> Halley’s intervention saved science from being reduced to “things fall down because they do” for another century.
Well, that might be stretching it. Speaking as someone who has done a little university level physics the understanding still seems to be basically that things fall because they do - we haven't made much progress beyond a firm strike-through of the word "down".
Newton's contribution was a very precise description of how rapidly they fall, and how we can calculate and understand the direction that things fall in complex multidimensional settings.
You’re correct. Newton wasn’t proposing a mechanism or deeper cause for gravity; he just described its effects. Einstein did add a “why” of sorts, with general relativity, he reframed gravity not as a force but as the curvature of spacetime caused by mass and energy. That’s closer to a mechanism, but even there we might ask: why does mass curve spacetime? And we don’t have a deeper answer to that.
Is "why" really a meaningful question? These are all models. The best we can do is to show how to derive the phenomenon from the (hopefully simple) rules of our model.
It's relative. I operationalize "why is X true?" as "update my worldmodel to the point that X is not surprising". The typical way to do that is to show a more general rule that applies, and which implies X. But yes, you can keep asking the question about the more general rule.
Why do things fall? -> A special case of the general law of gravitation.
Why does reality adhere to the general law of gravitation? A implication of matter distorting the shape of spacetime.
Why is reality such that matter distorts space-time?
I think so. The why can be a powerful and compact way to express the elements of a model, when a model can be applied, and when the model might break down. A complicated model without a WHY might not be easily understood by others. A surprising, new result with a good WHY can point the way to other aspects of the model that might be confirmed or disproved.
Think about how much our understanding of atoms has changed. I think the why is an important part of the development. If you're interested in that topic, how about a 35 min nuclear physics primer from Angela Collier (I love her videos!): https://www.youtube.com/watch?v=osflPlZdF_o
If you want something that won't fall down, your only options are (1) luck into noticing something that already does that, or (2) understand why things fall down, so you can prevent your thing from falling.
No, it isn't. That sounds strange, right? But here is an explanation what
eternauta3k probably meant: In modern physics, there is a kind of consensus that asking “why” has often been inappropriate or even misleading and should be therefore left to philosophers. The real questions are: Does our current model describe all observations? If not, can we find a model that does? And, even better, can that new model make predictions that we can verify?
> The real questions are: Does our current model describe all observations? If not, can we find a model that does? And, even better, can that new model make predictions that we can verify?
But every prediction your model can make comes from a "why" question that the model answers.
Newton’s title has philosophy in it. It’s probably a modern error to separate all the fields and ignore philosophy when doing science etc and vice versa.
It’s interesting how it’s not turtles all the way down (as I understand it at least). The things you learn of one scale do not completely translate to the next but serve as the “shadow” projected by the next level down the line. And this probably brings up the complexity that lets us exist and perceive. I say probably because who knows what else is going on in the universe.
All this to say who knows if we are ever going to learn the fundamental why if there is one.
Yes, general relativity explains gravity pretty darn well, tying it to the fundamental fabric of causality that makes up the universe. It goes from “it just happens” to “it must happen and there is no other way it could be.”
There is no notion of causality in our laws of physics, and from all we know gravity could certainly be different, or absent. You could have a universe with just the other three forces.
Modern physics says little about causality. The fundamental laws are all field equations. Those set constraints about how the physical state of a system can evolve between two points in time, but there is no notion of causation, only of consistency or (im)possibility.
I think we are talking past each other. Defining how the system evolves is defining how initial conditions must lead to (cause) intermediate and final states. That’s causality in the logical sense.
What I’m talking about is most similar to the “Causal Explanation” section at the end of the article you link.
Maybe? That's one theory - but what is "spacetime", and what does it mean to "bend", or "fall into" something like that? In many ways we've just given things names as if that suddenly means they're understood.
And even then how can that be measured and proven vs other theories? Is it some other mechanism that simplifies to "close enough" that it measures similarly? We didn't really see the effects of relativity until we got to a sufficient accuracy of measurement, Newtonian mechanics was sufficient to explain things to the accuracy they could reproduce for a very long time.
Einstein couldn't have done anything like what he published if he didn't have evidence from new (at the time) equipment suggesting there was something wrong with the current model. And then testing proposed new models against those same measurements.
It was less of a counterexample and more of an inaccuracy. It wasn't immediately obvious as to whether the theory was at fault or our measurements or even knowledge of celestial objects (e.g. maybe Mercury had some nearby object that was changing its orbit).
Also, there wasn't any alternative, so a theory that explains almost everything is going to be accepted. Modern theories are also accepted if they explain things with more accuracy or over wider ranges than alternatives - often it's the shortcomings of theories that gives us clues as to a better theory. (e.g. the ultraviolet catastrophe)
I've always been a bit flummoxed we haven't expanded a ton on this given how long it's been.
I'm not sure if it's wrong or right, and not smart enough to posit much, other than it -feels- wrong. But it wouldn't take a ton to convince me otherwise.
You'd think by now we'd have more supporting evidence of such a concept.
Do you mean evidence for general relativity? Because there's a lot: gravitational waves, black holes, gravitational redshift, the bending of light by gravity, the expansion (or contraction, depending on density) of the universe, are all new phenomena predicted by GR and experimentally verified
Oh I meant expanded it down, if that makes any sense, applying it to more things that we'd normally learn in high school.
At least in my case, you're taught gravity as this standalone force, and even the Newtonian idea in high school. Not until later college do they then throw that away and go into gr and the ideas you mention.
Though I guess this is more a failing of public education than science at large.
Ah, right. I would guess that's because it's just a much more complicated theory, and is well-approximated by Newtonian gravity for weak fields, e.g. like in the solar system. So applying GR to more everyday phenomena would just be a much more complicated way of saying almost the same thing.
GR is written in the language of differential geometry, and before even beginning you need a good grasp of Newtonian gravity, special relativity, multivariable calculus, and ideally electromagnetism too, so it needs a fair bit of preparation. And in fact the methods of Newtonian mechanics aren't thrown away, but incorporated into the more general framework. In that sense Newtonian mechanics is a conceptual foundation for GR, so that's why it's still taught first at school
I think it’s kinda more that (in 3D with time moving at a constant rate) space “falls” into the gravity well, and matter goes with it. In 4D this looks like spacetime being bent.
The "river model" you mean isn't very general, as one eventually becomes interested gravitating systems where there isn't a suitable congruence, e.g. in close binary compact objects. In such systems, one has to add terms analogous to turbulence, frustrating calculability (and the development of relativistic intuition). It also doesn't deal well with tides: for example, Schwarzschild infaller worldlines (even on a body like the moon, where there is no horizon) on widely separated radial trajectories converge in a way that is unlike the confluences of rivers and their tributaries. These models really only assist in understanding a single (spatial) radial line with possibly multiple successive "rafts" of matter bound to it (at different times), and in a set of PG-like coordinates useful for a particular distant observer. From there one symmetrizes: all observers and all radial lines are identical (speherical symmetry) and successive "rafts" all take the same radial line (static spacetime). Without this symmetrization, a black hole is an infinite number of slightly different rivers, and then you might as well solve the equations of motion in the standard way.
For understanding a handful of highly symmetrical systems, it might help a student understand some intuitions about what Killing vector fields and congruences (notably those made by choosing the velocity vector field of a set of geodesics) are, and tends to lead into an investigation of what the shift vector in a 3+1 decomposition represents.
For calculating things like the spherical orbits around or the photon surface of a real black hole like our galaxy's central Sgr A*, the river model seems outright unhelpful. For example, how does a river model help to understand https://duetosymmetry.com/tool/kerr-circular-photon-orbits/ ?
> time moving at a constant rate
This is another way of saying slicing of a Lorentzian (4d) spacetime into non-overlapping spaces organized along an arbitrarily chosen future-directed non-spacelike worldline. That is, this is a 3+1 slicing. We can slice along your worldline, or on that of a neutral hydrogen atom floating in intergalactic space, or on that of a high-energy cosmic ray, or on that of a CMB photon. It's arbitrary, and each can give markedly different spatial slices through the same spacetime (in particular particle counts on slices will differ where the choices of index axes are anywhere accelerated with respect to one another).
When we decompose in this way, and take an <https://en.wikipedia.org/wiki/ADM_formalism> approach, we will tend to think of the shift vector as how we associate a point one one slice (everywhere in space at a coordinate instant in the spacetime) with its successor slice (everywhere in space at the next coordinate instant int he spacetime), which is helpful when spacetimes expand or contract in one or more spatial directions along the arbitrarily chosen time axis.
Oh come on. Things fall down because matter has a property called gravity that attracts other matter, and below us is a giant earth with a lot of matter. And it has more of a net effect on us than any other matter in the universe because gravity scales with distance and mass in that particular way. That's as darn good of an explanation of why we fall down as one could possibly give. "But why does mass have gravity?" Why does Newton have to have all the answers to every other question too? Maybe ask that and someone will answer that question in a few hundred years? He answered your original question, he didn't claim he can answer every subsequent question you think of. It's quite ridiculous to suggest Newton just tautologically concluded "things fall down because they do" just because he doesn't go on and explain the "why" of every sub-question ad infinitum.
I'd just throw in that this idea of universal gravitation coupled with the laws of motion and dodgy ideas like force and momentum enabled a wide range of phenomena to be described and some predictions to be made that could be compared with observations.
Yes, ideas like 'force' and 'momentum' were a bit dubious but the resulting theory was effective[1] within its domain of applicability.
Newton’s physics is still taught and used everywhere because it’s simple and accurate enough for 99% of practical situations. Einstein’s relativity isn’t a better explanation, it just extends it to extreme conditions. NASA still uses Newtonian law to launch rockets.
Well, that might be stretching it. Speaking as someone who has done a little university level physics the understanding still seems to be basically that things fall because they do - we haven't made much progress beyond a firm strike-through of the word "down".
Newton's contribution was a very precise description of how rapidly they fall, and how we can calculate and understand the direction that things fall in complex multidimensional settings.