The Porter Zone

Philosophical musings and more

Quantum Mechanics stole my car

Quantum ordinaryness

I have lost count of the number of ways people abuse the word ‘quantum’.  From describing every breakthrough as a ‘quantum leap’, to Terry Pratchett saying that something is very strange, but it’s all probably due to quantum, to one book I read that attempted, rather desperately I thought, to argue that God was the thing that made quantum mechanics so weird.

And here’s the thing.  Quantum theory is used as a lazy excuse to justify more or less everything, and its probabilistic nature is viewed as a deep, deep mystery that we mere mortals cannot hope to fathom (going back to the God = quantum argument for a moment).

Now, as I hope to show, actually quantum mechanics is actually very simple, and its statistical nature makes perfect sense.  You just need to be a bit less anthropocentric in your reasoning than we like to be.

Wave-particle duality and quantum measurement

First let’s look at the so-called paradox of wave-particle duality, as evinced in the dual-slit experiment. Now ontology is determined both by knowledge and one’s perceptual capabilities.  

To see this, let us imagine a thought experiment.  I have three cards, and I show them to two people, one with normal vision, the other red-green colour-blind.  They agree that one of the cards is a different colour, but disagree as to which one it is (this is because, as has recently been discovered, red-green colour-blind people can distinguish shades of green that the rest of us can’t).  So perception shapes ontology.

Another thought experiment.  I look into the sky and I see two surprisingly identical looking stars quite close to one another.  I try to build a theory that might allow creation of groups of identical stars. Then I learn general relativity, and I realise that there is only one star, but there is also another thing, to act as the gravitational lens, that I cannot otherwise see.  So my ontology now includes ‘things I can’t see but that bend light’.  So knowledge shapes ontology.

Back to quantum mechanics.  Pretend for the moment that quantum mechanics is the ultimate reality. We are big, slow-moving, fairly light objects, and the other objects we interact with are the same. We inhabit the macroscopic world, where quantum effects get averaged out. So our perceptions, our philosophies are based on the presupposition that everything behaves like macroscopic objects. Then we see things that don’t fit the conceptual scheme and they, well, don’t fit the conceptual scheme.

Now imagine what would have happened if we had evolved on a neutron star. We’d be very small. We’d be used to enormous gravitational fields, and our senses would almost certainly work at the level of wave functions (or something equivalent). In which case the whole ‘electron in two places at once’ thing would be meaningless to us, because of course an electron is an extended object, not a point.

I made an assumption that quantum mechanics is correct here, but you can see that it isn’t really needed. All that’s needed is the observation that we have a point-object conceptual scheme, whereas (at least at our current level of observational acuity) we are beginning to see things that conflict with that conceptual scheme due to our having extended our sensory range. But we still in our hearts want to stick with the point-object paradigm. Hence the so-called paradox.

Quantum mechanics and probabilities

‘But’, you say, ‘I thought quantum mechanics was probabilistic.  That’s weird.’  Well, no, actually.  Imagine I have a field and I divide it up into lots of squares, and the only information I have about things in the field is whether they are in one of the squares (in other words, I have deliberately limited myself to a coarse perceptive scheme – you’ll see why in a moment).  Now say there’s a cow in the field.  It shows up not as a cow, but as a selection of squares.  But if I try to grab the cow by reaching into  a square, I won’t always succeed, because the cow isn’t square.  I will succeed with a certain probability.  In other words, because I have thrown information away, the cow has abruptly become probabilistic!

Now, in quantum mechanics it’s not that we’re throwing information away, it’s just that our conceptual scheme is so at variance with whatever is going on, that when we try to look at quanta we lose information.  So we end up with the cow syndrome.  The quanta are perfectly ordinary objects, when viewed with an appropriate conceptual scheme and we don’t have, and never will have, that scheme.

I can go one stage further.  I have divided the field into squares.  Say you divide it into chunks of your own devising.  We both register the cow as a set of probabilities associated to chunks.  What relation is there between those two sets of probabilities?  A very complex one.  In fact, there need be no relation.  Say I divide the field into strips running north-south, and you divide it into strips running east-west.  Say the cow is facing north.  Then I see the cow as being in one strip with probability 1 (certainty) while you see it as being spread across several strips.  Now say the cow starts to walk towards me.  You see it change; I do not.  There is no relation between the probabilities I see and the probabilities you see.  In other words, we have not the Heisenberg uncertainty principle, but something far worse: a complete failure of our theories (the division into strips) to interoperate.

(Technical note for QM cognoscenti: the argument above is not an example of hidden variables, because I am not saying there need be anything other than the wave function evolving according to QM’s rules.  My point is that we try to partition the space of wave functions into point particle shaped chunks, and this is not a good partition.)

And for my next trick . . .

Coming up: why I hate ‘Twilight’ and what I intend to do about it . . .


One response to “Quantum Mechanics stole my car

  1. Anne Jensen 17/07/2010 at 18:27

    [below was meant to be posted in response to our discussion on Facebook, but I can’t seem to post to Facebook at the moment! Now, I’ll be heading to the post above to read]Thank you! Advice is exactly what I need. I have noted down books and pointers in my notebook. I take your point re the impossibility regarding definitions. Just too many variables and unknowns. Also just read your Posterous post re qm. Really succinct, especially liked and understood the sectionings of field and/or cow. A problem with qm is that it is very ‘now’. Postmodernists, deconstructionists, mystics, zen practitioners, all can ‘take up’ elements of qm and make it that little bit sexier. Inevitable of course – any theory that describes fundamental laws accurately will be adoptable by all; but it becomes very easy to misquote and misunderstand and misrepresent, especially by us non-techies who only grasp the outer layers of the theory. So, when you say… oooh just checked back to Posterous to check my references to your post and there’s a new post up!I’m off to read!

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