What happens when a high-speed jet displays motion that is essentially higher than the velocity of sound? One would hear a cracking sound commonly known as a sonic boom. Analogous to this phenomenon, there might exist something similar in the case of electromagnetic radiation as light and sound have a lot of commonalities considering their physical effects. In fact, there does exist a similar phenomenon in the case of light.
When a charged particle like an electron travels faster than the phase velocity of light inside a water-bound nuclear reactor, there is an intense emission of blue light. This effect is called the Cherenkov effect named after the Soviet physicist Pavel Cherenkov who observed it for the first time in 1934. In a way, this is an optical analogue of the sonic boom effect that essentially relies upon shock waves. A full mathematical theory of the Cherenkov effect was developed using elements from classical electrodynamics and all the major physicists involved in describing the theoretical and mathematical aspects of it were awarded the Nobel prize in 1958.
Applications of Cherenkov Radiation
Cherenkov radiation can be considered as one of the faster-than-light (superluminal) effects but a very special one as it is subject to constraints of the medium inside which the effect takes place. The frequency spectrum of the Cherenkov effect is given by the Frank-Tamm formula. Since the time the effect was discovered, it has led to diverse applications in fields ranging from medical science to nuclear physics as well as in various particle detection techniques.
A particle (red line) propagates through a medium with a velocity β higher than the speed of light in this medium. As a consequence, a cone of radiation known as Cherenkov radiation is produced. The opening angle θ of this cone gives an estimation of the energy of this particle. Figure and description: Sera Markoff
So far Cherenkov radiation and its interactions with different systems have been observed in 3D environments. Now for the first time, researchers based at the Technion in Israel have detected this phenomenon in the 2D regime and discovered that it behaves quite differently than previously observed [1]. Up until now, its description based on classical electromagnetic theory was dominant and sufficient but with the current discovery, the quantum behaviour of radiation seems essential to explain the empirical results.
Experimental Advancements: Increased Photon Emission Efficiency
Since this was observed as a low-dimensional effect, it became easier to probe the quantum nature of the emission process and take note of the intensity of photons emitted during the process of interaction with electrons. Another interesting observation resulting from the investigation was that indirect evidence of quantum correlation or entanglement was also detected between the properties of the electron and that of the light emitted as a result of the dynamical motion of the electron. This observation essentially came up as a surprise for the researchers.
Furthermore, the latest empirical probe observed a significant spur in the efficiency of the phenomenon. In the previous experiments on Cherenkov radiation, approximately one out of 100 electrons emitted the Cherenkov photon. However, in the current work, it was observed that every electron emitted photons, which essentially is an advancement by more than 2 orders of magnitude. To learn more about the details of the experimental work as well as the original author views, look at the article by Phys.org.
A bonus fact for the reader here is that previous data shows the emission spectrum of Cherenkov radiation varies with water depth, the graph below represents the same. As one can explicitly see, the intensity of Cherenkov photons increases with decrease in the water depth. Additionally, one can also notice from the representation that most of the Cherenkov photons lie in the bluish-violet region of the spectrum, thus making the effect highly monochromatic and specific.
Cherenkov radiation produced in the water is emitted across a broad, continuous spectrum, and most of the light is produced predominantly in the blue, violet, and ultraviolet regions of the electromagnetic spectrum. Source: A.Vargas/IAEA
In conclusion, we can say that apart from finding a novel dimensionality where the Cherenkov effect is applicable, the latest work by the Technion research group further provides a strong confirmation of the discrete nature of radiation which is very well known as quantized wave packets or photons. The discovery provides promising assurance that substantially old concepts like the photonic nature of electromagnetic radiation can still have renewed verifications and new empirical tests (perhaps new effects too) which can provide convincing proof of such phenomena.
References
[1] Yuval Adiv et al, Observation of 2D Cherenkov Radiation, Physical Review X (2023). DOI: 10.1103/PhysRevX.13.011002
