Watching Photons Interfere: “Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer”

ResearchBlogging.orgIt’s been a long and brutally busy week here, so I really ought to just take a day off from blogging. But there’s a new paper in Science on quantum physics that’s just too good to pass up, so here’s a ReasearchBlogging post to close out the week.

Aw, c’mon, dude, I’m tired. What’s so cool about this paper that it can’t wait until next week? Well, the title kind of says it all: they measured the average trajectories of single photons passing through a double-slit apparatus. By making lots of repeated weak measurements at different positions behind the slits, they could reconstruct the average trajectories followed by photons on their way to form the interference pattern. They look like this:


Whoa! That got my attention. So, like, these guys are going to get a Nobel, right? What do you mean?

Well, they’ve measured definite positions and momenta for photons in a double-slit experiment. Which means they must’ve proved orthodox quantum mechanics wrong, because you’ve always said that quantum particles don’t have a well-defined position and momentum. And if orthodox quantum physics is wrong, that’s got to be good for some dynamite money, right? Not so fast. What I said, and what orthodox quantum physics tells us, is that a quantum particle like a photon of light doesn’t have a well-defined instantaneous position and momentum. You can perfectly well construct an average position and momentum for a quantum particle, though, by making lots of measurements of identically prepared systems. If you string a whole bunch of those average measurements together, you can construct an average trajectory, which is what they did here.

They haven’t done anything to prove orthodox quantum mechanics wrong, though I can predict with confidence that there will be at least one media report about this that is so badly written that it implies that they did. In reality, though, their measurements are completely in accord with ordinary quantum theory.

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The Real Point of Zero Point

While Kenneth Ford’s 101 Quantum Questions was generally good, there was one really regrettable bit, in Question 23: What is a “state of motion?” When giving examples of states, Ford defines the ground state as the lowest-energy state of a nucleus, then notes that its energy is not zero. He then writes:

An object brought to an absolute zero of temperature would have zero-point energy and nothing else. Because of zero-point energy, there is indeed such a thing as perpetual motion.

This is really the only objectionable content in the book, but he certainly made up in quality what it lacks in quantity. When I read that, on an airplane, it made me want to bash my head repeatedly against the seat in front of me (I didn’t, because it probably would’ve been interpreted as some sort of terrorist act, and I don’t want to be on the no-fly list).

Zero-point energy, of course, is the modern justification for all manner of perpetual motion and “free energy” schemes. Every lunatic or scam artist with a web site, it sometimes seems, proposes some scheme to extract the zero-point energy from some quantum system. While I understand what he’s trying to get at, it would be extremely difficult to come up with a worse way to phrase that.

This is another place where I think the odd ordering of the book comes back to bite him. This comes forty-odd pages before he discusses the wave nature of matter, which means that zero-point energy is something whose existence is just asserted, rather than demonstrated. In reality, zero-point energy, like the uncertainty principle is a natural and unavoidable consequence of the wave nature of matter. You can’t extract that energy from a system because doing so would require you to fundamentally change the nature of every object in the universe.

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