Quantum Gravity
M-theory
M-theory is a proposed framework in theoretical physics that unifies the five consistent versions of superstring theory through dualities and is expected to have an eleven-dimensional low-energy limit described by eleven-dimensional supergravity. First articulated by Edward Witten in 1995, the framework incorporates extended objects such as two- and five-dimensional branes and has seen nonperturbative formulations proposed via matrix theory and holographic dualities.
Planck length
The Planck length (ℓP) is the fundamental unit of length in the system of Planck units, defined by ℓP = √(ħG/c³). Using the 2022 CODATA adjustment maintained by NIST, its value is 1.616255(18) × 10⁻³⁵ meters with a relative standard uncertainty of 1.1 × 10⁻⁵. It is widely treated as a characteristic scale at which quantum and gravitational effects are expected to become comparable, motivating research programs in quantum gravity.
Quantum Foam
Quantum foam, also known as spacetime foam, is a theoretical concept in quantum mechanics, conceived by John Archibald Wheeler. It describes the fabric of spacetime at the subatomic, Planck scale as a turbulent, fluctuating medium where virtual particles and microscopic black holes are constantly created and destroyed, giving spacetime a "foamy" texture.