Beyond the void: New experiment challenges quantum electrodynamics

The HZDR team proposed improvements to an experiment designed to study physical boundaries.

Absolute emptiness—that’s how most of us imagine a vacuum. In reality, however, it is filled with flickers of energy: quantum fluctuations. Scientists are currently preparing a laser experiment aimed at verifying these vacuum fluctuations in a novel way, which could provide clues to new laws of physics.

A research team from the Helmholtz Zentrum Dresden Rosendorf (HZDR) has developed a series of proposals designed to help conduct experiments more efficiently and thus increase the chances of success.The team published its findings in the journal Science Physical Review D.

Physics has long recognized that vacuum is not entirely empty but is instead filled with vacuum fluctuations—ominous quantum flickers of light across time and space. Although it cannot be captured directly, its effects can be observed indirectly, such as through changes in the electromagnetic field of tiny particles.

However, it has not been possible to verify vacuum fluctuations in the absence of any particles. If this can be achieved, one of the fundamental theories of physics, quantum electrodynamics (QED), will be demonstrated in a hitherto untested area. However, if such an experiment reveals deviations from theory, it could indicate the presence of new, previously undiscovered particles.

Dr. Ulf Zastrau leads the European XFEL’s HED (High Energy Density Science) experimental station. In the HED beam chamber, a flash from the world’s largest X-ray laser must meet a pulse of light from the ReLaX high-power laser operated by HZDR to detect vacuum fluctuations. Image credit: XFEL Europe/Jan Hosan

Experiments aimed at achieving this are planned as part of the Helmholtz International Extreme Field Beamline (HIBEF), a research consortium led by HZDR at the HED experimental station of the European XFEL in Hamburg, the world’s largest X-ray mine launcher. The basic principle is that a super powerful laser emits a short and intense flash into a vacuum stainless steel chamber. The aim is to manipulate vacuum fluctuations so that they seemingly magically change the polarization of Europe’s XFEL’s X-ray flash, that is, rotate the direction of its oscillations.

“It’s like sliding a clear plastic ruler between two polarizing filters and then bending it back and forth,” explains HZDR theorist Professor Ralf Schützhold. “The filters are originally set up to prevent light from passing through. Bending the ruler now changes the direction of the light oscillations, allowing some of the results to be seen.” In this analogy, the ruler corresponds to vacuum fluctuations, while super-powerful laser flashes bend them.

Flash twice instead of once

The original concept was to shoot an optical laser flash into a chamber and use specialized measurement techniques to record whether it changed the polarization of the X-ray flash. But there’s a problem: “The signal can be extremely weak,” Schutzhold explains. “Probably only one in a trillion X-ray photons changes their polarization.”

But this may be below current measurement limits—the event could easily slip through without being detected. So Schutzhold and his team relied on a variation: Instead of just one, they aimed to fire two optical laser pulses into the vacuum chamber at the same time.

Both flashes will hit there and actually collide. Europe’s XFEL’s X-ray pulses are programmed to hit their collision points precisely. The decisive factor: The colliding laser flash affects the X-ray pulse like a crystal. Just as X-rays are diffracted (i.e., deflected) when passing through natural crystals, XFEL X-ray pulses are supposed to be deflected by two “photocrystals” where the flashes of colliding lasers exist briefly.

“This not only changes the polarization of the X-ray pulse, but also deflects it slightly,” explains Ralf Schützhold. The researchers hope that this combination could increase the chances of actually measuring the effects. The team calculated various options for the impact angle of two laser flashes colliding indoors. Experiments will demonstrate which variant is most suitable.

For ultralight ghost particles?

The prospects can be improved even further if the two laser flashes fired into the chamber are not of the same color but of two different wavelengths. This will also cause a slight change in the energy of the X-ray flash, which will also help with the measurement. “But this is technically quite challenging and may only be implemented later,” Schutzhold said.

The project is currently in the planning stage with the European XFEL team at the HED experimental station in Hamburg, with the first trials scheduled to start in 2024. If successful, they can confirm QED again.

But perhaps experiments will reveal deviations from established theory. This could be due to previously undiscovered particles—for example, ultralight ghost particles called axions. “This would clearly indicate the existence of additional, previously unknown laws of nature,” Schutzhold said.

Reference: “Detection schemes for quantum vacuum diffraction and birefringence” by N. Ahmadiniaz, TE Cowan, J. Grenzer, S. Franchino-Viñas, A. Laso Garcia, M. Šmíd, T. Toncian, MA Trejo and R. Schützhold, October 10, 2023 Physical Review D.
DOI: 10.1103/PhysRevD.108.076005