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The Quantum Speedboat
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A friend brought up the whole issue of quantum mechanics and the uncertainty principle the other day, mentioning that someone had evoked it in a book he was reading. I've brought this issue up before, and it's one that still intrigues me. I think the only thing that's certain about talking about quantum uncertainty is that most people who talk about it don't much know what they're talking about.

But anyway, I've brought it up before and I'll probably bring it up again. And I think I've used this particular analogy, but I'd like to flesh it out and see if someone can explain to me why it's qualitatively different from what's going on at the quantum level.

Here goes...

The Quantum Speedboat

Let's say we're on the dock of a small lake. It's nighttime, with no moonlight, so it's perfectly dark. There's a robot-controlled speedboat out on the lake, and its engine is perfectly silent, so we can't tell where it's at by sight or sound.

Let's say we've got a special harpoon, rigged with a sponged tip full of sensors. We can fire this harpoon in any direction we want, and when it hits something, a computer system can tell us the momentum and coordinates of whatever we hit.

So we want to know where the mysterious speedboat is, even though we can't see it or hear it. We fire our harpoon randomly out across the lake, over and over again. When we hit the speedboat, we get a reading of its position and momentum. But by hitting it with the spongy harpoon, we change both the position and momentum of the boat.

Our "observation" has directly influenced what we were trying to measure. Thus, we can no longer be sure of the boat's momentum or position.

This is analogous to measuring an electron:

As an example, [Heisenberg] considered the measurement of the position of an electron by a microscope. The accuracy of such a measurement is limited by the wave length of the light illuminating the electron. Thus, it is possible, in principle, to make such a position measurement as accurate as one wishes, but only by using light of a very short wave length, e.g., -rays. But for -rays, the Compton effect cannot be ignored: the interaction of the electron and the illuminating light should then be considered as a collision of at least one photon with the electron. In such a collision, the electron suffers a recoil which disturbs its momentum. Moreover, the shorter the wave length, the larger is this change in momentum. Thus, at the moment when the position of the particle is accurately known, Heisenberg argued, its momentum cannot be accurately known.

Now the problem is with the interpretation. Does the uncertainty that arises from altering the measurable attribute by our observation mean that the speedboat did not have a particular momentum or location before our harpoon struck it?

This is what some philosophical interpretations insist is going on at the subatomic level. That our inability to measure particular aspects of reality is a reflection, not on us or our ability to measure, but on reality itself.

This seems to me a modern reformulation of the old tree-falling-in-the-woods question. If we cannot empirically observe something, does it exist?

I still have yet for someone to explain to me how what's going on at the subatomic level is qualitatively different from the speedboat analogy above. Anybody out there understand it?

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