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Cloud of atoms goes beyond absolute zero
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positive. To create a system with increase in the energy spread a range of negative temperatures, means higher entropy – but only Schneider and his colleagues used until the particles are evenly moving atoms, following a recipe distributed between all possible first proposed in 2005. energies, corresponding to They began by cooling atoms to maximum entropy (see diagram). a fraction above absolute zero and This is the maximum possible positive temperature, but you can placing them in a vacuum. Lasers still increase the particles’ energy. then placed the atoms along the curve of an energy “valley”, However, from now on, there with the majority of the atoms are more particles with higher in lower energy states. Next, the energies than lower energy, so entropy will go down with further researchers turned this positive system negative by adjusting the energy hikes, a sign that the lasers to change the atoms’ energy particles have flipped into the realm of negative temperature, “This amazing experimental which, bizarrely, is hotter than work will give us a deeper its positive counterpart. understanding about what “The temperature scale as we temperature really is” know it starts at zero and goes –Forget hot and cold– up to infinity, but it doesn’t stop levels so that the majority of the there,” says Ulrich Schneider particles were high energy. The of the Ludwig Maximilian atoms can’t lose energy and “roll University of Munich, Germany. down” the hill because doing The resulting thermometer is so would mean first increasing mind-bending. It starts at zero, their kinetic energy, which is not ramps up to infinity, then jumps possible because they are in a to the negative side of infinity vacuum and there is no outside before increasing through the energy source. So this flipped energies are distributed within a negative scale to zero. the valley into an energy hill, sample, which sets their entropy, Previously researchers entered producing the inverse energy a thermodynamic property. the negative realm by flipping distribution that characterises At absolute zero, all particles a magnetic field so that an array negative temperatures (Science, have minimum energy, and zero of mostly aligned atomic nuclei doi.org/j55). What’s more, unlike in entropy. As they heat up, some become mostly misaligned. But the magnetic system, lasers can set particles gain lots of energy but this only offered two discrete the hill at different heights, giving most just a small amount. This energy states – one negative, one a range of negative temperatures. Cold atoms are already used The weirdest thermometer in the world to simulate the interactions of Temperature is dependent on entropy, but when energy gets really high, subatomic particles, including absolute temperature turns negative and entropy drops quarks. The new set-up could Energy create simulations not possible 2 with positive temperatures. Negative temperature may also have implications for cosmology. 1 3 Dark energy, thought to explain the accelerating expansion of the universe, exerts negative Absolute temperature pressure, which suggests it might have negative temperature. 0 0 Positive Negative ∞ “It is amazing experimental 3 LOW ENTROPY 2 MAXIMUM ENTROPY 1 LOW ENTROPY work,” says Allard Mosk of In the positive Adding more energy Once the spread is even, the University of Twente in temperature realm, reduces entropy. positive temperature the Netherlands, who helped heating increases the Welcome to the weird reaches a peak. spread of particle world of negative From here on the outline the 2005 theory that energies (entropy) temperature system turns negative drove the work. “It will give us a deeper understanding about what temperature really is.” ■
Jacob Aron
NOTHING is colder than absolute zero, but there’s a whole world beyond that point. For the first time, atoms have been put in a state where they can take on a range of negative absolute temperatures. This could deepen our understanding of subatomic particles – and perhaps even mysterious dark energy. Although we’re used to talking about negative temperatures, such as −10 °C, all temperatures on an ordinary thermometer are positive when measured in kelvin. On this scientific scale, which starts at absolute zero (−273.15 °C), temperature is determined by the kinetic energy of particles. So a gas of slow particles is colder than a gas of fast-moving ones, and absolute zero corresponds to the point at which particles stop moving completely, which is why nothing can be colder. That does not tell the whole story, though. Temperature also depends on how the particle 12 | NewScientist | 12 January 2013
Entropy
Atoms go beyond absolute zero
positive. To create a system with increase in the energy spread a range of negative temperatures, means higher entropy – but only Schneider and his colleagues used until the particles are evenly moving atoms, following a recipe distributed between all possible first proposed in 2005. energies, corresponding to They began by cooling atoms to maximum entropy (see diagram). a fraction above absolute zero and This is the maximum possible positive temperature, but you can placing them in a vacuum. Lasers still increase the particles’ energy. then placed the atoms along the curve of an energy “valley”, However, from now on, there with the majority of the atoms are more particles with higher in lower energy states. Next, the energies than lower energy, so entropy will go down with further researchers turned this positive system negative by adjusting the energy hikes, a sign that the lasers to change the atoms’ energy particles have flipped into the realm of negative temperature, “This amazing experimental which, bizarrely, is hotter than work will give us a deeper its positive counterpart. understanding about what “The temperature scale as we temperature really is” know it starts at zero and goes –Forget hot and cold– up to infinity, but it doesn’t stop levels so that the majority of the there,” says Ulrich Schneider particles were high energy. The of the Ludwig Maximilian atoms can’t lose energy and “roll University of Munich, Germany. down” the hill because doing The resulting thermometer is so would mean first increasing mind-bending. It starts at zero, their kinetic energy, which is not ramps up to infinity, then jumps possible because they are in a to the negative side of infinity vacuum and there is no outside before increasing through the energy source. So this flipped energies are distributed within a negative scale to zero. the valley into an energy hill, sample, which sets their entropy, Previously researchers entered producing the inverse energy a thermodynamic property. the negative realm by flipping distribution that characterises At absolute zero, all particles a magnetic field so that an array negative temperatures (Science, have minimum energy, and zero of mostly aligned atomic nuclei doi.org/j55). What’s more, unlike in entropy. As they heat up, some become mostly misaligned. But the magnetic system, lasers can set particles gain lots of energy but this only offered two discrete the hill at different heights, giving most just a small amount. This energy states – one negative, one a range of negative temperatures. Cold atoms are already used The weirdest thermometer in the world to simulate the interactions of Temperature is dependent on entropy, but when energy gets really high, subatomic particles, including absolute temperature turns negative and entropy drops quarks. The new set-up could Energy create simulations not possible 2 with positive temperatures. Negative temperature may also have implications for cosmology. 1 3 Dark energy, thought to explain the accelerating expansion of the universe, exerts negative Absolute temperature pressure, which suggests it might have negative temperature. 0 0 Positive Negative ∞ “It is amazing experimental 3 LOW ENTROPY 2 MAXIMUM ENTROPY 1 LOW ENTROPY work,” says Allard Mosk of In the positive Adding more energy Once the spread is even, the University of Twente in temperature realm, reduces entropy. positive temperature the Netherlands, who helped heating increases the Welcome to the weird reaches a peak. spread of particle world of negative From here on the outline the 2005 theory that energies (entropy) temperature system turns negative drove the work. “It will give us a deeper understanding about what temperature really is.” ■
Jacob Aron
NOTHING is colder than absolute zero, but there’s a whole world beyond that point. For the first time, atoms have been put in a state where they can take on a range of negative absolute temperatures. This could deepen our understanding of subatomic particles – and perhaps even mysterious dark energy. Although we’re used to talking about negative temperatures, such as −10 °C, all temperatures on an ordinary thermometer are positive when measured in kelvin. On this scientific scale, which starts at absolute zero (−273.15 °C), temperature is determined by the kinetic energy of particles. So a gas of slow particles is colder than a gas of fast-moving ones, and absolute zero corresponds to the point at which particles stop moving completely, which is why nothing can be colder. That does not tell the whole story, though. Temperature also depends on how the particle 12 | NewScientist | 12 January 2013
Entropy
Atoms go beyond absolute zero