Alchemy

Alchemy, the ability to transmute matter into other forms of matter, is a skill highly developed by most space-faring species. It has two main laws - Equivalent Exchange (or the Conservation of Mass and Energy), and the Law of Reactionary Force (or the idea that the more something is changed the more it pushes back on the alchemist). It can be performed by mechanical systems as well as organic life forms, allowing for alchemical engines in star ships for war.

The Philosopher's Stone bypasses both of the first law, and allows for the creation of mass and energy or the destruction of mass and energy.

History of Alchemy (Human-based)
Alchemy as a science was first discovered by humans on Terra in the 5st century Pre-Atomica by the scientist Willhelm Reichenbach. It was alchemy that led to humanity first understanding the working of atoms, and eventually to the first manmade atomic fusion. In the 1st century Anno Atomica, alchemy rapidly advanced. Humanity was far from the first species to discover alchemy or achieve atomic fusion, and when it did so there were hundreds of species already doing so and even some already space-faring.

It was humanity's rapid research in atomic power and numerous inventions for war and peace though that makes its history of alchemy of primary interest. Equivalent Exchange, the most inherent property of the Universe, was discovered by 2nd century Anno Atomica, and within the same century the Law of Reactionary Force was also discovered. Warfare rapidly changed with alchemy and atomic power, and by 249 Anno Atomica the Philosopher's Stone was discovered.

Alchemical research was continued, and its application to mechanical systems was introduced during the colonization of the Terra system in the 2nd century AA. While theories for interstellar travel were worked on prior to the discovery of the Philosopher's Stone, it allowed for it to be done far sooner. Yet despite its part in the first interstellar ship in the 270s, in 298 interstellar travel using basic alchemy instead of the Philosopher's Stone was achieved.

Equivalent Exchange
The basic concept of alchemical reactions is Equivalent Exchange, or the Conservation of Mass and Energy. Essentially, matter and energy can only be changed, not created or destroyed. This makes changing elements extremely difficult, but possible.

The classical transformation of lead to gold cannot make the gold into the same weight the lead was in unless atoms from outside are brought in to make up for the difference of sub-atomic particles. However, transmutation to lower elements is extremely easy (especially in larger quantities) - the extra particles are either released or made into more atoms of the desired element.

Law of Reactionary Force
Well known in thermodynamics is that for every action there is an equal and opposite reaction. This law applies to alchemy as well, albeit in a much stranger way. When an individual performs alchemy, the changes require a certain amount of potential energy to be converted into kinetic energy from them. When the energy flow goes on, the person exerting it is drained of energy. If not enough material is there for the transmutation, the alchemist's body can be used to supply extra material.