If we discover aliens, we’ll probably see them because of their farts.
The first signature of extraterrestrial life is likely to be unnatural compositions of gasses
in the atmosphere of a distant exoplanet. Think of the Earth, the atmosphere is full of oxygen,
and this is only true because it keeps on being replenished by plant and microbial life.
If life disappeared so would the oxygen. So if we see a distant planet with an atmosphere where the
proportions of gasses are inexplicable by the laws of chemistry alone, like lots of oxygen, or a mix
of carbon dioxide and methane but with little carbon monoxide, this would be a biosignature,
a strong indicator that at least microbial life exists there. To confirm any large creatures,
we’ll just have to visit. But how do we know what chemicals the atmosphere of a distant planet
contains? The answer is atomic spectroscopy. If we see a planet passing in front of it’s star,
some of that starlight is absorbed in a very specific pattern called an atomic absorption
spectrum. Each element has a specific pattern like a barcode, so through careful analysis of
the light it can tell us which gasses are in the atmosphere and their proportions.
We already use this technique for other space objects like stars and nebulae,
measuring properties like temperature, density, ionization and relative velocity. Think about how
cool this is! We can look into deep space and get so much information, just from the light.
It is like a gift that the laws of physics have given us and we’ve used it, most of our knowledge
of the universe comes from this technique. I’ve previously mentioned atomic spectra in my map
of quantum physics video and because this subject is so cool I made this video to dig deeper.
There are two kinds of atomic spectra: absorption spectra which is where the light source is behind
a gas, and the gas absorbs and scatters certain wavelengths of light leaving dark lines in the
spectrum of the star. The opposite is emission spectra, which is the same process but seen from
a different angle. Now you are only looking at the re-radiated light of this nebula.
Here it is worth noting a few things. A hot gas on its own emits light as an emission spectrum,
here I’m drawing the intensity of the light on the vertical axis. A hot solid emits a spectrum
at all wavelengths called a thermal spectrum, and so you can’t tell what the object is made of from
only how hot it is. And a star emits a combination of a thermal spectrum with an absorption spectrum
superimposed on top which comes from their upper layers. So in atomic spectra, why do we get these
very specific lines? To answer that we need to delve into the world of quantum mechanics.
Light is absorbed and emitted by the electrons in atoms and these electrons can only exist in
certain energy states which, plotted for hydrogen, look like this. The reason for
these discrete energy states is that in quantum mechanics electrons are described by a wave called
a wavefunction, and this wave can only vibrate at certain frequencies in the constraint of the atom.
Just like a guitar string only vibrates at certain frequencies in the constraint
of being tied to the guitar. These waves are standing waves, and each higher energy wave
includes an extra half-wavelength and oscillates at a higher frequency. I’ve simplified the drawings
to one dimension but in reality these are actually three dimensional waves because the electron cloud
is three dimensional. There are an infinite number of energy levels because you can keep on adding
one more half-wavelength to the electron wave, but the difference in energies between these levels
gets smaller and smaller because the difference between the waves gets smaller and smaller.
The biggest difference in energy is between the ground state and the first excited state.
When light shines on atoms, if it contains light with the same energy as one of the distances
between energy levels of the electron it can absorb that light and the electron jumps to
a higher energy. Then later it will jump back down to the lowest energy, emitting the light
in a random direction. This is why we see the dark bands in the absorption spectrum, because
the electrons are absorbing the light then scattering it mostly in a different direction
to the original light. So it is taking out those frequencies of the light. And an
emission spectrum is just made of that scattered light. Light is made of particles called photons,
and the energy of a photon is based solely on its frequency which you can see from this equation
E=hf where h is a constant called Planck’s constant. Also the frequency and wavelength are
related to each other inversely by this equation, where c is the speed of light in a vacuum.
Here we have part of the emission spectrum for hydrogen. You’ll notice that there are three groups
of spectral lines. There is a good reason for this. The first group, called the Lymann group,
is made from all the decays from high energy states down to the ground state. The other
groups come from energy decays from above down to higher energy levels in a similar way.
That’s why the lines are grouped like this. Note that this is not to scale, in reality these lines
are far more spread out in the wavelength scale, and there are more series at longer wavelengths.
You should also note that I’ve been using hydrogen as examples because it has only
got one proton and one electron, so it has got a relatively simple spectrum.
But the spectra of other elements are more complex and a real observation of many many different
gasses overlapping is incredibly complex. Now you know all the basics of atomic spectra,
but it is also worth knowing about fine structure and hyperfine structure. These are where,
if you look at a spectral line really closely you’ll see that they are often made of two or more
spectral lines that are incredibly close together. The atomic spectra we’ve talked about so far are an
idealised model, but when you add in the spin of the electron and also relativistic effects you get
the fine structure splitting of energy levels. And beyond that the hyperfine structure is seen
as small shifts and splittings of these energy levels caused by interaction between the electrons
and the nucleus through their mutual magnetic or electric fields as well as other effects.
Now it is important to note that what I’ve described so far isn’t the only way that light
interacts with matter. There are a few other ways that you should know about. The most common is
scattering which happens when electrons are free to wiggle about when light shines on
them called Rayleigh scattering. You don’t need the rules of quantum physics to describe this,
it is a classical effect of light being a vibrating electromagnetic wave which interacts
with the electric charge of the electrons causing them to wobble and then become
an oscillating source of more electromagnetic waves which radiate out in all directions.
This scattering called Rayleigh scattering is what causes the blue colour of the sky and the
red colour of sunsets. It is also why metals are very reflective, because they have loads
of free electrons that move around very easily. We’ve also got Raman scattering when light shines
on molecules, and can vibrate the molecules in different modes and has its own spectroscopy
called Raman spectroscopy. And there’s also Brillouin scattering where light can lose or
gain energy when interacting with vibrational waves in the atomic lattices of solids.
Finally let’s look at the applications of atomic spectra. I’ve already mentioned the array of uses
in astrophysics. The other main use is to see what elements and compounds something is made of.
This is used in chemistry labs to see what a sample is made of. Or even using the
hyperfine structure to delve into the nucleus with nuclear magnetic resonance and other techniques.
Lasers come from controlling the emissions from specific energy levels.
We can also harness the accuracy and repeatability of the energy levels. For example atomic clocks
all need a standard oscillator to keep the same time, so use the light from a specific hyperfine
transition of the cesium-133 atom which is also how the SI units of distance the metre
and time the second are defined, and so this also sets the definition of the speed of light.
So that’s my introduction to atomic spectra, very cool stuff.
I’ll be looking at other topics from the map of quantum physics in the future, so
consider subscribing if you want to catch those, this poster is still available on DFTBA as well.
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