Searching for Free Quarks

By the late 1960s there were a huge number of "elementary" particles, over a 100. Gell-Man and Zweig showed that all of the baryons and mesons (that is, all but about 10 of the particles could be accounted for as made up of even more elementary particles, "quarks," and that you would only need, depending on how you count, fewer than 10 quarks to do it. The baryons would each be made of three quarks, and the mesons of two quarks. In order to make it work, the quarks would have to have charge of 1/3, 0, or -1/3 times the charge of an electron. Everyone "knew" that what the Cavendish lab had believed, that there is a smallest unit of charge was true, and that it was the charge of an electron. Millikan had looked long and hard, and not found any fractional charges. There were two ways to explain that. First, free quarks are incredibly rare, since they like to combine, and so Millikan had just missed the fractionally charged droplets, or free quarks are impossible, the energy with which they combine is so great that there never are free quarks. That is known as "quark confinement," and it is the accepted version today.

Morpugo looked for free quarks. Unlike other quark experiments, all that that meant was that he looked for things with charges that are multiples of 1/3 the charge of an electron, not an integral multiple of the electron charge. Since free quarks were supposed to be rare (and it is not clear whether that is an empirical fact or a theoretical prediction), Morpugo was going to have to look at a lot of stuff. Stuff really just means stuff: Millikan's oil drops were tiny. Morpugo needed to measure the charge on much bigger objects. He used, in the end, bits of iron that weighed several tenths of a gram, as opposed to Millikan's drops that weighed hundredths of a picogram (%$10^{-11}$%), in fact much less than a trillionth as much.

He knew how to measure charge very accurately: Millikan had already done it, but he needed to work with something heavy. Millikan suspended his drops electrically and used a microscope to measure their height precisely. That ain't gonna work for something heavy: the charge on the plates required to suspend something heavy will be huge, hard to maintain, and hard to measure accurately. He decides to try a combined approach: float the objects nonelectrically, and add a small electric field.

How do you float things? On water. In fact, water is a conductor, so that is a terrible idea, but he tried floating them in a nonconductive liquid that is cheap, readily available in quantity, simple to model, and pure: dry cleaning fluid. His particles floated back and forth all over the place, because they kept exchanging charge with the fluid, and the dry cleaning fluid made him sick. So, try a different way of floating: you can float anything (even a frog) in a magnetic field using diamagnetism, and so that is what he tried next, and he did it in a "vacuum." This worked. He started making readings. There was some problem: the charge changed over a period of hours, but that is good enough. He soon found a grain that seemed to have a fractional charge. Unfortunately, it was not near a third, more like a quarter the charge of an electron. He tried reversing the field, and the charge came out the same, which indicates that there is a real charge being observed, not a nonuniformity in the field. He got excited. But he soon found other grains with other fractional charges, and calmed down again. It took a while to discover that moving the charged plates apart change the observed size of the charge, indicating that the measurement is an artifact of the nonuniformity of the plates.

That both changes the experimental design and the protocol: Mopurgo now tries to build an apparatus with the plates as far apart as possible, and measures charges with the field reversed and with the plates at different distances. He doesn't find anything. He switches from diamagnetic levitation (like floating frogs) to ferromagnetic levitation: floating bits of iron. He also switch from static electric fields to oscillating ones. He got charges that varied by 1/10 the charge of an electron over a period of many hours. What to do? He finally worked out that an oscillating field on a piece of iron has effects much like turning the piece of iron into a motor, that induces voltages, magnetization, and generally messes things up. He fixes that by making his grains spin, neutralizing those effects, and he still doesn't find anything.

So what is going here? Mopurgo knows that unit charges are a negative result, 1/3 charges are a Nobel prize, and anything else is "impossible," including changing or continuously variable charges and charges of any other value.

Mopurgo doesn't observe "nature," his apparatus and see what it reports. He observes it, and if he gets anything other than one of two expected results , he tinkers with it until he gets results of the expected sort. When he gets results of the expected sort, he doesn't tinker until after "observing" for a while, and if he hasn't gotten his Nobel yet, then he tinkers.

If Millikan had done that (and he did) it would have been outright cheating. (He did have a good argument available, but didn't report it.)

So, in what sense is scientific theory constrained by experiment, if experiment is itself constrained in the kind of way we described? Pickering's answer is "interactive stabilization": For an experiment to count, it must fit with the picture of what is being measured (not 'phenomenal," but "phenomenological"), with the understanding of the apparatus, and with the available theoretical context. This is a detailed story making sense of how science is empirical despite what is usually oversimply described with the slogan "observation is theory laden." We already knew the truism that "theory is observation laden." We take a theory to be "confirmed" and an experimental result to be "real" when we can get them to match up. That is not a trivial trick, it involves real cooperation from the apparatus itself, and so the stabilization is a genuine, though not independent, input from experiment to theory.

-- ShaughanLavine - 18 Apr 2006