I How can an atom reach less than absolute zero?

[Quantum Velocity]

I read a page about making atom under absolute zero.
https://www.livescience.com/25959-atoms-colder-than-absolute-zero.html
I don't understand very much, if you know pleas help me. Thanks!

 

[A. Neumaier]

They are actually hotter than infinite temperature. The point is that the physical relevant quantity is inverse temperature. Thus negative absolute temperatures are hotter than any positive one. I have given a more detailed explanation at https://www.physicsoverflow.org/28487

 

[hilbert2]

The article uses misleading terms, as is common in popular science. A thermodynamical absolute temperature can't be negative. That would mean a closed system that goes lower in entropy when you pump heat into it in a reversible process, and that has never been observed in any experiment. In the statistical mechanical definition of temperature, an apparent "negative temperature" is possible in a nonequilibrium system where you have a large set of atoms or molecules that are more likely to be in an excited state than in the ground state (this doesn't happen in any system that's in thermal equilibrium, no matter how high temperature it's in).

Talking about the temperature of a single atom doesn't make any sense no matter what definintion of temperature is used, the whole concept requires a very large number of atoms to be useful.

 

[A. Neumaier]

A thermodynamical absolute temperature can't be negative.

This only holds for systems that can move. See the references given in the link posted in #2.

 

[hilbert2]

This only holds for systems that can move. See the references given in the link posted in #2.

Would a system formed by the nuclear spins of small atoms locked in a solid matrix (i.e. interstitial sites of some crystal lattice) count as an immobile system? I saw something like this discussed on an MIT website.

 

[A. Neumaier]

Would a system formed by the nuclear spins of small atoms locked in a solid matrix (i.e. interstitial sites of some crystal lattice) count as an immobile system? I saw something like this discussed on an MIT website.

yes. The key question is whether the spectrum is unbounded. If it is, negative $\beta$ and hence negative energy is impossible. For Hamiltonians with a bounded spectrum, a canonical ensemble makes sense even for negative $\beta$, and this has been realized experimentally (not only recently).

 

[vanhees71]

This only holds for systems that can move. See the references given in the link posted in #2.

Formally it holds for systems whose energy is unbounded. For temperature in the usual sense to make any sense, the energy must be bounded from below, i.e., there should be a stable ground state. Then using the canonical ensemble you have
$$\hat{\rho}=\frac{1}{Z} \exp(-\beta \hat{H}), \quad \beta=\frac{1}{k_{\text{B}} T},$$
and if the spectrum of $\hat{H}$ is unbound from above, this has only a well-defined trace (normalized to 1 via the partition sum $Z$) for $T>0$.

For systems with a bounded Hamiltonian, $T$ can take any real value, and negative temperature means "population inversion".

 

[Quantum Velocity]

How can they hotter than infinite

 

[Gigaz]

The concept of negative temperature is controversial. Systems may have properties where you could attribute a temperature T<0, but those are not equilibrium states and usually, temperature is something which requires an equilibrium.

 

[Nugatory]

How can they hotter than infinite

That's answered partway through the LiveScience article in the first post.

We say that A is hotter than B if heat will flow from A to B when they are brought together.

Heat will flow from a system with negative temperature to a system with positive temperature, no matter how great the temperature of the positive-temperature system. In other words, no matter how hot the positive-temperature system, the negative-temperature system is hotter.

 

[Quantum Velocity]

ohhhhhhhhhhhhh

 

[A. Neumaier]

Systems may have properties where you could attribute a temperature T<0, but those are not equilibrium states and usually, temperature is something which requires an equilibrium.

One only needs local equilibrium, and the systems with $T<0$ are in local equilibrium.

 

[hsdrop]

hey guys,
I'm a little more than confused on this subject. Please bear with me as I stumble along with the limited education that I have and the questions that I'm struggling to answer. Also before anyone tells me that this thread is marked for undergrads when I'm asking for a high school level explanation. I just didn't want to start a new thread on the topic when there was one already up.
First off, I thought that you can only heat something so far (to vibrate at Planck length) and therefore can not be infinitely heated. I'm also having trouble understanding how to think of negative temperatures. Is it something that can only be shown with math or can someone describe what physically happens when hitting negative temperatures? Also where does the negative scale pick up at on the kelvin scale?

If it would be easier to start a new thread on the topic, let me know and I will.
Thank you to anyone ahead of time that will help me understand the concept.

 

[A. Neumaier]

I'm also having trouble understanding how to think of negative temperatures. Is it something that can only be shown with math

It needs math, but only simple high school math. The physically relevant scale is inverse absolute temperature $\beta=1/T$ (in appropriate units), which is a measure of coldness rather than heat. Because of historical accidents, hotness and not coldness was formalized first. In therms of coldness, the temperature is therefore $T=1/\beta$. Thus $T=0$ corresponds to infinite coldness; it cannot get colder. Zero coldness is already very hot and corresponds to infinite $T$, negative coldness is even less cold, i.e., even hotter. Due to the singularity of the inverse transformation at $\beta=0$, the resulting temperature scale is split into two differently arranged infinite parts, going from 0K (infinitely cold) through 273 K (freezing point of water) through 373 K (boiling point of water) to $\infty$ K = $-\infty$ K (extremely hot) to $-0$ K (the hottest conceivable state).

 

[hsdrop]

It needs math, but only simple high school math. The physically relevant scale is inverse absolute temperature $\beta=1/T$ (in appropriate units), which is a measure of coldness rather than heat. Because of historical accidents, hotness and not coldness was formalized first. In therms of coldness, the temperature is therefore $T=1/\beta$. Thus $T=0$ corresponds to infinite coldness; it cannot get colder. Zero coldness is already very hot and corresponds to infinite $T$, negative coldness is even less cold, i.e., even hotter. Due to the singularity of the inverse transformation at $\beta=0$, the resulting temperature scale is split into two differently arranged infinite parts, going from 0K (infinitely cold) through 273 K (freezing point of water) through 373 K (boiling point of water) to $\infty$ K = $-\infty$ K (extremely hot) to $-0$ K (the hottest conceivable state).

I'm sorry, I'm not understanding the math formulas at all or what each letter means in them. Is it possible to break it down even feather for me please. Also what's the difference between measuring hotness compared to measuring coldness? I am under the impression that hot and coldness is a matter of perspective.

 

[A. Neumaier]

measuring hotness compared to measuring coldness?

You measure one, and you take the inverse to get the other. $T$ is the temperature (what I called hotness) in Kelvin, $\beta$ the corresponding coldness.
Draw the function $\beta=1/T$ in a $(T,\beta)$ diagram, to see that the physically natural coldness order is transformed into a less natural hotness order, since positive and negative coldness transforms differently.

 

[hsdrop]

ok I believe I understood the math this time around. However I'm still having a bit of trouble picturing what it would look like physically (off the graff paper that is)
again thank you for the patient with my eager uneducated mind

 

[Quantum Velocity]

Heat will flow from a system with negative temperature to a system with positive temperature

i think heat must flow from the positive to negative for

no matter how hot the positive-temperature system, the negative-temperature system is hotter

 

[A. Neumaier]

heat must flow from the positive to negative

heat always flows from lower coldness $\beta=1/T$ to higher coldness. This is generally true. In case of positive temperature, it therefore flows from higher temperature to lower. In case of negative $T$ it also flows from small negative temperatures to large negative temperatures. But the pole at T=0 implies that all these negative temperatures are less cold and hence hotter than all the positive temperatures.

 

[Quantum Velocity]

i don't get it

 

[Quantum Velocity]

What is a

temperature system

 

[Nugatory]

What is a "temperature system"

Something that has a temperature. "positive-temperature system" is a just a short way of saying "something that has a positive temperature".

 

[Quantum Velocity]

So the cold one is hotter than the hot one so why heat flow from the cold to hot one

 

[hsdrop]

Ok, really dumb question then guys, if the cold is hotter than the hot why did they call it cold then?
Is there a way to describe the concept by comparison of some sort. Or perhaps a more visual representation of what -K might look like in what the concept is referring to?
Sorry for asking but the topic sounds fascinating and I would greatly appreciate a better understanding of it.

 

[Quantum Velocity]

i think that the temp = heat / entropy
so the cold is hot but it entropy is - so it temp is -

 

[hsdrop]

ok that makes a little more sense to me
so we have not gotten below 0K then ??

 

[Quantum Velocity]

just read the link in the beginning slowly and you will understand more

 

[Quantum Velocity]

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.

How?

 

[DrClaude]

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.

How?

You tell me. There is nothing magical about negative temperatures, and they do not lead to engines that are more efficient than the Carnot cycle.

In a sense, there is a lot of confusion ere that is simply based on an arbitrary choice for describing temperature. If instead of using $T$, we used $\tilde{\beta} \equiv -\beta = -1/k T$, then everything would make more intuitive sense. The coldest one could get to would be $\tilde{\beta} = - \infty$, and that temperature would actually be impossible to reach (3rd law). Then, as $\tilde{\beta}$ increases, things go from cold to hot. Normal thermodynamics systems would be limited to negative temperatures, up to $0_-$, while systems with a finite energy could have a positive $\tilde{\beta}$, up to $+ \infty$.

 

[Quantum Velocity]

You tell me. There is nothing magical about negative temperatures, and they do not lead to engines that are more efficient than the Carnot cycle.

But just read the link in the beginning they said so not me.

 

[DrClaude]

But just read the link in the beginning they said so not me.

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.

What can I say? Don't learn science from news articles. This is 100% genuine cow manure.

 

[f95toli]

But just read the link in the beginning they said so not me.

That part of the text was probably written by whoever is in charge of PR at their university. Press releases do -unfortunately- have a tendency to exaggerate the impact of results.

Personally I wouldn't even want to use the concept of temperature in this case. There are many non-equilibrium systems where the temperature is either ill defined or simply not meaningful.

 

[Quantum Velocity]

What can I say? Don't learn science from news articles. This is 100% genuine cow manure.

Umm... That why i asked you guy.

 

[Telemachus]

Here you have some references, it is in spanish, but you can look for the referenced papers. I think it is easier to understand if you think of the system of spins that was treated in the earlier works. In that case you can easily picture the population inversion for the absolute negative temperature.

http://www.fisica.uns.edu.ar/asignaturas/ver_archivo.php?q=ZqY&r=Y6Wh&f=1683514120_apuntes.pdf

 

[Telemachus]

i think heat must flow from the positive to negative for

Heat will flow from the hotter system to the colder one. That is the second law of thermodynamics. Actually, negative absolute temperature systems are hotter than positive ones, and are actually hotter than the infinite positive absolute temperature. You should take a look at the papers referenced in the link I gave below, it is pretty clear in there.

You have to think it this way: the definition for temperature can be given as:

$\displaystyle \left(\frac{\partial S}{\partial U}\right)_x=\frac{1}{T}$

But S is a concave function, this comes from thermodynamics (U is of course the internal energy). So to reach from positive to negative absolute temperatures, you must pass through a maximum of S:

$\displaystyle \left(\frac{\partial S}{\partial U}\right)_x=0\rightarrow T=\infty$.

With the system of spins is actually really easy to picture what happens. In the first place, you must have bounds in the energy so you can normalize the probability distributions.

If you place a system of spins in a magnetic field, the lowest energy state will be given with all the spins pointing in the same direction given by the external magnetic field. This is a zero entropy state, you have only one microstate for the given macrostate at T=0K. When you rise temperature, you will have some spins pointing in the direction of the external magnetic field, and others in the opposite direction, until you reach a maximum in entropy, a very disordered state, where there are many microstates compatible for the given macrostate at that temperature.

Now think of this situation, you start with the system in a positive absolute temperature state, close to T=0K, and suddenly you invert the external magnetic field: now the system will be trapped in a state of negative absolute temperature, with most spins pointing in the direction opposite to the external magnetic field. This is the population inversion that was mentioned before.

The thing is that the laws of thermodynamics and the usual statistical mechanics works in this negative absolute temperature states. You just change T by -T, and you see that the population of a given state will be:

$\displaystyle P_i \propto \exp{\left(\frac{E_i}{\kappa_B T}\right)}$,

so now the system tends to occupy the higher energy states, instead of the lower ones (which is the situation given by the sudden change in the direction of the external magnetic field).

 

[Quantum Velocity]

thx a lot Telemachus