Tuesday, November 3, 2009


In the previous post particles appeared which are said to be exchanged between other particles. These particles are also called 'force' particles, in contrast to those objects exchanging them, the matter fields.

To the matter fields belong the leptons, the neutrinos and the quarks.

The force particles are the photons, the gluons, and the W and Z bosons.

The Higgs takes a role in between. On the one hand it can exchange force particles, on the other it is itself exchanged.

But how can one imagine the 'exchange' of a particle?

It is a little bit like when two boats pass by each other on a quiet sea. If they move, the generate waves which travel from one to the other, and are very much felt by each other. Anyone having traveled in a boat can confirm that it can get quite rocky if another fast boat comes close by.

The situation in particle physics is somewhat similar. The boats are the particles. The water is essentially a medium made up of force particles. If a particle now crosses this medium, it generates disturbances in it, which can travel and can be felt by other boats.

Why are then the force particles are called particles? The waves are indeed very much different from the boats. The reason is that the medium is very much different from water. If the waves in the medium are strong enough, they actually become very narrow, and look very much like a boat (a particle) themselves. Therefore, at strong waves, or if much energy has been invested in creating a wave, the wave looks like another boat (another particle). In fact, some of these can then exchange waves themselves, the force particles become matter particles in their own right.

So there is an interesting duality between the force carriers and those affected by the force, similar to the Higgs particle itself.

As a consequence, it has become common to talk even of the medium as an ensemble of particles, though this is not entirely right: If the waves are shallow, there is no structure which could be recognized as a particle. It is exactly this domain, which is least understood. The reason is that the medium is also in another respect different from water. If the waves are shallow, they affect the remaining medium much stronger than do water waves. In fact, only on the contrary, if the waves are strong, they more or less ignore the remaining medium, but only then.

To understand how this medium behaves is one of the central questions in my own research, but also one of the great unsolved questions in the standard model during its more that thirty year old history.