Why removing the uncertainty principle accidentally creates perpetual motion

Werner Heisenberg's uncertainty principle tells us it's impossible to know simultaneously the location and the velocity of a subatomic particle. It's one of the main ideas that famously prompted Albert Einstein to declare God doesn't play dice with the universe.


After Einstein's repeated failures to disprove quantum mechanics, physicists have more or less come to accept that quantum weirdness is here to stay, even if not all experts are happy with something so resolutely non-intuitive. Disproving the uncertainty principle isn't really a mainstream pursuit anymore, but there's been some work centering on the idea of finding "hidden variables" that make it look like the uncertainty principle holds when, in fact, it's just an elaborate illusion of classic physics.

But all the evidence still very much says that the uncertainty principle describes the intrinsic workings of the universe, that we actually need it for the universe to continue making sense. Which brings us back to the headline, that it's impossible to do away with the uncertainty principle without also unleashing a perpetual motion machine, something that has been proven repeatedly is a laughable impossibility.

Now, I'm pretty sure I know what some of you are thinking - if all we need to get our hands on a perpetual motion machine is to do away with some silly bit of quantum theory, then let's do it! But that's not how it works, I'm afraid. The impossibility of perpetual motion is enshrined in the laws of thermodynamics. Even if you could somehow get rid of the uncertainty principle, removing the laws of thermodynamics would pretty much reduce all physics into gibberish.

That's why a new paper by Stephanie Wehner and Esther Hänggi of the Center for Quantum Technology in the National University of Singapore is so fascinating. The good folks over at New Scientist have the lowdown:

First, they suggest that the two properties of a single object that cannot be known simultaneously can be thought of as two streams of information encoded in the same particle. In the same way that you can't know a particle's momentum and location to an arbitrarily high level of accuracy, you also can't completely decode both of these messages. If you figure out how to read message 1 more accurately, then your ability to decrypt message 2 becomes more limited.

Next the pair calculate what happens if they loosen the limits of the uncertainty principle in this scenario, allowing the messages to be better decoded and letting you access information that you wouldn't have had when the uncertainty principle was in force. Wehner and Hänggi conclude that this is the same as getting more useful energy, or work, out of a system than is put in, which is forbidden by the second law of thermodynamics. That is because both energy and information are needed to extract work from a system.

You can check out the rest of the article for more, and the original paper is up on arXiv.


Anekanta - spoon denier

Perpetual motion is a laughable impossibility? Are you sure? Because my two-and-a-half-year-old nephew never stops moving.

I also wanted to say something about how the more you have a yang interaction with a particle (i.e. the more accurately you know one of its properties), the more yin (mysterious / chaotic) your interaction gets with the other property. Tao of Physics and all that.

I'm certain the hardcore physics wonks will tell me this is all metaphorical nonsense. But that won't deter me from the sense that there's a certain poetry to how the universe is put together.