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; , and for the weak force responsible for radioactive decay; and photons for the electromagnetic force—have all been detected long ago and studied in a great deal of detail for decades now. Therefore, in the framework of the modern quantum theory of fields, gravity is a force mediated by the graviton; not an epiphenomenon that manifests as a consequence of the energy distribution dependent curvature of space-time. Furthermore, most attempts to reconcile General Relativity with Quantum Field Theory are based on the scientific framework defined by the second of these theoretical pillars of present-day physics in which forces are forces carried by gauge bosons.
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.