Friday, September 14, 2012

Not So Uncertain After All?

This has turned out to be a good week for science posts. One of the ideas that I keep coming back to is that for a true "consciousness measure" to be constructed, we need to get a better handle on the relationship between quantum-level interactions and the brain. One of the biggest engineering challenges to overcome there is dealing with the Heisenberg Uncertainty Principle, which states that only so much information can be obtained when measuring a particle. The position of the particle and its momentum seem to have an inverse relationship - the more you determine about one, the less you can determine about the other. Back in June I discussed a possible method being explored by quantum physicists that might allow more accurate measurement of quantum states, and this week a group of Canadian scientists announced that may have discovered another. By using so-called "weak measurements," the latter group was able to measure the state of photons while inducing considerably less disturbance than that produced by more conventional techniques.

The researchers, a team led by Lee Rozema and Aephraim Steinberg, experimentally observed a clear-cut violation of Heisenberg's measurement-disturbance relationship. They did this by applying what they called a "weak measurement" to define a quantum system before and after it interacted with their measurement tools — not enough to disturb it, but enough to get a basic sense of a photon's orientation.

Then, by establishing measurement deltas, and then applying stronger, more disruptive measurements, the team was able to determine that they were not disturbing the quantum system to the degree that the uncertainty principle predicted. And in fact, the disturbances were half of what would normally be expected.

Although this new method still cannot measure quantum effects with absolute certainty, it demonstrates that the uncertainty principle is not as much of a hard limit as it was previously thought to be. Based in part on the uncertainty principle, Roger Penrose wrote in The Emperor's New Mind that we may never be able to uncover the relationship between the brain and the quantum realm. Now it seems that measuring those interactions is starting to look more a difficult engineering problem than an unbreakable physical law, and while we have a long way to go from measuring single photons to tracking quantum events within a living brain, this new research suggests a possible path forward where once there was none.

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Mister Li Liu said...

I'm pretty sure the Uncertainty Principle is built into the fabric of quantum theory. You can't simultaneously measure speed and location with 100% accuracy. If I recall correctly, the sum of the combined probabilities of speed and location measurements must always be 100%, so an increase in the probability (or precision) of one measurement must lead to a decrease in the probability of the other.

So in that sense, uncertainty is more than just an engineering problem.

Scott Stenwick said...

That's still true, even with this latest data. There's still some uncertainty in the measurements, but it's about half what the uncertainty principle predicts. It suggests that the problem is more like trying to measure the location and momentum of a pool ball by bouncing golf balls off it rather than an absolutely irreducible limitation. If we can reduce uncertainty by using weaker measurements (that is, say, ping pong balls in this analogy) it means that the pool ball can be measured with greater precision. I'm hoping that at some point that precision can be increased to the point where we can determine something useful about the nature of consciousness.