We’ll be looking first at the concept of entropy, in preparation for our AP-level look at the mysterious ‘flow’ of time. So strap in, get yourself a coffee (that’s an important part of the lesson, trust us) and get ready to rethink everything you thought you knew about Time.
« The more I learn, the more I realise how much I don’t know. »Albert Einstein
The greatest scientific minds in the world have an unlikely hallmark: humility. And so perhaps it should come as no surprise that the further we humans delve into the realms of Quantum Mechanics, the more mysteries and paradoxes we seem to encounter. There are a handful of as-yet-unsolved problems that plague (or delight, depending on their mood) the intrepid explorers at the frontiers of quantum physics.
One such confounding conundrum is what’s known as the arrow of time, or time’s arrow. This concept states – or ‘posits’, if you’ve got your lab coat on – what’s known as the one-way direction or asymmetry of time. In short, it says that time goes from past to future via the present. Obvious, right? Well, perhaps not, if you’re a quark with an attitude. But more on that later.
Developed in 1927 by British astrophysicist Arthur Eddington, the concept of Time’s Arrow isn’t just an unsolved mystery for Quantum folk – it’s also a widely-studied question in general physics, and even haunts the dreams (and waking nightmares) of philosophers and poets.
According to Eddington, who, incidentally, was widely ridiculed and snubbed for his outlandish claims, the direction of time could be determined by studying the relative ‘organization’ of atoms, molecules, and bodies.
Physical processes at the microscopic (very small) level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true. In other words, if you went back in time from present to past, they’d behave in equally ‘correct’ ways, like a palindrome (think the words ‘deified’, ‘racecar’ and ‘madam’… or the more familiar phrase ‘a man, a plan, a canal, Panama’).
Yet at the macroscopic (larger) level, it often appears that this is not the case: there is an obvious direction – or flow – of time. But then the question remains, why does time have a direction for ‘larger’ bodies and processes? The theory rests on the observable existence of a property known as entropy.
Entropy is the phenomenon that drives time ‘forward’, and is roughly defined as meaning that the degree of ‘disorder’ in the universe only increases; there is no way to reverse a rise in entropy after it has occurred. As any good watchmaker will tell you, things only get more complex as the minutes, days, and years go by – and we’re literally watching the time pass.
Although it might seem like an obvious illustration to say that, left to their own devices, a teenager’s room, global politics, or the methods required to access your online bank account become more chaotic, confusing, and disordered as time goes on… that’s not exactly what we’re talking about here. While not technically untrue, we’ll leave those interpretations to the philosophers and social historians.
We get a bit closer with the observation that cream, poured into coffee, disperses and intermingles with the coffee itself until it becomes virtually impossible to distinguish the two substances. (Experiment time: here’s where you try it out and prove science right, while hopefully giving yourself the strength to carry on reading. Examine coffee. Pour Cream. Watch time elapse on your handy TAG Heuer timepiece. Re-examine coffee-cream mixture and see if you can separate out the molecules of each substance. Fail to do so…).
Journalism on the subject of entropy, meanwhile, is fond of pointing out that an egg smashed on the pavement becomes a splattered mess that is, essentially, impossible to reassemble into its original, ordered form. We see no need to waste (even hypothetical) eggs; you could just as easily say that three eggs scrambled with plenty of butter have lost their original ‘ordered’ form and become mingled, jiggled, and dispersed into a (delicious) but more ‘random’ state.
…Insert snack break if necessary, and take a moment to ponder this simple summing-up of the previous section…
« The increase of disorder or entropy is what distinguishes the past from the future, giving a direction to time. »Stephen Hawking, A Brief History of Time
The thing is, if you were to ask a physicist for a bit more detail, they’d probably roll their eyes and explain to you patiently that at an atomic level, entropy is a bit more complicated than merely making breakfast would imply. It’s not just about order. Unfortunately, there’s some maths involved.
Specifically, there’s another important piece of the puzzle: the fact that entropy increases is also a matter of logic. Entropy measures the ‘quantum states’ of particles. Without bending the mind too much around what this means, the conclusion goes something like this: because there are more possible ‘mixed up’ arrangements of particles than there are ‘separate’ arrangements, it means that as things (atoms, molecules, bodies) inevitably change, they tend to fall into intermingled disarray.
And now we come to the High School Science Recall section: can you dig back into distant memory and dredge up the Second Law of Thermodynamics? We’re sure you can, but just in case, here’s a little reminder…
One of the most inviolable laws in the Universe, the second law of thermodynamics states that in any isolated physical system, entropy only increases and never decreases. This is true not only of a closed system within our Universe (i.e. your trusty cup of coffee, with a lid on it), but of the entire Universe itself.
And if you understood that recap about as much as you did the first time, when your exasperated Physics teacher waved their hands over and over at a bewildering diagram of blobs and arrows, it’s ok. Because here’s a little secret about entropy that Physics 101 probably never mentioned: you already understand it.
Despite being a hazy abstract concept, entropy is an idea for which pretty much every human alive has an intuitive sense. Even all the way back in the 1920’s, Eddington himself was adamant that the phenomenon was ‘vividly recognised by consciousness’ and ‘equally insisted on by reasoning faculty’. In other words – DUH.
Much as with temperatures rising and falling, humans can tell the difference between things that go ‘forwards’ vs. ‘backwards’ in time, based on the behaviour of physical matter. An (old-school VHS) video of a wood fire melting a nearby block of ice would, played in reverse, appear to show a puddle of water freezing itself while turning a cloud of smoke into a pile of wood.
Weirdly, both versions of the tape obey the majority of the laws of physics. With one notable exception: our old friend, the Second Law of Thermodynamics. So in a crazy twist, it’s only your inherent understanding of entropy that allows you to determine whether the video is playing forward or backward. As you already knew, when time goes forward, entropy must increase. You little genius, you.
Alright, we’ve now categorically proven your innate brilliance in the realm of Quantum Physics – so we’d better quit for today while we’re ahead. Join us next time in Part Two to apply your Mensa-worthy mind to the tricky question of Time’s Arrow. You’ve got this. So strap on your TAG Heuer Formula 1 Chronograph, and get ready to get more complicated by the microsecond.