Since it's already stirred up so much interesting discussion, I thought I'd give a little more background on this quote. It's the opening paragraph of a now-famous paper by Einstein, Podolsky, and Rosen (EPR), titled, "Can quantum-mechanical description of reality be considered complete?" As many of you probably know, Einstein objected to quantum theory on the grounds that it *cannot* be a complete description of objective reality (as he conceived of "objective reality.") So EPR raise this point about the distinction between objective reality and the description of a theory in order to set the stage for their argument that quantum theory leaves out some elements which are present in objective reality.

As a simple example, if "reality" consisted of a single particle, your "theory" might specify the position and velocity of that particle, and it would be a good theory if measurements of the postion and velocity always agreed with the predictions of your theory. But if, for example, the particle was found to split in two, or to change colors between blue and red or something, then this would indicate that your theory was incomplete. There are obviously physical properties of the system that were not accounted for in your theory.

The EPR paper is a very cleverly contrived thought experiment intended to force the reader to accept that there are properties of particles in nature which must *have* specific values, while quantum theory does not specify these values. Therefore, according to EPR, quantum theory must be "incomplete" - it leaves out of its description some elements which must exist out there in "objective reality." Interestingly, even more cleverly arranged real experiments (done fairly recently - early 80's) seem to show that EPR were wrong - the properties which quantum theory does not specify, really do NOT exist in nature.

Anyway, it's an interesting example of a very abstract-looking philosophical discussion about the nature of reality, which is actually subject to experimental test.

If anyone really wants to delve into any of these things, a good starting point is a book by Jim Baggott, "The Meaning of Quantum Theory," Oxford U. Press, 1992

Todd
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* Todd Duncan *
* Science Integration Institute *
* duncan@scienceintegration.org *
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> From: Science Integration Institute <info@scienceintegration.org>
> Date: Sat, 25 Nov 2000 20:45:08 -0800
> To: SII listserv post <science@lists.pdx.edu>
> Subject: quote of the week
>
> "Any serious consideration of a physical theory must take into account the
> distinction between the objective reality, which is independent of any
> theory, and the physical concepts with which the theory operates. These
> concepts are intended to correspond with the objective reality, and by means
> of these concepts we picture this reality to ourselves."
>
> - Einstein, Podolsky, and Rosen (Physical Review, 47, 777, 1935)