photon was "unlikely" to be. So, each "probability wave" would pass through both slits, and when it hit the photographic plate, the photon would find its location based on the probability of it being at any one location. This view may sound unusual, to say the least, but many experiments have confirmed its conclusions. What's more troubling is that it seems as though all matter is also made up of probability waves: for example, sending a stream of electrons through the double-slit experiment comes up with the same conclusion.

An example might make it clearer how such quantum probabilities can affect normal-scale objects. Say that you have a box which contains a radioactive atom, a Geiger counter, a vial of poison, and a cat. When the Geiger counter detects that the atom has decayed, it will break open the vial of poison and kill the cat. However, the decay of radioactive atoms is governed by quantum physics, and so one cannot say exactly when this will take place. One cannot say for certain whether or not the cat is alive. One can only say the probability that the cat has been killed. As it so happens, even that is not a sufficiently accurate description. If, after a given time, there is a 30% chance that the cat has been killed, one would say that the cat is 30% dead. This isn't just making an idle statement about the cat's chances of being dead; the cat is literally 30% dead. This state of being in two different positions at once is known as a quantum superposition.

Of course, when you open the box, you won't find the cat to be both dead and alive; it will be one or the other. Opening up the box "breaks" the quantum superposition, and forces the cat to "choose" one state or the other. Physicists aren't entirely certain how a quantum superposition is broken. Most are uncomfortable with saying, "Whenever a sentient being observes a quantum superposition, it breaks," because this wouldn't work in places without sentient beings. One theory is that when a quantum superposition takes place, it forms an infinite set of new universes, and that the distribution of living cats versus dead cats is determined by the probability in the superposition. When the observer opens up the box, all she does is check which universe she is in; nothing actually "changes" at this time. If this theory were true, than every time a quantum superposition breaks down, a host of new universes is formed. If such events happen frequently, then there must be an infinite number of universes, in which literally everything imaginable has happened. For any mistake you made in your life, there is another universe in which you did not make that mistake. Any regret you might have, another "you" does not have. It's somewhat comforting, to think that no matter how much you mess up, there's a version of you who did not – and a version of you who made a far more grievous error.