# Reaching Absolute Zero.

As I’ve already said on these boards, it has been a while since I took chemistry or even physics for that matter. But some parts of the subject still fascinate me. One thing that has always fascinated me is absolute zero. In case you don’t already know, absolute zero is the coldest anything can get. It is also the point when all molecular motion ends IIRC. According to my dictionary, absolute zero is minus 273.15 degrees Celsius, minus 459.67 degrees fahrenheit, or simply zero degrees Kelvin, of course.

Now is where it gets interesting. According to my high school physics teacher, theoretically nothing can ever reach absolute. IIRC the way he explained it is in order for all molecular motion to end, molecules have to stop. And yet whenever they bump into each other, they are deflected and move, thus no molecule can pass on the energy of absolute zero to another (forgive me if my description seems unscientific–as I’ve said, it’s been a while ). But what if they bump into each other while they are both moving in the same direction one student asked the teacher. No matter, he said, they’d still be deflected and continue moving. My classmates and I didn’t believe him.

My question is simple this, two-part: (1) what is the closest any man-made conditions have got to being absolute zero and (2) what is the closest any natural process in the universe has gotten to this temperature? As you can see, these are important questions, because if anything ever reached absolute zero, it would disprove this scientific theory.

Looks like MIT got to 450 picoKelvins:Absolute zero - Wikipedia, and that Boomerang Nebula is the coldest known place outside of Earth.

As has been pointed out on these Boards before, having an object – even a single particle – reaching absolute zero would mean that both its position and its momentum (zero) would be perfectly defined and, in principle, knowable. That violates the Heisenberg Uncertainty Principle. the HIP isn’t just an odd conjecture or a consistent observation, but is, if the de Broglie Wave Theory of matter is correct a pretty much inviolable condition of matter. Everything seems to indicate that the wave picture of matter IS correct, so that implies that you can’t ever reach absolute zero. as the previous post shows, though, it still lets you get damned close.

I was taught that the reason you couldn’t get anything to absolute zero was a simple result of the laws of thermodynamics (1st law?). If you have a region whose temperature is 0 Kelvin, then it has to be next to another region which is >0K. Since nature seeks equillibrium, heat energy will flow from the hotter region to the colder.

Come to think of it, that’s probably a pretty weak reason. Plus, the Heisenberg Uncertainty Principle is a much cooler explaination.

I think it’s more correct to say that the HUP is a consequence, rather than a cause. That is, one of the reasons you cannot measure certain pairs of quantities is because of things like the lack of perfect thermal isolation that you mention, and not a reason unto itself for our inability to attain absolute zero.

Nova did a show on this very subject recently. You can watch it online. If you want to cut to the chase go to chapter 9.

I’m not very knowledgeable about this but I have seen threads on it before, and I seem to recall that absolute zero does not mean the absence of all molecular/atomic motion but rather the lowest possible energy level (which I think is not zero energy). Any physicists out there able to fill in the gaps?

If a system is at absolute zero it is in it’s ground state, the HUP does not stop a system from being in it’s ground state.

The HUP is fundamental, so can be used to consider perfectly isolated systems.

Yes, you’ve pretty much hit the nail on the head.

And that is why “the temperature at which all motion ceases” is NOT the correct explanation of Absolute Zero. But how would you define the “ground state” for a complex assembly of particles, rather than a simple electron in a well, for instance?

The ground state is simply the lowest allowable energy state of the system. A ground state must exist even if it is degenerate and no matter how complex the system.

OK, exactly how did scientists extrapolate the find a value for absolute zero? Is it a simple linear relation? Perhaps they determined the limit of Pv = nRT as v appraoches zero volume, or something?

I’m pretty certain it’s a defined temperature on the ITS-90, i.e. when matter is at its lowest possible energy level, it is defined to have a temperature of 0 K.

That just begs the question of how they found the Celsius measure of absolute zero, though.

Because 0 degree Celsius = 273.15 degrees Kelvin. Simple subtraction dictates 0 degrees Kelvin is equal to -273.15 degrees Celsius.

Same way they found AS in Farenheit, they used the C-F conversion formula.

Swell. Where did that figure come from, then?

I’m sorry, you’re right I’m stupid. I didn’t realize that.

Can a moderator ban me for being so stupid. I’ll bite myself if it makes you feel better.

And I’m serious, I’m stupid and deserve to die.

Absolute zero was first determined in Celcius by linearly extrapolating the Pressure-Temperature curve of a gas to find at which temperature the gas has pressure 0. Turns out the P-T curves of all gases when approaching P=0 converge to -273.15 C.

An absolute temperature scale can also be determined independently of any thermodynamic substance using the idea of Carnot efficiency. Details here for the interested: http://www.maxwellian.demon.co.uk/art/esa/temperature/carnotcycl.html

Er, yes, I’m aware of this. I wasn’t really asking, I was pointing out a logical flaw, y’see. Mea culpa for not being more clear on that point.