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> we should note that even Einstein himself cautioned against believing spacetime was actually curved. [...] I don't want to appeal to authority

Another example that comes to mind is Max Planck believing that light being absorbed in discrete packets of energy was only a neat mathematical hack he came up with. It took Albert Einstein to say "but what if light is discrete packets". And then as you say Einstein having major reservations against the field of quantum physics that he himself spawned.

> if you think gravity isn't a force, drop a brick on your toe

By that logic the centrifugal force has to be a force. If you don't believe it, just drive a vehicle around a curve, or whirl a rock on a string.

> By that logic the centrifugal force has to be a force.

Are you trying to claim it's not?

Not the OP, but I think I know what wongarsu is referring to.

In order to make an object turn, it needs to experience an centripetal acceleration (towards the centre of rotation). This is the force causing objects to change trajectory.

If there is another object inside the turning object (like clothes inside a washer, or a person inside a car) they will "feel" like they are being flug out as if a centrifugal force existed, but actually that is just the effect of Newton's first law: the natural tendency of every moving body is to continue to move in a straight line, so when the containing object is changing direction (due to the centripetal force), Newton's first law tends to push you outwards.

All of the above is from the static (world) frame of reference.

It is also possible to put a coordinate system on the rotating object, in which case something like a centrifugal force will exit, but we kind of created it by choosing an accelerating reference frame, so it's not real. Sometimes called a pseudoforce.

"There is no centrifugal force" is often found together with "centrifugal forces appear as a term when using the frame of reference of the object going around", and this implicitly says that some reference frames are more privileged than others which -in a world that accepts the principle of Relativity- is just not acceptable.

So of course gravity is a force, and of course centrifugal forces are real. These dogmas serve to do nothing more than to scare away students, and to make the dogmatic seem like geniuses because only they can understand these things.

Relativity, classical Galilean relativity, special relativity, and general relativity, all say that all inertial frames of reference are equivalent. Accelerated frames of reference are not equivalent to inertial ones, nor to each other.

That is, there is a difference in all of these theories between an observer who is at rest with a train accelerating towards them, and the same observer accelerating towards a train that is at rest.

This doesn't mean that you can't do coordinate transforms to look at the world from the point of view of the accelerated observer, and get apparent forces. But different accelerated observers will come up with different apparent forces, while all intertial observers agree on the same forces experienced by any object.

> But he didn't believe in the non-local nature of quantum theory either, which we have now, since Einstein's death, proven to be true.

We have not proven quantum theory to be non-local. We’ve only proven that it can’t be both local and contain particles, as opposed to particles being emergent from the wavefunction’s interactions.

MWI chooses the latter, and is therefore a local theory.

(Alternately, collapse. But collapse theories are largely nonsensical.)

> First off, the author doesn't appear to be a crank.

Is this the first most people are hearing about this? In the 90's, I had a physics textbook for the layperson (i.e. non-STEM fields). It had a fantastic chapter on general relativity, and it also went with "not a force but a spacetime curvature".

Popular scientists have been saying it for decades and always use the flexible rubber sheet as the learning aid. Serious physicists tended to stay out of the fray, preferring to just "shut up and compute", as they do for quantum mechanics.

It's a relatively recent development that physicists have entered the fray and say maybe curved spacetime isn't a model, maybe it's reality. I urge caution to all as we have no evidence that the model of curved spacetime is indeed reality. I think the caution is warranted considering that GR contains singularities, which is a sign the theory has issues.

GR is a theory of the bulk. It’s like fluid mechanics, which treats fluids as infinitely divisible and doesn’t acknowledge the existence of molecules: There’s no way it’s accurate at small enough scales that molecules matter, which is what would be happening inside a black hole.

At larger scales, however, fluid mechanics is extremely reliable.

What if singularities do in fact exist? What issues does that create?

They might not be observable if superextremal black holes don't exist and thus naked singularities do not exist.

We’d have no way to predict what they do, since the math breaks down. This is fine if you treat the math as only a model, and you’re willing to accept that it can’t predict the behaviour of a thing that can’t affect you anyway, but given your wording — “do in fact exist” — I assume that’s not what you mean.

A theory that purports to describe how the universe actually functions, can’t have places where the theory says “and now the universe bluescreens”.

At a minimum it would need additional postulates about computational order that allows the inside of the black hole to be NaN without that causing external reality to seize up.

> “and now the universe bluescreens”.

I love it!

> A theory that purports to describe how the universe actually functions, can’t have places where the theory says “and now the universe bluescreens”.

A theory that provides useful predictions in some conditions and doesn't provide useful predictions in other conditions is a useful theory if you can determine which conditions are which. My lay understanding is general relativity provides useful predictions as long as mass isn't too dense (black hole) and as long as nothing breaks the speed limit; which is pretty general for everyday use, even if it doesn't cover the whole universe.

Singularities mean that the math stops working, so we don’t know what’s really happening there, the mathematical model is failing. We’d like to have a working model.

It would be fascinating if the universe actually has a “NaN” phenomenon that conveniently can never be observed or directly interacted with because it’s always inside an event horizon.

Event horizons aren't physical objects, they're mathematical constructs.

A fun fact about black holes that isn't often mentioned is that a black hole has two singularities. One is in the center — the one people always talk about — and the other one is at the event horizon. My impression is that the second one is eliminated by choosing to represent the calculations with a coordinate system to eliminates it at the geometrical level, but it's unclear if this is a "hack" or not. But I find it interesting (as a layman) that one can be handwaved away as irrelevant, while the other cannot.

It is possible to unify the electromagnetic field into a geometric framework via the Kaluza-Klein theory which is just rewriting GR + ED. Then electromagnetic forces are also of geometric origin. How do you interpret this? Is electromagnetism then also not a force? I am not a physicist, so I am asking the experts.

https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory

I found this paper which details how a charged particle falling into a black hole radiates. How is this possible when there is no acceleration on the particle?

https://adsabs.harvard.edu/full/1971PASP...83..633R

Title: Radiation from Particles Falling into Black-Holes

Authors: Ross, D. K.

Journal: Publications of the Astronomical Society of the Pacific, Vol. 83, No. 495, p.633

Bibliographic Code: 1971PASP...83..633R

I'm going to have to read that one. Thanks for the link!

> First off, the author doesn't appear to be a crank.

Perhaps, but the author is being dogmatic. See commentary elsewhere in this post.