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Super-cooling and a Brief History of Temperature

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Physicists have, throughout history, spent many years of their lives painstakingly searching through the world to find something to name after themselves. There are few areas where this is more true than the study of temperature. In the beginning, the first mass producer of thermometers, Fahrenheit was the first sensible person to put a scale to temperature. Then Celsius took the thermometer, measured the freezing temperature and the boiling point of water, divided it by 100, and created the Centigrade scale.

The Appearance of 273

In 1802 a French physicist called Joseph Louis Gay-Lussac determined the coefficient of cubical expansion. With this he calculated what would happen with a change in temperature for a change in volume. He proposed that the change in volume was equal to the volume multiplied by the change in time over 273.

This is the first appearance of 273 in an equation. This was also the first indication for a value of absolute zero. Then why do we not know anything of Gay-Lussac? Well, another French physicist, Jacques Alexandre Cesar Charles, claimed he'd known about the value since 1787. However, he hadn't published any of his work and publishing is required before a discovery can be acknowledged. Anyway, the above equation is now known as Charles' law1.

The final person to really invent a really useful scale was William Thomson. He pointed out that Charles' law indicated that an object would have zero volume at -273°C so why don't we call it zero. The point about zero volume had already been made, but he was the first to call it zero. This idea was good but his name was a bit far back in the alphabet so it became the Kelvin scale, and he became Lord Kelvin (although he was made a baron). However, not quite happy with this, the term 'absolute zero' came into being, putting this fact at the front of every dictionary of physics.

Absolute Zero

We know that absolute zero is -273.16°C - at this temperature molecules are motionless. Or not. Molecules will still have a small amount of kinetic energy at absolute zero. We know that near absolute zero metals become super-conductive. Again this is not accurate, we can have alloys that super-conduct at 120K, which is a lot easier to reach. What we do know happens at absolute zero, is, on the whole, nothing. We have never reached it and may never reach it.

The process currently used to cool temperature involves conducting heat away from an object. This reduces the object's kinetic energy and heat - the principles are similar to that used in fridges. We then take this object and look at it and say, 'look it's cool'. We can then cool it further using magnets. We order atoms in the object, (which will make the object magnetic, but we will come back to that) and this increases the heat of the system and we then remove this heat.

This exercise is, on the whole, pointless, leaving you with a very small space at 0.00001K, which you can't do anything to. Lancaster University currently holds the world record for the coolest temperature on Earth. The apparatus they have is mounted on air cushions to prevent vibrations from the nearby motorway, the A6. It is in a room shielded by the same materials used to shield the windows on Microwaves, to prevent radiation from the Sun entering it. Also, no one is allowed to touch its outer casing, again, to prevent vibration. Yet, they can do nothing practical with it. The question then is, why bother?

Why Bother?

Well, super-conductivity would probably be a good thing to aim for, and it's achievable at 120K. The uses of super-conductivity are really just magnetic levitation, unless a really good reason for putting a charge in to a super-conductor and taking out later without losing charge is found. Magnetic levitation, or Maglev, relies on the laws of induction. As a magnet falls towards a conductor it induces current and the conductor creates a magnetic field which opposes its falling. This does not prevent the magnet falling but rather, slows it down. When a magnet falls towards a super-conductor, the magnetic field created around it is of equal magnitude to the actual magnet, preventing it falling and effectively making it float. The only people interested in this technology are train designers2.

We were often told at school that the only magnetic substances on Earth were Iron, Nickel, and Cobalt. This is wrong. Magnetism is a totally heat-dependant property. Heating Iron causes it to lose its magnetic properties. Cooling Copper causes it to become magnetic. This is due to the fact that magnetic dipoles order themselves as they lose energy.

All that is written above is really what has been done. But physics is driven on by what can't be done. We really want to reach zero degrees Kelvin so that we can make liquid Helium. We can make liquid Helium, but at around zero degrees Kelvin it becomes a frictionless environment. The conventional laws of physics break down and we go into quantum mechanics. In fact, particle physicists are using cold atoms, about 20mK, to detect dark matter. If dark matter exists it would interact with this atom causing both heat and ionisation, which we can detect.

Super-cooling

Well, finally on to super cooling. Super-cooling is the cooling of a liquid to below its freezing point but keeping it a liquid by virtue of having an extremely pure sample of that liquid. Liquids do not normally super-cool, as they are often not pure and they form crystal structures. This is the problem faced when bodies are preserved in fridges. As the blood cools, it crystallises, which in itself is not a problem. The problem comes when the body is taken out of the fridge: when the blood froze it expanded causing tissue damage, so on thawing the body is so damaged it becomes unusable.

In an effort to understand super-coolingand how we could freeze someone without damage, a research project has taken to freezing turtles. This specific species of turtle can survive at -14.6°C, which is often its natural habitat. The margin of error for this temperature is ±1.9°C, when this margin of error is exceeded the turtles then, instantly freeze.

1 Perhaps the French really didn't want future generations making silly remarks about the equation.2This technology could theoretically be used by the army for a rail-gun, but there doesn't really exist a substance to withstand the acceleration it would undergo.

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