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Mon Feb 4, 2013, 08:49 PM

 

Scientists Send A Cloud Of Atoms Plunging Below Absolute Zero

http://www.popsci.com/science/article/2013-01/scientists-send-cloud-atoms-plunging-below-absolute-zero



Absolute zero--thatís zero degrees Kelvin, or -459.67 degrees Fahrenheit--is understood by textbook definition to be the absolute coldest anything can be, a temperature threshold at which atoms actually lose all of their kinetic energy and stop moving completely (or at which entropy reaches its lowest value). There can be nothing stiller than completely still, and hence absolute zero is as low-energy as something can go. Right? But researchers have discovered thatís not exactly the case. By messing with the distribution of high- and low-energy atoms within a system, a team of physicists at the University of Munich in Germany has created what it defines as a negative temperature system--one that has a temperature south of absolute zero.

The researchers describe their system in terms of hills and valleys (picture this). At absolute zero, a group of atoms has no energy and is motionless, and thus all atoms are at the bottom of the valley. As the temperature rises above absolute zero that changes, but not all at once--some particles gain a lot of energy, and some gain just a little, so now the atoms have different energies and are spread along the slope of the hill, stretching from valley to hilltop. Physics says the most disordered state of this system occurs when there are an equal number of particles at every point along the slope, and thatís the top of the positive temperature scale--increase the energy any further and the particles would no longer be evenly spread, lowering the systemís entropy (for a more detailed description of all this, click through to New Scientistís write-up).

The point is, youíre in the positive temperature scale when you have some number of high-energy particles atop the energy hill and some larger number of particles in the valley. So to reach their theoretical negative temperature scale the U. of Munich researchers forced that model to flip, placing more high-energy particles atop the hill than in the valley. Says New Scientist:

The resulting thermometer is mind-bending, with a scale that starts at zero, ramps up to plus infinity, then jumps to minus infinity before increasing through the negative numbers until it reaches negative absolute zero, which corresponds to all particles sitting at the top of the energy hill.

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Reply Scientists Send A Cloud Of Atoms Plunging Below Absolute Zero (Original post)
2on2u Feb 2013 OP
caraher Feb 2013 #1
Posteritatis Feb 2013 #2
eppur_se_muova Feb 2013 #3

Response to 2on2u (Original post)

Tue Feb 5, 2013, 01:14 AM

1. *sigh*

Science journalists need to take senior-level undergraduate statistical physics. There is NOTHING new or amazing about "negative temperatures" and examples of physical systems that can be described by them have been known for decades. Yes, the concept is not immediately intuitive based on our ordinary experiences with temperature, but it's pretty simple mathematically.

It seems like a quite a few research groups have caught on to science writers' weak spot for negative temperatures... just about once a month there's some breathless prose about the amazing breakthrough of some physicists that turns into a lengthy pop science piece on negative temperature, completely obscuring whatever actual accomplishments the research may represent.

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Response to caraher (Reply #1)

Tue Feb 5, 2013, 06:24 AM

2. I dunno if I'd blame the researchers

Science journalists go far out of their way to make up interpretations of relatively mundane stories all the time. I keep thinking of that one a few weeks ago where some researchers said that the formation of galaxy filaments looked roughly similar to neurons, which spawned a bunch of hysterical stories about how "science proves that the universe is a giant brain."

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Response to 2on2u (Original post)

Tue Feb 5, 2013, 09:51 PM

3. "WHAT IT DEFINES AS a negative temperature system" ...

You can define anything as whatever you want, but that doesn't make it meaningful.

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