The Quantum Picture of Forces

Electromagnetic Fields Again

Quantum physics changes everything. At the beginning of the twentieth century, physicists began to discover that the laws that work in describing the behavior of everyday sized objects no longer apply when we enter the microscopic world. I won't go into too much detail on quantum physics here. (If you want more on quantum physics, you might try The Page of Uncertainty.) We just need to look at how the discovery of quantum physics affects our concept of fields.

Let's consider an electromagnetic interaction between a proton (p) and a positron (e+). In the "classical" field view of forces, the proton is surrounded by an electromagnetic field. When the positron (or any other electrically charged particle) enters that field, it experiences a force. Similarly, the positron is surrounded by an electromagnetic field, and when the proton enters that field it experiences a force.

An interaction of two charged particles, via fields.

But what does quantum physics say about this electromagnetic field?

We need to get sidetracked a bit, onto the subject of light. People have always wondered just what light is. Around 1800, a man named Thomas Young showed that light was a kind of wave. He did this by shining light through very narrow slits, and observing that it spread out afterwards, in a pattern such as only a wave would do. Later in the nineteenth century, James Maxwell, in one of the greatest achievements in all of physics, gathered everything that was known about electricity and magnetism, and a few things that weren't known, into a set of four equations, known today as "Maxwell's equations." (Kind of appropriate, eh?) Anyway, one of the implications of Maxwell's equations turned out to be staggeringly unexpected. Maxwell discovered what kind of wave light is. It's an electromagnetic wave. That is, it's a wave of oscillating electromagnetic fields.

The Photon

This takes us to the twentieth century. Around 1900, Max Planck and Albert Einstein postulated that light was "quantized." That is, light is emitted in tiny bursts of electromagnetic waves: particles which we now call "photons." This idea developed into quantum physics, with its postulate of wave­particle duality. According to wave­particle duality, light consists of particles (photons) whose location and momentum are governed by the Heisenberg uncertainty principle, and are described by a "wave function" which follows from Maxwell's equations. That's a bit of a mouthful for one sentence. To see this idea spread out over a little more space, as I've mentioned, try The Page of Uncertainty.

Anyway, for our purpose here, all you really need to know is that electromagnetic fields are made up of particles called photons.

So here's how we now view the interaction between the positron and proton we started to describe above. The positron is surrounded by an electromagnetic field, which really means that it's surrounded by photons which it continually emits and absorbs. As the proton nears the positron, one of these photons is absorbed by the proton. The photon (denoted by the Greek letter gamma in the picture below) has energy and momentum which it carries from the positron to the proton, so that the positron recoils from the loss of energy and momentum, and the proton recoils from the gain. The end result looks about the same, at least for the simple interaction we've described here, but the description of how it happened is different.

An interaction of two charged particles, via a photon.

Virtual Particles

The photon here is not present at the beginning of the interaction, and is gone when the process is over. In fact, according to quantum physics, it can't actually be observed by any experimental means at any time during the process. (In fact, in their more philosophical moods, physicists will argue over whether the photon in the process above can really be said to exist at all.* I'll just assume it does in this discussion.) It's what we call a "virtual photon." Not all photons are virtual, mind you. Ordinary light, for example, is made up of "real photons." And we observe these with our eyes. A real particle is one which can actually be observed (at least in principle) by some sort of instrument. A virtual particle is one which is created during an interaction between two real particles, or during the decay of a real particle, and which is gone when the process is over. All the matter particles and gauge bosons I listed a couple of pages ago can exist both as real particles and as virtual particles.

The Gauge Bosons

The example above is an electromagnetic interaction. We can tell this by the presence of a photon, one of the gauge bosons I mentioned a few pages ago. An electromagnetic interaction is any interaction involving a photon. The photon can be either virtual, as in the example above, or real. For example, an electron and a positron (the electron's anti-particle) can annihilate, creating a pair of photons in the process. This is an electromagnetic interaction.

Each of the four forces is defined by its gauge boson:

Electromagnetic force Photon
Strong force Gluons
Weak force W±, Z0
Gravity Graviton

So, for example, any process involving a gluon is a strong force process. Any process involving the weak force will include a W or Z particle (either real or virtual). These gauge bosons are said to "mediate" the corresponding forces.

Here are a couple more examples of processes involving subatomic particles, displayed in their full quantum mechanical glory, with virtual particles.

Muon decay. (The decay products are an electron, a muon neutrino, and an electron anti-neutrino. By tradition, photons and the W's and Z are represented by wavy lines. Far be it from me to break with tradition.)

Muon decay, with a virtual W.

Electron-positron annihilation. (In this one, the two photons produced are real. The virtual particle is the "middle electron.")

Electron-positron annihilation

The first of these is a weak force process, while the second is an electromagnetic process. You can tell this because the first one contains a W particle, while the second one contains photons.

So this is how things happen in the microcosmos. And now that we know that, we can look at each of the four forces in a little more detail.

The Electromagnetic Force
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Dave's Microcosmos

* One can argue over whether or not the photon is actually present in this process, but one thing's for sure. If there was no such thing as a photon, the process could not take place. This can be said of any virtual particle in any process. Back.