杏吧原创

ON 17 April, North Korea鈥檚 deputy ambassador to the UN gave a tense press conference. The US was insisting that North Korea scale down its nuclear programme, and this, said Kim In-ryong, had created a situation in which 鈥渢hermonuclear war may break out at any moment鈥�.

It was not the first time such warmongering talk had come from North Korean diplomats, but concerns over nuclear war have rarely been higher. Donald Trump has warned of a 鈥渕ajor, major conflict鈥� with the country.

On the face of it, such tensions might seem to bolster the case for maintaining a nuclear deterrent in the West. Recent arguments in the UK in particular about replacing the ageing submarines that carry the country鈥檚 Trident nuclear missiles are part of that wider debate.

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But what if those submarines are lame ducks? A rumour has circulated since the cold war that subs, often considered the epitome of military stealth, can in fact be tracked. If that鈥檚 true, and there are fresh hints that it could be, it overturns an opinion that has largely held sway among military analysts for nearly 50 years 鈥� and changes the terms of debates about nuclear deterrence entirely.

It was the 1960 book The Strategy of Conflict by economist Thomas Schelling that put forward the first rigorous analysis of strategies for preventing nuclear war. Schelling used game theory to introduce the idea of a 鈥渃redible commitment鈥�. This principle says the US must be locked into ordering a retaliatory strike if it is hit by a nuclear missile. This guarantee of a counter-strike is supposed to deter the use of nuclear weapons in the first place.

For the commitment to be credible, the nuclear missiles must be constantly available at a moment鈥檚 notice. This led the US to introduce a 鈥渘uclear triad鈥�, in which nuclear missiles are kept at military bases, on aircraft and on submarines. The thinking goes that no state could disable them all at once, and render themselves immune to retaliation. In theory, Shelling鈥檚 principles still apply today.

Ear on the deep

Submarines are the critical leg of the triad because they are supposedly well nigh impossible to pinpoint. So much so that the UK doesn鈥檛 bother with the other legs at all, keeping all its missiles aboard submarines. 鈥淭he maritime element remains the most survivable part of any nuclear triad 鈥� a fact unlikely to change in the immediate future,鈥� says Peter Roberts at the UK鈥檚 Royal United Services Institute (RUSI), a defence think tank. Disappearing into the vast ocean, 鈥渟ubmarines on patrol are remarkably difficult to find鈥�.

That鈥檚 not to say subs are undetectable. Sonar can reveal a sub if it is within about 15 kilometres, depending on the ocean conditions. Starting in the 1950s, the US also used an to listen for Soviet submarines. This exploited a layer of water about 1 kilometre down where sound carries especially well, and allowed subs to be heard hundreds of kilometres away. That still left a lot of ocean to hide in, though.

The USSR explored an alternative way to find subs during the cold war when it was lagging behind with sonar technology. This involved identifying the wake of disturbed water submarines leave behind them, which would create a pattern that rises to the ocean鈥檚 surface. According to the cold war historian and ex-US Navy analyst Norman Polmar, a task force was set up in the early 1960s within the Soviet Union鈥檚 council of science to look into whether it was possible. The Soviets later claimed to be able to track wakes with ship-based radar, with aircraft and even from satellites. Their own submarines were fitted with comb-shaped SOKS or Snegir (meaning 鈥渂ullfinch鈥�) sensors, which supposedly allowed them to secretly tail NATO submarines for days at a time.

杏吧原创s in the West were sceptical of all this. Submarines certainly leave a trail of turbulent water immediately behind them, but according to the accepted understanding of turbulence it ought to disperse and merge with other currents within minutes. Think sloshing your hand around in a bath tub. The turbulent flow you produce breaks down into smaller and smaller streams, eventually fading into nothing.

But is that true under all circumstances? Turbulence is one of the trickiest unsolved problems in physics. When a thick fluid like honey flows, we know where we are. But with less-viscous fluids like water, the movements can become unpredictable. We have equations that describe this turbulent flow, but they can鈥檛 be solved exactly, meaning that we鈥檙e not certain they capture what is happening.

In 1963, , an oceanographer at the University of California, San Diego, looked at turbulence in wind tunnels, water tunnels and tidal channels and became convinced that the data didn鈥檛 fit the expected pattern. Instead, he points to the existence of 鈥渇ossil turbulence鈥�, an idea first mentioned in the 1950s by George Gamow, a Russian physicist living in the US.

Gamow was talking of of the turbulent gas flows in the young cosmos. But Gibson has extended the idea: 鈥淭urbulence always starts with eddies forming at small scale and cascades to larger scales,鈥� he says.

This is the reverse of the accepted understanding of turbulence 鈥� that energy dissipates from large to small scales, says who studies turbulence at the University of Cambridge. But this is a net effect and recent computer modelling studies now suggest 鈥渂ackscattering鈥� of a small amount of energy to larger scales is possible.

Gibson鈥檚 idea is that those spinning whorls of turbulence you can create with your hands in the bath will combine to produce larger and larger whorls over time. And he does have a little evidence to support these controversial thoughts.

In 2002, he joined an environmental research project studying the impacts of a waste water pipe in Mamala Bay, Hawaii. The team deployed a series of sensors to measure water velocity in three dimensions and Gibson used this to map the turbulence. He says the sensors picked up patterns of turbulence that increased in size as they rose to the surface.

He then analysed satellite photographs of the sea, looking for tiny changes in the brightness of the surface. Despite the pipe being 70 metres underwater, he reported in a 2005 paper that those anomalies were there, and that they . Signs were visible on the surface as far as 12 kilometres away from the pipe.

This means fossil turbulence is real, says Gibson, and submarines could be tracked via their wakes. True, subs typically cruise at a depth of about 300 metres, but Gibson insists his extrapolation should hold. 鈥淭he fossil turbulence remnants from a submarine persist for many days,鈥� he says. 鈥淭he Russians have understood this from the beginning, but have considered all their progress to be important state secrets.鈥�

Recently there have been hints that the US Navy is also waking up to wakes. It funds firms to carry out research through a government scheme called Small Business Innovation Research. from show that the Navy contracted a firm called in Falls Church, Virginia, to develop models of various ways a sub might be detected (see 鈥�How to track a sub鈥�). One of them is how underwater currents and turbulence might manifest themselves on the ocean surface.

The company declined an interview with New 杏吧原创. But military analysts are also sounding the alarm. 鈥淭he Russian interests in this field are real,鈥� says Polmar. 鈥淪ome of their accomplishments are impressive, and a concern to US and British officials.鈥�

Even if he鈥檚 wrong, the worries over submarine tracking aren鈥檛 over. at Georgetown University鈥檚 Center for Security Studies in Washington DC points out that a , including underwater drones, have evolved to the point at which submarine undetectability is no longer a given.

鈥淚f fossil turbulence is real, submarines could be tracked via their wakes鈥�

This casts a shadow over the NATO efforts to create stealth submarines that are invisible to sonar. If Polmar and Gibson are right, then perhaps more focus should be on finding ways of reducing the wake that submarines create. That, it seems, is what the Russians have done. 鈥淢any Russian submarines have vortex attenuators on their screws and small vortex-unwinding propellers,鈥� says independent naval analyst Jacob Gunnarson in Williamsburg, Virginia.

It鈥檚 enough to give us pause over UK government plans to upgrade its nuclear deterrent. The planned four new Dreadnought class subs will cost upwards of 拢31 billion. A released in 2016 says: 鈥淚t is unlikely there will be any radical technological breakthrough which might diminish materially the current advantages of the submarine or make the oceans transparent.鈥� That statement now looks just a tiny bit less certain.

This article appeared in print under the headline 鈥淒ead in the water鈥�