Record Temperature Set: Colder than Absolute Zero

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When an object is heated, its atoms can move with different levels of energy, from low to high. With positive temperatures (blue), atoms more likely occupy low-energy states than high-energy states, while the opposite is true for negative temperatures (red). CREDIT: Image courtesy of LMU / MPQ Munich

Content provided by Charles Choi, LiveScience

Absolute zero is often thought to be the coldest temperature possible.

But now researchers show they can achieve even lower temperatures for a

strange realm of "negative temperatures."

Oddly, another way to look at these negative temperatures is to consider them hotter than infinity, researchers added.

This unusual advance could lead to new engines that could technically

be more than 100 percent efficient, and shed light on mysteries such as dark energy, the mysterious substance that is apparently pulling our universe apart.

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An object's temperature is a measure of how much its atoms move — the

colder an object is, the slower the atoms are. At the physically

impossible-to-reach temperature of zero kelvin, or minus 459.67 degrees

Fahrenheit (minus 273.15 degrees Celsius), atoms would stop moving. As

such, nothing can be colder than absolute zero on the Kelvin scale.

Bizarro negative temperatures

To comprehend the negative temperatures scientists have now devised,

one might think of temperature as existing on a scale that is actually a

loop, not linear. Positive temperatures make up one part of the loop,

while negative temperatures make up the other part. When temperatures go

either below zero or above infinity on the positive region of this

scale, they end up in negative territory. (What's That? Your Basic Physics Questions Answered)

With positive temperatures, atoms more likely occupy low-energy states

than high-energy states, a pattern known as Boltzmann distribution in

physics. When an object is heated, its atoms can reach higher energy

levels.

At absolute zero, atoms would occupy the lowest energy state. At an

infinite temperature, atoms would occupy all energy states. Negative

temperatures then are the opposite of positive temperatures — atoms more

likely occupy high-energy states than low-energy states.

"The inverted Boltzmann distribution is the hallmark of negative

absolute temperature, and this is what we have achieved," said

researcher Ulrich Schneider, a physicist at the University of Munich in

Germany. "Yet the gas is not colder than zero kelvin, but hotter. It is

even hotter than at any positive temperature — the temperature scale

simply does not end at infinity, but jumps to negative values instead."

As one might expect, objects with negative temperatures behave in very

odd ways. For instance, energy typically flows from objects with a

higher positive temperature to ones with a lower positive temperature —

that is, hotter objects heat up cooler objects, and colder objects cool

down hotter ones, until they reach a common temperature. However, energy

will always flow from objects with negative temperature to ones with

positive temperatures. In this sense, objects with negative temperatures

are always hotter than ones with positive temperatures.

Another odd consequence of negative temperatures has to do with entropy,

which is a measure of how disorderly a system is. When objects with

positive temperature release energy, they increase the entropy of things

around them, making them behave more chaotically. However, when objects

with negative temperatures release energy, they can actually absorb

entropy.

Negative temperatures would be thought impossible, since there is

typically no upper bound for how much energy atoms can have, as far as

theory currently suggests. (There is a limit to what speed they can

travel — according to Einstein's theory of relativity, nothing can

accelerate to speeds faster than light.)

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Wacky physics experiment

To generate negative temperatures, scientists created a system where

atoms do have a limit to how much energy they can possess. They first

cooled about 100,000 atoms to a positive temperature of a few

nanokelvin, or billionth of a kelvin. They cooled the atoms within a

vacuum chamber, which  isolated them from any environmental influence

that could potentially heat them up accidentally. They also used a web

of laser beams and magnetic fields to very precisely control how these

atoms behaved, helping to push them into a new temperature realm. (Twisted Physics: 7 Mind-Blowing Findings)

"The temperatures we achieved are negative nanokelvin," Schneider told LiveScience.

Temperature depends on how much atoms move — how much kinetic energy

they have. The web of laser beams created a perfectly ordered array of

millions of bright spots of light, and in this "optical lattice," atoms

could still move, but their kinetic energy was limited.

Temperature also depends on how much potential energy atoms have, and

how much energy lies in the interactions between the atoms. The

researchers used the optical lattice to limit how much potential energy

the atoms had, and they used magnetic fields to very finely control the

interactions between atoms, making them either attractive or repulsive.

Temperature is linked with pressure — the hotter something is, the more

it expands outward, and the colder something is, the more it contracts

inward. To make sure this gas had a negative temperature, the

researchers had to give it a negative pressure as well, tinkering with

the interactions between atoms until they attracted each other more than

they repelled each other.

"We have created the first negative absolute temperature state for

moving particles," said researcher Simon Braun at the University of

Munich in Germany.

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New kinds of engines

Negative temperatures could be used to create heat engines — engines

that convert heat energy to mechanical work, such as combustion engines —

that are more than 100-percent efficient, something seemingly

impossible. Such engines would essentially not only absorb energy from

hotter substances, but also colder ones. As such, the work the engine

performed could be larger than the energy taken from the hotter

substance alone.

Negative temperatures might also help shed light on one of the greatest mysteries in science. Scientists had expected the gravitational pull of matter to slow down the universe's expansion after the Big Bang,

eventually bringing it to a dead stop or even reversing it for a "Big

Crunch." However, the universe's expansion is apparently speeding up,

accelerated growth that cosmologists suggest may be due to dark energy,

an as-yet-unknown substance that could make up more than 70 percent of

the cosmos.

In much the same way, the negative pressure of the cold gas the

researchers created should make it collapse. However, its negative

temperature keeps it from doing so. As such, negative temperatures might

have interesting parallels with dark energy that may help scientists

understand this enigma.

Negative temperatures could also shed light on exotic states of matter,

generating systems that normally might not be stable without them. "A

better understanding of temperature could lead to new things we haven't

even thought of yet," Schneider said. "When you study the basics very

thoroughly, you never know where it may end."

The scientists detailed their findings in the Jan. 4 issue of the journal Science.

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