2010-01-11 13:33:28
It is very easy to get ice cubes, even in summer, just open the refrigerator. Cooling a substance is so habitual for us that it is unimportant, what’s more, seeing the ease with which the thermometer registers sub-zero temperatures in our kitchen, we could come to the idea, wrong by the way, that we can cool a substance to infinity . No it’s not true. In a matter of temperatures, nature surprises us with a magic number: 273.15 ºC below zero. The absolute zero. Nothing can be colder.
It was the French physicist Charles who for the first time arrived at that strange number and he did it by pushing his reasoning about a curious phenomenon to the limit. The French physicist had verified that gases, when cooled, contract. It is easy to verify: inflate a balloon and put it in the refrigerator, you will see how, when it cools down, it decreases in size; then leave it outside, at room temperature, and you will observe how it inflates again until it recovers its original size.
Charles measured the contraction of gases with temperature and found that any gas at 0ºC contracts 1/273 of its volume for every degree drop in temperature. He experimented with various gases and with all of them he obtained the same result. Intrigued, he reasoned as follows: “If we take 273 liters of a gas at 0ºC and begin to cool it, for each degree the temperature drops its volume will be one liter less, at -2ºC it will occupy two liters less and, continuing like this, at 273 ºC below zero, the gas will lose its last liter. It will completely disappear.”
That things disappear, as if by magic, is a very unpleasant thing in science, so everyone suspected that, when the temperature dropped a lot, Charles’s law would no longer hold. It was only a guess because no one, until then, had managed to come close to that figure to verify it. A coherent explanation of the phenomenon was necessary and that came hand in hand with an old theory, established 2,400 years earlier by the Greek Democritus, and forgotten since then: the atomic theory.
According to the atomic theory, gases are made up of small molecules that move at considerable speed, like a huge swarm of bees around a honeycomb. The higher the temperature of the gas, the greater the speed of its molecules and “the more space they need to move”, that is, they occupy a larger volume. As the temperature decreases, the molecules move more slowly and the volume decreases.
In the 1860s, the British Lord Kelvin suggested that it is the energy of the molecules that decreases by 1/273 for each degree of cooling. Applying this reasoning, he came to the conclusion that at 273 degrees below zero, the gas molecules do not disappear but simply remain motionless. This temperature is set today at 273.15 degrees below zero and, in honor of Kelvin, a new temperature scale was created that starts counting from that point: absolute zero.
From that moment began a race to reach absolute zero. The general idea was that gases first liquefy and then freeze until their molecules are absolutely immobile. Playing with the pressure and the evaporation of some substances and others, one by one, the gases liquefied as lower temperatures were reached. Chlorine, carbon dioxide, air, oxygen, etc. In the process, industrial methods were developed to produce ever lower temperatures. In the year 1900 hydrogen liquefied at 240ºC below zero, just 33 degrees above absolute zero. The goal was getting closer.
The victory over hydrogen was a great major achievement, but there was still a ways to go. The noble gases, especially the lightest of all, helium, refused to be allowed to liquefy. It was achieved by the Dutch physicist Heinke Kammerlingh Onnes and for this he had to reach temperatures of 272.3 ºC below zero, less than one degree above absolute zero. It was then that matter showed its most capricious side. Some substances lose their resistance to the passage of electric current and become superconducting, others flow unimpeded through the smallest openings and become superfluid. Once again, science opened the doors to a surprising, almost magical world.
Currently, scientists have managed to get within a few trillionths of a degree of absolute zero, but the goal remains unattainable. Like the tale of Achilles and the tortoise, when scientists cover a distance, the goal is always farther away, making fun of them.
For his achievements, Heinke Kammerlingh Onnes received the Nobel Prize in Physics in 1913. Today we offer you the story of his life.
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