Researchers force a gas to a temperature below absolute zero

Jan 04, 2013 by Bob Yirka weblog

(Phys.org)—A team of physicists in Germany have succeeded in forcing a gas to become colder than absolute zero. Using lasers and a magnetic field to manipulate an ultra-cold gas, the researchers, as they describe in their paper published in the journal Science, managed to coax the temperature of the gas to a few billionths of a Kelvin below absolute zero.

was first defined by Lord Kelvin back in the mid 1880s, as the lowest possible temperature state, where atoms stop moving. The temperature scale bearing his name starts at that lowest point, but over the past several decades, scientists have discovered that there are exceptions to the rule and that at least theoretically, it should be possible for a system to produce conditions where temperatures fall lower than absolute zero. This is possible, they say, because the temperature of a system is generally considered to be the average energies of the particles in it. Most hover around a certain point, with a few moving to higher levels. But, when the system is turned upside down, with most of the particles exhibiting higher , and just a few have lower energy, the system is reversed as are the temperature signs, indicating temperatures below absolute zero.

To turn such a system upside down in the real world, the started by chilling a made up of potassium atoms to near absolute zero. They used lasers and magnetic fields to force the atoms into a lattice pattern. At temperatures above absolute zero, the atoms naturally want to repel one another, keeping the system stable. But by adjusting the lasers and , the researchers were able to force the atoms to attract one another, essentially, turning the system on its head. At positive temperatures, they note, such a system would quite naturally be unstable – to force it to be stable, the team also adjusted the lasers that held the atoms trapped in place. Doing so, they report, resulted in the gas transitioning to a temperature below absolute zero.

While the achievement won't likely result in the creation of such systems for practical purposes, it does help better understand the principle of temperature, and may, some suggest, help explain other still mysterious phenomenon, such as why the universe is continuing to expand, despite the pull of gravity – which some have attributed to a force called dark energy. There are parallels, the researchers say, between the way the gases in a sub-absolute zero system want to collapse, but don't due to the negative absolute temperatures and the negative force preventing the universe from doing the same.

Explore further: New approach to form non-equilibrium structures

More information: Negative Absolute Temperature for Motional Degrees of Freedom, Science, 4 January 2013: Vol. 339 no. 6115 pp. 52-55 DOI: 10.1126/science.1227831

ABSTRACT
Absolute temperature is usually bound to be positive. Under special conditions, however, negative temperatures—in which high-energy states are more occupied than low-energy states—are also possible. Such states have been demonstrated in localized systems with finite, discrete spectra. Here, we prepared a negative temperature state for motional degrees of freedom. By tailoring the Bose-Hubbard Hamiltonian, we created an attractively interacting ensemble of ultracold bosons at negative temperature that is stable against collapse for arbitrary atom numbers. The quasimomentum distribution develops sharp peaks at the upper band edge, revealing thermal equilibrium and bosonic coherence over several lattice sites. Negative temperatures imply negative pressures and open up new parameter regimes for cold atoms, enabling fundamentally new many-body states.

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Chromodynamix
3 / 5 (2) Jan 05, 2013
Is it April 1st?

If you go below Absolute Zero, it's not Absolute Zero then, is it?
VendicarD
3 / 5 (2) Jan 05, 2013
That would depend on your definition of temperature wouldn't it?
VendicarD
4 / 5 (4) Jan 05, 2013
The definition of temperature is..

T = k*(d(Entropy)/d(Energy))**-1

k is positive.

Therefore, if the rate of change of energy or entropy have opposite signs, then the absolute temperature of the system is negative.

Normally if you add energy to a system it's entropy also increases, and hence it's temperature increases.

If you can contrive a situation in which entropy decreases as you increase a aystem's energy... ie., the system becomes more ordered as you add energy then it's Absolute temperature is negative.

It all comes down to definition, and how precise definitions do not necessarily map expectations exactly onto real world behaviour.

There is nothing magic here. No extraction of energy from the vacuum, or from alternate universes, or violation of the principles of thermodynamics.
johanfprins
1 / 5 (3) Jan 05, 2013
Temperature = kinetic-energy: To have a temperature below absolute you must have negative kinetic-energy: The expression for kinetic energy is a quadratic form. It can only be negative if you can have an imaginary momentum. Basically it is BS!

The atoms in this experiment attract one another because they are in the process of forming a single, holistic matter-wave which will not exist of separate atoms after it has formed. I suspect that all matter-waves will do this at absolute zero. Since there can then not be any separate entities which can move relative to one another; there can only be potential energy; not temperature: Positive or negative!
niels_walet
not rated yet Jan 09, 2013
This seems to be a case of defining temperature from the microcanonical ensemble (fixed N and E), and then deriving a temperature using thermodynamic relations (steepest descent approximation). For finite particle number, you can thus obtain all kind of counter-intuitive effects, including the one quoted in their title. The physics is interesting, but not that strange...
I think the authors have chosen their words wisely to get accepted in Science; Any other journal would probably have rejected this idea as not sensible and overhyped.
This is not temperature as we know it, Jim!
VendicarD
not rated yet Jan 09, 2013
Wrong.

"Temperature = kinetic-energy" - Johanfprins

The definition of temperature is..

T = k*(d(Entropy)/d(Energy))**-1

k is positive.
johanfprins
1 / 5 (2) Jan 09, 2013
Wrong.

"Temperature = kinetic-energy" - Johanfprins

The definition of temperature is..

T = k*(d(Entropy)/d(Energy))**-1

k is positive.


Entropy is defined in terms of heat (kinetic-energy) and temperature (caused by kinetic energy) (S=Q/T). You are thus using a circular argument to define temperature in terms of entropy; which is nonsensical.
antialias_physorg
not rated yet Jan 09, 2013
Entropy is defined in terms of heat (kinetic-energy)

Only in ideal gases which this is expressly not (since it has a population inversion and limited number of modes)

For those gases you have to use the definition of temperature according to statistical mechanics:
http://en.wikiped...perature

And there you can very well end up with negative Temperature:
http://en.wikiped...perature

So yes: if you use definition of temperature that doesn't apply to what you are measuring then there is no temperature lower than 0K. But using inapplicable methods for an argument makes very little sense.
johanfprins
1 / 5 (2) Jan 09, 2013
For those gases you have to use the definition of temperature according to statistical mechanics:
Temperature is a statistical parameter!
So yes: if you use definition of temperature that doesn't apply to what you are measuring
An example please! It is impossible to have temperature unless you have separate entities moving relative to one another
But using inapplicable methods for an argument makes very little sense.

What is inapplicable? You are posting BS as you usually do! You are a master at BS-SING!

antialias_physorg
not rated yet Jan 09, 2013
An example please!

Read the wikipedia link. It's explained quite plainly. You can state "it's impossible" or "you're BS-ing" all you want - that doesn't make it so.

That definition of temperature is the one used in science. If you want to use a different one then you may do so - but don't expect that an argument along the lines of:
"According to my personal definition of temperature taken from an unrelated field it ain't so" impresses anyone.
johanfprins
1 / 5 (2) Jan 10, 2013
An example please!

Read the wikipedia link. It's explained quite plainly.
I have read it and it is unadulterated bullshit.

That definition of temperature is the one used in science.
Which science? Voodoo I suppose! You are the best Voodoo scientist I have ever come across. Why is that definition of temperature not used when students are informed for the first time what temperature is?
antialias_physorg
not rated yet Jan 10, 2013
Why is that definition of temperature not used when students are informed for the first time what temperature is?

Because the study of entities with limited modes is not the first thing you learn. Getting to grips with tmperature is hard enough at that point. I've been teaching a lot and it's not productive to dump everything on a student at once.

That's how it is, you know? First you learn the basic stuff and then you study the more advanced stuff. (Pratchett calls it very fittingly "lies to children". You have to tell them 'easy' lies first so that they can gradually understand the more complex stuff.)

Sure it'd be 'right' to teach them the correct stuff right away - but student can't handle e.g. relativity right away. So you start slow (by going for Newton first, then Dirac, etc.) and build up to it - even though they aren't 'right'.

Telling kids in first grade that time isn't constant will just confuse them.
VendicarD
not rated yet Jan 10, 2013
Wrong again!

"Entropy is defined in terms of heat." - Johan

Entropy is defined as S = -B * SUM over i (P sub i ln(P sub i))

Where P sub i is the probability of the system being in the i the state.

B = Boltsmann's constant.

If the probability of the system being in any state is equivalent then ...

S = B * Ln(Ox) where Ox is the number of states available.
VendicarD
not rated yet Jan 10, 2013
Originally, temperature was what thermometers measure, but since that is the expansion of a liquid (typically alcohol or mercury) other more fundamental definitions were sought.

With the atomic theory of gasses we discovered that PV/nR = T

PV/n = k*(dE/dS)

It was precisely the fact that T <> E that lead to the above relation in an effort to discover what T actually was.

"Why is that definition of temperature not used when students are informed for the first time what temperature is?" - Johan
VendicarD
not rated yet Jan 10, 2013
Then you don't understand it, and should read it again.

"I have read it and it is unadulterated bullshit." - Johan

Science is filled with examples of where terms have been re-defined over the years.

You have to be careful to identify where those instances are, and what their ramifcations are.
johanfprins
1 / 5 (2) Jan 10, 2013
Science is filled with examples of where terms have been re-defined over the years.
If it is necessary to do this so that the original definition is wrong, the original definition must be discarded. The original definition is still correct in that only zero kinetic-energy means zero temperature. To get negative temperature, you need negative kinetic-energy. Since the latter is impossible, negative temperature is impossible, no matter which new Voodoo semantics you want to employ!

You have to be careful to identify where those instances are, and what their ramifcations are.


I am! But you are not! There is no need to generate Voodoo physics in order to claim the physically impossible!
antialias_physorg
5 / 5 (1) Jan 10, 2013
The original definition is still correct in that only zero kinetic-energy means zero temperature.

This definition is correct ONLY in the case of infinite possible modes. Definitions are only correct within the context which they are defined in.

(Another example would be the word "Entropy" which has different meannigs in thermodynamic or information-theory contexts. They seem similar in many respects, but you have to know which context you're in to understand which definition to use)

The context your definition requires is not present in the stuff described in the article - therefore that definition is not the correct one to use in this case.

It's a bit like saying: "Acceleration increases linearly with force applied" (Newton). This is pretty much 'true' in any but relativistic contexts.
Go to a relativistic context and you have to use a different definition. You can't just stick to the definition you feel 'sounds good'. You have to use the definition that applies.
johanfprins
1 / 5 (2) Jan 10, 2013
Another example would be the word "Entropy" which has different meannigs in thermodynamic or information-theory contexts.
It should not be so since entropy has nothing to do with information!

The context your definition requires is not present in the stuff described in the article - therefore that definition is not the correct one to use in this case.
So you are saying that since the article is Voodoo you require a Voodoo definition?

It's a bit like saying: "Acceleration increases linearly with force applied" (Newton).
This is NOT the original definition: The original definition states that force is equal to the time-change of momentum. This is still true when using Einstein's Special Theory of Relativity. One does not need to replace Newton's original definition with a Voodoo definition!
johanfprins
1 / 5 (2) Jan 10, 2013
Don't worry if you don't understand the quantum mechanics - the best physicists don't understand it too...


You are the guy who has proved time and again that you cannot even understand Kindergarten physics: Physics is not about what Nature is not, but what it probably is: To define an electron as not being a wave and not being a "particle", defines what you are! A moron?
antialias_physorg
5 / 5 (1) Jan 10, 2013
It should not be so since entropy has nothing to do with information!

Entropy is a term in information theory which was named that way because the formula looks exactly like the one in thermodynamics.

So much so in fact that this lead to some research into whether the universe could be described based on information (which produced some good insights in both areas but ultimately wasn't a success).

Naming something because of similarities is not uncommon (E.g. stuff like 'flow' or 'spin' are used in various disciplines in a different manner - depending on context. That's not really a problem because if you know your way around these disciplines you can't get them mixed up. )

So you are saying that since the article is Voodoo you require a Voodoo definition?

No. I'm saying the article uses the definition that is the correct one for this discipline.

If you want to use a definition from another discipline that doesn't apply here then that's your problem.
johanfprins
1 / 5 (2) Jan 10, 2013
Entropy is a term in information theory which was named that way because the formula looks exactly like the one in thermodynamics.
That does not mean that it is entropy!

So much so in fact that this lead to some research into whether the universe could be described based on information
I know about this stupid research: My response is the same as that of John Bell: "Whose information?"

but ultimately wasn't a success
Not surprising!!

No. I'm saying the article uses the definition that is the correct one for this discipline.
That does not mean that it is temperature just because you want to call it temperature! It is foolish to argue as you do! And utter nonsense.

antialias_physorg
5 / 5 (1) Jan 10, 2013
That does not mean that it is entropy!

It's a measure of order in inofrmation thory and it's a measure of order in thermodynamics - so it probably seemed sensible to the guys at the time to name it entropy, too.

That does not mean that it is temperature just because you want to call it temperature!

No. It is temperature because what they used IS the definition of temperature. If you used the false definition then you're screwed up.

You'd be like a guy commenting on a paper in information theory using the entropy definition by saying "that's not what entropy is because thermodynamics defines entropy thus...".

Get it through your head: This isn't about ideal gases - and harping about that it should use the definition of temperature that only applies to ideal gases is stupid.
antialias_physorg
5 / 5 (1) Jan 10, 2013
but ultimately wasn't a success


Not surprising!!

Well, you have to look at it in historical perspective. There's all kinds of things that are tried on other fields when a new field crops up.
When topology was new it was tried on spacetime (string theory).
Fractals were tried for all sorts of things.
When holograms were en vogue we had people try out the holographic principle on the universe.
Wave theory was tried out on quantum mechanics
etc.

You may remember some of those - the fractal and topology ones weren't that long ago.

That's really what happens in science a lot: you find something new and then you go back over all the old stuff and see if it teaches you something about it - where it fits in.
Sometimes it works, sometimes it doesn't. But you always learn (as opposed to when you don't even try). Because at the very least it allows you to look at problems from another point of view.
johanfprins
1 / 5 (3) Jan 10, 2013
It's a measure of order in inofrmation thory and it's a measure of order in thermodynamics - so it probably seemed sensible to the guys at the time to name it entropy, too.
No!! Only an idiot will confuse entropy in this manner! It is like saying "All elephants are grey", and then concluding that all grey animals are elephants". Only a total clown will reason in this way!

No. It is temperature because what they used IS the definition of temperature.
No it is not! Why do you need an alternative definition?
You'd be like a guy commenting on a paper in information theory using the entropy definition by saying "that's not what entropy is because thermodynamics defines entropy thus...".
This is so obvious that it is amazing that a bunch of clowns could call a formula in information theory "entropy" just because it turns out that the same formula might apply for another reason.
johanfprins
1 / 5 (2) Jan 10, 2013
@ antialias-physorg,

Your "historical perspective" is such nonsense that I cannot believe that anybody who can read and write can spout this!!

The only parts that should not be rejected out of hand are so obvious that I already knew it when I was lying in my cradle: For example: "That's really what happens in science a lot: you find something new and then you go back over all the old stuff and see if it teaches you something about it - where it fits in.". When did you realize this for the first time? Although it is amazing that with your limited intellectual capacity you can realize this; it is not an excuse to conclude that apples and pears are the same because they both grow on a tree!

antialias_physorg
5 / 5 (1) Jan 10, 2013
. When did you realize this for the first time?

Well - it certainly sounded like you needed a refresher.

Your "historical perspective" is such nonsense that I cannot believe that anybody who can read and write can spout this!!

Shannon (who's sometimes called the father of info theory) coined the term information enttropy in his seminal paper from 1948: "A Mathematical Theory of Communication" - which makes direct reeference and comparison to entropy from statistical mechanics.

So, yeah, I'd say the historical reference is apt and you're just pulling stuff out of thin air (as usual).
johanfprins
1 / 5 (2) Jan 10, 2013
Shannon (who's sometimes called the father of info theory) coined the term information enttropy in his seminal paper from 1948: "A Mathematical Theory of Communication" - which makes direct reeference and comparison to entropy from statistical mechanics.
Really!? Now I am going to use an Afrikaans word: Any POEPHOL knows this. It still does NOT make it entropy! I am getting sick and tired of your pompous arrogance which causes your patronising posts as if only you know what has happened in the past and what is happening in physics. Just accept it that by stating what you have read, without being able to evaluate it (which is your shortcoming) you only keep on confirming that you are an IDIOT!

frajo
5 / 5 (3) Jan 12, 2013
ValeriaT (a clone of Zephir) gives natello (another clone of Zephir) a score of 5.
Zephir (the AWT preacher) just can't stop masturbating in front of other users.
ValeriaT
1 / 5 (1) Jan 12, 2013
I've nothing to do with johanfprins posts. This is my first post in this thread.
johanfprins
1 / 5 (1) Jan 12, 2013
I've nothing to do with johanfprins posts. This is my first post in this thread.


Thank God for that: I do not mind to be wrong, but to post idiotic stuff like AWT, I will need to be demented !!!
ValeriaT
not rated yet Jan 12, 2013
You'll die soon anyway, it has no meaning to oppose you.
johanfprins
1 / 5 (1) Jan 12, 2013
You'll die soon anyway, it has no meaning to oppose you.
You might be wrong!! I have prayed many times that people like you should die at a young age, and then it happened! I will start to pray for you VERY SERIOUSLY! Good luck on the other side.