My 11 month old daughter loves electric lights. If you visit my home, you may often find me standing near the switch and flicking it on and off while the daughter watches and squeals in delight. Today morning we were playing our switching game and I explained to her with much drama how photons are expelled from the atoms in the filament, how they travel down to her eyes, how the arrow of time and principle of least action guides the photons and the whole world, etc. She, of course, giggled watching my mouth make all the funny sounds.
When my hand reached her eyes, I gave a tickle, then traced the photons back to the light and explained to her that Time was now running backwards. The vase she broke was going to be alright, the milk she threw up will be going back in, photons were getting back into the filament.. Wait a minute. That is not how nature works. Or maybe, it does work that way? Perhaps, Time can run any way it well pleases – forwards, backwards, sideways.. I say this because at the atomic level, at the level where the atom ejects or absorbs a photon, there is no way to discern the arrow of time. All interactions are symmetric with respect to time (in other words, energy is conserved). If I had not traced my hand from the filament to my daughter’s eyes and thereby had not irreversibly altered her perception of the world, there would be no arrow of time to speak of. Or atleast that’s what I think. What is this arrow of time? Does it exist? Does it have a preferred direction?
There’s more to this than I originally thought. Nevertheless, it is always startling to find such beguiling things lurking right under the thin surface we call ‘mundane reality’.