The Moving Finger

There is no future, there is no past
…
No Day But Today
…………………………………Rent, Finale B., by Jonathan Larson

Bee asks us if we believe that the past, present, and future exist in the same sense.

Few notions of physics are as recalcitrant as time. The Newtonian notion of time as always and everywhere uniformly flowing may have temporarily (!) tamed time, but it didn’t domesticate it. Special relativity, which showed that time can’t be clearly separated from space, loosed some demons, and general relativity is worse. There was trouble from quantum mechanics too, which really prefers to treat time and space differently – unlike other observables, time doesn’t correspond to any Hermitian operator.

I don’t want to get into those very interesting issues, however, since there is a more elementary one that is even more difficult. No aspect of time is more fundamental to our ordinary understanding than its division into past, present, and future, but no aspect of our world is less captured by physics. The known laws of physics, both classical and quantum, are almost entirely symmetric in time. No distinction between past, present, and future seems to be embodied in them. The reason for the ‘almost’ is that there is inferred a slight past – future asymmetry in the weak interactions. I say inferred, because the asymmetry has not, to my knowledge, been measured directly, but is inferred from the assumption of CPT (combined charge conjugation, parity reversal, and time reversal) invariance and measured CP violation. As far as I know, no one has a clue as to how such an asymmetry could produce the “present.”

For the most part, physicists have reacted to this mysterious fact as Einstein did – “Nothing to see here folks, move along now.” Usually this is put into a form something like: “the present is an illusion, due to consciousness.” For me, that is akin to saying “sound is an illusion, created by ears.” These aren’t explanations because they beg the question.

In the case of sound, we can relate the real and the illusion to a larger world. Real sound originates in mechanical disturbances and its transduction into electrochemical signals in the ear and brain, and we can (in principle) distinguish illusory sounds produced by malfunctions of the auditory apparatus from real one by looking for their correlates in the larger world. The illusion depends on the partial mimicry of the reality, and the larger reality discriminates between illusory and real.

If “the present” is called an illusion, the question to ask is what is it an illusion of? By what mechanism could such an illusion arise? If this special moment, “the present” is singled out by consciousness, how can it do that? That, I think, comes back to physics.

In one of his debates on quantum mechanics, Einstein related a conversation with another prominent physicist:

PP: I’m inclined to believe in extrasensory perception.

AE: This has more to do with physics than psychology.

PP: Yes.

Indeed.

UPDATE: Don't miss Wolfgang's Magical Mystery Tour of Time and other things.