“How are matter and energy distributed?” requested Peter Schweitzer, a theoretical physicist on the University of Connecticut. “We don’t know.”

Schweitzer has spent most of his profession fascinated with the gravitational aspect of the proton. Specifically, he’s all for a matrix of properties of the proton known as the energy-momentum tensor. “The energy-momentum tensor knows everything there is to be known about the particle,” he mentioned.

In Albert Einstein’s concept of normal relativity, which casts gravitational attraction as objects following curves in space-time, the energy-momentum tensor tells space-time learn how to bend. It describes, for example, the association of vitality (or, equivalently, mass)—the supply of the lion’s share of space-time twisting. It additionally tracks details about how momentum is distributed, in addition to the place there can be compression or growth, which may additionally calmly curve space-time.

If we may be taught the form of space-time surrounding a proton, Russian and American physicists independently labored out within the Sixties, we may infer all of the properties listed in its energy-momentum tensor. Those embrace the proton’s mass and spin, that are already recognized, together with the association of the proton’s pressures and forces, a collective property physicists confer with because the “Druck term,” after the phrase for stress in German. This time period is “as important as mass and spin, and nobody knows what it is,” Schweitzer mentioned—although that’s beginning to change.

In the ’60s, it appeared as if measuring the energy-momentum tensor and calculating the Druck time period would require a gravitational model of the same old scattering experiment: You fireplace an enormous particle at a proton and let the 2 trade a graviton—the hypothetical particle that makes up gravitational waves—moderately than a photon. But as a result of excessive weak spot of gravity, physicists anticipate graviton scattering to happen 39 orders of magnitude extra not often than photon scattering. Experiments can’t presumably detect such a weak impact.

“I remember reading about this when I was a student,” mentioned Volker Burkert, a member of the Jefferson Lab crew. The takeaway was that “we probably will never be able to learn anything about mechanical properties of particles.”

Gravity Without Gravity

Gravitational experiments are nonetheless unimaginable at present. But analysis within the late Nineteen Nineties and early 2000s by the physicists Xiangdong Ji and, working individually, the late Maxim Polyakov revealed a workaround.

The normal scheme is the next. When you fireplace an electron calmly at a proton, it normally delivers a photon to one of many quarks and glances off. But in fewer than one in a billion occasions, one thing particular occurs. The incoming electron sends in a photon. A quark absorbs it after which emits one other photon a heartbeat later. The key distinction is that this uncommon occasion includes two photons as a substitute of 1—each incoming and outgoing photons. Ji’s and Polyakov’s calculations confirmed that if experimentalists may gather the ensuing electron, proton and photon, they might infer from the energies and momentums of those particles what occurred with the 2 photons. And that two-photon experiment could be basically as informative because the unattainable graviton-scattering experiment.