Thread: Achieving absolute zero through radiative cooling?

I understand that a material can’t typically reach absolute zero through radiative cooling because the cosmic background radiation will always warm the material to around 2.7 K. But suppose you have a crystal of material that doesn’t have any phonons that would be accessible with energy on the order of 1.9 mm (the CMBR wavelength). Wouldn’t the material then be transparent to the CMBR, and therefore not subject to heating by it? What’s to stop such a material from simply radiatively cooling until its entire lattice is at absolute zero?

Absolute zero is simply not possible or practical. You can get closer and closer, but absolute zero is the limit, which means it is not obtainable. There are many reasons for this. Any particle is always surrounded by other particles, no matter how far away they are. These particles always have fields of force acting between them which cause them to move. So if there are any particles which have a temperature, all other particles must too, because these other particles will induce motion in them, no matter how slow this motion may be.

Originally Posted by Waveman28

Absolute zero is simply not possible or practical. You can get closer and closer, but absolute zero is the limit, which means it is not obtainable. There are many reasons for this. Any particle is always surrounded by other particles, no matter how far away they are. These particles always have fields of force acting between them which cause them to move. So if there are any particles which have a temperature, all other particles must too, because these other particles will induce motion in them, no matter how slow this motion may be.

Absolute zero does not mean that there is no motion at all; it simply means that everything is in the lowest possible vibrational state. Even in the ground state, there will still be some zero-point vibrational energy. I don't see any reason why a crystal couldn't radiatively cool until all its phonons were in the ground state.

Originally Posted by Scifor Refugee

Absolute zero does not mean that there is no motion at all; it simply means that everything is in the lowest possible vibrational state.

That is laughably wrong. Absolute zero means that there are no vibrations.

Originally Posted by Waveman28

Originally Posted by Scifor Refugee

Absolute zero does not mean that there is no motion at all; it simply means that everything is in the lowest possible vibrational state.

That is laughably wrong. Absolute zero means that there are no vibrations.

That is laughably wrong. Absolute zero does not mean that there are no vibrations.

Originally Posted by Waveman28

That is laughably wrong. Absolute zero means that there are no vibrations.

No offense Waveman28, but I don't think you understand what I'm asking about well enough to discuss this. Absolute zero most certainly does not mean that there is no vibrations - it merely means that the vibrations are all at the lowest possible energy state.
http://en.wikipedia.org/wiki/Absolute_zero

A system at absolute zero would still possess quantum mechanical zero-point energy. While molecular motion would not cease entirely at absolute zero, the system would not have enough energy for transference to other systems. It is therefore correct to say that molecular kinetic energy is minimal at absolute zero.

Also, maybe you should educate yourself about physics a little better before you go around calling things "laughably wrong," especially when it's something that any undergraduate physics or chemistry student should know. It's fine to not understand zero point energy or how quantized energy states in phonons work, but maybe you should refrain from arrogantly calling things "laughably wrong" when you don't actually understand the topic of discussion?