
To make the perfect mirror, physicists confront the mystery of glass - sohkamyung
https://www.quantamagazine.org/to-make-the-perfect-mirror-physicists-confront-the-mystery-of-glass-20200402/
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metaphor
> _...Ediger and his team dropped molecules one by one onto a surface as if
> they were Tetris pieces, allowing them to find snug fits._

Let's assume a 40kg super mirror was made from the same base
silica[1]/tantala[2] substrate as the legacy mirror. Let's also assume 50%
distribution by mass.

That's a (20kg/60.08g/mol)+(20kg/441.893g/mol) ~= 378.1mol tetris endgame.

So if we took this analogy quite literally and wanted to build 1 mirror in 10
years, you'd have to play precision atomic tetris a mere 17 orders of
magnitude faster than this guy[3].

Not going to lie...sure does make watching grass grow sound a lot more
pragmatic.

[1]
[https://en.wikipedia.org/wiki/Silicon_dioxide](https://en.wikipedia.org/wiki/Silicon_dioxide)

[2]
[https://en.wikipedia.org/wiki/Tantalum_pentoxide](https://en.wikipedia.org/wiki/Tantalum_pentoxide)

[3]
[https://www.youtube.com/watch?v=v3TJXSFnVS4](https://www.youtube.com/watch?v=v3TJXSFnVS4)

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lachlan-sneff
Totally possible through convergent assembly.

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metaphor
Ahh, so that's the scalable magic trick they're playing. Thanks for the
insight!

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at_a_remove
I have often thought that technologically advanced civilizations would not
have FTL travel or transporters or the like, rather, they would be masters of
specced-out materials engineering, producing items by placing just the right
atom _here_ and a different one over _there_ to uniquely suit the purpose of
whatever it was.

~~~
N1H1L
Technically we already do that. Molecular beam epitaxy is layer by layer
growth of crystals, some of which like superlattices can only be engineered
from the ground up.

Source: My PhD is in Materials Science

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sunstone
I do this in my sleep. But then, so does everyone else.

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mannykannot
I was wondering whether a design using total internal reflection within
90-degree prisms at each end of the arms would be feasible for an instrument
of this precision. I assume it is not, and I have a few guesses as to why not:
1) the Fabry-Perot resonator arrangement of mirrors in each arm creates a much
higher intensity between the mirrors, and this is important for getting a
measurable signal out; 2) the resonator rejects a lot of noise; 3) there's no
good way to get power into, and a signal out of, an arrangement of prisms; 4)
an arrangement of prisms puts too much glass in the path. I would be
interested in hearing what the actual answer is.

Update: there's a lot more information here (though it is about the Advanced
LIGO concept), but I don't think it addresses the failings of a prism
arrangement directly:

[https://iopscience.iop.org/article/10.1088/0264-9381/34/1/01...](https://iopscience.iop.org/article/10.1088/0264-9381/34/1/015001/ampdf)

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chris_va
AR coatings are not perfect at the interface boundary (kind of the same
problem as a perfect mirror in reverse), plus glass has attenuation.

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mannykannot
I get the point about glass attenuation, but with regard to AR (anti-
reflection?) coatings, are you referring to the surface through which light
enters the prisms? If those surfaces are normal to the laser beam, would the
reflected light not still be contained within the beam path, just as it is in
the case of a Fabry-Perot resonator?

~~~
chris_va
Yes, but at the wrong path length (and offset by some vector, if you had a TIR
cube)

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newfeatureok
Reminds of me the Three Body Problem series. The "droplet" was a perfect
mirror because a mystery reflective material was tied together using the
strong force.

So I guess that means the "perfect" mirror must be bound by strong forces.

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NikolaeVarius
For some cool stuff about mirror development, I recommend reading up on the
Hale telescope.
[https://en.wikipedia.org/wiki/Hale_Telescope](https://en.wikipedia.org/wiki/Hale_Telescope)

Some massive improvements to mirror making techniques were derived from the
construction.

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ebg13
If the ideal state is perfectly flat and the proposed mechanism is to place
individual atoms one at a time, why are they still talking about glass and not
crystal lattice sheets?

> _ideal glass are theoretically packed together in the densest possible
> random arrangement_

Why would you want it to be random at all?

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VorticesRcool
You would like it to have a lot of entropy. It's not about being random in the
sense if being up to chance, it's about making the interactions between the
electrons so complicated that any passing photon does not get absorbed or does
not produce stimulated emission.

When there is enough entropy/complexity/randomness the electrons have no state
that they can jump to of a similar range of energy to the frequencies of light
that you would like the glass to be transparent to.

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moonliscuits
That was pretty interesting -- although a bit short. Happen to know of
something that's a bit more in-depth about the two-level motion of glass?

~~~
Phaune421
Turns out that this was my PhD thesis subject.

To put it simply two-level systems (TLS) are groups of atoms in a disordered
solid that present two stable configurations, that is two physical states of
similar potential energy separated by a small energy barrier. The barriers
need to be small for it to be overcome by thermal fluctuation alone and thus
for the TLS to resonate at a frequency close to the frequency of interest.

If you are interested by what exactly are these TLSs at the atomic level, have
a look at this paper:
[https://journals.aps.org/prb/pdf/10.1103/PhysRevB.97.014201](https://journals.aps.org/prb/pdf/10.1103/PhysRevB.97.014201)

