Now and Then

Unless we think about it, time seems pretty straightforward. The future is stuff that hasn’t happened yet, the past is the stuff we remember or can discover, and the present is what we’re experiencing right now. Couldn’t be simpler, unless…. What does ‘right now’ mean?

Let’s look at an example. Lightning splits the sky, two distinct bolts in one flash. Can we say that they happened ‘Now?’ They strike our consciousness at the same moment, so at least in that sense they share a present, but do they share the present?

It’s a matter of perspective.

We see the moon by light reflected over a second ago. The sun we see now has already slipped some nine minutes into the past. The nearest starlight began its journey over four years ago. Everything we sense—the mountain outside our window, the annoying buzz of a fly in the room, the pinch of an uncomfortable shoe, even the screen we’re reading on—exists in our past: the tiny fraction of a second it took the light or sound to reach our nervous system and a longer fraction of a second it took to integrate the sensations into our consciousness. Given all this, what does now mean?

Our intuition is that some magical instant exists in which things happen: stars explode, a planet spins, leaves fall. We see things happen now even as we realize they are already part of then. We assume, when we aren’t thinking about it, that the moment we live in is now, even if we won’t see the star explode for thousands of years, the planet spinning for hours, the leaf falling for a fraction of a second after it separates from the tree. When we do think about it, things get really confusing. Light strikes our eyes and sound our ears a tiny fraction of a second before the nerve pulses can arrive at our brain. Once the signals reach our brain, we spend about a fifth of a second processing them, and then the resulting sensations must be integrated into our awareness. As a result, we have to conclude that the magical instant when things really happen exists just a little way into our future. In other words, if we define now as the moment in some universal time when the different things we experience actually happen, we have no direct evidence that now exists. We only reason that it must, that the sequence of sensations impinging on our awareness must have a source in the external world, a source hidden a few hundredths of a second in our future.

This conclusion becomes even more troubling if we ask a physicist for clarification. It might have been easier when we lived in a nice, simple Newtonian universe and time flowed smoothly from past to future, second by second. Einstein changed all that when he announced that time moves at a different pace depending on how fast we move or how strongly gravity curves the space-time continuum around us, and it moves at a different pace for everyone. We might see two lightning bolts as striking simultaneously but, for an astronaut speeding by fast enough, one bolt would strike a tiny, but measurable, time before the other. And we’d both be right.

What does that mean? We can accept, sort of, that our psychological present is not exactly the same as the real present, what is actually happening out there in the real world, but it is disturbing to realize that scientists, who should know, admit that real time is relative. It depends on where you are and how fast you’re moving. Then we realize that while all those scientific equations describe how a system will change over time, they have no preferred time. No now.

For a physicist, time is what clocks measure, no more and no less. Their theories are expressed in equations that describe change over time and are evaluated on how well predictions match measurements. It makes no difference what time is entered into an equation or if the time variable increases or decreases. The equations are just as accurate when we look back in time as when we look forward. We can measure the mass, location and velocity of a comet, for instance, and be very sure where it was yesterday and where it will be tomorrow, but only have a general idea where it was a hundred years before our measurement or where it will be in another hundred years. For the purpose of tracking comets, at least, now is not the moment the light we observe left the sun, and it is not the moment it was reflected from the comet. For the scientist, now is the moment we take the measurement, the moment we have maximum information about the system. That is time zero in the equations. Not the moment the comet was actually at the place shown by the measurements and definitely not the approximate location projected by the equations a day or a week later.

Like our psychological now, the scientist’s now lags behind events. Both incorporate sensations on the one hand and measurements on the other that presumably originated sometime between a fraction of a second and 13.8 billion years ago. Events inform our psychological now as they enter our awareness, our memory. Events impact the scientist’s now as they are measured. In both cases, they have already passed into then by the time we can think now, and everyone involved, scientist, philosopher, or man on the street, takes the existence of a magical intersection between anticipation and memory on faith.


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