There’s plenty of controversy about where to go from here. The issue, in other words, is that if our statistical reasoning is correct, worlds like ours are far more likely to have arrived in their current state by spontaneously descending from equilibrium (eg, randomly forming from dust) than by rising from a state of much lower entropy (as our fossils and memories and leading theories of astrophysics suggest). We can derive that almost all trajectories from worlds that look like ours right now lead toward equilibrium in the past-to-future direction (which is good) but also, by the same token, in the future-to-past direction (which is a problem). As Boltzmann’s contemporary Johann Josef Loschmidt pointed out, the statistical derivation is neutral as to whether we evolve time forward or backward. Think of it as an Anna Karenina principle: for each way for everything to be in its place, there are many ways for some things to be out of place.Īs Ludwig Boltzmann and others first noticed in the late 19th century, we can use this asymmetry between order and disorder to show that almost all trajectories of systems not in equilibrium (that means: systems not yet maximally messy) lead fairly directly toward equilibrium (maximum messiness) as time evolves.īut there’s a snag. There are far more ways for a room to be messy than there are for it to be clean. Worlds like ours likely arrived by spontaneously descending from equilibrium, not by rising from much lower entropy So where does the asymmetry of entropy come from? If a video of a house of cards collapsing presents a scene that’s compatible with the laws, then that video played in rewind, of a pile of cards spontaneously jumping into a balanced tower, does too (with quantum details suitably edited). The funny thing is that this asymmetry of entropy doesn’t follow from the laws of physics as we usually understand them (except for a few quirks about particle decay, which we can edit out when we reverse the videos). These are different expressions of the fact that entropy goes up as you travel in the past-to-future direction (and accordingly, it goes down if you travel in the future-to-past direction). In summer, ice cubes tend to melt, but warm water doesn’t tend to spontaneously form into ice cubes. Houses of cards tend to collapse spontaneously piles of cards don’t tend to spontaneously build themselves into houses. The second law basically says that your room will get messier, not cleaner, as time goes on.Įntropy has a role to play in accounting for most or all the ways that the two directions of time (past-to-future and future-to-past) differ from each other. The messier the room, the higher the entropy. Entropy is the measure of disorder of a system, like how messy your room is. The answer is – because of the second law of thermodynamics: the law stating that entropy rises. So there’s a legitimate question of why we don’t encounter time-reverse twins in real life. If the video is consistent with the laws played one way, it’s also consistent with them played the other. As we usually understand them, the relevant laws of physics are time-reversal symmetric, meaning that if you have a video recording of a system evolving over time, it doesn’t matter which end you label ‘start’ and which end ‘finish’. That’s to say, the laws of physics don’t rule her out. Strange though it might sound, your time-reverse twin is physically possible. In particular, the early Universe isn’t just part of the story of how you got here it’s part of who you are. In fact, your consciousness is about as extrinsic as anything could be: it constitutively involves the entire cosmos. Either we’re thinking about the laws of nature all wrong, or your own consciousness isn’t intrinsic to what’s going on inside your head. My reasoning reveals something important about you. I think that your time-reverse twin would not experience anything at all: there wouldn’t be anything it is like to be her (though I’m not the first to say so: check out Tim Maudlin’s article ‘ On the Passing of Time’ ). What would it be like to be such a creature, a genuine time-reverse twin? Would such a twin feel the same as you’d feel if the rest of the world were running in reverse? Would she have experiences at all? And why does it matter? In contrast, if you record yourself and play it in reverse, you’ll see someone who walks backwards (without slipping) and talks backwards (without slipping up). Merlin and Button both walk and talk normally. These things are hard to imagine, but imagine something even stranger: someone whose life is like yours, but played fully in reverse, frame for frame. Merlin remembered the future and anticipated the past.
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