QUANTUM GRAVITY, set of theories that attempt to reconcile Einstein’s theory of general relativity with quantum mechanics in order to describe how gravity functions at the subatomic level. While relativity seems to accurately describe the behavior of very large objects and quantum theory the behavior of very small objects, as of yet there is no clear consensus as to how these two theories can be reconciled.
Einstein’s theory of relativity, developed between 1905 and 1916, upended many of the basic tenents of Newtonian physics. Previously, space and time were thought of as separate, universal, and static phenomena; objects moved through an unchanging container of three-dimensional space; time moved at the same pace everywhere; and gravity was a force emitted by objects themselves.
Einstein demonstrated that time actually slows down for an object in motion, especially as it approaches the speed of light. Time is dependent on how fast an object is moving relative to the observer. Time therefore cannot be truly distinct from space. Instead, space and time should be considered together, woven into four-dimensional “spacetime.” This conclusion led Einstein to further insights about the nature of gravity. Very large objects warp the fabric of spacetime, like a bowling ball placed in the center of a trampoline. The curvature of spacetime itself is what creates a gravitational pull.
Einstein’s theories have proved extremely accurate when describing large cosmic events, but physicists have struggled to reconcile this theory of gravity with the behavior of subatomic particles. Reconciling quantum theory and general relativity becomes especially relevant when thinking about singularities, such as the center of a black hole or the beginning of the Big Bang.
Quantum mechanics was developed in the 1920s by a group of physicists including Werner Heisenberg and Erwin Schrödinger. It is currently the best method physicists have for modeling the behavior of particles at the subatomic level, which do not seem to follow the rules of classical physics.
Central to quantum mechanics is the process of quantization. Quantization refers to breaking down smooth continuous forces into their smallest measurable
units. Light, for example, behaves both like a smooth continuous wave and like a collection of tiny particles known as photons. When energy and matter are treated as being made up of small discrete units, their behavior at this tiny scale can be more accurately described.
According to quantum theory, gravitational fields, and potentially spacetime itself, should break down into similar discrete units, hypothetically called “gravitons.” So far this has not been borne out mathematically. It remains an open question whether the problem is one of finding the right experiments and mathematical descriptions or if the two theories are truly incompatible.
It is extremely difficult to test theories of quantum gravity because gravity is a relatively weak force; the Earth pulls you towards its center, but not so hard that you cannot jump. Your body also has a gravitational pull, but it is so faint that you do not notice time slowing down ever so slightly as you walk across the room. Imagine, then, how difficult it is to measure the gravity of something smaller than an atom.
The most well-known conceptual framework for how to resolve this conflict is string theory. String theory describes the subatomic world as made up of one-dimensional, vibrating strings. These strings could look and behave like quantum particles, but these vibrational states could also explain gravity. String theory implies the existence of at least ten dimensions, if not more. While string theory has been refined over time, there exists no clear iteration of the theory that works for all cases.
Einstein was always skeptical of quantum theory and particularly of what he called “spooky action at a distance”: if two electrons are generated with interdependent qualities, altering one electron seems to alter the other instantaneously, even after the pairs are separated. Einstein completely rejected this as a possibility because it would imply that electrons are somehow communicating with each other faster than the speed of light. That the speed of light is constant and serves as the “universal speed limit”— the fastest speed anything is capable of—is essential to the theory of relativity. And yet “spooky action at a distance” has been proved in multiple experiments. At the quantum level, physical properties like position, velocity, and momentum are described in terms of probability. They do not seem to have fixed values until they are measured. Applying this quantum uncertainty to spacetime itself—the very fabric of reality, which even quantum mechanics treats as the background container against which other properties are measured—raises philosophical as well as mathematical questions about the nature of time, existence, and reality itself. This has led to speculation about multiple universes, whether time itself is an illusion, and other hypotheses worthy of science fiction.