What is your opinion about Freeman Dyson's opinion on the unification?

[LCSphysicist]

TL;DR Summary

What is your opinion about Freeman Dyson's opinion on the unification of General relativity and quantum physics?

In this video, Freeman Dyson shows a peculiar opinion when the subject comes about the unification of GR and Quantum physics. In fact, while the most currently opinion is to support or String theory, or QLG, Dyson claims that maybe nature has two different aspect of itself. With this he means that it is not necessarily true that QM and GR need to be unified, since they are two different aspect of physics.

I would like to know your opinion about this. I see this as one of the most interisting and maybe, radical, point of view of the nature and the laws of physics. While i am not sure if i support that opinion or not, i think this is really a justifiable way to see the science. Considering physics is actually a experimental science, both QFT and GR describe pretty well the phenomenons in their respective "domain". So, considering this, in fact it does not seems necessary to search a way to join both fields, but maybe just improve each one individually and separatelly.

But, if i see by the theoretical point of view, this way to see the nature is really radical.

What is your opinion?

Of course Freeman is a mathematician, but i am sure he knows the impact of this point of view in physics.

 

[George Jones]

Of course Freeman is a mathematician, but i am sure he knows the impact of this point of view in physics.

Dyson, a polymath, was as much or more a physicist than a mathematician. Weinberg said that Dyson was "fleeced" out of a physics nobel prize.

Also, Dyson was a renowned contrarian. From Wikipedia
'Friends and colleagues described him as shy and self-effacing, with a contrarian streak that his friends found refreshing but intellectual opponents found exasperating. "I have the sense that when consensus is forming like ice hardening on a lake, Dyson will do his best to chip at the ice", Steven Weinberg said of him. His friend the neurologist and author Oliver Sacks said: "A favourite word of Freeman's about doing science and being creative is the word 'subversive'. He feels it's rather important not only to be not orthodox, but to be subversive, and he's done that all his life."'

As far as I know, there is no empirical evidence that gravity is quantized. Some folks think that measurement of polarization of the Cosmic Microwave Background could provide empirical evidence for quantum gravity, e.g.,

https://arxiv.org/abs/1404.0634

 

[Demystifier]

How do you unify sound and quantum mechanics? You don't. But you still have a theory of sound quanta, namely phonons. In that sense my guess is that gravity and quantum mechanics may not be unified, but that there is a consistent (and physically correct) theory of gravitons.

 

[thidmir]

I'm not sure that GR and QM can be considered to be right for only two entirely separate and exclusive domains, since there are cases where their domains overlap and not just considering classical mechanics. There is also the case of how elementary particles behave in strong gravitational fields (with QM being how elementary particles behave in weak gravitational fields and GR being how massive particles behave in strong gravitational fields). There ought to be a subtle difference between this case and QM and GR. QM describes the non-deterministic motion of elementary particles and GR describes the motion of elementary particles as deterministic, which would lead to errors because the motion of elementary particles should be non-deterministic even in strong gravitational fields. But there is no theory at present to account for this, although a complete theory of Quantum Gravity would.

 

[Fra]

I've got a mixed attitude towards Dysons thinking.

On one hand I am symphatetic to thinking that gravity is simply a different beast than other forces. Its universality and the fact that you can't shield yourself against gravity. Also the equivalence between inertial and gravitational mass, implies that this is just very different than other forces. Also, the notion of quantum, explicitly relates to a vacuum or "zero energy level". That notion makes no sense in gravity. In this sense I think that we can not expect much progress from regular quantum theory in merging with gravity in terms of gravitons etc. Because regular quantum theory requires a background spacetime, and a classical reference. His references to Bohrs classical world is also thus relevant. So given then we talk about GR, and QM/QFT as it stands today, the attituude makes sense.

But that said, we can hopefully do better, and I think it is natural to expect a unification of the foundations of the theories. And the key there would be to explain how the background emerges and evolves. And then we need to consider the whole CONTEXTUAL background of the observer, no just the spacetime background. But this would likely requires modification of the framework of QM. The linearity of QM is certainly handy, but I think QM will hold only for "differential expectations", we yet need to figure out how to evolve the context. Differential expcetations hold even finitesimally for cases where we study small subsystems from the point of a massive classical background. So QM will likely still hold, but I think we should understand it similar to a perturbation along something else, that we do not understand yet.

/Fredrik

 

[MathematicalPhysicist]

I've got a mixed attitude towards Dysons thinking.

On one hand I am symphatetic to thinking that gravity is simply a different beast than other forces. Its universality and the fact that you can't shield yourself against gravity. Also the equivalence between inertial and gravitational mass, implies that this is just very different than other forces. Also, the notion of quantum, explicitly relates to a vacuum or "zero energy level". That notion makes no sense in gravity. In this sense I think that we can not expect much progress from regular quantum theory in merging with gravity in terms of gravitons etc. Because regular quantum theory requires a background spacetime, and a classical reference. His references to Bohrs classical world is also thus relevant. So given then we talk about GR, and QM/QFT as it stands today, the attituude makes sense.

But that said, we can hopefully do better, and I think it is natural to expect a unification of the foundations of the theories. And the key there would be to explain how the background emerges and evolves. And then we need to consider the whole CONTEXTUAL background of the observer, no just the spacetime background. But this would likely requires modification of the framework of QM. The linearity of QM is certainly handy, but I think QM will hold only for "differential expectations", we yet need to figure out how to evolve the context. Differential expcetations hold even finitesimally for cases where we study small subsystems from the point of a massive classical background. So QM will likely still hold, but I think we should understand it similar to a perturbation along something else, that we do not understand yet.

/Fredrik

What does this term "Differential Expectation" mean?

 

[Fra]

What does this term "Differential Expectation" mean?

It's an admittedly informal term, but I mean a contextual "expectation of the future", based on the current, not accounting for the actual evolution of the context, using a linear model. So our expectation of the future is formulated extrapolating in the tangent plan, like a "differential change".

/Fredrik

 

[Melbourne Guy]

Dyson's view is possibly wistful, as he opens with: "I like a universe to be more diverse, more subtle..." and closes with status quo being "a much more interesting universe". He also presents an apparent separation of GR and QM using 'past' and 'future' as the dividing line, and notes - perhaps playfully - that Bohr "believed in keeping them separate, so I'm naturally just following him." His view also seems to be that the quantum world it is not observable...is that true?

On GR, Dyson says early in the video, "It would spoil it if you bring them together." His perspective seems to be philosophical, or perhaps related to the 'beauty' of the equations because he dismisses the unreconcilable aspects of QM / GR at the moment of the Big Bang without a hint of irony that the reason "we don't know" is because our two best theories of such physical environments don't play nicely together. (I feel the interviewer played softball with that, giving Dyson a free pass on what could have been a key test of his reasoning.)

Taking the view of another physics luminary, Richard Feynman stated: “I think I can safely say that nobody really understands quantum mechanics,” and that still holds. QM is a very successful black box, and until we understand the gears and cogs inside the box, I feel that any definitive position regarding the unification - or otherwise - of QM and GR is supposition and should be treated as such.

 

[MathematicalPhysicist]

Dyson's view is possibly wistful, as he opens with: "I like a universe to be more diverse, more subtle..." and closes with status quo being "a much more interesting universe". He also presents an apparent separation of GR and QM using 'past' and 'future' as the dividing line, and notes - perhaps playfully - that Bohr "believed in keeping them separate, so I'm naturally just following him." His view also seems to be that the quantum world it is not observable...is that true?

On GR, Dyson says early in the video, "It would spoil it if you bring them together." His perspective seems to be philosophical, or perhaps related to the 'beauty' of the equations because he dismisses the unreconcilable aspects of QM / GR at the moment of the Big Bang without a hint of irony that the reason "we don't know" is because our two best theories of such physical environments don't play nicely together. (I feel the interviewer played softball with that, giving Dyson a free pass on what could have been a key test of his reasoning.)

Taking the view of another physics luminary, Richard Feynman stated: “I think I can safely say that nobody really understands quantum mechanics,” and that still holds. QM is a very successful black box, and until we understand the gears and cogs inside the box, I feel that any definitive position regarding the unification - or otherwise - of QM and GR is supposition and should be treated as such.

It's as if nature puts a black screen which no one who can watch or pass through it; at least not rationally, you need to pass through some belief of some sort.

 

[Melbourne Guy]

It's as if nature puts a black screen which no one who can watch or pass through it; at least not rationally, you need to pass through some belief of some sort.

And there I was thinking Dyson was the philosophical one, @MathematicalPhysicist But yeah, there's a curtain, for sure. Parting it is not trivial, obviously, but my belief is that we will eventually do so, because irrespective of QM and GR being reconciled, I would like to know what's up with time!

 

[PAllen]

Here is a paper suggesting that the question of whether to quantize gravity is ultimately an experimental question:

https://arxiv.org/abs/0803.3456

 

[Fra]

It's as if nature puts a black screen which no one who can watch or pass through it; at least not rationally, you need to pass through some belief of some sort.

The "agents/observers belief" while interacting is one of the possible interpretations of all this.

"The thought experiments of Eppley, Hannah, and others do, however, suggest that a fundamentally classical theory of gravity is likely to require changes to quantum mechanics as well. As I shall argue below, once one allows a coupling between classical and quantum systems, quantum mechanics almost inevitably becomes nonlinear, suggesting the possibility of sensitive new experimental tests."
-- https://arxiv.org/abs/0803.3456

For me, the coupling between classical and quantum, represents the feedback between what the agent/observer THINKS it is "certain" of (and thus what constrains its actions), and the environments backreaction to the agent (ie. it is LEARNING). Ie. it's about inferring as rational degrees of beliefs from interactions as possible. This takes as straight to a reconstruction of QM IMO.

/Fredrik

 

[MathematicalPhysicist]

@Fra when you search for nonlinear quantum mechanics in google, you get one of the following paper:
https://arxiv.org/pdf/1901.05088.pdf
From the bibliography it seems Nonlinear Quantum Mechanics is as old as String Theory.

 

[Fra]

@Fra when you search for nonlinear quantum mechanics in google, you get one of the following paper:
https://arxiv.org/pdf/1901.05088.pdf
From the bibliography it seems Nonlinear Quantum Mechanics is as old as String Theory.

Yes these are not new ideas. Non-linear models also adds computational trouble. But IMO the emphasis is not just on linear vs non-linear, but why the current formalism of QM is limiting. As I see it, the heart of the problem is that when you consider a evolving context (an observer/agent) that is evovling as is participates int the interaction, the notion of a timeless statespace is doomed to failure. The argument is also part of Smolings evolution of law argument. One is thus lead to an unphysically large statespace, that is computationally unmanagable to the actual observer (and thus useless). In the evolutionary picture, the state of possibilities will grow. IF you start out with a mathematical embedding, which should "fit" any future state, then one gets into a fine tuning problem that also lacks explanatory power. One "explanation" could be that the state space itself evolves, but is constrained bt the complexity of the context. These ideas are as far as I know now part of the old ideas on non-linear QM. The old ideas have an idea to capture all this, in non-linear but still deterministic models. And the evolutionary picture will have to give up deerminism, but trade it in for self-organisation. The relation between determinsim and self-organisation is in my view like the one betwen loosy and non-lossy compression. Agents must use lossy compression of their world view, due to information capacity limits. It's up to use, to yet EXPLAIN, why such a thing converges to stable rules, where the CHOICE of lossy compression is "optimal".

/Fredrik

 

[stevendaryl]

I’m not sure if I understand what Dyson meant in saying QM and GR don’t need to be unified. If there is some situation in which both gravity and quantum effects must be taken into account then there has to be some kind of way of combining them.

It may be, though, that for practical purposes they will forever be separate domains. Maybe there are no practically achievable circumstances where both effects are important.

 

[stevendaryl]

On the other hand, uniting gravity and QM may not mean developing a quantized theory of gravity. There could be some theory that reduces to GR in certain situations and reduces to the standard model of QFT in others.

 

[Melbourne Guy]

I’m not sure if I understand what Dyson meant in saying QM and GR don’t need to be unified. If there is some situation in which both gravity and quantum effects must be taken into account then there has to be some kind of way of combining them.

It is obviously hard to glean his entire worldview from a ten minute interview, @stevendaryl, but Dyson skirted this aspect and the interviewer did not press him on it. For instance, the interview asks about the Big Bang and Dyson merely says we "know nothing about the Big Bang in detail" and shrugs off that unified QM and GR might help increase our knowledge.

On the other hand, uniting gravity and QM may not mean developing a quantized theory of gravity

Dyson agrees, at least in the sense that GR can't be quantized, and references compensation of EM field measurement apparatus to make the case that you cannot 'compensate' for mass with a negative mass, so "there is a real difference there" [compared to EM] and theory fails when it comes to quanitzing gravity. He also notes that "even in principle" you cannot detect a single graviton, saying that the device required to measure it would "collapse into a black hole."

That could have been a neat launching point into singularities and firewall paradoxes, and perhaps the discussion did lead there and whoever edited the video deliberately cut it off, but from the conversation, I doubt it. I think Dyson genuinely believed that the classical 'past', where everything is fixed and therefore knowable, and the quantum 'future', which is mere probability and cannot be known, were truly separate realms that do not need to be unified.