The Quest for Absolute Zero

I could just about keep up with the science in this talk at the Science Café! It had equations in it! And graphs!
For this session, we were in the bigger back room at the Swan, and the speaker was John Tyler, who had a background working with liquid oxygen.
So, as I vaguely remembered from my A level Chemistry days, the coldest temperature it's possible to be is -273.15 degrees Celcius, which is the same as 0 degrees Kelvin, and John's talk outlined the history of how scientists had attempted to get down to that temperature through history.
The first problem was how to measure temperature at all. The first accurate measurements were with the scale devised by Mr. Farenheit in the 18th century, followed by M. Celcius, who devised the scale that started at 0 degrees for the boiling point of water, and 100 degrees for the freezing point of water - until someone else decided that it would be more sensible to do it the other way round, as we do today. To take the measurements they needed mercury, and good enough glass blowers to make the glass tube to put the mercury in.
There are other methods of temperature measurement that are more useful at very low temperatures, such as measuring the resistivity of a platinum wire.
The theory was that, if the temperature was low enough, a gas would turn into a liquid, and eventually into a solid, as can be easily observed with water, becoming steam or ice according to the temperature. Another important variable is pressure - which is why it gets more difficult to boil water the higher up a mountain you are. So more pressure helps to turn your gas into a liquid.
This is where I learned of the "triple point", the one place in a graph of temperature against pressure where an element would be in all three states - solid, liquid and gas - at the same time! There's an international committee which fixes the official measurements, and which fixed the official temperature for Absolute Zero.

Steam engines were used to power the experiments, and at first the only object of the experiments was to see if it could be done - there was no practical application for liquid oxygen, hydrogen or nitrogen. There is now, of course - MRI scanners in hospitals use liquid helium as a coolant. Helium wasn't even known as an element when the experiments began, but in the middle of the nineteenth century there was an eclipse, centred in India, where astronomers used spectroscopy for the first time to detect the different gases in the corona of the sun as it went behind the moon. This gives coloured lines for each element, and there was a bright yellow one that had never been seen before. The name it was given, helium, comes from helios, Greek for the sun. Later, helium was detected on earth, coming from uranium ore, and the scientists who had been experimenting with making liquid hydrogen also wanted to try to make liquid helium. One of the foremost of these was Sir James Dewar, who gave demonstrations before audiences at the Royal Society. There is a picture of him in the 1890s making liquid hydrogen before an audience which included famous scientists like Marconi - without any safety precautions whatsoever. Apparently Sir James' two assistants each lost an eye in experiments that went wrong!
Meanwhile at the University of Leiden, a chap called Onnes had the resources and the inclination to make Leiden a world centre of cryogenic research, setting up a school to train glass blowers and instrument makers who were essential to build the equipment needed. By this stage they were using glass vacuum vessels as part of the method of lowering the temperature, first making liquid air, and using that to cool down hydrogen until the gas became liquid.
There was some discussion at the end of the talk about the feasibility of producing hydrogen powered cars in the volume necessary to replace petrol driven ones, and they weren't very optimistic that it was practical. Another member of the audience had also worked with cryogenics, at Llanwern, where they processed 5,000 tons of air a day into liquid oxygen and other elements! Argon is one of the trace elements in air that has a commercial use (welding, I think). And apparently the Dutch are experimenting with introducing hydrogen into the gas supply with some success.
The picture used at the beginning of the talk was of the Boomerang nebula, which is the coldest place known in nature, at about 1 degree Kelvin. Most of space is at about 2 degrees Kelvin. The coldest recorded temperature in nature on Earth was -81 degrees C.

The next talk will be on Monday March 23rd, when Brian Henderson will be talking about PCR, which I think stands for Polymerase Chain Reaction!