The natural clocks that can pinpoint someone's time of death

But Libby had to prove that carbon-14 existed in the wild in concentrations that matched his estimates. Other scientists had only ever detected carbon-14 after synthesising it in a lab. Libby reasoned that living things would deposit it in their excrement, which is why he turned to sewage. Sewage produced by the people of Baltimore, to be precise. And he found what he was looking for.

Libby didn't know it then but the idea that you could use radioactive carbon – radiocarbon – to date things would have all kinds of applications. Since the mid-20th Century, radiocarbon dating has confirmed the age of countless ancient artefacts, helped solve missing person cases and put ivory traffickers in jail. It has even enabled scientists to understand the intricacies of Earth's climate. Radiocarbon dating is one of the keys that unlocks our world.

But how does carbon-14 come into existence in the first place? Libby understood that it was being produced constantly by cosmic rays striking nitrogen atoms in the Earth's atmosphere and changing their structure. The resulting carbon-14 atom quickly combines with oxygen to make radioactive carbon dioxide (CO2).

Back on the ground, plants absorb some of that radioactive CO2 in the air as they grow, as do the animals – including humans – that eat them. While a plant or animal is alive, it keeps replenishing its internal store of carbon-14, but when it dies, that process stops. Because radiocarbon decays at a known rate, measuring how much is left in organic material will tell you the material's age. A clock that starts ticking the moment something dies.

Once Libby confirmed there was carbon-14 in the methane gas from Baltimore sewers, he went on to detect radiocarbon in many different things, allowing him to prove how old they were – from the linen wrappings of the Dead Sea Scrolls to a piece of a ship found in the tomb of Sesostris III, an Egyptian king who lived nearly 4,000 years ago. "This is a problem where you won't tell anybody what you're doing. It's too crazy," Libby later said. "You can't tell anybody cosmic rays can write down human history. You can't tell them that. No way. So we kept it secret."

But once he had proved it worked, he let the world know. And, in 1960, Libby won the Nobel Prize in Chemistry. His technique works on organic material up to 50,000 years old. Older than that, and there is too little carbon-14 left. Carbon-14's gradual decay is what makes radiocarbon dating possible – but that also means you can only go back so far. Even so, radiocarbon dating is now central to our understanding of history.

"In terms of putting things in order, in terms of being able to compare between different regions in particular, and understand that pace of change, it has been really important," explains Rachel Wood, who works in one of the world's most distinguished radiocarbon dating labs, the Oxford Radiocarbon Accelerator Unit.

She and her colleagues date materials including human bones, charcoal, shells, seeds, hair, cotton, parchment and ceramics, but also stranger substances. "We do the odd really unusual thing, like fossilised bat urine," she says.

The lab uses a device called an accelerator mass spectrometer to directly quantify the carbon-14 atoms in a sample – unlike Libby, who was only able to measure the radiation emitted and thereby infer how much carbon-14 a sample contained. The accelerator can also date tiny samples, in some cases a single milligram, whereas Libby needed much more material.

Removing carbon-containing contaminants can take weeks, but once done the accelerator readily spits out a sample's estimated age. "It's really exciting to be able to see the results immediately," says Wood.

Radiocarbon dating has settled some long-standing arguments. Take the human skeleton discovered by theologian and geologist William Buckland in Wales in 1823. Buckland insisted it was no more than 2,000 years old, and for more than a century, no-one could prove he was wrong. Radiocarbon dating eventually showed it was actually between 33,000 and 34,000 years old – the oldest known buried human remains in the UK.

Wasser has analysed radiocarbon dating results from ivory samples as part of efforts to crack down on the illegal wildlife trade

More recent human remains have also revealed their secrets thanks to this technology. In 1975, a 13-year-old girl called Laura Ann O'Malley was reported missing in New York. Remains found in a California riverbed in the 1990s were thought to have originated in a historic grave until radiocarbon dating earlier this year showed they belonged to someone born between 1964 and 1967, who most likely died between 1977 and 1984. This fitted the timeline of O'Malley's disappearance, and DNA analysis confirmed the remains were hers.

Forensic analyses often rely on the"bomb pulse" method of radiocarbon dating, which is possible due to the hundreds of atmospheric nuclear weapons tests that occurred during the 1950s and 1960s.

The blasts sent vast quantities of additional carbon-14 into the air, but these artificially high levels have been falling ever since. And so, by comparing carbon-14 measurements with that downward-sloping curve, it is possible to date materials from the mid-20th Century onwards very precisely – to within a year or so, in some cases.

"I don't know of any other technique that comes close to that," says wildlife biologist Sam Wasser at the University of Washington. "It's extraordinarily useful."

Wasser has analysed radiocarbon dating results from ivory samples as part of efforts to crack down on the illegal wildlife trade. The data can show whether the elephants died before or after the 1989 ban on ivory sales, whatever traffickers may claim.

One man jailed on this evidence is Edouodji Emile N'Bouke, convicted in Togo in 2014. While DNA tests uncovered the geographic origin of the ivory he trafficked, radiocarbon dating showed exactly when the elephants were poached. These two strands of evidence were "the smoking gun critical to bringing N'Bouke to justice", the US State Department later said.

The same techniques have exposed artworks as forgeries. Take the painting of a village scene that one forger claimed was made in 1866. Radiocarbon dating confirmed that it had actually been painted, and artificially aged, during the 1980s. Radiocarbon dating has also shed light on climate change by helping scientists understand the effect of fossil fuel emissions on Earth's climate. Studies of glaciers and ancient ecosystems, for example, are made much more accurate thanks to radiocarbon dating technology.

This research has informed reports by the Intergovernmental Panel on Climate Change (IPCC), which in 2007 was awarded the Nobel Peace Prize – along with former US Vice President Al Gore – for its work disseminating information about climate change. "It's also very useful for people who want to use climate models to predict what the climate may potentially be like in the future," says Tim Heaton, professor of applied statistics at the University of Leeds. Scientists can use radiocarbon records to establish how Earth's climate changed over time, and check climate models against those results, validating the models' accuracy.

But another clock is ticking. Fossil fuels contain copious quantities of carbon but no carbon-14 – the organisms that became coal, natural gas and oil, died so long ago that the carbon-14 they once contained has long since decayed. That means fossil fuel emissions are diluting the carbon-14 in Earth's atmosphere today, which has a direct effect on how much radiocarbon ends up in living things.

Heather Graven, professor of climate physics at Imperial College London, says that in the worst-case scenario of extremely high emissions during the next century or so, the accuracy of radiocarbon dating could crumble. "Something that's freshly produced will have the same [radiocarbon] composition as something that's maybe 2,000 years old," she says. Radiocarbon dating wouldn't be able to tell the two apart.

Rachel Wood argues that these problems won't arise any time soon, but Paula Reimer, professor emeritus at Queen's University Belfast, thinks fossil fuel emissions do "put a damper" on radiocarbon dating and ultimately threaten its accuracy. She spent many years working to heighten the precision of radiocarbon dating, by making painstaking measurements of the radiocarbon found in tree rings, for example, to reveal variations in atmospheric levels of carbon-14 across millennia.

Extremely precise curves of radiocarbon levels are now available dating back 14,000 years or so. But fossil fuel emissions may eventually bring this era of incredible precision to an end.

Chris Baraniuk