Comment by philipkglass | Hacker News Reader

Comment by philipkglass | Hacker News Reader

There is something missing from this press release.

In their new study, he and his colleagues used techniques they developed previously to first freeze a cloud of fermions — in this case, a special isotope of lithium atom, which has three electrons, three protons, and three neutrons. They froze a cloud of lithium atoms down to 20 microkelvins, which is about 1/100,000 the temperature of interstellar space.

“We then used a tightly focused laser to squeeze the ultracold atoms to record densities, which reached about a quadrillion atoms per cubic centimeter,” Lu explains.

They're freezing lithium 6 vapor and increasing its density. But lithium 6 is normally a solid at room temperature. Solid lithium 6 would contain about

(6.02 * 10^23 / 6.0151 [0]) * 0.460 [1] = 4.6 * 10^22

atoms per cubic centimeter -- a density 7 orders of magnitude greater than what has been attained in this work. Is there something special that prevents this frozen atom cloud from condensing to an ordinary solid? Is it just a very short lived state observed before it condenses to an ordinary solid?

[0] Atomic weight of lithium 6

[1] Specific gravity of lithium 6: https://aip.scitation.org/doi/abs/10.1063/1.1743927

The ELI15 explanation is that at these temperatures, quantum mechanical effects take over and the atoms themselves become indistinguishable from each other. They no longer have a definite position in space and experimentally act as matter waves [1].

This diagram is for bosons, but the top two boxes still apply for fermions (Bose-Einstein condensates are different but closely related): https://condensed-matters-cmcdt.wp.st-andrews.ac.uk/2019/02/...

The process is 1) cool until the de Broglie wavelength is larger than the typical interparticle distance and 2) compress.

If you did 2) before getting 1) then you would risk forming a classical liquid/solid. More info on the process: [2].

[1] https://en.wikipedia.org/wiki/Matter_wave [2] https://www.uni-muenster.de/Physik.AP/Demokritov/en/Forschen...

> Is there something special that prevents this frozen atom cloud from condensing to an ordinary solid?

Yes. The fact they are Fermions and the Pauli exclusion principle is what stops them from condensing into a solid. Compared to the effect of the Pauli exclusion principle in solids it is very much a case of same same but very different. In solids there are a limited number of 'states' the electrons can occupy. With a gas of free atoms, there are infinite states for the atoms to be in, however the Pauli exclusion principle still affects the probability field of the atoms with respect to each other. Specifically, the Pauli exclusion principle ensures that no atoms are ever stationary with respect to each other. This stops them from colliding with each other.

The best way to think of this is as one of those nbody planet simulations. If you place two stationary planets near each other they start moving together and eventually smash into each other. However, if the planets are moving they just keep zipping around each other, never actually hitting (unless you are very unlucky). The same is true with a cold gas of Fermions. This is often referred to as a 'centrifugal barrier' that keeps them apart.

The missing piece is that the coldness of the atoms isn't a random stat. It's an incredibly important feature of this research field. The density in the context of a quantum gas is what's interesting

In general their article's use of "Fermion" clashes with my understanding. I've understood fermions[1] to be a type of sub-atomic particle, like an electron. The article routines refers to them as if they are atoms themselves, including your quoted test.

I presume the MIT literature knows more than i do here, but i'm unsure what i am missing. Anyone have an idea?

1. https://en.wikipedia.org/wiki/Fermion

Read the “Composite Fermions” part of the Wikipedia article you posted. Anything, including many atoms, composed of an odd number of fermions is, itself, a fermion (and anything composed of an even number of fermions is a boson.)

If it's only an odd-even difference, why are composite bosons (like, whole atoms) so more.. exotic than composite fermions?

That is, why do atoms normally obey the Pauli exclusion principle?

> If it's only an odd-even difference, why are composite bosons (like, whole atoms) so more.. exotic than composite fermions?

> That is, why do atoms normally obey the Pauli exclusion principle?

Whole atoms can be fermions (the Wikipedia article gives the example of ³He), and the Pauli exclusion principle applies to fermions within a given quantum system, not to bosons (though sometimes the quantum system itself is a composite boson, though it could also be a composite fermion—the most common set of systems in which it is discussed, atoms, can be either.)

What I'm asking is, everyday materials usually obey the Pauli exclusion principle

Why is the situation where an whole atom doesn't obey the Pauli exclusion principle much more exotic and unusual?

I mean, take a look at the wikipedia pages https://en.wikipedia.org/wiki/Composite_boson and https://en.wikipedia.org/wiki/Composite_fermion - when one talks about composite bosons, it's exotic materials like superfluid helium, bose-einstein condensates, cooper pairs.

Since it's only a even/odd difference, I would expect that roughly half materials form composite bosons, and the other half form composite fermions, but this doesn't seem to be the case.

> when one talks about composite bosons, it's exotic materials like

Atoms with both even atomic and even mass numbers, nuclei with an even mass number.

I mean, both wikipedia articles you point to focus on exotic examples, but there are far more common mundane examples of both, too.

> everyday materials usually obey the Pauli exclusion principle

In the everyday case, this is pretty much always in reference to electrons (which are fermions). Even when we're talking about atoms or solids, the effects of the PEP are due to electrons: https://en.wikipedia.org/wiki/Pauli_exclusion_principle#Appl...

The quantum-mechanical wavelength of everyday whole atoms is much smaller than their physical size, so they behave as classical particles (and the PEP doesn't really apply). In contrast, electrons have wavelengths large enough that they exhibit macroscopic quantum mechanical effects in everyday scenarios.

> Since it's only a even/odd difference, I would expect that roughly half materials form composite bosons, and the other half form composite fermions, but this doesn't seem to be the case.

Every element has bosonic and fermionic isotopes. Neutral atoms have equal numbers of protons and electrons, so any neutral atom with an odd number of neutrons is a composite fermion, and any neutral atom with an even number of neutrons is a composite boson.

Every day objects do not really bother with the Pauli exclusion principle. The amount of phase space available at room temperature is vast and there are staggering amounts of degrees of freedom.

What everyday objects obey is electromagnetism. The electric bond between things we think of as objects is much much much stronger than the force you can apply by normal means. So objects which come in contact are excluded from the same space not by Pauli principle but because the force to combine them is astronomical in human terms

I believe it was a Leonard Susskind lecture[1] where he said, if the electromagnetic force were unitary like gravity (not positive and negative) and therefore couldn't be balanced to near-0 in a single object, the force of attraction between two grains of sand at each end of the lecture hall (say, 15 meters) would be 3,000 tonnes. I'm probably butchering the numbers, but the electromagnetic force is wildly strong, and this thought experiment always stuck with me.

Another consequence is that, while black holes can have charge, we don't expect them to in nature—they'll very quickly suck up whatever it takes to balance them from surrounding space.[2]

[1] One of these: https://www.youtube.com/playlist?list=PLQrxduI9Pds1fm91Dmn8x...

[2] Via one of Sean Carroll's "Biggest Ideas in the Universe" videos

Is that why when a star starts fusing iron, it explodes into supernova? The electromagnetic force overcomes gravity?

The term "fermion" is often used for everything with a half-integer spin. This is because their wave-functions are still anti-symmetric and therefore the Pauli exclusion principle applies leading to Fermi-Dirac statistics.

A lot of terms are reused for what are technically compound particles, especially since it isn't always too obvious which properties of a particle are intrinsic and which are caused by interactions.

In physics you often find rules for modeling composite objects in terms of simple ones. For example a rigid body can be modeled for many purposes as a point mass, whose value is the rigid body's total mass, located in its barycenter.

In quantum mechanics you can also form composite objects (in the most common formalism it will be a "wavefunction") and you still can determine a total fermion/boson-ness as a function of the elementary objects that form it. So yes, an atom, a proton, a molecule, a table all can be bosons or fermions once you decide to ignore some internal degrees of freedom.

In fact even those subatomic particles you mention could be composite objects for all we know.

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