Seeing is Believing

Alpha particles aren’t usually greeted with a round of applause, but my physics class just couldn’t contain itself after we saw them for the first time. We had dragged ourselves out of the sunshine and into one of the last Physics for Poets classes of spring semester, and we were pleasantly surprised to be greeted with a cloud chamber, a piece of equipment that allows you to literally see into the subatomic scale.

Cloud chambers were one of the earliest ways of “seeing” particles. When charged subatomic particles race through the saturated alcohol vapor in the chamber, their electric charges ionize the gas particles around them, causing them to bunch up and condense into visible tracks for a few moments. The result looks something like a ghost of a spider web, with the ephemeral tracks forming and disappearing constantly, showing you where particles had just traveled. The characteristics of the track, like its direction and thickness, point to which kind of particle it is.

Observing tracks in a cloud chamber is the closest we can get to actually seeing a particle. Talking about “seeing” particles at all is misleading, as we will never be able to see an atom (let alone anything smaller) in the way that we see people’s faces. Instead of piecing together visual information and recognizing characteristics, we must rely on the effects particles have because of their charge and mass (like their tracks). The best way we currently have to determine their characteristics is to observe how particles interact with each other in colliders. Millions of collisions occur each second, and particles’ interactions usually happen so quickly that by the time the detector observes an event, the mass and energy that resulted from the collision have already changed forms many times. Identifying particles based on a series of interactions is daunting—imagine if instead of being able to recognize someone by his or her face, you had to base your identification on the reactions of a number of people who had recently seen and talked to that person. Now imagine that you’ve never met the person you are trying to identify.

Colliders are cutting-edge technology, but cloud chambers are almost a century old. By now, the particles usually visible in a cloud chamber have all been identified. Where they come from, however, is more complicated. While beams of high-energy particles are controlled and directed in accelerators, cloud chambers pick up whatever particles may be passing by at the moment. Tracks appear as if by magic, with nothing provided except the surrounding air. Most of the tracks are the result of cosmic rays, high-energy particles that have been whizzing around the universe since the Big Bang. Looking at the hundreds of electron, proton, and muon tracks in the cloud chamber made physics seem more real than it ever had. Proof of the Big Bang was literally right before our eyes. When the lab assistant put a piece of material that emitted alpha particles into the chamber and let us see radiation in action, we were understandably impressed.

Alpha particles, which contain two protons and two neutrons and are the equivalent of helium nuclei, made an even more impressive appearance in the early 20th century. In one of the most famous experiments in the history of physics, Ernest Rutherford fired alpha particles at a sheet of thin gold foil in order to study the structure of the atom. Based on the model of the atom accepted at the time, which called for a “soup” of positive charge with negatively charged electrons scattered within it, he expected the alpha particles to be only slightly deflected. What he saw, however, was a small percentage of the alpha particles bouncing straight back at him, which, he said, was “as incredible as if you fired a 15-inch shell at a piece of tissue paper, and it came back and hit you.” He concluded they must have been colliding with a tiny, positively charged part of the atom, now known as the nucleus.

Perhaps even more incredibly, Rutherford’s experiment was based on actually seeing the particles. The pattern of alpha particles showed up as tiny blips on a screen, much like an early version of a television screen. Rutherford spent hours each day in a room dark enough to see the blips, and apparently, he used to sing “Onward Christian Soldiers” at the top of his lungs while waiting for his pupils to fully dilate—much to the dismay of his graduate students.

As experiments have become more complex and high-tech, they’ve lost a little of their charm. While it is clearly more efficient and powerful to have computers, rather than singing scientists in a dark room, to analyze data, cloud chambers are still breathtaking. Seeing one in action after studying particle physics through graphs and abstractions is comparable to finally hearing an orchestra after years of studying music theory. While being able to see particles race through a cloud chamber might not teach us anything new about the subatomic world, it is perhaps still the best way we have of showing that it actually, tangibly exists. Our eyes may have become obsolete in particle physics, but they are still good for one thing—inspiration.

Elizabeth Wade is a Barnard College senior majoring in comparative literature.
Fear of Physics runs alternate weeks.
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