From: hal@finney.org
Date: Mon Mar 11 2002 - 11:32:59 MST
The Washington Post writes this morning recapping the controversy over
the recent tabletop fusion experiment at Oak Ridge:
http://www.washingtonpost.com/wp-dyn/articles/A5592-2002Mar10.html
There's not much new in the article, mostly a bunch of scientists saying
nasty things about each other. But I thought one thing mentioned at
the end was interesting.
With sonoluminescence experiments you need to get a bubble in the liquid.
This will then be alternately compressed and expanded by the sound waves
and if all goes well, it will emit a very short burst of light at the
point of highest compression.
The "amateur" experiment I pointed to earlier formed the bubble simply
by lifting a few drops of water into the air and dropping them into
the flask. But most labs do it with a tiny heating filament. They give
it a burst and essentially boil the water in a small area to form a
tiny bubble. The ORNL experiment was different; they used neutrons.
They fire a beam of neutrons into the water and when they hit, that
vaporizes the region and forms a bubble.
That seemed strange to me, because it ends up making the results harder
to interpret, since one of the signs of fusion is emission of neutrons.
Now, the emitted neutrons are a different energy than those they were
using to create the bubbles, but still it means that you have to be
careful in your measurements. And indeed some of the critics claim that
the researchers were not measuring their neutrons properly.
The WP article explains why it was done this way. The long term hope
is that the neutrons emitted by the fusion could themselves trigger
other bubbles. Presumably this would require a larger tank of liquid,
or perhaps some neutron-capturing elements mixed into the heavy water.
But if you could do this then you could give it an initial burst and then
you would get a continual series of bubbles, each triggering others,
to produce a self-sustained fusion reaction. That would really be
impressive.
BTW, did you see "The Sound of Neutrons" movie from the links Scerir
provided? Look at
http://www.eurekalert.org/pub_releases/science/2002-03/taleyarkh-3-8-02.html,
which has a link to a QuickTime movie showing the experiment in action.
You can hear the rapid-fire popping as each bubble forms, grows and then
collapses. Plus you can see many of the bubbles - they are much larger
than I would have expected. I'm not sure what the scale is but the
bubbles supposedly grow to 6.5 millimeters, which is much larger than
I thought was usual for sonoluminescence.
Hal
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