The Penrose gravity thesis/interpretation of QM

[user30]

TL;DR Summary

Roger Penrose proposes an interpretation of quantum mechanics that takes everything at face value, which includes objective wave function collapse.

The Penrose interpretation postulates that the wave function is real and that there is objective collapse without departing from reality as we know it. It does not make any assumptions; the observations of quantum mechanics and classical mechanics are exactly what they appear to us.

"Accepting that wavefunctions are physically real, Penrose believes that matter can exist in more than one place at one time. In his opinion, a macroscopic system, like a human being, cannot exist in more than one place for a measurable time, as the corresponding energy difference is very large. A microscopic system, like an electron, can exist in more than one location significantly longer (thousands of years), until its space-time curvature separation reaches collapse threshold"

"Despite the difficulties of specifying this in a rigorous way, he proposes that the basis states into which the collapse takes place are mathematically described by the stationary solutions of the Schrödinger–Newton equation.""Penrose speculates that the transition between macroscopic and quantum states begins at the scale of dust particles (the mass of which is close to aPlanck mass). He has proposed an experiment to test this theory, called FELIX (free-orbit experiment with laser interferometry X-rays), in which an X-ray laser in space is directed toward a tiny mirror, and fissioned by a beam splitter from tens of thousands of miles away, with which the photons are directed toward other mirrors and reflected back. One photon will strike the tiny mirror moving en route to another mirror and move the tiny mirror back as it returns, and according to conventional quantum theories, the tiny mirror can exist in superposition for a significant period of time. This would prevent any photons from reaching the detector. If Penrose's hypothesis is correct, the mirror's superposition will collapse to one location in about a second, allowing half the photons to reach the detector.[2]"https://en.wikipedia.org/wiki/Penrose_interpretation

What is the current standing of this interpretation?

 

[PeterDonis]

The Penrose interpretation

Can you give a reference that is by Penrose himself? It would be better to discuss what he actually says than what a Wikipedia article says he says.

 

[user30]

Can you give a reference that is by Penrose himself? It would be better to discuss what he actually says than what a Wikipedia article says he says.

https://link.springer.com/article/10.1007/s10701-013-9770-0

"This paper argues that the case for “gravitizing” quantum theory is at least as strong as that for quantizing gravity. Accordingly, the principles of general relativity must influence, and actually change, the very formalism of quantum mechanics. Most particularly, an “Einsteinian”, rather than a “Newtonian” treatment of the gravitational field should be adopted, in a quantum system, in order that the principle of equivalence be fully respected. This leads to an expectation that quantum superpositions of states involving a significant mass displacement should have a finite lifetime, in accordance with a proposal previously put forward by Diósi and the author."

 

[user30]

Similar objections to those leveled at other interpretations. The inability to account for the Born rules. Even when made to fit, it has to explain how the Born Rules arrises naturally. I very much doubt the latter can ever be done. We may need to let this go, similar to the arbitrary values of the cosmological constants. It's a bit harsh ruling out theories based on that.

https://iopscience.iop.org/article/10.1088/1367-2630/16/11/115007

"According to the Schrödinger–Newton equation without the standard collapse postulate, on the other hand, a superposition state as in (20) implies a gravitational attraction also between the spatial wave packets Φ1 and Φ2representing the massive pointer. The wave-function of the pointer would always 'collapse' to the average position https://static.iopscience.com/2.31.0/img/lazy-loading-placeholder.gif, simply due to the symmetry of the deterministic dynamics and the initial state. Numerical simulations confirm this behaviour of spatial superpositions collapsing to an average position [45]. Such a behaviour is however in obvious contradiction with the standard collapse postulate, as well as with our everyday experience, where the pointer is found with equal probability either at https://static.iopscience.com/2.31.0/img/lazy-loading-placeholder.gif or at https://static.iopscience.com/2.31.0/img/lazy-loading-placeholder.gif, and never in the middle.

Moreover the Schrödinger–Newton equation is deterministic and as such it cannot explain why quantum measurements occur randomly, distributed according to the Born rule. Therefore, the Schrödinger–Newton equation explains neither the standard collapse postulate nor the Born rule; one still needs both to describe experimental results, as long as no additional collapse prescription is added."

 

[PeterDonis]

Similar objections to those leveled at other interpretations.

There are objections to every QM interpretation. As I have previously noted, arguing back and forth about such objections is off topic for this forum, since all such objections are matters of opinion as far as QM interpretations are concerned, because all interpretations make the same predictions for all experimental results.

Some interpretations do suggest possible new theories--i.e., new mathematical models that make different predictions from standard QM for some experiment--that could be potentially tested, because of the difference in predictions. As far as I can tell, Penrose's proposal falls into this category, since it predicts that there should be a new fundamental theory, to which current QM and current GR are approximations, in which there is a potentially testable limit beyond which quantum superpositions cannot be maintained. Such a new theory would not be an interpretation of current QM; it would be a different theory.

However, as far as I can tell, neither Penrose nor anyone else has actually developed such a theory; Penrose has only suggested a potential experiment (with a photon separated by a beam splitter and directed into two cavities, one of which has a mirror at the end) that could test potential limits to maintaining quantum superposition. (The Schrodinger-Newton equation proposal might be the start of such a new theory, but I don't know that it has been developed enough to make an actual quantitative prediction for something like the mirror experiment.)

 

[user30]

There are objections to every QM interpretation. As I have previously noted, arguing back and forth about such objections is off topic for this forum, since all such objections are matters of opinion as far as QM interpretations are concerned, because all interpretations make the same predictions for all experimental results.

Some interpretations do suggest possible new theories--i.e., new mathematical models that make different predictions from standard QM for some experiment--that could be potentially tested, because of the difference in predictions. As far as I can tell, Penrose's proposal falls into this category, since it predicts that there should be a new fundamental theory, to which current QM and current GR are approximations, in which there is a potentially testable limit beyond which quantum superpositions cannot be maintained. Such a new theory would not be an interpretation of current QM; it would be a different theory.

However, as far as I can tell, neither Penrose nor anyone else has actually developed such a theory; Penrose has only suggested a potential experiment (with a photon separated by a beam splitter and directed into two cavities, one of which has a mirror at the end) that could test potential limits to maintaining quantum superposition. (The Schrodinger-Newton equation proposal might be the start of such a new theory, but I don't know that it has been developed enough to make an actual quantitative prediction for something like the mirror experiment.)

What's interesting is Penrose and Dioisi have independently of each other proposed the same idea

Diósi-Penrose (DP) model[12][13]https://en.wikipedia.org/wiki/Objective-collapse_theory#cite_note-:3-13

"Diósi and Penrose formulated the idea that gravity is responsible for the collapse of the wave function. Penrose argued that, in a quantum gravity scenario where a spatial superposition creates the superposition of two different spacetime curvatures, gravity does not tolerate such superpositions and spontaneously collapses them. He also provided a phenomenological formula for the collapse time. Independently and prior to Penrose, Diósi presented a dynamical model that collapses the wave function with the same time scale suggested by Penrose."

On gravity's role in Quantum State Reduction
https://link.springer.com/article/10.1007/BF02105068

 

[user30]

"Since then, other researchers have repeated the experiment with electrons, atoms, even with relatively bulky molecules containing as many as 70 carbon atoms. The results never vary. Individual atoms and molecules go through both slits at once. Yet for some reason the laws of physics take away that ability for large objects like paper clips, people, and planets.

“Something has got to go wrong with quantum mechanics somewhere,” Penrose says. “I regard this as a major problem that is going to require another revolution. But rather few people seem to agree with this viewpoint.”

https://www.discovermagazine.com/th...ron-can-be-in-two-places-at-once-why-cant-you

 

[romsofia]

If you have access to the book, Penrose has a chapter in "Quantum concepts of space and time" edited by Penrose and Isham called "gravity and state vector reduction" that explores this idea [it's actually where my avatar comes from :^) ]

The book has other chapters on this concept too, from some great physicists. For example the chapter (essay?) before titled "On the possible role of gravity in the reduction of the wavefunction" does propose some experiments.

It's a great book if you haven't seen it, but, is over 30 years old at this point, but should point you in the directions of authors who would write on this topic.

 

[PeterDonis]

other researchers have repeated the experiment with electrons, atoms, even with relatively bulky molecules containing as many as 70 carbon atoms. The results never vary. Individual atoms and molecules go through both slits at once.

Actually, I believe the current record for "number of atoms in a superposition-type state" is in experiments with SQUIDs, which involve, IIRC, about a trillion atoms. I can't find a good online reference right now.

 

[user30]

Weinberg mentions it in this very old interview at the start of the clip

 

[user30]

If you have access to the book, Penrose has a chapter in "Quantum concepts of space and time" edited by Penrose and Isham called "gravity and state vector reduction" that explores this idea [it's actually where my avatar comes from :^) ]

The book has other chapters on this concept too, from some great physicists. For example the chapter (essay?) before titled "On the possible role of gravity in the reduction of the wavefunction" does propose some experiments.

It's a great book if you haven't seen it, but, is over 30 years old at this point, but should point you in the directions of authors who would write on this topic.

Yeah he never gave up on this idea, it seems. It is intriguing since his predictions are quite specific.