Tuesday, December 01, 2015

Seeing Photons in a New Light

If I asked you to picture a photon, an electromagnetic wave, I’d expect the image that popped into your head to look something like the one below, with the characteristic intertwined sine curves of the electric and magnetic field vectors.
Image Credit: Wiki user P.wormer, CC BY-SA 3.0

This model of electromagnetic radiation dates back to the work of James Clerk Maxwell, whose differential equations laid the groundwork for the pre-Einsteinian understanding of electricity and magnetism, and to this day, this is still how most students first learn to think about the quantum of light—as a plane-polarized wave, composed of perpendicular 2D components.

Pictured this way, however, the photon behaves unlike any familiar 3D system, with its components shrinking out of existence before popping back in, upside-down. As for analogies, waves on the surface of a pond fall short by a full dimension, leaving the student wondering which way is “up”, and what provides the restoring force equivalent to gravity. Once a student understands the derivative relationship between electricity and magnetism—that one is induced by a change in the other—the nature of light starts to make more sense mathematically, but this doesn’t make it any more intuitively palatable, and accepting the peculiar undulations of the sine wave as a real, physical thing can be a difficult stumbling block for people to surmount. But there’s an easier, more fundamental way to think of photons—it’s surprising at first, but ultimately a great relief to learn that the plane-polarized electromagnetic wave pictured above is not one photon, but two! In truth, neither part of the photon ever vanishes, or even changes in amplitude.

At the quantum-mechanical level, every photon—regardless of wavelength and energy—is circularly polarized. All this means is that, rather than oscillating up and down, like the tip of a bird’s wing, it spirals through space, like the blades of an airplane’s propeller. The electric field vector still oscillates perpendicular to its direction of propagation, fitting the definition of a transverse wave, but the crucial thing to note is that it's moving in both perpendicular directions. If we were flying alongside a plane with lights on the tips of its propellers, matching its speed, the lights would trace out sine waves against the sky behind them, oscillating straight up and down, changing direction when they reach their maximum amplitude. However, this fact arises simply from the choice of a perspective that ignores one of the propeller’s degrees of freedom. From any other angle, we’d see that the propeller’s tips don’t slow to a stop and change direction twice per cycle, they simply continue on their path around the circle.
To an observer situated in the plane of rotation, the electromagnetic field vector appears to oscillate sinusoidally, as below.

Like the classic "Spinning Dancer" illusion, this could be seen as a side-view of either clockwise or counterclockwise rotation.
Image Credit: Wiki user ikaxer, CC BY-SA 3.0 

Ordinarily, students learn about the linearly-polarized sine wave first, in part because of the mathematical simplicity, and in part because it makes for a convenient analogy between polarizing filters and the slats of a picket fence—only light oscillating vertically can make it through the gaps between the pickets. When they’re advanced enough in their understanding to handle the concept of superposition, the teacher can introduce the idea of circularly-polarized light as a superposition of perpendicular plane-polarized states, a quarter-cycle out of phase.
Image Credit: Wiki user Averse, CC BY-SA 2.0

However, it’s possible—and perhaps even preferable—to teach it the other way around. If two circularly polarized photons are in superposition, but spinning opposite directions, they’ll appear as a single plane-polarized photon, as their components cancel out at one point in the cycle and add together a quarter-cycle later. Not only is it more correct from a quantum-mechanical point of view, thinking of the linearly-polarized photon as a superposition of two circularly-polarized photons with opposite handedness also offers a better intuitive understanding of why a linear polarizer inserted at 45 degrees between two perpendicular linear polarizers allows some light to pass through all three.

When I was in school, I was taught to think of a "ray" of light as containing linearly-polarized photons oscillating at all different angles, each staying within its plane. Ultimately, my displeasure with this description stems from the fact that, to generate a linearly-polarized photon, you need a charge moving straight up-and-down, or side-to-side, which is a very unnatural thing for electrons to do outside of an antenna: far more natural is the kind of circular motion displayed in the Bohr atom, which can generate a circularly-polarized photon with ease. It may be the easiest to teach from a mathematical perspective, but describing the fundamental quantum of light as a linearly-polarized sine wave strikes me as counterintuitive and, ultimately, counterproductive.

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Monday, November 23, 2015

Hot and Cold, All at Once

New research slated for publication in Physical Review B shows that “cold spots” can be localized within a molecule, leaving single atoms with temperatures near absolute zero, while other parts of the molecule rest around a comparatively balmy 100 Kelvin.

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Tuesday, November 10, 2015

What Can a Blob-eating Game Teach Us About Biblical Plagues?

Swarming behavior has always fascinated physicists, biologists, and behavioral scientists alike—as well as anyone who’s seen a sky-darkening flock of starlings twist into its mesmerizing shapes. It’s hard not to wonder how such elegantly concerted behavior arises on the fly, or how on earth the birds keep from running into one another. But birds aren’t the only things that swarm like this, and while the idea of The Birds acting as a collective is scary enough to merit a Hitchcock film, this might just be a psychological sublimation of the instinctive fear of a very real and far-more-threatening swarm: Locusts. Now, research from the University of Bath gives us some understanding of how this swarming behavior happens in insects, and how we might disrupt it.

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Friday, November 06, 2015

Ask a Physicist: FTL Communication With a Very Long Stick

Amy from Hull, England asks:

"Imagine a tube structure stretching a large distance (say a light year) encasing a row of ballbearings that are lined up inside the full length of the tube. If I push one more ballbearing in from one end, would a ballbearing at the far end instantaneously drop out? Or for millions of years until the information is transferred would there be more ball bearings than the tube would normally fit? Or would it simply take me millions of years to push an extra ballbearing in the tube?"

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Tuesday, November 03, 2015

Two by Two

For close to a decade now, two of the hottest buzzwords in technology have been “Quantum Computing”—the promise of storing a information by manipulating the spin of a single electron, and the associated prospect of harnessing quantum entanglement for faster computation has captured the imaginations of physicists and computer scientists alike. As exciting as the theoretical possibilities are, much of the nitty-gritty work of constructing a functional quantum computer has yet to be done.

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Thursday, October 29, 2015

What's in a (Martian) Name?

If you’re a fan of The Martian, then you’re familiar with the alien landscape of Acidalia Planitia and Arabia Terra. But you may be wondering: Where did these strange names come from? On this week’s podcast we set out to answer that question, in a fun (spoiler-free) romp through fictional astronaut Mark Watney’s Martian neighborhood. Behind every name, there’s a story, and many of them are tied to the history of physics and astronomy down here on Earth. Here’s a taste of what we uncover in the podcast:

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Tuesday, October 27, 2015

Slippery Lipids Give Snakeskin its Slither

Snakes can slither smoothly over almost any surface, from jungle branches to desert sands, without damaging their skin – an ability that has fascinated researchers.

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Thursday, October 22, 2015

Scaling Down the Solar System

“I sort of missed the science boat entirely,” says Wylie Overstreet, one of the creators behind the new short film To Scale: The Solar System. “It was only a couple of years ago that I discovered science as a story...and it was transformative. I suddenly became totally sucked into the story of nature, and in doing so, in reading more about it, learning about it, I discovered that there's this massive discrepancy between our notion of where we are in the universe...and the reality of it.”

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Wednesday, October 21, 2015

Back to the Present

It's here, folks: today is the day we officially enter "the future", at least according to a certain wildly-popular 1980s film trilogy. The movies in question are much-beloved here at PhysicsCentral, so after ascertaining that today is in fact Marty McFly's "destination date" in "Back to the Future Part II", it seemed a special tribute post was in order. (We had to double-check, because there's a blog that's been churning out photoshopped screen captures claiming that "today's the day!" every single day for the past two years.)

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Thursday, October 15, 2015

One Small Step for Kinesin

Adenosine triphosphate, or ATP for short, is the universal currency of energy among living things. It’s the gasoline that drives our cellular motors, the necessary intermediate step between chemical and kinetic energy. By and large we’re still figuring out the details of how that conversion process works, but a new result from the Polish Academy of Sciences, slated for publication in PRL, brings us one step closer to understanding the mechanics of motor proteins called kinesins.

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