Quantum Mechanics and Philosophy III: Implications

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

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

We’ve already discussed some of the experimental phenomena that inspire competing interpretations or theories of what’s going on in the real world during quantum-mechanical experiments. (In brief: Observing microscopic particles seems to either: cause them to randomly take one result or the other; create a branching world for every possible outcome; or require hidden variables and allow for faster-than-light, nonlocal communication. Particles seem not to take one path, not the other, not both, and not neither, and even act as if they “know” when we’re observing them.)

In this final installment of a three-article series, we’ll look in very broad strokes at some of the philosophical implications of these views of quantum mechanics.

I. Logic

Standard logic is two-valued. That just means that each sentence in the logic is true or false, not both, and not neither. ‘My cat’s breath smells like cat food’ is either true or false; it can’t both smell that way and not smell that way at the same time to the same person, and surely it either does or it doesn’t. But as we’ve seen, some interpretations of quantum mechanics might suggest adding in a new value.1 Perhaps Copenhagen-style interpretations indicate that we should have a value of neither—a truth-value “gap”—and Copenhagen and many-worlds interpretations indicate that we should have a value of both: a truth-value “glut.”2 As it happens, there are independent philosophical reasons to explore three- or four-valued logics (and their semantic counterparts: their claims about the metaphysical laws of the real world)3 as well.

Nevertheless, defenders of standard logics can reply that Copenhagen-style interpretations might simply show that it is sometimes false that the particle took one path and false that it took the other; that doesn’t mean it’s actually true and false at the same time that it took a particular one. Many-worlds interpretations might suggest that all possibilities are realized, but not that all possibilities are realized in a single branch of the world, and traditionalists about logic can simply say that their logic is always intended to range across one branch at a time.

II. Time

Most philosophers of time hold that backward causation is impossible.4 That is, nothing that happens now can change or affect what happened in the past; necessarily, if x causes y, then x happened before or at the same time as y, not after. But a set of interesting quantum-mechanical experiments may also throw this view into doubt. Briefly put, these experiments seem to show that a choice about measuring a particle now can affect whether the particle was measured in the past.5

It’s worth noting that the backwards causation we’ve discovered here—if that’s what it really is—only applies to very small systems and very short distances in time. No one can use an experimental apparatus to, e.g., prevent the Kennedy assassination.

III. Modality

Philosophers often interpret modal operators (‘possible,’ ‘impossible,’ ‘necessary,’ ‘contingent’) in terms of possible worlds: ways the world could have been, one of which is the way the world actually is. The dominant view is that non-actual possible worlds aren’t concrete; they are hypothetical, or abstract, or otherwise ‘ersatz.’ But a minority viewpoint has it that every possible world is concrete; they’re just separate from the actual world. A version of the many-worlds view might be thought to lend support to this sort of modal concretism, a theory that is sometimes called ‘modal realism.’6

In reply, it’s not at all clear that the best many-worlds theories commit us to saying that all possibilities are realized. Instead, at best, all physical possibilities in quantum-mechanical experiments are realized. This doesn’t provide a concrete branch for every logically possible outcome of every event ever, which is what modal concretism actually needs.7

IV. Personal Identity and Survival

Returning again to many-worlds theories: If every possibility is realized in some branch of the world, then this may make room for something philosophers have called ‘quantum immortality.’8 Suppose a nuclear bomb goes off right next to you. In principle, according to some many-worlds theories, it’s possible that the energy from the bomb passes through you harmlessly, without disturbing any of the particles in your body. (Possible, but of course extremely unlikely.) So there’s a branch of the world, then, in which you survive the blast, no worse for wear. If so, then perhaps no matter what happens to you, as long as it’s possible for you to survive, then you will survive in some branch or other. Recall that no branch is any more real than any other, so it’s genuinely you who will survive everything that happens to you.

This intriguing possibility is by no means certain, however. One problem is that, as mentioned above, there’s debate about whether every observation leads to branching, or instead, merely observations in experimental conditions.9 Another is that this immortality may be cold comfort, given that in many branches, you will survive, but severely wounded.

V. Determinism and Free Will

Suppose we accept an indeterministic interpretation of quantum mechanics, such as Copenhagen. Does this imply that we have libertarian free-will, since for some of our choices, we really are choosing between multiple, live options?10

At the very least, this indeterminism seems to make more room for free will than we thought there was. But of course, many have argued that indeterministic choices aren’t really “free” anyway; they’re random in an important sense. Maybe those who believe in free will should hope that determinism is true.11 So there is still more work to do to use indeterminism to defend libertarian free will.

VI. A Caveat

As we’ve seen, quantum mechanics may tell us interesting things about traditional philosophical debates. Yet there is still enormous debate about which, if any, of the prevailing views of quantum mechanics is correct. Indeed, the prevailing views are actually incompatible with another very well-tested theory: relativity.12 So the jury is certainly still out on the truth of the prevailing interpretations, and certainly about their philosophical implications. But those implications are clearly important enough to be worth exploring. Further scientific and philosophical work will shed additional light on this exciting topic.


1Cf. Putnam (1968).

2For much more, cf. Wilce (2014).

3Cf. Priest (1997); Beall and Ripley (forthcoming). By ‘semantic counterparts,’ I mean views of the world according to which some sentences really are true and false (or neither) in real life.

4Cf. Dummett (1954).

5E.g. Kim (2000).

6Lewis (1986).

7Vaidman (2014): §6.1.

8Tegmark (1998): §IV.B.

9See, for example, Tegmark’s (2015) remarks on the ideal experimental apparatus.

10Atmanspacher and Rotter (2011). See also Balaguer (2010).

11Cf. Smart (2003): 63. See also Dennett’s (2003) view of freedom here.

12For more, see Monton (2011).


Atmanspacher, Harald and Stefan Rotter. 2011. “On Determinacy or its Absence in the Brain.” In Richard Swinburne (ed.), Free Will and Modern Science (Oxford: Oxford University Press/British Academy), pp. 84-101.

Balaguer, Mark. 2010. Free Will as an Open Scientific Problem. Cambridge, MA: MIT Press.

Beall, JC and D. Ripley, forthcoming, “Non-classical Theories of Truth.” In Michael Glanzberg (ed.), The Oxford Handbook of Truth (Oxford: Oxford University Press).

Dennett, Daniel C. 2003. Freedom Evolves. New York: Viking Penguin.

Dummett, Michael. 1954 “Can an Effect Precede its Cause?” Proceedings of the Aristotelian Society 28 (Supplement): 27–44.

Kim, Yoon-Hoo. et al. 2000 “A Delayed Choice Quantum Eraser.” Physical Review Letters 84: 1–5.

Lewis, David. 1986. On the Plurality of Worlds. Oxford: Blackwell.

Monton, Bradley. 2011. “Prolegomena to any Future Physics-Based Metaphysics.” Oxford Studies in Philosophy of Religion, Volume III (Oxford: Oxford University Press), 142-65.

Priest, Graham. 1997. “Sylvan’s Box: A Short Story and Ten Morals.” Notre Dame Journal of Formal Logic 38: 573-82.

Putnam, Hilary. 1979. “The Logic of Quantum Mechanics.” In Hilary Putnam (ed.), Mathematics, Matter and Method: Philosophical Papers Volume 1 (Cambridge: Cambridge University Press), pp. 174-97.

Smart, J. J. C. 2003. “Atheism and Theism.” In Smart, J. J. C. and Haldane, J. J., Atheism and Theism, Second Edition (Malden, MA: Blackwell), pp. 6-75.

Tegmark, Max. 1998. “The Interpretation of Quantum Mechanics: Many Worlds or Many Words?” Fortschritte der Physik 46: 855-62.

Tegmark, Max. 2015. “Quantum Immortality.” URL = <http://space.mit.edu/home/tegmark/quantum.html#immortality>, accessed 21 February 2015.

Vaidman, Lev. 2014. “Many-Worlds Interpretations of Quantum Mechanics.” In Edward N. Zalta (ed.), The Stanford Encyclopedia of Philosophy (Winter 2014 edition), URL = <http://plato.stanford.edu/archives/win2014/entries/qm-manyworlds/>.

Wilce, Alexander. 2014. “Quantum Logic and Probability.” In Edward N. Zalta (ed.), The Stanford Encyclopedia of Philosophy, (Winter 2014 edition), URL = <http://plato.stanford.edu/archives/win2014/entries/qt-quantlog/>.

Tom is a visiting assistant 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. Website: http://colorado.academia.edu/ThomasMetcalf