One shows the remains of a dying star four thousand light years away which has thrown off a cloud of glowing hydrogen gas.
VOA: special.2009.09.30
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.
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.
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.