Why Time Appears To Move Forward (9/19/2008)

This was originally posted on a horrible site called Myspace. When Myspace underwent a redesign in Fall 2010, hundreds of insightful reader comments that had been left over the years were lost. I have since deleted my account there.

We think of time as something that's in constant motion. Unlike anything else we can imagine, time could never stand still: It must always "flow," passing us by (and passing through us) at a constant rate. Even if we remain completely motionless, time passes inexorably, like water in a river -- and once a moment in time passes, it's gone. Much has been written about this phenomenon, as well as the question of why there is an "arrow of time." Why does time move in a specific direction, and only one direction? We can remember the past, but the future is a total mystery, a void of darkness. Why such a marked difference between the past and future?

Going through time is a bit like traveling through space, at a constant speed, in a ship that rides on a giant stationary ruler. There's one narrow window on the ship, through which the only thing you can see is a sliver of the ruler. Inches, feet, and miles pass by at a constant rate, and even though you can't see the inch marks and mile marks coming, from experience you know that they will come eventually. The length markings are moving in a specific direction -- toward the ship and then past the ship -- and disappearing. But this singular direction of time may not be as real as it seems.

Many physicists like to think of time in terms of the "block universe," where the past and future merge as a single entity. Any event in the history of the universe can be defined as a point in this block, in terms of its location along four dimensions -- three of space and one of time. For example, the first human reaching the summit of Mt. Everest is an event that can be defined by its three-dimensional location in space and its one-dimensional location in time (May 29, 1953). Time doesn't need to actually pass by in order for a date to help define an event, any more than miles need to "pass by" in order for New York to be described as being 100 miles northeast of Philadelphia.

What is it that connects these events together in the block universe? Information. When a massive star explodes, the explosion creates information about the event, in the form of light and other radiation emanating outward. Another event -- the observation of a supernova -- occurs when that information reaches humans on Earth. The explosion is an event that causes the observation event. This same principle of causation between events can be seen on the human scale (causing event: I drop a hammer; resulting event: floor gets dented), as well as on the scale of the very small (causing event: photon interacts with atom; resulting event: electron changes energy state). In all of these cases, the resulting event happens "later in time" than the causing event, as information flows from one place to another. How much time elapses between events? That depends on the speed of the information. Information can travel as fast as the speed of light, or much slower, as with the falling hammer. But as far as we know, no kind of information can travel faster than the speed of light. So, causally related events cannot occur simultaneously; there always has to be a time interval between them.

The block universe is like a stack of events, with newer events at the top. In this stack, causing events are located below events that result from it. In other words, the events in the stack aren't arranged randomly; in terms of vertical placement at least, all events are arranged in a specific order of causation, with every resulting event located somewhere above its respective causing event. Causally related events are connected by channels of information flowing. So what we have is an unimaginably complex web of events and the information that connects them.

Now, as a thought experiment, put yourself in the place of one single event in our stack. You are frozen in both time and location; below you in the stack are "previous" events, and above you are "future" events. From this position, what would you be perceiving? You would perceive only past events -- the ones below you. This is because perceiving any event has to be a result of the event, the event being exactly the thing that caused the perception. Causation can never come from the future, because we've defined a cause as something that must occur earlier in time than its result. However, from your spot in the event stack, where you're frozen in time and location, you wouldn't perceive all past events. You'd only perceive events that are at specific distances and times, relative to you, for their information to be reaching you at that precise moment.* What about future events? They would all be hidden from view, owing again to the very definition of cause and effect. So, some past events could be perceived, but most of them wouldn't be. And all future events would be imperceptible.

In the real universe where you and I live, existing is like sitting in a movie theater. Turning around in your seat and trying to see the back of the theater, which is like looking into the future, there is always darkness. But from the "past" direction of the screen, information comes to us constantly, and always from this one direction. The result is the distinct sensation that time is advancing relentlessly forward, just like the action in the movie, or the slivers of the ruler in the first example. But we could never find out what the future is in advance, because information does not travel from the future; that would require inverting the very definition of cause and effect. The future will always reveal itself only as fast as we can receive information about it.

One thing I like about this informational view of time is that it solves things like the "grandfather paradox" -- the question of whether you could go back in time and kill your grandfather when he was a youth, preventing your parents from meeting. This would require information (you with a knife) coming to your grandfather from the future, and causation does not allow information to go from the future into the past. Information only goes in the past-to-future direction. As a result, in our universe, you could never successfully travel into the past, at least not to participate in causation in any way. You can travel into the future, however -- in fact, there's no way not to; you're time-traveling into the future right now. And as Einstein discovered, if you also move through space, you'll increase the rate at which you travel into the future, relative to someone who isn't moving.

Interestingly, the idea suggested in this essay -- that of a timeless, eternal universe that "knows all," including every event of the past and future -- sounds something like the traditional idea of "God." However, nothing here calls for a sentient being that can direct or change events, listen to our prayers, or judge us after we die.



* Physicists use the terms "light cone" or "plane of simultaneity" to describe the set of events perceivable from a single point in space and time. For example, the Sun eight minutes ago and the Moon 1.5 seconds ago lie on the same light cone as the Earth now, so right now we are receiving information about the Sun and Moon as they were at those times in the past. Hermann Minkowski (1864-1909) was a leading theoretician in this field.