“At the precise moment heat is produced, the process is irreversible: the past differs from the future,” writes Carlo Rovelli in Reality Is Not What It Seems. “It is always heat and only heat that distinguishes the past from the future.”
I find this fascinating – the notion that heat is inextricably tied to our perception of time and how things march on in one direction. Entropy always increases – a piece of wood can burn but it can’t be unburnt. A glass smashes (it’s just entropy heat/entropy increasing) but it can’t be unsmashed. But what really blows my mind is that technically, a reverse of entropy is possible – it’s just incredibly unlikely on a macroscopic scale.
If you put a cold spoon in a warm cup of tea, the spoon always warms up. But according to the laws of physics, it is possible for this not to happen. Due to the high number of molecules jumping around between the spoon and the tea, the probability is heavily weighed in favour of a more even dispersal of those heat packets. But technically there are arrangements where all those heat particles could stay in the tea and not exchange into the spoon. It’s just dazzlingly unlikely.
It’s all about probability – life really is random. Entropy basically just informs us that there are a certain number of different internal arrangements of a state; different probabilities. As Rovelli writes: “We are small systems within the world, interacting only with macroscopic variables that average among innumerable, small microscopic variables.” At the quantum level, such things as time are irrelevant. But for us, creatures giving out heat all our lives until we perish, this passing of time and heat feels very real. There appears to be no life without heat. If you enjoy reading about entropy, please check out this post later – it inspired me to read more into the subject and is beautifully constructed. It makes quite challenging concepts entirely accessible: Entropy explained
What if we could go backwards?
So life moves forwards – or as Kierkegaard put it: “Life can only be understood backwards; but it must be lived forwards.” When the universe began, entropy was low – it has grown and life has flourished. But eventually the stars will all burn out and the universe will slowly reach equilibrium – where no life is possible. At least not life as we know it. And so based on the macroscopic nature of entropy and thermodynamics, it would seem probability isn’t in our favour to achieve some kind of time travel. If we had the mechanical means, we could technically time travel based on the laws of Special Relativity – if you were travelling on a spaceship going close to the speed of light while your companions remained on the Earth, you would experience your passage of time far more slowly than they would theirs; in five of your years you’d return to find they’d raced through 10 times as many. They’d all be old while you’d be young. Sweet deals! Actually, no. That would be awful.
Anyway, due to another of Einstein’s amazing discoveries (General Relativity), we know that warped spacetime, and the the gravitational pull of planets, also affects time; time slows down for objects in gravitational fields compared to those further away from these fields. And there are theories out there which suggest wormholes in space could offer shortcuts through spacetime. Whether or not this is ever realised, it would seem that the laws of physics don’t allow for the kind of high-jinks Marty got up to in Back To The Future – the fading picture of you with your siblings when your nemesis stopped your dad shacking up with your mum appears to be implausible. Phew!
But if you – and I mean ONLY YOU – could travel back in time to correct errors like Sam in Quantum Leap or Domhnall Gleeson in About Time, would you want to do it? I’m sure we’d all be tempted, but I stand by Kierkegaard (and Nietzsche’s Amor Fati – an appreciation and acceptance of fate). Life is lived forwards. What happens in those lives, both good and bad, is part of the journey. That is life. I like the fact entropy increases – I take comfort in the fact that we are burning up existence in one direction. We can look back to help us understand, as Kierkegaard said, but we need to continue to move forwards, and we need to embrace that momentum. We will all burn out one day, but let’s appreciate how lucky we were to become a product of this wonderful warming.