Oppenheimer: The worryingly real fears scientists have about a single nuke ripping open Earth

There is a scene in Christopher Nolan’s new film Oppenheimer (a biopic of J Robert Oppenheimer, the man who invented the nuclear bomb) in which Leslie Groves, an army engineer played by Matt Damon, worries about destroying the world.

This is just before the Trinity test, the first-ever detonation of an atomic bomb, and Oppenheimer says he’s confident that the chances of annihilating all life on Earth are near zero. “Near zero?” splutters Groves. “Zero would be nice!”

In reality, Groves’s concerns were those of Manhattan Project physicist Edward Teller. According to Dr Steven Biegalski, Chair of Nuclear and Radiological Engineering at Georgia Institute of Technology, Teller was worried that the heat of the explosion “would cause the hydrogen in the atmosphere to undergo fusion, setting off a catastrophic chain reaction that would continue around the globe and destroy Earth.”

In other words: there was a fear that it would set the world on fire. “This obviously did not happen,” adds Biegalski. “The density of fusible atoms and the energy balance prevent it from happening.”

Still, it raises the question: how big and destructive can nuclear weapons get? Could a single bomb feasibly end the world?

“The bomb dropped on Hiroshima during World War Two was 15 kilotons, equivalent to 15,000 tonnes of TNT,” explains Dr Tara Drozdenko, a director of the Global Security Program at the Union of Concerned Scientists.

“The Nagasaki bomb was 20 kilotons. Most current US nuclear weapons are 15 to 20 times more powerful than those bombs."

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For context, the Hiroshima bomb devastated 13km² of the city, destroyed 63 per cent of its buildings and killed at least 70,000 people. But the most powerful nuclear bomb ever made was tested by the Soviet Union in 1961. Named Tsar Bomba, it had a yield of over 50,000 kilotons, which is equivalent to 50 million tonnes of TNT.

The blast was monstrous, destroying houses in the village of Severny, 55 kilometres (34 miles) from the testing site at Mityushikha Bay. If such a bomb were dropped on Manchester, people in Liverpool, Leeds and Sheffield would suffer third-degree burns.

Incredibly, Tsar Bomba was designed to be twice as powerful (100 megatons), but was scaled back to mitigate radiation effects.

As Drozdenko explains, however, there is such a thing as too powerful. “That bomb is just not practical for an arsenal,” she says. “There is no practical delivery system for it.”

Such a bomb would be too big to be deployed as a missile and carry enormous risks for any crew hoping to drop it by plane.

“Even at 50 megatons, there was some question of whether the Tsar Bomba plane would be able to escape the area before it exploded,” says Drozdenko. “Why would you manufacture something that big if you could get 50 other ones that are smaller?”

According to Biegalski, there is a “general consensus that fission-fusion weapons can go to much higher yields than Tsar Bomba”. There is even speculation that in the future they could be used to knock an asteroid off course. Yet there are limits to how powerful a single bomb can be.

“There is always a maximum for two reasons,” says Biegalski. “One is that you can run out of material to put in the bomb. Two, at a certain stage the bomb blows itself apart and the reaction stops.”

As for whether a “theoretical-maximum-size nuclear weapon” could destroy the world, Biegalski is uncertain. “All I know is that I hope we never go there,” he says.