Gravity makes apples fall from trees. But what is it, really?
Einstein’s general relativity describes how very large, high-mass things warp the fabric of space and time that all matter in our universe exists in. Every experimental test of general relativity matches its predictions. But what is gravity on a small scale?
Early in the 20th century, quantum mechanics was used to explain the colors of objects as they heat up or cool down and the structure of atoms. Continuing down this path of scientific exploration, researchers discovered fundamental particles inside of atoms. These discoveries lead to a framework that describes the fundamental particles that make up matter and the forces that mediate their behavior: the Standard Model of particles and their interactions. Central to this theory is an understanding of how the various kinds of forces work: the electromagnetic force explains light and electricity; the strong force explains how the nucleus inside atoms work; and the weak force explains rare decays and the elusive neutrino particles.
Conspicuous by its absence, however, is gravity. The puzzle persists: what is gravity?
Why can’t we combine quantum mechanics and general relativity?
The problem is, while these theories work in tandem, at the most basic level, they aren’t a complete description of physical reality.
The speed of light, c, is a constant of nature. The size of the quantum realm is defined by Planck’s constant, h. The strength of gravity is set by G, Newton’s gravitational constant, which also is used in general relativity. A combination of these constants yields a fundamental length scale for all physical reality. This limitation, realized early on, was elegantly described by Arthur Eddington in the March 1918 edition of Nature:
It seems to be inevitable that this length must play some role in any complete interpretation of gravitation… In recent years great progress has been made in knowledge of the excessively minute; but until we can appreciate details of structure down to the quadrillionth or quintillionth of a centimetre, the most sublime of all the forces of Nature remains outside the purview of the theories of physics.
Quantum gravity seeks this reconciliation. There have been, and continue to be, many theoretical approaches to this conundrum. In this extremely active field of research, a challenge is studying the minute scale in which direct effects of the theories are manifest—the quintillionth of a centimetre, or Lmin, of which Eddington wrote.
Consider this: The size of the smallest fundamental particles is about a billion billion times smaller than we are. Lmin is a billion billion times smaller than that! What would we use to build microscopes to see that small? You would need to do that to figure out what gravity is.
Resolution of what gravity is
GQuEST resolves this dilemma by testing a group of compelling theoretical approaches to quantum gravity that predict side effects that we will be able to measure.