Coffee cup

Coffeeshop Physics
by Jim Pivarski

Rotavirus as seen in an electron microscope, reconstructed by computer [image source].

Viruses Have No Color

1 Feb 2013

Some things are too small to see. Microscopes help us to zoom in on some of them, but only up to a point. Objects smaller than a few hundred nanometers don’t even reflect visible light: a 1,000,000× magnifying glass wouldn’t be able to show us anything, despite its magnification strength. To study the shapes of things smaller than this, scientists bounce electrons (or other particles) off of them, but this technique is a bit more like sonar than sight.

To be specific, concepts such as color have no meaning for objects this small. Color is the pattern of wavelengths of light that a substance likes to reflect— for instance, grass absorbs red light (570–750 nm) and blue light (380–495 nm) but reflects green (495–570 nm). A strawberry reflects red but absorbs green and blue. Most viruses are between 20 and 300 nm, smaller than all visible wavelengths, so they don’t reflect much visible light. They are without color in a more fundamental sense than something that is merely gray.

This may cause consternation for virologists who want to explain what the critters look like, but it derives from a deep principle of physics that relates to the bandwidths of radio stations, the Heisenberg Uncertainty Principle, and why you can put staples in the microwave. read more »

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Lost in Hyperbolia

22 Dec 2012

Before I studied physics in college, I was captivated by two of the things physicists talk about: quantum indeterminacy and curved space-time. I spent a lot of time thinking about what it might mean for a particle to be both here and there, and how something as insubstantial as space could be bent up and stitched together. Even as I learned about these things rigorously, it irked me that I couldn’t visualize them.

Eventually, I came up with ways of visualizing these things that made sense of them without doing too much violence to the underlying formalism. When I talk about curved space now, for instance, I’m imagining the contorted fabric of a pair of pants I once sewed, and how they couldn’t lay flat. I presented this explanation in a previous article on this website, but the “space-time as a sheet” metaphor is an old one that might only be helpful after a course in Riemannian geometry and another in sewing.

Computers have no trouble imagining curved spaces, and multitouch devices such as iPads let the user engage the computer’s abstractions in a palpable way. So I got to thinking, what if I write a program to directly interact with curved space? This article presents the result of that tinkering: a hyperbolic portal that runs in your browser (no need to download anything), intended to give you a direct experience of spatial curvature. The code is on GitHub, and I’d love to see (and link to) anything that you might do with it. read more »


17 Nov 2012

(This is an expanded version of an article originally written for Fermilab Today.)

A few weeks ago, I wrote an article for Fermilab Today about the spin of fundamental particles and the Higgs boson in particular. My cats were eager to demonstrate how photons emerge from a Higgs decay in an anticorrelated state, so I included them as Figure 1. It was wildly popular. I was even asked to present it as a talk, which gave me a chance to expand on the topics that I had raced through to stay within my 800 word budget.

Spin is interesting for a lot of reasons. At the moment, it is perhaps the most important unknown parameter of the new particle discovered last July. Spin is also at the heart of quantum weirdness, because it’s an “amount of rotation” that is somehow quantized like a light switch: on or off, and never in between. It seems like we could just put a particle on a slow enough turntable to dial up a non-quantized angular momentum, but nature has a way of enforcing its rules. Talking about spin also makes for a nice bridge between the world of subatomic particles and the world of everyday experience, since it has some macroscopic consequences.

I don’t harbor the illusion that the popularity of my article was due to anything but Cats On The Internet, however. read more »

The Bouncing Baby Boson

4 Jul 2012

Two weeks ago, I learned something amazing and couldn’t tell anybody. I saw a plot like the one on the right: a new particle emerging from freshly unblinded data.

To avoid bias, scientists “blind” their data, hiding the most interesting part from themselves while they make sure that they understand all of the experiment’s calibrations and finalize the analysis procedure. When the physicists of the CMS Collaboration believed that the experiment was well-controlled, we all breathlessly looked at the Higgs data to see if last year’s hint might be reinforced. It was. In fact, it is now statistically robust enough to say that a new particle exists and it has some of the properties of the long-sought Higgs boson. This morning, the CMS and ATLAS Collaborations revealed their results to the world (and each other). The fact that such different experiments reached the same conclusion solidifies it: we’re probably looking at the first appearance of a particle hypothesized 48 years ago.

Why are physicists excited by one more particle? Statements like, “It gives mass to all the other particles,” are correct but beg the question, “Why should mass come from a new particle?” And what does that mean, anyway? Some physics topics are easy to motivate: a conversation about extra dimensions could begin with, “You know how there’s length, width, and height? Well, what if there are more than these three?” The Higgs Mechanism, however, solves a technical problem at the core of modern theories about matter and forces, and it requires a lot of background to even know that there’s a problem that needs to be solved. This boson’s “God Particle” epitaph hints that it plays a central role, but the name explains nothing and embarrasses physicists.

This article is intended to fill in what the newspapers leave out. Without assuming technical background, I’m going to try to explain why Peter Higgs and five other (uncredited) physicists independently invented the Higgs Mechanism, what that means, and why it’s so exciting to learn that they seem to have been right. The topic ties into so many different areas of physics that I have to fight the temptation to talk about all of them. I’ll try to get at the heart of the matter, if only by cutting away the rest of the body. read more »

The Gnat Who Walks Through Walls

25 Mar 2012

Imagine being invisible: what would that mean, physically? None of the skin, muscles, or bones in your body could absorb light in any way, including your eyes. Vision works because light is absorbed on the backs of your eyes, so to be invisible, you would also need to be blind.

Now imagine being intangible as well, imagine matter could pass through you as light does. You’d probably drop through the floor and orbit the center of the Earth like a yo-yo— that is, if you were insensitive to all forces except gravity. Take away gravity and there would be nothing left to tie you to this world. When light, matter, and gravity are not communicating with something, it might as well be in a separate universe. The extent of the physical world is defined by the interactions that connect our senses to phenomena: if something is truly undetectable, does it even make sense to say it exists?

Neutrinos inhabit a world that is almost, but not quite, disconnected from our own. They are insensitive to electromagnetism, the force that makes things visible and tangible, as well as the nuclear strong force that holds nuclei together in atoms. They ought to feel gravity but have so little mass that this has never been proven. We only know they exist because of their involvement in the weak nuclear force, which governs some but not all radioactive decays.

The picture above is the Earth— as seen in neutrinos. Neutrinos are emitted by uranium products in the Earth’s crust, but then pass pass through the Earth as though it were a soap bubble. The bright spots are new: they are nuclear power plants. To a weakly-interacting being who only sees neutrinos, nuclear reactors and atom smashers are the only evidence of life on Earth. read more »

Spinning Tops, Protons, and Planets

14 Jan 2012

Magnetic field lines induced by a rotating fluid, cherubs, archangels, and Valkyrie

When I was little, I tried to make an electromagnet by winding a thin wire around a nail and connecting it to a battery. I must have seen this on Mr. Wizard’s World. But instead of magically picking up paper clips, it just got hot and wasted the battery. What I didn’t understand is that the wire must be insulated, not bare metal: instead of flowing around the nail in many circular loops, the electric current flowed through the whole thing as a bumpy metal blob.

This fact that circulating electric currents produce magnetic fields can be seen everywhere in nature. Electromagnets, whether they flip bits in a hard drive or cars in a junkyard, are essentially just (insulated) wires wound around nails. Neutron stars spin faster and faster as they collapse, generating the strongest magnetic fields known in the universe. Elementary particles such as electrons are haloed by tiny magnetic fields due to their intrinsic spin, a kind of internal rotation they can never stop. Even refrigerator magnets are not as stationary as they seem: their magnetic fields are due to a partial alignment of electron spins.

Sometimes, though, the flow can be so turbulent and complicated that its dynamics are a mystery. The underlying equations are known, but even supercomputers are not powerful enough to determine the implications of those equations. Only simplified versions of these systems can be calculated, so the predictions don’t exactly match the real systems in all their messy glory. The two examples I have in mind are a proton’s magnetic field and the Earth’s. read more »

The Discovery of Rainbows

30 Oct 2011

Why are there so many songs about rainbows?

I was stuck in traffic one day when a glorious double rainbow appeared over the highway. It had been a drizzly day; the whole sky was covered with clouds except for a little gap along the horizon, and it was just about sunset. As the sun slipped between the grey above and the ground below, the Chicago skyline was briefly golden with horizontal light, and two concentric rainbow rings encircled I-290 like a kind of tunnel.

Most of the rainbows I’d ever seen were faint wisps; this was an intense glow, as bright as a flask of electrified mercury. Fortunately, the cars weren’t moving, so I got a good, long look. Rainbow-like color separation happens a lot in physics classes, and I thought I understood what caused the second rainbow. I was wrong. I was thinking about first-order and second-order rainbows from diffraction gratings. If the rainbows in the sky were due to the same mechanism, the second rainbow would have to be twice as big as the first (it isn’t) and the colors would have to be in the same order (they aren’t). I stared at that second rainbow until the car behind me started beeping. Were my eyes deceiving me? Did the colors really go in the opposite order?

When I got home, I read all about rainbows and how they work. It’s fascinating: the story of its discovery spans twenty centuries. read more »