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Name: Jamie
Status: student
Grade: 9-12
Location: IL
Country: USA
Date: Winter 2012-2013


Question:
I just heard (January 2013) that there has been a discovery of a material that was taken to a negative Kelvin temperature. How is this possible?

Replies:
Hi Jamie,

Thanks for the question. The concept of negative temperatures has been around since the 1950's when lasers and masers were invented. A negative temperature is really a mathematical artifact. Temperature measures the average speed of a particle. A particle cannot have a negative speed since speed is always greater than or equal to zero. A negative temperature signifies what is called a "population inversion" in which there are more particles at a higher energy than at a lower energy. One can prove, that under ordinary circumstances (not involving masers or lasers), there are more particles at a lower energy than a higher energy.

I hope this helps. Please let me know if you have more questions. Thanks Jeff Grell


Without trying to give a point-by-point counter argument about negative absolute temperatures, there is no mystery. The discovery of a “new material”, isn’t a “new material”. This is an example of hitting the news headlines with an, “OH! WOW!” when the response should be, ”OH! SO!” There is nothing new.

Systems composed of a small number of particles are governed by different “rules”, than systems composed of a large number of particles.

At low temperatures, the mechanics of a small collection of particles is determined by the “rules” of quantum mechanics. These “rules” are governed by populations of the occupancy of (quantum mechanical) states. Qualitatively, this means that “states” of higher energy contain fewer molecules/atoms than “states” of lower energy. This is no surprise.

The temperature of the small collection of molecules/atoms is determined by the population of the various quantum mechanical states. Now, it is possible “freeze” some states, so that the population of the higher energy states is greater than the population of the lower energy states. This is the way that lasers work – even the simple annoying pointers used in large lecture rooms. This is because the processes that “excite” atoms/molecules are faster than the reverse processes that “de-excite” atoms/molecules from the higher energy state to a lower state.

This, of course, is not an equilibrium condition.

So without even knowing the details, it is the obligation of the contender to show that their system involves conditions that are different than conditions that have been known for a generation or so.

Vince Calder


Hi Jaime -- yeah, it is one of those interesting paradoxes of physics... the material they created was both very hot *and* negative temperature. How could this be? The answer has to do with how you define temperature. There is a 'practical' definition, and there is a more complex mathematical definition. In this case, the 'negative' temperature is actually hotter than positive temperatures -- kinda like flipping a car's odometer when it reaches a maximum. The temperature is so hot, it 'flips' to negative (weird, huh?).

The most common (and common-sense) definition of temperature has to do with the vibrations of atoms or molecules -- the hotter the substance, the more bouncing around the atoms do, the higher the temperature. This is a very user-friendly, practical definition and it works in most cases. And, once those atoms cool down to having 'no' motion, they are at absolute zero, and they cannot get any colder. But that is the wrong end of the scale for this question... we need to get *hotter* to get to the negative.

It is important to keep in mind also that, in the world of physics, math sometimes dominates the hands-on interpretation. Here, we use a different definition of temperature to explain how it became 'negative'. Another way to define temperature has to do with thermodynamic, mathematical relationships that relate entropy and temperature. Basically, in 'normal' systems, if you add energy, you add entropy. This gives rise to another, mathematical, definition of temperature. (you can read more about the math in any thermodynamics text - if you wish!). Conversely, under this definition (increase in energy gives increase in entropy), if you create a system where adding energy *reduces* the entropy, it, by definition, has negative temperature.

The physicists that did this realized that a specially-arranged, extremely hot collection of atoms would not consist of a random assortment of high and low energy atoms (the way a normal gas would - called the Boltzmann distribution), but instead would have nearly all of its atoms in a narrow band of extremely high-energy states. At this point, adding energy would cause the atoms to become more ordered and uniform (pushing them into an ever-narrower band of configurations), thereby *reducing* the entropy. Adding energy, reducing entropy -- voila, you have negative temperature!

I have skipped over a lot of the details of "how" they did this -- I have focused on trying to explain how a negative temperature could be. However, if you want more detail, the best article I found explaining the experiment (and the concept) is here:

http://phys.org/news/2013-01-atoms-negative-absolute-temperature-hottest.html

It has got some very good pictures to help explain the concept, and it also describes some interesting thermodynamic consequences of this result as well! (although, many readers may find it a bit dense and hard to understand... you've been warned!)

Hope this helps, Burr Zimmerman


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