Light Versus Light: The Secret Physics Battle That Could Rewrite the Rules

In a fascinating dive into the strange world of quantum physics, scientists have shown that light can interact with itself in bizarre ways—creating ghost-like virtual particles that pop in and out of existence.

This “light-on-light scattering” isn’t just a theoretical curiosity; it could hold the key to solving long-standing mysteries in particle physics.

Quantum Light: Why Lasers Don’t Clash Like Lightsabers

Under normal conditions, light waves can pass through one another without any interference. Based on the principles of electrodynamics, two beams of light can occupy the same space without affecting each other. They simply merge and continue on their paths. This means that the dramatic laser duels seen in science fiction would be much less exciting in real life.

However, quantum physics introduces a twist. It predicts a phenomenon known as “light-on-light scattering.” This effect cannot be detected with typical laser systems, but it has been observed in high-energy environments like the CERN particle accelerator.

In this process, virtual particles can momentarily appear from the vacuum, interact with photons, and alter their trajectory. Although the effect is incredibly subtle, understanding it with precision is essential for testing particle physics theories, especially in sensitive experiments involving muons.

Researchers at TU Wien (Vienna) have now demonstrated that one overlooked factor plays a significant role in this process: the influence of particles known as tensor mesons. Their findings were recently published in Physical Review Letters.

Ghost Particles That Leave Real Marks

When photons collide or interact, virtual particles can briefly come into existence. These particles vanish almost instantly and cannot be observed directly. In a strange way, they both exist and do not exist at the same time. Quantum mechanics allows for this kind of paradox, where different states can coexist even if they seem contradictory from a classical perspective.

“Even though these virtual particles cannot be observed directly, they have a measurable effect on other particles,” says Jonas Mager from the Institute of Theoretical Physics at TU Wien, lead author of the study. “If you want to calculate precisely how real particles behave, you have to take all conceivable virtual particles into account correctly. That’s what makes this task so difficult – but also so interesting.”

What Happens When Light Hits Light

When light scatters off light, a photon may transform, for example, into an electron-positron pair. Other photons can then interact with these two particles before the electron and positron annihilate each other and become a new photon. Things become more complicated when heavier particles are created that are also subject to strong nuclear forces – for example, mesons, which consist of a quark and an antiquark.

“There are different types of these mesons,” says Jonas Mager. “We have now been able to show that one of them, the tensor mesons, has been significantly underestimated. Through the effect of light-light scattering, they influence the magnetic properties of muons, which can be used to test the Standard Model of particle physics with extreme accuracy.” Tensor mesons did appear in earlier calculations, but with very rough simplifications. In the new evaluation, not only does their contribution turn out to be much stronger than previously assumed, but it also has a different sign than previously thought, thus influencing the results in the opposite direction.

Gravitons and Holograms: A 5D Approach

This result also resolves a discrepancy that arose last year between the latest analytical calculations and alternative computer simulations. “The problem is that conventional analytical calculations can describe the strong interactions of quarks only well in limiting cases,” says Anton Rebhan (TU Wien).

The TU Wien team, on the other hand, used an unconventional method – holographic quantum chromodynamics. This involves mapping processes in four dimensions (i.e., three spatial dimensions and one time dimension) onto a five-dimensional space with gravity. Some problems can then be solved more easily in this other space, and the results are then transformed back again. “The tensor mesons can be mapped onto five-dimensional gravitons, for which Einstein’s theory of gravity makes clear predictions,” explains Anton Rebhan. “We now have computer simulations and analytical results that fit well together but deviate from certain previous assumptions. We hope that this will also provide new impetus to accelerate already planned specific experiments on tensor mesons.”

Putting the Standard Model Under the Microscope

These analyses are important for one of the biggest questions in physics: How reliable is the Standard Model of particle physics? This is the generally accepted quantum physical theory that describes all known types of particles and all forces of nature – except gravity.

The accuracy of the Standard Model can be investigated particularly well in a few special test cases, for example, by measuring the magnetic moment of muons. For many years, scientists have been puzzling over whether certain discrepancies between theory and experiment point to “new physics” beyond the Standard Model, or whether they are simply inaccuracies or errors. The discrepancy in the muon magnetic moment has recently become much smaller – but in order to really search for new physics, the remaining theoretical uncertainties must also be understood as precisely as possible. This is exactly what the new work contributes to.

Reference: “Longitudinal short-distance constraints on hadronic light-by-light scattering and tensor-meson contributions to the muon 𝑔−2” by Jonas Mager, Luigi Cappiello, Josef Leutgeb, and Anton Rebhan, Accepted, Physical Review Letters.
DOI: 10.1103/dxwr-gpsl

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