Quantum gas temperature drops below absolute zero

[Alan C.]

Quantum gas temperature drops below absolute zero

Physicists have created a quantum gas capable of reaching temperatures below absolute zero and paving the way for future quantum inventions.

The chilly substance was composed of potassium atoms which were held in a lattice arrangement using a combination of lasers and magnetic fields. According to a report in the Nature journal, by tweaking the magnetic fields the research team were able to force the atoms to attract rather than repel one another and reveal the sub-absolute zero properties of the gas.

"This suddenly shifts the atoms from their most stable, lowest-energy state to the highest possible energy state, before they can react," said Ulrich Schneider of the Ludwig Maximilian University in Munich. "It's like walking through a valley, then instantly finding yourself on the mountain peak."

Previously absolute zero was considered to be the theoretical lower limit of temperature as temperature correlates with the average amount of energy of the substance's particles. At absolute zero particles were thought to have zero energy.

Moving into the sub-absolute zero realm, matter begins to display odd properties. Clouds of atoms drift upwards instead of down, while the atomic matrix's ability to resist collapsing in on itself echoes the forces causing the universe to expand outwards rather than contracting under the influence of gravity.

The ability to produce a relatively stable substance at several billionths of a Kelvin below absolute zero will allow physicists to better study and understand this curious state, possibly leading to other innovations.

"This may be a way to create new forms of matter in the laboratory," said Wolfgang Ketterle, a Nobel laureate at MIT, commenting on the results.

[TheRobin]

Scientists reaching new levels of coolness again. Sweet

[ribuck]

These scientists have not achieved a temperature below absolute zero in the "familiar" sense, where temperature is proportional to the kinetic energy of an object's molecules. Obviously that kinetic energy can never drop below zero, so the temperature can never drop below zero.

What these scientists have done is very cool nevertheless. They have cajoled some matter into a state where there are more particles in higher-energy states than in lower-energy states, rather than the other way around as is usual. Scientists working with such systems use a more general definition of temperature, which gives the same answer as the "familiar" definition in the usual case, but gives a negative number when there is a deficit of particles in the lower-energy states.

As the Wikipedia Article explains, these "negative" temperatures are actually hotter than temperatures with a positive sign. It's just an artifact of the way the signs are handled in the equations.

[Metric]

 

These scientists have not achieved a temperature below absolute zero in the "familiar" sense, where temperature is proportional to the kinetic energy of an object's molecules. Obviously that kinetic energy can never drop below zero, so the temperature can never drop below zero.

What these scientists have done is very cool nevertheless. They have cajoled some matter into a state where there are more particles in higher-energy states than in lower-energy states, rather than the other way around as is usual. Scientists working with such systems use a more general definition of temperature, which gives the same answer as the "familiar" definition in the usual case, but gives a negative number when there is a deficit of particles in the lower-energy states.

As the Wikipedia Article explains, these "negative" temperatures are actually hotter than temperatures with a positive sign. It's just an artifact of the way the signs are handled in the equations.

 

Exactly right.  In thermodynamics, the defining relationship for temperature 1/T = dS / dE, where T is temperature, S is entropy, and E is energy (and dS / dE means "the change in entropy as you add energy").  Now the entropy is, roughly speaking, "the number of possible states" of all the constituent atoms.  So, positive temperature means that if you add energy to the system, there are more things each particle could randomly do (more states become available).  This makes sense for systems with low total energy -- the atoms spend most of their time sitting around at low energy, and only occasionally get bumped around into higher energy states, so adding energy means "more possibilities" for each atom.

However, in a system where the number of high-energy states for each atom is limited, you can end up with the opposite situation -- if you add a lot of energy to the entire system, nearly all the atoms have to spend all their time in their own high-energy state, and only occasionally fall back down to their low-energy state.  Adding even more energy to the system makes the situation even more restricted -- i.e. there are fewer possible "low energy" states available for each atom.  So in this case dS / dE becomes negative -- more energy = fewer possible states.  And so that means negative temperature.

[Dylan Lawrence Moore]

However, in a system where the number of high-energy states for each
atom is limited, you can end up with the opposite situation -- if you
add a lot of energy to the entire system, nearly all the atoms have to
spend all their time in their own high-energy state, and only
occasionally fall back down to their low-energy state.  Adding even more
energy to the system makes the situation even more restricted -- i.e.
there are fewer possible "low energy" states available for each atom. 
So in this case dS / dE becomes negative -- more energy = fewer possible
states.  And so that means negative temperature.

Cool stuff. Do you know which type of atomic energy states? Electrical, rotational, translational, spin, all of the above? Just curious.

-Dylan

[Metric]

 

However, in a system where the number of high-energy states for each
atom is limited, you can end up with the opposite situation -- if you
add a lot of energy to the entire system, nearly all the atoms have to
spend all their time in their own high-energy state, and only
occasionally fall back down to their low-energy state.  Adding even more
energy to the system makes the situation even more restricted -- i.e.
there are fewer possible "low energy" states available for each atom. 
So in this case dS / dE becomes negative -- more energy = fewer possible
states.  And so that means negative temperature.

Cool stuff. Do you know which type of atomic energy states? Electrical, rotational, translational, spin, all of the above? Just curious.

-Dylan

 

I don't know what system was used in this particular experiment, but the simplest example of a system like this would be a fixed lattice of spins in the presence of a magnetic field.  Each atom then has just two possible states -- one "high energy" state and one "low energy" state.  So for a low energy of the whole system, the temperature would be positive, but at a high enough energy of the whole system, the temperature would become negative.