Quantum Physics for the Simple-Minded

By Ari Gilder

 

          A little more than one hundred years has passed since the first notion of quantum physics was introduced. Before the end of the 19th century, physicists believed that they had discovered all there was to be found in the realm of physics, and the only thing that remained was to carry out the accuracy of numbers to a further decimal place. However, there were some oddities that remained unsolved. For example, the physics of the late 19th century had not predicted the correct light spectrum emitted by a glowing-hot object. Rather, when you looked at the heating coils inside a toaster, it predicted an “ultraviolet catastrophe” – it should emit ultraviolet light and x-rays enough to blind you.

          In 1900, Max Planck had succeeded in deriving the correct spectrum that is emitted. However, this involved making a strange assumption: that energy is only emitted in finite bundles, or “quanta.” This assumption was once again used in 1913 by Niels Bohr when he explained the reason why the electron in an atom of hydrogen didn’t spiral into the nucleus. Bohr stated that electrons were confined to certain fixed orbits, and the absorption of energy would raise the electron to a higher orbit. Consequently, when an electron falls back to its original orbit, it emits energy in the form of a photon, or particle of light.

          In 1923, Louis de Broglie proposed an explanation for this strange idea of quanta in his doctoral thesis. He said that electrons and other particles act like waves with certain frequencies. Two years later, Erwin Schrödinger went on to develop equations for these waves, which later on became the basis for many scientific advancements and modern physics today. However, Schrödinger’s equation described a particular unknown quantity, a “wave function.” Max Born had thought to explain this wave function in terms of probability. For example, you could predict the probability of finding an electron in a given region of space. However, this produced an underlying notion of randomness, to which Albert Einstein remarked, “I can’t believe that God plays dice.”

          Schrödinger himself was puzzled by this. His equation described certain states or positions that were combinations of 2 or more states, or superpositions. For example, an electron could be in two places at once. Since large objects are made up of electrons and atoms, they too should be subject to these superpositions.

          Schrödinger’s famous example of this is known as “Schrödinger’s Cat.” It is an experiment (a fictional one, so worry not animal-rights activists) in which a cat is in a sealed environment with a device that will release poison gas and kill the cat if a certain radioactive atom decays. Since the radioactive atom enters a superposition of decaying and not decaying, the cat should also be in a superposition of being dead and alive at the same time.

          Another example is if you balance a card perfectly on its edge, classical physics dictates that it should remain balanced forever. However, according to Schrödinger’s equation, the card should fall down – in both directions. If you were to perform this experiment, you would find that indeed classical physics is wrong, and the card does fall down shortly – but apparently only in one direction, face up or face down, contradicting the predicted superposition.

          The Copenhagen interpretation of quantum physics offers one possible explanation of this. As long as the card is unobserved, it continues to evolve smoothly and obeying the superposition of Schrödinger’s equation. However, by observing the card, it causes a “collapse” of the wave function of the superposition, and nature selects one position at random based on its probability, which is what you see.

          This had satisfied most physicists at them time. However, in the 1950s, a Princeton student named Hugh Everett III proposed an alternate approach, thus resolving the “collapse” of the wave function (which violates Schrödinger’s equation). Everett believed that the entire universe itself is described by one very complicated wave equation. In this way, there really is no collapse of any wave function, and the superpositions indeed do exist in large objects. Even more, your mind would enter a superposition of seeing the card face up and face down. This is because each possible state in the superposition perceives its own separate world. This comprises the many-worlds theory. Essentially, for every possible outcome, there is a separate parallel world for every possible outcome, existing at the same time as this one. However, each of these parallel worlds, or perceptions of different states, is just as real as the one we are in right now.

A result of the many-worlds theory is that things can be in two places at once – which has been proven experimentally with electrons and photons, as well as large 60-atom carbon molecules. Scientists in Vienna have even begun to discuss trying this with a virus.

          However, Everett’s theory left some questions to be answered. The first is if superpositions really do exist, why don’t we see them in day-to-day life? In 1970, H. Dieter Zeh proposed a solution to this known as decoherence (a superposition is called coherent, and thus removing the superposition would decohere it). Since this quantum card of ours is constantly being bumped into by stray air particles or photons, this interaction with the environment causes the superposition to be reduced to such a minimal probability that it virtually is inexistent. Decoherence is similar to the Copenhagen interpretation in this way, except that there is no ultimate collapse ever, just a large reduction. Also, decoherence does not require that the card be observed per se, but any interaction with the surrounding environment will produce this reduction of the coherent or superposition state.

          Even if the card were to be observed in a dark vacuum chamber at absolute zero (i.e. no stray photons, air molecules or heat), the act of observing the card would trigger at least one neuron in your brain to fire, which is a sufficient interaction with the environment for the card to decohere in about 10-20 seconds, which is not enough time for our brains to perceive the card being both face up and face down.

          The second question about the many-worlds theory that decoherence resolves is why these many worlds divide up exactly along the distinct states we know as either “face up” or “face down”? This is because the “face up” and “face down” states are the states that produce the maximum reduction of the superposition without a total collapse. The interactions with the surrounding environment don’t interfere with these distinct states, but they force any combinations of states into either “face up” or “face down.”

          Quantum mechanics is currently being researched to produce the most powerful computer ever, a quantum computer. This would make use of superpositions instead of traditionally using just 0s or 1s in current computer processors. However, a lot of research is also being put into resolving the conflicts between quantum physics and Einsteinian relativity in the ultimate quest for a “Grand Unification Theory” which will (supposedly) be a purely mathematical theory that can predict anything and everything in the universe.[1]

 


[1] Source used:

Wheeler, John Archibald and Tegmark, Max. “100 Years of Quantum Mysteries.” Scientific Amercian. pp. 68-75, February 2001 issue.