

Relativity gives gold its color - _delirium
http://www.fourmilab.ch/documents/golden_glow/

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kurtosis
"If someone tries to sell you non-relativistic gold you are being had!"

This is mostly a concern for theorists who prefer to go as far as possible
with non-relativistic theories.

This relativistic shift also has large implications for the bonding of gold
nanostructures. The relativistic correction shifts the energy of the core
electrons and they hybridize - giving the Au - Au bonds directionality. Small
gold clusters can thus take on planar, cage, and tubular structures -
completely contrary to what you would expect from surface tension.

see these wonderful papers:

Evidence of hollow golden cages:
<http://www.pnas.org/content/103/22/8326.full>

These small structures are famous for the fact that they are _not_ chemically
inert! Many people are exploring applications as catalysts to complete the
oxidation of CO in automobiles.

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_delirium
Here's a plot of the aluminum/silver/gold reflectance spectra, fwiw (visible
range is ~390-750 nm): [http://commons.wikimedia.org/wiki/File:Image-Metal-
reflectan...](http://commons.wikimedia.org/wiki/File:Image-Metal-
reflectance.png)

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millar
If you have some time to kill and an interest in Chemistry and its history
check out Donald Sadoway's excellent lectures at MIT -

Introduction to Solid State Chemistry
<http://www.youtube.com/watch?v=R90sohp6h44>

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roundsquare
IANAP but I thought that applying relativity to objects that quantum mechanics
traditionally studies crates some contradictions. This, as I understand it, is
why there is a search for the "Grand Theory of Physics" or whatever the cool
name is for it these days. Am I wrong?

~~~
hugh3
Yes and no. The short answer would be that the interface between quantum
mechanics and special relativity is fairly unproblematic but the interface
with general relativity has some huge problems. Since this is a special-
relativity effect it's pretty well understood.

~~~
roundsquare
Thanks for the answer. I didn't realize it was only GR that caused the
problem.

Do you know any good places I can get a fairly simple description of the long
answer? (I.e. a good place to learn about why the interface between GR and QM
is problematic)?

~~~
btilly
Any simple explanation you find will be of the form "lies for children". The
real explanation is, of course, "None of the things we've tried have succeeded
in making testable predictions." But to understand that you need to understand
what we have tried, which means understanding the theories, which means a very
considerable investment of effort.

That said, I can illustrate the conflict. The fundamental idea of general
relativity is that mass warps the structure of space-time, and we perceive
this warpage as gravity. One fundamental idea of quantum mechanics is that a
quantum mechanical system is always in some superposition of possible states,
and the superposition adds. (Since we're dealing with a complex wave,
sometimes adding 2 possibilities leads to cancellation in one place and
strengthening in another, creating interference patterns. But fundamentally
we're just adding waves.)

One of the things that nobody knows how to do is add different kinds of
warpage of space-time in a linear way and get a sensible answer.

Let's try something else. In QM perturbations of any kind of field must have
an associated particle with describable characteristics. For a random instance
vibrations in a lattice of atoms can be described by a field that must have an
associated particle. That particle is called the
<http://en.wikipedia.org/wiki/Phonon>. (Phonons are not fundamental physical
particles, but they do exist mathematically and their effects have been
experimentally measured.)

Now in QM the exchange of particles actually gives rise to various forces.
Furthermore the ability for virtual particles to arise and disappear as long
as you stay under the Heisenberg limit causes the prospect of particles
interacting with various things, including themselves. When you try to
calculate all of these interactions you wind up getting infinities added and
subtracted with each other. There is a mathematical trick called
renormalization that lets you cancel these out.

Now in General Relativity you have gravitational waves. (They have never been
directly detected, but binary star systems have been observed losing power in
accord with the predictions of GR.) If you try to fit these waves into QM you
should wind up with particles called gravitons. But when you try to do
renormalization on them, the procedure fails and you get infinities out.

So we have failures both ways. Attempts to state QM within the framework of GR
fails because we don't know how to add perturbations in coordinate systems.
Fields that GR predicts exist cause QM to break down and give impossible
answers. The theories just don't merge. Experiment doesn't help because each
theory is a very good description in its own region, and we can't get data on
how the theories break down between their areas of specialization because we
can't create regions with high gravitational curvature on a scale where
quantum mechanical effects matter.

If you want to know more about some of the attempts to reconcile the theories
I'd suggest reading up on "string theory", "quantum loop gravity", and trying
to understand John Baez. I'm not a physicist and don't understand John Baez
myself, so I can only wish you good luck if you choose to dive more deeply
into this.

~~~
coconutrandom
Thanks for the relatively (lol) concise response. But really, thanks.

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Gonsalu
<http://www.webexhibits.org/causesofcolor/9.html>

I find this article provides a better explanation (and thorougher) of the
peculiar properties of gold, and some practical manifestations of those
properties.

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RevRal
Wow, the alchemists had no idea how far off they really were.

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ww520
Naive question. If you bombard gold with high energy particle beam, would you
be able to bounce off some electrons and get gold isotope? What color would it
be?

~~~
eru
You don't get isotopes by adding or removing electrons. You'd need to alter
the number of neutrons for isotopes. And that doesn't normally change the
color.

If you would add or remove electrons from your gold particles, you'd need to
keep the electrons somewhere close-by e.g. keep them with your anions. If you
separate a lot of atoms from their electrons--you get a blob charged matter.
To get all the atoms in 1 mol of gold (around 200g, say in a small cube) to
each lose one electron and put the mol of electrons somewhere else, you'd need
some non-trivial amount of energy. (Sorry, I am currently to lazy to calculate
it..)

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marknutter
I would that light gives gold its color. Or better yet, the human eye.

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mhb
What would the color of gold be if there was no relativistic effect?

~~~
quantumhobbit
Silver. The article explains that without the effects of relativity the
electron transition that absorbs blue light(leaving a combination of colors
that appear golden) would instead absorb ultra-violet light. With no visible
colors absorbed, it would reflect all colors the same like silver.

