Now: The Physics of Time

by Richard A. Muller

Quantum physics is perhaps the most successful theory of all time—with agreement between predictions and observations reaching ten decimal places—yet this theory is both disconcerting and distressing.

Human perception of the movement of now, the flow of time, seems to be determined by the number of milliseconds it takes to send a signal from the eye or ear or fingertip to the brain and to record and notice and remember. For humans, that’s a few tenths of a second; for a fly, a few thousandths of a second. That’s why it is hard for a human to catch a fly. To a fly, your threatening hand approaches in slow motion—just as in Clockstoppers.

Augustine, in his Confessions, bemoaned his inability to understand time’s flow. He wrote, “What is time? If no one asks me, I know; if I wish to explain, I do not know.”

You know more than you can say. MJP 6/9/22

For humans, Augustine says, there is no past or future, but only three presents: “a present of things past, memory; a present of things present, sight; a present of things future, expectation.” (Was this an inspiration for Dickens’s A Christmas Carol?)

The glory of relativity is that everyone everywhere agrees.

This is what a thoughtful observer on Earth would say: “The time interval between the two birthday parties (the two events) was three months in the Earth frame, and one year in the rocket frame.” The observer on the rocket would say exactly the same words. Observers don’t disagree on time intervals any more than they disagree on velocities.

Lightspeed is defined as velocity divided by the speed of light, just as Mach number is velocity divided by the speed of sound. Light (in a vacuum) travels at lightspeed 1. Move at half the speed of light and you have lightspeed 0.5. The time dilation factor, the stretching that takes place when you compare time intervals in two reference frames, is called gamma (the Greek letter γ), and its formula is , where b is the lightspeed.

My iPhone’s internal clock runs at 1.4 billion cycles every second. It can do thirty-six elementary computations in the 26 nanoseconds it takes the pion to decay.

(Excel won’t do, but the iPhone app called Calculator works. Orient the iPhone horizontally for the scientific calculator mode.)