Decoherence Mechanics: My Journey Into Quantum Gravity.
Asking about the quantum nature of gravity is to ask a fundamental question about the nature of reality. It’s tempting to think, therefore, that physicists could be guilty of saying almost anything in their theories to try and make sense of the world. String theory, for example; one of the major theories of quantum gravity; is often accused of being inaccessibly abstruse, and an area of theory in which physicists seem to be ‘making it up as they go along’.
This is not an article about string theory per se, but it relates to string theory and other theories of quantum gravity tangentially. A key insight I’ve managed to glean after many years of thinking about quantum gravity is that other quantum gravity models might not mutually exclude string theory. Loop quantum gravity, for instance, the theory of spin foams and spin networks which atomises the spacetime of general relativity, is not cancelled out as a possibility by string theory. Here’s what physicist Sabine Hossenfelder has to say on this issue. Additionally, I suspect that a Euclidean framework; the framework postulated by Stephen Hawking; is also not mutually exclusive of string theory or loop quantum gravity.
This is an interesting conundrum therefore, and one that requires detailed and careful thinking. Why are physicists proposing models that seem to be opposed but which, on further inspection, do not mutually exclude? Why are we drawing lines in this way between our theories? And, furthermore, do we need to establish some new theory; some ‘master’ theory; or some new set of assumptions going forward about the fundamental nature of the world in order to successfully reconcile our seemingly opposed theories into a single, cohesive framework? I would have said that, yes, we do need to do that — more than that, however, what we are clearly in danger of losing is intelligibility.
The new theories are becoming not merely unintelligible to lay readers, but also seemingly to physicists themselves. And this is clearly indicative of a broken paradigm in research. Crucially, it must be remembered that what we’re looking for first of all is a theory of quantum gravity. But what does that mean? It means that we’re studying gravitational fields, or this tendency that objects composed of matter have to attract one another. We might examine the gravitational field of a planet for example and ask the question: what is the quantum basis for this attractive tendency? The follow-up question, since we’re dealing with quanta — i.e. the smallest indivisible unit of something (a particle, in all cases) — ,would therefore be: what particle, associated with a gravitational field, causes a force of attraction between two gravitating objects?
We might therefore go in search of a new particle, and indeed this is what most quantum gravity theories do. They propose a new particle called the graviton to explain the force of gravity. However, there might be another mode of thinking that we could employ whereby we simply use the particles of physics’ standard model that we have at present. That is, we could imagine that there exists some as yet unobserved behaviour (or some behaviour only hinted at in the evidence) of fundamental particles that explains the force of gravity. This is the approach I decided to take when first considering the problem of quantum gravity in 2016. Additionally, to keep the as-yet-unnamed new theory consistent with both loop quantum gravity and string theory we might say that their graviton is simply some configuration of the particles we’re familiar with behaving in a way that’s unfamiliar to us.
This is what I proposed. I proposed that electrons tunneling from one object exchange a W- Particle for a W+ particle with the protons from another object and these exchanges thereby create a force of attraction between the two objects. Therefore the quantum of the gravitational field is the electron and the W particles. We might see, in this light, the gravitational field as being a sort of cloud of electrons exchanging particles with the background which surround all gravitating objects (stars, planets, galaxies, etc). We cannot see this cloud of electrons of course since the wavefunction of the electrons would be constantly collapsing due to their interactions with other quantum particles (photons mostly). In this model, we’d be very much encouraged to think of electrons as being part of a cosmic soup of particles whereby quantum particles are constantly interacting with other particles.
This means that what we observe of nature is quantumly decoherent. That is, that quantum states do not last very long in nature and that the wavefunction of a given particle has very little time to exist prior to being collapsed again by another particle from the background. For instance, gravitational electrons would be mostly interacting with photons (particles of light) from the background and this would cause the wavefunction of these electrons to be constantly collapsing. They would also be interacting with neutrinos and exchanging Z bosons with them, thereby causing the so-called oscillations between their different flavours (types of neutrino). The fact that these electrons interact so often with photons makes this model consistent with general relativity (our current, best theory of gravity) since we know from general relativity that gravitational lensing occurs and that the background reflects particles of light.
We also anticipate, from quantum field theory, that a massless, spin-2 particle is the particle we’re looking for responsible for the force of gravity. In what I’m here proposing, this particle emerges in light of the fact that the background electrons exchange a W- for a W+ with an extraneous proton. The W particles each have a spin of one and a charge of either +1 or -1, and they have mass; however, due to their proximity during this simultaneous exchange of particles, their masses cancel and we’re left with a composite, spin-2 gauge field without any mass. This proposal seems to be undermined by the Weinberg-Witten theorem, though I suspect there are ways around this problem. In any case, this decoherent way of looking at things, filled with quantum fluctuation, seems to me to have great utility in explaining nature at its most fundamental level.
And these ways of looking at the world that I propose are not cancelled out as possibilities by either string theory or loop quantum gravity. They might finesse the ideas of those two bodies of theory to some extent, and they certainly seek a synthesis between them. The graviton of string theory is the W particle and the graviton of loop quantum gravity is the electron; what’s closer to the truth is that both particles have a role to play in the macroscopic force of gravity. We might therefore say that neither theory is wrong, merely that they are both incomplete. What seems evident though is that we need radically new thinking in theoretical physics before any progress can be made since we’re currently witnessing a broken paradigm that has become unintelligible even to most physicists themselves.
