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Why is everything made from matter rather then anti-matter?

Why is everything made from matter rather then anti-matter?
Andy Palmer from Greater London (Age 25-34)


2 Responses

  1. Most physicists or cosmologist assume that at the beginning of the Universe at the Big Bang there was complete symmetry and that there was the same amount of matter and antimatter. More precisely we assume that for every electron there was a positron, for every up-quark there was an anti-up-quark etc. For every particle of matter which has an antiparticle partner there was equal numbers of both. As the Universe expanded the energy per particle and hence the corresponding temperature of the Universe fell. Whereas in the early Universe all pairs of particles and antiparticles could be destroyed or created in interactions, as the energy per particle fell, the heavier particle and antiparticle pairs could no longer be created and only the lightest particles such as up and down quarks and electrons were left.

    These are what normal matter in the form of molecules and atoms and their nuclei is made of. But why are there no corresponding atoms of anti- matter and so on. For this to be the case there must processes in which matter and antimatter behave differently, in such a way that what is left predominates and we subsequently call matter. This subtle difference in the way matter and antimatter behaves is first observed in particles known as neutral kaons. The decays of these particles proceeds via the weak interaction, the same force that leads to the radioactive beta decay of nuclei. It is know well known that the weak interaction is not symmetric in the way it deals with left-handed and right handed things at the particle level. This is known as parity- violation (P). It is also known that the weak interaction is not symmetric in the way it affects particles and antiparticles either. This is known as charge-conjugation (C).

    What was observed for some years was that under the joint symmetry (CP) it was symmetric. However more careful observation showed that a small fraction of the time it violated this joint symmetry. This was known as CP-Violation. The effect of direct CP-violation has now been seen in heavier neutral particles known as B-mesons. CP-violation is one of the three essential ingredients that Sakarohov showed was necessary for the dominance in the present Universe of matter over ant-matter.

    Observation of these CP-violation effects is essential to the understanding of a matter-dominated Universe.

    Posted by Alan Walker, School of Physics, University of Edinburgh

  2. The Russian physicist Andrei Sakharov showed that three things have to be true for the Universe to have end up containing different amounts of matter and antimatter.

    1. The reactions that produce or destroy particles have to be different from the ones that produce or destroy antiparticles.

    The weak interaction does this for us because it can create only left-handed particles or right-handed antiparticles but not right-handed particles or left-handed antiparticles. (We can picture a left-handed particle as one that spins clockwise as it moves towards us; a right-handed one spins anticlockwise.) This violates the symmetry known as charge conjugation (C) which replaces particles by their antiparticles.

    2. But this isn’t enough, we need something to prevent weak interactions creating equal numbers of left-handed particles and right-handed antiparticles. A much smaller effect related to the weak interaction means that these are created at different rates. This violates a symmetry which replaces left-handed particles by right-handed antiparticles. Since reflection in a mirror turns a left-handed particle into a right-handed one, this symmetry is a combination of charge conjugation (C) and reflection (known as “parity” or P). We know that this CP symmetry is violated because of the experiments described above by Alan Walker.

    3. But even this isn’t enough! If the Universe were in a steady state at some definite temperature, then the number of particles of any type would just depend on their energy. By Einstein’s famous equivalence of energy and mass, this means that the number of each type of particle would depend only on their masses. When people combined quantum mechanics and special relativity, they were able to prove a theorem that particles and their antiparticles must have equal masses. In a steady state, this means that the numbers of particles and antiparticles must be equal, which looks like a disaster if we want the Universe to contain more matter than antimatter.

    Fortunately the Universe just after the Big Bang was not in anything like a steady state: it was expanding extremely rapidly. During this expansion, the differences between the reaction rates were able to create slightly more particles than antiparticles.

    These differences between the production rates are very small and so the original imbalance was tiny: roughly one extra particle for every billion matched pairs of particles and antiparticles. But as the Universe expanded and cooled down, all those pairs annihilated into radiation and the tiny excess was left over to become the matter we’re made of.

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