By the equivalence principle, Einstein inferred that being under either gravitation or constant acceleration are indistinguishable experiences that might share a physical mechanism. The suggested mechanism was "progressive" length contraction and time dilation—a consequence of the local energy density within 3D space—establishing a progressive tension within a rigid object, relieving its tension by moving toward the location of greatest energy density. Special relativity would be a limited case of a gravitational field. Special relativity would apply when the energy density across 3D space is uniform, and so the gravitational field is scaled uniformly from location to location, why an object experiences no acceleration and thus no gravitation.
In 1915, Einstein's general theory of relativity newly explained gravitation with 4D spacetime modeled as a Lorentzian manifold. Time is one dimension merged with the three space dimensions, as every "event" in 3D space—2D horizontally and 1D vertically—entails a point along a 1D time axis. Even in everyday life, one states or implies both. One says or at least means, "Meet me at building 123 Main Street intersecting Franklin Street in apartment 3D on 10 October 2012 at 9:00PM". By omitting or missing the time coordinate, one arrives at the correct location in space when the sought event is absent—it is in the past or future at perhaps 6:00PM or 12:00AM.
By converging space and time and presuming both relative to the energy density in the vicinity, and by setting the only "constant" or absolute as not even mass but as light speed in a vacuum, general relativity revealed the natural world's previously unimagined balance and symmetry. Every object is always moving at light speed along a straight line—its equivalent, on a curved surface, called "geodesic" or "worldline"—the one pathway of least resistance like a free fall through 4D spacetime whose geometry "curves" in the vicinity of mass/energy.
An object at light speed in a vacuum is moving at maximal rate through 3D space but exhibits no evolution of events—it is frozen in time—whereas an object motionless in 3D space flows fully along 1D time, experiencing the maximal rate of events' unfolding. The displayed universe is relative to a given location, yet once the mass/energy in that vicinity is stated, Einstein's equations predict what is occurring—or did occur or will occur—anywhere in the universe. The popularized notion that "relative" in Einstein's theory suggests "subjective" or "arbitrary" was to some regret of Einstein, who later thought he ought have to named it "general theory".
The electromagnetic field's messenger particles, photons, carry an image timelessly across the universe while observers within this field have enough flow through time to decode this image and react by moving within 3D space, yet can never outrun this timeless image. The universe's state under 400 000 years after the presumed big bang that began our universe is thought to be displayed as the cosmic microwave background (CMB).
In 1915, the universe was thought to be entirely what we now call the Milky Way galaxy and to be static. Einstein operated his recently published equations of the gravitational field, and discovered the consequence that the universe was expanding or shrinking. (The theory is operable in either direction—time invariance.) He revised the theory add a "cosmological constant" to arbitrarily balance the universe. Nearing 1930, Edwin Hubble's telescopic data, interpreted through general relativity, revealed the universe was expanding.
In 1916 while on a World War I battlefield, Karl Schwarzschild operated Einstein's equations, and the Schwarzschild solution predicted black holes. Decades later, astrophysicists identified a supermassive black hole in the center of perhaps every galaxy. Black holes seem to lead galaxy formation and maintenance by regulating star formation and destruction.
In the 1930s, it was noticed that according to general relativity, galaxies would fall apart unless surrounded by invisible matter holding a galaxy together, and by the 1970s dark matter began to be accepted. In 1998 it was inferred that the universe's expansion, not slowing, is accelerating, indicating a vast energy density—enough to accelerate both visible matter and dark matter—throughout the universe, a vast field of dark energy. Apparently, under 5% of the universe's composition is known, while the other 95% is mysterious—dark matter and dark energy.
By the 1920s, to probe the operating of the electromagnetic field at minuscule scales of space and time, quantum mechanics (QM) was developed. Yet electrons—the matter particles that interact with the photons that are the electromagnetic field's force carriers—would appear to defy mechanical principles altogether. None could predict a quantum particle's location from moment to moment.
In the slit experiment, an electron would travel through one hole placed in front of it. Yet a single electron would travel simultaneously though multiple holes, however many were placed in front of it. The single electron would leave on the detection board an interference pattern as if the single particle were a wave that had passed through all the holes simultaneously. And yet this occurred only when unobserved. If light were shone on the expected event, the photon's interaction with the field would set the electron to a single position.
By the uncertainty principle, any quantum particle's exact location and momentum cannot be determined with certainty, however. The particle's interaction with the observation/measurement instrument deflects the particle such that greater determination of its position yields lower determination of its momentum, and vice versa.
By extending quantum mechanics across a field, a consistent pattern emerged. From location to adjacent location, the probability of the particle existing there would rise and fall like a wave of probability—a rising and falling probability density. When unobserved, any quantum particle enters superposition, such that even a single particle fills the entire field, however large. Yet the particle is not "definitely" anywhere in the field, but there at a definite "probability" in relation to whether it was had been at the adjacent location. The waveform of Maxwell's electromagnetic field was generated by an accumulation of probabilistic events. Not the particles, but the mathematical form, was constant.
Setting the field to special relativity permitted prediction of the complete electromagnetic field. Thus arose relativistic quantum field theory (QFT). Of the electromagnetic field, it is relativistic quantum electrodynamics (QED). Of the weak and electromagnetic fields together, it is relativistic electroweak theory (EWT). Of the strong field, it is relativistic quantum chromodynamics (QCD). Altogether, this became the Standard Model of particle physics.
When the Standard Model is set to general relativity in order to include mass, probability densities of infinity appear. This is presumed incorrect, as probability ordinarily ranges from 0 to 1—0% to 100% probability. Some theoretical physicists suspect that the problem is in the Standard Model, which represents each particle by a zero-dimensional point that in principle can be infinitely small. Yet in quantum physics, the Planck's constant is the minimum energy unit that a field can be divided into, perhaps a clue to the smallest size a particle can be. So there is a quest to "quantize" gravity—to develop a theory of quantum gravity.