[Jimbee (OP)]

Absolute zero of course is when all molecular motion stops. Scientists say we will never reach it.

My HS chemistry teacher said it is because all molecular collisions are elastic. And when an atom hits another one, it always bounces off and starts moving again. Another message board said that absolute zero is impossible because there is no such thing as a perfect vacuum.

So is absolute zero possible to reach? Will we reach it someday? I think, like air flight and the Wright Brothers, we will.

[alancalverd]

The real problem is basic thermodynamics.

The efficiency of a theoretical Carnot engine (the most efficient engine conceivable) is 1-T(cold)/T(hot). This implies that as you approach absolute zero, the efficiency of any heat transfer device tends to zero. So you need to do an infinite amount of work to remove the infinitesimal last bit of heat from your sample.

[Eternal Student]

Hi.

    The simple answer is the one your High School chemistry teacher gave you:   No.   That would be sufficient for most exams at most levels.

   It looks like you've already had a few comments or read some replies in other places (you mention other forum boards). 

    A lot of arguments will consider temperature with definitions based on kinetic theory (hot things have fast moving particles in them,  cool the thing and the particles slow down,  at zero Kelvin all motion of the particles should have stopped).   This is not the only way you could define "temperature", for example we can take a thermodynamic approach like the one suggested by @alancalverd involving Carnot engines.  We can also take an approach based on statistical mechanics:  Entropy can be directly defined as proportional to the number of microstates a system can occupy at a given energy   and temperature can then be defined as a derivative of entropy with respect to a change in energy.

The details are not important, it's just important to note that "temperature" is NOT a simple thing to define and there are several approaches that are all useful and equally valid.

    In typical situations, all of these definitions of "temperature" will be equivalent, they will all be measuring and describing the same thing.   However, they each describe what it would mean to be at zero Kelvin slightly differently.
The Kinetic Theory definition of temperature is often the easiest one to work with and has become the approach incorporated into the standard set of units for describing quantities  - the S.I. units.   S.I.  = "Syst?me International d'Unit?s"   [French.  Late editing: this website doesn't want to reproduce accents over the e].    So this is the definition of "temperature" that we would normally assume but it is not without problems.    For example, the basic models associated with this kinetic theory are all based on classical mechanics and not quantum mechanics.    So,  when you obtain a numerical value, say a temperature of 100 K in the SI units, for a substance it's telling you about some properties related to a classical model of the substance only.

     I expect you've already seen lots of discussion and arguments to state that a tempertaure of 0 kelvin could never be reached.  So I won't repeat that here.   It's true enough when you recognise that temperature was always set up to tell us only about a classical property of a substance under a conceptualised (and fully classical) kinetic theory model.   Instead we'll try to go around the problem and consider a situation where quantum mechanics is going to win the day.

     There are some things and situations where classical models just aren't useful or relevant any more.   In particular there are some substances (like a simple gas) that can form a Bose-Einsten condensate.   This is a situation where all the particles are occupying the lowest possible energy state.   This can happen at what we describe as a low "temperature" using the SI units for quantifying temperature   (e.g. close to but NOT exactly at 0 Kelvin).   When the gas has reached this state,  trying to cool it further is irrelevant, everything is in the lowest energy state, there is no lower energy condition it can have.     The particles do not have a "kinetic energy" that varies in proportion to the "temperature" in the region of space that some other instruments or thermometers you may have are reporting this "temperature" to be.   Whether it was 1 K  or  0.1 K  doesn't make the slightest bit of difference to the condensate, nothing about it can change, all particles are in the lowest energy state.
     So we are in a situation where the conventional definition of "temperature" just doesn't apply to the condensate.   It doesn't have this temperature as any sort of intrinsic property.   For example, lowering this "temperature" doesn't make the particles move more slowly.  Similarly, increasing the temperature (provided you keep it below the critical condesation temperature) doesn't make the particles move any faster.
     The "temperature" which your other equipment in the region is reporting and we nominally assign as the temperature of the condensate is just an artefact of the way we define "temperature" in SI units.  It's saying  "  if the condensate was represented by a classical system in kinetic theory, then the particles would have this much energy"  -  but the key point is that the condensate is NOT in any way behaving like a classical system any longer.   It is absurd to assign a value for the temperature of the condensate as if it was a classical system, because it just isn't one.    There is no physical difference in the condensate at 1 K,  compared to the condensate at 0.1 K.   Whatever this nominal temperature is describing it isn't any physical property that the condensate has.   I have probably laboured the point but I just wanted to try and make it clear.   When we consider quantum mechanical behaviour, very low temperatures can lose their meaning and just be artefacts of something that would happen if the system was a classical one.

    Anyway, I hope that's a positive note to end on.   We have cooled some things down enough to form a Bose-Einstein condensate and so they have achieved their absolute zero temperature  - the point at which no lower energy or physically meaningful internal temperature could be assigned to the substance.

Best Wishes.

[paul cotter]

Just the same as trying to accelerate a massive object to the speed of light, we can get arbitrarily close by expending more and more energy but we can never get to the target speed. Similarly with absolute zero the closer one gets the harder it becomes to produce a further decrease. Mathematically one can only asymptotically approach either the speed of light(with mass) or absolute zero. |Oh dear Eternal Student beat me to it!