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By Hamish Johnston
Everyone knows that water in a draining sink or toilet swirls in opposite directions on opposite sides of the equator…or does it? For the answer, watch the instructions in the above video and then go to “The truth about toilet swirl”.
Physicists at CERN are a lucky bunch. As well as having the world’s most energetic collider at their disposal, they are also surrounded by the natural beauty of the Alps and the Jura mountains. However, I’ve always felt that the CERN site itself and the flat farmland that overlays the Large Hadron Collider (LHC) are rather dull.
Not so for two University of York academics: the physicist David Jenkins and the archaeologist John Schofield have written a paper called “A journey to the heart of matter”. Published in the journal Landscapes, the paper is subtitled “The physical and metaphysical landscapes of CERN” and looks at how both archaeology and physics can be understood as explorations of time and space. As well as exploring obvious locations such as the underground hall of the CMS experiment, the pair also highlights equations that appear to have been casually inscribed onto a standing stone at CERN’s Prevessin site. A weathered wooden hut associated with the LHC – arguably the world’s most sophisticated piece of technology – makes it clear that CERN is all about science, not good looks.
Earlier this week we celebrated 60 years of the atomic clock and now blogger Chad Orzel is reminding us that it is also the 20th anniversary of the first Bose–Einstein condensate (BEC). Writing on the Forbes website, Orzel points out that on this day in 1995 Carl Wieman, Eric Cornell and colleagues at JILA in Boulder managed to cool a cloud of rubidium atoms to such a cold temperature that all the atoms were in the same quantum state. Orzel explains why this is such a big deal in “Twenty years of Bose–Einstein condensation”.
Although not a complete answer, one place to start is with the coldest naturally occurring place in the universe, which is the Boomerang Nebula, a planetary nebula that is around 1 K. As best as I can tell, this cooled below the CMB temperature simply by adiabatic expansion, and is insulated in its interior from CMB heating. Is this a feasible way to get to ultracold temperatures?
For a monoatomic gas, recall that adiabatic expansion is TV2/3=const. So something that cools to 100 nK from 3 K would have to expand in volume by a factor of ~1011. Which is, obviously, a lot, but space is big and has more than enough room. The Boomerang nebula is about 1 light-year across and is expanding out at about 164 km/s. So we could imagine, for example, a similar object that starts out with a radius of around 10^(-4) LY (which is still 10,000 times larger than the Sun) and expands to the same size at the same rate, which would take around 1000 years. This doesn’t seem particularly implausible, although I’m no astronomer.
The harder question to answer is what the heating rate from the CMB would be in the interior of this cloud. It would only have to be very small, of course, to counteract the adiabatic cooling. Looking at one of the papers on the Boomerang nebula, the authors there estimate the cosmic ray heating as 4∗10−28 erg/s, while the cooling rate is around 10−25 in the same units. So since adiabatic cooling goes slower as the gas gets colder (indeed, in this simple model we have T˙(t)=−T/t), we would probably expect that by the time the gas has cooled to about 1/1000th of the CMB temperature, if not sooner, the heating rate would match the cooling rate.
All in all, my very crude best guess then is that adiabatic expansion of this sort could not lead to a temperature below the mK scale.