Getter Rays are a fictional cosmic ray radiation, typically green in color, prevalent throughout the Getter Robo franchise. It is a mysterious source of energy that had a part in the evolution of life on Earth and possibly several other planets and realities.
Getter Rays appear to have been prevalent since the beginning of time. Millions of years before mammals evolved, when the dinosaurs walked the Earth, a shower of Getter Rays penetrated the atmosphere. This drove the dinosaurs to near-extinction, the survivors forming the Dinosaur Empire within the depths of the Earth. With most of the Earth's surface thus emptied for the taking, the surviving mammals evolved under the influence of the Getter Rays, resulting in many life forms - including humans.
Getter Rays for most of the series is seen as a boundless energy source that can be harvested to power complex machines. As long as Getter Rays exist in the universe, whatever is powered by them never runs out (this was the primary reason why the Getter Robo was originally designed for space travel). The reactions take place in the Getter Core which acts similar to a reactor that can reach a critical state and cause a meltdown.
Getter Ray pollution is an aftereffect of this meltdown, but it is unknown what the effects are and does not seem to be as hazardous as nuclear radioactive contamination as shown when people entered zones without any adverse affects on them. It can also cause artificial fusion energy that is capable of absorbing a total of 15 satellites and changing Jupiter into a sun.
Getter Rays are deadly on certain lifeforms including dinosaurs and the Invaders, working in a way similar to accelerated process of Darwinian evolution taking out life forms that cannot survive the effects of Getter Rays and causing other species to evolve into higher forms. Getter Rays also causes metal to chip away which is why the Getter Robo is made with a special shape memory alloy dubbed the Getter Alloy that reacts to Getter Energy and also allows the Getter Robo's transformation abilities.
Strangely, Getter Rays also seem to be alive when given a vessel such as the Getter Robo prototypes with the ability to dream and even allow its pilot to see visions of the future which is what happened to Ryoma during a test where he sees a post-apocalyptic world ruled by Getter Robos. In the Getter Robo Go manga, Go Ichimonji even speaks with Getter Rays in the form of the Shin Getter Robo where it explains that life exists in matter, spacetime, and evolution itself. It also stated that it is a means to spread life through space after the seed of the universe was planted. This can be shown and theorized by Ron Schweitzer at the end of the Getter Robo Go manga where the Shin Getter Robo and its pilots sacrificed themselves to melt the polar ice cap of Mars and start the process to terraform the planet.
It is also able to bring back the spirits of the dead who perished or were affected by the Getter Rays such as Musashi Tomoe, Dr. Saotome, Burai, and Emperor Gore. On a similar note, Getter Rays are able to hold information on an individual to the point of producing clones and artificial lifeforms based on the template as was done with Musashi in Getter Robo Arc and the Super Robot Wars novel. Some publications such as Getter Robo Anthology - Shinka no Ishi and Getter Robo Devolution imply that Getter Rays even tie themselves to certain individuals and their counterparts across the multiverse, almost always in contact with some version of the primary Getter Team.
There have been a few instances where Getter Rays cause a type of metamorphosis that changes the appearance and power of a machine. Examples include the Getter Dragon turning into a more powerful form in Shin Getter Robo and Getter Robo Armageddon, the prototype Getter Robo into the Getter Robo Hien, and (in some cases) Mazinger Z into Mazinkaiser.
Getter Rays have provided a source of energy with limitless potential and a number of hazardous risks. In moderate use, Getter Rays appear harmless but an overload or massive amount can cause large scale destruction that can destroy an entire city and the lives within it. Because of this, measures had to be taken to prevent further catastrophe such as stopping research on Getter Rays until threats that could be best dealt with Getter Rays surfaced.
It also came to be feared by all of the villainous groups in the Getter Robo franchise for the dangers it presents to them and some groups (or individuals like Kamui Sho) have even extended this to humans seeing how far humans could progress with Getter Ray technology and its apparent favoritism of humanity.
Some humans like Ryoma Nagare and Go Ichimonji showed an animosity towards Getter Rays after seeing their comrades die from it as well as the apocalyptic future where people became obsessed with the power of Getter Rays and the appearance of the Getter Emperor. Ironically these humans were the ones most favored by the Getter Rays.
However as seen in the various media, Getter Rays are neither good nor evil and instead are seen as a force that has society question themselves asking philosophical questions such as "the existence of humanity" and "why human beings kill each other". Getter Rays themselves appear to want to find someone who can overcome the effects of Getter Rays and witness how far those people can go.