The Universe is not symmetric. The laws of physics obey certain… | by Ethan Siegel | Starts With A Bang! | May, 2025
The laws of physics obey certain symmetries and defy others. It’s theoretically tempting to add new ones, but reality doesn’t agree.
When you wave at yourself in the mirror, your reflection waves back. But biologically, there are many ways it’s painfully obvious that your reflection is fundamentally different from you. When you raise your right hand, your reflection raises its left. If you were to examine your body with X-rays, you’d find your heart is in the center-left of your chest, but for your reflection, it’s in the center-right. When you close one eye, your reflection closes its other eye. And while most of us are largely left-right symmetric, any apparent differences will manifest in the completely opposite fashion for our mirror-image counterpart.
You might think this is only a property of macroscopic objects made out of large collections, or composites, of fundamental particles. As it turns out, however, the Universe is not symmetric at even an elementary level. If you allow an unstable particle to decay, you’ll discover many fundamental differences between the allowable decays in the Universe and the decays you’d observe in the mirror. Certain particles, like neutrinos, only have left-handed versions, while their antimatter counterparts, the antineutrinos, only come in…
