Saturday, 20 April 2013

Will the real Reality please stand up...

I sometimes find myself in this recurring debate ... it's not terribly serious, it's the old one about waves and particles but in field theory.

I have long held the position that, in QM, what you can detect is what is (physically) real - everything else is just maths. I've thought this position is a form of empiricism.

In fundamental physics, what you detect is, oversimplifying a tad, particles.

But I keep running into people who will tell me
  "but hang on - just because you don't detect something, does not mean it does not exist."
Which is to say, the Universe exists outside our experience of it. That's, what?, realism? Existentialism?

The idea is that the field of Field Theory is more fundamental than what we detect as it gives rise to the particles. We may not be detecting fields - but we do detect their effects.

I'm thinking that, then, the only way to distinguish a "real" fundamental field from one that just happens to produce the right numbers is Occam's Razor ... and we may not even have that because many of the same people will expound on the physical reality of those "virtual particles" that mediate forces in particle physics while I'd call them "math" because they crop up as a step in a calculation process called "perturbation theory".

I could point out that "more fundamental" is not the same as saying "real". It could be a more fundamental maths.

One of my old professors points out that the concept of "reality" may not mean much in Quantum Mechanics ... that's a lot of help!

In QM, "steps in a calculation" have this way of biting you. For instance, there is a "sum over many paths" formulation for reflection famously demonstrated by Feynman which has the anle of reflection and incedence equal only on average (after all the other possible angles have added and subtracted out).

That's just a calculation - one where there are lots of intermediate steps. Does light really follow unequal angle paths? Well, it's easy to check - just remove the bit of the mirror where the angles would be equal and see. Sure enough, you get a reflection! It's dimmer, but it's there. The light that makes the reflection could not follow the rules you learned in secondary school.

You can take this further - but you get the point. So what is "real" here?

1 comment:

  1. I know it's an old post, but I'm just catching up on reading around. I couldn't resist posting to a good old "what is reality" discussion.
    IMHO reality is that which we can detect, either directly or by proxy. Everything else is theory. But things are never so simple. Detection is direct if our human senses can discern it, or by proxy if it must be shifted or converted into the realm of said senses. We go to great lengths to build devices to convert the undetectable into sound or vision so we can see and measure them. But our senses can be fooling us (and "ya mustn't be fooled"). I guess that's one way that maths can help. Then there's the theory, the mathematical relationships that describe what's been detected, then go on to predict that another piece of the jigsaw must exist e.g the Higgs. So we go looking for it.
    I guess it all comes down to what's useful to us. Venturing down this rabbit hole has produced some amazing technologies like semiconductors in their various forms.
    So we're effectively hacking the universe to reveal it's nature and to make it more useful to us.
    As for the simulation arguments - if this is all a simulation, then what's running it? etc.. as in the movie "The Thirteenth Floor" (1999).
    Occam's Razor would suggest this is not a simulation, in the same way it suggests that the Teleological Argument doesn't stack up.
    An old lady was once asked about the nature of the universe. She said it rests on a turtle's back. When asked what the turtle rests on, she replied "well, it's turtles all the way down"!