What is Time?

I enjoy teaching, as anyone who has had the misfortune of asking me a simple question that set me off on a half-hour lecture can attest, but teaching to someone else’s curriculum can be…frustrating.  Just recently, I was reviewing a lesson guide on the topic of time that instructed me to begin the lesson by asking students ‘what is time,’ and proceeded to say that the students must reach the answer that time is based on the rotation and revolution of the Earth.  Putting aside for a moment the fact that this question was supposed to lead students to an erroneous conclusion about the nature of time, I am actually more perturbed by the prescriptiveness of the desired answers.  Such an open-ended question begs for creativity in answers, and I would be thrilled to hear what students could conjure to a question that has challenged scientists and philosophers for centuries.  So thrilled, in fact, that I intend to subject you to an entire post discussing my answer to the question ‘what is time?’ 

It is first necessary to establish what, precisely, is being asked by that question, since most casual answers are actually responding to the perceived question ‘how is time measured,’ and not to the actual question ‘what is time’.  Answers involving the rotation of the Earth, the Earth’s revolution, the phases of the Moon, the half-life of cesium, or the vibrations of selenium all pertain to the measurement or perception of time, not to the nature of time.  Whether it is done with a crude sundial or an atomic clock, measurements of time are ultimately indirect, measuring a periodicity, not a fundamental quantity.  We count the number of times that a predictable measurable event occurs, and we use that as a proxy for the passage of time.  It is not, though, time. 

Saying that time is the rotation of the Earth or the vibration of an atom is like saying that space is a yardstick.  We can measure space with a yardstick, or with a measurement of how far light travels in a certain period of time, but that does not make space the yardstick.  Instead, space is a physical span.  We all have an intuitive understanding of what space is, and we use measurements to help us establish standards for interaction with it.  Similarly, we all have an intuitive understanding of time.  If we strip out the distraction of measurements, and reduce time to its fundament, we find that time is actually a manifestation of change. 

Consider a thought experiment.  You are in a sealed box in empty space.  There are no gravitational fields or accelerations of any kind acting upon the box, and there is no means by which you can interact with the universe outside of your box.  Nothing is in the box; it is entirely static.  In fact, I don’t even want you to be in the box, because humans inevitably change.  This is a box of empty space floating in empty space.  Instead of a person, imagine that there is a notionally perfect, unchanging entity.  It exists in this box of empty space, which itself also does not change.  Does this entity in any way experience the passage of time?  No.  Time is change.  It may also be money, which makes for a semi-clever bit of wordplay with the double meaning of “change.” If there is no change, there is no time.  What we call time is a way of characterizing the change that surrounds and permeates our lives. 

Special relativity exhorts us to treat time as a dimension, like the three spatial dimensions in which we live day-to-day. That makes sense if you read Relativity (which we will be reviewing in a few weeks), but it has led to a certain misunderstanding of time as a more…physical concept than it really is. The logic of relativity demands that time be treated akin to a spatial dimension, but that does not mean that time is like space. There is not a timeline along which we can walk, or that contracts or expands the ways spatial dimensions may, or some kind of oceanic Instant. Time is an expression of change, a byproduct, something we perceive as existing because of change when in reality we are just seeing change.

Theoretical physics took many years searching for “time’s arrow,” an explanation for the apparently monodirectional progression of time, but the answer lies in an old physical principle: entropy. Entropy is an expression for the level of discernibility in a system. A system that is very organized, in which you can clearly identify one point from any other point, is a low entropy system. A system that is very chaotic, in which any given point may be indistinguishable from any other, is considered high entropy. Imagine that you have a very organized dresser. You know exactly what clothes are in which drawer, and they are neatly folded in discrete piles based on color. That is a low entropy system because you can easily differentiate the red pile in the shirt drawer from the blue pile in the pants drawer and the yellow pile in the sock drawer (though why you would have yellow socks is a very good question). When all of those clothes are thrown haphazardly into the laundry basket, the system becomes high entropy because the red shirts are mixed in with the yellow socks and the blue pants and you can no longer readily distinguish their respective locations.

Because the universe moves from a state of low entropy to a state of high entropy, irreversibly on its overall scale despite the ability of isolated regions to demonstrate reductions in entropy, change, that is, time, appears to progress in only one direction: from a state of low entropy to a state of high entropy. This does not mean that entropy is the answer to our original question about the nature of time, but it does explain why time appears to be monodirectional. If you saw an egg unbreak itself, it would look to you as if time were running backwards, because that is a blatant reduction of entropy (in a localized area, so it is possible, just highly unlikely). Imagine that the breaking of the egg is the only change in your environment (and that you don’t have a heartbeat, metabolism, et cetera that would indicate that you are a) alive, and b) experiencing the passage of time). The egg breaking would be an indication that time is moving forward, the egg unbreaking would be an indication that time is moving backward, and the egg remaining unbroken would be an indication that time is not moving. If that is your only indication of the passage of time, time stops when the egg is not changing.

Realistically, the egg is never not changing, which is why this is a thought experiment, and not a real experiment. Consider the ultimate end of the universe, if nothing untoward occurs to interfere with it: the great heat death of the universe. This is when entropy has risen so high that every single point of the universe is indistinguishable from any other. There is just a uniform distribution of energy, with no swirls, no eddies, nothing to differentiate any one place from anywhere else. In this maximum entropy state, time is meaningless. I will not go so far as to say it does not exist, but I do posit that it cannot be measured, cannot be perceived, cannot be interacted with, and is, effectively, still. When there is now change, the steady progression of time is paused.

Think about all the different ways we measure time, like we discussed at the beginning of this post. Whether it is a hyper-accurate hydrogen maser clock, or the simple rising and setting of the sun, all of these are brought about by change. It is impossible to imagine time as anything but fundamental because we ourselves are inevitably changing all the time, even without external indicators of time’s passage, but consider if that stopped. If you ever stopped, all the little changes in you froze, would it not be as if time stopped? Because…time would have stopped. Because time is change.