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
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?
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].
> 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?
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.
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.)
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]
> 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.)
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.
> 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.
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.
Didn't understand if it stops but do not bounce back light or if it lets light to pass as there was nothing there. Those are different ways to be invisible.
>“An atom can only scatter a photon if it can absorb the force of its kick, by moving to another chair,” explains Ketterle, invoking the arena seating analogy. “If all other chairs are occupied, it no longer has the ability to absorb the kick and scatter the photon. So, the atoms become transparent.”
by that logic neutron stars should be highly transparent/invisible too.
> by that logic neutron stars should be highly transparent
Neutron stars aren’t uniformly crushed to the limit where they couldn’t interact with photons. That would be the point at which they’d be on the verge of collapsing into a black hole, and photon scattering would be irrelevant.
I think a neutron star is fully degenerate; effectively it's all core. It's not made of atoms; it's more like a huge atomic nucleus. It's much more dense than the "super-dense" gas these researchers are playing with.
A newly-formed neutron star should contain about as much heat as the stellar core that collapsed to form it, but in a much smaller space; so I'd expect it to be super-hot. WP says it's temperature is around 600,000K.
So rather than becoming transparent, I'd guess that a neutron star would be super-reflective. It's surface is ultra-smooth, and I can't see a photon getting far into a neutron star without encountering a neutron. Newly-formed neutron stars lose heat very quickly, but they do that by emitting huge numbers of neutrinos, not photons.
Hmm. A farfale phase? (butterflies) Perhaps a chonchiglie phase? Made of the hardest substance in the universe? Mmm - tasty.
Thanks - I knew neutron stars had some kind of "atmosphere", but I didn't realise they had layers and interlayers.
[Edit] I didn't even realise that NSs had protons in them. I thought protons all degenerated into electrons and neutrons. I have new stuff to learn about - I knew NSs were about the weirdest places anywhere, but they're weirder than I thought.
Crazy thought I has was what if you could make a supercooled wall to the point you could see through it then you could realistically only see it if you perturbed it and what you'd be able to see is the "wave" as the energy travels from the perturbation. Kinda like those anime shields. Maybe a little too crazy of a thought.
If I understand this correctly, couldn't this explain dark matter?
The article didn't say how dense this cloud was, but if it was still a cloud it couldn't be that dense right? Surely something that could exist in the cold vastness of space.
Fair, but the article implied that it's a somewhat linear effect- you don't need absolute zero for it to make the cloud dim. The article didn't do a great job of saying what temps they were testing at.
Still, I'm not surprised. I doubt some armchair scientist is going to solve the riddle of dark matter. I was curious and got some good answers. Thanks!
That was fun to read. Even funner to realize that the huge relative number is actually more useful too. Saying 2.72 degrees sounds small, but a difference that large on average throughout the universe would be absolutely insane And break basically all of physics
The temperature variance across the CMB is incredibly small. It depends on what you mean by uniformly spread, of course, but the fancy colorful images you often see are showing variance out to like 4 or 5 decimal places, if I recall. Like thousandths or ten-thousandths of a degree K.
I would assume this is because of Heisenberg. If the Atom does no longer move you should be able to nail it down to a specific position yet at this tinyness we are entering the land of probable positions instead of definite ones so the atom has to disappear so laws of physics still hold.
Agree it's an exaggeration, but still seems fascinating - that photons with the right energies go straight through, as if the matter wasn't there?! Does sound quite dark-matter-ish, but as someone else pointed out, the temperatures seem off. I prefer to believe dark matter is mostly something we don't yet quite understand about gravity/matter/space.
Physics is a highly speculative field due the limitation of our technology.
Physicists know this. But laypeople don't.
With the anti and pro science folks, this part is getting worse where you are called multiple insults for even casting doubt on any physics theory/law like "nothing can be faster than speed of light" or "energy is always conserved".
A) Superconductors can conduct a theoertical infinite amount of electricity when supercooled (super-cooling being the key to this phenomena)...
B) That there is a corollary; a relationship between electricity and information; that is, that electricity can carry information (unless you're reading this in the far future, the computer you are reading this on is proof of that! <g>), and a theoretical infinite amount of electricity can carry a theoretical infinite amount of information...
C) If A and B are true, then that would imply that super-cooling -- is the key to being able to transfer a theoretically infinite amount of information, that is, there's a link, a relationship between super-cold things (super low temperatures) -- and the ability to pass information through it...
So, that brings us to the title of this article: "Ultracold, superdense atoms become invisible".
If this article is true, then not only is there a link between super-cold temperatures, the ability to pass theoretically infinite information through a superconducting material (which really just consists of atoms of the superconducting substance, which really just consists of probable repeating structure of that substance's atoms in space...) but also atom invisibility...
So, if true, we have ultra-cold, invisibility, and theoretical infinite information... all at the same place, at the same time...
OK, so with that background, I'm going to go for "full crackpot" here... <g>
If atoms (or heck, any subparticle really, this would include any and every subparticle) -- become invisible when super-cooled -- then:
Question #1: Are they really there, at all? (When super-cooled?)
Question #2: If the answer to Question #1 is that they are not (again, when super-cooled), that they somehow collapse (for the time that they are super-cooled) and then re-emerge when heated (boy, wouldn't this solve all problems of all kinds of atoms and sub-particles "disappearing" and "reappearing" if true?) -- then here's "full crackpot":
If all of that were true -- then couldn't any single atom or sub-particle (any of them, no matter how small) -- be modeled and/or viewed as INERTIA -- but INERTIA RELATIVE TO SCALE, INERTIA RELATIVE TO TEMPERATURE... or more specifically INERTIAL FIELDS RELATIVE TO TEMPERATURE, or even more specifically INERTIAL FIELDS RELATIVE TO TEMPERATURE, RELATIVE TO SCALE...
Well, if one or more things, one or more aspects of the above logic is false, then the answer is 'No'...
But who knows?
Maybe it's possible...
And then again, maybe it isn't... <g>
In other words, if true -- all atoms, particles and subparticles, all of them -- are INERTIA (force) -- at their relative scale...
They are only invisible for a laser with a color with low energy that can only "move" the atoms. When light colides with an atom and changes direction, the atom has some speed after the colission.
If the atoms can't move because there are other atoms blocking them, then the can't redirect the light. [I oversimplified a few details here.]
If you use a laser with other color, then the light can be absorbed by the electrons in the atoms, and the electrons can jump and later emit the light. So it will not be visible with another laser. [Probably an ultraviolet laser is enough.]
> Question #1: Are they really there, at all? (When super-cooled?)
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