Let's Talk Physics

A stochasticly updated blog about interesting topics in Physics & Astronomy


Today, I’m going to start on a journey to talk about how our Universe came to look like what it is today. To start with, I’m going to talk about matter, anti-matter and annihilation. This will lead us onto a mystery of the Universe, and will then allow us to began formulating the history of the Universe.

Nature is very big on symmetry, especially when it comes to matter. For every species of particle that exists in the Universe, a species of anti-particle also exists. Simply put, an anti particle is a particle with all of the exact same characteristics as a normal particle, except it has the opposite charge. For example, an electron, one of the smallest particles known, has a charge of  -1.19\times10^{-19} C. An electron’s anti particle, called a positron, is the exact same, except its charge is +1.19\times10^{-19} C.

So, what happens when a particle meets its anti-particle? Well, we know from electromagnetism that if you have 2 charges, then like charges repel and opposites attract (meaning a positive charge repulses other positive charges, but attracts negative charges). This means that a particle and its anti-particle attract each other. And when they hit each other, they undergo a process called annihilation. What this means is that the particle and anti particle combine, and give their energy to become a photon, as you can see in the figure below.

A positron and electron annihilating

A positron and electron annihilating

The energy conversion here is the purest known in the Universe. It’s more pure than nuclear fission, fusion, anything, as the only by product from 2 particles annihilating is simple photons of light. This is why anti-matter is such an exotic research topic – the ability to produce anti-matter and then have it annihilate at will would give a beautiful method of storing and converting energy (as was discussed and then butchered in Angels and Demons).

Maybe not quite how annihilation would work, but it was still damn pretty

Now, for some more interesting physics! Just like how a particle and an anti-particle can combine to give a photon, a photon can spontaneously decay into a particle and its anti-particle. However, this only works if it obeys the following rules

  1. The photon must have enough energy so that, when it decays into the particle and anti-particle, no energy has been created! That is, the energy before the decay must equal the energy after the day
  2. Charge has to be conserved! Since a photon does not have a charge, then whatever it produced must have equal and opposite charges (which is why we get a particle and it’s anti particle)
  3. Momentum has to be conserved!

And then there are a few more finicky rules that don’t really concern us yet.  So, this process (along with the initial process) can be seen in the figure below.

An electron and positron annihilating, and then the resulting photon decaying into an electron and a positron

An electron and positron annihilating, and then the resulting photon decaying into an electron and a positron

And that’s really all there is to it. So now to talk about our Universe. As I said, Nature loves symmetry. And at the beginning of the Universe (some 13.8 billion years ago), we believe that there was equal amounts of matter and anti-matter in the Universe. But from the picture I just painted you above, all of this matter and anti-matter should have been constantly recombining, producing photons, which would then decay into more matter and anti-matter (in equal parts) and then they’d annihilate again and so on and so on in a cycle that would never end.

But it obviously did. Because, we’re here, and we’re all made of matter. Which means that the anti-matter, which by rights should have kept annihilating matter at the beginning of the Universe and stopped the production of anything more than protons and electrons in the Universe, had to go somewhere. And that is a big mystery of physics. One of the hypothesis at the moment is that, just as our Galaxy is composed of matter, there are galaxies out their composed of entirely anti-matter, which means over the entire Universe, there is the same amount of matter and anti-matter in the Universe. However, it doesn’t explain why the anti-matter and matter drifted apart at the start of the Universe. But, we’re luck that they did.


About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

5 comments on “Annihilation!

  1. Thomas Kelly
    May 4, 2013

    Hey, good to see a return to the blog. There can never be too many physics blogs. Also nice to see Feynmann diagrams (though I think the positron arrows are pointing in the wrong directions).

    You might find this paper interesting (It has the greatest title of any science paper).


    The maths is heavy, but what it says is as well as having electrons and positrons annihilate and scatter of each other, a quantum theory of gravity implies you could have entire universes doing something similar! (albeit with a few caveats and limitations)

    • irishphysicist
      May 6, 2013

      Hey Thomas,

      I’m glad you like that piece. I’m sorry I haven’t updated since, but my final year exams start tomorrow so all of my energy is going into passing those at the moment!

      That paper looks pretty interesting – will give it a read when I get some time over the summer. As for it having the greatest title ever, I’m not so sure – this one is definitely a contender. (http://arxiv.org/abs/0805.3991)

      Will be posting again in June (I hope).


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