Is nuclear fusion a pipe dream? Scientists celebrated yesterday after producing 10 quadrillion watts of nuclear power in a fraction of a second, a leap towards unlimited energy.

The key to this remarkable feat is nuclear fusion. Hydrogen has two isotopesAtoms from the same element with the same number of protons and electrons, but with different numbers of neutrons. The result is that various isotopes have different masses., deuterium (which has two neutronsA subatomic particle with no electric charge, found in the nucleus of an atom.) and tritium (which has three). When these are smashed together in the heart of the Sun, they combine to form a new element, heliumA very light gas used in balloons to make them float. Old airships used hydrogen, but it burnt easily. Helium does not burn., with one neutron left over. This neutron is converted into energy, which ultimately makes its way out through the solar system to warm the Earth.

Scientists have long hoped that we can recreate fusion to generate energy on Earth. Now, a major breakthrough has some scientists convinced that it might finally be within reach. Researchers at the National Ignition Facility in San Francisco were able to achieve a fusion reaction by training massive lasers on a tiny spot of hydrogen, no wider than a human hair.

The reaction only lasted 100 trillionths of a second. But in this time, it became self-sustaining: each time two hydrogen atoms smashed together, the neutrons they produced made other atoms collide as well, without needing more energy input. This is essential to achieving fusion on a grander scale.

Fusion could revolutionise the world's energy supply. So far, the only kind of nuclear power we have been able to harness is fission, where large atoms are broken apart, instead of small atoms being smashed together. Fusion could produce four times as much energy as fission, without producing any dangerous radioactive wasteNuclear fission uses uranium which stays radioactive. Fusion can only take place in the unique conditions of the reactor.. And since true fusion produces more energy than is put into it, it could be a source of unlimited energy.

But others are sceptical. The energy output from the reaction the researchers achieved was only 70% of the energy they had put into it. This suggests we are still far from being able to use fusion as an energy source.

Some worry that fusion could be a distraction from the transition to renewable energy. If we believe that fusion is just around the corner, we might become complacent about the need to stop burning fossil fuels.

Is nuclear fusion a pipe dream?

Absolutely, say some. The running joke among scientists is that fusion is always 20 years away. This experiment proves that fusion is possible, but the scientific work needed to unlock nuclear fusion is just the tip of the iceberg: turning it into a viable source of fuel across the world will require unprecedented financial and political investment.

Of course not, say others. Many scientific breakthroughs seem impossible until they happen. In 1895, respected scientist Lord Kelvin claimed that heavier-than-air flight was unthinkable; just eight years later, the Wright brothers proved him wrong. As late as 1934, Albert Einstein argued that human beings would never be able to harness nuclear fission. Time and again, scientific ingenuity has won out over scepticism.