What is time and what does it mean for it to “move forward”?

In my first post, I alluded to concepts I call the three realms and the four ideas. Last week, we talked at an abstract level about life in each of the three realms, a framework for segmenting reality according to relative size scale. For the next several weeks, I’ll be publishing a series of posts that get more specific about how different realms experience the four ideas. These are the fundamental physics concepts of time, space, matter, and energy.

If the prospect of multiple blog posts on “time” doesn’t raise your heart rate, you probably took a high school physics class that dealt little with time beyond including the letter t in a bunch of equations. The same can probably be said of matter, represented proudly by the letter m. Energy? Just convert potential to kinetic (and sometimes rotational) and call it a day. And space, well… what is there to say about space, anyway? Space is nothing! Wait, distance maybe? Did you mean to say distance? Would you like to borrow my x?

Suffice to say I think we can do better. For each of these series, I hope to begin by reflecting on the meaning behind the idea “at our level,” i.e. in the household realm. Then, we’ll explore the phenomena at the bottom (i.e. the quantum realm) out of which the idea manifests. Finally, we’ll head to the top, or the cosmic realm, where the idea reaches its limits and starts to break down. I hope it will be a conversation-provoking journey.

How do we measure time in the household realm?

I want to talk about time first; of the four ideas, it is probably the most familiar to our conscious experience of the world while being the most difficult to define from first principles. By “define from first principles,” I really mean, “define in a non-circular way.” Consider the prompt “Time is a measure of ____,” and you may (quite reasonably) end up saying something useless, like “Time is a measure of how long it takes between two events.” Of course, all you’ve really said is, “Time is a measure of the time between two events.”

Well, who could blame you? Time is what it is! Even physicists appear to have punted this question – they say that time is a measure of what a clock reads! This definition sounds trivial and nonscientific, but the more I think about it, the more I like it. Through the “clock” lens, time starts to resemble money.

Money has value because everyone is using the same currency (or at least, there are agreed-upon rules for converting between the various currencies people use). You accept a $20 bill from me because you know that somebody else will accept it from you later. Similarly, we can talk about time in a standardized way because we have all agreed on a system for recording it. When you ask me to meet you at noon, we are making the implicit assumption that our clocks will both read the same value – 12:00 – at more or less the same instant.

Of course, the metaphorical structures we use to think about time (or money, for that matter) are based on the tools we have for measuring it. In antiquity, there was no way to mark the passage of time with precise instruments, so repeating natural phenomenon like sunrises and the movement of constellations were used. The ubiquity of ancient solar and lunar calendars spanning tens of thousands of years speaks to this expansive but imprecise view of time. More reliable mechanical clocks in the Middle Ages allowed the solar day (one rotation of the Earth) to be divided up into conceptual –if not always measurable– pieces. Today, the international standard unit of time, the second, is defined as the duration of 9,192,631,770 vibrations of the atom Cesium-133*, yet another repeating event from the natural world!

In this way, time is really less akin to “money,” and more analogous to a word like “value” or “worth.” The value of someone’s possessions (or their time!) is measured by counting what they will accept in exchange. Likewise, time is measured by counting the events that occur within that time. Even the atomic clocks that synchronize events across the entire planet work this way – they count to 9 billion and say, “A second has passed!” and the GPS in your phone, or the stock market, or a globally distributed data center run by Google says, “Roger that!”

I get that this might not be the most mind-boggling stuff; everyone knows time is sort of a made-up construct. My point really is just to underline the notion that time is entirely a made-up construct, and even the most advanced means of defining and measuring it rely on a metaphorical understanding of repeating events that we as one species agreed to!

How do we experience time in the household realm?

  Of course, the measurement of time isn’t the thing that makes it so immediate and intuitive. What really matters to us humans is the sense of the passage of time, the “forwardness” of our existence. We live moment to moment, and there is always a sense that the present moment is taking place after some previously experienced moment. Although the perceived rate of this passage can certainly vary –probably because of how our brain encodes memories– its direction is resolutely onward.

But what does that actually mean? The metaphor I like to use is of a game played between you and a film projectionist. Imagine you’re sitting in a theater, and a series of short films are projected on the screen in front of you. Behind you is the projection booth, in which the projectionist sits with two piles of film reels. The reels in the left pile were filmed normally, but those on the right were filmed with the reel moving the opposite direction inside the camera, and so will show reversed motion when projected. The game begins when the projectionist selects a random film to play. If you can correctly guess which pile it came from, you win.

Now, most of the time, the movie won’t have to play very long before you’re able to identify with certainty whether or not it was filmed in reverse. You might see something spill or break, say (or implausibly reconstruct itself). Smoke may drift up from a fire (or back down into it). Bullets could fly into or out of a gun. These would be instant red flags.

Notice that I didn’t say “you’d notice all of the actors walking and talking backwards.” That can be (and has been) faked. In fact, there would be an awful lot of footage that you couldn’t bet with absolute certainty came from one pile or the other. Trees swaying in the wind, cars rolling along a road, practically any movement from a human or animal; you may have a clear intuition about the direction in which these actions were filmed, but not the absolute certainty you would have about, for example, a block of butter melting in a frying pan.

You might reasonably conclude that the laws of physics preclude such “backwards” behaviors, but that’s not the case! There is no physical rule preventing the requisite forces and heat from conspiring to push a puddle of melted butter back up into a solid cube. Taken a step further, consider something as open-and-shut as an egg falling off a countertop and breaking on the floor: we know from conservation of energy that the potential energy from the egg’s initial height converts into kinetic energy as it falls, which, when it lands, is transferred to the floor and to the flying shards of egg shell. But conservation of energy also requires that, in principle, the floor could push back up on the egg, and the molecular bonds off its shell could be restored, and this would provide the egg exactly enough kinetic energy to lift off from the floor and land back onto the countertop. Such a sequence of events would be totally legal, so to speak. So why do we never see it happen?

This is the point at which your brain might remind you of something called an “irreversible process,” or something else called  “entropy” that always increases. In fact, understanding entropy is exactly what we need to do in order to unlock the fundamental question of time’s arrow. But to understand entropy, we need to leave the household realm, and I want to linger here for just a moment longer.

To return to the prior paragraph, and indeed to the theme of this entire post, I want to underline that last question: “Why do we never see it happen?” As I said at the end of the time-as-value discussion, I know that this in itself is not the most profound thing you’ll read all week. I’m arguing that this question is hugely significant simply because we have to ask it at all. Much as measurements of time are equivalent to the regular processes we use as timepieces, so our ability to spot a seemingly irreversible process is not just a symptom of our experience of time; it is the experience! The arrow of time is the fact that there are things we know to be possible but never witness. And that’s it! That’s what it means for time to move forward. There’s no hidden complexity. 

To put a “human” touch on this discussion (and to go out on a bit of a limb) my theory of why we experience the arrow so intuitively is that the materials that make up our bodies and brains constantly undergo irreversible processes. As a result, we are especially attuned to such events when we observe them in nature. On a deep level, we recognize when objects transition permanently from one state to another. And the vehicle for that transition? We call it “time.”

Next week, we’ll take a deep dive into the quantum realm, wherein lies the root cause of time’s steady march from the past toward the future.

* It’s not exactly as simple as vibrations, turns out. I’m making a mental note to devote a short post to exactly how atomic clocks work (I guess I’m actually making a written note!)

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