[Bored chemist]

Do laser cutters decrease entropy?

no.

So, they don't have negative temperature.

I already explained that there's nothing magical about lasers.
You can use a focused beam of sunlight to cut stuff with.
No laser required; no negative temperature required.

There's also nothing special about lasers for cooling.

[hamdani yusuf (OP)]

There's also nothing special about lasers for cooling.

Using a laser is by far the most sensible way but, in principle you could use a different light source.

What other kinds of light can be used for cooling? Do you have any source?

How to cool things using light other than laser?

[hamdani yusuf (OP)]

I'd have to do lots of complicated maths but, for the sake of discussion, let's pretend that I have done it and the answer is minus 5000K.

What use are you going to make of that information?
Because, if you don't have a good answer, I'm going to take it as further proof that you are a troll.
It will prove that you were just trying to trick me into doing lots of pointless work.

It seems like you are trying so hard to look like you know more things than you actually do.

I wanted to verify the justification for assigning a negative value for temperature of laser. If in an isolated container an object  with the same heat capacity but have positive 5000K temperature is let to exchange heat with the object having negative 5000K temperature, the result is expected to be objects at 0K. If that's not the case, then saying that the laser has negative 5000K is erroneous in the first place.

[Bored chemist]

with the same heat capacity

Heat capacity isn't well defined here.
The heat capacity depends on the available degrees of freedom.
Since, in the case of the negative electron temperature in  lasers we are talking about the heat capacity of a specific electronic transition it's not gong to be possible to get equilibrium within that transition without getting a more general shift towards equipartition.

It seems like you are trying so hard to look like you know more things than you actually do.

I do know the stuff; I just know that there's not much point trying to explain it to you  because you don't understand and or don't listen.

If that's not the case, then saying that the laser has negative 5000K is erroneous in the first place.

or it means that your understanding is faulty.
In principle, what you actually get is an infinitely negative temperature.

Please try to realise that it isn't my fault that you do not understand this.
I am not your teacher.

[Bored chemist]

How to cool things using light other than laser?

The same way that you would do it using a laser, but using a different light source.
The fact that you ask this proves that you don't understand how laser cooling works.

[alancalverd]

Imagine we have sheep in a field. All wandering about in different directions and doing different things. Lots of entropy.

Now introduce an energetic dog who rounds them up into a pen, where they all stand still and face the same way. He has done work and reduced their entropy.

Therefore if you define temperature in terms of entropy, T = (dS/dE)^-1, he has reduced the temperature of the sheep by doing work on them, so T_sheep is negative.

[hamdani yusuf (OP)]

How to cool things using light other than laser?

The same way that you would do it using a laser, but using a different light source.
The fact that you ask this proves that you don't understand how laser cooling works.

Is there any report of the experiment? Or is it just your hypothesis?

[Bored chemist]

Or is it just your hypothesis?

No, it's a deduction.
There is no way for the atoms being cooled to know if the photons hitting them came from a laser or from some other source, is there?
So the idea that they might behave differently is a bit silly, isn't it?
So, once again, we are into the territory of you asking questions that would not be asked by anyone who understood the science.
so, once again, you are only proving your own lack of understanding.

Why do you keep doing this?

[hamdani yusuf (OP)]

No, it's a deduction.

Until it's confirmed experimentally,  your deduction will remain as a hypothesis.

There is no way for the atoms being cooled to know if the photons hitting them came from a laser or from some other source, is there?
So the idea that they might behave differently is a bit silly, isn't it?

What are the differences between laser and other light sources?

A laser differs from other sources of light in that it emits light which is coherent. Spatial coherence allows a laser to be focused to a tight spot, enabling applications such as laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over great distances (collimation), enabling applications such as laser pointers and lidar (light detection and ranging). Lasers can also have high temporal coherence, which allows them to emit light with a very narrow spectrum. Alternatively, temporal coherence can be used to produce ultrashort pulses of light with a broad spectrum but durations as short as a femtosecond.
https://en.m.wikipedia.org/wiki/Laser

[Bored chemist]

What are the differences between laser and other light sources?

The difference is (1) irrelevant, and that's the point (2) the way in which the light is produced- by stimulated, rather than spontaneous emission.

Until it's confirmed experimentally,  your deduction will remain as a hypothesis.

Not really, no.
We know from other experiments that a photon is a photon is a photon.
If it has the same wavelength and polarisation it will act the same as any other photon with that wavelength and polarisation.

Did you not realise this?
I tried to explain it to you.
"There is no way for the atoms being cooled to know if the photons hitting them came from a laser or from some other source"
But it seems the point went over your head.

[Bored chemist]

A laser differs from other sources of light in that it emits light which is coherent.

Again, this is in the "lies we tell to children" category.
Unlike  "laser" cooling, in the field of holography you actually need to concern yourself with coherence.
You can't really make holograms without coherent radiation. (Ok, technically, you can but it's doing things the hard way)
And yet, Gabor invented and demonstrated holography without using a laser.
https://en.wikipedia.org/wiki/Dennis_Gabor
So we know that the wiki article isn't entirely correct.

However, it is nearly correct.
By far the easiest way to get coherent light is to use a laser.
It would be very unusual to use any other source of coherent radiation in an experiment today.


But, here's the point you missed.
You do not need coherence to do "laser" cooling.

So why did you quote the (not entirely accurate) wiki article about something irrelevant?
Are you trolling, or did you just not understand?

[hamdani yusuf (OP)]

The closest thing I can get for light cooling is in this article.
https://www.esa.int/gsp/ACT/projects/electroluminescent_cooling_using_LEDs/

Electroluminescent cooling using light-emitting diodes
There is a multitude of areas where sub-zero temperatures are required in space, from the storage of lunar material [1] to the cooling of infrared (IR) cameras and other sensors [2]. Because the transportation of equipment and materials from Earth results in huge costs, with an estimated price of around 500 000 €/kg to the Moon in 2008 [3], lightweight cooling systems are highly sought [1].

For almost 60 years [4], we have known that when an electron-hole pair recombines in certain light-emitting diodes (LEDs), the energy of the emitted photon (light, energy Eout = hν) can exceed the energy of the injected charge carriers (applied current, energy Ein = qV). For this phenomenon, hν > qV, to make physical sense, a portion of the energy of the photon must be taken from somewhere - in this case phonons (heat) in the crystal lattice. The result is that the LED can function as a cooling element, drawing heat from the crystal. In this situation, the wall-plug efficiency (WPE), defined as the ratio of the total optical output power to the input electrical power and therefore also called the electrical-to-optical power-conversion efficiency, has a value above unity.

Electroluminescent cooling (ELC) is only feasible, however, if the combined cooling effect from all radiative recombination events exceeds the heating caused by all parasitic, non-radiative recombination events. For every time an injected electron fails to recombine with a hole or a photon is absorbed in the structure, heat is produced.

Only if the external quantum efficiency (EQE, the ratio of successfully extracted photons to injected charge carriers) of the LED is close to unity or the photon energy hν is considerably higher than the applied voltage qV to the LED, the WPE = EQE × ( hν / qV ) can exceed unity [5].

Although the theoretical potential of ELC has been known for a long time [6], it was only recently that ELC was demonstrated in the laboratory - although for extremely low cooling powers [5]. With the increase in the efficiencies of LEDs in general, several groups have started looking into the use of ELC for practical purposes [7-10].

The main difference from laser cooling, is the light cools the LED body, instead of the cooled gas molecules.  Have you ever thought why can't we just simply replace the lasers in laser cooling with LED or other incoherent light sources?

[Bored chemist]

Have you ever thought why can't we just simply replace the lasers in laser cooling with LED or other incoherent light sources?

Yes, I have.
I didn't need to think about it for very long before I realised the answer.
Now it's your turn.
Can you think of some property of laser light that might explain why lasers are typically used for cooling and other light sources are not?
Here's a hint
Nd YAG 1064 nm
NeNe 632.8nm
HeCd  422 nm

[hamdani yusuf (OP)]

Not really, no.
We know from other experiments that a photon is a photon is a photon.
If it has the same wavelength and polarisation it will act the same as any other photon with that wavelength and polarisation.

Photon model can explain some experimental results, but has difficulties to explain some others. We should not take it as a complete representation of physical reality, and assume that it will produce correct predictions for experiments yet to be done.
 

[hamdani yusuf (OP)]

Yes, I have.
I didn't need to think about it for very long before I realised the answer.

What's the answer?

[hamdani yusuf (OP)]

Can you think of some property of laser light that might explain why lasers are typically used for cooling and other light sources are not?
Here's a hint
Nd YAG 1064 nm
NeNe 632.8nm
HeCd  422 nm

Laser frequency and wavelength can be tuned at will, by changing the length of light-emitting cavity or its temperature.
Temperature can also change frequency of LED.
Incandescent light produces continuous spectrum. Prism or diffraction grating can be used to select desired frequency.

[Bored chemist]

Can you think of some property of laser light that might explain why lasers are typically used for cooling and other light sources are not?
Here's a hint
Nd YAG 1064 nm
NeNe 632.8nm
HeCd  422 nm

Laser frequency and wavelength can be tuned at will, by changing the length of light-emitting cavity or its temperature.
Temperature can also change frequency of LED.
Incandescent light produces continuous spectrum. Prism or diffraction grating can be used to select desired frequency.

Good, you are apparently learning.
I wonder how long it will last.

[Bored chemist]

but has difficulties to explain some others.

Which?

[Bored chemist]

Yes, I have.
I didn't need to think about it for very long before I realised the answer.

What's the answer?

In order to get "laser" cooling to work, you need to produce a beam of light with a very carefully defined wavelength.
That's relatively easy with a tuneable laser.
You can do it with a  conventional light source and a monochromator and collimator , but it's horribly inefficient.

But, in principle, you can do it.
So, do you now (a few days later than everyone else) understand that you do not need a laser to produce that sort of cooling, so that sort of cooling can't be anything to do with a negative electronic temperature in a laser?

[hamdani yusuf (OP)]

but has difficulties to explain some others.

Which?

Diffraction, interference, polarization, and refraction are better explained using wave model.