The Quest | The Free Press of Reed College

Cool Thesis of the Week: Quantum Mechanics and General Relativity

All too often, the complaint over the impracticality of the humanities resounds through Hauser: “All these abstract ideas, none of them really exist. Not like what they do in the hard sciences.”

Not so for Gabriel Barello ’12, a Physics major from Seattle, Washington. Gabriel, who is working on his thesis with Professor David Latimer, is using nonexistent particles in an exercise in theoretical physics that will estimate the behavior of particles that have been scattered by the gravitational field of a large object.

Scattering, generally, is when “a particle or object has force applied to it by another object and changes direction,” says Gabriel. In this case, Gabriel will estimate the effect of the gravitational force exerted by a black hole on theoretical particles called “massive scalar fields.”

However, says Gabriel, these particles “do not actually exist,” and “are really more fields”—physical quantities associated with points in spacetime—than they are particles. Instead, they are abstractions of quantum theory. “Quantum mechanics predicts a mathematical structure for particles,” Gabriel explains. These mathematical structures, he goes on, can be simplified to yield the massive scalar field, “the simplest example of a quantum mechanical field.” Though the massive scalar field does not actually describe any object in the empirical world, its mathematical properties, similar to those of actual, physical particles, should allow Gabriel to gain insight into what would happen with “more physically relevant cases” like electrons or neutrons.

However, there are more problems for Gabriel than the nonexistence of the particles he is studying. Usually, scattering is treated by quantum mechanics, the branch of physics that deals with the lowest possible discrete amounts of energy and mass. However, Gabriel says, there are apparent incompatibilities between general relativity, the physical theory that describes gravity, and quantum mechanics, which would make treating gravity with quantum mechanics impossible. “Essentially,” he says, “you’re measuring infinite quantities.” Thus, the interaction of gravity and quantum mechanics “is a historically hard problem.”

To get around this issue, Gabriel will combine two different branches of physics. He is treating the scattered particles with quantum mechanics, and the gravitational field with classical mechanics. He will rely on approximations to develop a theoretical mathematical model of the interaction. “The same techniques have been applied with other forces,” he says, “but not to my knowledge with gravity.”

Even though, Gabriel says, the processes under examination are not physically observable—he will not be conducting any lab experiments for his thesis—his work is necessary “preparation” for studying other cases, laying down “the meta-theory of the abstract structure.” Indeed, he hopes to expand his work, either later this year or at another time, into the same scattering phenomenon acting on electrons. More generally, Gabriel hopes that his work will pave the way to a better understanding of the fundamental forces of the physical world—one of the last issues that modern physics has still yet to fully elucidate. “It’s exciting to me,” he says, “because the combination of quantum mechanics and general relativity is going to be where the next revolution in physics will come from.”

Do you have or know of a thesis that compels attention? Just want to see your face in The Quest? Email with “Cool Thesis” in the subject line.

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