This is a moment that scientists have dreamed of for well over half a century. The US’s National Ignition Facility (NIF) has smashed the longest-standing goal in the quest for carbon-free energy from fusion, the nuclear process that powers stars. Researchers from NIF used the world’s most energetic laser to fire 2.05 megajoules (MJ) of energy into a millimetre-sized capsule of hydrogen fuel. Reaching temperatures many times those found in the sun’s core and pressures 300bn times those normally experienced on Earth, a wave of nuclear reactions ripped through the fusion fuel, releasing 3.15 MJ of fusion energy – 1.1 MJ more than was put in – over a few tens of nanoseconds. Now this is not exciting because of the absolute energy released — that was small, only enough to boil two or three kettles. And it’s not even exciting because of any new physics: fusion experts have long argued that you just need a hammer of a certain size to make the thing “go”, and NIF has obliged by upping the input laser energy considerably. (That said, the physical precision behind this machine is astonishing: as little as a 0.1% error in the timing of the laser energy can degrade the conditions needed for fusion by as much as 50%.) No, this is exciting because it’s the first scientific proof that fusion can produce more energy out than is put in, also known as “net energy gain”. If the numbers check out, the experiment generated 54% more energy than was put into it. Releasing energy through fusion reactions isn’t unusual in the wider universe: the sun produces 4bn kilograms’ worth of pure energy from fusion reactions every single second. But, despite decades of hopes pinned on fusion as a clean and plentiful energy source on Earth, no one has ever shown it can release more energy than is needed to set it off – pretty fundamental for a power source. That is, until now. What does it all mean? As ice and snow grip the UK, I hardly need say why energy is a good thing. It makes our lives better in a million and one ways. As a planet, we need a lot more of it. Nuclear fission and renewables are absolutely part of that story, but if the technology can be perfected, fusion offers carbon-free energy for everyone on the planet for thousands, probably millions, of years. It doesn’t create long-lived radioactive waste, and there’s no chance of meltdowns such as those at Chornobyl and Fukushima. Fusion would complement renewables by providing baseload energy, rain or shine, while taking up little precious land. So the prize is big – which is why scientists and engineers have stuck with it for decades. This nuclear breakthrough is, in many ways, what every fusion scientist has been waiting for. Before this, they couldn’t even claim the scientific principle was empirical fact. That made it hard to build momentum behind turning fusion into a power source. Now, they can say “it works!”. Producing star power on Earth is no longer a dream. Of course, that doesn’t mean fusion power that we can use is a reality yet. This is a single result on a single experiment. A commercially viable plant would need to produce 30 times energy out for energy in (30x), rather than the 1.54x seen in this experiment. Even with that magnitude of energy release, there would be engineering and economic challenges to overcome, such as firing the laser 10 times a second, rather than once a day. Gigantic lasers may not even be the best route to economical fusion power: promising alternative approaches are being explored that use magnetic fields to trap the 150mC fuel. And when fusion pessimists say NIF hasn’t produced more energy than it took to charge the laser batteries, they’re right – but this facility was only ever meant to demonstrate scientific feasibility; no government would fund a prototype power plant without hitting this milestone first, and there’s still a long road between this experimental result and a power plant. So this stunning achievement may not appear to bring us much closer to fusion power being available on the grid … at least, not directly. Indirectly, psychologically, the effect is akin to a trumpet to the ear: we now know fusion for energy is possible. Knowledge that fusion can work changes everything, including how willing everyone from governments to entrepreneurs will be to invest. With more resources, higher gains in energy can be pursued, and a virtuous cycle of development is now likely to be set in motion. Nature has seen fit to make the science of nuclear fusion such that it can produce energy. Whether we now turn that fusion energy into a power source – well, that’s down to us. Arthur Turrell is author of The Star Builders: Nuclear Fusion and the Race to Power the Planet