On how not to emulate Einstein only *partially*

On how not to emulate Einstein only *partially*

Einstein’s quest for developing a mathematically beautiful and physically correct field theory, out of pure thought and nothing else, was successful in the discovery of the general theory of relativity. However, his pure thought based quest failed in the attempt to unify gravitation with electromagnetism. But this failure of Einstein has not prevented us from emulating him – searching for a theory of quantum gravity, and a unified theory of known interactions, based on pure thought, without there being any experimental evidence to support quantization of gravity,  or to support its unification with the other forces. We have tried this for more than half a century, but like Einstein, we have also failed. At least we have not succeeded thus far. And yet, on physical grounds we know that there must be a quantum gravity, as well as unification. 

Could the reason for our failure be that we have been emulating Einstein only `partially’?

In hindsight, we know good reasons why general relativity was successfully discovered. Maxwell’s electrodynamics was relativistic to start with – it in fact propelled the discovery of special relativity. Because Galilean-invariant Newtonian mechanics was inconsistent with electrodynamics. Once special relativity was discovered, Newtonian gravitation had to be made relativistic too; hence general relativity was inevitable. The same cannot be said about unifying electrodynamics and gravitation as a geometric theory. Because the world is not classical. Enter quantum theory. Electrodynamics must be quantized, for it to agree with experiments. There goes the Einstein-style unification. Attempts at unification must take quantum theory into account. However, Einstein was not satisfied with quantum theory, and believed it to be an approximation to a more general  theory. 

In trying to pursue Einstein style thought based unification  program, while at the same time ignoring Einstein’s  concerns about quantum theory, we are emulating him selectively – this could be risky.

Einstein objected to the spooky action at a distance, through the EPR argument on quantum non-locality. He was not saying that quantum theory allows superluminal signaling. Rather, he was saying that there was a quantum influence outside the light cone, which is not causal. And this meant  that either the quantum mechanical description of reality is incomplete, or that special relativity and its related description of space-time structure would have to be modified so as to make it compatible with quantum theory. Since Bell’s theorem rules out local hidden variable theories, and since quantum non-locality has been confirmed by experiments, it is indeed special relativity which needs a rethink, in the quantum context.

Einstein also objected to the occurrence of probabilities in a deterministic mechanical theory: God does not play dice, he famously said. 

Can we then, in our quest for a quantum theory of unification, in the Einstein style of pure thought, also  address his concerns about quantum theory? Why do we pick just one half of Einstein? Maybe emulating him all the way will pay dividends? 

If we decide to emulate the full Einstein, how shall we do it? In true Einstein style, we must look closely at the quantisation procedure. The world of classical dynamics works perfectly, almost, in the macroscopic domain: classical bodies and fields on a classical space-time. If we are to quantise, we must quantise everything in one go: matter, gauge fields, AND space-time degrees of freedom [special and general relativity, not just the latter]. Quantising only matter and gauge fields, and leaving spacetime classical, while very successful, is at the heart of the spooky action at a distance that bothered Einstein. If we quantise everything, we will get a pre-quantum, pre-spacetime theory. This is what removes the incompleteness of quantum mechanics, because it makes pre-spacetime compatible with pre-quantum theory.

Through this generalized quantisation, we have gone pre-. How do we recover the classical world of matter and space-time from the pre-theory? Spontaneous localization is the answer. Classical macroscopic bodies and classical space-time emerge *simultaneously*. We say that space-time arises from the collapse of the wave-function.

Those matter particles and gauge fields which do not undergo spontaneous localization must be described by the pre-quantum pre-spacetime theory. This is the world of elementary particles undergoing standard model interactions. Even if these interactions are at low energies, the pre-theory must be used, if we are to avoid the spooky action and the probabilities of quantum theory. The pre-theory has an IR sector. However, we can to an excellent approximation describe quantum systems by using not the pre-theory, but quantum field theory on a classical space-time background, as we conventionally do. The approximation consists of dropping the very tiny quantum correction to space-time, caused by quantum systems, and assuming  spacetime to be classical. This is our beloved quantum theory, the one Einstein correctly calls incomplete. It has spooky action at a distance. The pre-theory is also deterministic [though non-unitary]. It has no probabilities – these arise only in the approximate description.

Moreover, when we examine the pre-quantum pre-spacetime theory, we find evidence for the standard model symmetries. Maybe the pre-theory can explain things about the standard model which we do not otherwise understand.

Maybe it pays to emulate Einstein fully.