Millikan, Einstein, and Planck: The Experiment io9 Forgot

A couple of weeks ago, io9 ran a piece about the old accusations that Robert Millikan manipulated his data for the electron charge with the headlineDid a Case of Scientific Misconduct Win the Nobel Prize for Physics? that got a lot of attention. I wasn't as impressed with this as a lot of other people, mostly because it's mostly just adding a clickbait headline to a story that's been around for decades, and doesn't even really engage with the various responses and defenses of Millikan, including this PDF that offers a (to my mind) fairly convincing argument that most of the argument turns on a misreading of Millikan's paper, despite the fact that it's linked at the bottom of the io9 story. But I object to the headline for reasons beyond the cheap sensationalism-- it's also overlooks half of the citation for Millikan's 1923 Nobel Prize in Physics:

for his work on the elementary charge of electricity and on the photoelectric effect

As my own physics background is all about quantum optics, I regard this as a grievous oversight. And while I wasn't annoyed enough to clear my schedule of more important day job stuff in order to respond in blog time, I do want to say something to highlight Millikan's other great work.

The photoelectric part of Millikan's prize is centered around two papers: "Einstein's Photoelectric Equation and Contact Electromotive Force" from January of 1916, and "A Direct Photoelectric Determination of Planck's "h"" from March of the same year (those are paywalled links, but given the full titles you might be able to find copies that fell off the back of a digital truck...). These are both tour de force examples of precision experimental physics and a clear demonstration of Millikan's integrity.

To put them in a bit of context, the photoelectric effect was discovered as a sort of by-product of Heinrich Hertz's experiments to demonstrate that light was an electromagnetic wave. Hertz was creating and detecting waves using spark gaps-- discharging a current through a small gap to make a spark in the air produced a blast of radiation, which he then detected by its ability to trigger a spark between another pair of electrodes some distance away. He noticed that he got bigger sparks when there was a clear path for the light from the first spark to travel to the second spark gap. This was a result of photoelectric emission: ultraviolet light from the first spark knocked loose some electrons from the metal electrodes of the detector, and having those free electrons there made it easier to make a spark.

This seems like it ought to be easy to explain, but classical physics turned out not to do a very good job of it, as I explained back in the pre-ScienceBlogs days of this blog. One of Einstein's famous 1905 papers was "On a Heuristic Viewpoint Concerning the Production and Transformation of Light", which famously suggested that the whole business could be explained by treating light as a stream of photons rather than a classical wave. This was a pretty radical suggestion-- Abraham Pais quotes Einstein calling it the only truly revolutionary thing he did-- and a lot of people didn't like it very much. Max Planck, whose idea Einstein was adopting and adapting, wrote in a recommendation for Einstein that he had "missed the target" here, but that "it cannot really be held to much against him, for it is not possible to introduce really new ideas even in the most exact sciences without sometimes taking a risk." (Well, Planck presumably wrote it in German; I got this wording from here.)

Einstein's model makes some very concrete predictions, so a bunch of people went out to do experiments. Which mostly confirmed his theory, except some of them didn't, and the whole picture was sort of muddled over the next decade. Which is where Millikan comes in-- having wrapped up his electron-charge experiment, Millikan began "subjecting this experiment to some searching experimental tests from a variety of viewpoints" (quote from the January paper linked above). This led to the two papers above, and contributed to Nobel Prizes for both Einstein (the photoelectric effect is the only specific theory mentioned in his prize citation) and Millikan.

There's a bit of irony in this, because Millikan didn't much care for Einstein's theory. The January paper features maybe the greatest passive-aggressive introduction in the history of science:

Einstein’s photoelectric equation for the maximum energy of emission of a negative electron under the influence of ultra-violet light… cannot in my judgment be looked upon at present as resting upon any sort of a satisfactory theoretical foundation. Its credentials are thus far purely empirical, but it is an equation which, if correct, is certainly destined to play a scarcely less important role in the future development of the relations between radiant electromagnetic energy and thermal energy than Maxwell’s equations have played in the past.

I have in recent years been subjecting this experiment to some searching experimental tests from a variety of viewpoints, and have been led to the conclusion that, whatever its origin, it actually represents very accurately the behavior… of all the substances with which I have worked.

But whatever his opinion, Millikan was above all else dedicated to getting experimental physics right, and in that paper, he clears up a lot of the confusion surrounding the previous attempts to test the theory. This involves a lot of extremely fussy experimental work-- he performed all his experiments in very high vacuum, including forming the metal surfaces under vacuum, thus eliminating the effects of surface contamination. He also used optical filters to eliminate the effects of stray light at wavelengths other than the one he was trying to test-- a precaution that anybody who's ever tried this in a modern physics course can tell you is absolutely essential-- and investigated the effects of contacts between the metal being studied and the different wires used to connect to the measuring apparatus in great detail.

Thanks to all of these precautions, he showed that the previous experiments that seemed to contradict Einstein's model were, in fact, hopelessly contaminated. The stray light issue in particular makes an enormous difference in the results. He's largely responding to experiments by Carl Ramsauer, and there are some places where his comments about Ramsauer's measurements read as fairly scathing. When you do everything properly, as Millikan did, Einstein's model is brilliantly successful.

Having established that the model worked, Millikan then put it to use, in the March paper. This also includes some harsh descriptions of Einstein's theory-- "This hypothesis may well be called reckless first because an electromagnetic disturbance which remains localized in space seems a violation of the very conception of an electromagnetic disturbance, and second because it flies in the face of the thoroughly established facts of interference"-- but also an extremely precise confirmation of it. More than that, it establishes very clearly that the energy of the emitted electrons depends on Planck's constant h, and provides a measurement of the constant that agrees with previous values but offers much better precision.

Again, this all works out because of Millikan's exceptional skill and dedication to precision experiments. He works with everything under vacuum, blocks out stray light, tests the effects of contact potentials, and investigates second-order sources of error like reflected light and possible photoemission from the metal cylinder used to screen out external electric fields. He tests multiple alkali metals over a wide range of wavelengths, finding consistent results, and combines them to get the best measurement of Planck's constant to that point, within a few percent of the best modern value. It's a beautiful piece of work, if you're into experimental physics and precision measurement.

It also demonstrates Millikan's professional integrity, because he was clearly conflicted about the theory. In the conclusion of the March paper, he opens by saying:

Perhaps it is still too early to assert with absolute confidence the general and exact validity of the Einstein equation. Nevertheless, it must be admitted that the present experiments constitute very much
better justification for such an assertion than has heretofore been found, and if that equation be of general validity, then it must certainly be regarded as one of the most fundamental and far reaching of the equations of physics...

and in the very next sentence he writes:

Yet the semi-corpuscular theory by which Einstein arrived at his equation seems at present to be wholly untenable.

and goes on to try to find some alternative way to get the same predictions. This isn't the action of a guy who would be prone to manipulating data, even subconsciously, to fit a preferred theory. He's doing exactly what a scientist is supposed to do: subjecting a proposed model to "very searching tests" to the very best of his abilities, and going with what the data say.

Now, there are still some issues with Millikan from a modern perspective. His is the only name on these papers, as was the style in those days, but he clearly had help from a couple of students, A. E. Hennings and W. H. Kadisch, and he thanks Walter Whitney for doing spectroscopy to determine the wavelengths of light from his source. By modern standards, those people would probably be co-authors. But then, he also wrote:

To construct tubes in which these operations can all be performed in rapid succession, particularly when the substances to be studied are, as in this case, the inflammable alkali metals, sodium, potassium and lithium, is not at all easy, and such success as is being obtained in these experiments is due in no small degree to the skill and experience of the mechanician, Mr. Julius Pearson, who has from the start made all the tubes and contributed not a little toward their design.

That's a pretty gracious acknowledgement of a technician, who most likely wouldn't be listed as a co-author even by modern standards.

Would this have been worth a Nobel by itself, without the disputed electron charge experiments? Maybe not. It is an extremely important and unjustly overlooked bit of work, though, which cleared up some controversies about earlier measurements, and played an important role in locking up the case for quantum theory. The fact that this went against Millikan's own preferences at the time is also a great demonstration of how science is supposed to be done.

If you're going to tell the story of Robert Millikan, the photoelectric effect needs to be a major piece of it, particularly when considering accusations of shady dealings. So, while it's too late to really tie into the io9 article and its attendant hype, I offer this post to fill in the gap. Now you know, as they say, the rest of the story.