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Much like the game of go, the basic rules of the electromagnetic force are simple, yet they play out in complex ways.
Electromagnetism, the simplest force
29 Mar 2013
This is a reprint of an article from Fermilab Today (which are limited to 300–500 words).
Electromagnetism is the training ground for modern physics, both in its historical development and in classrooms today. It is the simplest of the four forces, compared to the nuclear weak force, nuclear strong force and gravitation, but it gives rise to intricately rich patterns. All of the complex phenomena of everyday life, except for gravity and radioactivity, are due to the workings of electromagnetism. It makes chemical bonds, forming the basis of life, gives structure to solid and liquid matter, and makes lightning and aurora borealis twist across the sky.
And yet the fundamental rules of electromagnetism are startlingly simple: Like charges repel and opposite charges attract. If fundamental forces were board games, electromagnetism would be go, in which the rules can be learned in a few minutes but for which the strategy takes a lifetime to master. The other forces are more like chess, with more complicated fundamental interactions. The only problem with this analogy is that electromagnetism took physicists two centuries to learn and has never been mastered. read more »
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Top quark cross-section
8 Mar 2013
Although cross section has little to do with a literal slice of the top quark, it can tell us about the quark's fundamental properties.
This is a reprint of an article from Fermilab Today (which are limited to 300–500 words).
Last week’s Physics in a Nutshell described the strange way that particle physicists use the term “cross section” to mean a reaction rate. For instance, the proton-proton to top-antitop cross section is related to the probability that two protons will interact and produce a top quark and an antitop quark. This idea of cross section has historical roots in the fact that the collision probability of a stream of spherical balls is proportional to the cross sectional area of those balls. Today, the term is used because it specifies the collision probability in a way that is independent of the number of particles in the stream, so that results from one accelerator can be compared with results from another. read more »
Explain it in 60 Seconds: Spin
7 Mar 2013
Illustration: Sandbox Studio, Chicago
This is a reprint of an article from Symmetry Magazine.
Spin is the amount of rotation an object has, taking into account its mass and shape. This is also known as an object’s angular momentum.
All objects have some amount of angular momentum. A spinning coin has a little angular momentum; the moon orbiting the earth has a lot. Like energy, angular momentum is a conserved quantity: The total amount is constant, though it can flow from one object to another. When a spinning figure skater contracts her arms and rotates faster, her angular momentum is unchanged because a narrow object rotating quickly has the same angular momentum as a wide object rotating slowly.
Particles, as far as we know, are infinitesimal points of zero size. Yet they have measurable amounts of angular momentum. Does the concept of rotation even make sense for a featureless speck? Angular momentum seems to be a more foundational concept than rotation itself. read more »
Cross-section
1 Mar 2013
A beam of particles is like a shower of arrows— the probability of any one hitting the target depends on their cross-sectional area and the space between them.
This is a reprint of an article from Fermilab Today (which are limited to 300–500 words).
Sometimes, everyday words are co-opted by scientists and used as technical terms. One of these is the word “berry.” Talking to a botanist friend of mine, I learned that tomatoes are berries, but strawberries are not—the scientific meaning of a berry has more to do with the reproductive structures of the plant than the way it tastes. The term “cross section” is a berry of particle physics—its technical meaning is very different from the common usage. read more »
Viruses Have No Color
1 Feb 2013
Rotavirus as seen in an electron microscope, reconstructed by computer [image source].
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 »
The Atom Splashers
26 Jan 2013
In some Civil War battles, the shooting was dense enough and prolonged enough for bullets to collide. The atoms of the metal bullets redistributed themselves as a liquid, much like the quarks and gluons of heavy ion collisions. Image source: brotherswar.com.
(This is a combination of two articles from Fermilab Today: The Atom Splashers and Experiments on quark matter.)
In most particle physics experiments, physicists attempt to concentrate as much energy as possible into a point of space. This allows the formation of new, exotic particles like Higgs bosons that reveal the basic workings of the universe. Other collider experiments have a different goal: to spread the energy among enough particles to make a continuous medium, a droplet of fluid millions of times hotter than the center of the sun.
The latter studies, often referred to as heavy-ion physics, require collisions of large nuclei, such as gold or lead, to produce amorphous splashes instead of point-like collisions. Lead ions, for instance, contain 208 protons and neutrons. When two lead ions hit each other squarely head-on in the LHC, many of the 416 protons and neutrons are involved in the collision, unlike the single-proton-on-single-proton collisions used to search for the Higgs boson. With so many collisions in such close proximity, the debris of the nuclei mingles and re-collides with itself like atoms in a liquid. Instead of just splitting in half, the nuclei literally melt.
This is a bit like what happens when two bullets collide in mid-air. Immediately after impact, the atoms in the bullets have enough energy to temporarily melt. Similarly, the quarks and gluons in the colliding lead nuclei spread and mingle as a droplet of fluid before evaporating into thousands of semi-stable particles. read more »
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 »
