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quantum system. If such a theory is true, it would have to be nonlocal—
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parts of the system would have to directly interact with parts at other loca-
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tions in space.
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An even more radical approach is to simply deny the existence of an
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underlying reality altogether. This would be an antirealist approach to
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quantum mechanics, since it treats the theory as merely a bookkeeping de-
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vice for predicting the outcomes of future experiments. If you ask an anti-
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realist what aspect of the current universe that knowledge is about, they
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will tell you that it’s not a sensible question to ask. There is, in this view, no
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underlying “stuff” that is being described by quantum mechanics; all we are
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ever allowed to talk about is the outcomes of experimental measurements.
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Antirealism is a pretty dramatic step to take. It seems to have been ad-
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vocated, however, by no less of an authority than Niels Bohr, the grandfa-
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ther of quantum mechanics. His views were described as “There is no
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quantum world. There is only an abstract physical description. It is wrong
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to think that the task of physics is to find out how nature is. Physics con-
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cerns what we can say about nature.”
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Perhaps the biggest problem with antirealism is that it’s hard to see how
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it could be a position that one holds with perfect consistency. It’s one thing
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to say that our understanding of nature is incomplete; it’s another thing
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entirely to say that there is no such thing as nature. For one thing, who is it
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that’s doing the saying? Even Bohr, in the quote above, speaks of what we
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can say “about nature.” That would seem to imply that there’s something
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called “nature” that we can say things about.
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Fortunately, we have not yet exhausted our possibilities. The simplest pos-
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sibility is that the quantum wave function isn’t a bookkeeping device at all,
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nor is it one of many kinds of quantum variables; the wave function simply
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represents reality directly. Just as Newton or Laplace would have thought
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of the world as a set of positions and velocities of particles, the modern
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quantum theorist can think of the world as a wave function, full stop.
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The difficulty with this robust brand of straightforward quantum real-
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ism is the measurement problem. If everything is just wave function, what
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makes states “collapse,” and why is the act of observation so important?
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A resolution was suggested in the 1950s by a young physicist named
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Hugh Everett III. He proposed that there is only one piece of quantum
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ontology— the wave function— and only one way it ever evolves— via the
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Schrödinger equation. There are no collapses, no fundamental division
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between system and observer, no special role for observation at all. Everett
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proclaimed that quantum mechanics fits perfectly comfortably into a de-
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terministic Laplacian view of the world.
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But if that’s right, why does it seem to us that wave functions collapse
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when we observe them? The trick, in modern language, can be traced to a
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feature of quantum mechanics called entanglement.
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In classical mechanics, we can think of every different piece of the world
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as having its own state. The Earth is moving around the sun with a particu-
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lar position and velocity, and Mars has a position and velocity of its own.
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Quantum mechanics tells a different story. There is not a wave function for
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the Earth, another one for Mars, and so on through all of space. There is
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only one wave function for the entire universe at once— what we call, with
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no hint of modesty, the “wave function of the universe.”
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A wave function is simply a number we assign to every possible measure-
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ment outcome, like the position of a particle, such that the number tells us
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the probability of obtaining that outcome. The probability is given by the
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wave function squared; that’s the famous Born rule, after German physicist
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Max Born. So the wave function of the universe assigns a number to every
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possible way that objects in the universe could be distributed through
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space. There’s one number for “the Earth is here, and Mars is over there,”
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and another number for “the Earth is at this other place, and Mars is yet
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somewhere else,” and so on.
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The state of Earth can therefore be entangled with the state of Mars. For
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big macroscopic things like planets this possibility isn’t realized in a de-
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monstrable way, but for tiny things like elementary particles it happens all
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the time. Say we have two particles, Alice and Bob, each of which could be
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spinning either clockwise or counterclockwise. The wave function of the
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universe could assign a 50 percent probability to Alice spinning clockwise
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and Bob counterclockwise, and another 50 percent to Alice spinning coun-
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terclockwise and Bob clockwise. We have no idea what answer we would
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get were we to measure the spin of either particle; but we know that once
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we measure one of them, the other is definitely spinning the other way.
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They are entangled with each other.
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Everett says that we should take the formalism of quantum mechanics
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at face value. Not only is the system you’re going to observe described by a
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wave function, but you are described by a wave function yourself. That
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means that you can be in a superposition. When you make a measurement
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of a particle to see whether it’s spinning clockwise or counterclockwise,
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Everett suggests, the wave function doesn’t collapse into one possibility or
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the other. It evolves smoothly into an entangled superposition, part of
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which has “the particle is spinning clockwise” and “you saw the particle
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spinning clockwise,” while the other of which has “the particle is spinning
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counterclockwise” and “you saw the particle spinning counterclockwise.”
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Both parts of the superposition actually exist, and they continue to exist
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and evolve as the Schrödinger equation demands.
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At last, then, we have a candidate for a final answer to the critical onto-
10
logical question “What is the world, really?” It is a quantum wave function.
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At least until a better theory comes along.
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•
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Everett’s bare- bones approach to quantum mechanics— just wave functions
15
and smooth evolution, no new variables or unpredictable collapses or deni-
16
als of objective reality— has been dubbed the Many- Worlds Interpretation.
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The two parts of the wave function of the universe, one in which you saw
18
the particle spinning clockwise and the other in which you saw it spinning
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counterclockwise, subsequently evolve completely independently of each
20
other. There is no future communication or interference between them.
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That’s because you and the particle become entangled with the rest of
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the universe, in a process known as decoherence. The different parts of the
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wave function are different “branches,” so it’s convenient to say that they
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describe different worlds. (There’s still one “world” in the sense of “the
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natural world,” described by the wave function of the universe, but there are
26
many different branches of that wave function, and they evolve indepen-
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dently, so we call them “worlds.” Our language hasn’t yet caught up to our
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physics.)
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There’s a lot to love about the Everett/ Many- Worlds approach to quan-
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tum mechanics. It is lean and mean, ontologically speaking; there’s just the
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quantum state and its single evolution equation. It’s perfectly deterministic,
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even though individual observers can’t tell which world they are in before
33
they actually look at it, so there is necessarily some probabilistic component
34
when it comes to people making predictions. And there’s no difficulty in
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explaining things like the measurement process, or any need to invoke
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conscious observers to carry out such measurements. Everything is just a
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wave function, and all wave functions evolve in the same way.
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There are, of course, an awful lot of universes.
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Many people object to Many- Worlds because they simply don’t like the
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idea of all of those universes out there. Especially unobservable universes—
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the theory predicts them, but there’s no practical way of ever seeing them.
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This is not a very thoughtful objection. If our best theory predicts that
08
something is true, we should place a relatively high Bayesian credence that
09
it actually is true, until a better theory comes along. If you have some vis-
10
ceral or a priori bad feeling about multiple universes, then by all means
11
work on better formulations of quantum mechanics. But a bad feeling is
12
not a principled stance.
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The secret to making your peace with Many- Worlds is to appreciate that
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the approach doesn’t start with the formalism of quantum mechanics and
15
add in a preposterously big multiverse. All those other universes are already
16
there, at least potentially, in the formalism. Quantum mechanics describes
17
individual objects as being in superpositions of different measurement out-
18
comes. The wave function of the universe automatically includes the pos-
19
sibility that the whole universe is in such a superposition, which we then
20
choose to talk about as “multiple worlds.” It’s all the other versions of quan-
21
tum mechanics that have to work to get rid of the extra worlds— by chang-
22
ing the dynamics, or adding in new physical variables, or denying the
23
existence of reality itself. But you gain nothing in explanatory or predictive
24
power, and have unnecessarily made a simple framework more elaborate—
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at least as Everettians see things.
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Which isn’t to say that there aren’t very good reasons to be concerned
27
about Everettian quantum mechanics. According to Everett, the branching
28
of the wave function into different parallel worlds isn’t an objective feature;
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it’s simply a convenient way of talking about the underlying reality. But
30
what exactly determines the best way of drawing the line between uni-
31
verses? Why do we see the emergence of a reality that is well approximated
32
by the rules of classical mechanics? These are perfectly respectable
33
questions— though ones that seem quite answerable to partisans of Many-
34
Worlds.
35S
There are two important things to take away from this discussion, as far
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as the big picture is concerned. One is that, while we don’t have a finished
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understanding of quantum mechanics at a fundamental level, there is noth-
01
ing we know about it that necessarily invalidates determinism (the future
02
follows uniquely from the present), realism (there is an objective real world),
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or physicalism (the world is purely physical). All of these features of the
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Newtonian/ Laplacian clockwork universe can easily still hold true in
05
quantum mechanics— but we don’t know for sure.
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The other important takeaway is a feature common to all interpreta-
07
tions of quantum mechanics: what we see when we look at the world is
08
quite different from how we des
cribe the world when we’re not looking at
09
it. As human knowledge has progressed over the centuries, we have occa-
10
sionally been forced to dramatically rearrange our planets of belief to ac-
11
commodate a new picture of the physical universe, and quantum mechanics
12
certainly qualifies as that. In a sense it is the ultimate unification: not only
13
does the deepest layer of reality not consist of things like “oceans” and
14
“mountains”; it doesn’t even consist of things like “electrons” and “pho-
15
tons.” It’s just the quantum wave function. Everything else is a convenient
16
way of talking.
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The Core Theory
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Quantum mechanics is, as far as we currently know, the way the
universe works. But quantum mechanics isn’t a specific theory of
the world; it’s a framework within which particular theories can
be constructed. Just as classical mechanics includes the theory
18
of planets moving around the sun, or the theory of electricity and magne-
19
tism, or even Einstein’s theory of general relativity, there are an enormous
20
number of particular physical models that qualify as “ quantum-
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mechanical.” If we want to know how the world really works, we need to
22
ask, “The quantum- mechanical theory of what?”
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Your first guess might be “particles and forces.” When we talk about
24
atoms, for example, the central nucleus is a collection of particles called
25
protons and neutrons, while orbiting around the nucleus are particles called 26
electrons. The protons and neutrons are bound to each other by a force (the
27
nuclear force), and the electrons are bound to the nucleus by a different
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force (electromagnetism), and everything pulls toward everything else be-
29
cause of yet another force (gravitation). Particles and forces are reasonable
The Big Picture Page 30