Nice explanation of how Einstein's spacetime curving explains gravitational motion as well as does Newton's force laws. What seems not explained is why Einstein's model is better, and where Newton's laws fail to make the correct prediction. What were Einstein's thought processes that made him move to this new model? The opening and title of the video ask 'why' and not 'how', so I thought this part would be covered.

The only experimental evidence at the time was the anomalous precession of Mercury's orbit. That had to be it, but not mention of this I think in the video. Later, bending of light around large masses, and gravitational redshift were verifications, but neither of these had been done at time of GR publishing.

This isn't an ad, I promise haha. — flannel jesus

But it has an ad built in that you have to wade through or forward past.

he has a huge video series that looks at different aspects of relativity. He might have one that goes over the reasons why it had to be thought of, I'll see if I find one like that.

I just watched two videos of his that explain the original inspiration for the idea of quantizing energy that led to quantum mechanics, so he does go into that kind of topic

Hmm I had a look and I'm not sure he has done that for relativity. He has many videos going into depth on many aspects of relativity, but I don't think he has one that explains the origins of relativity itself.

since I couldn't find a video, I did ask an ai for a summary. https://g.co/gemini/share/11f0f523d7b3

In short, special relativity had to be derived as a consequence of Michelson Morley experiment as well as Maxwell's equations, and then General Relativity because he needed a way of keeping gravity fully local (in contrast to Newtonian gravity which involves instantaneous arrival of updated gravity information). Also he had this idea - that was explained in the video - about how a guy falling wouldn't feel that he is falling. The "happiest thought in his life", right?

In short, special relativity had to be derived as a consequence of Michelson Morley experiment as well as Maxwell's equations, and then General Relativity because he needed a way of keeping gravity fully local (in contrast to Newtonian gravity which involves instantaneous arrival of updated gravity information). — flannel jesus

M&M was pre-existing evidence, yes, and everybody knew a new theory was needed because of that. Several were working on it and Einstein put out SR shortly after LET, both valid explanations. Neither dealt with gravity and neither were geometric solutions.

OK, keeping GR local was perhaps a major motivation for the geometric approach. Not sure how far Einstein would have got on his GR work had not Minkowski re-cast SR as a geometric model first, but that move was essential to the completion of GR.

Also he had this idea - that was explained in the video - about how a guy falling wouldn't feel that he is falling. The "happiest thought in his life", right?

Yes, the EP is based on that, that gravity and acceleration are locally indistinguishable, thus he could take the mathematics of accelerated frames from SR and derive things like gravitational time dilation. I think that part could have been done with the non-geometric model. Maybe. Not like I've worked through the derivations myself.

Totally interesting video and great teacher. Will watch more videos.

So now that gravity isn’t a force, what the hell is space- time (with a curvature to it)?

We go from abracadabra to hocus-pocus.
Job security for people way smarter than me.

what the hell is space- time — Fire Ologist

The arena upon which events happen

The arena — flannel jesus

But “arena” has to be analogy - it’s not an actual arena. It’s not like “space” can be a “thing-in-itself” like an arena is a thing.

So I’m not trying to argue against Einstein, or Newton, or you. I’m just saying it is all still, in my mind, “full of holes” (if holes can be used to fill something).

But “arena” has to be analogy — Fire Ologist

sure

It’s not like “space” can be a “thing-in-itself” like an arena is a thing. — Fire Ologist

Not so sure about this one though. Why are you sure? Why couldn't space be a thing? Relativity tells us spacetime can be stretched, compressed, and warped, and that doing so has measurable effects (effects we have actually measured, in real life).

Relativity tells us spacetime can be stretched, — flannel jesus

Ok - so invisible gravity has been replaced by space which stretch and compressed etc can be measured. Like things can be measured. Ok, I sit corrected. :up:

I've been struggling to fully wrap my head around it, hence why I've been watching videos like the above. It's fascinating and confusing

Seems a very good presentation. Have you encountered Matt O'Dowd and the channel PBS SpaceTime? He also has a large collection of lucid videos on physics and cosmology.

Later, bending of light around large masses, and gravitational redshift were verifications, but neither of these had been done at time of GR publishing. — noAxioms

Those were the subject of the measurements carried out by Sir Arthur Eddington, which provided observational validation of Einstein's theories. Specifically, the 1919 expedition aimed to measure the bending of starlight as it passed near the Sun. The position of stars observed during the eclipse was compared to their known positions when the Sun was not in the sky. Eddington's team found that the starlight was indeed deflected, and the amount of deflection was consistent with the predictions of general relativity, not the predictions of Newtonian gravity. This finding propelled Einstein to international fame and was a crucial piece of evidence in favor of his new theory. It also made Eddington a notable scientific hero - his books sold hugely in the period between the World Wars, and helped make Einstein's theories understood by the broader audience.

Einstein's first theory Special Relativity was an attempt to resolve the contradictions resulting from from The Michelson–Morley experiment.
Basically the idea was to measure the speed of light (or other electromagnetic radiation) in various directions and if speeds were additive, the speed of light would be different measured forwards and backwards and we could measure our direction and speed from that. but it turned out to be constant though we know the Earth is moving.

@flannel jesus @Wayfarer @Fire Ologist @noAxioms I should note that there are two lines of thinking going on here I feel which are dripped in metaphorical language and analogy. Something covered somewhat in the substantivalism vs. relationism dispute.

I will call them the 'expanded inertia' picture and the 'separate substance' picture.

One way of interpreting GR is to see it merely as Newton's first law but. . . expanded. Instead of axiomatically dictating that objects are following 'natural' trajectories or inertial ones because they follow straight line ones you merely stipulate that it just needs to satisfy such and such system of PDE's. After all. . . Newton's first law was a postulate not a god given truth and also not one that is strictly even empirically verifiable but asserted/assumed. Numerous philosophers/physicists have made this point rather clear given the circularity in trying to test it which requires us to first assume its truthfulness in practice. Ergo, the attraction as of 'late' to pragmatic and constructive definitions of 'inertial-ness'.

Why didn't Newton add in uniform acceleration which hide their presence via the action of 'Universal forces'? He knew about it in his sixth corollary. It's even the subject of many papers which easily use it to geometrize even Classical Newtonian gravity into Newton-Cartan gravity. No differences in observation there in.

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The other way of interpreting this is something favored by substantivalists and one which usually uses metaphorical language talking about spacetime curvature. This language is meant to indicate that space or spacetime are words referring to a 'thing' which is responsible for moving things around ergo why Einstein was rather conflicted in asserting the re-emergence of an Aether albeit one with. . . and I want to emphasize this. . . rather peculiar characteristics. It's a thing. . . whose only purpose and role is to specify how far away things are. . . when they occur. . . or how they move with no further 'normal' physical properties.

You can imagine it as if space or spacetime is this fluid which we are immersed in and interpenetrated by but which has no properties from the entities that lie within it that it dictates their motion for. It's not strictly solid, has no colors, no charge, etc.

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So what is the difference? Well in the inertia picture we are trying to give an answer to the age old question of Aristotle's prime mover argument. At least the childish mentality of it. If something moves something then something most move. . . or change. . . that thing. . . and so on. An infinite regress results unless we somehow end it in some fashion or make some object 'self-sufficient' in its motion without anything external. The point of the concept of inertia is to postulate just this. . . that an object can move or retain its properties without having something force it to do that externally.

This opens the door to 'natural' states of objects and 'un-natural' states of objects. Forced and un-forced. Following geodesics and not following geodesics.

Under this picture there is no need of a substance necessarily as it seems we are making a lawful statement about how physical things just are or just behave. It's like asking for the physical thing responsible for conservation of energy. . . energy is just conserved and we don't look further for the 'thing' responsible for it.

However, under the substantivalism picture we still desire to explain why things move the way they do and might feel at odds with bluntly just assuming the ways things move is just a law of nature not to be further explained by any other 'thing'. So we might choose to assert there is an entity who is responsible solely for grounding those familiar spatial/temporal intuitions of ours and explain why objects move the way they do.

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So which picture is better? The one that says matter brute fact just behaves this way 'naturally'. . . or the picture that says there is another entity which as a matter of brute fact behaves 'naturally' in such a way that matter behaves the way it does or appears so?

Another question might be how either perspective influence philosophical/scientific pedagogy.

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I got this line of thinking from somewhere but I can't remember where but clearly there is a difference between Einstein's field equations being merely a lawful abstract relation similar to conservation of energy and asserting nothing over, or above, matter itself.

Contrasted with the popular substantivalism picture which asserts that the field equations are telling us something about the properties or behavior of a peculiar entity.

Remember that Newton's laws apply to inertial frames only, and don't usually work in other kinds of frames. This also goes for most of the conservation laws.

So what is the difference? Well in the inertia picture we are trying to give an answer to the age old question of Aristotle's prime mover argument. At least the childish mentality of it. If something moves something then something most move. . . or change. . . that thing. . . and so on. — substantivalism

This makes it sound like A causes B to accelerate (effect), which is wrong. Both interact with each other, with neither being cause nor effect. There is no regress.

An infinite regress results unless we somehow end it in some fashion or make some object 'self-sufficient' in its motion without anything external. The point of the concept of inertia is to postulate just this. . . that an object can move or retain its properties without having something force it to do that externally.

Motion under either model is an abstraction, a change in coordinate position over time, both of which are frame dependent. Thus velocity is not a physical property that gets mucked about by some other object. Contrast this with proper acceleration which is physical, and zero in gravitational inertial picture.

This opens the door to 'natural' states of objects and 'un-natural' states of objects. Forced and un-forced. Following geodesics and not following geodesics.

An object not tracing a geodesic is due to some force (EM say). So sure, using the term 'forced' seems weirdly applied, but appropriate. Calling it un-natural is deceptive.

As for spacetime being a sort of aether/fabric, well, you described that pretty well, and contrast it gravity being a sort of material force rather than geomery. I don't see much difference between the views, just different ways of wording the same thing.

It's like asking for the physical thing responsible for conservation of energy. . . energy is just conserved and we don't look further for the 'thing' responsible for it.

Conservation is a property of laws with certain symmetries. Newtons laws of motion exhibit that symmetry. So yes, it isn't a physical thing that 'causes' conservation. We'd probably just not have a name for energy if it wasn't useful to reference.

However, under the substantivalism picture we still desire to explain why things move the way they do and might feel at odds with bluntly just assuming the ways things move is just a law of nature not to be further explained by any other 'thing'. So we might choose to assert there is an entity who is responsible solely for grounding those familiar spatial/temporal intuitions of ours and explain why objects move the way they do.

Entity-of-the-gaps can be done with any view, including the geometric one. It isn't an explanation, it's hand-waving away to the realm of magic that flies in the face of methodological naturalism, the lack of which kept science pretty much at a standstill for pretty much all of the dark ages.
It makes for an even more complicated thing in need of explanation and as such, solves no problems.

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So which picture is better?

Hands down the one without the magic entity. I didn't know where you were going with all this and was surprised when that came up.

Remember that Newton's laws apply to inertial frames only, and don't usually work in other kinds of frames. — noAxioms

I was giving an example of something that was considered a physical law. If conservation principles don't count as truly frame independent then a simple tensor adventure could fix that and we'd find something that was respected in a frame independent manner therefore deemed significant enough to our understanding of things in nature. Even to the point of calling it a law perhaps.

The point is that there are certain relations/properties of physical systems that are sometimes deemed as 'brute' facts. Sometimes not.

This makes it sound like A causes B to accelerate (effect), which is wrong. Both interact with each other, with neither being cause nor effect. There is no regress. — noAxioms

There are, however, motions which were considered 'caused' and those which were not.

Two objects which exert mutual forces on each other are each the cause for the effect they imbue on the other as result of their reaction to said influence.

B causes A to accelerate and exhibit internal stresses as an effect. In response, A causes such a response likewise to B which exhibits a similar effect.

As history has gone on the number or kind of motions for which objects can partake in yet still be considered inertial/natural/un-forced/free/etc has grown. From mere rest, to uniform motion, to acceleration, and beyond.

Motion under either model is an abstraction, a change in coordinate position over time, both of which are frame dependent. Thus velocity is not a physical property that gets mucked about by some other object. Contrast this with proper acceleration which is physical, and zero in gravitational inertial picture. — noAxioms

All motion is relative, end of story.

The great absolutists and relativists who fought over whether the Earth moved or not disagreed over what they dubbed 'true motion'. They well understood that their models were perceptually indistinguishable but attributed some dynamical difference. Despite the fact that they agreed on their relative motions.

DYNAMICAL not KINEMATICAL distinctions are important here. Proper acceleration is a bit of a misnomer then.

An object not tracing a geodesic is due to some force (EM say). So sure, using the term 'forced' seems weirdly applied, but appropriate. Calling it un-natural is deceptive. — noAxioms

No less or more than saying its following the 'contours of a curved spacetime'. Or that it 'follows the laws of physics'.

Caused and un-caused pops up here as well as explained or un-explained. Different terms, different metaphors, but some what the same intentions.

As for spacetime being a sort of aether/fabric, well, you described that pretty well, and contrast it gravity being a sort of material force rather than geomery. I don't see much difference between the views, just different ways of wording the same thing. — noAxioms

Exactly, so its irrelevant. The ontology is irrelevant.

Hands down the one without the magic entity. I didn't know where you were going with all this and was surprised when that came up. — noAxioms

I think you misunderstand this then. . . analogies such as the 'law of inertia' or thinking of their being an entity to dictate how things should move (spacetime tells matter how to move, matter tell spacetime how to curve) serve strong heuristic purposes. Their distinct in their purposes and utility.

The substance picture or law picture.