Mar
20
Posted by jns on
March 20, 2007
Speaking of the Vernal Equinox, many people were — speaking of it — yesterday but occasionally with some imprecision, saying that spring “officially” arrived at about 2007 EDT. They would be better off saying “astronomically” arrived, since there’s nothing “official” about it: no international committee meets to set the time of the arrival of springtime. Instead, we have chosen to relate the change of seasons to clearly and precisely defined events related to the apparent motion of the sun, events that have been noted since antiquity.
Because of the tilt of the Earth’s axis of rotation (about 23.5°) relative to the plane of its orbit around the Sun, the zenith of the Sun (i.e., it’s largest angle above the horizon each day) changes with the seasons; it’s higher in the sky during summer and lower during winter.* In other words, the Sun appears to move not only along a path across the sky (the “ecliptic”), but that path appears to move higher and lower as the year progresses.
Now, imagine a line from the center of the Earth to the Sun; where the line passes through the surface of the Earth is the point at which the Sun, at that moment, can be said to be directly overhead. Let’s call this line the “Sun Chord” — so far as I know it has no generally recognized name, and this name sounds harmonious.
As the days of the year pass, the apparent motion of the sun in the sky — or the intersection of the Sun Chord with the Earth’s surface — traces out a squished figure-eight (the “analemma“);# the exact shape of the analemma depends on the location of the observer.
As the Sun executes its stately analemmic dance, there are fixed extremes to its motion. When the Sun appears at its northern-most point, the Sun Chord passes through the “Tropic of Cancer”, which is at a latitude of about 23.5°N; similarly, when the Sun appears at its Southern-most point, the Sun Chord passes through the “Tropic of Capricorn”, is at a latitude of about 23.5°S. It is not a coincidence that these latitudes have the same angles as the tilt of the Earth’s axis; it is a consequence of geometry. The Sun’s passing through these extreme points is called a “solstice”. In the northern hemisphere we often call the solstice that occurs in June the “summer solstice”, and the solstice in December the “winter solstice”.
Also during the year there are two times — very nearly 6 months apart and 3 months separated from each solstice — when the Sun Chord passes through the Earth’s equator, i.e., when the sun is directly overhead at the equator. When the Sun appears to be moving in a northerly direction this point is the “vernal equinox” (or “spring equinox”); when the Sun is moving in a southerly direction this point is the “autumnal equinox”. “Equinox”, of course, means “equal night”, a name given because the amount of daylight and nighttime are [roughly] equal everywhere on the Earth (except extremely near the poles, which are singular points in this geometrical picture).
And now to the point of this essay. Equinox and solstice times are mathematical concepts describing astronomical events. They occur at well-defined times that can be determined with as much precision as one would care to take. We can calculate to any number of decimal places the exact moment when the Sun Chord intersects the equator, making it possible to say that the vernal equinox occurred at seven minutes past eight (EDT) last evening.
Now, whether spring “officially” started then is another matter entirely, a matter of convention and history, but not a geometric necessity.
[After I'd started writing this piece, Isaac sent me a link to an essay on the vernal equinox in the New York Times by Natalie Angier, "The Tilted Earth at Its ‘Equal Night of Spring’ ", which adds some cultural considerations to the topic. The illustration is kind of cute, too, although it does suggest something more along the lines of a "martini equinox".]
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*This statement is true in both northern and southern hemispheres, but the seasons are reversed, which should become clear with a moment’s reflection.
#Yes, this is true at the equator as well: the apparent motion of the sun describes an analemma. It is a common misconception that the sun at the equator is always overhead; what is true is that day and night at the equator are always equal, but there is apparent motion of the sun by about 23.5° to both sides of vertical.
Mar
09
Posted by jns on
March 9, 2007
Today in Bob Park’s What’s New (9 March 2007 edition) was this tidbit:
OPENNESS: THE MARCH MEETING OF THE AMERICAN PHYSICAL SOCIETY.
The commitment of physicists to the principle of openness was tested this very morning in Denver at the APS March meeting, as it has been every year for 108 years. Roy Masters, author of “God Science and Free Energy from Gravity,” was to deliver “Electricity from Gravity” at 9:36 a.m. Anyone can deliver a paper at the March Meeting. What if Masters actually succeeded in using up our gravity to keep the lights on? Not to worry.
I think all physics graduate students, sometime during their years’ long hazing, have noticed this phenomenon. It’s always good for a few giggles. But what Park says is true: any member of the APS may submit an abstract and deliver a 10-minute paper at general meetings of the society. Especially in the days when programs of abstracts were printed and distributed on paper, it was common for a few abstracts to appear for which no speaker materialized at the appointed time.
In my day there was someone who submitted an abstract at every opportunity, but who never appeared at the meetings; I don’t remember his name or where he was from. In those days one had a piece of paper on which a rectangle was inscribed; one’s abstract would be photographically reproduced and everything that was to be printed must appear withing the bounds. Said person always included a photograph of some geological feature, around which he typed his abstract, and then he filled the remaining bits of space with arrows to bits of the photo and handwritten notes. In the physics world I suppose it’s what passes for conceptual art.
Similarly, for years while I was in graduate school, there was every month, without fail, a small advertisement in the back of Physics Today from a person whose name now escapes me, who was searching for his “gamma-gamma correlations”. None of us knew what “gamma-gamma correlations” were — mostly because there is no such thing — but the advertiser never gave up hope.
Then, when I was nearing the end of graduate school, the advertisements disappeared. We were all a bit bereft at the loss of this institution. Then, after a couple of months another advertisement appeared in which the previous advertiser now promised to sell, for a small fee, something like all the secrets of the universe based on his theory about “gamma-gamma correlations”, or something like that. He had apparently found them and we could all rest again, knowing that the integrity of fringe science was safe again.
Mar
09
Posted by jns on
March 9, 2007
There has been lots of talk, relatively speaking, this week about a now-famous event that took place at the annual meeting of the American Physical Society 20 years ago. The first piece that I saw was in the New York Times (Kenneth Chang, “Physicists Remember When Superconductors Were Hot“, 6 March 2007 — his piece is fine, but I think I’ll scream if anyone mentions mag-lev trains again in the same breath as superconductors, or anything else for that matter) about what quickly became known as “The Woodstock of Physics”, if you can imagine.
Today it’s the lead story in my e-mail’s “Physics News Update” (9 March 2007 edition), by Phil Schewe and Ben Stein of AIP (the American Institute of Physics is an umbrella organization that encompasses the American Physical Society, and publishes Physical Review and Physics Today, among others).
So this is the story that got everyone all excited twenty years ago. I wasn’t at that meeting — I usually attended a smaller local meeting the next month where most of my low-temperature colleagues congregated by tradition — but I certainly remember the buzz it created in the hallways near my lab. This is probably the event I will recall when people start talking again, as they seem to every generation or so, about how physics is pretty much played out and all important discoveries have already been made.
It was rather more excitement than you might expect to see among a group of typically staid physicists. By the way, this gives you a chance to see the differences between a news story written for the public, and one written with an audience of physicists in mind.
“THE WOODSTOCK OF PHYSICS,” the famous session at the March 1987 meeting of the American Physical Society, earned its nickname because of the rock-concert fervor inspired by the convergence of dozens of reports all bearing on copper-oxide superconductors. The 20th anniversary of this singular event was celebrated this week at the APS meeting in Denver.
Why such an uproar over the electrical properties of an unlikely ceramic material? Because prior to 1987 the highest temperature at which superconductivity had been observed was around 23 K [i.e., "Kelvins", centigrade sized degrees where 0 K is "absolute zero"]. And suddenly a whole new set of compounds–not metallic alloys but crystals whose structure put them within a class of minerals known as perovskites–with superconducting transition temperatures above 35 K and eventually 100 K generated an explosion of interest among physicists. Because of the technological benefits possibly provided by high-temperature superconductivity (HTSC)—things like bulk power storage and magnetically levitated trains—the public was intrigued too.
This week’s commemoration of the Woodstock moment (the months of feverish work leading up to the 1987 meeting) provided an excellent history lesson on how adventurous science is conducted. Georg Bednorz (IBM-Zurich), who with Alex Mueller made the initial HTSC discovery, recounted a story of frustration and exhilaration, including working for years without seeing clear evidence for superconductivity; having to use borrowed equipment after hours; overcoming skepticism from IBM colleagues and others who greatly doubted that the cuprates could support supercurrents, much less at unprecedented temperatures; and finally arriving at the definitive result–superconductivity at 35 K in a La-Ba-Cu-O compound. In October 1986 Bednorz and Mueller prepared a journal article confirming their initial finding in the form of observing the telltale expulsion of magnetism (the Meissner effect) from the material during the transition to superconductivity. Submitting this paper, however, required the approval of the IBM physics department chairman, Heinrich Rohrer who, that very week, had been declared a co-winner of the Nobel prize for his invention of the scanning tunneling microscope (STM). Afraid that he would not be able to obtain the preoccupied Rohrer’s attention, Bednorz obtained the needed signature by thrusting the approval form at Rohrer as if he (Bednorz) desired only a celebratory autograph. A scant year later Bednorz and Mueller pocketed their own Nobel Prize.
The IBM finding was soon seconded by work in Japan and at the University of Houston, where Paul Chu, testing a YBaCuO compound, was the first to push superconductivity above the temperature of liquid nitrogen, 77 K. Very quickly a gold rush began, with dozens of condensed matter labs around the world dropping what they were doing in order to irradiate, heat, chill, squeeze, and magnetize the new material. They tweaked the ingredients list, hoping to devise a sample that superconducted at still higher temperatures or with a greater capacity for carrying currents. At this week’s APS meeting Chu said that he and his colleagues went for months on three hours’ sleep per night. Several other speakers at the 2007 session spoke of the excitement of those few months in 1987 when-according to such researchers as Marvin Cohen (UC Berkeley) and Douglas Scalapino (UC Santa Barbara)-the achievement of room-temperature superconductivity did not seem inconceivable.
The Woodstock event, featuring 50 speakers delivering their fresh results at a very crowded room at the New York Hilton Hotel until 3:15 am, was a culmination. In following years, HTSC progress continued on a number of fronts, but expectations gradually became more pragmatic. Paul Chu’s YBaCuO compound, under high-pressure conditions, still holds the transition
temperature record at 164 K. Making lab samples had been easy compared to making usable power-bearing wires in long spools, partly because of the brittle nature of the ceramic compounds and partly because of the tendency for potentially superconductivity-quenching magnetic vortices to form in the material. Paul Grant, in 1987 a scientist at IBM-Almaden, pointed out that HTSC applications have largely not materialized. No companies are making a profit from selling HTSC products. Operating under the principle of “You get what you need,” Grant said, superconducting devices operating at liquid-nitrogen temperatures weren’t better enough so as to displace devices operating at liquid-helium temperatures.
Nevertheless, the mood of the 2007 session (Woodstock20) was upbeat. Bednorz said the 1986/87 work showed that a huge leap forward could still take place in a mature research field whose origins dated back some 70 years. Bednorz felt that another wave of innovation could occur. Paul Chu ventured to predict that within ten years, HTSC products would have an impact in the power industry. Paul Grant referred to the study of superconductivity as the “cosmology of condensed matter physics,” meaning that even after decades of scrutiny there was still much more to learn about these materials in which quantum effects, manifested over macroscopic distances, conspire to make electrical resistance vanish, a phenomenon which at some basic level might also be related to the behavior of protons inside an atomic nucleus and the cores of distant neutron stars.
(Photographs and an original summary press release from the 1987 meeting is available at our Physics News Graphics website, www.aip.org/png)