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Turning Matter Invisible!

Image: Pixabay

By Inés Urdaneta, Physicist and research scientist at RSF

By manipulating quantum properties in atoms, scientist at MIT were able to prevent a sample of Litium atoms (6Li) from scattering light, therefore, turning it invisible! In other words, the capacity of the atoms to scatter light, was suppressed ...

This effect was predicted theoretically 30 years ago, and it is an example of a phenomenon called Pauli blocking, based on the Pauli exclusion principle, where electrons in atoms are forbidden to occupy the same quantum state. In standard conditions, electrons in an atom are arranged and localized in such a way that they are all distinguishable from each other; they cannot superpose. This is a property of fermionic particles; they all have different quantum states (identified by quantum numbers) and so they are distinguishable.

If the electrons in an atom where like people in a hotel room, each person would be identified with 4 numbers (the four quantum numbers): the first one would stand for the floor in the hotel, the second one would stand for the room in the floor, the third one would stand for the bed in the room, and the fourth would stand for one of the two possible positions in the bed. A couple may have the first three identical numbers, meaning that they are lodging in the same floor, room, and bed, but the fourth would be different. If all 4 numbers where the same, they would be the same person, or both would be completely superposed and therefore, indistinguishable from each other. This later case happens with particles of light called photons, which can superpose. But matter (electrons and atoms) can't superpose.  

“Like concertgoers in an arena, each electron occupies a single chair and cannot drop to a lower tier if all its chairs are occupied. This fundamental property of atomic physics is known as the Pauli exclusion principle, and it explains the shell structure of atoms, the diversity of the periodic table of elements, and the stability of the material universe.” -Jennifer Chu, MIT.

Atoms have allowed energy levels with orbitals occupied by electrons. An energy level would be the floor in the example of the hotel given above. An energy level has groups of orbitals, which would be the rooms in the floor, and the orbitals in each group (the beds in a room) can be occupied by at most two electrons with opposite spins (these would be the two possible positions in a bed) as to preserve the Pauli exclusion principle that guarantees electrons remain distinguishable. Based on this feature, atoms and elements have particular electron distributions that are specific to each atom, and the diversity of the periodic table of elements is explained by this.



When light encounters these atoms, generally the photons of light and the atoms are bouncing around, like billiard balls, and photons impart part of their momentum (kinetic energy) to the atoms, with a partial loss of their own energy. With the impact, photons also get deflected in every direction, radiating light all around, as shown in the figure below by the curved red arrows. This makes the gas visible, just as you would see the specks of dust when light goes through an apparently empty volume of space.

What happens if atoms were almost completely frozen and were squeezed into a tight enough space? Then the atoms themselves would behave like electrons in packed energy levels or shells, with no room to shift their velocity, or position. Under such circumstances, if light were to stream in, it wouldn’t be able to scatter.

Figure taken from the original preprint.

The degenerate gas is an extremely cooled sample of atoms, where the kinetic energy of the atoms remains confined in a range with maximum kinetic energy called Fermi energy -in reference to metals, where only the electrons with this amount of kinetic energy are free to move in the metal and interact with light-. The difference in kinetic energy between the incident photon and the scattered photon, in this case is noted by q, and it would correspond to the kinetic energy that the atoms would acquire during the impact with photons. As explained by Ketterle, an atom can only scatter a photon if it can absorb the force of the photons kick by moving to another place. If all other places where the atom may move are occupied, it no longer can absorb the kick and scatter the photon; the atoms become transparent.

The MIT physicists have observed for the first time this Pauli blocking, happening when atoms are cooled and squeezed extremely, meaning that the particles effectively have less room to scatter light, such that if the cooling to create a density of atoms is high enough, photons stream through without being scattered. The sample then becomes transparent to light, hence, invisible.


RSF in perspective

This is a remarkable example of the combination and exchange of behaviors between light and matter. It is as if the cooling and squeezing of the atoms gradually erased their “individuality” and therefore, the most important feature of materiality, which is interaction with light, has been suppressed, so we “cease” to see it. The atoms are being pushed into indistinguishably, sort of speak.

In the frame of the generalized holographic theory, an electron is not a separate entity occupying an orbital, instead it is a field of charge resulting from the dynamics of the proton, that we call an orbital. Module 7, section 7.1 The electron in the frame of the generalized holographic model of our free unified science course, gives a detail explanation of the model for the bounded electron in the hydrogen Bohr atom.  Free course in

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