cern-summer-webfest/readmore/the_electromagnetic_interaction.html

<p>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". 

<h2>What is interaction?</h2>


<p>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?</p>


<p>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.</p>

<img src="EM_spectrum.svg">

<p>[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.</p>


<p>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.]</p>  

<h2>Electromagnetism and Light</h2>

<p>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.</p>


 
<p>[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!] </p>

<p>[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.]</p>


<h2>What is a photon?</h2>

<p>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).</p>



<p>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. </p>

<h2>Outlook</h2>

<p>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.</p>