“How are matter and energy distributed?” asked Peter Schweitzer, theoretical physicist at the University of Connecticut. “We do not know”.
Schweitzer has spent most of his career thinking about the gravitational side of the proton. Specifically, he is interested in a matrix of proton properties called the energy-momentum tensor. “The energy-momentum tensor knows everything there is to know about the particle,” he said.
In Albert Einstein’s theory of general relativity, which projects gravitational attraction as objects following curves in space-time, the energy-momentum tensor tells space-time how to bend. It describes, for example, the arrangement of energy (or, equivalently, mass), the source of most of the twisting of space-time. He also tracks information about how the momentum is distributed, as well as where there will be compression or expansion, which can also slightly curve spacetime.
If we could learn the shape of the space-time surrounding a proton, Russian and American Physicists worked out independently in the 1960s, we could infer all the properties indexed in its energy-momentum tensor. These include the proton’s mass and spin, which are already known, along with the arrangement of proton pressures and forces, a collective property that physicists refer to as the “Druck term,” after the word It means pressure in German. This term is “as important as mass and spin, and no one knows what it is,” Schweitzer said, although that is starting to change.
In the 1960s, it seemed as if measuring the energy-momentum tensor and calculating the Druck term required a gravitational version of the usual scattering experiment: shoot a massive particle at a proton and let the two exchange a graviton, the hypothetical particle. that forms gravitational waves, instead of a photon. But because of the extreme weakness of gravity, physicists expect graviton scattering to occur 39 orders of magnitude less than photon scattering. Experiments cannot detect such a weak effect.
“I remember reading about this when I was a student,” he said. Volker Burkert, a member of the Jefferson Laboratory team. The conclusion was that “we will probably never be able to learn anything about the mechanical properties of particles.”
Gravity without gravity
Gravitational experiments are still unimaginable today. But research conducted in the late 1990s and early 2000s by physicists Xiangdong Ji and, working separately, the late Maxim Polyakov revealed to alternative solution.
The general scheme is as follows. When you shoot an electron lightly at a proton, it usually delivers a photon to one of the quarks and it bounces back. But in less than one in a billion events, something special happens. The incoming electron sends a photon. A quark absorbs it and then emits another photon a heartbeat later. The key difference is that this rare event involves two photons instead of one: incoming and outgoing photons. Ji and Polyakov’s calculations showed that if experimenters could collect the resulting electron, proton, and photon, they could infer from the energies and momenta of these particles what happened to the two photons. And that two-photon experiment would be essentially as informative as the impossible graviton scattering experiment.