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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.arch
Subject: Re: Is Intel exceptionally unsuccessful as an architecture designer?
Date: Sun, 22 Sep 2024 13:40:22 +0200
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On 22/09/2024 11:41, Niklas Holsti wrote:
> On 2024-09-22 7:02, Chris M. Thomasson wrote:
>> On 9/21/2024 6:28 PM, MitchAlsup1 wrote:
>>> On Sat, 21 Sep 2024 23:55:13 +0000, Lawrence D'Oliveiro wrote:
>>>
>>>> On Sat, 21 Sep 2024 13:43:55 -0700, Chris M. Thomasson wrote:
>>>>
>>>>> On 9/21/2024 1:22 AM, Lawrence D'Oliveiro wrote:
>>>>>
>>>>>> On Fri, 20 Sep 2024 15:33:23 -0700, Chris M. Thomasson wrote:
>>>>>>
>>>>>>> Is there any activity going on at absolute zero?
>>>>>>
>>>>>> No, because the Third Law of Thermodynamics says you can’t get there
>>>>>> anyway.
>>>>>
>>>>> How close can one get?
>>>>
>>>> Arbitrarily close. I heard of experiments already being done in the
>>>> microkelvin range.
>>>>
>>>> Correction: just checked, and the Guinness World Record site reports a
>>>> figure of 38pK.
>>>
>>> Using lasers to slow the particles down !
>>>
>>> When a particle is vibrating towards the laser, a picosecond blast
>>> of energy slows it back down. Using heat to achieve cold.
>>
>> Targeting a single particle without casting any effect on any other 
>> particle? Can that be done?
> 
> 
> It's not done that way - the laser beams are continuous, but they are 
> tuned and/or polarized to interact more with particles moving the "wrong 
> way", slowing them down on the average, which means cooling them. The 
> particles "self select" to interact with the beams, based on Doppler 
> effects or other effects that depend on particle movements.
> 
> https://en.wikipedia.org/wiki/Laser_cooling
> 

Yes.  I only learned about that recently - previously I had some vague 
(and wrong) ideas that about lasers hitting above-average temperature 
particles that moved faster and further than the rest.

To give Chris a little more detail, the atoms will absorb light at 
particular frequencies, where the photon energy matches energy levels 
for its electrons.  The closer the frequency matches, the higher the 
probability of an absorption.

You can imagine the atom vibrating back and forth, with a speed 
dependent on its kinetic energy (its temperature).  If you shine a laser 
with a particular frequency at a stationary atom, it will "see" that 
exact frequency of light.  But if it is moving towards the light source, 
it will "see" a higher frequency, while if it is moving away from the 
light source, it will "see" a lower frequency - that's the Doppler effect.

So you pick a laser frequency so that when a hot atom is moving quickly 
towards the light, it has a high probability of being absorbed.  When 
that happens, the light exerts a force on the atom against its direction 
of motion, slowing it down and thus reducing its kinetic energy and 
temperature.  Of course some other atoms will be hit too - they have a 
lower but non-zero probability of absorbing the photons.  Overall, 
however, you reduce the average kinetic energy.