h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Of course, you realise that Absolute Zero was proved to be utter nonsense about 3 years ago.

The value for Absolute Zero was found by plotting a graph; kinetic energy of particles against their temparature. The graph showed an apparently linear relationship. Kelvin extended the graph back until it crossed the axis (i.e. zero kinetic energy) and found that the temparature at which this occurs is -273 Celcius. This became 0 Kelvin.

Recent experiments in low temparature physics have show, however, that the relationship isn't linear. As the graph in question approaches Absolute Zero, it curves exponentially and doesn't cross the axis until it reaches minus infinity Celcius - i.e. never.

The chemists just cheerfully ignored this.

/Mike

*Cheerfully ignoring this*

NEXT!!!

Erm... *that* sounds like absolute twaddle to me. Sorry.

Given that 'absolute zero' actually means all the particles are completely stopped, there is definitely an absolute zero and given that scientists have cooled things within a few milliionths of a degree of it, I'd think they're pretty sure where it is.

Either way... isn't the temperature of a substance just the kinetic energy of its particles? In which case, plotting one against the other wouldn't make much sense...

There *has* to be a temperature which is the temperature of something which has zero energy, and that temperature, naturally, is absolute zero. I'm fairly sure about this one...

26199

No I'm quite serious. I work for a Chemistry firm, it is the honest truth.

The point is, it isn't until you get to that few zillionths of a degree that you mention that the graph begins to curve. Which is why it wasn't noticed until recently.

The practical upshot of this, of course, is that you can NEVER remove ALL the kinetic energy from a mass, no matter how cold you make it. Nature won't allow a body to have finite mass but zero energy.

/Mike

Er... you're going to have to explain what definition of 'cold' you're working from, because as far as I'm concerned temperature is basically a measure of the kinetic energy the particles have. Which means that no kinetic energy and no temperature (ie, absolute zero) are the same thing.

So... what am I missing?

26199

Okay.

If particles have kinetic energy (which they all do), they have a temparature. Because, as you say, temparature is a measure of kinetic energy.

When particles have no kinetic energy, they would have a temparature of Absolute Zero. According to Kelvin's graph, this would occur at -273 Celcius.

The problem lies in the fact that nature will not allow a mass to have zero kinetic energy. When you approach -273, the amount you have to cool a body to reduce it's kinetic energy increases exponentially. so instead of the linear relation you get at (relatively) warm temparatures, you get an expontential curve. So that if you got to minus 273, there would still be kinetic energy. At -500 there is still kinetic energy.. not alot.. but it's still there. In fact it's only at minus infinity that the graph will actually touch the axis and the body have zero kinetic energy. So minus infinity is the true Absolute Zero.

When they discovered this, they invented some new thing called Lattice Energy (I think, or I might be confused with somethine else). The temparature reduction goes into reducing this lattice energy instead of reducing this Kinetic Energy.

Is that easier to understand?

I doubt it.. I'm no teacher. Sorry.

/Mike

But... if temperature and kinetic energy are the same thing, how can a temparature of negative infinite degrees be the same as an energy of zero? It doesn't make sense.

Okay, I can accept that it's impossible to cool something to absolute zero... but why have a scale starting with 0 celcius and going down to minus infinity? Why not start it at absolute zero? Surely it's quite *easy* to warm something up from absolute zero?

But having absolute zero at minus infinite degrees implies that it takes infinite energy to heat it up, which just plain sounds daft...

26199

Kinetic Energy and temparature *aren't* the same thing. That theory went out with the discovery that Absolute Zero wasn't at -273. Temparature is Lattice Energy PLUS Kinetic Energy. My word this is all coming back to me now. *L*

And I know.. things do start to sound daft when you get infinities creeping in don't they? But then again, the Universe cooled from infinity degrees to a few billion billion degrees within a very very short space of time.

All the chemistry equations that use the Kelvin scale as a measure of absolute temparature still work, because once you get above 273, Kinetic Energy and Temparature are - as you say - to all intends and purposes the same. Which is why -273 is still taken as Absolute Zero.

/Mike

Ok... so now you are saying Thermodynamics is wrong? Temperature is defined as the effect of the internal energy of the substance. Therefore plotting temperature against KE isn't valid.

The graph used to form the Absolute scale is pressure against temperature for an Ideal gas. The graph is indeed linear, the points do approach a temperature (at -273.15 C) where all motion can be said to stop. Of course we can never reach this point for then particle would cease to exist. That doesn't stop the point existing though.

The universe was never at infinity degrees in whatever scale since that requires infinite energy which there isn't. Anyway, what size infinity?

Lattice energy (or more accurately, from my A level chemistry course in energetics), Lattice Enthalpy, is the energy required to break a given ionic lattice. It is the ionic (and maybe metallic, I'm not sure) equivalent of bond enthalpy and has been known about for decades.

Yes thermodynamics is wrong. But then again Newtonian Gravity is wrong, the speed of light isn't 3x10^8 and neutrons are heavier than protons. It doesn't stop these principles being of use in everyday laboratory situations. The point is that whilst you're above -273C then equations based on the Kelvin scale work marvellously. This is why Absolute Zero is still taken as a valid value. No real-life situation will ever involve temparatures below -273.

Ideal gas you say? Pressure vs Temperature? Yes that sounds familiar. Take a look at kinetic theory though.. which says that pressure is caused by elastic collisions of particles with kinetic energy against the walls of the container.

Still.. whatever the graph was.. I'm telling you it curves as it approaches -273.15 C. I was mid-way through my chemistry A-Level when the teacher came in all embarrassed saying "Oh dear I just found out that half of what I've been teaching you is nonsense. Still we've checked with the exam board and they say to press on". He was a lovely old fellow. Mr Bailey his name was.. used to be an industrial chemist and had a glass eye. He spilt Bromine all over the lab desk once.. we had to evacuate.. 'twas fun.

As for the lattice energy bit.. well that was pulled from memory. And I did say at the time I could be confusing it with something else. But there is some other source of energy within a mass that they didn't know about before, that is tapped instead of KE when the KE reaches a certain level.

Oh and the Universe *was* at infinity degrees, according to Big Bang theory, which says the Universe started as a point of infinite mass, infinite temperature and zero volume.

Right, so we are saying that Temperature is a measure of how much the particles jiggle about. i.e. their kinetic energy. If we take something that is colder and bring it into contact said kinetic energy is shared between the "cold" and the "hot" objects, until they have the same temperature, i.e. random jiggling of particles, i.e. kinetic energy of any average particle. We can keep doing this via many a cunning method until the temperature approaches 0K. It then gets harder and harder to cool our object, as you have to find something to suck out the energy, expressed as jiggling (although not very much) particles. Hence you can't get there. However, if you could remove all the energy responsible for these jiggling particles the temperature is, by definition, 0K, -273.16C. You can't then further reduce the jiggling to get a lower temperature, surely. There is the fudge expressed in the Negative absolute temperature thread, which I don't like, but works, but that has nothing to do with jiggling particles.
I have the feeling that I just talked my way round this, without expressing properly what I think is going on. I often have this problem so I will shut up now.

No... you made perfect sense. The coldest temperature possible (zero on an absolute scale) is obtained when the particles have no motion. It can't actually happen (because of Pauli's Exclusion Principle and Relativity) but the theorhetical point is there. You can get closer and closer and closer, but never reach that point. It gets much harder to slow the particles down.

Okay, I agree with more or less everything you said.. except where you said that when the particles stop jiggling the temparature is
-273.16 C

As I have said, the data Kelvin used to plot the graph from which he derived that figure is flawed. What Kelvin couldn't have known (because the equipment to measure it was only just developed) is that as you approach 0K the apparent linear relationship of his graph breaks down. The graph curves exponentially.

So at -273.16 there IS still some Kinetic Energy in the mass. At -400 there is still some. Not a huge amount. Almost a negligible amount. But there is still some. It isn't until you reach minus infinity degrees that there is truly zero Kinetic Energy.

Hmm?

I think we have a definition problem here, I really ought to go and look it up. I always thought that the definition was, Zero Kinetic Energy equals Zero degrees Kelvin which further equals -273.16 degrees Centigrade (Celsius) and rather a lot of minus Fahrenheit. Hence, if you have a small amount of kinetic energy, you have a small amount of positive degrees Kelvin. If, of course, you define 273.16 degrees Kelvin as the freezing point of water, and 373.16 degrees Kelvin as the boiling point of water then kinetic energy has bog all to do with it. Thus I presume it is possible to have differences at both high and low temperatures, away from this defining range.
I still go with the first definition, but perhaps I was not paying attention when reading all those cryo books a couple of years ago. Certainly not unlikely

Indeed.. but if you look at the top of this thread.. it was begun by a post wherein I mentioned that the definition of 0K = no kinetic energy was wrong, thanks to this recent discovery of Kelvin getting the graph wrong.

No-one believes me, of course. Why I should lie about it, I have no idea.

Funny... cos in physics we have a theoretical physicist teaching us thermodynamics who normally tells us all the latest discoveries. He normally tells us to pretend they weren't there for our working. He said that "no one knows if the graph curves"

You don't expect me to actually read stuff from the beginning before saying something! Where is the fun in that?
Anyway, I now see where you are coming from. I don't believe it, but I am going to have to go and look it up now. Unfortunatley, no longer having the resources of a university behind me, this could take a while. Hey, ho!

It was in 1996/7 I heard about this.. while I was doing my chemistry A-Level. I saw it on the news at the time and told my teacher. He said he'd have to go away and look it up.

The following week he went through with the class what was going all (all that curving of the graph business) and said he'd spoken to the NEAB who said the syllabus hadn't changed and he was still to teach us the Kelvin scale as being Absolute.

/Mike

Not really surprising the syllabus hasn't changed when Newtonian theory is still taght. Basically, like quantum theory and relativity, this only has an effect in situations so far removed from everyday life, that they can be ignored in most experiments.

When I first read this thread I thought it was a load of rubbish, now I have to admit that it may make some sense. I have no book about thermodynamics here but maybe someone else can look it up and explain more accurately.

Unfortunately our beloved temperature isn't the real thing but entropy S. Entropy is clearly defined microscopically (very complicated though). Temperature is defined via entropy: T=(delta S)/(delta Q) (Q: heat energy).

As a result, temperature of matter under non-pathological conditions is kinetic energy. Yet this is a _conclusion_, not the definiton of temperature.

So if entropy does strange things this affects temperature. Hell knows what it does at extremely low temperatures.

However I still don't like this idea of negative temperature. People are interested in the energy left in their sample not in bizarre entropy effects. And it does definitely depend on the exact way you indicate your entropy (have a look at the external link in the neighbour thread).

I always hear about new low temperature records (millikelvin, microkelvin, ...). These people would be really stupid if there was no zero point.