5 ms·

Quantum gas goes below absolute zero

117 points by thisjepisje 12y ago

heydenberk 12y ago
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
- deleted 12y ago
  [deleted]
- MrBra 12y ago
  Yes, as I read the article at the second part you quoted, a flying saucer immediately popped up into my mind... I am sure I am not the only one.. :)
- AnimalMuppet 12y ago
  >> There are a lot of cool things about this... Pun intended? If so, nicely done.
omegant 12y ago
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.
- avoid3d 12y ago
  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.
  - omegant 12y ago
    I´ve been reading the negative temperature link, I understand that you reduce entropy reducing energy, till a point, where you start "holding" the molecules down adding energy (I don´t know if that makes sense). But still is there a way that you could start affecting the forces inside the atom this way?. Or then we are talking about something different?.
    - avoid3d 12y ago
      This is really on the edge if where I don't know what I'm talking about anymore but I would think not. The forces holding atoms together and the energies associated with changing those forces are far larger than the amounts that are presented in this study.
- forgotketchup 12y ago
  To expand on what avoid3d said: 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.
eloff 12y ago
Needs 2013 tag, this is old news.
lelf 12y ago
https://en.wikipedia.org/wiki/Negative_temperature https://en.wikipedia.org/wiki/Negative_temperature
- 1ris 12y ago
  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.
  - thisjepisje 12y ago
    That's a very nice and clear explanation.
  - m_mueller 12y ago
    Am I guessing correctly that this isn't at all related to negative energy? When calling this a temperature below 0K - isn't this shoehorning the concept of temperature onto something it wasn't meant to do? Isn't this transition from a positive temperature to a negative temperature system non continuous, as in it needs to be set up with different properties? I'm thinking a more intuitive approach would be to put an additional system defined factor into equations defining a relation between temperature and entropy - this factor would usually be +1, but can be -1 under certain circumstances. But hey, IANAP.
    - archgoon 12y ago
      > Am I guessing correctly that this isn't at all related to negative energy? Yes. > When calling this a temperature below 0K - isn't this shoehorning the concept of temperature onto something it wasn't meant to do? No. Suppose you're doing a Fluid Dynamics Simulation. You need to assign a temperature and pressure to each point in space to simulate gas flows. Using the negative temperature will correctly predict the evolution of the gas flow. This is actually a strength of physics, having equations that can take on values that to your knowledge are unphysical (such as negative refraction indices for example) but still work. This means that when someone manages to figure out how to build such a system; we don't need to rework all of our theorems (conservation of energy; momentum; etc) to see which are still valid. We also have the ability to investigate the applications of such materials; should we ever be able to find a way to create them. http://en.wikipedia.org/wiki/Metamaterial http://en.wikipedia.org/wiki/Metamaterial > Isn't this transition from a positive temperature to a negative temperature system non continuous, as in it needs to be set up with different properties There are plenty of discontinuous transitions in thermodynamics, phase diagrams are an example. This is hardly an argument to that the concept of negative temperature is a broken one. http://en.wikipedia.org/wiki/Phase_diagram http://en.wikipedia.org/wiki/Phase_diagram
michaelochurch 12y ago
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)?
- abecedarius 12y ago
  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.
- sampo 12y ago
  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.
idlewords 12y ago
A curiosity of physics is that you can set up systems with a negative temperature, but you can't ever get to zero temperature, from either direction.
- waqf 12y ago
  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. http://en.wikipedia.org/wiki/Thermodynamic_beta http://en.wikipedia.org/wiki/Thermodynamic_beta
resdirector 12y ago
This was published 18 months ago. 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=4128309624818941529&as_sdt=2005&sciodt=0,5&hl=en http://scholar.google.com.au/scholar?cites=41283096248189415... [1] http://www.physik.uni-augsburg.de/theo1/hanggi/Dunkel_Nature.pdf http://www.physik.uni-augsburg.de/theo1/hanggi/Dunkel_Nature...
- MrBra 12y ago
  So they did not actually attain below-absolute-0 temperatures?
  - pagejim 12y ago
    Isn't it impossible as per laws of physics to achieve less than absolute ZERO temperature?
    - dragonwriter 12y ago
      No. https://news.ycombinator.com/item?id=8094173 https://news.ycombinator.com/item?id=8094173
    - sampo 12y ago
      It depends on what we mean by "temperature". If we think that temperature is a measure of how much the atoms move around then it is not possible to have less than zero movement. But if we think temperature as related to the ratio of how much a system's entropy changes when a certain amount of energy is added (or removed) to the system [1], 1/T = dS/dq then it's possible to construct systems with a negative T. [1] Eqs. 8–10 in http://en.wikipedia.org/wiki/Temperature#Second_law_of_thermodynamics http://en.wikipedia.org/wiki/Temperature#Second_law_of_therm...
      - simias 12y ago
        If I understand correctly, it's like saying a car has negative speed because it's going backwards. If you accelerate it forwards you actually reduce its speed towards 0, but that does not mean that the car is more immobile at -5km/h than at 0km/h. It's just a matter of convention, a negative temperature just means that the temperature gets closer to 0 as you add energy. Did I get that correctly?
KONAir 12y ago
Bots in the comments section of the article are pretty interesting.
Zikes 12y ago
If we could achieve a negative energy/temperature state couldn't we use that to harness zero point energy? [1] [1] http://en.wikipedia.org/wiki/Zero-point_energy http://en.wikipedia.org/wiki/Zero-point_energy
ThePhysicist 12y ago
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". http://en.wikipedia.org/wiki/Negative_temperature http://en.wikipedia.org/wiki/Negative_temperature
ericfrederich 12y ago
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"

117 points by thisjepisje 12y ago

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

deleted 12y ago
[deleted]
MrBra 12y ago
Yes, as I read the article at the second part you quoted, a flying saucer immediately popped up into my mind... I am sure I am not the only one.. :)
AnimalMuppet 12y ago
>> There are a lot of cool things about this... Pun intended? If so, nicely done.

omegant 12y ago

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.

avoid3d 12y ago
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.
- omegant 12y ago
  I´ve been reading the negative temperature link, I understand that you reduce entropy reducing energy, till a point, where you start "holding" the molecules down adding energy (I don´t know if that makes sense). But still is there a way that you could start affecting the forces inside the atom this way?. Or then we are talking about something different?.
  - avoid3d 12y ago
    This is really on the edge if where I don't know what I'm talking about anymore but I would think not. The forces holding atoms together and the energies associated with changing those forces are far larger than the amounts that are presented in this study.
forgotketchup 12y ago
To expand on what avoid3d said: 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.

eloff 12y ago

Needs 2013 tag, this is old news.

lelf 12y ago

https://en.wikipedia.org/wiki/Negative_temperature https://en.wikipedia.org/wiki/Negative_temperature

1ris 12y ago
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.
- thisjepisje 12y ago
  That's a very nice and clear explanation.
- m_mueller 12y ago
  Am I guessing correctly that this isn't at all related to negative energy? When calling this a temperature below 0K - isn't this shoehorning the concept of temperature onto something it wasn't meant to do? Isn't this transition from a positive temperature to a negative temperature system non continuous, as in it needs to be set up with different properties? I'm thinking a more intuitive approach would be to put an additional system defined factor into equations defining a relation between temperature and entropy - this factor would usually be +1, but can be -1 under certain circumstances. But hey, IANAP.
  - archgoon 12y ago
    > Am I guessing correctly that this isn't at all related to negative energy? Yes. > When calling this a temperature below 0K - isn't this shoehorning the concept of temperature onto something it wasn't meant to do? No. Suppose you're doing a Fluid Dynamics Simulation. You need to assign a temperature and pressure to each point in space to simulate gas flows. Using the negative temperature will correctly predict the evolution of the gas flow. This is actually a strength of physics, having equations that can take on values that to your knowledge are unphysical (such as negative refraction indices for example) but still work. This means that when someone manages to figure out how to build such a system; we don't need to rework all of our theorems (conservation of energy; momentum; etc) to see which are still valid. We also have the ability to investigate the applications of such materials; should we ever be able to find a way to create them. http://en.wikipedia.org/wiki/Metamaterial http://en.wikipedia.org/wiki/Metamaterial > Isn't this transition from a positive temperature to a negative temperature system non continuous, as in it needs to be set up with different properties There are plenty of discontinuous transitions in thermodynamics, phase diagrams are an example. This is hardly an argument to that the concept of negative temperature is a broken one. http://en.wikipedia.org/wiki/Phase_diagram http://en.wikipedia.org/wiki/Phase_diagram

michaelochurch 12y ago

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)?

abecedarius 12y ago
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.
sampo 12y ago
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.

idlewords 12y ago

A curiosity of physics is that you can set up systems with a negative temperature, but you can't ever get to zero temperature, from either direction.

waqf 12y ago
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. http://en.wikipedia.org/wiki/Thermodynamic_beta http://en.wikipedia.org/wiki/Thermodynamic_beta

resdirector 12y ago

This was published 18 months ago. 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=4128309624818941529&as_sdt=2005&sciodt=0,5&hl=en http://scholar.google.com.au/scholar?cites=41283096248189415... [1] http://www.physik.uni-augsburg.de/theo1/hanggi/Dunkel_Nature.pdf http://www.physik.uni-augsburg.de/theo1/hanggi/Dunkel_Nature...

MrBra 12y ago
So they did not actually attain below-absolute-0 temperatures?
- pagejim 12y ago
  Isn't it impossible as per laws of physics to achieve less than absolute ZERO temperature?
  - dragonwriter 12y ago
    No. https://news.ycombinator.com/item?id=8094173 https://news.ycombinator.com/item?id=8094173
  - sampo 12y ago
    It depends on what we mean by "temperature". If we think that temperature is a measure of how much the atoms move around then it is not possible to have less than zero movement. But if we think temperature as related to the ratio of how much a system's entropy changes when a certain amount of energy is added (or removed) to the system [1], 1/T = dS/dq then it's possible to construct systems with a negative T. [1] Eqs. 8–10 in http://en.wikipedia.org/wiki/Temperature#Second_law_of_thermodynamics http://en.wikipedia.org/wiki/Temperature#Second_law_of_therm...
    - simias 12y ago
      If I understand correctly, it's like saying a car has negative speed because it's going backwards. If you accelerate it forwards you actually reduce its speed towards 0, but that does not mean that the car is more immobile at -5km/h than at 0km/h. It's just a matter of convention, a negative temperature just means that the temperature gets closer to 0 as you add energy. Did I get that correctly?

KONAir 12y ago

Bots in the comments section of the article are pretty interesting.

Zikes 12y ago

If we could achieve a negative energy/temperature state couldn't we use that to harness zero point energy? [1] [1] http://en.wikipedia.org/wiki/Zero-point_energy http://en.wikipedia.org/wiki/Zero-point_energy

ThePhysicist 12y ago

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". http://en.wikipedia.org/wiki/Negative_temperature http://en.wikipedia.org/wiki/Negative_temperature

ericfrederich 12y ago

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"