Possible to reach negative kelvin?

Question

http://en.wikipedia.org/wiki/Negative_temperature
This article confused the heck out of me; can someone explain this to me?
http://en.wikipedia.org/wiki/Negative_temperature

Answer 1

The answer is, actually, yes. It's true that a system in thermal equilibrium cannot achieve negative Kelvin. It's usually cited that temperature cannot be defined for a system in nonthermal equilibrium, but this is technically not the case according to its strict definition. Temperature of a system is defined as the inverse of the partial derivative of entropy with reference to energy, holding volume constant. There are systems of nuclear spin states that can be made to have a negative temperature out of equilibrium with the background electron and phonon temperature (what we normally measure). Confusing enough for you?

The confusion most have is that they assume energy always increases with temperature. In fact, there is only a positive correlation for most systems since entropy *usually* increases with internal energy (recall definition of temperature above). Entropy is defined as the natural log of the number or states that correspond to the system's overall "macroscopic" description. Suppose you had a system of three atomic nuclei, for example, that could each have (quantized) angular momentum 1, 0, or -1 that, due to an imposed magnetic field, corresponded to energies 1, 0, or -1 units, respectively. A system with energy 2, then, would have three possible states: 1,1,0 or 1,0,1 or 0,1,1. Its entropy then would be ln(3)=1.1. Meanwhile, both the highest and lowest energy configuration have only one possible state: (-1,-1,-1) and (1,1,1), respectively. So, their entropies are both ln(1)=0. For the highest energy state, then, a change in entropy of 1.1 corresponds to a *negative* energy change of 1. The temperature of the highest energy state, then, is -1/1.1, or about -0.9 in our units. This is based on a discrete derivative. For a larger system, one would use a continuous approximation.

Nuclear spin demagnetization is used to refrigerate minuscule samples to microkelvin *equilibrium* ("real") temperatures by bringing the whole system into equilibrium with the spin state. That is, spin temp goes up while electron temp goes down, and both settle on an intermediate equilibrium, which must be positive. Phonons follow. I'm not sure the negative entropy vs. energy relationship is exploited for this, though.

Answer 2

I agree with the other answers as there is no way to remove more energy from a system once you have removed all the energy.

However, the negative temperature that they are talking about in this article is a special case of what occurs when you have reached the maximum temperature for a system and then add additional energy. It is no longer dealing with our concept of temperature from only the standpoint of heat or one object simply being hotter than another.

For us normal people, we normally only think of heat from 0 degrees K up to the point that something melts or vaporizes. 0 degrees Kelvin means there is no heat (no energy in the system). If we add energy, the molecules begin to move and the temperature goes up. This continues as long as you continue to add energy, the temperature should go up. In the normal range of what we are typically used to, the increase in temperature is related to an increase in entropy. For the cases they are describing, there comes a point where certain types of energy can be used to alter the entrophy of the system where the addition of more energy reduces the entrophy instead of increases it. They then call this a negative temperature.

So, can you reach negative kelvin? Based on the desctiption of what they are discussing, yes. However, the negative Kelvin they are describing never occurs at or below the temperature that we consider absolute zero. In fact, it lies on the other end of the scale above the hottest temperature that we could normally raise the temperature to.

To me, a lay person trying to grasp the significance of this article, I believe the best way for me to understand this is this way:

The kelvin scale measure from 0 degrees kelvin(absolute zero) to an infinitely hot object (infinite kelvin). Up to this point, additional energy increases the temperature and entrophy. At that point, certain types of energy can be used on the system such that the entrophy of the system decreases as more and more energy is added. When this additional energy is added, initially, the temperature is now called negative infinite Kelvin. Now, the "temperature" will continue to go down as more and more energy is added and the entrophy continues to decrease. When the entrophy approaches zero, the temperature will approach -0 degrees kelvin. So, by using this description, at point 0 kelvin and -0 kelvin, the entrophy is approaching 0. The difference is that an object approaching 0 kelvin would freeze you and one approaching -0 kelvin would burn you.


So, I believe I will leave any remaining discussion or correction to the experts. I wouldn't let this information confuse the point for you as no one will ever question you again about this, except perhaps in some specialized graduate level classes.

Answer 3

Yes, according to the article it is theoretically possible to reach negative kelvin. The important thing to note is at this point the substance being measured is HOTTER then 0 kelvin.

I read the article as such: 1) The kelvin scale measures the kinetic energy of a system. 2) At 0 kelvin there is no kinetic energy in the system. I think of it as, at this temperature the molecules/atoms stop vibrating, spinning, moving. 3) In the negative scale the kinetic energy is greater then +infinity (I know, an oxymoron). I think of it (probably incorrectly) as, at this point the molecules/atoms stop moving due to the sheer magnitude of the kinetic energy.

Answer 4

No, it's impossible to have an object's temperature go lower than absolute zero, which means reaching a negative Kelvin measure. Here's how my chemistry teacher back in high school explained it:

Heat always travels from a warmer body to a colder body. So to make an object colder, you have to place it in a surrounding colder than itself. So in order to make an object colder than 0 Kelvin, you'd have to put it in a room that's lower than 0 Kelvin, which you can't do because absolute zero is the coldest anything can get.

Having said that, there is a strange phenomenon called "negative temperature". This has to do with what "temperature" means, typically the average kinetic energy of something. This link explains things a little better.

Answer 5

The kelvin scale is an absolute temperature scale, which is tied to a zero that represents total cessation of atomic motion.

As quoted from your link:
"Temperatures that are expressed as negative numbers on the familiar Celsius or Fahrenheit scales are simply colder than the zero points of those scales. By contrast, a system with a truly negative temperature is not colder than absolute zero; in fact, temperatures colder than absolute zero are impossible. "

Answer 6

I dont think you can reach negative kelvin because zero Kelvin is the absolute minimum energy and atom have

Answer 7

its says that some temp systems have a negative temp, like farenheit or centigrade,
but kelvin does not.

kelvin is like this,
a bag of popcorn
( where each kernal is a tempature)
once its empty, you can't take any more tempature away.
the reason the other ones can go negative, is that their zero point is not a empty container.

farenheight is based on the freezing point of salt water in Norway I think. it freezes at 0 f.

I don't remember what C is based on.

Answer 8

Even article states you cant get colder then 0 kelvin. What is this question even asking. 0 K is it. Thats all she wrote. Finish, kaput. the end

Answer 9

lowest is -273 kelvin