Gregory Snow - 1984


Diffractive photon dissociation in a high pressure hydrogen time-projection chamber


The student I am about to present to you today for the degree of Dr. of Philosophy, Gregory Snow, is the first student to be graduating from The Rockefeller University in the field of Experimental High Energy Physics. His dissertation addresses the very nature of a particle well known to all of us, the particle of light, known as photon. The investigation of the properties of light has played a key role in understanding the physical laws of our universe. The controversy over the wave versus particle nature of light gave birth to quantum mechanics, and the discovery that the speed of light is constant, independent of the motion of the light source, led to the theory of relativity.

In recent times, research on light has focused on questions concerning the internal structure of the quantum of light, the photon. For over 100 years, photons were thought to have no internal structure. They were viewed as quanta of energy with zero rest mass, traveling with the maximum speed allowed in the universe, and interacting only with electrically charged particles through the feeble electromagnetic force. However, on studying the behavior of high energy photons produced in modern particle accelerators, it was discovered that at the level of a fraction of a percent, they also interact strongly like protons, neutrons, and pions. Through their strong interaction, photons acquire internal structure; and although they are massless, in interacting with matter they can convert to particles with mass.

The spectrum of these particles, the excited states of the photon, represents in some sense a snapshot of the photon's internal structure. A few of these states have been known for some time. In his thesis experiment, Gregory Snow studied the complete excitation spectrum of the photon. Careful analysis of this spectrum revealed important aspects of the structure of the photon and its interaction with hadronic matter, showing that there is more to the photon than what meets the eye.

After such giants of physics like Huygens, Newton, and Einstein, Greg, a student, still found something exciting to do in this field; and he did it well, as he does everything else he attempts. His serious, hard driving, enthusiastic and always pleasant personality is a model to all of those who know him. As our first student, he became the life of the laboratory, and in a way we felt unhappy as the time of his graduation approached. Personally, I went as far as telling him that I wouldn't let him graduate unless he let me beat him in the New York City Marathon, which we ran together several times. I thought that his competitive nature would not allow this to happen and thus he would remain with us a student, who knows, perhaps for ever. But this competitive nature also compelled him to graduate. And as usual, he was quick in finding a solution to this problem: he offered to stay and work with us as a postdoctoral fellow. So, he has been allowed to graduate and is now a member of our faculty working on an experiment at the European Center for Nuclear Research, in Geneva, Switzerland, comparing proton-proton with antiproton-proton interactions.

Mr. President, I am very happy and proud to present to you for the degree of Dr. of of Philosophy an accomplished physicist, a valuable colleague, and a great friend, Gregory Snow.