In astronomy we talk about billions of years like it’s no big deal. But how can we be
sure about timescales so far beyond the capacity for human intuition? Our discovery of what
we now call deep time is very recent - as recent as our discovery of the true spatial
vastness of our universe. And it came as scientists tried to measure the age of the Earth. What
they found was as shocking and humbling as anything ever seen through the telescope.
“How long has the world been here?” The question is surely as old as human civilization
- and perhaps older. The earliest written record we have of a cosmogony - an origin story for
the cosmos - came from the Sumerians, followed soon by the Babylonians. There were floods,
dismemberments of gods - all that good stuff that was later adopted by other traditions.
But the main thing was this notion that the world had a beginning. The early Jews adopted
this idea, and in turn it was picked up by the early Christians, and the idea of a single,
unique creation event became a cornerstone of some of the earliest religious texts – like
the Book of Genesis.
The Genesis story was so influential that many people used it to try to figure out the
age of the world. Probably the most famous guess came from the Irish bishop James Ussher,
in the early part of the 17th century. Ussher looked not only at the Bible, but at hundreds
of other ancient texts, trying to align the different – and often conflicting -- histories
that they presented. Usher eventually concluded that the world began at 6:00 p.m. on Saturday,
October 22, 4004 BC. An oddly precise prediction given the source material - and obviously
a bit lower than the value given to us by science.
The French naturalist Georges-Louis Leclerc, Comte de Buffon, was among the first to calculate
the age of the Earth using what we would now call scientific methods, publishing his result
in 1778. It was ingenious really. He assumed the Earth started as a ball of molten rock,
which subsequently cooled down to its current temperature. Buffon spent six years measuring
the cooling rates of materials in his lab, and in the end calculated that the Earth was
74,832 years old. And yes, that’s also a weirdly precise number. Both Ussher and Buffon
are docked half a point for excessive significant figures. Buffon’s premise was clever, if
flawed. His 75 thousandish years was too small - and we now know why. I’ll come back to that.
At around the same time as Buffon was staring at warm lumps of iron, the Scottish geologist
James Hutton wandered Britain, pondering the ages of its rock formations. He became convinced
that they were formed when molten rock pus hed through the crust from Earth’s interior.
This directly gave us igneous rocks like granite, while sedimentary layers resulted when these
materials eroded and were deposited on ocean floors - to be later pushed up again in an
endless cycle.
Hutton’s biggest breakthrough was that reasoned that these processes were driven by the same
forces operating in the world today - so-called uniformitarianism. But those processes were
excruciatingly slow, and so Hutton realised the Earth must be unthinkably old.
We can thank Hutton and his 1788 book the Theory of the Earth for opening scientific
eyes to the possibility of an ancient planet, and the idea of what we now call deep time.
The initial temporal vertigo was shocking. Hutton’s collaborator, John Playfair, put
it well: “the mind seemed to grow giddy by looking so far into the abyss of time.”
Hutton didn’t propose a beginning for the Earth - instead he assumed an infinite series
of cycles. This was also the notion of the great German philosopher Immanuel Kant, a
few decades earlier. Kant wasn’t afraid to throw out some numbers, based on pure speculation
- to quote: “There has mayhap flown past a series of millions of years and centuries,
etc. etc. ... Creation is not the work of a moment.” By the way, Kant was also the
first to have speculated about the existence of galaxies beyond the Milky way - Island
Universes, as he called them.
Deep time and deep space. We’d known since Copernicus and Galileo that earth was just
one planet among several in our solar system. Astronomers now swear by the Copernican principle
- Earth is not in a privileged position in the universe, and so the laws of physics work
the same everywhere. Hutton’s uniformitarianism is a sort of temporal Copernican principle
for geology. The idea was further developed and popularized by another Scottish geologist,
Charles Lyell. In Principles of Geology, published in the 1830s, Lyell talks about millions of
years of geological processes. He speculates on a true beginning for the Earth, but doesn’t
try to date it. Instead he imagines that it must be vastly older than geology can yet
see, and he draws an astronomical analogy - that the universe must be vastly larger
than what was visible at the time.
Lyell’s work was incredibly influential - and not least to a young scientist named
Charles Darwin. Darwin read Lyell’s book during his famous voyage on the HMS Beagle.
Lyell’s ideas gave Darwin the millions of years he needed for his theory of evolution,
and its painfully slow mechanism of natural selection. But Darwin was also into the geology.
In his Origin of Species, he estimated a minimum age of the earth based on the erosion timescale
for chalk formations in Southern England. His figure of 300 million years was actually
too high for that formation, but we now know much lower than the true age of the planet.
Of course geology and evolutionary biology are now intimately connected. We can trace
the progress of evolution by mapping the fossil record to the geological clock. The ordering
of the appearance of ancient species is found when we date the rocks in which their fossils
are found. We’ve now found fossils as old as 3.5 billion years - but to understand
how we can possibly know that age, we have to turn from geology to physics.
The discovery of X-rays in 1895, and the discovery of radioactivity a year later, would open
up a new world within the atom – and yield new tools for probing vast stretches of time.
Ernest Rutherford, discovered that certain
elements released energy from radioactive decay at an ever-decreasing rate. And that
some of those elements would slowly leak that energy
over thousands or even millions of years. We now know that radioactive decay
is responsible for keeping the interior of the Earth hot - and explains why Earth
is much older than Buffon’s 75,000 year estimate based on the passive cooling of a ball of
molten rock.
But the discovery of radioactivity also gave us our most accurate way to figure out the
age of chunks of the Earth through radiometric dating. Unstable atomic nuclei decay into lighter
nuclei by splitting or by ejecting particles. The rate of decay is expressed in terms of
“half-life” - which is the amount of time for a given radioactive nucleus to have a
50% chance of decaying; or equivalently, the amount of time it takes for half of a large
number of radioactive nuclei to decay. In principle, if you know how much of the stuff
there was to start with you can figure out how long the radioactive material has been
decaying.
Figuring out the initial content is typically impossible - at least directly. Instead, there
are some clever workarounds. Perhaps the most well known example is carbon dating. In this
case we’re interested in the decay of carbon-14 - that’s the version or “isotope” of
carbon with 8 neutrons. It’s radioactive, and decays with a half-life of 5,700 years.
Carbon-12 and 13 isotopes are stable, and so much more abundant. Carbon-14 exists on
the surface of the earth because it’s produced when cosmic rays hit nitrogen in the atmosphere,
resulting in a constant proportion of C-14 within the atmospheric CO2.
That carbon is incorporated from the atmosphere into living organism by photosynthesis, but
when the plant, or whatever ate the plant dies, the C-14 content gradually drops. By
measuring the current C-14 content relative to the stable C12 and C13 content, the age
of once-living material - including fossils can be determined. But this is only accurate
to around 10 half-lives - or 50,000 years.
Move useful for measuring the age of the earth is uranium-lead dating. Uranium decays on
much longer timescales - 710 million years for the U-235 isotope and 4.5 billion years
for U-238, with both decaying to different isotopes of lead.
You can figure out how much uranium a given sample had to start with by looking at the proportion
of uranium to lead. This only works if you can be sure that no lead was in the sample
to start with - and in some cases you CAN be sure. Some crystals like zircon tend to
incorporate uranium atoms into their crystal structure when forming, while at the same
time repelling lead. So any lead you find in those crystals came from uranium decay.
By looking at the ratios of each type of uranium to the corresponding isotope of lead you get
two independent ages - which will match up if the crystal has not lost any of its lead. But
the comparison of the two ratios one something called a Concordia Diagram. Allows you to correct for any lead loss.
The other cool thing about this technique is that the different half-lives of each uranium isotope means this
radiometric technique is useful between hundreds of millions of years to many billions of years.
By the 1920s, British geologist Arthur Holmes would declare that the Earth was between 1.6
to 3.0 billion years old, based on his radiometric dating. This was around the same time that
astronomers proved that Immanuel Kant’s Island Universes were indeed other galaxies,
many millions of light years away. The world simultaneously got a lot older and the universe
a lot bigger.
Older rocks were discovered, pushing back Earth’s age further and further. But beyond
a few billion years it starts to get tricky. There aren’t many patches of land left from
way back then - most of the original crust has been recycled into the mantle. One of the few remaining
OG patches is in Western Australia. There, zircon crystals have been found as old as 4.4 billion years,
based on uranium-lead radiometric dating. But to go beyond that date we have to look
beyond the Earth. We believe that the moon formed at the same time as the Earth - both
coallescing after a giant planetary impact in the early solar system. Now the moon is tectonically
inactive, so rocks on its surface now where there when it formed. The Apollo missions
brought back lunar specimens that have been radiometrically dated to 4.5. billion years.
That jibes with our measure for the age of solar system. Nearly 4.6 billion years - we
get the same number from radiometric dating of solar system meteorites and also from our
calculations of the age of the Sun.
I should add that science took its time getting to these large numbers for the age of the
world. The Mayan long-count calendar includes cycles of 63,000 years. Hindu tradition also
has a cyclic cosmology, with nested epochs lasting millions of years, In fact a single
day of Brahma is 4.32 billion years. Oddly close to the age of the Earth - although it
looks like we survived one day of Brahma AND the last long-count transition, so I’d say
we’re good for now.
So the picture has come together. There’s consistency in that 4.5 billion-ish year age
across several independent measures. That gives us some confidence in the result. But
confidence in the number may be justified, but perhaps we’ve become a bit too comfortable
with it. These days we stare down at the unfathomable gulf of the past and shrug, and we forget
that entire span of human civilization is the width of a single hair on our heads compared
to the stratified depths of the Grand Canyon. Not that our ancient Earth cares - it’s
revolved around the sun 4.5 billion times through deep space and out of deep time, and it’ll do the same
again. Into what I guess you could call deep spacetime.