What is Antimatter all About?
“Antimatter may seem strange, but in the same sense that Belgians are strange. They are not really strange; it is just that one rarely meets them.”
- Lawrence M. Krauss
I thought that following up from a previous post on dark matter (and dark energy), it would make sense to cover antimatter! So firstly, what’s the difference between matter and antimatter?
Matter is the particles that make up everything around us that we can see (all the different atoms). The periodic table can essentially be built from protons, neutrons and electrons. Whereas, antimatter is basically “the mirror” of normal matter. It is the opposite to everything we naturally deal with.
A brief history
Scientists began to suspect the existence of antimatter in the late 1920s. A scientist from Bristol, Paul Dirac, can be thanked for where we are today with our knowledge of it.
In 1928, Dirac and Cambridge wanted to work out what happened when Einstein’s special theory of relativity (which deals with when things travel close to the speed of light) was combined with quantum physics (which deals with the behaviour of tiny particles, e.g. electrons). Specifically, what Dirac wanted to solve was what happened when an electron moves close to the speed of light — how much energy does it have? The confusing thing was that two answers were received: one positive and one negative.
The positive number was simple enough, it’s just the positive energy of the electron itself. But negative energy doesn’t make sense? Most physicists, therefore, just disregarded this second answer — they saw it as meaningless. However, Dirac didn’t. He was focused on the mathematical equations that correspond to such events, and he didn’t feel comfortable just throwing away one answer.
So he had a breakthrough when he suggested: what if, hypothetically, there is another particle that was opposite to the electron in every way, and thus was responsible for this negative energy. He had just predicted the existence of the positron… and no one believed him.
It was only when images of cosmic rays were taken and analysed that some mysterious tracks could be seen that curved the wrong way? These were found to be the signature of positrons.
With just a few mathematical equations, Dirac discovered an entirely new class of matter… antimatter. At the age of 31, he received the Nobel Prize.
Then, after the discovery of the positron, many other antimatter particles started to be discovered. It was realised that all normal matter particles had antimatter coefficients.
Where is all the antimatter?
For the first fraction of a second at the start of the universe, there was an even amount of matter and antimatter. In theory, this should have led to “mutually assured destruction” where both annihilated each other and left nothing (this event is known as “the great annihilation”). However, somehow matter was able to outlive antimatter by a tiny fraction: for every billion particles that annihilated each other, one matter particle managed to survive. To this day, it is still unknown how or why matter was able to “win the battle” against antimatter.
Studying antimatter
As mentioned, antimatter was only present for a fraction of a second 13.8 billion years ago. Therefore, to study antimatter, you have to make it.
At CERN, there is a machine 100 metres underground which is a 27km circumference tunnel that is about the same width as the London underground. In that tunnel is the Large Hadron Collider (LHC) which is essentially a set of accelerator units and magnets that can speed up a beam of protons in the middle of the tunnel. They get sped up to a point where they circle the entire tunnel 11,000 times every second (i.e. very close to the sped of light).
One beam of protons travels in one direction while another beam in the other, and at certain points, their paths will cross and they collide head to head. This leads to the creation of very heavy exotic particles.
The detectors (which are basically just huge digital cameras) take photographs of these events millions of times a second. Most of this captured data is not useful, and so the useful data must be extracted. Very often the protons do not even collide, and so no useful data is captured. The analysis of this data is a very slow process (“a needle in a haystack”).
Does it have a use?
When it was initially discovered, no one thought antimatter could be used for anything. However, it can. In hospitals, antimatter is routinely used in a scanning technique called positron emission tomography (PET scan). This is where antimatter (i.e. positrons) are used to detect where cancer cells are in the body, allowing for very precise surgery.
In the same way that Dirac did not know that his discovery would lead to cancer treatment, we do not know any further uses of antimatter currently, although there are likely many.
But what we do know, is that antimatter is a piece of a bigger puzzle that physicists are slowly building upon as they try to solve the riddle of the universe.
