Book Reviewed: Experimental Search for Quantum Gravity, edited by Sabine Hossenfelder

What is Spacetime?

In this edited book, physicists discuss the nature of spacetime and the experiments that would substantiate a theory of quantum gravity. Understanding quantum gravity has been elusive despite numerous theoretical models that exist in physics literature, but none makes any testable prediction. This includes, String theory, Loop Quantum Gravity, Supergravity theory, Twistor theory, and others that provide mathematical descriptions of quantum gravity. Part of the problem is that we don’t know what to look at in cosmology. It stems from the fact that most theories of quantum gravity appear to predict departures from general relativity at very high energy scales, of the order of 10exp(19) GeV. In addition, gravity is so weak that there is no physically realistic way to measure properties of particles such gravitons (force particles that mediate gravitational force).

In one approach, products of cosmic events such as, Cosmic Microwave Background (CMB), black holes, naked singularities, neutron stars, cosmological constant and gravitational waves are used as a laboratory for gravitation research. The second route is the particle colliders like Large Hadron Collider (LHC) where high energy particles are produced that can be studied to support existing theories of quantum gravity. New technologies may allow to detect gravitons, the force particles of gravity or new particles that may lead to other laws that operate in the universe.

A brief description of this book is as follows: The physical reality we observe, and experience is due to the interplay of matter and energy in spacetime. The theory of relativity describes the reality at macroscopic scale, and the quantum theory explains the reality of elementary particles. Gravity is understood from cosmic structures, and the quantum gravity is studied at the lowest physical scale, i.e. the Planck scale, 10e(-35) M. At this range, the universal constants, c (the velocity of light in vacuum), ℏ (the Planck's constant), and G (Newton's constant), come together to form units of mass, length, and time.

In general relativity, mass and energy are treated in a classical manner, i.e. the strengths and directions of various fields and the positions and velocities of particles have definite values. These quantities are represented by tensor fields (that has direction and magnitude). Each point gives the distance to neighboring points by a set of real numbers (not complex or imaginary) numbers. But the laws of quantum physics describing a physical force do not have definite values because of uncertainty principle. In addition, physical quantities are described by quantum states which gives a probability distribution over many different values, and increased specificity (narrowing of the distribution) of one property (e.g., position, electric field) gives rise to decreased specificity of its canonically conjugate property (e.g., momentum, magnetic field). A fluctuating metric implies continually changing causal structure and spatiotemporal ordering of events. The conceptual difficulty is that since we represent gravity as a property of spacetime rather than as a field propagating in a (passive) spacetime background. When one attempts to quantize gravity, one is subjecting some of the properties of spacetime to quantum fluctuations. This question our experience of matter/energy in spacetime; the quantum spacetime suggests that matter distributed in spacetime is just an approximation.

Several scenarios are described in this book to probe quantum gravity; the acceleration of the universe’s expansion. Gravitational ether scenario to capture effects of the quantum regime in a thermodynamic manner; connectedness of spacetime due to quantum entanglement (EPR) and wormholes (Einstein-Rosen bridge); probing the ER=EPR equivalence, the spacetime connections between objects are the geometrical manifestation of their entanglement; the entanglement seems to be shaping spacetime. This is an “emergent geometry” phenomenon. Studying black holes as the condensates and computer simulation of unifying spacetime, condensed matter and quantum information. Another way to probe events that does not preserve symmetries permitted by Lorentz invariance of general relativity. A sensitive experiment that would detect a field in the cosmos that exerts a force on electron spin.

I found the following chapters very illuminating; Spacetime Structure: Analogy in Condensed Matter and Quantum Information by Martin Seltmann; Experimental Search for Quantum Gravity Using Cosmology by Manon Bischoff and Vincent Vennin; The Cosmological Constant and Its Problems: A Review of Gravitational Ether, by Michael Florian Wondrak; and General Relativity, Black Holes and Planck Stars, by Matteo Trudu.