Actually, if you think about it, you can see air even at normal
density. The blue sky just comes from light bouncing off air molecules-
so that's air you see. When the setting sun looks reddish, that's
because you're seeing it through a thick layer of air, and a larger
fraction of the blue light bounced off of air molecules than the red,
and so a larger fraction of what's traveling straight toward you is
red.
If you want to get air dense enough to see samll amounts of it, the
easiest way is to cool it down until it turns liquid. In principle it
could be compressed that much without cooling, but that would take a
lot nof pressure. Liquid nitrogen (the main component of air) is very
common around labs. It's just a clear liquid. Liquid oxygen is too, but
since it tends to promote explosions you don't ordinarily come across
it. Carbon dioxide (a trace component of air) is also common in
condensed form. It's called dry ice, and you've probably seen it (it's
white, but that's mainly because a typical block of it is made up of
many little crystals oriented in many different directions). At
ordinary pressures it CO2 goes straight from gas to solid without
forming a liquid. High pressure CO2 forms a fluid which can be used for
toxin-free dry cleaning. So far as I know, it's also clear.
None of these materials have any nuclear funny business going on.
Mike W.
You can squeeze gas hard enough to make nuclear reactions happen,
but gosh, that's really really hard. It's easier to get the nuclear
reactions to happen if you heat up the gas. The reason for this is that
nuclei are all positivly charged and repel each other. To get them
close enough so that the short-range strong nuclear force is more
important than the repulsive electrostatic force, you either have to
push really hard or throw the nuclei at each other really fast. Pushing
really hard probably means pressures that exist only on neutron stars.
And not just the surface (which is made up of a crust of ordinary
material), but in the core where it is energetically favorable for
nuclei to interact with each other strongly.
The high temperatures are much more easily obtained in laboratories
here on earth. Plasmas can be heated to phenomenal temperatures with
lasers or electrical discharge in order to try to get light nuclei to
fuse together, in big projects which attempt to design a fusion reactor
that produces more energy than it uses (so far no luck, but they are
slowly improving). Anything that is that hot glows with a
characteristic spectrum called the "blackbody" law. The sun is an
example of an object which has at its core gases which are at high
enough pressure and temperature for nuclear fusion to happen, and it
glows bright.
If you ask me, it's far safer to look at a styrofoam cup of liquid
nitrogen. It looks a bit like water, although the refractive index is
different and the density's different and the viscosity's different,
but still it's plenty visible.
If a change in refractive index is enough for you to be hapy that
you can see air, just thing of the shimmering, wiggling image
distortion you see when looking at something through some air that's
been heated (say, over a stove, or over a road on a hot day, or through
the exaust of a jet engine).
Tom
(published on 10/22/2007)
