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Author Topic: Less than absolute zero???  (Read 1125 times)
Agent D.
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« on: January 05, 2013, 11:19:42 AM »

So apparently, scientists have figured out how to get stuff colder than 0° kelvin (aka "Absolute Zero").

http://newrisingmedia.com/all/2013/1/4/scientists-reach-a-temperature-lower-than-absolute-zero.html

Interesting story, honestly. Feels like the makings of mass effect...
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Mickeymac92
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« Reply #1 on: January 05, 2013, 11:26:42 AM »

Now this is the kind of discovery I was hoping to hear within my lifetime. It's also interesting to hear that these Sub-Zero-Kelvin particles apparently float? Now this could be quite useful.
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Hathen
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« Reply #2 on: January 05, 2013, 11:31:41 AM »

colder than 0° kelvin
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0° kelvin
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°

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MeshGearFox
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« Reply #3 on: January 05, 2013, 12:11:46 PM »

http://en.wikipedia.org/wiki/Absolute_zero#Negative_temperatures

Wiki'd it. Apparently this is something different, not colder than absolute zero, and kind of a different thing altogether.
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Hathen
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« Reply #4 on: January 05, 2013, 01:13:03 PM »

From what I'm reading it's not actually that they created an all-new type of particle that literally went into negative temperature, but rather fired some magic lasers that made the average temperature of every particle in the gas closer to true absolute zero (since measured temperature is just a probability distribution of sorts), which ends up being read as being lower than absolute zero without actually being so.

That is, particles with 0 energy is still the theoretical minimum, they didn't actually create particles that had negative energy or something like that.
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Kevadu
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« Reply #5 on: January 05, 2013, 02:49:47 PM »

So I read the actual paper and now I want to smack the author of that link silly.  He wrote, "For example, while atoms are usually pulled downward by gravity, those below absolute zero float upwards instead."

There is no mention of anything of the sort in the actual paper.  This is complete fiction.

So what do they actually mean by negative temperature?  The definition they are using for temperature is that the probability of being in a state i with energy E_i is proportional to exp(-E_i / (k_B * T)), where k_B is Boltzmann’s constant.  You might note that since we're talking about discrete energy states this is an inherently quantum mechanical definition, but the idea is simply that there is a decaying exponential distribution that favors lower energy states.  So what if T goes negative?  Well, it's not a decaying exponential anymore, and for that to make any sense you need a system will a well-defined upper bound energy, which the paper spends a while talking about.

But is this "negative temperature" distribution the same as being cold?  Actually, no.  The paper even says, "...emphasizing that negative temperature states are hotter than positive temperature states, i.e., in thermal contact, heat would flow from a negative to a positive temperature system."

So is this just semantics?  Maybe in some sense.  It's interesting because the negative temperature state is actually stable (under the right conditions), not some rapidly-decaying pseudo-state.  They mention some potential applications in the paper, but this isn't anything that's going to change the world.  But I guess that's less exciting than antigravity...
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Hathen
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« Reply #6 on: January 05, 2013, 03:22:00 PM »

I'm a bit confused, how can the negative temperature system be hotter than an absolute zero temperature system if the idea is that they're making it so the upper bound temp isn't reached as often, thus the average temperature of the system should be cooler?

And yeah, "journalists" that don't bother reading a single word of the source material need to go to a special level of hell where they spend eternity playing Chinese Whispers, always on a losing team.
« Last Edit: January 05, 2013, 03:29:38 PM by Hathen » Logged
Kevadu
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« Reply #7 on: January 05, 2013, 04:32:51 PM »

It might be better to think about energy rather than temperature for a moment.  Temperature is a parameter which describes a statistical distribution of energies.  It only has meaning in an ensemble.  Energy is a bit more universal and more easily understood.

In a system at absolute zero, every single particle is in its ground state.  That means the energy of the system is the smallest it can possibly be (that's kind of the definition of a ground state).  So there is no way for energy to decrease from the absolute zero system without introducing some sort of exotic (and completely hypothetical at this point) negative energy or something (which I guess is what the author of the original link thought was happening here...).  That's why absolute zero is typically viewed as an absolute limit.

But that's in terms of energy, not temperature.  Again, temperature is a parameter that describes a statistical distribution.  In turns out that for some distributions, it is possible to have a well-defined system in which that parameter is negative.  The shape of the statistical distribution is quite different from a positive temperature, but make no mistake here:  there is no question that the "negative temperature" system has more energy than at absolute zero.  The distribution is different, but there are still particles not in their ground state, and nothing has somehow gone below ground state energy.

So is this just semantics?  A funny definition of temperature?  Well, it's not quite that simple.  You might ask why the shape of the energy distribution is so important in the first place.  Certainly it is possible to artificially construct a system with a different energy distribution, but such a system is not stable.  It's not in equilibrium.  Turns out that the mathematics behind temperature are quite universal, and given enough time to reach equilibrium all systems will eventually reach a thermal distribution.  So it really is a special distribution.

So here is a system that's stable and just happens to be described by a thermal distribution with a negative temperature.  It's very hard to think that's just a coincidence, thus describing this system as having negative temperature isn't unreasonable.  But it probably differs from people's intuition about temperature quite a bit.
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Hathen
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« Reply #8 on: January 05, 2013, 06:41:47 PM »

I understood the idea that Temperature being a slightly different from the measurement of energy, I'm just not quite understanding why the negative temperature is actually at a higher energy state than the positive temperature, because if I'm understanding this correctly, they basically forced more particles at any given moment (within the probability distribution) to be in the ground state. If that's the case, wouldn't the new negative temperature system have, on average, lower energy? Why would it be hotter than the positive temperature system, then?
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Kevadu
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« Reply #9 on: January 05, 2013, 07:45:49 PM »

It's 'hotter' because when temperature goes negative the probability distribution is actually emphasizes high-energy states over low-energy states (which is why an upper bound energy is so important, because otherwise the probability distribution would just explode to infinity).  So no, there aren't more particles in the ground state.  At absolute zero all particles would be in the ground state, but if you keep going 'down' in temperature you start having particles in excited states again, but now with a totally different shape to the probability distribution than a positive temperature.
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Aeolus
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« Reply #10 on: January 05, 2013, 10:20:38 PM »

I came here to post that that article is a load of bunk but I got beaten to it.

Still, that article is a load of bunk just by virtue of the fact that it's relatively impossible to measure absolute zero due to the fact that anything you put into a 'zone of absolute zero' to measure said 'zone of absolute zero' is putting energy back into the 'zone of absolute zero' which raises the temperature and now you no longer have a 'zone of absolute zero'.

Not to mention fun things like that anything next to a 'zone of absolute zero' is putting energy back in said zone. It's like trying to dig your way to the bottom of the sand in a desert. Unless you take out enough sand from the desert so that the remaining sand can no longer cover the desert as well as stop all wind, rain, and other forms of movement that might cause neighboring grains of sand from slipping back into the hole you're never going to accomplish it. Especially if you depend on said sand to function.

Amusingly though, everything slows down in lower temperatures. Even light if you get it low enough.
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Kevadu
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« Reply #11 on: January 05, 2013, 11:13:34 PM »

In case there's anyone who doesn't want to read the paper (or can't, 'cause you need journal access...), here's a general news article that actually does a pretty decent job of explaining it:

http://www.huffingtonpost.com/2013/01/04/absolute-zero-record-setting-negative-temperature_n_2404666.html
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Agent D.
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« Reply #12 on: January 06, 2013, 02:19:29 PM »

So basically, it's like reversed Lufia 2 hp logic. When you heal an enemy who has maximum hp (i.e. the egg dragon), it restarts their hp counter, effectively giving them whatever you healed their hp for minus 1 as their new hp total.

Sorry, I felt this conversation needed some overall silliness added.
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MeshGearFox
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« Reply #13 on: February 18, 2013, 02:33:45 AM »

[02:06] <HotdogNub> Lately I've been thinking of binary oppositions/binary systems
[02:06] <HotdogNub> and how they're often describe in a boolean sense
[02:06] <HotdogNub> Either 1 or 0; presence or absence.
[02:07] <HotdogNub> Life, and death as the absence of life. A marked phoneme, and an unmarked phoneme as the absence of the mark.
[02:07] <HotdogNub> But if you look at it as a 1/-1 system, with no 0...
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