monster of the milky way

Today I was watching an excellent PBS NOVA episode, Monster of the Milky Way, about recent cosmological theories that all, or almost all, galaxies have supermassive black holes at their center, including the Milky Way galaxy, our own little home in the universe. I don’t consider the theory problematic; in fact, I think it makes perfect sense. Just as our solar system revolves around the sun, so too the much larger structure of galaxies, especially spiral galaxies, would seem to be revolving around something much more massive than your typical, run-of-the-mill main sequence star. So I’m not going to address supermassive black holes in this post.

In the NOVA episode, astrophysicist Andrew Hamilton makes the following statement:

“Albert Einstein had this crazy idea that space and time were curved, and it was the curvature of space that gave the appearance of gravity.”

I found this to be of great interest. To speak of “the appearance of gravity” suggests a contrast to the reality of gravity, and the appearance/reality distinction is deeply embedded in Western metaphysics since Parmenides and Plato (and given life again in the recent recrudescence of metaphysics). Physicists often speak loosely, and I doubt that Andrew Hamilton intended to propound a philosophical thesis within cosmology, but it is a thesis worth exploring, even if unintended.

Is gravity the mere appearance by which a deeper reality manifests itself? And is that deeper reality the structure of spacetime? Is gravity less real than spacetime? Can gravity be reduced to the structure of spacetime? All of these questions can be reformulated as their opposite number: Is the structure of spacetime a mere appearance manifesting the deeper reality of gravity? Is spacetime less real than gravity? Can spacetime structure be reduced to gravity?

It would be strange indeed if gravity were epiphenomenal to the cosmos. Contemporary physical theory distinguishes four physical forces at work in the nature of things: gravity, electromagnetism, the strong force, and the weak force. Unified field theories have done a passable job of providing a common framework for electromagnetism, the strong force, and the weak force, but gravity has proved resistant to these unified field theories, not least because of the difficulty of giving a quantum account of gravitation. There are plenty of quantum gravity theories, but none of them are yet considered definitive, and their connection to the other forces and a unified theoretical framework is more speculation than physics.

The problem of appearance and reality is an old one in the philosophy of science. Russell caricatured F. H. Bradley as the “classical” tradition in philosophy (in his Our Knowledge of the External World, and elsewhere as well I think), and Bradley is remembered for his treatise Appearance and Reality. But Russell himself opens his The Problems of Philosophy with a chapter on appearance and reality, where, writing about a table, he says:

With the naked eye one can see the grain, but otherwise the table looks smooth and even. If we looked at it through a microscope, we should see roughnesses and hills and valleys, and all sorts of differences that are imperceptible to the naked eye. Which of these is the ‘real’ table? We are naturally tempted to say that what we see through the microscope is more real, but that in turn would be changed by a still more powerful microscope. If, then, we cannot trust what we see with the naked eye, why should we trust what we see through a microscope? Thus, again, the confidence in our senses with which we began deserts us.

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Similar difficulties arise when we consider the sense of touch. It is true that the table always gives us a sensation of hardness, and we feel that it resists pressure. But the sensation we obtain depends upon how hard we press the table and also upon what part of the body we press with; thus the various sensations due to various pressures or various parts of the body cannot be supposed to reveal directly any definite property of the table, but at most to be signs of some property which perhaps causes all the sensations, but is not actually apparent in any of them. And the same applies still more obviously to the sounds which can be elicited by rapping the table.

Thus it becomes evident that the real table, if there is one, is not the same as what we immediately experience by sight or touch or hearing. The real table, if there is one, is not immediately known to us at all, but must be an inference from what is immediately known. Hence, two very difficult questions at once arise; namely, (1) Is there a real table at all? (2) If so, what sort of object can it be?

Russell often expresses himself in the language of contemporary science, but the distinctions he makes in this passage (which I have greatly shortened) are not dependent upon science. But science does add another layer to the distinction between appearance and reality. The instruments of scientific research give us unprecedented ways to extend our senses, and with each novel perspective on things that science opens up, there is another way to describe these things. Moreover, scientific theory appeals to non-observable entities to explain the way the world is and naïve scientific realism assures us that the elementary particles are truly real and calls into question the manifest realities of macroscopic experience.

The problem of gravity, however, can’t even be settled by naïve scientific realism. Scientific realism would hold that the elementary particles that make up the objects studied by cosmology and astrophysics are real, and it would not deny the collections of elementary particles into atoms, molecules, stars, and galaxies to possess a certain reality. But whether gravity is epiphenomenal to spacetime structure, or whether spacetime structure is epiphenomenal to gravity is not readily settled by an appeal to scientific realism. Asking “What comes first, the gravity or the structure?” is a lot like asking, “What comes first, the chicken or the egg?”

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