A paradigm shift in quantum mechanics may be needed to solve quantum gravity
Wave and particle dualities
A paradigm shift is a useful term to explain how a change in perspective can open new routes to problem-solving. The classic example is the shift from the geocentric to the heliocentric model of the Solar System. If you have spent time observing the planets with a telescope, you know that the Earth's rotation is the primary motion you have to deal with through the night. The ancients, however, believed that the Earth was stationary (no, they did not believe it was flat) and the planets, moon, and Sun orbited around the Earth. This belief persisted partly because, without telescopes, ancient people could not see evidence to the contrary. The main problem with the belief is that the planets occasionally appeared to wander backward against the fixed stars throughout the year. The word planet means “wanderer” in Greek. Nevertheless, the ancients devised solutions for this problem and maintained the paradigm until the early modern period when telescopes caused it all to fall apart.
Since then other paradigm shifts have ushered in new eras of discovery. General relativity shifted our concepts of space and time from fixed to dynamic. Quantum mechanics questioned the underlying reality of measurements and the exactness of prediction.
Each of these shifts occurred even in the absence of sufficient evidence to back up the associated belief system. The Heliocentric model of Copernicus and Galileo was a poor predictor and needed Tycho Brahe’s observations and Johannes Kepler’s theories of orbits to establish. General relativity took decades because atomic clocks, lasers, radio astronomy, and other equipment were needed to corroborate its predictions. The shifts occurred, not because the weight of the evidence demanded them, but because new theories resolved inconsistencies in earlier theories while explaining or predicting some new evidence that the older theories could not account for without clunky additions.
The current great inconsistency now is how to formulate a consistent theory of quantum gravity. Unfortunately, while there are new theories, there are few solid predictions. We do not have a theory of quantum gravity that does for either gravity or quantum physics what Einstein’s theory did for the perihelion precession of Mercury or the bending of starlight nor what quantum theory did for blackbody radiation or the double slit experiment.
It is possible that scientific instruments are not sufficiently sensitive to usher in any new paradigm, but they one day will be. On the other hand, I argue that the main problem with current theories is that they all stick to the familiar paradigm of quantum mechanics while attempting to hammer the square peg of general relativity into the round hole of quantum field theory.
After all, this is the program that worked for all other fields and the particles that belong to them.
Some researchers recognize that General Relativity, since it is a theory about space and time, the stage upon which all other forces and matter play, is different. Yet, I think they struggle to work their way out of the old paradigms anyway.
The old paradigms are roughly in lay terms these:
Quantum theory is a mysterious and difficult to interpret property of reality that we get when we quantize classical theories such that measurements of, e.g., particles, are the sum total of many possible histories and influences from all spacetime. In mathematical terms, this is the path integral. (I could go on about other aspects like Wilson loops and S-matrices, but the path integral is my main target.)
Space and time are manifestations of a 4 dimensional manifold on which is defined a field called a metric tensor which is the gravitational field. Matter and spacetime interact via the Einstein field equations which are derived from the Einstein-Hilbert action.
Quantum gravity has suffered for decades because it is impossible to take the paradigm from #2 and apply #1 to it. There are fundamental problems with both the mathematical approach and the physical interpretation of such an approach.
Most attempts to get around the problem involve changing #2 so that it fits into #1.
This is the basis of Effective Field Theory (EFT) approaches to quantum gravity. Essentially, #2 is simply an EFT of some other theory—string theory, loop quantum gravity, Causal Dynamic Triangulation, asymptotic safety (of which CDT is an example), or whatever else we can dream up as the “small-scale” representation of space and time.
So far, this program has failed and has done so for about 40 years.
The prevailing opinion of most of the proponents of these theories, however, is that without experimental data we cannot know which one is correct, and, therefore, each should be judged based on theoretical merits. Meanwhile, researchers keep pushing each theory forward.
I have no real problem with this. I have even lauded the praises of more than one approach. On the other hand, my nagging feeling is that they are all falling into the same trap.
The trap comes from sticking to the same old paradigm and assuming that quantum theory is fine as it is. After all, why mess with one of the most successful theories of all time? In particular, physicists have solved this problem before with other field theories like the weak force. Gravity is just more complicated.
Yet, this might be as pointless as trying to merge the Ptolemaic theory of celestial motion with Newton’s laws.
Nevertheless, I think it is time for a shift in quantum theory as well, not because we have problems fitting gravity into it, but because quantum theory has its own internal inconsistencies.
The main issue with quantum theory is the measurement problem. That is, quantum theory is incomplete because it does not include a theory or model for how predictions turn into measurements. Although there are numerous interpretations for how this occurs (I have written about Copenhagen, Many Worlds, and so on ad nauseam) no evidence has supported any of them.
Could this missing piece be the key to resolving quantum gravity? That is what I propose. Moreover, to find the solution, a paradigm shift must occur in which we think about reality—both the reality of measurement and that of space and time—quite differently.
The funny thing about paradigm shifts, however, is that they appear, at first glance, to be ridiculous. The Heliocentric model was ridiculed in its day. Surely the Earth does not move! Copernicus published his theory posthumously to avoid repercussions. Galileo was forced to recant the theory. Quantum theory was also ridiculed as was general relativity. (In fact, the Nobel committee explicitly avoided offering Einstein a prize for relativity even after Eddington measured starlight bending in 1919, choosing instead to award it for the somewhat lesser discovery of the photoelectric effect.)
Thus, I expect any proposal I make to seem ridiculous. Nevertheless, here it goes.
