Does gravity need to be quantized?
Transmission Control -
One question I see asked a lot but rarely answered (and never convincingly, for me) is whether or not space and time are actually quantized. Obviously particle interactions are quantized but this is really a separate issue.
It seems to me that quantization of space itself has some practical implications that could be tested. My first look would be at the inverse square law at small scales. A fully quantized universe suggests that space is divided up into a discreet grid of cubes — possibly 1 cubic Planck length but possibly smaller. This is known as a Voxel or volumetric pixel. Any field within one of those voxels would likely have to be at specific discreet intervals. This means that if you traced the edge of a sphere, measuring the forces, there should be noticeable aliasing of the type seen in computer graphics. The Inverse-square law would be an integral approximation of a sphere. The smaller the scale, the lower the resolution, and the less accurate an approximation it would be. Testing this would be tricky as it would be hard to detect much beyond an order of magnitude or two above the Planck scale.
What if the universe is analog as people always previously believed and quantization is an emergent property? The mathematics of Pilot Wave Theory (de Broglie-Bohm Theory) and the impressive hydrodynamic analog of it suggests this could easily be the case. Quantization in that system is an emergent property that arises from analog standing Faraday waves interacting, in a sense, harmonically. One particle’s standing wave will tend to move towards another until their pilot waves reach a sort of maximal constructive resonance. There are several discreet distances at which that happens depending on the momentum of the particles. In the hydrodynamic model, it’s not instantaneous but the clamping occurs quickly enough that it’s very noticeable.
This suggests to me that there is a narrow range within which quantization occurs where the timescales and distances are small enough so things move into resonance instantaneously for all practical purposes. Planets also fall into resonances for roughly analagous reasons but the scale is such that they can spend much of their time out of resonance and we don’t perceive the highly discreet orbits and energies seen in atoms.
I raise this question because I believe it’s important to determine which is the case in order to know exactly what unification should look like. The main thrust at present appears to be to quantize gravity but maybe we should be trying to “unquantize” quantum mechanics. This argument is a variation on an old one that goes back to the 1930s and culminated in the EPR Paradox and Bell’s Theorem — whether Relativity or Quantum Mechanics is incomplete because one of them has to be. I don’t believe it’s ever been truly settled definitively despite Bell’s success in shooting down the only serious argument presented. People mistakenly assume Bell’s Theorem implies QM is complete but it in no way proves that. All it shows is that we are missing something critical somewhere but it isn’t quite what Einstein, Rosen and Podolsky thought it might be.
I think that whether you quantize gravity or “unquantize” QM in some respects is just a matter of perspective. Perspective, though, is incredibly important in how one constructs a theory and what conclusions are drawn from the model. The best example of that is geocentrism vs heliocentrism. Mathematically the two are just different perspectives on the same problem. One can construct a mathematical model of geocentrism that, on small scales at least, is perfectly accurate. The problem is that in order to make that theoretical model apply to the entire universe you have to keep bolting on more and more complex epicycles until it’s effectively an infinitely complex theory. This point of view also leads to many wrong and often outlandish conclusions that have to be tested and adjusted for. It isn’t so much wrong; it’s just a useless way of reasoning about the problem and making predictions. A tiny shift in perspective is all it takes to simplify things and make your intuition a valuable guide to discovery.
The Copenhagen Interpretation (and most of it’s cousins), String Theory and Many Worlds Theory seem to me to be in a similar position to geocentrism. It’s giving us wrong intuitions of larger scales. Einstein pointed out one of these wrong intuitions most poetically with the question, “Do you really believe the moon is not there when you are not looking at it?” We appear to be bolting on more and more complicated structures as Ptolemy did with epicycles.
I firmly believe what we are missing is little more than the right perspective. Historically this has nearly always been the case with major scientific revolutions and we are overdue for another one.