By shining laser light onto hydrogen or antihydrogen and observing which wavelengths are absorbed, the energy levels of the two can be compared in detail.

ECONOMIST: Antimatter: Gotcha! | The

The barrier to using the devices in vehicles is no one knows how to carry enough hydrogen, a light and flammable gas that is difficult to compress, to be useful.

CNN: Bus showcases fuel cell technology

Neutrons ejected from a watery surface move much more slowly than those from a rocky one, because the hydrogen nuclei in water are light enough for an exiting neutron to knock around much like one billiard ball hitting another a process that slows the neutron down.

ECONOMIST: Divining water

The idea was this: If after burning a quantity of heavy gasoline the airship became light, the skipper had only to switch to hydrogen, thus reducing lift.

FORBES: The long, loud history of hydrogen as a motor fuel.

But as the first stars condensed out of the newly formed atoms, the heat and light they produced would have started to tickle the hydrogen clouds around them into emitting radio signals at a characteristic wavelength of 21cm.

ECONOMIST: Radio telescopes

It consists of 1, 000 tonnes of heavy water (the sort that has deuterium, a heavy form of hydrogen, in its molecules, instead of the normal, light form of the element).

ECONOMIST: The news that neutrinos have mass is both good and bad

It is still unclear whether the hydrogen represents the release of water by the heat of the impact, or whether it simply sheds light on the way that water is formed on the Moon from its chemical constituents.

BBC: Nasa reveals new information about moon probes crash sites

The CMB is the light that was finally allowed to spread out across space once the Universe had cooled sufficiently to permit the formation of hydrogen atoms - about 380, 000 years into the life of the cosmos.

BBC: Planck satellite: Maps detail Universe's ancient light