If will take a closer look at the Hydrogen atom, we already know that it is made of a proton with electric charge +1 and an electron with charge -1. Particles with opposite electric charge attract each other just like magnets. This is an example of the electromagnetic force, or "the electromagnetic interaction".

## What is interaction?

How do the proton and the electron know about each other? How do they "interact", and what do we mean by “interaction”? Particles interact with each other by exchanging other particles. How does this work?

Two astronauts in space, being outside of their spaceship, repairing their spacecraft. Both are floating around, and as it happens, are closing up. One of the two would now throw a tool, that the other would eventually catch. What happened? The throwing astronaut transferred momentum onto the tool he threw. He is not fixed to the spacecraft, but floating in space freely. The momentum he will achieve from throwing the tool will carry him in the opposite direction of the tool. The other astronaut, catching the tool, will receive the tools momentum, carrying him in the direction of flight. What happened? The two astronauts are now floating away from each other, whereas they were approaching each other before. They exchanged a tool, and transmitted momentum from one another, a process particle physicists call "scattering". Sadly, this example only works for a repelling interaction. However, there is a similar example, that also works for an attractive interaction, which is a little more complicated.

[READ MORE: Here is another analogy: Two people are sitting in small boats on a huge and quiet lake. Both boats are approaching each other slowly. If one of the two people now throws a ball to the other person, who will catch it. If the ball is very heavy, both boats will then move away from each other, because the inertia of the ball transmitted momentum to both boats. The one boat gained momentum by the person throwing the ball, and the other picked up momentum by the person catching the ball. What happened? The boats changed direction, they exchanged momentum by exchanging a basket ball. The boats interacted with each other, by exchanging a ball as their messenger.

The above analogy is useful to understand repulsion of particles by exchange of some kind of “messenger”, for example two electrons, due to their electric charge. The same analogy is possible for attraction, only requiring the change of the basketball for a boomerang and the directions that it is thrown and caught. If the person throwing the boomerang throws it away from the other boat, and it loops around so that the person catching it is facing away from the first boat, the exchanged momentum is just opposite to that for the analogy for repulsion.]

## Electromagnetism and Light

What is that object being thrown between the proton and the electron exchange in order to communicate their attraction to each other? The answer is simple, it’s the photon. These photons are the same particles that make up the light that comes to us from the sun and enables us to see the world around us. But the photons we can see are only a small part of the full spectrum of the photons that are out there. Visible light, X-rays, microwaves and radio waves are all photons. The only difference is that they have a higher or lower energy than the light we can see - much like the force that keeps us on the surface of our planet and the force that binds our planet to the sun is the same - only that there is a different amount of energy involved.

[http://en.wikipedia.org/wiki/File:EM_spectrum.svg -GFDL Version 2]

[The most energetic photon particle discovered is in the Tev range. So if the range of the visible photon our eyes can detect is scaled to the width of a human hair, the range of the electromagnetic spectrum is the distance between the earth and the moon!]

[The photons that are described in the figure are characterized by a wavelength, meaning that photons are waves. But why did we say that the photon is a particle before? The reason for this is that the physical laws we are familiar with (i.e. Newtonian mechanics) cannot be applied at very small scales. When at these small scales, quantum mechanics apply. One of those rules of quantum mechanics is that all particles have both particle and wave properties. This is referred to as the wave-particle duality.]

## What is a photon?

First of all a photon is an elementary particle, which means that - as far as we know - it doesn’t have any substructure. It is not a composite object, and not divisible into smaller building blocks. The photon belongs to a group of particles that we call bosons - we will encounter other members of that group later on. Bosons are characterised by the fact that they have integer spin, where spin is an intrinsic property of particles that can take different values, but never changes, much the like charge we have encountered earlier. Other particles, such as the electron and the quarks, have spin ½ and are called fermions (all particles with half-integer spin are fermions).

Like all other bosons, photons can - given that sufficient energy is available - be produced out of nowhere and can also vanish into nothingness, leaving behind nothing but energy. Therefore, the electrons and protons in the atoms - as opposed to the persons in the boats we used as an analogy earlier - do not need to carry around their messengers (in our case photons), as they can simply be produced by all particles that carry charge.

## Outlook

The exchange of photons between particles is what we call "electromagnetic interaction", The photon can create either attraction or repulsion between particles and transport momentum as well as energy. Now we know what binds together the electron and the nucleus to form atoms, such as the Hydrogen atom or the Helium atom that we used as examples - it is the electromagnetic interaction, the exchange of photons, communicating the attraction between the particles due to their opposite charge. In the next chapter, we will learn what glues together the quarks to form protons and neutrons.