Waves… at CERN?

CERN is one of the most famous places in the world for physics, and for science in general. It has been the site of numerous scientific discoveries, chiefly the experimental discovery of the Higgs boson, a particle theorised to exist since 1964 but not discovered until 2012. In a place so concerned with particle physics, the last thing you would expect to find would be experiments that deal with waves, which at least classically are almost the opposite of particles. Particles are localised (they exist in one place in time and space), they follow trajectories, and they do not undergo interference. Waves on the other hand, do interfere with each other, and are spead out continuously in space.

So what do they have in common? Well, to answer this question, we must go deeper than classical physics, into the realm of quantum mechanics. Quantum mechanics tells us that particles can be described as waves, and vice versa. How can this be? Well, it was first noticed when looking at electron beams – in some ways, the electrons behaved as particles (for example, they have mass like a particle). However, after other experiments, it was found that electrons can interfere and diffract, just like waves! Really, what we can say is that properties of the electron which we can measure (spin, position, momentum, etc.) are not precise like a particle until we measure the respective property. Before this, the electrons are described by a wavefunction – this is a function that gives us information about the relative probability of finding an electron in a particular position in space.

So, if waves and particles can almost be treated the same, then why do we do “particle” physics at CERN and not “wave” physics? The answer lies in energy. This was explained by de Broglie – he stated that the wavelength of a wave that could be associated with a particle was related to the momentum of the particle – and therefore its energy. Higher energy particles have a wavefunction that is extremely localised in space – this means that although we could technically look at the particles as waves, this would overcomplicate the physics we have to do, and it doesn’t give us an extra information. On the other hand, whilst we could think of light as a stream of particles called photons (and sometimes we do), low energy light such as radio waves are almost always referred to as waves! The fact that the light has low energy, means that the wave is more spatially spread out (or rather, the wavelength is longer) – this makes the wave behaviour more apparent.

The de Broglie equation

So, when can we use waves in particle physics? For one, particle accelerators essentially operate through electromagnetic waves. Traditionally, in an accelerator there will be certain cavities, or areas of empty space. These areas are filled with an electromagnetic field (waves) and when a particle enters the cavity, it experiences a force from the electromagnetic field which accelerates the particle. However there are other, more innovative ways of using waves to accelerate particles. At AWAKE (Advanced Proton Driven Plasma Wakefield Acceleration Experiment), research is being done on how to create plasma wakefields, and how to use these to accelerate particles. Firstly, plasma is just a collection of ionised particles, whether this be ions or electrons. A plasma wakefield is created when either a proton or a laser is used to create a ripple effect in the plasma, therefore moving many of the electrons away from their host atoms, but leaving behind heavier ions. In turn, the electrons bounce back to their atoms, but much too strongly. What is created is a very strong electromagnetic field from the seperation of the negative electron charges and the positive nucleus charges. It has been found that these waves can be used to accelerate electrons much more efficiently than the electromagnetic fields currently used inside the largest particle collider, the Large Hadron Collider. So far, AWAKE has provided promising results and personally I can’t wait to see what they do next!

The AWAKE experiment, CERN

#TP