Friday, June 17, 2005

The Story of O2

Banded Iron Formation

There's a really nice article in this week's Science about the history of gaseous oxygen in the earth's atmosphere. Everyone agreed that the earth started out without any gaseous diatomic oxygen. There was some debate about when free oxygen first appeared. Minerals older than 2.4 billion years or so showed no signs of oxidized iron (rust). But some researchers believed there was evidence of atmospheric oxygen even earlier. But now a detailed analysis of atmospheric sulfur chemistry supports the later date for oxygen. So there seems to have been a "Great Oxidation Event" around 2.4 billion years ago in which banded iron formations were laid down around the world. Photosynthetic algae appear at around 2.7 billion years ago and perhaps at that point they were producing enough oxygen to create the iron formations.

In the same issue of Science, there's also an article with evidence that the earth held on to its gaseous hydrogen for longer than previously thought, which would help explain some of the history of early life on the planet.

Neutrinos have Mass!

1987A supernova remnant near the center

Neutrinos are produced in several different ways: natural radioactivity here on earth; nuclear reactors; in copious quantities in the Sun and other stars; in supernovas (Wikipedia SN1987A article); and in the Big Bang (though these have yet to be detected, there's indirect evidence here Neutrino ripples spotted in space). In all cases they are ghostly particles, very hard to detect.

The Super-Kamiokande detector, half-filled with pure water. (Courtesy of the Institute for Cosmic Ray Research, the University of Tokyo.)

They were postulated theoretically by Wolfgang Pauli, but it took many years to actually find them.
For a long time one of the great mysteries of physics was the "Solar Neutrino Problem" - the actual number of neutrinos we could detect from the Sun was about a third of the expected flux. This was finally resolved by oscillations among the three types of neutrinos (hence the 1/3). This seems to imply, theoretically, that neutrinos have mass - which has yet to be measured, though it must be very, very small. Until the oscillations were discovered, it was usually assumed that neutrinos were massless, for both theoretical and experimental reasons.
Here are two "popular" articles by John N. Bahcall of Princeton:
Solar Neutrinos: A Popular Account and Solving the Mystery of the Missing Neutrinos
The three years 2001 to 2003 were the golden years of solar neutrino research. In this period, scientists solved a mystery with which they had been struggling for four decades. The solution turned out to be important for both physics and for astronomy. In this article, I tell the story of those fabulous three years.

Here's a recent, more technical review of Neutrino Physics in general by Boris Kayser.

There are many interesting questions about neutrinos which still need to be answered by new experiments (from Kayser).

1. How many neutrino species are there? Are there sterile neutrinos?
There's already an experiment (LSND) which suggests that there might be more than three types of neutrinos. If there are more than three types then there are combinations which do not even couple to the weak nuclear force - they would only interact via gravity. These are the so-called sterile neutrinos.

2. Are neutrinos their own antiparticles?
Charged particles cannot be their own antiparticle, an antiparticle must have the opposite electric charge. But neutrinos are, naturally, electrically neutral, so it's possible. It could be, however, that there's another conserved quantity the "lepton number". It's not clear whether there is such a quantity, but if there is, there thould be distinct antineutrinos.

3. Do neutrino interactions violate CP (charge/parity conservation)?
Why does the universe seem to be made almost entirely of matter, rather than equal amounts of matter and antimatter? CP violation occurs in quarks, but that doesn't appear to be sufficient to explain the observed matter/antimatter imbalance. Since we are made of matter not antimatter, perhaps neutrino interactions are the reason we exist!

Let us not forget neutrino poetry, by John Updike no less.

Recasting Mermin's multi-player game into the framework of pseudo-telepathy

Preprint by Gilles Brassard, Anne Broadbent, Alain Tapp
Entanglement is perhaps the most non-classical manifestation of quantum mechanics. Among its many interesting applications to information processing, it can be harnessed to reduce the amount of communication required to process a variety of distributed computational tasks. Can it be used to eliminate communication altogether? Even though it cannot serve to signal information between remote parties, there are distributed tasks that can be performed without any need for communication, provided the parties share prior entanglement: this is the realm of pseudo-telepathy.
One of the earliest uses of multi-party entanglement was presented by Mermin in 1990. Here we recast his idea in terms of pseudo-telepathy: we provide a new computer-scientist-friendly analysis of this game. We prove an upper bound on the best possible classical strategy for attempting to play this game, as well as a novel, matching lower bound. This leads us to considerations on how well imperfect quantum-mechanical apparatus must perform in order to exhibit a behaviour that would be classically impossible to explain. Our results include improved bounds that could help vanquish the infamous detection loophole.

In Mermin's game, n players are given a simple task which definitely requires communication in a classical setting. By using quantum techniques (involving the notorious concept of entanglement), the task can be performed without any communication at all - hence the "pseudo-telepathy".

The problem, by the way is very simple. It requires three or more players. Each is given an input bit (0 or 1). They are promised that the total number of input 1's is even. They each then produce an output bit. If the sum of the input bits is divisible by four, there should be an odd number of output 1's. If the sum of the input bits is not divisible by four, the sum of output bits should be divisible by two (an even number of output 1's).

Chi-hwa-seon a film by Im Kwon Taek

The Korean film festival continues with this period piece about the life of the 19th century Korean painter Ohwon. Beautiful photography but a somewhat chaotic story of a chaotic life and times.

By the way, in the shot on the cover of the DVD, I believe the characters are depicted in sexual intercourse. It's very hard to tell what's going on beneath those elaborate Korean costumes!

Chunhyang is another even more beautiful film by the same director.