Sunday, June 08, 2008

Creating the world

Norman Geras raises a question about this question, that was raised here:

Do we create the world just by looking at it?
The context is quantum mechanics, and whether or not a certain principle that many physicists hold to be true at that scale is also true at a larger scale: the idea that a photon, say, doesn't actually possess a given quality, let's say polarisation, until you actually measure it, which is to say until you observe that quality. Until then, the photon possesses the possibilities of a great many variations of that quality. Things only become definite in the act of measurement or observation, which itself affects the subject of the measurement in a concrete and testable way.

Does that principle apply, the article Geras links to (and the second link above) asks, on the scale of the universe we inhabit? Does it apply, for instance, to your sofa?

Some of the people interviewed thinks it does, but that we're unaware of that because the particular senses we possess are too crude to notice.

Geras poses this problem:
Either (1) we are part of the world we create by observing it, or (2) we are not. If (1) we are part of that world, then we do not exist till we've created it by our observation, and therefore either can't create it or must create it from a situation of not ourselves existing. In either case, there's something remaining to be explained. If, on the other hand, (2) we aren't part of the world we create by observing it, then there is a reality - call it the not-world - which escapes the finding being relayed to us here; and that is not the spirit in which the finding is set out.
I think this particular problem arises out of a misunderstanding caused by a use of language, rather than out of the issue itself, so have a comment to make. The question is posed in a particular language, journalese, that seeks to provide both clarity and memorability. This is a worthwhile ambition, but memorable (and necessarily brief) phrases don't always provide clarity. By 'create the world', the original question does not mean "bring into existence", but rather "determine the properties of". Measuring the polarisation of a photon does not "create" the photon, but it does "create" a particular polarisation - using the language of the question itself.

Therefore looking at a sofa does not, to use a metaphor in the piece, create the sofa, but it does affect the properties of the sofa - this last idea being one of the more novel ideas expressed in the article (which is worth reading). Geras's question, therefore, arises out of a misunderstanding of the sense in this context of the word 'create'. In this context, it does not mean "bring into existence", it means "determine some of the qualities of an object".

At any rate, that's how I understand it.

This reminds me of an analogy I once heard, and liked, about the problems of measurement at a very small scale. It's not an issue of quantum mechanics. We often measure things by throwing other things at them. For example, if I want to measure what's in a dark room I can shine a torch in, which involves hurling huge numbers of photons into the room. Some bounce off objects and pass through my pupils to hit the retinas of my eyes, which measure their properties.

If I want to measure the properties of a bus that's passing by me, I could throw a tennis ball at it. By measuring the direction and speed at which I throw it, and then the point the ball returns to and the speed with which it returns, I could figure out some of the properties of the bus. If I did this a few times at different angles, I ought to be able to figure out how fast the bus is travelling, how much friction there is at its surface, and so on.

It gets much harder to gather meaningful results, though, if a tennis ball is passing by, and you throw a bus at it.


Anonymous said...

You write that we "determine some of the qualities of an object." I'm curious about the limitation: "some of." If we determine some of the qualities of an object (e.g., its momentum or position), why can't we determine all of its qualities? It's as if quantum mechanics accepts realism about some qualities and not others, which seems oddly piecemeal to me. If, on the other hand, we can determine all of the qualities of any object, then Geras's problem re-emerges.

Can you please clarify?

Peter Risdon said...

Sorry to be slow, I've been away from my desk for a couple of days.

Two things. "Determine" has a couple of different possible meanings here: 1) To discover what a quality is without making it so, or 2) To make it so.

I believe we're using the first of those, so the word "create" is rather inappropriate and misleading.

Secondly, as I understand it, there is a theoretical proof for the thesis that we can't determine both the position and the momentum of a particle. This is the uncertainty principle, and I am not competent to do more than point it out.

In other words, we can't determine all the qualities of a particle; there are well established reasons for this; these reasons are logical; they are also counter-intuitive and at odds with our experience of the world on the scale we inhabit. It's for the latter reason that a theory that is internally consistent can appear to be superficially inconsistent.