You hear about parallel universes all the time in science fiction (see Fringe, probably a billion episodes of Star Trek, Stephen King’s Dark Tower series, etc.). But did you know that scientists take parallel universes seriously and consider them possible. There are several types of possible parallel universes. This is the second post in a series of five posts. The first can be found here.
Next up: quantum theory. Quantum theory is the crowning jewel of science in the 20th century. It has been proven incontrovertibly correct innumerable times. As many of you many know, quantum theory describes the behavior of particles at the atomic and subatomic level. Basically, quantum particles act both like waves and like particles. Sounds basic enough, but the results of the equations are just bizarre. For example, there is the classic double-slit experiment, as told by this creepy floating head:
Despite the theory being correct, there are lots of different ways of interpreting its results. Interpretation is needed because of the Heisenberg uncertainty principle. This idea is that, because particles are both waves and matter, the more accurately measure one aspect of a particle, such as its speed, the less accurately you can measure other aspects of that particle, such as its position. It seems bizarre, but this idea has been experimentally confirmed over and over again. To work around this, quantum mechanics uses a type of equation that will give you the probability that a particle has a particular speed, position, etc. That’s the best that the physical world will let us do to mathematically describe these systems. These probability equations are called a waveforms.
So, let’s say you have a waveform for a particular system, say the electrons in a helium atom. The waveform then describes the probabilities of all the properties of those electrons. Suppose you want to measure one property of the electron, its position. What happens to the other possible positions of that electron? The waveform says there are other possible positions, but your measurement says it is in one particular place. Here is where interpretations of quantum theory come in.
The most common interpretation is the Copenhagen interpretation. It holds that the very act of measuring a property, like position, causes a waveform collapse: the universe decides that this will be the position and no other positions exist. The waveform equation poofs out of existence. In this interpretation, the waveform is more like a guide rather than an actual description of reality, so the collapse of the waveform is not a big deal. Once you know that particular property, the other possible states of the property a no longer relevant.
Erwin Schrödinger did not like the Copenhagen interpretation. When scaled up, he thought the idea of the waveform collapse did not make sense. To illustrate, he created the now-famous thought experiment of a cat in a box with bottle of poison, a hammer and a radiation detector. If a particle decays, the radiation detector will sense it and trigger the hammer, which will break open the bottle of poison and kill the cat. Supposing the cat is in the box for an hour and you open the box at the end of the hour to “measure” if the cat is alive or dead. Until you measure whether the cat is dead or alive, there is a waveform to determine the probability of the cat being dead or alive. The Copenhagen interpretation would hold that, until measured, the cat is both dead and alive at the same time. This is what he thought was nonsensical: the cat cannot be both alive and dead at the same time and it cannot alternate between the two.
Another interpretation attempts to work with the dead-and-alive cat paradox: the many worlds interpretation. Instead of a waveform collapse, you have a decoherence. The waveform is universal and governs the system all the time. However, as soon as a measurement is made and it becomes incompatible with other probabilities in the waveform, the universe branches into two different parallel universes: it decoheres. The universe is like a giant tree in which every possible outcome occurs, each in its own separate universe. All possibilities exist, it’s just that our particular universe traces out one branch on the tree and other possible universes are constantly branching off from ours. In terms of the cat, the “measurement” of the cat’s life branches off into two universes: one where the cat is alive and one where it is dead. In terms of fiction, all possible alternative histories exist in some other universe somewhere. Somewhere, there is a universe where The Man in the High Castle is history, not fiction. And another universe where steampunk is history, not a modern lifestyle. This is a Level III type of parallel universe on Max Tegmark’s taxonomy.
However, this is only just one of several disputed interpretations and there is not yet a way to test whether or not this interpretation is more valid than any of the others.