Going Lower than Absolute Zero

When cooling atoms (e.g. to produce a Bose-Einstein Condensate, or BEC) there are at least three different methods of laser cooling employed to cool atoms. They are 1) simple radiation pressure, 2) Doppler cooling, and 3) evaporative cooling by selective removal of atoms. As I understand it, these are the three steps that are currently used in making BECs, and I'll go over them as three steps in cooling a bunch of atoms down to extremely low temperatures.
Before I start, though, it's important to be familiar with the idea that light carries momentum, and can exert pressure on things. For the purpose of this explanation, I think it's easiest to think of light as a sort of "wind" of photons (or "light-particles"). So a laser would be like a stream of photons. It's also important to know that these photons 1) carry momentum along with them, and 2) can get absorbed by atoms. And when they do get absorbed by atoms, the momentum has to go into the atom (since the photon is no longer there). So if a photon and an atom have a head on collision where the photon gets absorbed, the atom gets slowed down a bit.

Okay, now to make a BEC, first you need to accumulate a relatively small group of atoms for the condensate, and you've got to get them away from other atoms (or the other atoms would just heat them right back up), so you want to get them off of a lump of material and into a vacuum where you can isolate them. The general way of doing this is by heating a lump of the material in a vacuum, so that a bunch of atoms are essentially boiled off the surface. Then you channel them all into a jet and point it towards the place where you want to make your BEC.

Okay, because you just �boiled� these atoms, they are very hot (and moving very fast - remember temperature is proportional to the speed of the atoms squared), so first you want to just kinda stop them from shooting away from the oven at high speeds. This can be done simply by shining a laser head on at the atoms. As they're flying towards the laser, they absorb photons traveling in the opposite direction and are slowed down, and since they are going slower, they by definition have a lower temperature. Now there's a slight problem with this, which you may have spotted: once they stop, the photons will keep getting absorbed, and the atoms will start to accelerate back the way they came! So this only works for a little while, and then you need to switch to the second cooling method, Doppler cooling.

For Doppler cooling, we need another detail from quantum mechanics, and a bit of relativity. First the quantum mechanics. Basically, the atoms we're looking at don't absorb every photon that comes near them - they absorb photons somewhat randomly, depending on the wavelength of the photon. Also, each atom has a specific wavelength it "likes", or is more likely to absorb. So if you shine a laser of light at an atom, it has a probability of absorbing that light that is basically a Gaussian (bell) curve that peaks at the wavelength the atom likes. If the light is at a wavelength the atom doesn't like as much, it's less likely to absorb photons that pass by. As for the relativity part, we just need to know about the Doppler Effect, which says that if you are traveling towards a wave, it will seem to be a higher frequency (shorter wavelength) than if you're standing still, and if you're traveling away from a wave, it will seem to have a lower frequency (longer wavelength) than if you were standing still.

Right, so now for the Doppler cooling part. Imagine we've got one atom sitting in a vacuum, in what we'll call our "trap", which is really just a region of space. Now imagine we've got two laser beams hitting the atom, one coming from the left, the other from the right. At first glance, it seems like overall nothing would happen - whatever effect the beam from one side had, the beam from the other side would cancel it out. But there's a trick! The trick is to make the lasers operate at a frequency that is just below the frequency of light that the atom likes to absorb (this is known as "red-detuning"). That way something really neat happens. Say the atom is going moving to the right. Since it's moving into the beam coming from the right, that beam looks like it's got a slightly higher frequency (due to the Doppler shift) - and this means the atom sees photons that are closer to the frequency it likes to absorb, so it's more likely to absorb photons from the right, which means it will get a bunch of little pushes from the right (pushing it back left). What about the beam from the left? Well, the atom is moving away from it, so the photons from the left are shifted to a lower frequency - away from the frequency that the atom likes, so it's not absorbing them nearly as much, and not getting pushes towards the right. The same sort of thing happens when the atom moves towards the left, and voila! You've got a system where whichever direction the atom moves, it gets slowed down. (In a 3-D trap, you really have lasers coming from 6 directions - right, left, up down, and front and back - so that the atom truly does get slowed down whichever direction it's going).

Unfortunately, this method can�t cool the atoms down to cold enough temperatures and high enough densities to produce a BEC. The reason for this is that when the atoms get too dense, the blob of them becomes opaque to the Doppler cooling lasers � in other words, the clump of atoms becomes so thick that the lasers can only hit the atoms on the surface, and can�t cool the ones in the middle anymore. The limits of Doppler cooling are densities of around 10^11 atoms per centimeter, and temperatures around 10 to 100 microKelvin.

The final method of cooling is really sneaky. The basic idea is that you want to get rid of a few of the hottest atoms (i.e. the most energetic ones). When you do this, they take a bunch of energy out of the system, and when the rest of the atoms settle, they are at a lower temperature. Okay, so the idea in itself isn�t that sneaky � you do it all the time when you blow on a cup of coffee or tea to cool it down. The way you do it with atoms is the sneaky part.

To understand how to accomplish evaporative cooling with atoms, we need to first take a look at how these atoms are being held in the trap. The basic trapping mechanism is a magnetic field. So the atoms all have a magnetic moment (say for the sake of example, they are all pointing �up�), and you set up a magnetic field so that atoms with magnetic moments pointing upwards feel a force towards the center of the trap, wherever they are. For those who are familiar with magnetic potential wells, you basically just set up a potential well that is concave up for upwards magnetic moments, and the atoms with magnetic moments pointing up all congregate around the minimum potential. You might ask, though, how we know that all the atoms have their magnetic moments pointing upward? Well, we actually enforce that when we set up our trap; if you think about it, if we are forcing �up� atoms towards the center of the trap, atoms with opposite magnetic moments (�down�) would be forced away from the center of the trap � kicked out, in effect.

Here�s the sneaky part � we use that fact that atoms with the wrong moments are kicked out of the trap. Remember when I said that atoms absorb some wavelengths of light more than others? Well it turns out (to go more in depth with this, you need a course in quantum mechanics, or a couple�), that this absorption depends many things, including what happens to the atoms when they absorb the light. And there�s a very special case which physicists can exploit, where if you shoot exactly the right frequency of light into your trap, it will take atoms with a specific energy (for instance the atoms with the highest energy in your trap � the ones we want to kick out), and flip their magnetic moments! This is great, because once we�ve flipped the moments of these atoms, they are kicked out of our trap! And they take energy with them, which leaves the rest of the atoms cooled down! And then you re-tune your beam to take off the next layer of most energetic atoms, and repeat this until you have your BEC. Using these methods, physicists have achieved temperatures on the order of nanoKelvins (10^-9 degreed Kelvin) above absolute zero. And there are all sorts of neat things you can do with matter that cold, but I�ll leave that for the reader to explore! (You might want to search topics such as Bose-Einstein Condensates, superconductivity, superfluidity, and atom lasers to start with, and see where you go from there!)

I love embarrasing the so called scientific mind, thanks Lilblam.

Absolute zero is a theoritical limit. It is like the theorized distance I can drive on a tank of gas going at a certain speed. Who knows what happens when I pass that theoritical no man's land boundary?

Besides when matter stops all activity or loses all energy can it not still be cooled? does it compress or change structure? seemingly yes.

what i dont understand what ur saying THENEO?

do sum research. its not being cooled its losing energy in the form of 'heat'. Most of matters energy is stored in its mass i guess. how could you take away more heat energy than something has?

[Edited on 26-2-2004 by quiksilver]

Going below absolute zero is, by definition, impossible.
I also dont think that doing so would reverse electro-charges on the particles.

nice example about that ball.

However, if that ball had the same force to drive it forward therefore there must have be another force somewhere to drive it backwards.

I think that the author has read somewhere on this site that if you reverse matter you can go back in time by making it retrace it steps.

Heat energy, in the ball context, would not be a vector quantity. It doesnt matter which way the ball is going, but once it is stopped it is stopped.
The only way to get it moving again is to give it more heat energy.

Heat is a scalar, not a vector

Originally posted by THENEO
Besides when matter stops all activity or loses all energy can it not still be cooled? does it compress or change structure? seemingly yes.

Heat IS motion. So when it stops, no it can not be cooled further.

Just so everyone is clear on this, there is no such thing as "cold" or "cooling" just low heat or removing heat. Once all the heat or kinetic/thermal energy is removed from something it is at absolute zero, there is nothing left to remove. Talking about going below absolute zero is like saying you are going to keep removing water from and already empty glass.

for quicksilver.

[Edited on 26-2-2004 by Quest]

"Nuclear and electron spin systems can be promoted to negative temperatures by suitable radio frequency techniques."

While it is rather far removed from everyday reality idea of temperature, it has been done through manipulating spins. Here is the link:

math.ucr.edu...

Originally posted by DismalDream
"Nuclear and electron spin systems can be promoted to negative temperatures by suitable radio frequency techniques."

While it is rather far removed from everyday reality idea of temperature, it has been done through manipulating spins. Here is the link:

math.ucr.edu...

Thats a math negative... Not a real negative temperature.

Here is the explination.

Q: (A) I want to ask about this magnetic pole reversal. It's the current theory or understanding of magnetic field of planets in terms of
dynamo mechanism, where there is a liquid metal - iron - which is hot - there are convective currents, and there is self-excitation through
magnetic field. That's the present model. They were able to model this magnetic pole reversal using this kind of magneto-hydro-dynamics. Is
this model essentially correct?
A: Meow.
Q: (A) What is the main thing that is important, and that is lacking from this model?
A: Purr. Meow.
Q: (A) Everybody thinks that the core is a crystal iron; that's the present thinking. Say it's an ammonia core: is an ammonia core in all
planets with magnetic fields? Is this so?
A: [ignores]
Q: (A) When we speak about crystalline ammonia, do you mean a new kind of crystalline ammonia that is not yet known on Earth to our
scientists?
A: [licks own ass]
Q: (L) I think we need to find out something about this crystalline ammonia. (A) What would make it go into the very core? (L) I don't
know. We don't know enough about it to even know how to frame a question. I know we thought it was crazy when they were talking about
Jupiter and the ammonia, and then of course all this ammonia shows up on Jupiter. And I remember them saying something about this at the
time, but I don't think we ever followed up on it because I thought it was even to crazy to think about. Maybe we need to find out something
about ammonia, crystalline ammonia. (A) Is there a mini black hole in the center of the Earth?
A: Meow.
Q: (L) I remember when I was a kid - this is a funny thing - we got this kind of chemistry experiment. You put these chemicals together and
it grew crystals. I think ammonia was part of it. I think you had to use ammonia to grow crystals. (A) Okay, now this crystalline ammonia
core inside the Earth, can we have idea how big it is, what radius?
A: Purr.
Q: (L) What is surrounding it, what is the next layer? (A) Normally people would say it's an iron crystal. What is the next layer?
A: [ignores]
Q: (A) There is this ammonia - crystalline... (L) Surrounded by iron crystal. Is it crystal iron? (A) Probably at this pressure that is here, it
may very well be crystal. (L) Okay, is the iron surrounding the ammonia, is it crystalline?
A: [ignores]
Q: (L) What's the next layer?
A: [watches bug]
Q: (A) Okay, now we know that some planets have this crystalline ammonia, and some do not. When we consider planets that have this
crystal ammonia inside, how did it get there? Was it a kernel first around which the planet was formed, or first the planet was formed and
then during some processes the ammonia sank and crystallized inside? I would like to know how it got there?
A: [scratches ear]
Q: (L) I read somewhere - about supernovae - that the only reason we have iron is because it's produced in supernovas. That would mean
that our solar system is formed from a supernova, right? In which case what blew up and when? (A) I understand that this crystalline
ammonia core - 300 km radius - must have certain magnetic properties which are important. Because it was mentioned that it was lacking in
dynamo theory or certain very important properties concerning heat convection. So there are these two main things in dynamo theory -
conductivity and electric properties - on the other hand heat convection properties. Why is this ammonia important for the magnetic field
because of what properties?
A: Meow.
Q: (A) According to what we know it's very hot inside the earth because of the pressure. Now, is this ammonia also hot, as much as iron?
A: Meow.
Q: (A) Is it super conducting even if when it is very hot?
A: Meow.
Q: (A) When it gets cold, how cold does it get?
A: MEOW.
Q: (L) What is absolute zero? (A) That is something you can't get below. That's why it's called absolute zero. It's a new
thermo-dynamics. (L) How often does it alternate?
A: [ignores]
Q: (L) So when it gets so cold and becomes super conducting, the act of super-conducting is what heats it up? Is that it?
A: Purr.
Q: (L) Well once it heats up, how does it then get cold again?
A: Purr.
Q: (L) What is it conducting? When something is super conducting what does it conduct?
A: Meow.
Q: (A) The point is, you see, that when something is super conducting it offers no resistance. Which means that the current it flows through
it, is not heating it. Well we learned that it gets hot because it's super conductive, right? Which is somewhat contradictory because when it
is super-conducting there's no reason for it to be hot except it can become hot because there is the hot external shell of iron. So that is very
likely why it would become hot. Because by the very definition of super conductivity you don't become hot when you conduct, see? Well, if
there are big, very big currents, then okay, they can stop super conductivity, then it gets warm.
A: Purr.
Q: (A) Now, I want to go back to this 55 degree below absolute zero. And here I would like to have a confirmation of this 55 degree below
zero. Because. according to the current knowledge of physics, the absolute zero was set by definition, as the temperature on the scale,
according to the science of thermo-dynamics, which is - so to say - nothing moves so you cannot go below this temperature. If you say 55
degrees below zero it means we have to redo physics and redo thermo-dynamics.
A: [ignores]
Q: (A) What?
A: Meow.
Q: (A) What causes this appearance of new physics in the center of the planet? We do not see this need for new physics around us. But
somehow there are specific conditions, apparently, in the center of the planet that cause necessity of entering this new physics.
A: Purr.
Q: (L) Let me ask this, if it was possible to measure a temperature of something that was being subjected to a very intense electro-magnetic
field what would it show? (A) Well the question is different, you see, because we asked first about why there is this ammonia crystal inside,
okay? The answer was it was a natural process. But now we see there is this window inside. What is the reason that there is this window
inside? Now you suggest, honey, that the widow inside is because there are - or because who knows what causes what - but there are very
strong electro-magnetic fields. Is the window inside related to the fact that we have to go beyond standard physics? Is it related to the fact
that there are very strong electro-magnetic field inside?
A: Meow.
Q: (L) What is ammonia composed of? (A) Ammonia? NH3, one nitrogen and three hydrogen atoms, and it kind of rotates, and that's
ammonia. (A) What is nitrogen number? Six? Or seven? Seven is phosphorus, yeah? (L) I don't know, I don't remember, I'm too tired to
remember. (A) You're too tired.
Q: (L) Okay, anything else we can think of we ought to ask before we...is there going to be any kind of terrorist attacks tonight or tomorrow?
A: [licks own butt]
Q: Are there going to be any further terrorist attacks in the United States?
A: [ignores]

Well Like I said in the original post this is a theory. Just like the Big Bang Theory, Evolution, Gravity, and the whole list of the others. It is still just a theory that you cannot push past absolute zero.

Remember that it was not until 1947 that the first quark was identified. Until that point in time the atom was theorized to be the electron, neutron, and proton were thought to be the smallest form of matter. So keep an open mind.

Yes I think that if you went below absolute very you would get something that would contain a negative energy. I know that negative energy does not fall into line with today's science but I am just theorizing and I do not have the ability to test any of these theories so I am just bound to writing it all down and thinking it out.

Originally posted by BlackJackal
Well Like I said in the original post this is a theory. Just like the Big Bang Theory, Evolution, Gravity, and the whole list of the others. It is still just a theory that you cannot push past absolute zero.

Not really, the very definition of absolute zero means it cannot be passed.

To simplify (as many others have already in this thread). The temperature of a group of atoms is a measure of the average speed of the atoms. Absolute zero is defined as the point at which this motion ceases. So to go 'beyond' would infer the motion starting up again, thus is the same as raising the temperature. As someone mentioned, if something is stopped you cant stop it more.

For an easier to understand explanation of how near-0K temperatures are reached, go to this page:
www.colorado.edu...

(Its about Bose-Einstein, so it covers all relevant areas, also funky little animations )

you can't take something below absolute zero, temperature is a measure of the movement of an atom, if an atom stops moving, it's temperature is absolute zero, you can't make something stop moving more then no movement at all lol

actually, absolute zero itself is impossible to get to, because you cant make matter stop moving, if matter stops moving, then it ceases to exist, that's what happens in the singularity of a black hole

While I'm no physicist, I'm going to have to echo the above... Interesting idea, but the idea of matter suddenly starting to move again, yet in the opposite direction, doesn't seem to jive...

Originally posted by Gazrok
While I'm no physicist, I'm going to have to echo the above... Interesting idea, but the idea of matter suddenly starting to move again, yet in the opposite direction, doesn't seem to jive...

Thats the other thing, there is no 'opposite direction' as the speed of the atoms is measured as an absolute speed, not as a vector. Meaning the temperature is an average of the speeds the atoms are moving at 'in any direction'.

While it's an interesting theory BlackJackel, I tend to agree with Kano and Gazrok (and not my cat).

Well its just a theory divised in a sick mind . I agree its far fetched however, if it is ever accomplished maybe I will get credit for thinking of it first.

BTW RANT I think your cat was on to something

Originally posted by THENEO
Besides when matter stops all activity or loses all energy can it not still be cooled? does it compress or change structure? seemingly yes.

Cooling is the process of losing energy in the form of heat. Nothing more, nothing less. So, if an atom loses all of it's energy, then it can't be 'cooled' any more. So seemingly, no, you can't cool an atom down past the point of zero motion, since slowing the atom down is what cooling is.


[Edited on 26-2-2004 by Esoterica]

What you are not taking into account is what potentially happens when an atom is no longer in motion. The "spin" I am told is what determines the atoms energy state. The slower the spin the lower total energy. BUT what happens when it no longer spins, does it lose the atomic structure as we know it ? At truely AZ do the remaining Photons, Electrons, Protons, Neutrons etc just split in a mass exodus from the atomic structure. If so you have the potential to release enormous amounts of stored energy in any surrounding atoms near by.

This is why I think when you approach AZ you either A get a huge explosion or B just when the atom is ready to split it pulls electrons and photons from near by atoms and restores its energy state to just above AZ. Thereby preventing you from actually ever reaching AZ or lower (if possible).

AZ is only a theory no body knows what the REAL AZ is but i believe that somewhere in this universe something is absolute zero

in the analogy of the ball being stopped as representing absolute zero, obviously you couldn't stop movement or slow movement anymore on something already stopped, but what about contraction.. maybe shrinking it(atoms) and increasing the density.

Just an idea of something going below absolute zero.