Tuesday, 29 January 2013

On running videos backwards

Not so easy, of course, with DVDs. You can search backwards, of course, but it skips back in little still-frames, you don’t see the action run backwards as you could with a video cassette if you rewound it without stopping it first. We didn’t have a TV at our house, let alone a video, but we got to watch a movie on the last day of school one year (I think it might have been Herbie Goes Bananas) and they ran the tape backwards when it was finished and we six-year-olds just laughed and laughed, it was all so funny.
I’ve heard, though without any titles or names that would help me Google it, that someone once actually made an entire movie that way – acted everything backwards, and also filmed everything backwards, so that when it was played things happened forwards but everything was subtly weird. But what makes it weird? What were we all laughing at when I was six?
I guess it’s mostly the effects of gravity. Going backwards, everything looks so bouncy. Fruit, scattered around the street, leaps suddenly up, neatly slotting into place in a fruit-stand at the exact peak of its jump; the fruit-stands themselves stand at attention, and they too first move fast and then suddenly settle delicately at exactly the right place, while the car zips backwards between them.
Only... it’s not actually gravity that causes that effect. A video of a bouncing ball will look fine played backward, except that the bounces get bigger instead of smaller. In either direction, there’s a downward acceleration equal to roughly 10 metres per second per second. The difference is that, played backwards, the ball gains energy at each bounce instead of losing it. With the fruit-stand, what’s funny is not so much that things are moving upward as that they’re moving upward to exactly the right height.
It’s the same – ultimately – with the extreme case of a glass falling off a table and smashing on the floor. Played backward, the glass fragments first rush back to exactly the right places, before leaping up to the precise height of the table. Even the paint dissolving in the water is following the same principle; going backwards, the paint molecules seem to seek out their exact places on the surface.
This phenomenon is known as the Second Law of Thermodynamics. The “thermo-” part means “heat”, because heat and temperature are basically the same principle playing out at the molecular level. The hotter an object, the more its molecules jiggle randomly. Temperature is the average speed of the jiggling (or, in a gas, shooting about); heat is the randomized energy making them do that. I’m afraid the upbeat Flanders & Swann song gets it wrong—
You can’t pass heat from a cooler to a hotter,
You can try it if you like, but you’re far better not ta...
If you “try it”, if you put work in (work being non-randomized energy), you can quite easily pass heat from a cooler to a hotter. Fridges and heat-pumps both do exactly that, the work here being done by an electric current.
Some day, I need to sit down and really nut out thermodynamics. There are still a few things I’m not quite getting my head around. What I do get, is why it’s irreversible. Think of a pool game, just before it starts; the balls are all ready, arranged in a triangle, and the first player is about to “break” them – an apt metaphor. How often would you expect the balls, rolling about the table, to coalesce back together in a neat triangle? Never, that’s how often, even though it wouldn’t break any laws of physics operating at the single-ball level. There’s a little leeway in the triangle; you can swap balls around, you can turn them over, and it still counts as the triangle. But however many arrangements of balls count as “being in the triangle”, there are a phenomenally vaster number of arrangements that don’t count as being in the triangle. That’s why the probability of them returning spontaneously to the triangle is so tiny that it can safely be ignored.
The next question you should be asking is: why did the universe start off in such an unlikely configuration? You can go ahead and answer “God did it” if you like, but then you have to ask: why did the universe start off with a God? If you can’t answer that, all you’ve done is declare the question closed by fiat, not explained anything. With the pool game, of course, you can appeal to entities outside the system – to the players and the wooden frame of the triangle that they put the balls in and then lifted away. With the glass on the table, you can bring in a thirsty but forgetful person who set it down and then left it. In each case the regression of causes stops in someone’s brain, but that’s just because we can’t in practice trace all the pathways of activity in the brain in real-time. Plus, I chose familiar examples in the hope of being understood, and familiar examples generally involve things put there by people because we’re people and we’re used to putting things where we want them. The point is, if you’re talking about the universe, then – unless of course you have sneakily redefined the word “universe” to exclude something, such as God – there is nothing outside the system to appeal to for explanations.
I wonder... I wonder. I have a speculation. To explain it, I’m going to have to go back to the video running backwards again. But first, a bit of philosophical wibbling. Um, I mean an important distinction to grasp about cause and effect.
Philosophers distinguish between “necessary” and “sufficient” causes; this turns out not to have been Aristotle after all, as I thought it was before I looked it up on Wikipedia just now. For a lightbulb to light up, the necessary causes include an undamaged filament, intact wiring, a connection to the mains, and a working electrical generator or battery somewhere at the other end. The sufficient cause, however, is when you flick the light-switch to the “on” position.
But what if the lightbulb doesn’t go when you flick the switch? What if you go and get another lightbulb out of the cupboard and put it in, and you forgot to turn the switch off first so the bulb lights up the moment the contacts make, er, contact? Then the switch is still necessary, but no longer sufficient; the connection between bulb and socket becomes the sufficient cause. The sufficient cause, in fact, is simply the last remaining necessary cause.
Hang on a moment. Why do we say that the switch “causes” the light to shine, in any sense? Because, without the switch, you wouldn’t have the light – which is another way of saying it’s “necessary”. But the converse is also true. If the circuit is otherwise complete, and the generator is going, then when the switch is flicked the light must shine – which is another way of saying it’s “sufficient”. Without the light, there can’t have been a switch-flick (or else something else is broken somewhere). Do you see the conundrum?
You don’t? Let me lay it out.
  • Whenever A happens, B also happens —> A is a sufficient cause of B.
  • Whenever A happens, B also happens —> B is a necessary cause of A.
You see where I’m heading? The sufficient cause is the last remaining necessary cause, and last implies a time-sequence. You might argue that the flick of the switch was caused by a choice in a human mind, which itself had no physical cause. I would disagree on several levels, which I won’t go into here, because all we need ask is: so what about events where no human mind is involved? No. This is what makes the difference:
  • Whenever A happens, B happens afterwards —> A is a sufficient cause of B.
  • Whenever A happens, B happens first —> B is a necessary cause of A.
The central argument of Richard Dawkins’ The God Delusion is that, if it’s phenomenally unlikely that living things popped into existence “by chance” (and it is; natural selection is not a theory of chance), it must be still more unlikely that God would appear “by chance” since God would be an even more impressively non-random entity than any known living thing. Various theists have responded that there is no need to account for God’s existence because God is “outside time”, a doctrine going all the way back to C. S. Lewis. Quite apart from the fact that this misses the point – an impressively non-random entity requires an explanation whether it had a beginning or not – there’s also the issue that God is understood to be the uncaused cause of all that is, a concept which we now see depends upon him not being outside time. Outside time, you could still have rule-bound relations along the lines of “Whenever A happens, B happens”, but there’d be no way to determine which was the cause and which the effect, and so it would be just as meaningful to call God the ineffectual effect of all that is.
Mind you, there is another situation in which we commonly use words like “because”. I could rephrase the above slightly and say “God cannot be both outside time and the uncaused cause of existence because causality requires a time direction”. But I wouldn’t call that cause-and-effect, exactly. Causality requiring a time direction does not cause God’s inability to be both outside time and an uncaused cause; said inability is an instance of causality requiring a time direction. I don’t know how well it would work theologically to refigure God’s creative relationship with the universe to that kind of “because”, and frankly I’m not sure it’s worth the mental effort until someone comes up with a compelling reason to believe there is a God. A compelling reason being one which does not resort to arguments of the form “How else [phenomenon we don’t yet fully understand]?”
I’m starting to get tangled up in my words there. Back to that video. Suppose it has a close-up scene of someone turning a light on – or, at least, that’s what it is when played forward. Run backwards, effects become causes. Let’s pretend it’s a super-high-framerate video, so we can see things that happen in tiny blinks of time. Also, it will be able to look inside things and see electric currents.
First, the light arrives. The video will still look as if the light is coming out of the lightbulb, whereas if everything is truly time-reversed it should be going into it. But the radiating light is simply another special example of the thermodynamic rule that energy disperses outwards. You could argue that, going backward, all the lit-up areas are, in fact, receiving light from the camera, and elsewhere, and generating some themselves, and sending it all directly into the bulb. The filament (it’s an old-style incandescent bulb, because I have a better idea how that works) heats up with the incoming light. The heat generates an electric current, which courses through the wires and away back to the generator, where it turns a turbine which helps push a large volume of water up the local river.
Into this picture walks a human (backwards, of course). Just after each step, a decision occurs in the human’s brain recording where it just came from. This sends neuronal signals around which eventually come to the retinas of the human’s eyes. Light shoots out of the retinas, bounces off objects, and goes to join the light zeroing in on the bulb. The human progresses backwards across the room – but wait, I need to clarify. The decision to take each step comes after the step itself. The cause of it is an inward rush of energy, mostly in the form of heat and soundwaves, plus a slight bounce from the floor, which push the foot up into the air. Eventually, the human reaches the door-frame. At this point, two things happen. A set of very small and almost random-seeming air movements in the room conspire to push the human’s hand up to the light-switch. Also, the human’s brain reaches the end of the process of unravelling its visual picture of the room. In consequence of the latter, the light shot out by its retinas suddenly drops to a tiny fraction of its former level, and the same is true of the camera and of every other item in the room. There is no cause, in this backwards world, why all these things happen at once, any more than there is a cause for all the light converging on the lightbulb. The drop in light levels stops the electric current in the bulb, which soon propagates to the switch, where the human’s hand has just arrived. Propelled by the drop in current, the switch moves, pushing the human’s hand out of the way. Nerve impulses travel from the human’s muscles to its brain, where they coalesce into a decision to turn the light on, which in turn gives way to an impression that the room is too dark to cross safely.
We can know the past, but not act on it. We can act on the future, but not know it. In time-reverse, our actions on the universe become the universe’s actions on us, while the signals that bring us our knowledge instead propagate out from our sense organs. Each cause-effect link is still there, just flipped around. Our memories concern only the past because the past is where everything is collected together and knowable; we do not “remember” the future because in the future, the cause-effect pattern spreads out and can’t be captured. And now here comes the speculation. Remember our question: Why did the universe start off in such an unlikely configuration? Step outside of time and look at it. Do we have to picture time as a line all running one way? I don’t think we do. I think it could be more like a ball of wibbly-wobbly timey-wimey stuff. Spread out all spacetime, not with the Big Bang at the “start”, but with it somewhere wodged in the middle. It’s still a phenomenally unlikely configuration of matter, but spacetime is so big that even phenomenally unlikely things happen occasionally.
Now, the Big Bang is embedded, like everything else, in a network of cause-effect links. So which is which? What’s a cause, and what’s an effect? Here’s a related question. On most maps, “south” means “towards the bottom”, but what does it mean on a map of Antarctica? Why, it means “towards the South Pole”, of course. In our wibbly-wobbly picture of spacetime, what does “before” mean? It means “towards the Big Bang”. It is at the “start” because it is the most unlikely configuration of matter to have occurred, not vice versa.
That kind of reminds me of Stephen Hawking’s A Brief History of Time, where he said the Big Bang wasn’t a singularity after all.
That makes me think I must have got something wrong, because I surely can’t have understood Stephen Hawking with help from a line out of Doctor Who.
Anyway. Cosmology is awesome.

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