There are a lot of cool things about this, but here are two that caught my attention:
>> Exotic high-energy states that are hard to generate in the laboratory at positive temperatures become stable at negative absolute temperatures
and
>> ... whereas clouds of atoms would normally be pulled downwards by gravity, if part of the cloud is at a negative absolute temperature, some atoms will move upwards, apparently defying gravity
A little rephrasing: In a system with a negative temperature the particles have more energy than a system with 0K. A system has a negative temperature if making it hotter/adding energy increases the order in it (delta q/delta S is negative). For example if there is a maximum energy state for each particle increasing the average energy at some point makes it more uniform, as more particles reach said state.
Negative temperatures are not at all unheard of in physics, as lasers are based on this very principle: At positive temperatures, states with higher energy will always be (statistically) less occupied than states with lower energy. In a laser one inverts this for some states of the system by pumping energy to meta-stable high-energy states, thus creating a "population inversion".
But you can get to infinite temperature from either direction. So it's more a curiosity of notation: instead of temperature T, the more natural thermodynamic concept is β = 1/T.
By the description it seems like they have reduced the energy so that not only intermolecular energy is reduced (or somehow stop), but also it reduces interatomic energy some how. Is this possible? It´s there another atomic absolute zero still to be discovered beyond 0K?
I´m have no idea about physics, so surely what I say doesn´t make sense.
You can make a plot of entropy vs. energy of a system. Further, you can calculate the derivative (slope) of that plot and give it a name. You could also calculate the quantity 1/slope and give that a name, too--and the name we give it is "temperature." So, the temperature of the system is negative any time the slope of entropy vs. energy is negative.
How do we get a negative slope? We need to find a place where entropy decreases when energy is added to the system. Entropy is proportional to the natural log of something called the "partition function (see below)." That means we need to find a place where partition function has a negative slope.
The partition function tells us essentially "how many distinct ways can we arrange (store) a given amount of energy in this system?" For almost all macroscopic systems, there are a greater number of ways to rearrange the system each time a unit of energy is added. However, it is possible to construct a system where adding a unit of energy actually restricts the number of ways you can arrange the system. And that is the basis for negative temperature.
The reason that negative temperatures are hard to explain is that the definition of temperature in the sense used by physicists is not very accessible.
It has to do with how much a certain property of a system (object) changes when you add a certain quantity of heat energy.
In certain edge cases adding energy makes this quantity change in the opposite to usual way and therefore the temperature is negative.
Hoverboards... if Back to the Future was right, we'll have them in October of next year.
"For instance, Rosch and his colleagues have calculated that whereas clouds of atoms would normally be pulled downwards by gravity, if part of the cloud is at a negative absolute temperature, some atoms will move upwards, apparently defying gravity"
This result, described today in Science1, marks the gas’s transition from just above absolute zero to a few billionths of a Kelvin below absolute zero.
Correct me if I'm wrong, but doesn't this mean that the gas molecules (at near-0K from negative) were extraordinarily hot? My understanding is that -1/T is the "true measure", ergo super-low negatives are, in fact, "absolute hot".
Does anyone know what are the thermodynamic properties of negative temperatures? For example, a pin-sized blackbody at 10^9 K would probably kill everyone on earth if it remained at that temperature. Would that apply to macroscopic objects at negative temperatures (if that were even possible, and if blackbody mechanics make sense in such states)?
Systems at negative temperature are, in a way, hotter than infinite temperature. Yet, the contain much less energy than the same system at a very large (or approaching infinite) temperature would contain.
They are, after all, created in a lab, so their energy content is limited by how much electricity the lab equipments has consumed.
I believe so: to keep a thing at negative temperature while it's in thermal contact with us, you'd need to keep supplying power, just as you need to power a lightbulb. I didn't read the OP, but a familiar example of negative temperature is the population inversion in a laser.
[+] [-] resdirector|11 years ago|reply
A subsequent publication[0][1] argues that their notion of "negative temperatures" is invalid as they used a flawed definition of entropy.
[0] http://scholar.google.com.au/scholar?cites=41283096248189415...
[1] http://www.physik.uni-augsburg.de/theo1/hanggi/Dunkel_Nature...
[+] [-] androidb|11 years ago|reply
[+] [-] MrBra|11 years ago|reply
[+] [-] heydenberk|11 years ago|reply
>> Exotic high-energy states that are hard to generate in the laboratory at positive temperatures become stable at negative absolute temperatures
and
>> ... whereas clouds of atoms would normally be pulled downwards by gravity, if part of the cloud is at a negative absolute temperature, some atoms will move upwards, apparently defying gravity
[+] [-] MrBra|11 years ago|reply
[+] [-] AnimalMuppet|11 years ago|reply
Pun intended? If so, nicely done.
[+] [-] unknown|11 years ago|reply
[deleted]
[+] [-] lelf|11 years ago|reply
[+] [-] 1ris|11 years ago|reply
[+] [-] ThePhysicist|11 years ago|reply
http://en.wikipedia.org/wiki/Negative_temperature
[+] [-] idlewords|11 years ago|reply
[+] [-] waqf|11 years ago|reply
http://en.wikipedia.org/wiki/Thermodynamic_beta
[+] [-] omegant|11 years ago|reply
I´m have no idea about physics, so surely what I say doesn´t make sense.
[+] [-] forgotketchup|11 years ago|reply
You can make a plot of entropy vs. energy of a system. Further, you can calculate the derivative (slope) of that plot and give it a name. You could also calculate the quantity 1/slope and give that a name, too--and the name we give it is "temperature." So, the temperature of the system is negative any time the slope of entropy vs. energy is negative.
How do we get a negative slope? We need to find a place where entropy decreases when energy is added to the system. Entropy is proportional to the natural log of something called the "partition function (see below)." That means we need to find a place where partition function has a negative slope.
The partition function tells us essentially "how many distinct ways can we arrange (store) a given amount of energy in this system?" For almost all macroscopic systems, there are a greater number of ways to rearrange the system each time a unit of energy is added. However, it is possible to construct a system where adding a unit of energy actually restricts the number of ways you can arrange the system. And that is the basis for negative temperature.
[+] [-] avoid3d|11 years ago|reply
It has to do with how much a certain property of a system (object) changes when you add a certain quantity of heat energy.
In certain edge cases adding energy makes this quantity change in the opposite to usual way and therefore the temperature is negative.
[+] [-] Zikes|11 years ago|reply
[1] http://en.wikipedia.org/wiki/Zero-point_energy
[+] [-] ericfrederich|11 years ago|reply
"For instance, Rosch and his colleagues have calculated that whereas clouds of atoms would normally be pulled downwards by gravity, if part of the cloud is at a negative absolute temperature, some atoms will move upwards, apparently defying gravity"
[+] [-] michaelochurch|11 years ago|reply
Correct me if I'm wrong, but doesn't this mean that the gas molecules (at near-0K from negative) were extraordinarily hot? My understanding is that -1/T is the "true measure", ergo super-low negatives are, in fact, "absolute hot".
Does anyone know what are the thermodynamic properties of negative temperatures? For example, a pin-sized blackbody at 10^9 K would probably kill everyone on earth if it remained at that temperature. Would that apply to macroscopic objects at negative temperatures (if that were even possible, and if blackbody mechanics make sense in such states)?
[+] [-] sampo|11 years ago|reply
They are, after all, created in a lab, so their energy content is limited by how much electricity the lab equipments has consumed.
[+] [-] abecedarius|11 years ago|reply
[+] [-] KONAir|11 years ago|reply
[+] [-] eloff|11 years ago|reply