Saturday, April 21, 2012

What every fantasy writer needs to know about quantum physics

Do fantasy writers need to know anything about quantum physics? At last year's World Fantasy, I attended a panel on magic systems, where the topic of quantum physics came up, and I realized that there are a number of misconceptions about quantum physics that can affect how people write fantasy.

I have a Ph.D. in Electrical Engineering, which in itself doesn’t qualify me to talk about quantum physics, but I did my thesis research on superconducting quantum computation.  In other words, I investigated ways to use superconductors to make a computer based on quantum states.  I was always more of an experimentalist than a theorist, so I’ll be the first to admit that there’s a lot that I don’t understand, but I can at least talk about the basics.

The first thing to realize is that quantum physics is counterintuitive.  It doesn’t work the way we expect, because it doesn’t work the way that we observe the world to be in our daily experience.  The way that we interact with the world is not on a quantum level (at least as far as we can observe it), and therefore quantum physics seems strange and mysterious to us.  Sometimes quantum physics is cited as proof that the universe is magical, or that human consciousness is special, et cetera.  In reality, quantum physics is proof only that the universe is strange and mysterious to our experience.  It may also be magical; human consciousness may be special.  In my admittedly anecdotal experience, different scientists believe different things about the whole metaphysics of the universe, but that is usually based on reasons other than their knowledge of quantum physics.

Rather than focusing on the wave particle duality, or the Heisenberg uncertainty principle, or quantum entanglement, or any of a hundred other strange things about quantum physics, I’ll focus on the fundamental issue that causes so much consternation and so many interpretations.

In quantum physics, it’s possible to have a superposition of states.  For example, imagine that you have two metal plates.  You can place charge on one, which affects charge on the other, and you have a capacitor, which there’s really nothing quantum about.  However, suppose that instead of millions of electrons, you have a charge of one electron, which you place on one plate.  If you place your electron on the first plate, your system is in one state, let’s call it state 0.  If you place your electron on the second plate, your system is in state 1.  So what happens if you place your electron on both plates?

Wait a second, you say.  It’s only one electron, you can only place it on a single plate.  And here’s where quantum physics gets strange.  In quantum physics, you can place your electron on both plates.  In this case, it’s called a superposition of states, because it’s in both state 0 and state 1.  However, when you measure the superposition, it collapses.  It becomes either state 0 or state 1, not both.  Wait, you say again.  If every time you measure it, it’s only in one or the other state, how do you know that it’s ever in a superposition of states?  We can tell because of certain measurements which can characterize the state as a superposition rather than one or the other, but that would require more detail than I can give here.  You can read here for more information. 

The bottom line is that the system is in both states until you measure it, and then it becomes one.  Which one it becomes when measured is a matter of statistics.  The weight of each state in the superposition can vary—it can be equal amounts of state 0 and state 1, mostly 0 with a little 1, or vice versa.  When it is measured, the chance of finding it in one state or another is dependent on the weighting of each state.  If the superposition is weighted to 75% of state 1 and 25% of state 0, there is a 3 in 4 chance of measuring it in state 1 and a 1 in 4 chance of measuring it in state 0.

And this is one of the fundamental issues with quantum physics.  What does it mean that the superposition collapses when you measure it?  There are a number of explanations.

The Copenhagen interpretation says that observation is what causes it to collapse.  This is sometimes interpreted as proof that consciousness is real, that there is something special about people, since their observation causes a real, physical change to a system, but the Copenhagen interpretation was never meant to encompass such philosophical considerations.  Instead, it was proposed as an empirical explanation.  That quantum superpositions collapse when they are observed is what happens, and the reasons behind it are not a concern of the interpretation.  The idea that it’s our conscious knowledge that causes it to collapse is actually called the von Neumann/Wigner interpretation, which doesn’t have that much of a following. The most popular idea as to the reason for the collapse is decoherence, which I’ll discuss more in a moment.

Another interpretation, especially popular among sci fi and fantasy writers, is the “many worlds” interpretation.  This is much more popular in fiction than in physics, although it does have its adherents among physicists.  The many worlds theory states simply that the quantum superposition does not collapse.  It’s still in a superposition, only now, so are you.  There are now two of you, one of which observes the system in state 0, the other of which observes the system in state 1.  Now this concept, of coexisting worlds based on coexisting quantum states is often merged with the idea of alternate dimensions with alternate timelines—despite the fact that there’s no dimensional element to the many worlds theory.  The many worlds would co-exist in the same space and time.  The other issue with many worlds, at least as it corresponds to alternate timelines, is that events which change history are, for the most part, not quantum.  They’re on the large scale compared to quantum physics.  Physicists would say they’re based on classical physics.  It’s hard to see how the state of an atom would affect whether Booth shot Lincoln, for example.  Oh, it’s not impossible that if there was a change in a large enough number of atomic states that would have an effect, but it would have to be a huge number in aggregate, meaning that alternate history events would be very low probability events.  In a many worlds interpretation, that would not mean it didn’t exist, but it would be a very small weight in the superposition.   In an infinite number of worlds, most of them would be indistinguishable from our own.

Adherents of either interpretation are familiar with the concept of decoherence.  That’s the idea that any time you measure a system, you introduce noise into it.  This noise determines how quickly the superposition collapses, or decoheres.  This means that noise can be controlled for, feedback decreased, and coherence times lengthened.  If you can get quantum states to last longer despite interacting with them, you can do things with them.  Now measuring a state without collapsing it may be out of the question, but you can probably manipulate it, which allows you to do quantum computation with it—which was my field.  Decoherence works.  You can test in the lab how long it takes a quantum state to decohere, and increase it or decrease it, according to how much noise you couple into the system.  That doesn’t necessarily mean that there’s nothing to the other interpretations—you still can’t measure a state without collapsing it, which is the question the interpretations were dealing with in the first place—but in recent years, physics has focused on the mechanism causing them to collapse.

What does all this mean for the fantasy writer?  Should he stay away from alternate worlds, decry the existence of consciousness as a force which can influence systems, and the like?  No, of course not.  The fun of fantasy is that you can play with reality, rather than abide by it.  But many writers, when they want their characters to justify the existence of magic or the supernatural or alternate worlds, appeal to quantum physics as proof of the soul or multiple worlds.  These appeals are hardly necessary, and in fact can be quite damaging to the suspension of disbelief for those who know something about quantum physics. 

I used the Wikipedia article on the interpretation of quantum physics to review, and as a starting point, for writing this.

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