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Comment by pdonis | Hacker News Reader

> we know for a fact that we are getting gravity wrong

This statement is way, way too strong. We have a theory of gravity, General Relativity, which has had its predictions confirmed by countless experiments, in some cases to thirteen or fourteen decimal places. That is not "getting it wrong".

Many physicists believe that GR, as well confirmed as it is, can't be the final fundamental theory of gravity because it's not a quantum theory. But nobody has yet found a quantum theory of gravity that (a) is consistent with the rest of quantum mechanics, and (b) makes experimental predictions that have been confirmed. Unless and until both (a) and (b) are done, the claim that we are "getting it wrong", even in the restricted sense of not having a final fundamental theory of gravity, because GR isn't a quantum theory is just speculation and might itself be wrong. It should certainly not be stated with the confidence you are stating it.

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> Many physicists believe that GR, as well confirmed as it is, can't be the final fundamental theory of gravity because it's not a quantum theory. But nobody has yet found a quantum theory of gravity that (a) is consistent with the rest of quantum mechanics

It was my (lay) impression that we believe general relativity cannot be the final fundemental theory of gravity because general relativity itself is not consistent with quantum mechanics. Whether general relativity is a quantum theory or not is beside the point; we want to have a theory -- any theory -- that is consistent with itself. Right now we have two separate theories (general relativity / quantum mechanics) that can't be combined because they conflict with each other.

Obviously, that in itself doesn't prove that the problem is in the theory of gravity, but it does prove the problem is either in the theory of gravity or the theory of quantum mechanics, and we have to be getting one of them wrong.

> It was my (lay) impression that we believe general relativity cannot be the final fundemental theory of gravity because general relativity itself is not consistent with quantum mechanics.

That's the same as "not a quantum theory", which is what I was saying. "Consistent with quantum mechanics" requires "is a quantum theory", according to the many physicists I was referring to.

> Right now we have two separate theories (general relativity / quantum mechanics) that can't be combined because they conflict with each other.

Most physicists believe that they conflict, yes. But not all. Freeman Dyson, for example, has speculated that gravity might just be different and not require a quantum theory the way the other interactions do.

> the problem is either in the theory of gravity or the theory of quantum mechanics, and we have to be getting one of them wrong.

Not true: we could also be getting them both wrong.

There is no way to be getting both of them wrong but not be getting one of them wrong.

Pedantically that is true, but you should still word it as "at least one of them" to avoid being misunderstood.

Now, why am I seeing this factual comment in grey?

See here for instance a popsci take on the problem: https://www.scientificamerican.com/article/is-gravity-quantu...

Everybody expects a quantum theory of gravitation, but if we were drawing it from scratch no one would make it that weak. It is a difficult problem, people work in string theory because you can get Einstein's equations from strings and the alternatives look indeed worse. I don't think that means GR is a wrong take on gravitation, it's more like an amazing one to begin with.

It's grey because someone in their infinite wisdom downvoted you.

Most likely because they'd rather downvote rather than challenge your statement. It's a worrying trend.

>(a) is consistent with the rest of quantum mechanics

i think we're on a right way in general :

QM -> GR : https://en.wikipedia.org/wiki/Quantum_gravity#Quantum_mechan...

" In particular, contrary to the popular claim that quantum mechanics and general relativity are fundamentally incompatible, one can demonstrate that the structure of general relativity essentially follows inevitably from the quantum mechanics of interacting theoretical spin-2 massless particles (called gravitons)"

GR -> QM : We did observed gravitational waves of GR. A force field with waves mediating that force interactions - that requires just a bit of math to "quantize" it. It is a bit more complicated when classic EM -> QM because of added complication of non-stationary spacetime and just vastly different scales of spacetime/energy to run the experiments to establish/verify the gravitons parameters.

For the "true deep" nature of graviton I subscribe to the view that - given gravitational force not being the "real" force and the gravitational waves being really a ripples of spacetime - graviton is more like phonon, ie. quasi-particle from the point of view of QM/SR theories.

> one can demonstrate that the structure of general relativity essentially follows inevitably from the quantum mechanics of interacting theoretical spin-2 massless particles (called gravitons)

While this is technically true, it doesn't help much, since the quantum theory described is not considered a viable candidate for a fundamental theory of gravity for a number of reasons.

> We did observed gravitational waves of GR. A force field with waves mediating that force interactions

We have observed classical gravitational waves. We are many, many orders of magnitude away from being able to measure any possible quantum properties of those waves.

> that requires just a bit of math to "quantize" it

Math (more than "a bit") can quantize it in theory. But theory is not enough. We would actually have to observe the quantum properties experimentally to confirm the theory.

>We would actually have to observe the quantum properties experimentally to confirm the theory.

While not directly quantum properties of gravitational waves yet, we've already observed quantization of gravitational potential (and that naturally suggests "gravitational quantas" at least by similar machinery as with EM->QM) - at small scales/energies when the spacetime curvature can be not paid attention to, the gravity potential of an ultra-cold neutron quantizes just nice using the Schrodinger equation in full agreement with experimental observation - https://www.physi.uni-heidelberg.de/Publications/dipl_krantz... . In my view, giving the amount of clarity in gravitation as well as in QM that experiment produced, it is worth of a Nobel.

> we've already observed quantization of gravitational potential

No, we haven't. We've observed that, under appropriate conditions, gravitational potential has to be included in the potential term in the Hamiltonian. But the potential term in the Hamiltonian is not quantized; it's not part of the quantum state and it does not exhibit quantum properties. The only things in the experiment that exhibit quantum properties are the neutrons themselves.

>gravitational potential ... it does not exhibit quantum properties. The only things in the experiment that exhibit quantum properties are the neutrons themselves.

It is like saying that discrete orbits of electron say nothing about quantization of associated EM potential.

>The only things in the experiment that exhibit quantum properties are the neutrons themselves.

And those properties are position and momentum.

They specifically chosen neutrons to avoid effect of other forces and the paper is pretty clear that position and motion of neutrons were primarily affected by gravitational field, and as result the neutron position got quantized :

"... we conclude that the measurement manifests strong evidence for quantisation of motion in the gravitational field as is expected from quantum mechanics

...

the quantum theory we derived neglecting gravity is unable to reproduce the shape of the neutron height distribution even on the largest scale.

..."

Do you think the height of observed "steps" in neutron position would be the same or different if the experiment were repeated in different gravity, say Moon?

> It is like saying that discrete orbits of electron say nothing about quantization of associated EM potential.

That's right, they don't. There is no such thing as "quantization of EM potential". The electromagnetic field is quantized, if you're using quantum field theory, but in QFT there is no "EM potential". In non-relativistic QM, the EM potential term in the Hamiltonian is not quantized and does not exhibit quantum properties. Even in models which add relativistic corrections, such as the models used in the late 1940s to predict the Lamb shift, the "quantization of the EM field" only applies to the "quantum electromagnetic field" external to the atom; it does not apply to the EM potential due to the nucleus.

> the paper is pretty clear that position and motion of neutrons were primarily affected by gravitational field, and as result the neutron position got quantized

No, the neutron positions were not quantized "as a result" of the gravitational field. They are already quantized in the absence of a gravitational field, as shown by numerous previous experiments. The only thing this experiment shows, as I've already said, is that under appropriate conditions, you have to include the gravitational potential in the potential term in the Hamiltonian.

> Do you think the height of observed "steps" in neutron position would be the same or different if the experiment were repeated in different gravity, say Moon?

Obviously since the gravitational potential would be different, the neutron position "steps" would be different. That still doesn't change the fact that the experiment is not showing "quantized gravitational potential". It's only showing that "quantized neutron position", which is already well established by other experiments, is affected by the gravitational potential in the same way as it would be by any other classical potential in the Hamiltonian.

We also have the flyby anomaly where multiple space probes flying past the earth do not exactly match the predictions of GR. So is GR therefore falsified?

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

There could be unaccounted mass (dark matter) or magnified sensor error, just off the top of my head. Lots of ways to interpret this result.

>Lots of ways to interpret this result.

rounding error in The Grand Simulation.

I meant evidence based interpretation.

The predictions are based on what we know is out there, and how we would expect it all to react with us simultaneously. So as long as we have it exactly understood and haven't missed anything whatsoever, then yes, it would be therefore falsified. But we can't make such a claim.

I haven't really seen science work that way, since it isn't all that often we base our expectations solely on things we can completely account for. Science necessitates a certain amount of calculated projection. Theories are adjusted as new details are discovered, and only fools entertain themselves by resisting the bigger leaps needed to make actual discoveries.

I'm not quite sure what you meant in the 2nd para. But I find it interesting that multiple probes showed the same effect. It's so compelling I wish they would send some missions specifically to test this even more precisely.

The obvious parallel was https://en.wikipedia.org/wiki/Pioneer_anomaly which was finally solved. But who know at this point?

Personally I think GR has to be "wrong", since it conflicts with QM, and if the 20th century showed us anything it's that QM alway wins.

GR is no more or less wrong than Newtonian physics.

IANAP but I’m reasonably confident in asserting that GR is less wrong than Newtonian gravity, based on each models’ predictive power.

> GR is no more or less wrong than Newtonian physics.

Then why does GR make more accurate predictions?

At low speeds relative to the speed of light, they have the same accuracy don't they? And in that case, the Newton model is simpler so more useful. At high speeds, it becomes inaccurate and then you need the other model.

So there is no global "right" or "wrong", just different measures of usefulness depending the problem constraints.

> At low speeds relative to the speed of light, they have the same accuracy don't they?

When measuring signal (ie radio, light, gravity) from slow speed perspectives, newtonian physics is not as accurate - https://futurism.com/newtonian-physics-vs-special-realtivity - Eg we actually need to account for it between gps sattelites and we do so from the perspective of GR as well as the recent evidence of gravity propogation being restricted to localization (ie roughly the speed of light)

Yup. And when you need to measure where artillery shells land, the GR model is overkill and you'll waste less time by using a Newtonian prediction. I think this proves my point?

"Newtonion physics is a perfectly acceptable approximation in many circumstances" and "GR is less wrong than Newtonion physics" are not in conflict. Newtonion physics can absolutely be "more wrong" and "useful in many circumstances" at the same time- and in fact is!

So GR is more right in more scenarios and that makes it no more or less wrong?

i am a big fan of newtonian mechanics

If your point is that they're both equally wrong, then no, I think you've disproven your own point. There are no circumstances in which Newtonian math produces better results than GR. That the latter is more complex is not relevant to the discussion of whether one is more accurate for a wider range of phenomena than the other.

> I think this proves my point?

No, because "less accurate but in practical terms the difference can often be ignored" is not the same as "the same accuracy".

> there is no global "right" or "wrong"

You are wrong, so there is.

> At low speeds relative to the speed of light, they have the same accuracy don't they?

GR is always more accurate, but the increased accuracy is not always needed; at low speeds relative to the speed of light the difference is often too small to matter in practical terms.

Besides the valid points of the other commenters, I have a nitpick: It is special relativity that deals with things near the speed of light, you do not need GR for it. GR is necessary to account for strong gravitational fields.

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