The quest to map the inside of the proton

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“How are matter and energy distributed?” asked theoretical physicist Peter Schweitzer of the University of Connecticut. “We don't know.”

Schweitzer has spent much of his career thinking about the gravitational side of the proton. In particular, he is interested in a matrix of properties of the proton called the energy-momentum tensor. “The energy-momentum tensor knows everything there is to know about particles,” he said.

In Albert Einstein's theory of general relativity, which causes gravitational attraction as objects follow curves in space-time, the energy-momentum tensor tells space-time how to bend. For example, it describes the arrangement of energy (or, equivalently, mass) – the source of the lion's share of the curvature of space-time. It also tracks information about how momentum is distributed, as well as where compression or expansion will occur, which can even mildly bend space-time.

If we can know the shape of space-time around a proton, which Russian and American physicists worked on independently in the 1960s, we can predict all the properties indexed in its energy-momentum tensor. Are. These include the proton's mass and spin, which are already known, as well as the proton's pressure and arrangement of forces, a collective property physicists refer to as the “Druck word” after the word for pressure in German. The term is “as important as mass and spin, and no one knows what it is,” Schweitzer said, although that is beginning to change.

In the '60s, it seemed as if measuring the energy-momentum tensor and calculating the Druck term would require a gravitational version of the usual scattering experiment: You fire a massive particle at a proton and the two get a Gravitational exchange – hypothetical particle that creates gravitational waves instead of a photon. But because of the extreme weakness of gravity, physicists expect gravitational scattering to be 39 orders of magnitude more rare than photon scattering. Experiments cannot possibly detect such a weak effect.

“I remember reading about it when I was a student,” said Volker Burkert, a member of the Jefferson Lab team. The conclusion was that “we will probably never learn anything about the mechanical properties of the particles.”

gravity without gravity

Gravity related experiments are still unimaginable today. But research conducted by physicists Jiangdong Ji and the late Maxim Polyakov, working separately in the late 1990s and early 2000s, led to a solution.

The general plan is as follows. When you lightly fire an electron at a proton, it usually sends a photon to one of the quarks and deflects it. But something special happens in less than one in a billion occurrences. The incoming electron sends in a photon. A quark absorbs it and then emits another photon in a subsequent heartbeat. The main difference is that this rare event involves two photons instead of one – both an incoming and an outgoing photon. Gee and Polyakov's calculations showed that if experimenters could collect the resulting electron, proton, and photon, they could infer from the energies and momentum of these particles what happened to the two photons. And that two-photon experiment would be just as informative as the essentially impossible gravitational-scattering experiment.