Zenith Grant Awardee
Steven Giddings
University of California, Santa Barbara
Co-Investigators
Donald Marolf, University of California, Santa Barbara
Project Title
Locality and the emergence of space and time
Project Summary
Quantum mechanics and Einstein's general relativity have provided conflicting views of nature for nearly 100 years. Their reconciliation almost certainly involves ideas as radical at those underlying these theories. Time and space are no longer immutable and fixed in general relativity; with quantum mechanics they can melt away due to random fluctuations. When these ideas are combined in a complete theory, familiar space and time probably cease to exist as fundamental concepts in physics. These theories particularly clash in describing black holes and their evaporation, predicted by Hawking: theoretical study of high-energy particle particle collisions which produce black holes yield an apparent paradox, forcing us to abandon one of the cornerstones of our current physical theories of nature. Our investigation of this and attempts to describe space, time, and gravity in a quantum-mechanical framework suggest that the property of locality — independence of distant phenomena — must be revised at a fundamental level. This research will sharpen these ideas, which should particularly take force early in the evolution of the Universe, and have profound consequences for understanding its origin, its fate, and the basic structure of matter and laws of nature.
Technical Abstract
The nature of spacetime, locality, and causality in quantum gravity will be investigated. We take primary input from semiclassical gravity and local field theory but attempt to infer deeper principles. Conventionally, the problems of renormalizability and singularities have been thought the main challenge, and a primary clue to new physics. But lessons learned from studying the "problem of time," black holes, and cosmology strongly indicate that a quantum theory of gravity needs to incorporate significant departures from local quantum field theory and that modifications of locality will play a key role. We probe both the recovery of locality in the limit of vanishing Planck length and departures from strict locality at finite Planck length. A central question is the extent to which causality can be preserved if locality is lost. We seek clues from the structure of diffeomorphism-invariant observables, high energy gravitational scattering, and large infra-red effects in cosmology — all of which indicate limitations to the validity of local quantum field theory over large regions of spacetime. We also investigate the proposed AdS/CFT correspondence which, if true, may arise from general properties of gravity as opposed to details of string theory.
QSpace Latest
PressRelease: Precision experiment puts pressure on quantum collapse theories
Quantum mechanics, the theory governing the microscopic world, is famously counterintuitive. A particle can exist in a superposition of multiple states, such as different positions, until a measurement is performed. At that point, the wavefunction describing that particle appears to ‘collapse’ to a single outcome. This puzzle lies at the heart of the measurement problem, famously illustrated by Schrödinger’s cat, suspended between life and death until observed. The XENONnT detector, which was designed to be sensitive to rare physics events, has tightened constraints on one family of possible solutions to the measurement problem, known as ‘collapse theories.’ The work, which was partially funded by FQxI, was reported in Physical Review Letters in March 2026. Image credit: XENON Collaboration.