Below Absolute Zero

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Below Absolute Zero

Post by RedOranje on Sat Jan 05, 2013 12:46 am

By: Charles Choi, LiveScience Contributor
Published: 01/03/2013 02:34 PM EST on 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.

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.)

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.

Because 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.

http://www.huffingtonpost.com/2013/01/04/absolute-zero-record-setting-negative-temperature_n_2404666.html

100% efficiency engines and anti-gravity devices when? bounce

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Re: Below Absolute Zero

Post by spanky on Sat Jan 05, 2013 4:05 pm

yea i remember this from thermodynamics 2 years ago and if this is true in practice they will have to revamp the main 4 laws of thermodynamics.

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Re: Below Absolute Zero

Post by free_cat on Mon Jan 07, 2013 10:09 am

Had no idea bout this. Never heard of "negative temperatures". Still, I'm not toally sure this makes sense: you have a negative temperature but you are hotter than any positive temperature and you transfer energy to the surrounding matter? Sounds like stuff with very high positive energy. Where's the difference? Not sure...
You study physics Spanky?

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Re: Below Absolute Zero

Post by RedOranje on Mon Jan 07, 2013 10:21 pm

@free_cat wrote:Had no idea bout this. Never heard of "negative temperatures". Still, I'm not toally sure this makes sense: you have a negative temperature but you are hotter than any positive temperature and you transfer energy to the surrounding matter? Sounds like stuff with very high positive energy. Where's the difference? Not sure...
You study physics Spanky?

Exactly. You have to think of temperatures as circular rather than linear. Once a temperature reaches a certain point in either direction it basically doubles over to the opposite end of the scale.

As I understand it once such incredibly temperatures are reached not only does entropy trend opposite of the norm but so do reactions to gravity and other universal forces. It's a pretty crazy thought.

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Re: Below Absolute Zero

Post by CBarca on Mon Jan 07, 2013 11:18 pm

Very interesting, but I can't honestly wrap my head around this. It's just ridiculous.

The part about absorbing entropy is what gets me the most. I don't understand how that's possible.

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Re: Below Absolute Zero

Post by spanky on Tue Jan 08, 2013 7:53 pm

@free_cat wrote:Had no idea bout this. Never heard of "negative temperatures". Still, I'm not toally sure this makes sense: you have a negative temperature but you are hotter than any positive temperature and you transfer energy to the surrounding matter? Sounds like stuff with very high positive energy. Where's the difference? Not sure...
You study physics Spanky?

no i actually graduated last year in mech. engineering, where thermodynamics is probably one of the most important courses in the whole program. regarding the negative temperature, think of it this way, in the conventional temperature scale you have 2 temperatures T1 and T2, where T2 is hotter than T1 ( T2 > T1) and if you have either convection,conduction,radiation, energy will flow from T2 to T1, so basically T2 is giving away heat and this heat flux is considered positive.

Now in relation to this article we have energy flowing from a colder temperature to a hotter one and this is the case why they call it negative temperature. The term negative has nothing to do with the actual value of the temperature but more so of its properties.

Heres a quote from wikipedia on negative temperatures:

In physics, certain systems can achieve negative temperature; that is, their thermodynamic temperature can be expressed as a negative quantity on the Kelvin scale. In colloquial usage, "negative temperature" may refer to temperatures that are expressed as negative numbers on the more familiar degrees Celsius or Fahrenheit scales, with values that are colder than the zero points of those scales but still warmer than absolute zero. By contrast, a system with a truly negative temperature in absolute terms on the Kelvin scale is hotter than any system with a positive temperature. If a negative-temperature system and a positive-temperature system come in contact, heat will flow from the negative- to the positive-temperature system.

That a system at negative temperature is hotter than any system at positive temperature is paradoxical if absolute temperature is interpreted as an average internal energy of the system. The paradox is resolved by understanding temperature through its more rigorous definition as the tradeoff between energy and entropy, with the reciprocal of the temperature, thermodynamic beta, as the more fundamental quantity. Systems with a positive temperature will increase in entropy as one adds energy to the system. Systems with a negative temperature will decrease in entropy as one adds energy to the system.[1]

Most familiar systems cannot achieve negative temperatures, because adding energy always increases their entropy. The possibility of decreasing in entropy with increasing energy requires the system to "saturate" in entropy, with the number of high energy states being small. These kinds of systems, bounded by a maximum amount of energy, are generally forbidden classically. Thus, negative temperature is a strictly quantum phenomenon. Some systems, however (see the examples below), have a maximum amount of energy that they can hold, and as they approach that maximum energy their entropy actually begins to decrease.[2]

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