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Imagine it is 1750, and you want to know Earth’s density. Reading a book or asking an expert won’t help, as the value is still undetermined. The only way to find out is to do the work—the measurements and the calculations—yourself. Would it be worth the effort? If so, why? Would the value lie in the knowledge that you would gain or would it be in the process of working out the answer?

In the 1770s, building on a suggestion by Newton, a team spent two years on the rain-swept flanks of Schiehallion—a mountain in the highlands of Scotland—undertaking exactly that task. They were working on the premise that, if they could measure the mass of the mountain and determine the deflection of a pendulum hanging near its lower slopes, they could infer the mean density of Earth. They chose Schiehallion because its shape and presumed uniform geological composition promised to make the work easier. But it was still backbreaking labor, with enormous physical, mental, and material demands.

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The even shape and presumed uniform composition of Schiehallion made it a good target for efforts to measure the density of Earth.

In the end, they did manage to answer their question with a fair degree of accuracy. But as with most great research projects, their conclusion brought with it some unanticipated discoveries. To determine the deflection of the pendulum, they had to refine the use of the zenith sector—a telescope designed for precise measurements of astronomical latitude. To determine the mass of Schiehallion, they had to invent the concept of topographical lines.

In hindsight, these spin-offs seem to make the couple of years that the group invested in their project time well spent. Knowledge is power, and when we know things, we can do things—things we care about and things that are practically valuable to us. If this were the only justification for the effort, however, then whether it was worth doing would have to be assessed by purely practical benefits weighed against opportunity cost. What practical benefit came (and will yet come) from these discoveries? What else could the scientists have done with their time? Could their resources have been channeled more effectively?

Science is expensive, and we have to decide where and how to direct time, energy, and money. For all we know, the puzzles that interest physicists could have less practical benefit than those of interest to other scientists. Assume for a moment that that’s the case. Is there any reason to think that physics deserves special support?

I think there is, and my reason for thinking that lies not with physics but with philosophy, my discipline. I am not so reckless as to try to tell the full story here, in a magazine for physicists, but I will give a sketch of the answer, drawing on my own research and that of my colleagues.

All fields of inquiry yield knowledge, and knowledge is a good thing, something it is right to value and to seek. But physics is unique and unparalleled in the sciences for the quantity of knowledge it yields. This knowledge is not greater because the realm of physics is larger than, say, that of biology—as the cosmos is larger than the biosphere. Rather, it is because how much knowledge a theory contains is determined by the “fundamentality” of the objects, properties, and relations the knowledge concerns.

Here is an analogy that might give a clearer picture of my view: If you learn an apple is a fruit, you learn more about it than if you learn it belongs to your friend Affan. Each is a fact about the apple, to be sure, but being a fruit is a fundamental property of the apple whereas belonging to your friend Affan isn’t.

All fields of inquiry contribute to bringing the many-splendored world into focus. But physics descries its most fundamental aspects. By doing so, it can tell us more about the world than anything else can. If knowledge is a good thing, and more of it is better, physics deserves special support independently of the practical benefit it may bring. — Nick Treanor