Atoms Reach Record Temperature, Colder than Absolute Zero
2013-01-06 0:00


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.

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.

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.)

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.

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.


Related Articles

Latest News from our Front Page

Your Smartphone Could be Tracking You Every 3 Minutes, Study Says
2015-04-01 2:24
Your apps want to know where you are Smartphone apps regularly collect large amounts of data on users’ locations, sometimes as often as every three minutes, new research suggests. Researchers at Carnegie Mellon University conducted a study where they asked 23 people to use their Android smartphones normally, and tracked location data requests from each device with specially designed software, the Wall ...
Facebook accused of tracking all users even if they delete accounts, ask never to be followed
2015-04-01 2:14
A new report claims that Facebook secretly installs tracking cookies on users’ computers, allowing them to follow users around the internet even after they’ve left the website, deleted their account and requested to be no longer followed. Academic researchers said that the report showed that the company was breaking European law with its tracking policies. The law requires that users are ...
'Gay cake' bakery discriminated against client over sexual orientation, court told
2015-04-01 2:37
David Scoffield QC, acting for the bakery, said if Lee’s argument was right, a Muslim printer could not turn down a contract to print leaflets about the prophet Muhammad, an atheist could not turn down an order saying God made the world and a Roman Catholic printer could not decline making leaflets calling for the legalisation of abortion on demand. Judge ...
Gay rights groups criticize Indiana religious liberties law
2015-04-01 0:10
Editor's note: Would it be ok if a court forced a bakery operated by homosexuals, to make a cake for a Christian that says: "Homosexuality is a sin"? What would the reactions be? One way tolerance? Respecting peoples beliefs extends in all directions or in no direction. Indiana Gov. Mike Pence (R) signed a religious liberties bill into law Thursday that has been ...
Daily Show's Trevor Noah under fire for Twitter jokes about Jews and women
2015-04-01 0:01
Trevor Noah – the surprise choice to succeed Jon Stewart as high-profile host of satirical news program The Daily Show – has come under fire for a series of controversial tweets he posted before his appointment. The South African comedian – son of a Swiss-German father and half-Jewish South African mother – was criticised for having made tasteless jokes about Jews ...
More News »