I think the phrase “fabric of reality” unnecessarily prejudges possible answers to what the question is really asking, namely “What does reality consist of?”
(I suspect that the phrase is a remnant of a common description of gravitational waves as “ripples in the fabric of space time”, which is actually not a terrible metaphor, as physics metaphors go, but notice that when the phrase is used here, the questioner makes a leap: spacetime=reality, which may or may not be justified, depending on the context).
The short answer is that at this stage of our scientific knowledge, the question, as a matter of physics, is unanswerable. It is interesting, however, to contemplate what it would take to be able to answer this question.
We have currently two fundamental frameworks for understanding reality, and they seem to be at odds with each other.
One framework, almost single handedly developed by Einstein, tells us that the arena of reality is not space, nor time, but a sort of a fusion of the two which results in a four-dimensional “space” we call Minkowski space time. This is, in my view, the most profound lesson of special relativity.
Before I go on, I want to elaborate on this a bit because it is just incredible how much at odds it is with our perception of reality, and most people, I suspect, do not appreciate this sufficiently. We cannot but perceive reality by “slicing” space time into space like and time-like hyper-surfaces, but these “slicings” are completely arbitrary and do not reflect a deeper reality.
This is actually quite similar to the following analogy: say, I hold pencil on its tip on a piece of paper, first so that it makes a right angle with the paper, and then move the top while keeping the tip fixed so that it makes various other angles. The pencil does not change, but its shadow does. It is shortest, pointlike, when the angle the pencil makes with the paper is a right angle (assuming the light source is right on top), and gets longer as the pencil deviates from it, until it reaches the same length as the pencil when it lays flat on the paper.
In this analogy, we are like observers who are incapable of perceiving the pencil, but can only perceive its shadow. Changes in the shadow may not reflect changes in the pencil (Of course, they could, such as if we break the pencil, but I will ignore these situations) but reflect changes in how the pencil is rotated with respect to the paper. The well-known relativistic phenomena of length contraction and time-dilation are, in fact, a kind of rotation, but not the kind that we are used to, which is not circular rotation, but hyperbolic rotation.
(As an aside, there is an interesting relationship between circular and hyperbolic rotations which involves the square root of −1−1, and it is possible to reformulate special relativity so that the time dimension is multiplied by this factor, turning Minkowski space time into a four-dimensional Euclidean space. This reformulation, called `Euclidean Relativity’ used to be popular in the first few decades after special relativity was developed, but has since fallen out of favor.)
Going from the analogy to the real thing, special relativity tells us that, in reality, we are not highly localized bunches of mass-energy moving about in space over time, but extremely long and narrow branching 4-dimensional tubes, connected with other extremely long and branching tubes at various points. Our branchings are what we perceive as contact with other objects or people, consuming food, and excretions. At the moment of our birth, we branched out from our mother-tube, and after our death, our tube will diffuse into many smaller tubes, some of which may join yet other tubes, such as a worm-tube or a tree-tube (which only coincidentally also happen to be tubes in 3 spatial dimensions).
Just as we are in the analogy incapable of perceiving the pencil and can only observe its shadow, in reality we cannot perceive ourselves as these tubes, except in our imagination, and must be content with the “slicings” of space time our sense organs allow us to observe.
A decade after Einstein developed special relativity, he went further with his general theory of relativity, which reframes gravity as a change in the geometry of space time near where such tubes are present.
The main issue relevant to this discussion that general relativity raises is that the geometry of space time can change even in regions in which space is empty. That means that the picture of everything in reality consisting of 4-dimensional tubes cannot be the complete story, because there still is “something” that is not a 4-dimensional tube, yet which can give rise to a change in space time geometry. That, in turn, means that a possible answer we might have considered, “reality consists of 4-dimensional branching tubes” is, at best, incomplete.
In his later years, Einstein seems to have thought of space time in terms of a kind of an ether. A key excerpt of a 1920 speech entitled “Ether and Relativity” he gave at the university of Leiden says:
“What is fundamentally new in the ether of the general theory of relativity as opposed to the ether of Lorentz consists in this, that the state of the former is at every place determined by connections with the matter and the state of the ether in neighboring places, which are amenable to law in the form of differential equations; whereas the state of the Lorentzian ether in the absence of electromagnetic fields is conditioned by nothing outside itself, and is everywhere the same. The ether of the general theory of relativity is transmuted conceptually into the ether of Lorentz if we substitute constants for the functions of space which describe the former, disregarding the causes which condition its state. Thus we may also say, I think, that the ether of the general theory of relativity is the outcome of the Lorentzian ether, through relativation.”
I personally have an opinion on how to think about this, but in this article I wish to stick to what has been established and is (even if only indirectly) generally accepted, rather than my own speculations. I will say, however, that to me, an “ether” conception of space time is highly problematic because at best, it seems to merely postpone the answer we are seeking by one step: I feel that saying “Reality consists of an ether with x properties” only gives us another name for our ignorance.
Now, as mentioned, relativity is only one of two fundamental frameworks we have for understanding reality. The other is quantum theory. “Quantum Theory” is a general term which encompasses both non-relativistic quantum mechanics and relativistic quantum theories. Quantum mechanics in its modern version was developed about a decade after General Relativity, but the version of quantum theory that is relevant to our discussion is quantum field theory (QFT), which combines quantum mechanics with special relativity within a field theoretical context, and took another couple decades to develop.
Because QFT incorporates special relativity, it shares with it the most profound lesson of the latter, that the fundamental arena of our reality is space time. However, QFT has some features which cause a marked deviation from the classical geometric picture conveyed in the above discussion.
For one thing, there are no longer “objects”, aggregates of billiard ball-like particles, but only excitations of quantum fields. Quantum fields are abstract ‘things’ which permeate all of space and time. We may not notice that they are there but for the possibility that in some localized regions they give rise to “excitations” which we might have otherwise called “particles”. These excitations can persist and propagate, which means we have to make an adjustment to our picture of the 4-dimensional branching tubes insofar as we conceptualized them as “objects” distinct from what is around them. In the new picture, the tubes are (aggregates of) excitations of a field that is everywhere.
This picture has the added advantage that what we thought of as “empty space” is now also part of the quantum field, so if something happens that appears to us as a change in the geometry of space time near such a tube in empty space, it no longer seems as unfathomable.
This may then suggest a possible answer to the main question posed here: “Reality consists of quantum fields”. I think there are physicists who would be quite satisfied with this answer, but I have two problems with it:
First, while we understand very well what quantum fields do, we do not really have a good handle on what they are. It is easy to dismiss a demand for understanding what physical fields (of any kind) are as a purely philosophical one divorced from physics, but the truth is, until we can actually answer the question, we don’t know. There is nothing that prohibits the possibility that once we understand what quantum fields are, we will have a much deeper understanding of their physics than we do now, and I think that this possibility is actually more likely than the alternative. So, in my view it is premature to dismiss such a demand, and if so, then the claim that reality consists of quantum fields once again suffers from the problem of giving another name to our ignorance.
Second, there are still considerable problems in reconciling the picture of reality conveyed by Quantum Field Theory with that conveyed by General Relativity, and until we understand how they are reconciled, we cannot really claim to understand what reality consists of.
Attempts at reconciling the two frameworks have focused on a particular approach, unification, in which the two are thought of as limiting cases of a more fundamental theory which incorporates both. The general name for such a theory is quantum gravity, and there are many different attempts at formulating such a theory, of which the best known are string or M-theory and loop quantum gravity (but there are perhaps dozens of other, less well-known frameworks).
Most of the contemporary physics literature paints a picture in which unification is the only way to reconcile the two frameworks, but the possibility is not yet foreclosed that they could just peacefully co-exist, separated by a boundary in their respective domains of validity which we have not yet recognized.
(Without going into too much detail, what it would take to foreclose this possibility is measuring a non-zero gravity field of a source that is firmly within the domain of quantum theory. This has not yet happened, though many physicists may take other observations to infer this, in my view incorrectly. For instance, the gravitational bending of light, which is an aggregate of photons and therefore within the domain of quantum theory, can be explained by means of a quantum field theory against a classically curved space, so it fails to foreclose the possibility, but if we could measure a non-zero gravity field of a laser beam, then this possibility would be foreclosed. Such measurement has to my knowledge never been performed, but it is almost universally expected that such gravitational fields exist).
Until we understand how exactly to reconcile the two frameworks, there is always the possibility that whatever we think is the answer to the question of what reality consists of could be incorrect. This is especially so given the stark difference in the picture of reality painted by General Relativity and Quantum Field Theory: In the former, any 4-dimensional tube is just associated with one field configuration in a given observer frame*, whereas in the latter, it is associated with up to an infinite number of them, a consequence of what is called quantum superposition.
Quite apart from these issues, which are well understood, I see another problem that renders the question unanswerable at present and that is that reality, at bottom, consists of what is “out there”, or, in other words, what physically exists.
But “existence” is not at present a physics concept, even if many physicists have a penchant for using this word. What it would take for it to become a physics concept is a definition purely in terms of other physics concepts, and that is at present lacking. I think that is another deficit in our understanding which needs to be overcome before the question can be answered in a satisfactory manner.
So, in summary, we do have an inkling of what reality is like, as described by special relativity, but we cannot pin it down because general relativity and quantum field theory take this basic special relativistic picture in different directions, and we don’t yet understand how to reconcile them with each other. On top of that, we don’t have a physics concept of “existence” which is precisely what would characterize whatever it is we would find that reality consists of. What it takes to answer the question “what does reality consist of” is then, at minimum, solving these problems.
**The qualification is meant to address the fact that different observers can observe different electric and magnetic fields for the same system. However, the four-dimensional description, given by the electromagnetic field tensor, is the same for all observers.