“Time is a distension of the mind.” —St. Augustine of Hippo
“Time is an illusion. Lunchtime doubly so.” —Ford Prefect
This is sort of related to my previous posts on French decimal time, time travel, and even time crystals, but in a lot of ways, it’s a totally different topic. Scientific ways to measure or deal with time aren’t the same as pondering the nature of time itself, which is what I invite you to do with me today.
When I ask, “Is time real?” I’m not suggesting that maybe time is imaginary and therefore useless. In fact, I think pretty much everyone would agree that even if time is some sort of illusion, we imagine it exactly because it’s so useful. Also, it seems a little silly for a physicist to question the evidence of change over time that we see in, oh, everything around us.
No, what we’re talking about is whether time has some kind of absolute existence beyond the evidence we see in the physical world. This is a very philosophical question, so it’s more of an interesting companion to scientific inquiry than anything else. It’s certainly not something we can settle at this point, so we might as well enjoy it!
Why wouldn’t time have an absolute existence? Well, even though we often refer to time as a dimension like the three spatial dimensions, it’s obvious that we interact differently with time than with space. We don’t perceive the state of things from the past or future to be physically real alongside our present moment, and all we really have to prove the passage of time is evidence of differences from how things used to be. On top of that, we humans are pretty flawed observers of time. It’s understandable that it would be a hot issue with no easy answers, and it’s been that way for millennia.
Let’s start with the Augustine quote I opened with, which comes from his Confessions, pretty much the earliest example we have of what appears to be an honest, (mostly) unaffected autobiography. In its later parts, it also turns into something of a philosophical commentary on the nature of being, memory, and time. Both parts are delightful in their own ways, but as an undergrad reading the Confessions, I was really struck by the way he talked about time.
Augustine comes to the conclusion that he can’t explain what time is if he really tries to think about it, and goes beyond that to break down our conventions of talking about past, present, and future. Basically, he comes to the conclusion that the way we think about time, especially the present moment, is a lie to help us process life. (This is supported by modern science! Our brains process information slightly after the fact, so that what we think of as the present moment is already in the past by the time we can think about it.)
I could keep paraphrasing, but his own words are so much better:
If we conceive of some point of time which cannot be divided into even the minutest parts of moments, that is the only point that can be called the present: and that point flees at such lightning speed from being future to being past, that it has no extent of duration at all. (11.XV.20)
Augustine considers the present to be so incredibly short that it doesn’t really exist itself, being more of a thin dividing line between the unreal past and future. This is a slightly extreme but overall pretty good expression of a philosophy that’s still around today. It’s called presentism (which can have a few meanings, but we’re focusing on the time philosophy), and it says more or less what Augustine expresses so eloquently—only the present moment is actually real, and past and future don’t exist in any meaningful way.
The opposing view is eternalism, which says that every moment, whether present or future, is equally real. This is somewhat inspired by the way we talk about time as a fourth dimension, which would make it natural that every “point” in time would exist just as much as every point in space. Theoretically, if we could travel to those points, we’d find them just as real as the split second we’re living in now.
Another way to look at the “reality” of time is whether it actually exists on its own, or just in the relationships between physical objects. The major divide here is between relationalism and absolutism. Absolutism states that time exists as a flow or quantity regardless of the physical characteristics of space; relationalism prefers to think of time as an expression of the relationships between physical objects.
Gottfried Leibniz, one half of the calculus controversy, was a poster child for relationalism. He came up with some logical thought experiments arguing against the absolute existence of time and space. Shockingly enough, Isaac Newton disagreed with Leibniz. He believed that reference frames of time and space existed absolutely, independent of observers or objects. Absolutes of time and space have been shaken up in the last century by relativity, but there’s still room for absolute time, at least in some sense.
Personally, I didn’t know the term “relationalism” until recently, but I’ve been partial for years to the view that time is only meaningful in conjunction with change. Our current and past definitions of time are certainly based on change. Right now, the length of a second is based on the period of an energy level transition in cesium-133, replacing an older definition based on the length of Earth’s day, a reminder of the constant motion of the Earth. (We changed our definition because Earth’s day is changing along with its orbit—see, more change!)
Taking this to extremes, you could argue that if you had a system where literally no change whatsoever was happening, time would have no measure and thus be meaningless. Think of a video—in the absence of any clues like a time bar, you can’t tell the difference between a still or a paused video and one in which there’s simply no change in any of the objects on screen. (Wendy’s has a few fun promotional videos taking advantage of this concept.)
This might sound like a pretty unstable basis for time. What would happen if nothing moved, then? Luckily, that’s pretty much impossible to achieve. First of all, it wouldn’t be enough to freeze just one or two things. You really need to stop all motion in an entire system, whether that’s the whole universe or just a sealed volume.
That is incredibly unlikely, because we’re not just talking about the mannequin challenge here. In a totally closed, self-sufficient volume of space, the passage of time is marked by the decay of particles on small scales, and by the movement of planets, stars, and galaxies on larger scales. All of that would have to stop, which is pretty much unimaginable, so you don’t have to lie awake at night worrying about time getting messed up.
Still, the “what if?”s are a lot of fun. Many people who like to think about this sort of thing turn to the idea of ultra-low temperatures, since particle energies get lower as things get colder. Absolute zero is the temperature at which things would reach their lowest possible energy and move around as little as possible. One could argue that if there were a system at absolute zero where nothing moved, ignoring the existence of any outside objects at non-zero temperatures, time would literally freeze for that system.
I find that idea so cool. Also impossible, because we’d have to ignore zero-point energy, which is a ground-state energy that particles would still have at absolute zero, keeping them from being totally motionless. As far as we know, zero-point energy isn’t something we can just remove, so we might be stuck with slightly moving particles even at the lowest possible temperature. Stopping time may be beyond our reach.
Could I (and Leibniz, and other relationalists) be wrong? Absolutely! The current impossibility of testing this or other ideas about the nature of time is what gives philosophy a foothold in this area, when it’s given way to scientific evidence in so many other fields. To some, it may seem useless to talk about time, unless we come up with ways to test these different views, but I think there’s a lot of value in having these philosophical discussions. It’s a different approach than the purely scientific method we try to apply so often, and it’s good for imagination, creativity, and logic.
In fact, you might say its value is…timeless.
(Thank you, you’ve been a lovely audience. I’ll be here all week.)
[Read more about different philosophies of time and space! Also, find out more about how our brains trick us into thinking we understand this whole time thing.]