Gravitational-Wave Observatory (LIGO) provides evidence that ...

Byron Leigh Hat...
Joined: 18 Jan 05
Posts: 116
Credit: 2,037,008
RAC: 0
Topic 194445

Reaching microkelvin temperatures, the Laser Interferometer Gravitational-Wave Observatory (LIGO) provides evidence that interferometric gravitational wave detectors (designed as sensitive probes of general relativity andastrophysical phenomena) can also become sensitive probes of macroscopic quantum mechanics, say
MIT...

The work is reported in a research article published today, Thursday, 16 July, in New Journal of Physics.

LIGO is a huge experiment, funded mainly by the U.S. National Science Foundation and involving more than 600 astrophysicists worldwide, undertaken to detect gravitational waves and thereby help us monitor space through another valuable set of lenses - gravitational radiation.

http://www.physorg.com/news166941860.html

tullio
Joined: 22 Jan 05
Posts: 2,118
Credit: 61,407,735
RAC: 0

Gravitational-Wave Observatory (LIGO) provides evidence that ...

There is one thing I do not understand. AFAIK the mirrors are operating at room temperature. How can they be cooled to a microkelvin simply by reducing their mechanical vibrations? I remember the formula Kinetic energy=3/2 kT, where k is the Boltzmann constant. So if I lower the Kinetic energy I am lowering the temperature?
Tullio

Mike Hewson
Moderator
Joined: 1 Dec 05
Posts: 6,569
Credit: 294,712,709
RAC: 97,093

If I've read the article right, the temperature is an effective one for a particular mode within the interferometer. If my memory serves me correctly there is the 'equipartition' theorem in thermodynamics which states that energy is distributed, in equilibrium, equally between all independent degrees of freedom for a system. So yes, if you have say a large group of interacting particles - like the mirror's crystal lattice - without internal degrees of movement of the particles ( like rotations ) then that leaves you with the 3 spatial directions of motion. As you get kT/2 for each dimension that is thus 3kT/2 overall for 'ordinary' kinetic energy. Or put another way temperature is a measure of the average kinetic energy of the molecules.

It seems they have divided the motion of the mirror atoms into two components : that which is about the centre of mass of the mirror, and that which is the motion of the centre of mass. One can legitimately do this as it comes down to simple vector decomposition of each atom's velocity vector. You could interpret the motion about centre of mass as 'ordinary' temperature. The motion of centre of mass ( measured via changes in separation of the mirrors ) is allotted an equivalent temperature - the microKelvins quoted. I guess the point is that we don't have equilibrium in that the interferometer actuators can drain energy from the mode defined as differential mirror motion especially. So you assign what energy that is left in that mode as equal to kT/2, solve for T - getting those microKelvins.

This trick can be used to define a 'spin temperature', say, in magnetic materials. There you define some mode related to spin alignments in some external magnetic field. Find the energy attributable to that interaction, regardless of whatever else might be going on in the substance, set that equal to kT/2 etc. An interesting issue would be to what extent does kinetic motion alter the spin alignments, and hence how is the total energy of the system ( all forms ) distributed amongst the modes. Are we in equilibrium? Have we waited long enough compared to some characteristic 'relaxation' time and so on.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

MAGIC Quantum M...
Joined: 18 Jan 05
Posts: 1,784
Credit: 1,218,267,016
RAC: 2,042,331

Quantum gravity

tullio
Joined: 22 Jan 05
Posts: 2,118
Credit: 61,407,735
RAC: 0

Whatever the results of LIGO,

Whatever the results of LIGO, VIRGO, etc. I would like to point out the difference between the astronomers' and astrophysicists' method, that of observing the Universe at all wavelengths, and the experimental physicists', that of making matter colliding with other matter at always rising energies (and costs) as the CERN history shows. A great experimental physicist like Edoardo Amaldi has switched his interests from particle accelerators to gravitational waves detectors in the latter part of his life, Another great Italian physicist, Enrico Fermi, once convinced his Italian colleagues not to build another cyclotron in Pisa but the first Italian designed computer, which Olivetti later named Elea. He understood that the future of physics lied not in accelerators but in computers.
Tullio

Martin Ryba
Joined: 9 Apr 09
Posts: 48
Credit: 155,859,935
RAC: 33,517

Message 93822 in response to message 93819

Quote:
If I've read the article right, the temperature is an effective one for a particular mode within the interferometer. If my memory serves me correctly there is the 'equipartition' theorem in thermodynamics which states that energy is distributed, in equilibrium, equally between all independent degrees of freedom for a system. So yes, if you have say a large group of interacting particles - like the mirror's crystal lattice - without internal degrees of movement of the particles ( like rotations ) then that leaves you with the 3 spatial directions of motion. As you get kT/2 for each dimension that is thus 3kT/2 overall for 'ordinary' kinetic energy. Or put another way temperature is a measure of the average kinetic energy of the molecules.

I haven't read the specific article through, but I recall (dim memory) that a popular college physics exercise was to calculate the de Broglie (sp?) wavelength of a baseball. The fact you could define one doesn't mean a baseball exhibits quantum behavior on a measurable scale.

I'm curious, since my thermo memory would make me believe that you would have to prove few enough "degrees of freedom" for the effective temperature to be that low. Therefore, you would have to show the mode of interest is sufficiently weakly coupled to things like the phonon gas (lattice vibrations) that set the normal temperature of macroscopic solid objects. Otherwise, the entropy of the phonons should swamp the degrees of the other effect.

Theory + experiment = physics. If their analysis shows something testable that is undeniably quantum, then bully for them.

"Better is the enemy of the good." - Voltaire (should be memorized by every requirements lead)

Mike Hewson
Moderator
Joined: 1 Dec 05
Posts: 6,569
Credit: 294,712,709
RAC: 97,093

Message 93823 in response to message 93822

Quote:

I haven't read the specific article through, but I recall (dim memory) that a popular college physics exercise was to calculate the de Broglie (sp?) wavelength of a baseball. The fact you could define one doesn't mean a baseball exhibits quantum behavior on a measurable scale.

I'm curious, since my thermo memory would make me believe that you would have to prove few enough "degrees of freedom" for the effective temperature to be that low. Therefore, you would have to show the mode of interest is sufficiently weakly coupled to things like the phonon gas (lattice vibrations) that set the normal temperature of macroscopic solid objects. Otherwise, the entropy of the phonons should swamp the degrees of the other effect.

Theory + experiment = physics. If their analysis shows something testable that is undeniably quantum, then bully for them.

Yep, there's a burden of proof all right. How can we know that these damping mechanisms are so good at draining energy from that quoted mode on the one hand, while that same mode is also sufficiently isolated ( ie. away from equilibrium ) with respect to the other degrees of freedom. And these interferometers are gadgets that can lose some delicate configuration/lock if an airplane flies overhead .....

[ and while we're down at quantum level, where's the cold load to dump the energy at anyway. Gotta have a cold spot to go with the hot spot for the energy to flow. ]

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal