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The theory of relativity encompasses two theories of Albert Einstein: special relativity and general relativity. General relativity is a theory of gravitation. The development of general relativity began with the equivalence principle, under which the states of accelerated motion and being at rest in a gravitational field are physically identical. The upshot of this is that free fall is inertial motion; an object in free fall is falling because that is how objects move when there is no force being exerted on them, instead of this being due to the force of gravity as is the case in classical mechanics. This is incompatible with classical mechanics and special relativity because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed that spacetime is curved. In 1915, he devised the Einstein field equations which relate the curvature of spacetime with the mass, energy, and momentum within it.

In biology, evolution is change in the genetic material of a population of organisms from one generation to the next.

Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a process that can result in the emergence of new species.

The similarities among species suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence.

The basis of evolution is the genes that are passed on from generation to generation; these produce an organism's inherited traits. These traits vary within populations, with organisms showing heritable differences (variation) in their traits.

Evolution itself is the product of two opposing forces: processes that constantly introduce variation, and processes that make variants either become more common or rare.

New variation arises in two main ways: either from mutations in genes, or from the transfer of genes between populations and between species. New combinations of genes are also produced by genetic recombination, which can increase variation between organisms.

Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes. The study of evolutionary biology began in the mid-nineteenth century, when research into the fossil record and the diversity of living organisms convinced most scientists that species changed over time.

However, the mechanism driving these changes remained unclear until the theories of natural selection were independently proposed by Charles Darwin and Alfred Wallace. Darwin's landmark 1859 work "On the Origin of Species" brought the new theories of evolution by natural selection to a wide audience, leading to the overwhelming acceptance of evolution among scientists.

The Standard Model of particle physics is a theory of three of the four known fundamental interactions and the elementary particles that take part in these interactions. These particles make up all visible matter in the universe.

Every high energy physics experiment carried out since the mid-20th century has eventually yielded findings consistent with the Standard Model. Still, the Standard Model falls short of being a complete theory of fundamental interactions because it does not include gravitation, dark matter, or dark energy. It is not quite a complete description of leptons either, because it does not describe nonzero neutrino masses, although simple natural extensions do.