Sunday, January 20, 2019

Frontiers of physics - an underappreciated point

In what branch of physics are the most extreme conditions reached?  If asked, I'm sure the vast majority of people would guess particle physics. Enormous machines (and they want bigger ones all the time) are used to accelerate particles up to a hairsbreadth below the speed of light and smash the particles into each other or into targets.  The energy densities in those collisions are enormous and by intent are meant to rival conditions in the earliest moments of the universe or in extreme astrophysical conditions.  Still, while the details are special (nature doesn't collide directed bunches of ultrarelativistic protons head on), the fact is that there are, or at least have been, naturally arising processes that approach those conditions.  

The fact is, condensed matter physics (CMP) and atomic/molecular/optical (AMO) physics are actually more extreme, reaching conditions that do not ever happen spontaneously, anywhere.  Now-common laboratory techniques in CMP and AMO can produce experimental conditions that, as far as we know, simply do not occur in nature without the direct intervention of intelligent beings.  

The particular condition I'm talking about is temperature.  As I discussed a little here, temperature is a parameter that tells us the direction that energy tends to flow when two systems (say a coffee cup and a coaster) are allowed to exchange energy via microscopic degrees of freedom that we don't track, like the kinetic jiggling of vibrating atoms in a solid.  When the cup and coaster are at the same temperature, there is no net flow of energy between them, even though some amount of energy is fluctuating back and forth all the time.  

The cosmic microwave background, the relic electromagnetic radiation left over from the early universe, is described by an intensity vs. frequency distribution that we would expect from radiation in thermal equilibrium with a system at a temperature of 2.726 K.  What this means is, if you had some lump of matter floating in interstellar space, and you waited a very long time, the temperature of that lump would eventually settle down to 2.726 K, absent other effects.  It would never be colder.

In CMP labs around the world, however, macroscopic lumps of matter are routinely cooled to temperatures far colder than this.  With a conventional dilution refrigerator (see here) it is possible to cool kgs of material down to milliKelvin temperatures.  Through demagnetization cooling, particularly of materials with nuclear magnetic moments, microKelvin temperatures may be reached.  In AMO labs, laser cooling techniques can get clouds of atoms down to nanoKelvin temperatures, though typically the number of atoms involved is far smaller.  Pretty amazing, when you think about it!


8 comments:

SP said...

This reminds me of the flurry of articles (for example, this one from the BBC) that came out when the LHC was first cooled below CMB temperature in 2008. Cooling 27km of magnets to ~2K is obviously quite impressive, but implying that this is somehow an extreme temperature ("one of the coolest places in the Universe") strikes me as misleading.

Anonymous said...

Well, the fact that 2 K <2.726 K renders that statement from the BBC quite correct. There may be 1000 more places in the universe that are as cold or colder (all on Earth), but that is not much given the size of the universe.

PhDstudent said...

@Anon-

Natural processes can get you down to at least 1 K, see the Boomerang Nebula: https://en.wikipedia.org/wiki/Boomerang_Nebula

DanM said...

I suggest that the extreme conditions at the focus of an amplified femtosecond laser might be considered even more extreme than the extreme conditions of temperature in a dilution fridge. Peak optical intensities exceeding 10^19 watts/cm^2 are now not so unusual, and 10^22 has been achieved in more than one lab. I'm pretty sure that the natural universe never approaches within many orders of magnitude of those numbers.

Douglas Natelson said...

PhDstudent, thanks - that was really interesting. Yes, I suppose that adiabatic expansion of a gas can transiently cool the gas to below the CMB temperature if the initial conditions were correct. For temperatures of macroscale solids, though, I'm not sure how you could get down there (barring some sort of fluky natural demagnetization process or contact with somehow boiling off liquid helium).

DanM, I was wondering about that. My high energy astrophysics colleagues who like to do experiments, e.g., at the Omega facility at Rochester or the TX petawatt facility in Austin (apparently resuscitated to some extent) have made passing comments to me before that there are astrophysical situations where crazy intensities like that may actually happen (fireball generation in colliding neutron stars?). I don't know, but I could ask. (For those unfamiliar: at intensities like that, the electric fields are transiently so intense that it's possible to create e-positron pairs, and accelerate an electron up to relativistic velocities in a fraction of an optical cycle.)

Ted said...

I expected you to mention negative temperatures, which in a sense are even more extreme!

https://en.wikipedia.org/wiki/Negative_temperature

DanM said...

I'd be interested to know if the astrophysics people think that there are naturally occurring electric fields large enough to exceed the threshold for pair creation. But, to be honest, the main reason I mentioned it here was LASERS.

Douglas Natelson said...

DanM, sure :-) Amazingly, there can be naturally occurring lasers as well (http://laserstars.org/news/MWC349.html). I guess that means that there could be some version of negative temperatures, depending on definition, though in equilibrium temperatures must be non-negative.