In a container filled with hydrogen under normal conditions, i.e. at room temperature and atmospheric pressure, hydrogen molecules (H-H, H2) collide quite normally. However, nothing at all is formed - only their temperature and momentum change as they collide.

In order for hydrogen to become a different isotope, a change must occur in its atomic nucleus. The difference between ordinary hydrogen, deuterium and tritium is in the arrangement of their atomic nuclei. Under normal conditions, the interaction between the nuclei is effectively prevented by electrostatic repulsive forces. These must be overcome by providing sufficient energy.

One option is an accelerator. Protons, i.e. the nuclei of ordinary hydrogen, are accelerated to near the speed of light and collide with other protons. Such collisions usually produce a number of interesting subatomic particles that tell scientists a lot about the composition of the building blocks of matter. But the 7 TeV energy that protons acquire in an accelerator is too high to give rise to isotopes like deuterium or tritium.

Less destructive interactions can occur if the energy of hydrogen is much lower, around 1.2 keV. This corresponds to a temperature of 15 million kelvin, which is the temperature at the core of our sun. Here, a process called the proton-proton chain commonly takes place. Simply put, two protons combine to form a deuterium nucleus, producing a positron and a neutrino. Deuterium fuses with another proton to form tritium. Tritium can then fuse with a proton producing the nucleus of a helium atom and two protons. In this way, the Sun converts hydrogen into helium and generates the energy that allows life to exist on Earth.

Proton fusion is a surprisingly slow and inefficient reaction. In theory, the average proton in the core of the sun takes 9 billion years to successfully fuse to deuterium. The Sun's enormous energy output is thus only possible because of the unimaginable amount of hydrogen in its core. Every second, the Sun burns 600 million tonnes of hydrogen and converts it into helium. This reaction produces deuterium, tritium and beta radiation, which are positrons. It also produces gamma rays, which are high-energy photons used to strip the atom of excess energy. But to carry out this whole reaction in 1 milligram of hydrogen under terrestrial conditions is virtually impossible.

At the much higher temperatures that our Sun's core does not reach, around 150 million degrees, fusion between deuterium and tritium can occur. This reaction takes place much more readily and efficiently (expertly we say it has a higher cross section) and produces a helium nucleus and a neutron. The process of fusing atomic nuclei at high temperatures is called thermonuclear fusion. We can already reproduce this process on Earth and hope that one day it will power our power stations. The fusion of 1 mg of a mixture of deuterium and tritium produces approximately 0.6 milligrams of helium, 1020 neutrons and releases 2.8*108 Joules of energy.