
What Gives Gold that Mellow Glow? (2006) - ColinWright
http://www.fourmilab.ch/documents/golden_glow/
======
sqrt2
It may be forgivable to use dynamic mass in an article written for laymen and
using a simple model of the atom, but I feel I should mention that the concept
of objects changing mass depending on the reference frame is a very dangerous
one because substituting the dynamic mass for the mass in a classical formula
does not always lead to correct results.

The concept of dynamic mass is motivated by wanting to continue to write the
previously known three-momentum as p = m v, which does not conform to special
relativity, hence the definition of mass is changed. However, in a formula as
basic as F = m a (F and a being vectors), substituting the dynamic mass for m
does not yield correct results because in general, under special relativity, F
and a do not even have to be parallel.

Modern formulations of dynamics in special relativity use the more intuitive
invariant mass, and three-momentum is written as p = m gamma v, where gamma is
the factor previously included in m_r. This p is now the spacial components of
four-momentum p^\mu = m u^\mu, where m is the invariant mass and u is the
relativistic four-velocity of the moving object.

~~~
marvin
This went way over my head. Is the article's claim that relativistic effects
cause gold's color incorrect?

~~~
yk
The article is correct. The nit from grandparent is, that it uses some
outdated language. So if you do not look to closely in special relativity, you
will find that the mass quite often appears together with the Lorentz factor,
which describes time dilation. This lead historically to the claim that moving
objects are heavier than the same object at rest. But since this does not hold
in general relativity, it is nowadays usually assumed that mass is always the
rest mass. And so the article reads a bit outdated.

------
Zenst
Very interesting explanation as to how colour or elements are what they are,
least for explaining silver and gold.

Certainly for non oxided elements, Bismuth being a wonderful element colour
wise with oxidisation in effect.

I do enjoy articles like this as they explain the more common building blocks
of elements in a way that gives you a different perspective beyond the
periodic table list and would love to see this expanded into a colour chart of
elements and there relationships.

Had quick look and found this:
[http://periodictable.com/Properties/A/Color.html](http://periodictable.com/Properties/A/Color.html)

Which is interesting as many are describes as Silver in colour, though very
few yellow (sulphur, chlorine) and Gold being described as gold. Though that
is the only metal yellow and with that would possibly explain its unique
colour.

On a side note I recall in the early days the fuss about creating that perfect
gold colour with R,G,B and the arguments abound with whos gold was gold and
whos was yellow. With that it is a fascinating colour and one that not easily
replicated.

~~~
creamyhorror
It was an early source of amazement for me to learn that we could actually
calculate the colours of certain substances from their electron transitions
(albeit with some empirically determined constants). A diagram, an equation, a
few constants, and boom, you have the actual colour of a substance that
matches your real-life observations. For reasons like this, I was quite into
chemistry for a few years in high school.

It's a demonstration of our ability to discover the principles of reality and
make meaningful predictions, rather than merely observe and catalog nature as
our ancestors did. It's one of the wondrous aspects of science for me.

------
mmcconnell1618
NOVA had a show on medieval stained glass that showed the size of gold
particles trapped in glass created different colors. If the mass and electron
distribution of gold is solely responsible for its color then shouldn't the
size of gold particles (above 1 atom in size) have no effect?

[http://www.pbs.org/wgbh/nova/ancient/science-stained-
glass.h...](http://www.pbs.org/wgbh/nova/ancient/science-stained-glass.html)

~~~
tfgg
You're right, the colour of materials is much more complex than just the
available atomic transitions. The electronic states are changed and spread out
by being bonded to other atoms and you get bulk collective properties like
polarizability. The relativistic contraction mentioned in the article is still
relevant for the electrons' energy levels in bulk gold (a big lump much larger
than the wavelength of visible light) and so neglecting it would give the
wrong colour for bulk gold.

Structures up to a few hundred nanometers in size could also conceivably
affect the colour. This is kind of like how the atomic-level description of a
radio antenna doesn't really matter, more its bulk properties like
conductivity. The gold nanoparticle effect is due to Mie scattering
([http://en.wikipedia.org/wiki/Mie_Scattering](http://en.wikipedia.org/wiki/Mie_Scattering)),
which is the scattering of light off dielectric spheres approximately the size
of the wavelength of light. This effect isn't relevant if you just have a
brick of gold.

~~~
Osmium
Indeed. For another example, photos like this are a classic example of how
particle size can affect colour:

[http://education.mrsec.wisc.edu/background/quantum_dots/imag...](http://education.mrsec.wisc.edu/background/quantum_dots/images/blacklight-700.jpg)

(solutions of CdSe nanoparticles in order of increasing particle size)

Broadly speaking, since "colour" isn't a well defined for things like atoms,
but it is for macroscopic objects, it makes sense that there's some weirdness
that goes on for particles in between those two extremes.

~~~
alokv28
The color change of gold and CdSe particles with size are due to two different
effects.

CdSe is a semiconductor, and reducing the CdSe nanoparticle size increases its
band gap through a process called quantum confinement. For smaller particles,
it requires a photon to have higher energy (i.e. smaller wavelength) to be
absorbed.

Gold particles derive their color from the scattering mechanism mentioned in
the parent comment.

~~~
Osmium
Completely agreed; I was just providing another example for how the colour of
large objects does not necessarily have anything to do with the colour of
small objects. Thanks for the added context :)

------
gus_massa
Previous submission:
[https://news.ycombinator.com/item?id=1246065](https://news.ycombinator.com/item?id=1246065)
(117 points, 1342 days ago, 17 comments)

The second comment has a link to to the spectrum comparison between gold and
silver.

------
jrockway
I like how the article starts by saying GPS hides too many details from the
user to be a good relativity example, and then dives into the "easier" example
of gold being yellow by explaining the electron distribution probability
function. That's much easier to grasp, thanks!

------
ColinWright
Reflectance vs. wavelength curves for aluminium (Al), silver (Ag), and gold
(Au) metal mirrors at normal incidence.

[http://commons.wikimedia.org/wiki/File:Image-Metal-
reflectan...](http://commons.wikimedia.org/wiki/File:Image-Metal-
reflectance.png)

(Found via a comment in the earlier submission[0] and discussion of this item.
Thanks to gus_massa[1] for finding that[2].)

[0]
[https://news.ycombinator.com/item?id=1246065](https://news.ycombinator.com/item?id=1246065)

[1]
[https://news.ycombinator.com/user?id=gus_massa](https://news.ycombinator.com/user?id=gus_massa)

[2]
[https://news.ycombinator.com/item?id=6874210](https://news.ycombinator.com/item?id=6874210)

------
fexl
Nice.

You may also recall this article about the nature of mercury:
[http://blogs.scientificamerican.com/the-curious-
wavefunction...](http://blogs.scientificamerican.com/the-curious-
wavefunction/2013/07/31/what-does-mercury-being-liquid-at-room-temperature-
have-to-do-with-einsteins-theory-of-relativity/)

------
smoyer
I've read the (900 page) Autodesk Files a couple times, but didn't realize how
many other topics John Walker has written about. If you look through this
site, there's even some fiction.

~~~
brudgers
The _Autodesk Files_ are wonderful context for HN discussions.

[http://www.fourmilab.ch/autofile/](http://www.fourmilab.ch/autofile/)

------
guard-of-terra
But why is copper reddish-yellow? I guess it's wikipedia time.

------
ommunist
And what gives pyrith that mellow glow?

------
interstitial
What does this have to do with the price of bitcoin?

