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Comment by matt123456789 | Hacker News Reader

I think that the following paragraph cuts to the heart of an important question linking GR to QM:

“Take the case where a massive object is put into a superposition of two possible locations at the same time. General relativity says that any object with mass should bend the fabric of space-time. But what if that object is in a superposition? Is the geometry of space-time also in a superposition?”

I don’t have a PhD in anything let alone physics, but it seems to me that answering the above question could potentially lead to a reformulation of GR in a way that gracefully incorporates quantum mechanics, which is worth getting excited about.

But yes, you’re correct that the nuance is lost on me. Still, there seem to be plenty of people who find it interesting.

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First of all what you said is correct. Physics is looking for a quantum theory of gravitation and of course that would allow for superposition.

The first problem is that a quantum theory of gravity is not straightforward. It's not "just" a spacetime with superposition, it probably has an even richer structure, the way QED has a richer structure than an EM field with superposition. And from the little hints we have it might have "spin 2" representation, and thereshould be some quantum very complicated field that acts like the metric GR field at low energies. But not only there are many possibilities for that, they are also very complicated, basically untreatable, or have infinite degrees of freedom in their definition itself.

The second problem is that there is no current way of experimentally probing such behavior. You say put a massive object in superposition, but to have observable effects you need to have so massive an object that is tens of orders of magnitude above "impossible".

This experiment is nice but it probes "just" the gravitational field of the Earth (a ~10^25 kg object). It is very nice for accurate time measures, and maybe for measuring gravitational fields, it comes nowhere near to probe quantum gravity effects, just quantum effects coupled to classical gravity (classic in the sense of GR but not quantum)

They key part here that is not easy to set up in an experiment is "massive object in superposition". Superposition is generally something that we observe in mass-less objects (bosons) like photons on a macroscopic level. The spin state of an electron or any other massive objects (fermions) isn't going to effect it's mass distribution observably on a macroscopic level as far as I understand it...these things usually collapse upon interaction with other particles.

The curious thing in my opinion is how this will effect quantum computing. If gravity causes decoherence, then it is likely difficult to maintain coherence in large qubit devices that aren't in microgravity.

A bit of a tangent, but boson vs. fermion is not a massless/massive split. There are massive bosons. Rather, it's a split on the qualities of the objects' spin, and thus whether they obey the Pauli exclusion principle.

https://en.wikipedia.org/wiki/Boson

Ah yes, you're correct. I see that I remembered that incorrectly.

I get what you are saying, but I have no way to know whether or not it's valid to any degree. Maybe the "constructive" part of what I am trying to say is that we either need better education on the fundamentals here, better examples, better explanations, or should just admit that it's far beyond the layman.

For example, I get entanglement. I can get answers to questions like "Is FTL comm possible? No." or "What is a practical application for entanglement? Quantum radar - sorting your initial emission from jamming." etc.

This though? Just too abstract (for me).

I don’t understand. Isn’t that a pretty basic hypothesis? There’s no way we’ve gone almost 100 years and no one tried that.

Creating an object that is 1) massive enough to observably bend spacetime, and 2) still can be put in a quantum superposition is... not easy.

True... but you might be able to put an object in superposition with another object and separate those a bit within an existing gravity well and observe the implications.

It is. We haven't had the necessary experimental equipment to be able to measure it until recently. You need an extremely precise clock that you can put into a quantum superposition- that's difficult to make.

https://www.google.com/search?q=can+spacetime+geometry+be+in...

Seems like there is a lot of theory discussions around the Internet but not seeing any experimental evidence one way or the other in the first few results.

It's too difficult to actually carry out in practice. You increase the mass, it quickly becomes impossible to distinguish the states.

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