Quantum Mechanics & Philosophy II: Measurement and Interpretations

Author: Thomas Metcalf
Category: Philosophy of Science
Word Count: 1000

Editor’s Note: This essay is the second in a series authored by Tom on the topic of quantum mechanics and philosophy. Read the first essay here and the third essay here.

I. Measurement

The story in the previous article in this series corresponds to real experiments about properties of microscopic particles.1 Recall that these experiments seem to show that particles can be partly in one position and partly in others, and that measuring their positions seems to change other properties about them. Thus there seems to be something very strange about measuring the properties of these particles.

Let’s talk about what happens, physically, when someone makes a measurement. Suppose you’ve flipped a coin at t1 and haven’t looked at the result yet; it’s apparently in a superposition2 of Heads and Tails.3 You’ll look at the result at t2. Here’s what an analogue of the Schrödinger equation would say about what happens:

t1: The coin is in a superposition: a combination of 50%-Heads and 50%-Tails. Then …

t2You are in a superposition: a combination of 50%-observing-Heads and 50%-observing-Tails.

Of course, no one has ever seemed to find herself in a superposition of two observations.4

It turns out that there are roughly three5 things we could say as the physical story about what happens when you make the observation. When you look at the coin …

(Copenhagen) … the superposition “collapses” (indeterministically!)6 into 100%-Heads or 100%-Tails, but not both.7

(Many-Worlds) … the universe branches into:

U1:      100%-you observes 100%-Heads.

U2:      100%-you observes 100%-Tails.8

(Bohm) … you observe what was true all along: the coin was 100%-Heads (or 100%-Tails) even before you looked at it.9

Again, the Schrödinger equation predicts that measurement will not collapse a superposition; it predicts that the observer will now be in a superposition. But we don’t find ourselves to be in superpositions. So what is measurement, and does it really violate the best-confirmed equation we’ve ever used?

II. Interpretations

There are several ways of interpreting measurement itself, corresponding to several hypotheses about what’s actually going on in the physical world with these particles.

A. Copenhagen

When we look for particles, we don’t seem to find them in superpositions. But of course when we don’t look for them, they seem to stay in superpositions. So there must be something special about measurement; it must cause superposed things to stop being superposed. Copenhagen-theorists say that observation “collapses” superpositions, and as noted, this collapse is indeterministic; nothing predicts or can predict whether the particle will be found here or there.10

A nice thing about this interpretation is that it seems very much like classical physics. There are particles, and they might do strange things when we’re not looking for them, but when we do look for them, they “become” classical: they just are in a particular place. The coin just is ‘Heads’ or ‘Tails.’

A not-so-nice thing about this interpretation is that there is simply no direct experimental evidence that collapse ever actually happens.11 Indeed, collapse is incompatible with the Schrödinger equation. Copenhagen-theorists conclude that collapse must have happened (since otherwise, we’d see the particles in superpositions), but we actually don’t have a mathematical or a physical story that tells us how or why it happens.

This interpretation also makes measurement mysterious. How does the coin “know” I’m looking at it? Could a cat’s observation “cause” this collapse? A bacterium’s?12 It would be better overall if we didn’t have to say that observation itself causes physical changes in the thing observed.

B. Many-Worlds

Roughly speaking, the Many-Worlds interpretation says that superpositions remain after observation. When you look at the coin, the world evolves into a superposition of you observing ‘Heads’ and you observing ‘Tails.’

A nice thing about this interpretation is that the mathematical side is completely straightforward.13 The best-confirmed equation we have turns out to be true. Measurement and observation aren’t really “special”; they’re simply further ways of the world evolving. Nothing collapses.

A not-so-nice thing about this interpretation is that it’s incompatible with our experience unless we say that the universe itself is branching into an outcome for every observation. We don’t ever see superpositions, so it must be that each branch of the universe gets its own “outcome” of the observation. This conclusion seems very strange to many people.

As it happens, this interpretation also makes probability very mysterious.14

C. Bohm

The third interpretation to consider is the most “classical” of the lot. According to David Bohm and his followers, the coin was definitely ‘Heads’ or definitely ‘Tails’ before you measured it. The reason is that in addition to the coin, there was also another thing: a sort of guiding probability-wave that caused the coin to land on ‘Heads’ or ‘Tails.’ The world evolves deterministically, and superpositions, in a sense, aren’t real.15 Particles just seem to behave in a “superposition” way because we don’t have a way of monitoring everything about them.

A nice thing about this interpretation is, as mentioned, that it’s very classical. Most of the mystery in quantum mechanics evaporates. There’s nothing special about observation. The Schrödinger equation merely tells us how to predict how deterministic systems evolve.

A not-so-nice thing about this interpretation is that in the details, it turns out to need nonlocality.16 Basically, that means that things can affect each other at faster than the speed of light, even if they’re nowhere near each other. I observe ‘Heads’ on a coin here, and instantly, somehow, a coin ten light years away “becomes” ‘Tails.’ And there’s no obvious particle or mechanism to convey that causal signal, if it is a causal signal.

Another thing some people don’t like about this interpretation is that it seems to require the existence of an object we have no way of empirically detecting: the “pilot wave” that guides the particles to do what they do.17

IV. Next Steps

We don’t have any empirical tests that can easily decide between these and other interpretations. We might never.18 So again, the choice between interpretations is at least partly a philosophical choice.

It turns out that the choice between interpretations also has many other implications for traditional philosophical questions. The last article in this series will take a look at some of those questions.


1Usually photons and electrons, and most commonly, spin-properties; cf. Albert 1992: 1, n. 1.

2In the “party” metaphor, this is like watching a guest arrive (at t1) through the front door before you’ve seen which item they brought, and then looking (at t2) at which item they brought.

3Coins in the real world don’t actually end up in superpositions. The reason is something called ‘decoherence’: big objects such as coins interact with their environments in lots of ways, constantly, enough to push them out of superpositions. On this, see Polkinghorne 2002: 43-44 and Ghirardi 2014: § 5. However, our best physics says that in principle, a coin could be placed in a superposition of ‘Heads’ and ‘Tails.’ See, e.g., O’Connell et al. 2010.

4What would it look like, to the observer? I have no idea. If the coin is an American quarter, would you be seeing 50% of George Washington’s face and 50% of an eagle? Would it look like a double-exposed photograph? Cf. Albert 1992: 112 ff. and Greene 2011: 207-08.

5There are different ways of dividing things up, but this sort of division is one of the most common in introductory-level works. See, e.g., Polkinghorne 2002: 46-56.

6Indeterminism can be construed as the thesis that a particular state of the universe does not physically entail any future state. See, e.g., Hoefer 2014; Haramia 2014: § 3; and Nagashima 2014: § 2. On the indeterminism in the quantum world, see Greene 2011: 191-192.

7This is sometimes called the ‘Copenhagen’ interpretation, after its main proponent, Niels Bohr (Bohr 1987a; Bohr 1987b; Bohr 1987c; Greene 2002: 208-09). Cf. Albert 1992: 80 ff. Notably, no one has ever found any direct experimental evidence that collapse of this sort actually happens (Albert 1992: 110-11).

8This is sometimes called the ‘Many-Worlds,’ ‘Everett,’ or ‘Everett-De Witt’ interpretation, after its main proponents, Hugh Everett and Brice De Witt (Everett 1957; De Witt 1970; Albert 1992: 112-13).

9This is sometimes called the ‘Bohm’ interpretation, after its main proponent, David Bohm (Bohm and Hiley 1993; Albert 1992: ch. 7).

10Albert 1992: 36; Polkinghorne 2002: 24-25.

11Albert 1992: 110-11. It’s possible to chart the evolution of a system that looks the way it would if the wavefunction is collapsing, but this is not an observation of collapse; see Murch et al. 2013. See also Greene 2011: 201-02 on the incompatibility of this interpretation with the mathematical formalism.

12For some discussion of measurement and this “macro-objectification problem,” see especially Ghirardi 2014: § 3. See also Albert 1992: 79 on the “measurement problem” and Greene 2011: 202 for the “bacterium” example.

13Albert 1992: 112-13; Greene 2011: 203-09 and 212.

14Suppose a certain quantum-mechanical process is known (empirically) to be 10% likely to result in outcome X and 90% likely to result in outcome Y. Now we run the process 1,000 times, and sure enough, about 100 times, the outcome is X, and about 900 times, the outcome is Y. But according to Many-Worlds, each of those 1,000 iterations caused the universe to branch into two universes: one for X and one for Y. Why, then, did we not observe about 500 Xs and about 500 Ys? See Green 2011: 228-37 and Greaves 2007.

15 Polkinghorne 2002: 53-54.

16Bell 1964; Albert 1992: 155 ff.

17Polkinghorne 2002: 54-55.

18Polkinghorne 2002: 55-56; Ghirardi 2014: § 13.


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Related Essays

Quantum Mechanics and Philosophy I: The Superposition of Paths by Thomas Metcalf

Quantum Mechanics and Philosophy III: Implications by Thomas Metcalf

About the Author

Tom Metcalf is an associate professor at Spring Hill College in Mobile, AL. He received his PhD in philosophy from the University of Colorado, Boulder. He specializes in ethics, metaethics, epistemology, and the philosophy of religion. Tom has two cats whose names are Hesperus and Phosphorus. http://shc.academia.edu/ThomasMetcalf

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