The Nature of Time

The passage of time is probably an illusion.

For one, time is relative. Or perhaps I should say, simultaneity is relative. If I stand midway between two poles, with a right-angled mirror that allows me to observe both, and if a lightning bolt hits each pole "simultaneously", I will indeed observe the simultaneity of the two lightning strikes. Light from each event will reach my eyes at the same time. For another observer, moving rapidly past me just as the lightning strikes, the events will not be simultaneous. As he's moving towards one of the poles and away from the other, the light from the pole he's moving towards will reach his eyes before the light from the pole he's moving away from. This is due, of course, to the shorter distance traveled by the light from the pole he's moving towards. But neither observer can claim absolute authority as to the simultaneity of the events; neither person is more special than the other. Time, here, is relative. If asked to signal the arrival of the lightning bolt on the pole toward which the moving observer is approaching, the moving observer will signal before the stationary observer will. The moving observer's "present" is the stationary observer's "future". Therefore, it doesn't make sense to confer special status on the present moment, because whose "present" would that moment refer to?

Another argument against the passage of time is the fact that nothing in known physics corresponds to its passage. The equations upon which physics rest work equally well whether time runs forwards or backwards. The present moment has no special significance. It seems that time is laid out in its entirety, with all times equally real. Our perception of the past, present, and future is not a result of time passing over us but instead of the way our brains work.

Why do we perceive time to move in one direction: from past to present to future? We're confusing the passage of time with the "arrow of time." The arrow of time points towards an asymmetry between past and future. (We, by convention, label the direction in which the arrow points as toward the "future".) A drinking glass dropped on the floor shatters, but a shattered glass never automatically reassembles itself and returns to your hand. If you were to see such a thing in a movie, you'd think that the film was being played in reverse. This forward-pointing arrow of time seems to be related to the second law of thermodynamics, which basically states that the entropy (or, roughly, disorder) of a closed system will increase in time. A shattered glass is definitely less ordered than an intact glass. But the fact that the arrow is pointing forward does not mean that it is moving forward. Some physicists speculate that the unidirectionality inherent in the formation of memories - new memories add information and raise the entropy of the brain - might lead to our perception of the flow of time. Others speculate that this perception may have something to do with quantum mechanics.

Sean Carroll (2006) proposes that the reason our arrow of time points towards the "future" and not the "past" is just a quirk of chance. That is, there's nothing fundamental about it. It could have just as well pointed in the opposite direction. Our universe just happens to be moving from a low-entropy state to a high-entropy state, but other universes may be moving in the reverse direction. Supposedly, though, on an ultra-large-scale, the entirety of all universes would be moving towards increased entropy, for the simple reason that there are more ways to be high entropy (disordered) than low entropy (ordered).

And another thing about time. Recent studies suggest that our perception of the world may not be continuous but might instead be a series of discrete snapshots like frames in a film. Actually, "it seems that each separate neural process that governs our perception might be recorded in its own stream of discrete frames" (Fox, 2009). And these streams (which need not all progress at the same rate) are then fit together in a separate process within the brain that produces a consistent picture of the world.

Not everyone agrees on the ideas presented above.

Sources:
"That Mysterious Flow" by Paul Davies, Scientific American, Volume 16, Number 1, 2006.
"The Time Before Time" by Sean Carroll, Seed, Volume 2, Number 6, September 2006.
"The time machine in your head" by Douglas Fox, NewScientist, Volume 204, Number 2731, 24 October 2009.