Imagine that one day, as you walk down the street on your way to your favourite coffee shop, everything around you suddenly stops. Cars stop rolling, birds freeze in midair, and the people around you halt in their tracks, halfway through blinking or sneezing or whatever it is people do as they walk down the street.
You take a look around and observe that you seem to be the only thing in the world capable of movement. There's not a breath of wind, and utter silence reigns. You're not alarmed, because you assume you've stumbled by chance into either a weird temporary rift in space-time or a thought experiment in someone else's blog. You decide to look around, and proceed to enjoy yourself exploring your new world.
After a while though, you get bored and wish that something new would happen. You plop down in the unmoving shade of a tree bent by a wind you can no longer feel. You wonder idly how much time you've spent in this condition, and as you glance at your watch you realize that its hands are stopped. The sun hasn't moved either, and even if you had an hourglass handy its grains would be frozen mid-stream. You're aware, however, that the heart rate of a resting adult is about 70 beats per minute, and so you decide to count your own heartbeats in order to keep track of time.
But after two hundred heartbeats, or about three minutes, your heart and lungs inexplicably stop, and you find yourself incapable of moving a muscle. You too are now frozen, except for your brain, whose neurons are somehow still firing despite receiving no oxygenated blood from the rest of your body. Again, your bodiless consciousness is not alarmed because you know things will probably go back to normal in a couple of paragraphs. You resolve to count seconds and minutes in your head, and thereby prove to yourself that time is indeed still passing.
After about two and a half minutes, though, your brain freezes too, and even your thoughts cease altogether. Of course, you don't experience this third stage of frozenness at all because you don't have thoughts to experience it with, and thus it's no wonder that it's difficult to imagine. (Actually, it's impossible to imagine). Since there's no one to observe you sitting frozen under the tree, it's meaningless to ask how much time passes in this state before the world suddenly and inexplicably starts up again. You get up, stretch, and notice that no one else has noticed anything unusual, and are blinking, sneezing, and going about their business as if nothing has happened.
Of course, from their perspective, nothing has happened, and when you put the question to yourself you're forced to admit that even from your own perspective all that's happened is a whole lot of nothing. You decide to shrug off the experience, knowing that no one will believe you anyway, and continue on your way to your favourite coffee shop.
It's a silly story, but silly stories are best when it comes to illustrating concepts that are exceedingly abstract. This week's topic, our third and final way of looking at energy, can be a real head-spinner if we don't continually drag it back to the concrete by the scruff of its hypothetical neck. This week I'd like to come at the question that titles this series of posts by way of a parallel question: what is time, really, and how do we know that it's passing?
The answer, I propose, is simpler than we might at first think. We know time passes because things happen. If there existed a place where nothing was happening (and I mean nothing, which, as I hope I've made clear, is impossible as well as unimaginable) then time could not be said to be passing in that place. Things happen because they have energy: our thoughts happen because our neurons convert food energy into electrical impulses and waste heat; our watches work because their mechanisms convert electrical energy into mechanical motion and waste heat; the sun appears to move across the sky because the earth converts rotational energy very slowly into friction and waste heat, as the moon's gravity very gradually slows our turning
by about a millisecond each year. The movement of things around us, and
even the movement of our thoughts, is an expression
of the flow of energy.
That's why physicists have tried to understand time's trajectory as the trajectory of energy. You may recall from the first post in this series that energy flows in one direction only, that is, from highly concentrated and useful forms to less concentrated and less useful forms. We visualized this by picturing a stream whose concentrated flow can be harnessed to do work, such as turning a waterwheel. By the time the stream reaches the ocean its energy can't be easily harnessed; it just mingles with the chaotic energy of the ocean's waves and can't be regained. Like the stream, energy never flows from a less useful form into a more useful form; that is, it always flows 'downhill', unless an even more energetic system like the weather moves it back 'uphill' into a more useful form.
Similarly, time never flows backward (although under the right circumstances it can slow down), nor can we travel into the past except by inventing a silly story that involves obvious paradoxes and/or Michael J. Fox. The second law of thermodynamics, which is essentially the lesson illustrated by the flowing stream, says so, and it's no use arguing with nature when she's made up her mind about something. Another way of stating the second law is that over time, concentrations of useful energy always dissipate and entropy always increases.
Entropy is a well-known phenomenon to anyone who's ever mopped a floor. You start out with distinct concentrations of dirt on your floor and on the sidewalk outside: the floor is in a very neat, ordered state compared to the sidewalk, an expression of the energy you put into cleaning it last week. A physicist would say that the floor has low entropy. But after a few days of people tramping in and out, the cleanness of your floor and the dirtiness of the sidewalk have averaged out into a general griminess all around. Both now have high entropy, and will remain that way unless you put more of your own energy into mopping up again.
If this sounds to you like a never-ending battle that you can't win no matter how long you fight, it's probable that either you understand the nature of cleaning pretty well or you're familiar with popular explanations of the scientific concept of entropy. It's a discussion that often leaves off somewhat bleakly, since the facts of thermodynamics as presently understood imply that our universe is making a long and inevitable descent from a state of concentrated, useful energies (hot stars, cold space) toward a state of diffuse and useless energy (a lukewarm, beige soup of particles stretching from one end of the universe to another).
At this point it's worth remembering that facts are one thing, but how we view them is quite another. Science can tell us the facts about how nature works, but it is we who must decide how we feel about those facts. As my readers may or may not be aware, I mop floors for a
living, and so I have a special relationship with entropy.
It's thanks to entropy that custodians like me have continual
employment, and it's thanks to my continued employment that I'm able to
see entropy as the universe's gift rather than as a never-ending battle
I'm destined to lose.
Similarly, we can interpret the laws of nature as being rigged against us, and search for meaning in things that seem to transcend the physical universe, or we can choose to embrace what we can't change and learn to admire it, even to feel at home in it. If you choose the second way you begin to notice that nature's way on earth is to make energy's long descent as convoluted and as interesting as possible, like someone telling a story in which each new development is more fantastic than the last. (If you want a distillation of that story, read part one of this series- it's not half bad!) Next week we're going to start uniting the three ways of viewing energy discussed in the last three weeks by looking at a story of humans acting in time, and thereby begin to work our way back toward decoding the message of my short story The Messengers. It's convoluted, to be sure, but it's also sure to be interesting.