Symmetry Breaking and Asymmetry in the Universe

SYMMETRY BREAKING AND ASYMMETRY IN THE UNIVERSE BY NACHIKET GIRISH he big bang theory postulates that charges reversed. Why should the universe this question lies, surprisingly, in the most Tall matter that currently exists in favor one over the other? And yet, if matter fundamental symmetries of nature, lead- the universe was created at one moment, and antimatter had been created in exactly ing science to question its long held beliefs nearly 14 billion years ago, at the birth of equal amounts, they would have precisely about physical symmetry. It is in the uni- the universe itself. All the fundamental annihilated each other, leaving the universe verse’s departure from perfect symmetry particles we know, from the massive Higgs full of energy from their explosive demise that we may find the answer to one of the boson to the miniscule neutrino, were but nothing else—no atoms, no molecules, most fundamental questions of them all— formed in that one moment. This primor- no stars and galaxies, and no us.2 why do we exist in the first place? dial particle soup then interacted with itself Evidently, the clear symmetry between and combined in various ways. Out of the matter and antimatter, known as C sym- SYMMETRIES IN PHYSICS primeval chaos emerged the universe we metry, should result in a universe devoid live in today.1 of matter. But a quick glance at their sur- “How nice it would be if we could only However, there is a nagging prob- roundings should convince any skeptic that get through into looking glass house! I’m sure lem with this narrative: prima facie, the the universe is in fact not empty; moreover, it’s got, oh, such beautiful things in it! Let’s universe should have no reason to dis- it seems to have a great deal more matter in pretend there’s a way of getting through into criminate between matter and antimatter. it than antimatter. it, somehow.” Antimatter has all the exact properties of How do we explain this puzzling mat- Through the Looking-Glass, regular matter, with only the signs of its ter-antimatter asymmetry? The answer to by Lewis Carroll The story of matter-antimatter asymmetry begins with one of the most basic “The mirror world of classical physics was, in fact, once symmetries of nature: mirror symmetry. a fairly boring place, its monotone enforced by the prin- Our macroscopic world is in no way uni- versally mirror symmetric. Most of the ciple of parity symmetry, which demands that the mirror objects we see in everyday life would look world be exactly identical to the real one.” very different through a mirror. Things 8 Berkeley Scientific Journal | SPRING 2019 the magnetic field, which is in contrast to the observed effects of the magnetic field itself, which always appear at right angles to the field as discussed in the figure. This is significant, because if we have two mirror reflections of the Wu apparatus, we can tell which one is in the real world and which is in the mirrorscape! Even though observers in both worlds may agree that the current flows from left to right in their reference frames (due to lateral inversion), the left- right reflection does not affect the up- Figure 2: Magnetic field does depend on the left-right orientation of atomic currents. B de- down orientation of the observers and thus notes the magnetic field created due to current flowing along the direction of I. A left-right there will be a glaring difference between reversal of the spin of the currents causes the magnetic field to flip from up to down. How- the two worlds in the direction of motion ever, even in this case, while the magnetic field might gain an up-down flip which would of the electrons. In the real world we only cause its mirror image to appear different from itself, all its observable effects, such as the see electrons going downwards for this ori- exerted force, are still flipped in the left-right direction only, preserving parity symmetry. entation of currents, therefore the world where the electrons go up must be the mirror world. The weak nuclear force has thus get much more interesting (or much less of parity symmetry would lead physicists to given us a universal standard for left and interesting, depending on your point of declare parity conservation to be a funda- right, and equally significantly, shattered view) when we analyze the basic phenom- mental property of nature. one of the core beliefs of physics.3,4 ena of classical physics, such as fields and Having been suddenly deprived of their particle interactions. The mirror world of SYMMETRY BREAKING anchor of parity symmetry, adrift physi- classical physics is governed by the prin- cists now searched for the correct law for ciple of parity symmetry, which demands While the principle of parity symme- the weak nuclear force. Fortunately, there that the mirror world be exactly identical try is an irontight rule in classical physics, seemed to be a solution ready at hand— to the real one. To define it more formal- quantum physics presents a whole new sto- scientists found that if the weak nuclear ly, classical physics demands that a system ry. The development of quantum mechan- force acted on matter particles spinning be unchanged when the coordinate axes ics was motivated, after all, by a series of one way, it would act on the corresponding we use to measure it are reversed, which particle phenomena which threw a series antimatter particle spinning the opposite is called a parity transformation. Granted, of monkey wrenches into the exquisitely way. Back to the Wu experiment: if we take what is right for a person is left for their built structures of classical physics. In 1957, the mirror image of the experimental setup mirror self, but the concept of left and right a stunning experiment by experimental and replaced all the atoms with antimatter is itself a relative one, and without arbitrari- physicist Chien-Shiung Wu at the Univer- atoms, then it would again be impossible ly deciding on one direction as “right” or sity of Columbia showed that parity sym- to differentiate between the two copies of “left,” there is no way we can differentiate metry is, in fact, broken in the radioactive the experiment. This new symmetry was between these two worlds. In this sense, all decay of particles, which is governed by the dubbed “CP” symmetry, which is a com- the laws of physics must work the same way then newly-discovered weak nuclear force. bination of C symmetry (“charge conjuga- in the mirrorscape as they do in “real” life. Professor Wu and her group aligned the tion” symmetry, which is to say replacing The mirror world (which is to say a “par- spin (and thus the “atomic currents” and matter with antimatter), and P or parity ity-transformed world”) is no less “real” individual magnetic fields) of supercooled symmetry.3 than the normal one we live in. cobalt nuclei along an external magnetic This solution, however, only held up for At least until the twentieth century, all field, and found that when the nuclei de- so long. In 1964, scientists James Cronin of the known fundamental forces of nature cayed radioactively, they emitted electrons and Val Fitch at the Brookhaven National were understood to be invariant under a preferentially opposite to the direction of Laboratory experimentally confirmed a vi- mirror reflection. Gravity, for instance, their magnetic field. Magnetic fields are olation of CP symmetry in the weak nucle- would work the same way in a mirrored produced by spinning currents as shown ar force—which won them the Nobel Prize world as it does in ours. The force of gravity in Figure 2; therefore, if the axes of rota- in 1980 and threw the question of symme- is entirely described by the relative orientation of the currents were aligned vertically, try conservation wide open.5,6 tions of the interacting bodies. It does not a purely left-right reflection of the direc- If the discovery of P-symmetry break- depend on any absolute sense of a left or tion of their spin would cause the decay ing taught physicists that nature differenti- right direction, and thus is unaffected by a electrons to go up rather than down. The ates between left and right, the discovery it parity transformation. The universal obe- emission of electrons, as an effect of the led to, that of CP violation, led physicists to dience of physical systems to the principle weak nuclear force, is thus antiparallel to a much more surprising conclusion—the SPRING 2019 | Berkeley Scientific Journal 9 symmetry breaking to account for the ob- preprint arXiv:1812.02651 served ratio between the amount of mat- 3. Feynman, R., Leighton, R., & Sands, ter and antimatter.9 This is known as the M. (1963). The Feynman lectures on “strong CP problem.” Finding new sources physics: volume 1. (2nd ed.). Retrieved of CP symmetry violation is in fact a very from http://www.feynmanlectures. active research field, with a CP breaking in- caltech.edu/I_52.html teraction being discovered in new particles 4. Garvey, G. T., & Seestrom, S. J. (1993). as recently as 2019, which could possibly Parity violation in nuclear physics: hint at a solution to this question. Matter signature of the weak force. Los Alam- antimatter asymmetry still remains a mys- os Science, 21. tery though, and is considered one of the 5. Christenson, J. H., Cronin, J. W., Fitch, biggest unanswered questions of physics V.

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