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But there's another interpretation which is a kind of extremist Copenhagen view. It says that QM is a tool which calculates probabilities. And that's all it is.

It doesn't try to define what really happens, it doesn't assume that wave functions are 'real' or even that they evolve. All it says is that if you measure a system at time t, the probabilities of the different outcomes are the real part of etc.

The fact that states can be in various baroque non-local superpositions before the interaction doesn't change this. The probabilities will still be consistent and computable.

Migeru:

What none of them address is the (not quantum-mechanical but probabilistic) problem of actuality (which of the actual histories is realised), which is basically the (probabilistic, not quantum mechanical) problem of assigning a probability to a single event.

"It's random within probabilistic constraints" seems to be all that QM can tell you.

There may be a theory which defines the ontology in more detail. But QM doesn't seem to be it.

The only difference between an observer and a non-observer is that the observer is consciously aware of a measured value. But that doesn't define the value.

What's annoying about QM is that it tries to conflate different issues. There's a difference between the physical interaction needed to make a measurement and the mental process of experiencing the measurement. There's also non-locality, which supposedly makes everything very spooky and has somehow - for some reason which has never been properly explained, because it's not really needed - been linked to mental experience, even though it's completely distinct.

Consciousness
Micro to macro amplification ('measurement')
Non-locality etc
'It's all one' woo woo

are all different. I'll take the middle two. I won't accept the other two until someone does an experiment in which two observers measure two contradictory observables from the same quantum system at the same time.

Migeru:

Remember Bohr's "no phenomenon is a phenomenon unless it is an observed phenomenon".

That's almost makes sense, but doesn't, because in the limit it says that nothing in the universe exists unless it's observed. Which seems unlikely - otherwise you're back to solipsism again, with every possible wave function in the entire history of the universe converging on your viewpoint.

by ThatBritGuy (thatbritguy (at) googlemail.com) on Thu Jan 3rd, 2008 at 06:13:12 AM EST
[ Parent ]
Great comment.

Regarding Bohr, isn't that very similar to what Kand said about noumenon and phenomenon? Aren't there serious epistemological issues at the core of all this?

ThatBritGuy:

"It's random within probabilistic constraints" seems to be all that QM can tell you.

There may be a theory which defines the ontology in more detail. But QM doesn't seem to be it.

The only difference between an observer and a non-observer is that the observer is consciously aware of a measured value. But that doesn't define the value.

Which is why I want to make a diary referring to a number of out-there models and experiments from Quantum Mechanics and ask people to hash out the ontology. Because things like "what is an object" are not quite clear. And it is a lot easier to say "the universe is purely relational" than to build any useful model out of that insight, which is why a lot of physicist will talk in those terms around a pool table, but few papers get written.

We have met the enemy, and he is us — Pogo
by Migeru (migeru at eurotrib dot com) on Thu Jan 3rd, 2008 at 06:21:27 AM EST
[ Parent ]
And it is a lot easier to say "the universe is purely relational" than to build any useful model out of that insight, which is why a lot of physicist will talk in those terms around a pool table, but few papers get written.

sniff, sniff  Chris, is that you cooking up another batch of your pudding somewhere?

Truth unfolds in time through a communal process.

by marco on Thu Jan 3rd, 2008 at 06:48:39 AM EST
[ Parent ]
Migeru:
Regarding Bohr, isn't that very similar to what Kand said about noumenon and phenomenon? Aren't there serious epistemological issues at the core of all this?

That depends what you expect physics to do. There's a difference between explaining reality, modelling reality, and defining reality.

I think realistically (sic) the best you can hope for is models of increasing sophistication and usefulness. Explaining reality is best left to theologians. (Not that they have a clue either, but it keeps them busy.)

No one should be trying to define reality, ever, but it's easy to fall into the trap of believing that a theory with predictive power is what's going on ontologically.

Too much physics is still cursed by Platonism. According to Penrose et el., models supposedly float around outside reality telling it what to do.

But there's a huge gap between using experimental recipes to predict what's going to happen next, and assuming there's a Central Recipe Database running things behind the scenes.

One is pattern recognition, the other is metaphysics. One assumes that reality works consistently and the consistency can be enumerated. The other makes unwarranted assumptions about the mechanisms which create that consistency.

by ThatBritGuy (thatbritguy (at) googlemail.com) on Thu Jan 3rd, 2008 at 10:19:21 AM EST
[ Parent ]

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