A few days ago at the dinner table, my son mentioned that one of their Theory of Knowledge teachers had explained to them on that day that gravity was not a force, but instead that it was an epiphenomenon in the sense that it arose as a consequence of the presence of mass and energy in spacetime. My immediate reaction was to specify that this was true in the framework of Einstein’s Theory of General Relativity, but that as revolutionary, elegant, subtle, and incredibly successful as it was and is, General Relativity is, as all other theories are, a theory nonetheless, and that theories are descriptions of nature that we construct to explain and understand, at least partially, the phenomena we observe.
No matter what it is that we are observing, no matter how microscopically small or astronomically large, no matter how simple of complex, no matter how subtle or coarse, no matter how rudimentary or sophisticated the instrumental methodology, the observation or measurement is inherently distinct from the phenomena being observed, it is removed from it. This precedes conceptually the modern quantum mechanical tenet that the act of performing a measurement affects the system to which the measurement is applied. The former is a statement about the inherent distinction and separation between the phenomena, the observation and measurement of a manifestation of it, and thus also the interpretation that is given to the observation. The latter underlines the fact that, in the quantum mechanical view of the world, a system is a weighted probabilistic mixture of different states that coexist until a measurement is made, at which point the `wave function collapses’, forcing the system to be found in one of these possible states, and the instrument tells us which state that is.
The fundamental point I am referring to, which, when expressed plainly, is as obvious as obvious can be, is this: a description of a phenomena is not that phenomena—it is a description of it; a theory about the physical world, a theory about the physical reality we observe is not the physical world, it is not physical reality—it is a description of it. This is so easy to see that it is not debated and obviously shouldn’t be. However, we, as scientists and philosophers, regularly, and in fact, too often make statements, adopt stances and draw conclusions that undeniably demonstrate that this most fundamental point about the relationship of the theories (to which we tend to be so dearly attached) to physical reality is not well understood, and the point is muddled in our appreciation of the scientific process in which we are engaged.
To hold that gravity is not a force but the manifestation of the fact that objects follow geodesic lines defined by the curvature of space-time which in turn is defined by the distribution of matter and energy illustrates the point well: we have substituted a beautifully accurate and successful description of the physical world as it pertains to the motion of bodies and particles, a jewel of a theory that is as elegant, far-reaching and as awesome in its descriptive as in its predictive powers, for an expression of how reality actually is, what gravity in itself is.
This is the first point I raised and explained in response to his mention of what the teacher told them in class. The supportive argument I used as an illustration of this was that in quantum field theory, another very successful theory that underlies all of modern particle physics, does, in fact, in stark contrast to Einstein’s classical Theory of General Relativity, treat the forces of nature as acting through the mediators of that force, bosons, that travel back and forth between the two particles, `carrying’ the force which is quantised in these boson force mediators. This is why it is described as a quantum theory of fields: everything is quantised into particles, including all the forces of nature, all of these particles are treated mathematically as fields pervading space-time, and gravity is quantised and carried by the graviton, even if the latter is the only one of the bosons that has not (yet) been detected. The other ones—gluons for the strong force holding quarks and anti-quarks together; 
Each time we succeed in understanding an aspect of the physical world more deeply and in subtler details, even if this understanding is flawed in some way that is not apparent to us, each time we succeed in developing a consistent theory with greater descriptive and predictive powers than the previous theory we had for this aspect of the observable physical world, the natural tendency is to claim and actually feel that now we finally understand how this works and how things are. But by the very fact that we have witnessed a multitude of both large and remarkable as well as small and incremental advances in our theoretical descriptions of the natural world, we are forced to appreciate the fundamental point that descriptions are only descriptions and will never be in any way equivalent to the actual phenomena that they describe.
In the same way that scientists and philosophers have pondered, discussed and argued about the meaning and consequences of the General Theory of Relativity on how we view nature and physical reality, they have done this, and in fact most likely to a greater extent, in relation to the interpretation of quantum mechanics, coming up with various paradoxes and conundrums in the process, which on the whole, instead of elucidating or clarifying issues, have only made the doctrines and theoretical implications appear stranger and more difficult to grasp. But here again, we suffer from the same problem: taking a description of reality, extracting meanings from this description about how reality or nature actually is, and then being intrigued and surprised by the counterintuitive consequences and paradoxes that arise from doing this.
To take the example mentioned above that deals with the collapse of the wave function, the fact that we describe our partial knowledge of the state in which the hydrogen atom finds itself as a superposition or co-existence of a set of different states with different probabilities for manifesting themselves, does not mean that this is so, it does not mean that this is how nature is. And the fact that when we make a measurement we find a particular state does not mean that prior to the measurement the system was in a quantum mechanical mixture of all the states. It is a description that works very well to describe certain physically observed phenomena in our laboratory experiments and therefore we use it. But it should not be interpreted as a statement about how nature in itself or physical reality in itself actually is; it is only a clever description that works in certain settings when certain boundary conditions are fulfilled.
This inquisitive human mind has always sought to understand. This understanding has grown evermore sophisticated and subtle over the centuries and millennia. The inherent human trait of clinging and holding onto whatever seems most solid in an attempt to make it feel most solid has led scientists and philosophers time and time again to believe in scientific theories as being expressions of how nature actually is, to equate a successful description of a physical phenomena to a statement about what the phenomena in itself is. Pursuing the intellectually challenging but stimulating and satisfying exercise of seeking increasingly sophisticated and subtle, extensive and ideally even all-encompassing explanations of natural phenomena through modern scientific theories has muddled the point further by continuing to ascribe to nature qualities derived from the interpretations we make of these theories. I think we should be more careful about this.
rigorouslycurious 
While claiming no knowledge of Quantum Mechanics, I do have problems with the clear distinction between scientific theories and what the theory purports to describe. From one point of view it raises the question of the thing in itself – the Ding an sich. Hume pointed out that empiricism cannot account for causation (or time or space). There is nothing inherent in sense perception that tells us that one event causes another, simply coincidence of sense data. Then why do we sometimes interpret an event as causing another and another time not? Kant sought to solve this by arguing that, rather than passive receivers of sense data, we actively impose a prior structure on the world. This structure includes the “categories” space, time and causation, which are within us, not the world (“the world is as we are, not as the world is”). This “solves” Hume’s problem of causation, but drives a wedge between the world as we see it and the world as it “really” is – the Ding an sich. Philosophers sought to cross the abyss between our world and the apparently unknowable Ding an sich. But Wittgenstein argues in the Tractatus that this is pointless. If the Ding an sich is unknowable then it is irrelevant – the only world that exists is that expressed in our language and it is a world of statements not things (“the world is all that is the case. The world consists of facts not things”). This suggests to me that the clear distinction that you seek to draw between the theory and the phenomenon the theory theorizes about is a false distinction. Ultimately the theory can only be about other theories.
The flavour of this is also captured in Quine’s attack on Popper’s theory of falsifiability (“Two Dogmas of Empiricism”). Quine criticized Popper for believing that theories can be individually falsified, arguing that all theories depend on other theories and that the whole network of theories must be falsified together. In other words, there is no event in a “real” world against which an individual theory can be tested, which I think must be the assumption behind your argument. Kuhn of course picks this up with his concept of the paradigm, with the consequence that in normal science consistency within the paradigm is the main criterion for scientific theories.
I concede that very few scientists would conceive of theories in these ways, but a you have commented elsewhere, scientists don’t generally reflect on the epistemological underpinnings of their trade.
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Thank you Shaun for the thoughtful and provocative comments.
“There is nothing inherent in sense perception that tells us that one event causes another, simply coincidence of sense data.” Can this be shown to be objectively true? We have today the means to measure the firing of nerve signal from a stimulus somewhere on the body and see the cascade of electrical pulses coursing through the nervous system to reach the brain where certain parts can be seen to be activated under certain circumstances and not under others. Isn’t it then actually simple to objectively measure the chain of events and thus the sequence of events in this cascade, each being triggered by the preceding event, and clearly showing the relation between cause and effect in sense perception? Or are we talking about what we perceive, independently of the functioning of the nervous system and the information it contains, little of which we actually “sense”?
There is no question as to whether or not we impose a structure to the world we perceive. This is demonstrably true, and has surely already been demonstrated in experiments. From our own experience we can see that it is the configuration of attention that defines what we can perceive through the senses: every Zen practitioner will have had the same realisation by the simple observation that looking at a white wall and keeping the eye lids open whatever is “seen” on the wall—shifting surfaces, changing colours, movement of and various dynamic textures—is understood as being movements of attention imposing on and consequently distorting the sense of seeing, because obviously, the wall is not moving, it is only white, and the light is not changing to the extent that the eyes could detect it: all of it is what we usually referred to as our mind spilling on the wall. There are numberless such experiences that are seen through the process of Zen practice and that cannot be arrived at by intellectual understanding because it needs to be experienced. But once it has, it is undeniable, and greatly sharpens our ability to distinguish between what is relevant and useful and what is less so in regards to intellectual matters such as this one. So, I totally agree with Kant, and actually know that this is true: that we impose a structure to the world we perceive through the moment to moment shifts and chances in the configuration of our attention, which filters everything that the senses perceive physiologically and biochemically. What is necessary to recognise and that adds quite another dimension to this point is that attention can be worked with and through the process of working with it, opening it up to more of the information that is available to the senses, the structures that are imposed evolve and grow less and less restrictive. In the end, the world that is perceived by a mature practitioner has very little to do with that perceived by a non-practioner. And there is nothing mystical about this, it is all just down to the configuration of attention, basically how open or how closed it is to the information available.
How can the only world we know be the one that can be expressed in language? If the world is not a world of things, it is even less so a world of facts expressed in statements! Language is so incredibly limited and limiting. Just try to describe fully and completely the sensation of the feeling of the breath moving in and out of the body, the pressures, the coarser and the subtler movements, what happens to the perception of weight in the different body parts, the lightness, the rising, the heaviness, the falling, what happens to the seeing, and on and on. Volumes and volumes could be written using words and statements to describe this simple sensation that is felt throughout the body-mind with the body-mind and it would never be able to capture the actual perception of the sensations.
As for theories being made up or existing in a greater context constructed based on an ensemble of other theories and for this reason not having an existence of their own, and the generalisation of this through the concept of paradigm that formalises what this context of theories and ways of thinking can be thought of, it is clearly sometimes the case—a good example is the modern edifice of theoretical particle physics based on quantum field theory, but it is also sometimes clearly not the case—the perfect example is Einstein’s special theory of relativity whose foundational element is the single isolated fact that the speed of light is finite and not infinite from which everything else in the theory follows. In the first case, everything depends on everything else, and with time the more and more branches spring out in all directions, but all remaining attached and thus reliant on the trunk and main branches. In the second case, the theory is on its own dependent on nothing other than the experimental observation that the speed of light is finite and not infinite, and for this reason standing on its own feet without relying on any other theoretical construct. Defending Popper’s position, I would say that if any little piece of a theory is falsified, then the whole theory is also falsified. We don’t even need to know or understand which piece relates to which other piece and how they related to one another. It doesn’t matter. If one thing breaks down in the face of experimental evidence, then it breaks down. That’s it.
Yes, scientists rarely reflect on foundational issues. But some do, and that’s enough to keep the discussion alive. Unfortunately, there are two many philosophers that philosophise about science without having enough scientific training nor enough scientific experience in actually doing science, compared to the number of scientists who spend some time considering philosophical issues relating to the science they do. We need more connections: more philosophers doing science and more scientists doing philosophy. That’s what I think. And I also continue to think that it is not debatable that the description and the described are inherently distinct. There is simply no way around this. What is another matter is the connection that these have to what we call “reality”. What is real? That’s a good question…
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