Metaphysical Experiments

“As long as our philosophy works, it lurks undetected in the background, leading us to mistakenly believe that science is something separable from metaphysics.”

“But at the uncharted edges of experience — at high speeds and tiny scales — those intuitions cease to serve us, making it impossible for us to do science without confronting our philosophical assumptions head-on.”

“Suddenly we find ourselves in a place where science and philosophy can no longer be neatly distinguished. A place, according to the physicist Eric Cavalcanti, called “experimental metaphysics.””

“In experimental metaphysics, the tools of science can be used to test our philosophical worldviews, which in turn can be used to better understand science.”

“THE DIVIDING LINE between science and philosophy has never been clear. Often, it’s drawn along testability. Any science that deserves its name is said to be vulnerable to tests that can falsify it, while philosophy aims for pristine truths that hover somewhere beyond the grubby reach of experiment. So long as that distinction is in play, physicists believe they can get on with the messy business of “real science” and leave the philosophers in their armchairs, stroking their chins.”

“As it turns out, though, the testability distinction doesn’t hold. Philosophers have long known that it’s impossible to prove a hypothesis. (No matter how many white swans you see, the next one could be black.) That’s why Karl Popper famously said that a statement is only scientific if it’s falsifiable — if we can’t prove it, we can at least try to disprove it.”

“In 1906, though, the French physicist Pierre Duhem showed that falsifying a single hypothesis is impossible. Every piece of science is bound up in a tangled mesh of assumptions, he argued.”

““Poincaré, in my opinion, is right,” Einstein said in a 1921 lecture. He added, “Only the sum of geometry and physical laws is subject to experimental verification.””

“As the American logician Willard V. O. Quine put it, “The unit of empirical significance” — the thing that’s actually testable — “is the whole of science.” The simplest observation (that the sky is blue, say, or the particle is there) forces us to question everything we know about the workings of the universe.”

“But actually, it’s worse than that. The unit of empirical significance is a combination of science and philosophy.”

“The thinker who saw this most clearly was the 20th-century Swiss mathematician Ferdinand Gonseth. For Gonseth, science and metaphysics are always in conversation with one another, with metaphysics providing the foundations on which science operates, science providing evidence that forces metaphysics to revise those foundations, and the two together adapting and changing like a living, breathing organism. As he said in a symposium he attended in Einstein’s honor, “Science and philosophy form a single whole.””

“Cavalcanti decided to pursue a doctorate in quantum foundations and found a place for himself at the University of Queensland in Australia. His dissertation began, “To understand the source of the conflicts of quantum foundations, it is essential to know where and how our classical models and intuitions start to fail to describe a quantum world. This is the subject of experimental metaphysics.” A professor put the thesis down and declared, “This isn’t physics.””

“In the 1960s, the Northern Irish physicist John Stewart Bell had also encountered a culture of physics that had no patience for philosophy.”

“Bell, doing his heretical work in his spare time, discovered a new possibility: While it’s true that you can’t test a single hypothesis in isolation, you can take multiple metaphysical assumptions and see if they stand or fall together.”

“For Bell, those assumptions are typically understood to be locality (the belief that things can’t influence each other instantaneously across space) and realism (that there’s some way things simply are, independent of their being measured).”

“His theorem, published in 1964, proved what’s known as Bell’s inequality: For any theory operating under the assumptions of locality and realism, there’s an upper limit on how correlated certain events can be. Quantum mechanics, however, predicted correlations that busted through that upper limit.”

“As written, Bell’s theorem wasn’t testable, but in 1969 the physicist and philosopher Abner Shimony saw that it could be rewritten in a form suitable for the lab.”

“Along with John Clauser, Michael Horne and Richard Holt, Shimony transformed Bell’s inequality into the CHSH inequality (named for its authors’ initials), and in 1972, in a basement in Berkeley, California, Clauser and his collaborator Stuart Freedman put it to the test by measuring correlations between pairs of photons.”

“The results showed that the world bore out the predictions of quantum mechanics, showing correlations that remained far stronger than Bell’s inequality allowed. This meant that locality and realism can’t both be features of reality — though which of the two we ought to abandon, the experiments couldn’t say.”

““To my mind, the most fascinating thing about theorems of Bell’s type is that they provide a rare opportunity for an enterprise which can properly be called ‘experimental metaphysics,’” Shimony wrote in 1980 in the statement that’s widely believed to have coined the term.”

“As it happens, though, the term goes back further, to a most unlikely character. Michele Besso, Einstein’s best friend and sounding board”

“Einstein had always been a realist, believing in a reality behind the scenes, independent of our observations, but Besso introduced him to the philosophical writings of Ernst Mach, who argued that a theory should only refer to measurable quantities. Mach, by way of Besso, encouraged Einstein to give up his metaphysical notions of absolute space, time and motion. The result was the special theory of relativity.”

“Besso later befriended Gonseth; in Switzerland, the two took long walks together, where Gonseth argued that physics could never be placed on firm foundations, since experiments can always overturn the most bedrock assumptions on which it is built.”

“In a letter, which Gonseth published in a 1948 issue of the journal Dialectica, Besso suggested that Gonseth refer to his work as “experimental metaphysics.””

“Experimental metaphysics gained something of an official headquarters in the 1970s with the founding of the Association Ferdinand Gonseth in Bienne, Switzerland.”

“The association published an underground newsletter called Epistemological Letters, a kind of physics “zine,” with typed, mimeographed pages speckled with hand-drawn equations that was mailed out to 100 or so physicists and philosophers who comprised a new counterculture — the daring few who wanted to discuss experimental metaphysics.”

“In the wake of experiments violating Bell-type inequalities, several views of reality remained on the table.”

“You could keep realism and give up locality, accepting that what happens in one corner of the universe instantaneously affects what happens in another and therefore that relativity must be modified.”

“Or you could keep locality and give up realism, accepting that things in the universe don’t have definite features prior to being measured — that nature is, in some profound sense, making things up on the fly.”

“Cavalcanti and colleagues to make their own version. In 2020, in the journal Nature Physics, they published “A Strong No-Go Theorem on the Wigner’s Friend Paradox,””

“Their no-go theorem showed that, if the predictions of quantum mechanics are correct, the following three assumptions cannot all be true: locality (no spooky action at a distance), freedom of choice (no cosmic conspiracy tricking you into setting your detectors so that the outcomes seem to violate Bell’s inequality even though they don’t), and absoluteness of observed events (an electron with spin up for Wigner’s friend is an electron with spin up for everyone).”

“If you want local interactions and a conspiracy-free cosmos, then you have to give up on the notion that a measurement outcome for one observer is a measurement outcome for all.”

“Significantly, their no-go theorem “constrains the space of possible metaphysical theories more tightly than Bell’s theorem does,” Cavalcanti said.”

““It’s an important improvement,” Brukner said. “It’s the most precise, strongest no-go theorem.” Which is to say, it’s the most powerful piece of experimental metaphysics yet”

““The strength of these no-go theorems is exactly that they do not test any particular theory, but a worldview. By testing them and showing violations of certain inequalities, we don’t reject one theory, but a whole class of theories. That’s a very powerful thing. It allows us to understand what is possible.””

“AS I SPOKE with Cavalcanti, I tried to get a read on which interpretation of quantum mechanics he subscribed to by feeling out which metaphysical assumptions he hoped to hang on to and which he was ready to toss.”

“Did he agree with the Bohmian interpretation of quantum mechanics, which trades locality for realism?”

“Was he a “QBist,” with no need for the absoluteness of observed events?”

“Did he believe in the cosmic conspiracies of the superdeterminists, who attribute all correlated measurements in the present-day universe to a master plan set out at the beginning of time?”

“How about measurements spawning parallel realities, as in the many-worlds hypothesis?”

“Cavalcanti kept a true philosopher’s poker face; he wouldn’t say.”

“We can’t know whether nature violates Cavalcanti’s inequality — we can’t know, that is, whether objectivity itself is on the metaphysical chopping block — until we can define what counts as an observer, and figuring that out involves physics, cognitive science and philosophy.”

“The radical space of experimental metaphysics expands to entwine all three of them. To paraphrase Gonseth, perhaps they form a single whole.”