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Number 362, March 12, 1998 by Phillip F. Schewe and Ben Stein
TUNABLE CHEMISTRY IN BOSE-EINSTEIN CONDENSATES (BECs) has been demonstrated by an MIT group (Wolfgang Ketterle, 617-253-6815), allowing researchers to choose whether atoms in this new state of matter attract each other, repel each other, or hardly interact at all. A BEC is a gas of atoms so cold and so dense that they overlap and act as a single, unified entity (Update 233). To control the chemistry of a sodium BEC, the researchers turned on a magnetic field which slightly altered the shape of the electron clouds surrounding each atom. This in turn could modify the force that the atoms applied on each other (Nature, 12 March 1998). Controlling whether BEC atoms attract or repel will help researchers to test theoretical ideas about BECs and understand chemical reactions and collisions in ultracold gases. In addition, the researchers developed an all-optical trap for BECs rather than the magnetic fields previously used (Physical Review Letters, 9 March 1998). This in itself is an advantage because (1) researchers now have the chance to create BECs of atoms that don't respond to magnetic fields, and (2) a laser beam can control atoms to a high degree, for example by guiding them down a hollow optical fiber (Update 245). Once produced in just 3 laboratories in the US, BECs have now been created in Germany (2 labs), and 4 additional labs in the US. (See Georgia Southern University's BEC Page)
COMPLEMENTARITY PRINCIPLE DEMONSTRATED FOR ELECTRONS. When light waves pass through a pair of slits in a screen, an interference pattern will form at a detector further along. If one of the slits is closed, or if one tries to take a peek at which way the light went then the interference pattern starts to go away. Quantum reality is shy; if you look at it, it disappears. Now a group at the Weizmann Institute in Israel have done a sort of double slit experiment with electrons and observed (for the first time with fermions, spin-one-half particles) how the resultant interference pattern dissipates the more you watch the electrons as they go through the slits, thus demonstrating Niels Bohr's complementarity principle which states that objects can have wave and particle properties, but not both at the same time. In the Weizmann experiment, led by Mordehai Heiblum, the electrons (or electron waves, depending on whether you look or not) slalom through a two-dimensional obstacle course, where they must negotiate a pair of channels, one of which (via a separate circuit called a "quantum point contact," or QPC) gives a hint as to whether an electron passed that way. Essentially, as a wave the electron passes through both channels; but if it senses that it is being watched, the electron (as a particle) goes through only the one path, diminishing the interference thereby. (E. Buks et al., Nature, 26 Feb. 1998.)
WATER ON MOON, ASTEROID NEAR EARTH. The Lunar Prospector spacecraft has detected the presence of water ice, at a level of about 1%, in the soil at the Moon's two poles. Perhaps brought to the Moon by passing comets, the water ice lies in valleys away from the Sun's rays. Its density was inferred from the number of neutrons flung up when cosmic rays strike the lunar surface (NASA press conference, 5 March). Meanwhile, several observers have spotted an asteroid, named 1997 XF11, whose orbit might bring it to within 30,000 miles of our planet in the year 2028. Its diameter may be as big as one mile, making it one of the largest asteroids expected to have passed within a distance equal to the moon's orbit. (IAU press release, 11 March.)
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