Hans Halvorson Physics, Logic, Philosophy

Philosophy of Physics

Fall 2024 seminar, meeting Tuesday afternoons at 12:30pm.

In November, we have several sessions with Carlo Rovelli, based on this material: Princeton seminars. Leading up to November, we will focus primarily on topics that prepare us for the sessions with Rovelli — which will be a lot of ground to cover, since his lectures draw on many branches of physics including relativity theory, quantum mechanics, and statistical mechanics.

You’re encouraged to read Rovelli’s notes in advance and form your own opinion about what kind of philosophical view he takes. He is somewhat difficult to place in the landscape of recent philosophy! He’s an avowed naturalist, but resists simplistic reductions, e.g. of value to fact. He’s a scientific realist, but doesn’t think that physics describes “what exists and how it moves”.1 This is an interesting combination of views, and places Rovelli in relief against some of the main figures in recent philosophy. It also places Rovelli in an interesting line of natural philosophers including Aristotle, Descartes, Leibniz, Newton, and Einstein.

Here are some of the main topics that we plan to cover:

Spacetime

Substantivalism versus relationalism

Substantivalism is the view that space (or spacetime) exists independently of the matter it contains. At least that’s the view to the first approximation. There are many ways in which that definition is unsatisfactory, and it needs to be refined. For one of the most recent statements of the problem of formulation (and a proposal for how to fix it), see (North 2021).

HH: in my opinion, the real question is whether the current formulation of our spacetime theories (viz. GTR) is satisfactory, or whether we can do better, e.g. by not having a spacetime manifold anywhere in sight. (This is related to the way that Hartry Field formulates the substantivalist-relationalist debate, see (Field 1984).)

Relationalism is basically the negation of substantivalism, but more specifically, the belief that space (or spacetime) is nothing “over and above” relations between material objects. Rovelli is an outspoken relationalist about spacetime.

For historical background, we need to look at the arc from Descartes to Newton, culminating in the Leibniz-Clarke correspondence, and then in Kant’s chirality argument. (But beware, the latter changes the nature of the debate from purely metaphysical to epistemological/semantic!) Fortunately for us, we have some of the top people in the world on these topics in our department: Dan Garber on Descartes and Leibniz and Des Hogan on Kant.

  • Garber, D. (1992). Descartes’ metaphysical physics. University of Chicago Press.
  • Hogan, D. (2021). Handedness, idealism, and freedom. Philosophical Review, 130(3), 385–449. http://dx.doi.org/10.1215/00318108-8998838
  • Huggett, N. (2008). Motion and relativity before Newton. pdf

The next question is whether the discovery of the general theory of relativity (1915) changes the debate about substantivalism and relationalism. Our main source here is:

  • Earman, J. (1989). World enough and spacetime. MIT Press.

We will use the “hole argument” to focus our discussions.

  • Earman, J., & Norton, J. (1987). What price spacetime substantivalism? the hole story. The British journal for the philosophy of science, 38(4), 515–525. http://dx.doi.org/10.1093/bjps/38.4.515
  • Weatherall, J. O. (2018). Regarding the ‘hole argument’. The British Journal for the Philosophy of Science, 69 http://dx.doi.org/10.1093/bjps/axw012 (There are quite a few responses in the literature; see citations in Google scholar.)
  • Halvorson, H., & Manchak, J. B. (2022). Closing the hole argument. The British Journal for the Philosophy of Science http://dx.doi.org/10.1086/719193
  • Dewar, N. (2024). The hole argument and determinism(s). philsci:23302

The discussion of the hole argument raises questions about the meaning of determinism, which is itself a rather large subject. See e.g. (Earman 1986) and (Belot 1995).

Do shifts generate new possibilities?

  • Belot, G. (2018). Fifty million Elvis fans can’t be wrong. No^{u}s, 946–981. http://dx.doi.org/10.1111/nous.12200

Absolute objects

  • Earman, World Enough and Spacetime, Chap 3
  • Jacobs and Read, “Absolute representations and modern physics” philsci:23520
  • Nijenhuis, Theory of the Geometric Object
  • Friedman, Foundations of Spacetime Theories, p 60

Substantivalism becomes Platonism?

  • Field, “Can we dispense with spacetime?”

Eternalism versus presentism

Many philosophers have the impression that relativity theory — with its erasure of the distinction between space and time — tips the balance in favor of eternalism. Interestingly, Rovelli disagrees — although he does think that relativity theory rules out presentism.

  • Rovelli, C. (2019). Neither presentism nor eternalism. Foundations of Physics, 49(12), 1325–1335. http://dx.doi.org/10.1007/s10701-019-00312-9
  • Putnam, H. (1967). Time and physical geometry. The Journal of Philosophy, 64(8), 240–247. http://dx.doi.org/10.2307/2024493
  • Sider, T. (2001). Four-dimensionalism. Oxford University Press.

Quantum mechanics

Rovelli thinks that the lessons learned from spacetime physics (in favor of relationalism) can be generalized to quantum physics, and that it solves the interpretive problems (viz. the measurement problem).

  • Rovelli, C. (1996). Relational quantum mechanics. International journal of theoretical physics, 35, 1637–1678. http://dx.doi.org/10.1007/BF02302261
  • Laudisa, F., & Rovelli, C. (2019). Relational quantum mechanics. Stanford Online Encyclopedia of Philosophy https://plato.stanford.edu/entries/qm-relational/

There’s a piece of mathematics that really helps to understand the relational interpretation: the biorthogonal decomposition theorem. For more, see SEP: modal interpretation

Aside: Rovelli appears to be strongly antirealist about the quantum state. (See his paper on Heisenberg mechanics is the good mechanics.) But his view may be more subtle than it initially appears.

Statistical mechanics

Statistical mechanics (developed late 19th century) has the amazing feature that it can derive a description of changes of macroscopic objects by applying probabilistic considerations to a large collection of microscopic objects. As such, it has looked to many like a success story of reduction to the microphysical. For any overview, see:

  • Sklar, L. (1993). Physics and chance: philosophical issues in the foundations of statistical mechanics. Cambridge University Press.

Direction of time

In recent years, philosophers of physics have displayed some worries about this reduction. Most of them want the reduction to work — and believe that it can — but there is a prima facie problem: the fundamental theory is time-reversal invariant, whereas the emergent theory is not.

  • Callender, C. (1999). Reducing thermodynamics to statistical mechanics: the case of entropy. The Journal of Philosophy, 96(7), 348–373. http://dx.doi.org/10.2307/2564602
  • Sklar, L. (1999). The reduction (?) of thermodynamics to statistical mechanics. Philosophical Studies, 95(1/2), 187–202. https://www.jstor.org/stable/4320956

Causality

Bertrand Russell claimed that causality doesn’t appear in fundamental physics, and so is not a part of the fabric of the world (Russell 1913). Rovelli seems to agree that causality isn’t fundamental, but he thinks that it emerges at a higher level, and is an anthropocentric concept.

The human being in a physical world

  • Rovelli does not think that human beings play any special role in constituting facts. In one sense, he is even more radical than those who do. He thinks that all facts about one physical object are relative to some other physical object.
  • Rovelli seems to be a compatibilist: physical determinism is compatible with free will.

Additional resources

For the mathematics of relativity theory, our main sources are:

  • Malament, D. Notes on Spacetime and Geometry. pdf
  • Malament, D. Topics in the Foundations of General Relativity and Newtonian Gravitation Theory.

The classical book on GTR is by Robert Wald, and we also like Landsman’s Foundations of General Relativity.

If you want to follow the recent philosophical conversation, there are curated topical indexes at PhilPapers.org. For example:

References

Belot, Gordon. 1995. “New Work for Counterpart Theorists: Determinism.” The British Journal for the Philosophy of Science 46 (2): 185–95.
Earman, John. 1986. A Primer on Determinism. Vol. 37. Springer Science & Business Media.
Field, Hartry. 1984. “Can We Dispense with Space-Time?” In PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association, 1984:32–90. 2. Cambridge University Press. https://doi.org/10.1086/psaprocbienmeetp.1984.2.192496.
North, Jill. 2021. Physics, Structure, and Reality. Oxford University Press.
Russell, Bertrand. 1913. “On the Notion of Cause.” Proceedings of the Aristotelian Society 13 (1): 1–26. https://academic.oup.com/aristotelian/article/13/1/1/1772124.

  1. Phrasing borrowed from the introduction to Tim Maudlin’s Philosophy of Physics: Quantum Theory.↩︎