When an object is heated, its atoms can move with different levels of energy, from low to high. With positive temperatures (blue), atoms more likely occupy low-energy states than high-energy states, while the opposite is true for negative temperatures (red). 
 
 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. 
 
 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. 
 
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 Physicists don’t tend to use universal language freely, so since Lord Kelvin dubbed the base measure of his temperature scale “absolute zero,” that should be a sign that there is reason for the “absolute” nature of the measure to be taken seriously. 
 
 Unlike the centigrade scale, where 0 degrees marks the freezing point of fresh water, or the Fahrenheit scale, where 0 is the stable temperature of a mix of water, ice, ammonium chloride and sea salt, 0 Kelvin—absolute zero—is, theoretically, the coldest thing imaginable. This is the temperature when all motion comes to a crawl, when molecules stop quivering and atoms stop moving. 
 
 In Nature, Zeeya Merali reports on the work of Stefan Braun and colleagues, a group of physicists that managed to overthrow Kelvin’s absolutism by creating “an atomic gas with a sub-absolute-zero temperature for the first time.” 
 
 Through an elaborate approach—harnessing lasers and rapidly-flipping magnetic fields—the scientists figured out how to push atoms to temporarily take on energies below absolute zero. The scientists manipulated a cloud of potassium atoms trapped by lasers, says Merali: 
 
 At positive temperatures, the atoms repel, making the configuration stable. The team then quickly adjusted the magnetic fields, causing the atoms to attract rather than repel each other. “This suddenly shifts the atoms from their most stable, lowest-energy state to the highest possible energy state, before they can react,” says Schneider. “It’s like walking through a valley, then instantly finding yourself on the mountain peak.” At positive temperatures, such a reversal would be unstable and the atoms would collapse inwards. But the team also adjusted the trapping laser field to make it more energetically favourable for the atoms to stick in their positions. This result, described today in Science, marks the gas’s transition from just above absolute zero to a few billionths of a Kelvin below absolute zero. 
 
 Researchers imagine that if new materials could somehow be made out of below-freezing atoms, then they would probably act in some mighty strange ways. 
 
 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. 
 
 Even in physics, it seems, some rules are made to be broken. 
 
 More from Smithsonian.com: 
 
 The Coldest Place in the Universe 
 
 Absolute Zero 

 Over the past decades, researchers have made significant progress in cooling objects closer to absolute zero, the temperature at which all molecular motion reaches its minimum. This has allowed them to study unusual states of matter, like Bose-Einstein condensates, which behave quite differently from the materials we're familiar with. But absolute zero is as low as a temperature can get, and we can't actually reach it, so progress will ultimately be limited. 
 
 Maybe not. 
 
 As thermodynamics defines temperature, it's theoretically possible to have a negative value. Yesterday, a team of German researchers reported that they were actually able to produce a system with exactly that. They found that the negative temperature system was stable for hundreds of milliseconds, raising the prospect that we can study a radically different type of material. 
 
 To understand how temperatures can go negative, you have to think in terms of thermodynamics, which is governed by energy content and entropy. In a normal system, there's a lower limit on energy content—absolute zero—but no upper limit. If you start with a system at absolute zero and add energy, the atoms or molecules it contains start occupying higher energy states. With more energy, they start spreading out evenly among these states. This in turn increases the entropy of the system, since fewer and fewer atoms are in the same energy state. 
 
 Now imagine a system where there's an upper limit on the energy state an atom can occupy. As you add more energy, more and more atoms start occupying the maximum energy state. As this happens, entropy actually starts to go down, since an increasing fraction of the atoms begin to occupy the identical energy state. In thermodynamic terms, you've reached negative temperatures. 
 
 This has some pretty bizarre consequences. If you could maximize the entropy in the system, temperature becomes discontinuous—it jumps from positive to negative infinity. Strange things would happen if you bring it together with a system that has a normal temperature. "In thermal contact," the authors write, "heat would flow from a negative to a positive temperature system. Because negative temperature systems can absorb entropy while releasing energy, they give rise to several counterintuitive effects, such as Carnot engines with an efficiency greater than unity." 
 
 To create one of these systems, the authors set up an optical lattice of potassium atoms, chilled to near absolute zero. Under normal circumstances, these atoms repel, and thermodynamics behaves as we've come to expect. But the authors were able to switch things so that the atoms had attractive interactions. This created something that could be viewed as an "anti-pressure," which should cause the collection of atoms to collapse. It's only the negative temperature that keeps the cloud of atoms from collapsing in this set of circumstances. 
 
 It's important to emphasize that this negative temperature isn't some state "below" absolute zero. The atoms in this system still have energy, and the negative temperatures are reached through a sudden transition, rather than by gradually shifting to negative values by going past absolute zero. 
 
 Still, the system is more than just a quirky consequence of how we define temperature, since it really behaves quite differently from normal systems. The authors suggest it may help us model dark energy, in which the expansion of the Universe is a product of the sort of "negative pressure" that this system displays. 
 
 Science, 2012. DOI: 10.1126/science.1227831 (About DOIs).
– Science students learn that the coldest theoretical temperature isn't found on either the Fahrenheit or Celsius scales, but on the Kelvin scale—at "absolute zero," the point at which even atoms stop moving around. That might change, thanks to German physicists, though you might need a working knowledge of thermodynamics to understand exactly what they did. Samples from coverage: "Through an elaborate approach—harnessing lasers and rapidly-flipping magnetic fields—the scientists figured out how to push atoms to temporarily take on energies below absolute zero. ... Researchers imagine that if new materials could somehow be made out of below-freezing atoms, then they would probably act in some mighty strange ways." Smithsonianmag.com. "They found that the negative temperature system was stable for hundreds of milliseconds, raising the prospect that we can study a radically different type of material." ArsTechnica "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." LiveScience