The search for magnetic monopoles, the truest north

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The hunt for the truest north

Many theories predict the existence of magnetic monopoles, but experiments have yet to see them

For Symmetry Magazine:

If you chop a magnet in half, you end up with two smaller magnets. Both the original and the new magnets have “north” and “south” poles.

But what if single north and south poles exist, just like positive and negative electric charges? These hypothetical beasts, known as “magnetic monopoles,” are an important prediction in several theories.

Like an electron, a magnetic monopole would be a fundamental particle. Nobody has seen one yet, but many—maybe even most—physicists would say monopoles probably exist. (Read the rest at Symmetry Magazine…)

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Everything is a particle, but what does that mean?!

[ This blog is dedicated to tracking my most recent publications. Subscribe to the feed to keep up with all the science stories I write! ]

What is a “particle”?

Quantum physics says everything is made of particles, but what does that actually mean?

For Symmetry Magazine:

“Is he a dot or is he a speck? When he’s underwater, does he get wet? Or does the water get him instead? Nobody knows.” —They Might Be Giants, “Particle Man”

We learn in school that matter is made of atoms and that atoms are made of smaller ingredients: protons, neutrons and electrons. Protons and neutrons are made of quarks, but electrons aren’t. As far as we can tell, quarks and electrons are fundamental particles, not built out of anything smaller.

It’s one thing to say everything is made of particles, but what is a particle? And what does it mean to say a particle is “fundamental”? What are particles made of, if they aren’t built out of smaller units? [Read the rest at Symmetry Magazine]

 

If you shine light on a barrier with two openings, it produces a distinct pattern of light on a distant screen. Measuring that pattern is standard in introductory physics laboratories. (You could even do it at home, but I recommend a very dark room and a bright laser pointer if you hope to see anything at all.) Where things get fun, though, is if you have a light source capable of sending a slow stream of photons — particles of light — through: you still get the interference pattern, but it emerges slowly from individual points of light. In other words, the photons behave as though the entire wave interference pattern is already present, even though they are single particles.

My latest article for Nautilus shows how researchers have taken this classic experiment, but use single photons to manipulate the interference pattern via the phenomenon known as entanglement. The result is a mind-bending experiment known as the “quantum eraser”:

The best way to see the quantum eraser is to couple the double-slit experiment with another fascinating quantum phenomenon: entanglement. In a typical implementation, light from a laser stimulates a certain kind of crystal, which in turn emits two photons with opposite polarization—one could oscillate left-right, while the other oscillates up-down. (You can see how this works by putting one pair of polarized sunglasses in front of another and rotating one pair. At certain angles, the light going through both lenses will fade to almost nothing, a sign that the light is passing through two filters with perpendicular orientations.)

The polarization of each photon is unknown before measurement, but because of how they’re generated, they are entangled, and measuring one can instantly affect each the other. That holds true no matter how far apart the two particles are or when the measurements are taken. [read more….]

Quantum droplets in an ocean of light

O, what entangled photons we weave!

(OK, it doesn’t scan. So sue me.) Quantum entanglement is a challenging topic, and one which has tripped up a lot of people (including many physicists!) over the decades. In brief, entanglement involves two (or more) particles constituting a single system: measurement on one particle instantly determines the result of similar measurements on the second, no matter how far they are separated in space. While no information is transferred in this process, it’s still at odds with our everyday experience with how the world should work. I updated my earlier explanation of entanglement, which hopefully can help clear up some of the confusion.

Recent work either assumes entanglement is real and probes some of the more interesting implications, or tests some mathematical relations known as Bell’s inequalities. The latter are aimed at quantifying the difference between the predictions of quantum physics and certain alternative models. In that spirit, a group of researchers proposed using light from quasars to randomize the measurement apparatus in entanglement experiments, to eliminate the tiny possibility of a weird loophole in quantum theory.

If a detector has some correlation with the hidden variables of the particles being measured, then the two detectors don’t act independently. That’s true even if only a very tiny amount of information is exchanged less than a millisecond before measurements take place. The interaction would create the illusion that the particles are entangled in a quantum sense, when in fact they are influencing the detectors, which in turn dictate what measurements are being taken. This is known as the “detector settings independence” loophole—or somewhat facetiously as the “free will” loophole, since it implies the human experimenter has little or no choice over the detector settings. [Read more…]

Final note: this is probably the first paper I’ve covered that involves both my undergraduate research focus (quantum measurement) and my PhD work (cosmology), albeit in a much different way than both.

Cartoon showing X-ray laser probing of Rydberg states in argon atoms. [Credit: Adam Kirrander]

Cartoon showing X-ray laser probing of Rydberg states in argon atoms. [Credit: Adam Kirrander]

I really love how many experiments are beginning to probe to the limits of quantum measurement. I wrote about a pair of cool studies in December that revealed the quantum wavefunction — the mathematical structure governing the behavior of particles. Today, my latest article in Ars Technica examined a proposed experiment using X-ray lasers to study the dynamics of electrons in argon (and other inert gases) in both space and time.

Rydberg atoms have the electrons in their outer layers excited until the electrons are only weakly bound to the nucleus, making the atoms physically very large. The increased size allows light to scatter off the outermost electrons without much interference from the nucleus or from the inner core of electrons. In other words, it’s a way to isolate the electron dynamics from other messy phenomena. Noble gases like argon are particularly useful for this, since they are largely non-reactive chemically and relatively easy to model theoretically. [Read more….]

Studying electron motion in space and time

Danish physicist Niels Bohr, whose model of atoms helped explain the spectrum of light emitted and absorbed by different elements, as illustrated by the spectrum emitted by the Sun. [Credits: AB Lagrelius & Westphal, via Wikipedia (Niels Bohr photo); N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF (solar spectrum); moi (composite)]

Danish physicist Niels Bohr, whose model of atoms helped explain the spectrum of light emitted and absorbed by different elements, as illustrated by the spectrum emitted by the Sun. [Credits: AB Lagrelius & Westphal, via Wikipedia (Niels Bohr photo); N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF (solar spectrum); moi (composite)]

Many of us are familiar with the Bohr atom: a simple model with a nucleus and planet-like electrons orbiting in circular paths. It’s a useful picture, even though it’s not complete. Bohr proposed it in 1913, but it took about ten more years for physicists to work out why it worked — and to refine it into the quantum-mechanical picture of atoms we have today. However, we’re still probing the structure of atoms, especially the really bizarre behaviors under extreme conditions. Bohr’s contributions are still relevant today.

Despite a century of work, atomic physics is not a quiet field. Researchers continue to probe the structure of atoms, especially in their more extreme and exotic forms, to help understand the nature of electron interactions. They’ve created anti-atoms of antiprotons and positrons to see if they have the same spectra as their matter counterparts or even to see if they fall up instead of down in a gravitational field. Others have made huge atoms by exciting electrons nearly to the point where they break free, and some have made even more exotic “hollow atoms,” where the inner electrons of atoms are stripped out while the outer electrons are left in place. [Read more…]

A century of the Bohr atom

(This was my original title for my article, but my editors evidently didn’t like it. I guess I’m too old school. Ahem. Moving right along.)

As you may know, quantum physics shows that matter has both a wavelike and particle-like character. When you combine quantum physics and special relativity, you find that a particle at rest vibrates with a frequency that depends only on its mass: the Compton frequency. For most purposes, the Compton frequency is useless: it’s huge, even for low-mass particles like electrons, and scales up proportionally for more massive particles like protons. However, researchers have figured out a way to access the Compton frequency of cesium atoms by stimulating them with lasers in a particular way. This could lead to more precise atomic clocks, enabling even more detailed measurement of the second of time—and provide a new way to measure the masses of subatomic particles.

Shau-Yu Lan and colleagues exploited advanced techniques to construct an atomic clock based on a single cesium atom, a device capable of dividing the huge natural frequencies of the atom into more manageable quantities. This provided a strong demonstration of the ability to construct clocks based on a single microscopic mass. And, because we already have excellent clocks to compare them with, this can potentially work in the opposite direction, leading to accurate mass measurements in the future. [Read more…]

Straight outta Compton…