Devangshu Datta: Overturning absolute zero

Last Updated: Fri, Feb 08, 2013 04:11 hrs

One of the cornerstones of the scientific approach is the need to question everything and match it with available evidence. If new evidence arises and conflicts with theory, even the most long-established theories will be overturned, or modified.

An obvious example is the classical clockwork universe, a model developed by Newton in the 17th century. Newtonian mechanics is used even now to accurately calculate trips to Mars, and so on. Newton’s laws of motion and gravity were accepted as the absolute truth for over 200 years before being modified by Einstein’s Theory of Relativity.

Many new insights developed in the late 19th and early 20th centuries since scientists investigated entirely new areas, such as quantum physics and relativity. Now, some of those theories are being challenged.

Scientists have just succeeded in breaking the barrier of absolute zero. Early into middle-school physics, children are taught that absolute zero is inviolate. It seems that, by a quirk of quantum physics, this is not always true.

In the 19th century, investigations into the properties of gases led to the concept of absolute zero. The mobility of gaseous molecules depends on their energy. High-energy states translate into high degrees of motion and into expansion in gas-volume, while low energy states correspond to low motion and volume-contraction. The less the motion, the lower the temperature. When there is zero energy, motion ceases and the state of absolute zero is reached.

This concept can be derived from Charles’ Law, though much of the key investigations were done by Lord Kelvin. Absolute zero is calibrated at 273 degrees below the freezing point of water (-273 Celsius) on the Celsius scale. It’s the start point, or 0K on the kelvin scale (which uses the same grades as Celsius).

It was initially assumed 0K could not be achieved, though lab experiments have come very close. The average temperature of the universe is between 2-3K. Peculiar effects occur at close to absolute zero. Super-conductivity develops, for example, as electrical resistance almost disappears. Super-fluidity also occurs in liquids (only helium and hydrogen are naturally liquid at these temperatures).

It has been speculated since the 1950s that there may be a theoretical workaround for going below absolute zero. Another way to define temperature (rather than “hot” or “cold” ) is by the entropy of a system. Very roughly speaking, entropy can be described as the amount of disorder in a system. It has several head-spinningly complicated mathematical descriptions.

But in most systems, entropy tends to increase as energy is added to a system. So, absolute zero is the lowest state of entropy. However, by a quantum quirk, it is possible to add energy to a quantum system in certain specific ways and actually have a decrease in entropy.

To an extent, lasers exploit this property because a high-energy laser does not have higher entropy than a laser with lower-energy. If entropy decreases as energy is added, temperature may also fall. If this can be done at near-absolute zero, a “negative kelvin” system could be created.

The energy of a gas is derived from averaging out the energy of its particles. Most particles in the gas will have near-average energy levels and only a few will be higher energy. It may be possible via the use of magnetic fields or other methods to change this situation. If a large number of particles can be suddenly pushed into their highest possible energy state, without increasing entropy, absolute zero could be broken and a negative kelvin zone entered.

A team at the Ludwig Maximilian University in Munich, Germany, achieved this feat in December. They assembled a cloud of ultra-cold potassium atoms, and arranged them in a lattice of individual atoms by using a combination of lasers and magnetic fields. Since the atoms were very cold, they were near their lowest-possible energy states. Normally, at positive temperatures, atoms repel each other. However, by forcing these atoms to attract each other by manipulating the magnetic fields, the atoms were pushed into their highest-possible energy states.

This sudden reversal caused the temperature to drop from just above absolute zero to a few billionth of a degree below 0K. Ulrich Schneider, who was a member of the Maximilian team, described it poetically — “It’s like walking through a valley, then instantly finding yourself on the mountain peak”.

At positive temperatures, this kind of energy-state reversal would be unstable. The atoms would tend to collapse inwards owing to attraction. That’s where the laser field came into play. By judiciously arranging the laser field, the atoms were kept trapped in the lattice and the negative kelvin state was maintained.

Apart from being a stunning demonstration of the apparently impossible, negative-kelvin states seem to be fascinating in themselves. Energy will actually flow from a “colder” negative-kelvin object to a warmer positive-kelvin object.

Other rules also seem to reverse in the negative-kelvin state. At temperatures above 0K, maintaining a stable configuration of high-energy atoms is difficult since these repel each other and tend to fly apart. But below 0K, it seems that high-energy states are apparently stable. This allows these systems to be studied and it may eventually lead to the fabrication of exotic new materials.

Another theoretical prediction is anti-gravity. Achim Rosch, the theoretician who proposed the experiment performed by Schneider’s team has calculated that a cloud of negative energy may fly upwards. Finally, such a system may also mimic “Dark Energy”, the mysterious force that scientists postulate to make cosmology work.

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