An unexpected new move in an old physics debate

 

The fundamentals of the science of physics that the 21st century has inherited from the 20th consist of two grand theories.

On the one hand, there is the general theory of relativity [GR] set forward by Albert Einstein, confirmed by a wide range of experimental tests since. This holds (to put things very simply) that space-time is a continuous fabric that is distorted with all the matter and energy within it, while by the same token this background determines the flows of all that matter and energy. It is at its heart a theory about gravity and it is about certainties. 

On the other hand, there is a body of quantum mechanics [QM], advocated by Niels Bohr and opposed in its earliest formulations by the aforementioned Albert Einstein. Quantum mechanics is all about indeterminacy and uncertainty. We can be sure about the location of a particle only if we are willing to accept some vagueness about its velocity. Or, we can fix its velocity only if we are willing to leave some vagueness about precise position. Either way, there is some fuzz built into the universe, or perhaps into the relationship of observer and observed at the smallest level. 

Now, as it happens there is a tension between the two theories.  I have discussed this general point in this blog before: https://jamesian58.blogspot.com/2020/08/einstein-and-bohr.html.

Why can't the two theories co-exist peacefully? That is not an easy point for non-physicists like myself to grasp. The gist of the answer though seems to be something like this: scientists love unification. Physicists want a Grand Unified Field Theory that will allow them to treat of the four fundamental fields as one. Those fields are: strong nuclear force, weak nuclear force, electromagnetism, and gravity.

Gravity is the hold-out. Many scientists are convinced they've got a "three out of four" unified field theory. But the holy grail is four out of four. Bring gravity in. They can't do that unless they can do what they've already done with the other three fields: quantize them. Treat of them on the graniest level as discontinuous. Yet the success of GR in explaining gravity without quantization is not a success at all from this point of view.  It is an obstacle!

Another way of looking at it has to do with the physics of a Black Hole. At the center ought to be a "singularity," according to work based on GR.  But a singularity is an impossibility, according to work based on QM. It seems somebody is wrong, at this limit. 

AT any rate, two facts about the differences between GR and QM seem to be connected: Particularity and discontinuity. QM deals with particles and sees the world as made up of particles and so as discontinuous.  GR sees space time as not particulate at all, and it seems gravity as bound up with this truth about the fabric and likewise continuous. These differences feed into a third diference, the difference about determinism versus indeterminacy.  

Much in the above explanation may seem familiar to some of you. so ... what is new?  Ah, there is a new approach to reconciling quantum theory with GR. 

The new theory, as I understand it, says:

1. The reason gravity cannot be quantized is that gravity is not quantum -- there exist no "graviton" particles awaiting discovery.

2. What is needed for a reconciliation then is not a quantum theory but a "post-quantum theory of gravity," the label by which the new view has come to be known.

3. The post-quantum theory can leave space and so gravity as continuous matters, while making gravity uncertain or indeterministic (and thus more similar to QM than it has been) without making it particulate. 

4. This means that gravity should fluctuate. There should be some "decoherence" to it. And THAT could be tested by looking for otherwise inexplicable fluctuations in weight. 

The advocates of post-quantum gravity have ideas about how to look for those random fluctuations in gravity and, so, in weight. The measuring tools exist to look for this, though it may take twenty years and a lot of co-operative effort around this planet to pull it off.    

5. This post-quantum theory, as understood by advocates such as Jonathan Oppenheim, renders useless the key ideas of "string theory" and its usual rival, loop quantum gravity They are rival ways of doing something that Oppenheim says cannot be done and should no longer be attempted, quantizing gravity/space. 

So: how do we decide if space is in fact both continuous and indeterminate? Beyond the search for tiny fluctuations in weight, there will likely be experimental effort toward finding something called "gravitationally mediated entanglement," where "entanglement" refers to a quantum concept of respectable vintage. 

Enough Deep Thought for today.  I hope physics has found its "way forward." I bet that if Sheldon and Leonard of TBBT fame were real people they'd be bent over a journal, hotly debating the pertinent articles now. Like this journal:   Phys. Rev. X 13, 041040 (2023) - A Postquantum Theory of Classical Gravity? (aps.org)

One simple question: does Dr. Jonathan Oppenheim have to keep telling people "yes, I am a physicist but no, my name does NOT end with 'er,' thanks."?