II Through the Looking-Glass
"One hopes that nature possesses an order that one may aspire to comprehend. When we arrive at an understanding, we shall marvel how neatly all the elementary particles fit into the great scheme."
-Madame Chien-Shiung Wu
If you look into a mirror, you look out into a reversed world. Left and Right are interchanged but otherwise the world in the mirror looks much like our own.
What about the Laws of Physics? Are they the same in the mirror-world? It appears so. If I throw a ball in the air, my double in the mirror also throws up a ball and both fall according the same law of gravity. I can play with magnets or tops or engines and again the actions of my double are also possible in this world.
Chemists work with the notion of Chirality. The left-right orientation of molecules has a profound impact on their properties. So a left-hand molecule in our world becomes its right-hand counterpart in the mirror world which, according to chemical precepts, is an entirely different molecule. But all of this is surprisingly irrelevant. Since every other molecule in the mirror-world has also changed its handedness, any chemical experiments our double performs are also possible in our world.
So it would seem that if we, like Alice, were unsure whether we were inside a mirror there would be no way for us to discover the truth. Or so it seemed, even to physicists, who refer to this mirror-symmetry property of the Universe as "Parity Conservation."
In 1956, an incredible (and vastly under-appreciated) experiment was performed whose purpose was in fact to determine on which side of the Looking-Glass we live in. The more prosaic intent was to determine the Question of Parity Conservation. The experimenter was a Chinese-born woman - Madame Chien-Shiung Wu.
Her fellow physicists, T.D. Lee and C.N. Yang, had predicted that there might be certain interactions among subatomic particles whose mirror was improbable or did not exist at all. Parity tests such as these had already been performed for a wide variety of interactions and so most physicists were dubious but intrigued.
It was Madame Wu, setting up a laboratory at the National Bureau of Standards in Washington, who set out to confirm or disprove their hunch. The key was in how one particular atom, cobalt-60, decayed. Unlike other atoms studied it decayed (ejecting pieces of itself in the form of beta particles) in an asymmetric manner. The mirror-image of this process was not one that was observed.
If you had placed a large mirror against one wall of Madame Wu's laboratory, her double in the mirror was performing a similar experiment but obtaining results which do not appear in our Universe. The mirror world is not our world merely reconfigured, it is a distinct and alternate reality. Deep within the subtle textures of the Universe there is indeed a small asymmetry, a telltale, a crack, which can be used to distinguish ourselves from the mirror world.
The result was completely unexpected. Mirror-symmetry appeared to be true and appeared to continue to hold true across thousands of other experiments. Looking upon this event, the fall of Parity, the physicist I. Rabi remarked "A rather complete theoretical structure has been shattered at the base and we are not sure how the pieces will be put together"