Advent Calendar of Science Stories 20: Dot Physics 1976

We're going to depart from the chronological ordering again, because it's the weekend and I have to do a bunch of stuff with the kids. Which means I'm in search of a story I can outsource...

In this case, I'm outsourcing to myself-- this is a genuine out-take from Eureka: Discovering Your Inner Scientist, specifically Chapter 2, which tells two stories from the career of Luis Alvarez, who I've talked about before in the context of his experiment to x-ray one of the pyramids at Giza, and the time I wrote him a letter when I was nine about his theory that an asteroid impact killed the dinosaurs.

Those two experiments are featured in the book, but there used to be a third story, that got cut for length. This one is about how Alvarez was a sort of spiritual ancestor to Rhett Allain, using his physics expertise to do video analysis. The larger context of the chapter is about how scientists use expertise gained in working on one particular problem to make discoveries in a different field. Alvarez's expertise in particle physics-- he invented the hydrogen bubble chamber, which was the dominant detector technology in the field for decades, and won him the 1968 Nobel Prize thanks to the discoveries made using it.

In the 1970's, Alvarez put his years of work scrutinizing bubble chamber photographs to work in studying a very different problem: the 1963 assassination of JFK. Here's the section describing this that I cut out of the book in the late stages of editing:

Probably no segment of film outside a particle physics lab has been more scrutinized than the 27 seconds shot by Abraham Zapruder on November 22, 1963. The famous “Zapruder film” captures the moment when John F. Kennedy was shot while riding in an open limousine in Dealey Plaza. While this film was a critical piece of evidence for the Warren Commission investigation that declared that Lee Harvey Oswald acted alone, copies of the film have also provided fodder for countless conspiracy theories claiming that there was at least one more shooter on the famous “grassy knoll.” One claim is that the camera must have been running at a higher frame rate than the Warren Commission claimed, so the time between shots is too short for a single marksman to have fired three aimed shots. Another has to do with the motion of the President’s head after the fatal shot, when he appears to jerk backwards, toward the Texas Book Depository where Oswald was, rather than in the direction the bullet was supposedly moving. This is immortalized in Oliver Stone’s JFK, where Kevin Costner’s character spends an excruciatingly long time replaying the key frames, repeating “Back, and to the left… Back, and to the left…”

While Alvarez would later be part of a commission established by the National Academy of Sciences to study audio recordings of the assassination (where he played an important role), his most interesting association with the assassination comes from a 1976 paper in the American Journal of Physics. This was prompted by a former student who had become a conspiracy buff, and sparked Alvarez’s curiosity regarding the whole business. Like a good scientist, once he became interested in the question, he didn’t let it go until he had brought all his skills to bear on the problem.

The paper is a fascinating glimpse into the thought processes of a professional physicist, providing a detailed reconstruction of the motion of the limousine and the timing of the shots through careful examination of every frame of the film (as published in Life magazine and the Warren Commission report). His analysis goes beyond that of the professional photo analysts who testified before the Warren Commission (and he doesn’t shy away from expressing his low opinion of their work), showing the benefits of many years spent studying the minute details of bubble chamber tracks.

Two items in particular stand out. One of these is an ingenious solution to the frame rate question, using the images on the film itself(1). The frame rate had been in question because Zapruder’s camera offered two different settings, and there is nothing in the film that obviously serves as a “clock” to measure the frame rate. On careful inspection, though, Alvarez found a clapping man in the background of one sequence of twenty frames, and realized that the man’s clapping hands could serve as a clock.

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fn1: The frame rate questions was officially settled by examining the camera, at least to the degree that anything is settled regarding the Kennedy assassination, but Alvarez’s method is unique in relying only on the film.

The apparent rate of clapping, assuming the official value of 18.3 frames per second, is around 3.7 claps per second. Thinking about the physics of the clapping process, Alvarez concluded that the effort required to clap at a given rate depends on that rate cubed—so, doubling the rate from 3.7 per second to 7.4 per second would require eight times the muscle power. That restricts clapping to a rather narrow range of “natural frequencies:” very fast clapping requires a lot more work, enough so that most people won’t bother(2). Experiments with a metronome showed that 3.7 per second is within that range, but 7.4 per second is outside it—it’s just too much work to clap at that speed. This “cube law clock” helps confirm that the film was not running at the higher speed sometimes claimed by conspiracy theorists.

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fn2: It won’t surprise anyone familiar with the Internet to know that there are web sites and YouTube videos featuring people demonstrating their world record clapping speeds, claiming rates as high as 14 claps per second. Nobody would ever mistake these for ordinary clapping, though, such as one might do while watching the President’s motorcade go by.

The other, more impressive result was an explanation of the physics of that “Back, and to the left” head-jerk. At first glance, it seems impossible that an object hit with a rifle bullet could recoil toward the gun. The laws of physics seem to dictate that anything hit by a bullet must recoil in the same direction that the shot was fired: the bullet has momentum in the forward direction, which is transferred to the target, making it move forward.

This is true in collisions where the colliding objects do not change or deform due to the collision. This is a very strict criterion, but it works as an excellent approximation when thinking about collisions between billiard balls, or low-energy nuclear physics (as we’ll see in Chapter XXX). Such collisions are only one small class of collisions, though. Most collisions between real objects, including many of the high-energy particle collisions Alvarez made his career studying, involve some change of the colliding objects. In the particle physics case, this takes the form of the creation and ejection of new particles, producing tracks that physicists study to learn their composition. In the case of Oswald’s bullet striking the President’s head, the ejected material is blood and brain matter—the very disturbing frame of the Zapruder film capturing the fatal shot shows just such a spray.

When you consider the full physics of the collision, and take into account the momentum of the ejected material as well as the target and the bullet, you find that it’s not difficult to produce the counter-intuitive “backward recoil.” Alvarez worked out the necessary math on the back of an envelope, in keeping with tradition, but more than that, he devised an experimental demonstration.

With the assistance of some Berkeley colleagues, Alvarez went to a gun range with several melons wrapped in glass-fiber tape to simulate a human head, and fired rifle bullets into them. Exactly as predicted, bits of melon sprayed outward as the bullet exited, and the effect of these jets was to push the melon backwards, toward the gun. Six out of seven filmed melon tests showed backward recoil, proving that the backwards jerk of the President’s head is indeed consistent with physics, provided you consider it from the perspective of an expert in collision physics.

Of course, none of this is enough to convince the most dedicated of the crazy people who obsesses over the Kennedy assassination. But the 1976 paper Alvarez wrote is an excellent demonstration of the way that physicists approach problems, even when they originate well outside physics. And it's a useful lesson for applying scientific thinking to everyday life, as well: when confronted with a new kind of problem, think about what you already know how to do well, and it will often turn out that there are surprising applications of your existing skills that will let you make progress.

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(Part of a series promoting Eureka: Discovering Your Inner Scientist, available from Amazon, Barnes and Noble, IndieBound, Powell’s, and anywhere else books are sold.)

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