HOW TO SEE THE INVISIBLE

If on a good Summer’s evening you go for a walk in the Park, it is likely that you find groups of people playing football or other ball games.  If the ball comes in your direction and hits you, you will not be pleased.  In fact you may be hurt.

Do you realize that your body is being continuously hit and bombarded by millions of different particles?  Luckily you do not feel them because they are very small, so small that they go through you without stopping.

Do you not think that it is worth trying to find out about these invisible particles?  They are, after all, the very fundamental stuff that makes you and everything that you see around you. 

Did I say, ”everything you see”?   Yes, this is the most amazing thing; all that you see around is made with these invisible things.

In maths we are told that a million or even a trillion multiplied by nothing is still ZERO. Therefore although very, very small, these particles must be such that their sum is not zero, on the contrary, they make up the whole Universe that you can see and the one you cannot see.

There are people really interested in finding out about these particles, and they spend their time trying different ways of trapping the particles as they move around.  This is the only way to know about them.  Once they have been caught, they can be identified by what they do.

We are going to look at pictures showing the trails left behind as the particles pass through a “Bubble Chamber”.

We will focus our study on just three particles; electron and its antiparticle called the positron and the photon. 

I hope you have heard about antimatter before, because you are about to see what its trails look like.

LOOKING AT THE EVIDENCE

We start now a journey of investigative work, trying to decipher what the particles are telling us. For that, we need to know their rules or better still, their language.

What do you see in this picture?  Sometimes it is not easy to interpret the traces but definitely there are two spirals in it.

One of the rules is that a charged particle that arrives at a place where there is a magnetic field is forced to move in circles, unless it enters in a direction parallel to the field. This is the reason for the circles in the picture above. The radius of the path is related to the momentum of the particle and the size of the magnetic field.

So you can already say something about the particles that were caught in this photograph.

In fact there are lots of things that can be said about this picture and you will notice them later on.

Do you know the feeling you get when you first look at the picture in a complicated puzzle? After a while you recognize a lot of details that initially were not obvious to you.

At CERN there are amazing people that can identify the particles in the pictures as easily as you quickly point out your friends in a school photograph.

The next photograph is one of those that make particle physicists very excited and I hope you come to see why.

• If you look at the single spiral at the bottom left of the picture, this has been produced by a knock-on electron on an incoming track, known as “delta rays”. This trace tells us the direction of the magnetic field, which in this case is out of the paper.
• The lone electrons, called Compton electrons, were knocked out of the atoms of the liquid by gamma rays. These are all negative particles which, like the delta rays turn to the left.  Can you find a positively charged particle?
• How do we know that they were knocked by gamma rays?  We know that because there is no visible trace just before the appearance of the electrons.
• Consider the electron on the top left hand side of the picture.  What is its origin?
• It has a positive “partner” which looks like an electron.  It is an anti-electron or positron.  The positron-electron pair has been produced from a high energy gamma ray in the field of a nucleus.  The process is called pair-creation or, more loosely, “materialization” of a gamma ray.
• Where did this gamma ray come from?
• Can you trace the beginning of the pair production back to the path of the electron near it?  The electron is part of another electron-positron pair that was produced by a high energy photon in the field of a nucleus.
• Where did the photon that pair-produced to give the electron that emitted the photon that pair-produced to produce the electron that we started with, came from?
• Is it hard to tell?  It might have been emitted early by the electron or positron from an electron-positron pair produced by a high energy photon in the field of the nucleus.  If this were so, where did the photons come from?
• Another example of accelerated charges radiating is the positron in this picture.  It loses energy abruptly, emitting a photon tangentially (like mud off a wheel). This is synchrotron radiation. This gamma radiation knocks on an electron out of an atom (Compton effect).

We have witnessed a quite extraordinary display of quantum electrodynamics – a mini electromagnetic shower. Nowadays these showers are measured in electromagnetic calorimeters. 

The last point to make is the cosmic connection.

A high energy cosmic ray comes into the picture from the top and gives a knock-on electron, just like the knock-on electrons on the incoming track we started with.

  Francisca Wheeler