The forces of nature II - Electromagnetism

Electromagnetism is another force that everyone of us experiences in their daily lives. Of course, everything that is electric is based on electromagnetism. But it is not only that. When we knock on the desk, the force we feel is actually also electromagnetism. The force which keeps a crystal together, or the desk, or make water not spontaneously evaporate: All this and much more is electromagnetism. In fact, even light is electromagnetism. Our eye registers electromagnetic radiation and turns it with the brain into visual information.

As the name already suggests - Electro-magnetism - electromagnetism is the unification of the electric force, which make electric things work, and magnetic force, which is the force making magnets act like they do.

Indeed, both forces are not fundamentally different. If one takes special relativity, the theory of movements close to the speed of light, into account, they turn out to be just two sides of the same coin.

If looked at from the quantum perspective, this is even more manifest: Both forces are mediated by the same particle, the photon. The photon itself is not feeling the force itself it mediates. This makes electromagnetism fundamentally different from the weak and strong forces, which will be described next.

In turn, almost all matter feels electromagnetism: All quarks carry an electric charge. A charge, like the mass in case of gravity, is the fundamental docking port for the exchange particles. Also electrons, and their heavier cousins the muon and tau, carry electric charge. On the other hand, the neutrinos and the ever-searched-for Higgs particle are both neutral, and do not interact with the photon (there is a subtlety in this statement which will be discussed much later).

An amazing difference between mass and electromagnetic charge is that it is quantized. There appears to exist an elementary charge of which the charged particles carry a certain number. For historical reason, this is counted in thirds, instead of integer. So, the up quark carry 2/3 of an electric charge, while the electron carries -1, and so on. This is different in two respects from mass, the charge of gravity.

One is that it is quantized. Though we understand how to describe this quantization, the standard model of particle physics actually gives no explanation for it. In particular, the fact that electrons and up quarks have electric charges in a ratio of whole numbers is not understood, though it is required for the standard model in its current form to work. There are many unified theories which try to explain it, but none of them has produced yet observable effects which are different from the standard model and could be accessed in experiments.

The other property is that there is a positive and a negative charge. Mass comes only in positive portions. Electric charge comes in two varieties, + and -, which can compensate each other. As a consequence, the net charge of an array of objects can be zero, which is not possible for gravity. Such an exact zero is only possible because of the quantization. In general, almost all objects have almost zero charge. As a consequence, electromagnetism is screened at long distances: Net zero charge objects can only pull at each other, if for some reasons their internal charge array is such that positive and negative charges are not evenly distributed. Then another such object can pull, say, at the top side and push at the bottom. But this is a weak effect compared to the electromagnetism of charges objects. Only close by this can be important, and yields many of the everyday experiences with electromagnetism. And that is pretty good: As stated before, electromagnetism is very much stronger than gravity, and if it would not be screened, everything would pull or push at each other with enormous strength, creating an immediate collapse.

The only electromagnetic objects not affected by this screening are photons. Therefore, they can travel freely. The most common experience with this is light, but also the cosmic microwave background or radio is based on this. If photons would also be charged, this would not be possible.

Hence, electromagnetism introduces two new concepts: Quantization of charge and different signs of the charge, yielding the possibility of screening. Both will become very important for the strong and weak forces as well.

The quantum theory of electromagnetism, in contrast to the case of gravity, is very well understood. It is called quantum electrodynamics (QED), and it is one of the best and most successful theories today. As such, it is also part of the standard model of elementary particles, and there it is called the electromagnetic sector.

Unfortunately, it is not as good as it could be. If you increase the speed, and thus energy, of the particles, QED starts to get into trouble eventually. It appears that at very high energies the theory collapses, and electromagnetic interactions ceases altogether. However, this is likely stabilized when QED is embedded into the standard model.

Still, QED is more than only a successful theory. In fact, the structure of the theory, a so-called quantum gauge theory, is prototypical, and more complex versions of it are the theories describing the strong and weak forces, and also many attempts for quantum gravity. To our knowledge, it is the most important type of theories. What such a theory is precisely will be described after the introduction of the other two forces of nature: The weak and the strong one.

The forces of nature I - Gravity

After illustrating last time how a force can be created by the exchange of particle, it is about time to make a list of which forces there are in nature, and which of them are included in the standard model.

Actually, there is only one force in nature which we currently know and which is not included in the standard model of particle physics. This is gravity. That is the force which pulls one inevitably to the ground, as long as one is not actively working against it. And the one which makes it so hard to get up in the morning. Or so.

It is actually not only the ground, and thus the earth, that is pulling at you, but actually also the earth is pulled by you. However, since the earth is much heavier than you are, it is rather ignorant of your presence. However, it cannot ignore the pull of the moon, to which it reacts with the tides. Nor can it ignore the sun, and this makes earth orbiting around it. On a larger scale, the solar system feels the pull of the milky way, making the solar system orbiting the center of it. And our galaxy the center of the local cluster of galaxies.

In fact, any object which has mass pulls any other object towards it, which has also mass. Actually, this is not entirely correct: Mass is not necessary, it suffices if there is energy in the game. This will lead a bit too far astray now, as it is necessary to delve into the theory of relativity for why this is the case, and I will leave this to later.

However, the generic concept that some objects act a force on each other because they both have a certain property is far more general. It is the simplest example of a charge. Gravity is simple in that everything pulls everything else to itself. In other cases, which will be encountered next time, this is not always the case: Some charges pushes away other charges.

So, why is gravity not included in the standard model (yet)? The simple answer is that we do not yet know how to really do it. There are quite a number of ideas, going by the fancy names of string theory, quantum loop gravity, and many others. However, none of these ideas could have been yet made so precise that it would actually explain how gravity quantitatively fits into the standard model.

The major problem encountered is that it is very hard to make gravity a quantum theory. That has rather technical reasons, and there are some hot leads how we can possibly circumvent this in the future. But not yet. The basic problem is essentially that we do not yet know how to cope with a pileup of gravitons, the (hypothetical) particles carrying the gravitational force, which inevitable always occurs in a quantum theory. That is actually an involved technical problem. For that reason gravity is not yet part of the standard model of particle physics, but instead described by a classical theory, general relativity.

The question is whether this matters when we want to talk about particle physics. The fortunate answer is that it does not, in most cases. The reason is that gravity is a very weak forces. Compared to those described by the standard model, it is about 10000000000000000000000000000000000000 times weaker than the weakest other force of the standard model. Therefore, only if there is a large charge - thus mass or energy - gravity becomes important. That happens only at energy scales which are more than 10000000000000 times larger than accessible in any experiment so far. In nature, it only occurs very close to a black hole or very, very early in the history of the universe. So, for most purposes, and in particular the ones of this blog, gravity can be neglected.

However, there are a number of open questions related to our limited understanding of gravity which have to do with large scales rather than particles: E.g., why is the universe expanding today? Also these questions will not be discussed for the moment in this blog.

Particles

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.

The standard model

Let me introduce the players in the standard model. These are the so-called elementary particles. These elementary particles are the smallest objects we known in nature. And small means small: We are sure they are smaller than 0.0000000000000000000001 meters. That is really tiny. However, it would not be the first time that when we look closer they are actually consisting out of something even smaller. But let me for the moment assume that this is not so.

So, what are the players then. Well, they can be divided in a number of groups.

First, there are things we call leptons. An example for a lepton is the electron. Electrons are the things that make up electrical current, so we have to deal with them every day. There are two heavier copies of the electron: The muon and the tau, about 400 and 3600 times heavier than the electron. If they would be stable, we could use them also for electrical energy, but they are not: They decay in some other elementary particles after fractions of a second.

There is a second group of leptons, the so-called neutrinos. These are the lightest particles which have a mass. We are not yet sure what their mass exactly is, just that they are at least 500000-times less heavy than an electron. Again, there are three of them, one for the electron (called electron neutrino), one for the muon (muon neutrino) and one for the tau (tau neutrino).

The second group of particles are called quarks. Quarks make up composites of quarks, known as hadrons. The most prominent hadrons are the proton, the nuclei of a hydrogen atom, and the neutron. The latter two are composites of the up and down quarks. Besides these two, which have both approximately ten times the mass of an electron, there are four more. The strange quark, about 200 times heavier than an electron, the charm quark (neat names), 3000 times as heavy as an electron, the bottom quark (9000 times), and the big guy, the top quark (350000 (!) times). Again, the heavier quarks are not stable, they decay.

Then there is light. Yes, ordinary light (and X-rays, and infrared light, and so on) is made up out of particles, the photons. They are exchanged between anything that has an electric charge, e.g., a quark and an electron.

But they are not the only thing, which can connect particles. There are gluons, which connect quarks. Neither photons, nor gluons have a mass. And gluons are a bit strange, but this will be discussed much more in detail latter. Sufficies to say, we do not see gluons as we do see photons.

Then there are the W and Z, both of approximately half the mass of a top quark. They are important for radioactive decays, and they are also a bit strange. They connect both quarks and leptons. Also, because they are so heavy, they are not very stable.

Finally, there is an elusive guy, called the Higgs. We did not yet find it - perhaps we will at the next big particle physics experiment, the Large Hadron Collider LHC at the European Center of Particle and Nuclear Physics CERN. But it is important, because it seems to be connected with all the masses which have been floating around.

Introduction

So, this is the blog in which I will discuss my research. I will try to be as general as possible, though at times it might get a bit tricky.

The general scope of my work is the standard model of particle physics - that is our current idea of how the smallest objects, the elementary particles, work. Very nice general introductions to this topic can be found at the large particle physics laboratories in Europe, at CERN or, in German, at DESY. Here I will only discuss what is of direct relevance to my own work.

An additional companion to this blog is my twitter account, on which I push some insights, some news, or some general remarks on my research, and on what is going on in the world of particle physics from my perspective.

Enjoy!