> I'm not quite sure of that. What happens to one photon, or whatever,
> depends on other photons it's interacting with (good Buddhist
> "co-dependent origination," as someone pointed out a while back), but
> does it depend on an *observer* -- some sort of (pardon the expression)
> consciousness observing it?
In QM states of all systems are defined by probabilistic distributions, which are akin to waves. Schrodinger developed a very famous equation that defined the wave associated with every particle. For a particle in a 1-dimensional well, the wave function might look like this (pardon the ASCII art):
P |
r |
o | ++++
b | + +
a | + +
b | + +
i | + +
l | + +
i | + +
t | + +
y |+ +
__________________
Position
In this case there are positions that have low probability, and positions that have higher probability. Until we actually measure the position of the particle all that we can say about the particle is that it exists as a wave. And we know that this wave-nature is real because it is observable as interference patterns with other waves (see the famous two-slit experiment, which just about anyone can do themselves). However if we observe the particle for one of its particle-properties, we get a specific result, within the boundaries of the measurement tool's error. While we measure the position of the particle the wave function "collapses" to a specific value.
While this observer-experiment relationship may appeal to certain metaphysical theories, the QM explanation is rather simple. To observe something, one must interact with it, as far as physics is concerned. E.g., to "see" a particle, one must bounce light off of it. There is no way to actually measure anything without somehow interacting with it. Consciousness is irrelevant.
Taking this fundamentally obvious physical fact, Heisenberg concluded that there exists a certain minimum amount of uncertainty that exists for all measurements. If we are shining a light on something to see it, we want to use short wavelength light (since it is the crests of the wave that will allow us to see). But in QM, there can not be arbitrarily short wavelengths - the smallest is going to be Planck's constant (the foundation of QM is that energy is measured in units - quanta). So when seeing a particle, we are limited to the wavelength of the light. Heisenberg calculated the minimum uncertainty for any pair of non-compatible operators, position and velocity being the most common pairing used as in examples, which is called the Heisenberg Uncertainty Principle.
What this all "means" is subject to debate, of course, and doesn't really occupy much time from physicists. Most of what QM physicists do - at least most of what I did as an undergraduate - is math. The "Many Worlds" interpretation of QM is popular with science fiction authors. It says that each time a wave function collapses, all possible values exist and splinter off as parallel worlds. A convenient way to avoid time travel paradoxes, but of very little use for experimental physics or theoretical QM. Most physicists, if asked, will offer the Copenhagen interpretation, which says that QM math represents an inherently non-deterministic Universe. Those wave functions are not just approximations for hidden variables we are unable to measure, but real representations of reality. Einstein was not especially thrilled with the Copenhagen interpretation, which was the later named collective of metaphysical thoughts of Bohr, Heisenberg, and Max Born.
More on the CI of QM: http://plato.stanford.edu/entries/qm-copenhagen/
Matt
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