Inconvenient Facts

Meehl's Philosophical Psychology, Lecture 2, Part 2

This post digs into Lecture 2 of Paul Meehl’s course “Philosophical Psychology.” You can watch the video here. Here’s the full table of contents of my blogging through the class.

One of the more obvious objections to Popper’s falsification program is that the history of science is littered with scientists refusing to abandon their theories in light of experimental refutation. Meehl gives a few examples that suggest it might even be “rational” (his word) not to abandon a theory just because the facts disagree with it.

The Periodic Table

In the 1860s, Dimitri Mendeleev noted surprising repeated patterns in the properties of elements. He arranged them into what would become the periodic table. By ordering the elements by atomic weight, patterns of different elemental properties emerged as either vertical patterns or local similarities. The table was a great fit to the elements known at the time, but it wasn’t a perfect fit. In particular, the tellurium wouldn’t fit in the table. Rather than abandoning the table, Mendeleev assumed the weight was incorrectly measured. 

It would turn out that he was sort of right: the correct ordering in the table was by atomic number, not atomic weight. And he was right about the position of tellurium, just not the reason for why it should be there. That tellurium didn’t fit led to further fruitful investigations into the fundamentals of chemistry. And Meehl argues that it was perfectly rational for Mendeleev to stick with his table, exclaiming, “When you’ve got that much going for you and that much order, perfectly sensible to say something was wrong with a couple of numbers in there.”

Ether Drift

Meehl’s second example details the refutation of the aether wind. Before Einstein, physicists thought that light must travel through a medium called luminiferous aether. After all, all waves travel through some medium. If light were to travel through aether, and since Earth was moving around the sun, there should be a difference in the speed of light at perpendicular directions on the surface of the Earth.

Albert Michelson and Edward Morley devised a clever experiment to detect this speed difference using an interferometer. By sending light down two perpendicular paths, reflecting the light off mirrors, and then intersecting the beams, a difference in speed would result in interference patterns at the intersection. After careful and valiant engineering of their device, they found no evidence for the aether drift in the 1880s.

The apocryphal story goes that this inspired Einstein to invent special relativity, where light moves at the same speed in all reference frames. However, there was a significant problem. Michaelson and Morley’s results were not only consistent with the refutation of the aether drift, but also with the apparatus being broken. A broken interferometer would also certainly see no interference patterns and hence would also report zero aether drift. There was plenty of evidence that Michaelson and Morley were trying to estimate an effect far below the capabilities of their device.

In 1933, nearly 50 years after the first Michaelson-Morley experiment and well after the acceptance of special relativity, Dayton C. Miller announced evidence of an aether drift. Miller was a highly respected experimental physicist, and physicists took his results quite seriously.   He even presented his aether drift findings as part of his presidential address to the American Physical Society. 

Miller’s results were compelling. Surely, they couldn’t be dismissed out of hand. But, in fact, they were. Einstein blew off Miller’s results as a “thermal artifact,” and physicists just continued on as if Miller’s experiment hadn’t happened. In 1945, a more spectacular confirmation of special relativity lit up the New Mexico desert. It wasn’t until 1955 that physicists bothered to explain away Miller’s results, using computational statistics to argue that the result was likely due to thermal effects. But this was just a reanalysis of 30-year-old findings, not a new experiment! It was a post hoc statistical rationalization for why Miller was wrong. While plans for more interferometry experiments continued into the 1960s, most physicists moved on to more trendy topics.

The Eddington Experiment

Meehl doesn’t discuss this one in-depth, but he notes a couple of times that Popper’s main scientific inspiration was Eddington’s experiment, in which Newton was “refuted” and Einstein “confirmed.” 

In 1919, Eddington led a scientific collaboration to measure the gravitation deflection of light near the sun to test the predictions of Einstein’s theory of relativity. Two teams, one in Principe and one in Brazil, measured the position of stars whose locations would be near the sun in the sky during a solar eclipse. Einstein’s theory predicted a gravitational lensing effect that would change the measured position of these stars because of the sun’s gravity.

Did the measured deflections confirm Einstein? Most analyses of the data now say not at all. There were only 28 salvageable photographic measurements from the experiment, 26 from Brazil and 2 from Principe. Eddington discarded 18 of the Brazilian ones, declaring that telescope defective. The remaining 8 revealed a deflection confirming Einstein. The 2 from Principe, after a theory-infected heuristic adjustment, also confirmed Einstein.

But before discarding them, the team had measured deflection on the 18 other plates. These measurements were far more favorable to Newtonian gravity than relativity. Eddington argued the device that had taken these measurements suffered from “systematic errors.” The headlines would read “Einstein Confirmed. Newton Refuted,” but this was just Eddigton’s opinion, man.

But Eddington wasn’t wrong! In 1969, a team of scientists at Caltech used radio interferometry to measure the deflection of light in the solar gravitational field. Twenty years of refining this new technique yielded progressively tighter agreement with relativity.

If you’re interested in learning more about the weirdness of these relativity experiments and the social pressures of science, I highly recommend checking out Chapter 2 of The Golem by Harry Collins and Trevor Pinch.

What should we do?

From these varied examples, Meehl concludes, “The history of science doesn't act the way Popper says the good scientist ought to act.”

“It's quite clear in the history of science that the way the facts control the theories is collectively over the long haul, not individually in the short run. Whatever you think about it philosophically, empirically, or historically, it's crystal clear that in every science, the theories, when they are reasonably well corroborated, are allowed to control individual alleged facts. The alleged fact that would be a falsifier is simply not admitted into the corpus of belief.”

Now, Meehl is also not particularly sanguine about formalizing how a long run of facts can support a particular theory. When should we accuse a scientist clinging to their pet theory of being a bad scientist?

“At what point that kind of theoretical tenacity becomes sinful is not known. No philosopher or historian of science has been able to give any kind of a rule or even rule of thumb, which enables you to say ‘Now at time T3 from this point on theoretical tenacity is a scientific sin’ and probably nobody ever will be able to.”

So we’re left with a few questions: Why might a scientist like one theory more than another if both were unfalsified? When should evidence be reasonably sufficient to abandon a theory? How can you argue against theoretical tenacity? Popper tried to engage with these questions, and I’ll describe his approach of “corroboration” in the final post about lecture 2.