The Crowd and the Cosmos: Adventures in the Zooniverse
Page 4
the sky; in fact, Sloan ended up covering about a quarter of the
entire celestial sphere. For most of the time, the telescope fol-
lowed the simplest possible observing strategy, allowing the
sky to turn overhead while its sensitive camera grabbed images
of whatever passed across its field of view. In all, more than
300 million separate objects were recorded by the survey in its
eight years of operation, and the resulting database and the pile
of pictures that accompanies it are uniquely valuable. Among
the haul, nearly a million fuzzy objects were identified as galax-
ies which were likely large enough, bright enough, and above all
close enough to allow us to discern their structure. For many of
these galaxies, we had not only images but spectra, careful stud-
ies of their light at each wavelength, which revealed the distance
to the galaxy and much else besides.
This all sounds pretty impressive—and it really was—but the
really groundbreaking thing lay in how the team of thousands
who dedicated years of their lives to designing and operating
the Sloan survey treated the precious data that resulted from
their efforts. They would have been perfectly within their rights
and consistent with historical precedent to hoard it for their own
private use, taking the time to publish paper after paper while
safe in the knowledge that, without data to match, the rest of us
had no way of scooping them. Yet to my continued astonish-
ment, they chose to share the fruits of their labours with the
world; an astronomer like myself who had put in no work at all
has exactly the same right to use the data as those who had spent
every working—or waking—moment of the last decade or two
dreaming of what it might reveal. Not surprisingly, the Sloan
How Science iS Done 17
data quickly became one of the cornerstones of modern astron-
omy, triggering and then fuelling an explosion of interest in
studying galaxy formation and evolution, a field of study that
holds the key to understanding the history of the Universe.
I stepped into the changing world of professional research
when I spent a summer as a 17 year old at the University of
Hertfordshire, sponsored by the wonderful Nuffield Foundation
to take six weeks to experience what life as an academic was
really like. Nuffield still sponsor thousands of British students to
spend time doing research during the summer, an experience I
highly recommend to anyone thinking of research as a career,
and looking to do something independent.
I was nominally employed to look at the effect icy dust grains
had on light travelling through the environment about newly
formed stars, but in reality this meant running computer pro-
grams over and over again while eating an obscene number of
Danish pastries obtained from the university’s library cafe. The
work itself was tedious, and I wasn’t very good at it, but I did
enjoy the company of the astronomers and a glimpse into their
world. Having written up the summer’s efforts I found myself at
a ‘science fair’ organized by the British Association, and more
through a certain gift of the gab than any scientific skill ended up
as one of the UK’s representatives at the International Science
and Engineering Fair, an annual American jamboree held that
year in Philadelphia, and my first introduction to the weird cult
of school science fairs that prevails across the pond.
The aim of these events is laudable. Through a hierarchical
system of school, city, state, and national science fairs every pupil studying science could have a chance to get to grips with science
as it is really practised, not just as presented in a textbook. Science fairs are a big deal in the US, as much a part of the high-school
experience as rituals such as the prom (similarly foreign to me),
18 How Science iS Done
and the competition in Philadelphia was fierce. Scholarships
worth hundreds of thousands of dollars were available to prize-
winners, and it would be difficult to underestimate the competi-
tiveness of two thousand or so teenage overachievers. I knew I
was outmatched as I carefully stuck up the A4 pieces of paper
that described my project, watching out of the corner of my eye
as parent-assisted competitors assembled fully lit display booths
and prepared experimental demonstrations. (In my memory at
least, the winner that year was someone who had built a plasma
chamber in their back garden.)
When the judging started strange things kept happening.
Adult after adult looked at my pieces of paper, and then started
asking where my hypothesis was, and how I’d gone about testing
it. It’s not a completely crazy question, and you’ll be familiar, perhaps, with the idea of hypothesis testing from school science—
you write down the idea you’re trying to test and the alternative,
boring, ‘null hypothesis’, and then use data to distinguish between
the two. For a simple classroom experiment, you might have a
hypothesis like this:
Talking to plants will significantly improve their growth rate.
And a null hypothesis like this:
Talking to plants will make no difference (to the plants—effects
on humans are not the focus of this experiment).
You could then take two plants, talk regularly to one while keep-
ing the other in splendid isolation, and in measuring the differ-
ence between the growth rate between the two gain some
evidence in favour of either the hypothesis or its null partner.*
* I’m no botanical expert, but I did spend some time trying to find out what would happen if you actually did this. I’m sorry to have to report that the scientific literature on this vitally important question is somewhat sketchy, but it
How Science iS Done 19
It’s harder in astronomy than in basic botany to design simple
experiments, but in my case the judges were expecting the
hypothesis printed proudly on the first sheet of paper to be
something like:
Scattering of light off dust grains is responsible for the high
levels of circular polarization observed in star-forming regions.
The null hypothesis would have been something like:
Scattering of light off dust grains cannot be responsible for the
degree of circular polarization observed in star-forming regions.
According to the science fair judges, devoted to ensuring their
competitors headed off to university with a decent understand-
ing of the scientific method under their belt, having written down
these formal statements all I had to do was design the right
experiment to test them, but I couldn’t see that it was that simple.
To see why I was confused, I need to explain about the specifics
of the problem involved.
Unfortunately, this means understanding the concept of cir-
cular polarization, which is both slightly obscure and overcom-
plicated. For starters, think about light. Since the work of James
Clerk Maxwell and the other pioneers of nineteenth-century
physics, we’ve known that light can be described
as a wave,
which travels through space.*
Everyone’s familiar with waves, so thinking of light as a wave
sounds simple enough. We’re used to ocean waves, where a swell
moving towards the shore lifts the water as it passes, and sound
seems that while plants do respond to sound, only loud noises have any impact.
Plants, it seems, would prefer clubbing to a nice quiet chat in the pub. If you act on this information by taking your yucca out on a Friday night, do let me know how it goes.
* It can also behave like a particle, but that’s due to quantum weirdness which need not distract us here.
20 How Science iS Done
waves, where sound is transmitted by atoms in the air knocking
into each other. (This is the explanation for why in space no one
can hear you scream.) Those early pioneers of physics were much
occupied with the question of what sort of wave light could be; it
seemed obvious that it would need a medium to travel through,
but this isn’t true. We now know to describe light as a wave that
propagates itself, capable of travelling through even the vacuum
of space. Think of it as a bundle of related electric and magnetic
fields, each of which oscillates as light travels through space.
In this picture, the components of the light—the electric and
magnetic fields—have a direction. They can be oscillating up and
down, or right to left, or at any angle in-between, and in most
circumstances and from most sources we receive light that is a
mix of all possible directions of oscillation. There’s no particular
reason for a source of light to spit out aligned waves.
If the light scatters off a surface, like the ground, this can
change. Such scattering can produce light in which some or all of
the oscillations are aligned; we say that it has become ‘polarized’.
This ‘polarization’ can be useful; by making sunglasses out of a
material that only lets through light oscillating in one direction,
we can cut out the scattered light. Using such a material lets
drivers can see more clearly, undistracted by light scattering off
the surface of the road (Figure 5).
Because stars form deep in the middle of clouds of gas and
dust, the light from a newly formed star quickly encounters a
surrounding cocoon of dust, tiny particles of carbon or silicon
about a tenth of the size of an Earthly grain of sand (Plate 3).
These particles scatter the light and cause it to become polarized.
My summer dabbling in research was concerned with what hap-
pens next. If polarized light is scattered again, then, instead of
the oscillations all being lined up with each other, in the right
circumstances a large fraction of them will tend to rotate in
How Science iS Done 21
Clockwise
Circularly Polarized Light
Linearly Polarized Light
Unpolarized Light
Quarter Wave Plate
Linear Polarized
Figure 5 Schematic showing transformation of light as it becomes
polarized. Initially the electric field can appear in any orientation, but after linear polarization there is a preferred direction. Circular polarization favours rotation of the field.
either a clockwise or anticlockwise direction. This is what’s
known as circular polarization—we say that light is circularly
polarized when we get more clockwise than anticlockwise light
from a source, or vice versa.
For most purposes, the presence of circularly polarized light
makes little difference to anything, but there is one important
exception. Some complex chemicals care deeply about whether
the light hitting them is circularly polarized, and as these are pre-
cisely the chemicals that life on Earth depends on we too have a
vested interest.
This phenomenon happens when an atom such as carbon
makes four different chemical bonds, each with a different atom
or set of atoms (Figure 6). A bit of thought or a glance at a dia-
gram will show there are two possible configurations, each one a
mirror image of the other. No amount of manipulation will turn
one into the other, any more than you can rotate your left hand
to sit perfectly on top of the same shape as your right hand.
Such pairs are known as ‘chiral’ molecules, and because they
have the same structure—they have the same chemical formula—
22 How Science iS Done
F
F
Br
C
C
Cl
Br
Cl
H
H
Mirror
(R)-Bromochlorofluoromethane is not superposable on (S)-bromochlorofluoromethane (its mirror image). These molecules are chiral.
H
H
C
C
H
H
H
H
H
H
Mirror
Methane is superposable on its mirror image, and therefore achiral.
Figure 6 Two forms of bromochlorofluoromethane, which are mirror
images of each other. As the central carbon makes four different bonds, one form can’t be rotated or transformed into the other.
they will behave the same in chemical reactions unless they
encounter another molecule which has this property of having
different mirror images. When that happens, then left-handed
and right-handed molecules will interact differently. For example,
find some spearmint chewing gum; it owes its sickly sweet
spearmint smell to the presence of just one type of a mirror mol-
ecule called carvone. Swap every molecule of carvone for its
opposite and your chewing gum would taste not of mint, but of
caraway—caraway seeds have the mirrored form of carvone.
That might not seem too bad (I would buy caraway chewing
gum, I think) but in some cases a benign compound can be trans-
formed into a deadly poison by the substitution of its mirrored
opposite.
Such a case would provide a superb detective plot—it would
be difficult for a police chemist with modest equipment to tell
the two apart—but what these examples really reveal is that life
How Science iS Done 23
on Earth has made a choice to prefer one mirrored set of molecules
to another. Why this should be is somewhat mysterious, and it
seems that astrophysics may have the answer. Recent astrochem-
ical revelations have shown that the chemistry found in and
among star-forming regions is surprisingly complex; out in
the darkness of space, on the surface of the dust grains from
which planets will end up forming, chemicals as complex as
amino acids—the building blocks of proteins, and hence of life’s
chemistry—can form. We haven’t actually found amino acids
yet, but we’ve got close and believe that the chemistry is well-
enough understood to infer their likely presence.
If such complex chemicals naturally appear in star-forming
regions, could these space-forged complex chemicals have been
the building blocks for life on Earth? Perhaps. We suspect that
the Earth’s early years were rather unpleasant, with a tempera-
ture on t
he surface that would cause water and any other volatile
chemicals that were initially present to boil away. The water that
we drink—all the water, in fact, on Earth—may have been
delivered here by an immense bombardment of millions of com-
ets and asteroids later in our planet’s history. If that’s true, and
studies of at least one comet have shown that its water is a good
fit for Earth’s, then it seems likely that a whole molecular cocktail could have been delivered to the then lifeless surface of what was
rapidly becoming the blue planet.
This delivery mechanism may explain life’s preference for left-
handed molecules. If they formed in space, then they may have
been exposed to light that was at least slightly circularly polar-
ized. Light which is circularly polarized so that the electric field
rotates clockwise may find it easier to excite left-handed rather
than right-handed molecules, whereas an anticlockwise polariza-
tion might do the opposite. In such circumstances, if you start
with an equal (chemists would say ‘racemic’) mixture of both
24 How Science iS Done
left-handed and right-handed molecules, you might get chemis-
try happening in right-handed molecules than doesn’t happen
on the left-handed side of things. If that happens, then you can
see how the products of such chemical reactions—naturally, the
more complex molecules—might tend to be more right than left
handed or vice versa.
So if our mix of molecules in space was exposed to light with
a sufficient degree of circular polarization then even before it
ended up on our planet it may have been processed to produce a
bias towards left-handed or right-handed molecules. That’s why
the work behind my summer project was so interesting—it
promised a link between the worlds of star formation and astro-
chemistry on the one hand, and of astrobiology and the origins
of life on the other—and that brings us back to the hypothesis I
was supposedly engaged in testing. Could a high degree of circu-
lar polarization be produced by scattering light off dust grains?
Science fair etiquette suggests that what’s needed is an experi-
ment. Astrochemistry can sometimes be done in the lab, but in
this case setting up a star-forming disc of gas and dust and allow-
ing millions of years for it to evolve was a little out of my reach.
As astronomers have done for more than half a century, in lieu of