LogFAQs > #934867056

Symmetry or why we actually have stuff in our universe

Before the Big Bang, we assume that there was nothing, but why do we have anything at all? If we have equal numbers of particles and anti-particles, why doesnt the anti-part of our universe just annihilate with the part were living in? How could we actually tell the difference between particles and anti-particles to begin with?

The answer to all of those questions (at least in theory) boils down to violations of symmetry in nature. Lets talk a bit about what symmetry actually means. Wikipedia, the source of all knowledge tells us that In physics, a symmetry of a physical system is a physical or mathematical feature of the system (observed or intrinsic) that is preserved or remains unchanged under some transformation. Consider we have someone running along a road if we as an observer move along with them (i.e. transform the spatial coordinates), we wouldnt expect the physics to change. We can use some mathematical wizardry to show that any such symmetry corresponds to a physical quantity that shall be conserved. Typical examples you might have heard of are total energy (symmetry of time), spatial momentum (symmetry of space), angular momentum (rotational symmetry of space) and more (conservation of probability, etc).

Lets get back to particle physics: for each particle we shall define three essential symmetries, C (charge symmetry, replacing particles with anti-particles follows the same physics), P (parity, mirroring everything along the three axes still gives the same physics), and T (time, on a local scale switching past and future still gives the same physics). Not all of them seem intuitive, and some of them are indeed violated in some processes. Consider in more detail the notion of parity if we apply the parity operator (mirror operator) to any quantity, we would expect it to either stay the same or get inverted since applying it twice should return us to our original state (-1 x -1 = 1 && +1 x +1 = 1). Following from there (but omitting mathematics), we split particles into two states, left-handed and right-handed (as they can have parity components which give -1 and ones that give +1).

The foundation of the classical radioactive decay we know (beta to be precise) is mitigated by something called the weak force (one of the fundamental forces). Experimental evidence (very strongly) shows that the weak force acts to different degrees on LH and RH states. That basically means that if we start with an equal amount of LH and RH states (since we have no reason to think otherwise), some particles can follow certain decays while others dont. Furthermore, mathematics shows that a total CPT symmetry (i.e. applying all three transformations at once) can never be violated. That leads to the (experimentally proven) result that particles and anti-particles participate in different decays not only can we distinguish them by which decays they follow, but it also explains the symmetry break in matter and why we have particles (instead of anti-particles) to begin with. Interestingly enough, it also means that there are some particles in the universe which can not interact with any other form of particles (or do they really exist if we cant observe them?).

Apologies for the rather lengthy read, idk if anyone is interested in these things to begin with since they have both experimental and theoretical components and people usually only like one of those exclusively.

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