Book Review: How Experiments End by Peter Galison

There is this gap.

I know that the computer in front of me is made of atoms, and the atoms have nuclei, and the nuclei have protons and neutrons, and the protons and neutrons are made of quarks.

I’m very confident in this knowledge. There are few things in the world that I’m more confident of.

But if somebody were to ask me why I’m so sure, I would reply something like: let’s take our computer to the neighborhood particle accelerator, they’ll take a sample, heat it up to very high temperatures, convert it into a plasma, use electromagnets to accelerate it to very high velocities, scatter it off some other material, observe the scattered patterns of products using detectors, then carry the information to computers using complex electronics, do sophisticated data analysis using computers, match it with the best current theories of physics and then: conclude that quarks are present.

Really? My trust is based on such a long and complex process of steps? What if a single step fails? Why do I trust the community of experimentalists and theorists to come up with the right answer?

There is this gap: I know I’m confident in my knowledge, but I don’t know why I’m so confident. I’m an apprentice practitioner of physics theory. So for me, this gap is an aching void. It’s always troublesome to believe something and not know why you should believe in it.

Actually, something does partially fill the gap: I trust the statements of the physics community because of the impact of their ideas on engineering; and the impact of engineering in everyday life: planes fly, trains run, we land on the moon, laser discs work, computers work, phones and radios work, GPS systems work, buildings stand up, factories function, nuclear reactors work, radars work, sonar works and so on.

Thus, I reason that the community of physicists who came up with so much useful knowledge cannot just go completely astray when they deal with fundamental questions; even though cutting edge particle physics is not applied in any domain of engineering.

You or I may be convinced by the past successes of physicists. But the physicists themselves who get involved in the effort and perform the experiments, they need to convince themselves and each other. And they can’t just say: “oh we succeeded then, and therefore we’ll succeed now.”

So the question now becomes: why do physicists trust this complicated chain of reasoning?

And the gap remains.

The entire fields of philosophy of science, sociology of science and history of science are concerned with finding out how to fill this gap. But a lot of people involved in this effort sit in armchairs and theorize.

Peter Galison is unique in his approach. He decides that he’s going to go and look (shocking, right) at the intricate and messy details of how we acquire knowledge. He’s going to look into the processes of how experimental physicists work: how they decide on experiment design, how they gather data, how they make arguments, how they change their minds, how they take theory into account, and how they declare the birth of a freshly-minted piece of knowledge. In short, how do they decide that a sequence of experiments have reached their end?

He is interested in the history and sociological mechanics of humanity’s finest applied epistemology.


So why should physicists trust such long and convoluted chains of reasoning? The short answer: reality is stubborn. If you keep looking long enough and carefully enough, you are bound to hit upon reality. Here is Galison illustrating this point:

“Microphysical phenomena are not simply observed; they are mediated by layers of experience, theory and causal stories that link background effects to their tests. But the mediated quality of effects and entities does not necessarily make them pliable; experimental conclusions have a stubbornness not easily canceled by theory change. And it is this solidity in the face of altering conditions that impresses the experimenters themselves—even when theorists dissent.”

There are two ways experimenters become increasingly certain about the phenomena they observe: by increasing the directness of their experiments and by increasing the stability of their experiments.

To understand directness, let us take an everyday example. Suppose you want to know whether How Experiments End is available at the local library or not. First, you could check the online catalog and see if it’s checked out. This is like an indirect measurement. But suppose that quite often your library forgets to update its records. So, you could make a more direct measurement by calling up the library and asking them; or by asking a friend who lives near the library to go and see. These are progressively more direct measurements. The most direct measurement would be for you to go to the library and try borrowing the book.

Every discovery consists of a series of experiments; increasingly direct. The “moment of discovery”—glorified in popular accounts of science—is a gross oversimplification. In the example with the library book, it’s pointless to ask at what “moment” you discovered that the book was available. Instead, your confidence increased as evidence from increasingly reliable sources came in. Similarly, every experiment attempts to correct and improve the potential faults of other experiments or tries to get at the phenomenon from a new perspective.

An experiment that Galison documents in great detail is the search and discovery of neutral currents. For a long people time did not believe that neutral currents existed. An American experimental collaboration—E1A, running at Fermilab—had amassed evidence to the effect that neutral currents didn’t exist; they even wrote up a draft paper to that effect.

But in 1973, new evidence started coming in. On 13 December 1973, David Cline, a leading member of the collaboration wrote a memo with the statement: “At present I don’t see how to make these effects go away.”

When new evidence came in, they made every attempt to explain away the signal as some kind of noise. But nature is stubborn. The signal was stable to manipulations and variations in experiments and to different approaches in data analysis. They tried everything to make it go away. But despite how much they didn’t like it, they had to change their mind.

You need to make variations in the experiments to test stability. But in large experiments—like the Gargamelle which was where neutral currents were discovered; or the LHC—making variations in the experimental setup is very hard. The equipment is expensive and has been set up almost permanently. In these kinds of situations, the test of stability comes from a having many different teams with different experimental and theoretical backgrounds and different preferred modes of analysis. Then, different people take different aspects of the evidence as convincing. Subgroups within the collaboration have to argue, counter-argue, and improve arguments in order to reach a kind of reflective equilibrium. Indeed, non-variability of experiments is a problem also faced by astrophysicists, who take similar approaches to processing evidence.


The goal of all experiments, at the end of the day, is to find a signal in a background: to carve away every part of their data that doesn’t encode evidence of the phenomenon under question.  As Galison puts it:

“In this respect the laboratory is not so different from the studio. As the artistic tale suggests, the task of removing the background is not ancillary to identifying the foreground—the two tasks are one and the same. When the background could not be properly circumscribed, the demonstration had to remain incomplete, like Michelangelo’s St. Matthew, in which the artist is unable to ‘liberate’ his sculpture from its ‘marble prison’.”

In textbooks, discoveries are caricatured: we get the sense that there was one experiment that changed everyone’s mind. But actually, there was an intricate and complex process of experimentation and argumentation that is drawn out over of period of several years—maybe even decades—before experimenters elevate a signal to a discovery and knowledge becomes solidified.

There is much treasure in this book, and it deserves to be read and re-read. It contains in-depth historical accounts of the experimental processes behind three discoveries: the measurement of the gyromagnetic ratio of the electron, the discovery of the muon and the discovery of neutral currents. Further, it offers much analysis. The historical detail is extraordinary; the sociological eye with which it is analyzed is rigorous; and the philosophical common-sense is refreshing.

But most importantly, as a physicist, this book gave me a sense of pride. It gave me a sense of the history—the amount of rigor, the amount of experimentation, the amount of the argumentation, and the amount of effort put in by so many fine people—behind the creation of knowledge that we now take for granted.

As Feynman put it: “I’m at the end of 400 years of a very effective method of finding out things about the world.” This book gives you a sense of why that method is so effective.

Postscript: For a far more technically deep and professional review, see this by Allan Franklin.


One thought on “Book Review: How Experiments End by Peter Galison

  1. Ignacio Araya says:

    Very interesting. Your review reminded me of “The structure of Scientific Revolutions”, by Thomas Kuhn. However, this other book seems to be more rigorous in the analysis and in the historical account. I am definitely going to read it at some point.

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