• Question: What is antimatter and why is there so little of it?

    Asked by anon-251974 on 29 Apr 2020. This question was also asked by anon-253012, anon-257807.
    • Photo: David Sobral

      David Sobral answered on 29 Apr 2020:

      Every single “fundamental” particle has an anti-particle that has the same mass and overall properties but with the opposite charge.

      For example, you can have an “anti-hydrogen” atom: it is made of a negatively charged “proton” (and anti-proton) with a positron (anti-electron) going “around it”.

      We know that if a particle and its anti-particle meet they annihilate and turn into energy: their combined mass goes into E=mc^2.

      So that ties in really well with your question: if particles and anti-particles annihilate and turn into energy when they meet, how come there’s even matter? That’s one of the big unanswered questions we have: why was there a bit more of what makes us (that we call matter, but we could be made of “anti-matter” – who decides?) so that after matter and anti-matter turned into energy there was still matter left?

      Nowadays, as fas we we can tell, there’s no anti-matter around. We can produce it in labs and particle accelerators, but to keep it we need to keep anti-matter away from matter (otherwise it will just meet and transform into energy and it’s gone!). One way to do it for charged particles is to build and use magnetic “cages”: that way we make sure that for example positrons (anti-electrons) stay away from electrons!

    • Photo: Ry Cutter

      Ry Cutter answered on 29 Apr 2020:

      To add to Davids answer,

      Scientists have noticed that there is a special property shown by quarks, the tiny particles that make up Protons and Neutrons. Quarks exist in 6 ‘flavours’: Up, Down, Top, Bottom, Strange, and Charm. The latter is the particle of interest.

      Now lets introduce Mesons! Mesons are like ‘small’ protons, instead of being made of three quarks, they are made of one Quark and an Anti-quark!

      D-Mesons contain one charm and one anti-charm quark. A special feature of Mesons is that they flip-flop between being their own anti-particle and back to being the ‘regular’ particle. Here’s what’s weird! We would expect it to go from ‘regular’ to anti at the same rate that it goes from anti to ‘regular’, but it doesn’t! This means there is a CP-violation! Which means, there’s a physical preference for one type of particle. Scientists hope that the CP-violation may help explain why we see much ‘regular’ matter in the universe!

      Brilliant question,

      Ry

    • Photo: Susan Cartwright

      Susan Cartwright answered on 29 Apr 2020:

      As David and Ry have explained, antimatter is matter which has some of the quantum numbers that describe it reversed, most noticeably electric charge – so an antiproton has negative charge, and an anti-electron – a positron – has positive charge. More subtle things are also reversed: although neutrinos have zero charge, there is a difference between neutrinos and antineutrinos: an antineutrino can interact with a proton to produce a neutron and a positron (that’s how the neutrino was first discovered, back in the 1950s), but a neutrino cannot (though it could interact with an antiproton to produce an antineutron and an electron).

      “Why is there so little of it?” – now there is an excellent question! When we produce particles in particle accelerators, we get particle-antiparticle pairs, not just particles, so from that we would predict that the universe should be 50% matter and 50% antimatter – but it clearly isn’t. This is one of the big puzzles in cosmology. We know that the matter remaining is only a very tiny fraction of what the universe started with: in particle accelerators we get about the same number of charged particles and photons, but in the universe there’s about one proton per billion photons, so we think that most of the matter and antimatter did in fact annihilate each other in the early universe (if a particle and an antuparticle collide, what you normally get out is gamma rays: the particle and the antiparticle sort of cancel each other out and get converted to energy by E = mc squared). But there was a little bit of matter left over, which now forms the universe we see – and we don’t know why.

      Actually, the experiment I work on has recently found evidence that the neutrino and the antineutrino are not exact mirrors of each other, which may be the first step to solving this puzzle – see https://www.sheffield.ac.uk/physics/news/matter-anti-matter-1.886775

    • Photo: Sophia Pells

      Sophia Pells answered on 29 Apr 2020:

      There’s already been some really good answers to this great question! The reason why there wasn’t an equal amount of matter and anti-matter at the beginning of the universe is something that’s not known and still a big area of research. Like Ry said,the fact there is a different amount is known as C-P violation. When matter and anti-matter get together they cancel each other out or “annihilate”, so if there had been the same amount of each then they would have cancelled each other out and we would never have existed.

      Something that I thought might be interesting to add is that there are some new experiments starting soon at CERN which are looking at how antimatter behaves in a gravitational field. As we know, normal matter falls downwards in gravity but this hasn’t been tested before with antimatter. Will it fall downwards at the same speed as normal matter? or will fall down at a different speed? or will it fall upwards? Most scientists expect that it will behave like normal matter and fall downwards so if it doesn’t it will be a very exciting discovery! There’s more info here if you’re interested: https://home.cern/news/news/experiments/new-antimatter-gravity-experiments-begin-cern

    • Photo: Sarah-May Gould

      Sarah-May Gould answered on 1 May 2020:

      Just to add to everyone’s answers, I thought you might be interested in a real-world application of antimatter… I work in a hospital where we use antimatter to diagnose what’s wrong with patients.

      To do this we first use a piece of equipment called a cyclotron to make antimatter. We can do this because if you have enough energy (our electricity bill is pretty big!) you can generate antimatter from normal matter. The cyclotron produces a really high energy beam of protons and directs these onto a target. The protons in the beam and the atoms inside the target combine to create atoms that are radioactive, which means they are unstable (they break apart after some time). The radioactive atoms we make produce a positron (the anti-particle of the electron) when they break apart.

      The cool bit is that we then take these radiaoctive atoms and attach them to a substance (we call this substance a tracer) that goes somewhere interesting inside the human body, such as to cells that aren’t working well in a person’s heart or brain. We inject a tiny amount of tracer into a patient’s vein and wait for it to go to the cells that aren’t working well. Others in this thread have already mentioned that when matter meets antimatter you get annihilation, where both particles disappear and energy is created. We ask the patient to lie in a special scanner that allows us to detect the energy that is generated when the positrons from the tracer annihilate with electrons inside the patient. Our scanner therefore tells us where the tracer went inside the patient, which tells us where the problem is inside the patient and allows the doctors to diagnose and treat the patient.

      So, antimatter is not only really interesting for theoretical physicists, it’s also really useful!

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