Our universe is built upon symmetry. In fact, we believed that there were three symmetries to the universe that had to be unbreakable. Charge symmetry – this means that the universe wouldn’t care if all positive and negative charges were swapped, everything would behave normally. It is only the opposite interaction that counted. The next was Parity symmetry. This means that if you took our universe to be in the axis of x, y, and z. All values could be inverted (-x, -y and -z) and everything would remain the same. A spatial symmetry of sorts. And finally, time symmetry. This means that time could go backward or forward and we would have no idea which way it was going (with an exception to the 2nd of thermodynamics). Although through the past decades, each of these symmetries has been disproven individually and we will see how.
The big bang theory speculates the creation of both matter and antimatter at the beginning of the universe. When they meet, they annihilate each other releasing massive amounts of energy. Charge symmetry states that particles should behave exactly like their anti-twins. Matter to antimatter. However, if this is true, why don’t we see antimatter in our universe today? If there is an equal amount of both antimatter and matter, this should mean that nothing should exist. Could this discrepancy mean that the symmetry doesn’t hold as once speculated? This idea was developed by Nobel Laureate Andrei Sakharov in 1967.
Parity symmetry can also be broken on its own. In 1957, C.S Wu and her team discovered an obvious parity violation in the beta decay of cobalt-60 atoms. During the decay, a neutron begins to decay to a proton by emitting an electron and an electron antineutrino. The cobalt-60 nucleus becomes a nickel-60 nucleus through this decay. The nickel-60 atom then releases to gamma rays in order to come back to its ground state. So how does this determine whether P
-symmetry is violated or not? Well, if the emissions of both the electrons and gamma rays are emitted in a biased direction corresponding to the spin of the nucleus, then, when this reflection is placed in a mirrored world, the emissions would go against natures preference. Hence, parity symmetry would be violated. We would be able to tell whether an observed atom lived in a mirrored world or not. And this is exactly what they found. A bias in terms of the direction of emissions.
The last individual symmetry that remains is time symmetry. This means that events can take place in either direction of time and not know the difference in directions of time. Although, does this really hold? Well, to start off with, the second law of thermodynamics states that the entropy of a system always increases with time. This itself breaks the symmetry. However, digging deeper, can particles themselves on their own scales tell which direction of time they are moving in? Yes, and one experiment which shows this is the following. When a pair of quarks is held together by the strong force, there are two different possibilities of positions which the quarks can take. The quarks are able to switch back and forth between these positions/arrangements via the weak nuclear force. Surprisingly, scientists have found that switching the positions of the quarks in one direction takes longer than switching back to the original positions. This means that if the interaction between the two quarks is played back in time, there will be a difference between the two directions of time due to the uneven time intervals between the switching of quark positions.
Another final example which breaks time symmetry is the existence of black holes. Due to the extreme gravitational fields of black holes, nothing can escape them beyond their event horizons, including light. This in itself would allow for time to go backward at all. Because trapped light can’t escape, only Hawking radiation can. However, if time reversal were possible, the creation of a ‘white hole’ would be inevitable (predicted in general relativity). They would have the opposite characteristics. While a black hole is inescapable, a white hole can never be entered although matter and light can escape from it. Like all mass, they attract other matter, however, any object traveling towards a white hole would never reach its event horizon (this is a topic for another article all by itself). In conclusion, time symmetry, like the other two symmetries can be broken by itself.
Now we have established that all three symmetries can be broken individually. This changed the way a lot of scientists had previously seen the universe. Although, instead of re-writing their works to account for these broken symmetries, many thought that if to symmetries were tested together, the symmetry would hold. This is where the combined charge-parity 
(CP Symmetry) comes into play. The first violation of CP symmetry was discovered in the decay patterns of kaons (K-mesons). The products of the decay were entirely CP-symmetric versions of each other. For example, both an electron neutrino and an electron antineutrino were produced in two different decays which mirrored off each other. If this were replicated in the mirror world, the symmetry would hold as the broken parity symmetry was now solved with the broken charge symmetry. However, the violation occurs because the products of the two decays are not produced at the same rate. Scientists found that the kaons were more likely to decay into the path involving electron neutrinos by a very small fraction. This small difference is what drove this theory into the ground and scientists back to the drawing boards.
If each symmetry can be broken individually and when paired with each other, then there is one more option which scientists could look to. A combined charge, parity and time symmetry or CPT symmetry. To this day, no one has broken or disproved it. If it is broken, a lot of the work in physics which has been done over the past century would have to be rewritten. Theories such as quantum field theory, Lorentz symmetry, etc. would all have to be discarded. This is why it is the last theory of symmetry which scientists can throw at the universe. It must hold in order for our understanding of the universe to be right so far.
Author: Rehaan
Exponentially Diverging 