Author: Miles Qvale
Teacher: Redha Rubaie
[Image source: IAEA (International Atomic Energy Agency)]
It is impossible to ignore the relationship between physics and numbers. Not just any number, mind you. The heart and soul of the physics world is the myriad of constants that form the basis of all our calculations and predictions. For example, Caltech’s list of physics constants is 140 items long and covers a whole range of physics subdisciplines. However, there is no constant more famous than 299792458 meters per second - the speed of light. Whether it is for space travel or understanding that Sonic the Hedgehog is the fastest thing alive, the speed of light has always been viewed as a barrier to human movement impossible to overcome. However, what if I told you that the speed of light doesn’t exist at all, and only serves as a lazy generalization that physicists would consider inaccurate? In short, there is no ‘speed of light’ in the way that the general public understands it.
Though this might sound absurd, hear me out. Think about holding your hands above your head. For most people, this should be more than doable. Now, let's do the same action, but have the person holding 10 kg weights in each hand. Some people will not be able to do the raise at all, and most people will do it more slowly. The key to the difference between there two scenarios is the resistance that your body has to act against. Before, you just had to act against the gravitational force produced by the mass of your arms. Now, you have to overcome the gravitational force of the weights as well, making the task harder and slower. Though weightlifters and particles of light have little in common with one another, the thing they share is the presence of resistance.
The reason why resistance matters to particles is that they will not be traveling through the same density medium for the entirety of their journey. 299792458 meters per second is the speed of light if we are measuring it in a vacuum, and so serves as a theoretical maximum. However, the speed of light traveling through the air will be lower than this amount, and should we introduce water into the mix, the maximum speed will be lower still. Perhaps this is not all that remarkable to you, and for the moment, I understand. However, remember that both the processes of acceleration and deceleration require time. In some cases, nanoseconds, others microseconds, and in others milliseconds. These tiny fractions of a second are so interesting because, in those moments, particles can be going quicker than the speed of light in that medium (though undergoing rapid deceleration). Indeed, for those moments, an almost eerie blue light is produced in the same direction as these particles are traveling.
It took a few decades for people to figure out exactly what was going on with this ‘eerie blue light.’ Oliver Heaviside had been aware of cone-like emissions from charged particles as early as 1888, though his work would not be re-discovered for a century after that. In 1910, Marie and Pierre Curie noted that a vial of highly concentrated radium solution was prone to emission of ‘phosphorescence’ - giving off light after being near the solution. Once again, the phenomenon was not explored further, given just how many different bits of research the Curies were committed to at that time. It was not until two Soviet scientists in the 1930s decided to fire some radiation at a bottle of water that, once again, the blue light would emerge. These two scientists, Sergei Vavilov and Pavel Cherenkov, would go on to publish their findings, and the concept of Cherenkov radiation was born.
At this point, you might be wondering why you should care about the fact that Cherenkov radiation exists. I will give you three different reasons, and the first of them is that they are one of the most influential physics concepts in the history of pop culture! Think about Superman’s Kryptonite, or the iconic green rods in the opening credits of the Simpsons. The thing that they share in common is that they both have a glow (albeit, green) which is consistent to what you would observe from Cherenkov radiation. It is a good way of announcing to an audience that a material is radioactive or dangerous. However, this is pop culture, so they didn't get it totally right. Most objects, even highly radioactive ones, are unlikely to emit Cherenkov radiation in air. Normally, you need a more dense material (such as heavy water) to fully observe Cherenkov emissions.
However, there is one place where there is plenty of heavy water, and this is in the cooling systems of nuclear reactors. What scientists have realized is that they can visually measure the level of radiation leaking from the main chamber of the nuclear reactor. If the radiation counters are faulty for whatever reason, then Cherenkov radiation can be used as a back up. If the blue light in the heavy water begins to get brighter, this is a sign that the reactor is about to set off a critical chain reaction and ‘melt down’. As you might guess, this is not something we want to have happen, as it is very dangerous to both the people who work at the reactor and the people who live around it. As a result, any elevated levels of Cherenkov radiation means that the reactor gets shut down so it can be inspected.
Another way in which the idea of Cherenkov radiation has proven to be very useful is when we are looking for neutrinos. Imagine neutrinos as the shy subatomic guests at an atomic garden party - they will pass through atoms without disrupting anything at all. That makes them very hard to detect using conventional sensors, given that it would require incredibly sensitive equipment. However, scientists at the Radio Ice Cherenkov Experiment (RICE), based in the Antarctic, realized that Cherenkov light detection using holes 200 m into the ice would be able to isolate lots of the background electromagnetic noise and focus on looking for high-energy neutrinos. The experiment was successful in detecting a few neutrinos a year between 2000 and 2010. A few neutrinos might not sound like a lot to us, but was a major success at the time. The technology would be rolled into the later Askaryan Radio Array - which started working in 2011.
Whether it is comic books or the search for the origins of matter, it is clear that Cherenkov radiation is one of the unsung heroes of the physics world. From its early observations, this phenomenon has not only influenced popular culture but also proven instrumental in nuclear reactor safety. Its usefulness is further showcased in neutrino detection experiments, having the potential to change particle physics as we know it. Cherenkov radiation exemplifies the synergy between theoretical concepts and real-world applications, emphasizing the evolving landscape of physics and the continual pursuit of understanding the many mysteries of the universe.
Bibliography
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Grimus, W. and Neufeld, H. (1993) ‘Cherenkov radiation of neutrinos’, Physics Letters B, 315(1–2), pp. 129–133. doi:10.1016/0370-2693(93)90169-i.
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Office of Nuclear Energy (2023) Cherenkov radiation, explained, Energy.gov. Available at: https://www.energy.gov/ne/articles/cherenkov-radiation-explained (Accessed: 07 December 2023).