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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