Most people's first exposure to the ideas of modern atomic physics comes through the Bohr model of hydrogen, which treats the atom as something like a little solar system, with the positively charged nucleus as the sun, and negatively charged electrons orbiting in well-defined circular orbits. It's a very compelling picture, and works well for hydrogen (provided you make a couple of really odd assumptions), but it's completely wrong on the details. Electrons do not move in well-defined orbits, but rather exists as fuzzy wavefunctions spread over the space near the nucleus.
It turns out, though, that you can make atomic states in which an electron does move in a nice, circular orbit, like the electrons in the Bohr model of hydrogen. the most recent issue of Physics has a nice article by Carlos Stroud explaining how this works in a new experiment by Tom Gallagher's group at the University of Virginia. (If you go to the Physics story, you can download the paper for free, even if you're not a subscriber).
They have to work at it a bit-- they use laser pulses to excite the outermost electron of a lithium atom into a high-energy state, then a microwave pulse to put the electron into a superposition of multiple states that turns out to correspond to an electron moving back and forth in a linear orbit. Then they add a second microwave pulse to convert the linear orbit into a circle. As long as the circularly polarized microwave field is on, the electron will continue to move in a nice, regular circular orbit, just like the electrons in the Bohr model. They detect the motion by hitting the atoms with very precisely timed laser pulses that will ionize it at particular points in its oscillation, and in this way they can stroboscopically follow the orbit.
Why is this interesting? Well, for one thing, it's just plain cool to be able to control the states of atoms in this way. As Stroud points out, though, this is also interesting in a fundamental quantum way, as it touches on questions of how to make quantum objects look like classical ones. And the authors suggest that such electron states might be useful as phase-sensitive detectors of light pulses.
But mostly, it's just cool to be able to manipulate atoms in this way.
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How does this not violate the uncertainty principle? Seems to me that you know where the electrons are rotating and also their speed (otherwise it wouldn't be periodic...). Where did Hbar/2 go?
yorumsuz
Ben, I'm assuming there is still some uncertainty in the position and momentum of the electron. The important fact here is that the probability distribution of the electron is small compared to its orbital radius, and moves around the nucleus as a function of time.
correspond to an electron moving back and forth in a linear orbit
Passing through the nucleus, 10^15 g/cm^3, is way cool even if there are no rules enforcing interaction.
The comment at the end ties in with my comment on another thread about scientist branding by who your mentor was.
Niels Bohr's towering role in the history of physics can be difficult to appreciate. Robert P Crease explains why
http://physicsworld.com/cws/article/print/33963;jsessionid=6671E6098FAC…
In his book Niels Bohr's Times, the physicist Abraham Pais captures a paradox in his subject's legacy by quoting three conflicting assessments. Pais cites Max Born, of the first generation of quantum physics, and Werner Heisenberg, of the second, as saying that Bohr had a greater influence on physics and physicists than any other scientist. Yet Pais also reports a distinguished younger colleague asking with puzzlement and scepticism "What did Bohr really do?".
Man of integrity
We can sympathize with that puzzlement. In history books, Bohr's chief contribution to physics is usually said to be "the Bohr atom" â his application in 1912â3 of the still-recent quantum hypothesis to overcome instabilities in Rutherford's "solar-system" model of the atom, in which electrons travelled in fixed orbits around a positively
charged nucleus. But this brilliant intuitive leap, in which Bohr assembled several puzzling features from insufficient data, was soon superseded by more sophisticated models....
I'd add: Niels Bohr was one of John Archibald Wheeler's teachers. You may have noticed the obituaries a year or two ago for John "Black Hole" Wheeler, whose students included great scientists such as Richard Feynman, Max
Delbruck, Linus Pauling, and Kip Thorne. Hence part of the great influence of Niels Bohr was his role as mentor of mentor of people who continued to change the way that we understand the world. Neils Bohr [7 Oct 1885-18 Nov 1962], one of John Archibald Wheeler's teachers, was born in Copenhagen, Denmark. He studied at the University of Copenhagen, where he received his Ph.D. in 1911. He met Einstein in 1920. In 1921 he founded and directed The Institute of Theoretical Physics. He won a Nobel Prize [Physics, 1972]. His students who won Nobel prizes include: Felix Bloch [1952], Max Delbruck [1969, while at Caltech], Linus Pauling [1954, and later taught at Caltech], and Harold Urey [1934]. He said:
"Anyone who is not shocked by quantum theory has not understood it."
He was certainly influential. His atomic model is used as the iconic symbol of nuclear things, even though it is misleading (and usually presented with three electrons in the S orbital...). But the question "What did Bohr really do?" is quite valid, especially about what he did after the atomic model.
IMHO his importance is based on being the manager of his Institute. He was a hub, not a node. The Marin Mersenne of his times.
BTW, he won his Nobel in 1922, not 1972. (His son Aage won in 1975.) Einstein was supposed to be in Stockholm at the same ceremony to collect the 1921 price, but what happened is a story in itself...
Wow. That's just plane cool.
The name for the field before "Nanotechnology" was "Angstromics."
Taking it literally, that electrons as like planets, and the nucleus like a sun. This was a very old idea before Hollywood heard the news. As I wrote in the Ultimate Science Fiction Web Guide:
1858: Fitz-James O'Brien's "The Diamond Lens", with a beautiful girl living on a molecular-scale world within a drop of water. First use of the Microcosm in science fiction, inspiring many later stories such as Ray Cummings' (1919) "The Girl in the Golden Atom."
Hello Chad Orzel , I want to thank for this blog, where show valuable information.
Undoubtedly serious it brilliant be able to handle the conditions of the atoms, already be by means of laser or to expose them to temperature, achieving the excitation of these, in order to manipulate and to obtain the different spectra that these possess, but it is necessary to mention that serious impossible . To manage to determine the position and at the same time the speed, violating the beginning of uncertainty discovered by Heisenberg, which makes us think of that this to light years to determine and to manipulate an atom. I hope that he accepts my comment and that it is of his pleasure.