Stupid Question No2

How was absolute zero known without it being achieved in a lab?
and is there a maximum temperature?

Absolute zero is known as a theoretical concept. While absolute zero has never been measured (and never will, if the laws of thermodynamics are correct), all other measurements seem to be compatible with the idea.

As far as I am aware, there is no maximum temperature, though many compositie particles will collapse if heated to high enough temperatures (e.g. proton, neutron).

Originally posted by leestatic
How was absolute zero known without it being achieved in a lab?
and is there a maximum temperature?

…How was absolute zero known without it being achieved in a lab?
…

This wasn’t the historical reason why absolute zero was known without it being achieved in a lab but, if you think about it, there must be a limit to how low a temperature you can have because temperature can be thought of as the concentration of heat energy (although that is technically too simplistic because that totally ignores the complication of the “heat capacity” of a substance, it will suffice as an explanation here) therefore, if it wasn’t for the laws of thermodynamics, you should be able to have zero concentration of heat energy (which would make the temperature absolute zero) but not a negative value to the concentration of heat energy (because that would not make much physical sense).

Many attempts have been made to get closer and closer to absolute zero in the lab but, although temperatures extremely close to absolute zero have been obtained in the lab, it has never been reached nor gone below it.
The closer you get to making a substance have the temperature of absolute zero the harder it becomes to extract just a bit more heat out of the substance because it requires vastly more energy to extract that bit more heat energy out of it and would in fact require an infinite amount of energy to extract all the heat energy remaining in the substance.

Originally posted by Andrew Hamilton

The closer you get to making a substance have the temperature of absolute zero the harder it becomes to extract just a bit more heat out of the substance because it requires vastly more energy to extract that bit more heat energy out of it [...]

What do you mean?

Originally posted by KazetNagorra
What do you mean?

What do you mean “what do you mean?“? I thought I was being concise?

Originally posted by Andrew Hamilton
What do you mean “what do you mean?“? I thought I was being concise?

I was hoping you could illuminate what you are saying - as far as my knowledge of laser cooling and evaporative cooling goes it doesn't require energy being supplied to the system at all (well apart from powering the lasers and vacuum pumps etc., but that's obvious).

Originally posted by KazetNagorra
I was hoping you could illuminate what you are saying - as far as my knowledge of laser cooling and evaporative cooling goes it doesn't require energy being supplied to the system at all (well apart from powering the lasers and vacuum pumps etc., but that's obvious).

…well apart from powering the lasers
…

Precisely! Just try and think of a way of cooling towards absolute zero without in any way (not either through lazars or by rotating magnet beads in a magnetic field etc) using more energy than the energy (in the form of heat) you are trying to get out of the sample of the substance -I don’t believe there is any way around that because the laws of thermodynamics don’t permit it.

Originally posted by leestatic
How was absolute zero known without it being achieved in a lab?
and is there a maximum temperature?

I could be wrong, but it should be relatively easy to calculate absolute zero from the equations of thermodynamics. For example if you expand a gas it cools. If you expand it to infinite volume and zero pressure, what is the temperature?

Originally posted by leestatic
How was absolute zero known without it being achieved in a lab?

The record of lowest temperature are millionth of a kelvin above the absolute zero. (?)
How is it measured? It isn't as easy as sticking an termometre into the medium, is it?

Originally posted by FabianFnas
The record of lowest temperature are millionth of a kelvin above the absolute zero. (?)
How is it measured? It isn't as easy as sticking an termometre into the medium, is it?

You have to measure the kinetic energy of the molecules/atoms of the stuff you are cooling. Absolute zero is not zero energy left, however. Absolute zero is the minimum energy an atom can have. But due to the uncertainty principle, even at absolute zero temp, there is still some energy left, even if you could reach absolute zero. To measure this at that tempurature, you have to measure how fast the molecules/atoms are moving but even at absolute zero there is still some residual motion.

Originally posted by sonhouse
You have to measure the kinetic energy of the molecules/atoms of the stuff you are cooling. Absolute zero is not zero energy left, however. Absolute zero is the minimum energy an atom can have. But due to the uncertainty principle, even at absolute zero temp, there is still some energy left, even if you could reach absolute zero. To measure this at that tem ...[text shortened]... st the molecules/atoms are moving but even at absolute zero there is still some residual motion.

How do you measure the kinetic energy of a very cold medium?
Does it emit some radiation you measure the doppler broadening, or what?
Sounds that the measuring method is worth its own Nobel prize, as well as the cryo research itself.

Originally posted by Andrew Hamilton
[b]…well apart from powering the lasers
…

Precisely! Just try and think of a way of cooling towards absolute zero without in any way (not either through lazars or by rotating magnet beads in a magnetic field etc) using more energy than the energy (in the form of heat) you are trying to get out of the sample of the substance -I don’t believe there is any way around that because the laws of thermodynamics don’t permit it.[/b]

Of course. But that doesn't necessarily mean that cooling to lower temperatures requires more energy than is used now.

Originally posted by FabianFnas
How do you measure the kinetic energy of a very cold medium?
Does it emit some radiation you measure the doppler broadening, or what?
Sounds that the measuring method is worth its own Nobel prize, as well as the cryo research itself.

The method that is most commonly used is to simply let the cloud of atoms expand for a while and see what happens.

Recommended read on low temperature physics and, more specifically, the discovery of Bose-Einstein condensation:
http://nobelprize.org/nobel_prizes/physics/laureates/2001/ketterle-lecture.pdf

Originally posted by FabianFnas
How do you measure the kinetic energy of a very cold medium?
Does it emit some radiation you measure the doppler broadening, or what?
Sounds that the measuring method is worth its own Nobel prize, as well as the cryo research itself.

Here is a great link that explains it all: Basically measuring the size of an ultra cold cloud:
http://cua.mit.edu/ketterle_group/Popular_papers/Ultralow_temperatures.htm