You're right, a lot of explanations seem to be kicking the can down the road, in a sense. And that's interesting that gravity acts on energy-momentum. I'm aware that mass is a form of energy, so what's the difference between gravity acting on mass, versus it acting on energy-momentum? Does that more accurately encompass gravity's functioning?
How does gravity work?
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of course, even 'inert' mass would still be seen as hurtling around the carousel of its respective solar system & galaxy at fantastic speeds, from a 'stationary' observer's viewpoint in space. 🙂
Elroch wrote:
Gravity turns out not to be about matter (or mass). It's about energy-momentum (a Lorentz-covariant quantity, the relativistic analog of a vector). Energy-momentum determines the curvature of space-time. (Mass is of course a form of energy that isn't moving, and when it is viewed from a moving frame it has momentum, but there are forms of energy without mass).
Explanations of why things happen in physics always move the question rather than being the last word. For example, the electric force can be viewed as being caused by the exchange of virtual photons. That mechanism requires more explanation.
When one is talking to somebody that thinks that the earth's core has some special property that emits gravity like magic, I am happy if I can just get them to understand the mass part of it 
. But yes, of course, gravity bends light, black holes swallow light, and the intense gravity is strong enough to overcome the other 3 forces. etc. Although as somebody is bound to point out now, there's no way to directly test/observe what happens at the center of black holes...
Oh, gravity! It's a fascinating phenomenon. You see, when the universe was feeling particularly mischievous one day, it decided to invent gravity just to mess with us. So, here's how it works: everything with mass pulls everything else with mass towards it. It's like an intergalactic game of tug-of-war, except nobody really knows why or how it happens. Scientists have come up with all sorts of complicated explanations involving curved spacetime and the warping of dimensions, but honestly, it's all just a bunch of mumbo-jumbo. Gravity is simply nature's way of keeping us grounded and preventing us from floating away into space. So, don't worry too much about understanding it. Just be grateful that you're not floating off into the cosmos right now!
DiogenesDue wrote:
Elroch wrote:
Gravity turns out not to be about matter (or mass). It's about energy-momentum (a Lorentz-covariant quantity, the relativistic analog of a vector). Energy-momentum determines the curvature of space-time. (Mass is of course a form of energy that isn't moving, and when it is viewed from a moving frame it has momentum, but there are forms of energy without mass).
Explanations of why things happen in physics always move the question rather than being the last word. For example, the electric force can be viewed as being caused by the exchange of virtual photons. That mechanism requires more explanation.
When one is talking to somebody that thinks that the earth's core has some special property that emits gravity like magic, I am happy if I can just get them to understand the mass part of it . But yes, of course, gravity bends light, black holes swallow light, and the intense gravity is strong enough to overcome the other 3 forces. etc. Although as somebody is bound to point out now, there's no way to directly test/observe what happens at the center of black holes...
Seems to me that a key thing about black holes that is way too often ignored in popular discussions is that the event horizon of a black hole and anything inside it always lays in the future of all points outside the horizon. So "observing" the inside of a black hole would be observing the future. Most people seem to have the idea that the inside of a black hole is something that can exist "now", which is not so in any sense. This is even true of the classical black hole textbook Schwarzschild model - a spherically symmetrical solution of Einstein's field equations.
To observe the interior of a black hole requires going somewhere some of it is in the causal past. That means falling into it, crossing the horizon to a place infinitely far in the future of the outside universe. I believe it is accurate to say there is uncertainty about what this would be like. There is agreement about there being massive tidal forces for smaller black holes (before the horizon), but only weak tidal forces for supermassive black holes, but one hypothesis involves absolute obliteration in a firewall at the horizon while the classical viewpoint says crossing the horizon is uneventful except for the tidal forces!
Elroch wrote:
Seems to me that a key thing about black holes that is way too often ignored in popular discussions is that the event horizon of a black hole and anything inside it always lays in the future of all points outside the horizon. So "observing" the inside of a black hole would be observing the future. Most people seem to have the idea that the inside of a black hole is something that can exist "now", which is not so in any sense. This is even true of the classical black hole textbook Schwarzschild model - a spherically symmetrical solution of Einstein's field equations.
To observe the interior of a black hole requires going somewhere some of it is in the causal past. That means falling into it, crossing the horizon to a place infinitely far in the future of the outside universe. I believe it is accurate to say there is uncertainty about what this would be like. There is agreement about there being massive tidal forces for smaller black holes (before the horizon), but only weak tidal forces for supermassive black holes, but one hypothesis involves absolute obliteration in a firewall at the horizon while the classical viewpoint says crossing the horizon is uneventful except for the tidal forces!
With the curvature of spacetime that extreme I would assume that past/present/future are all mashed together, but since existence inside a black hole is completely different and passing in and out is not really an option I am not sure that the distinction matters. If you could actually apply "infinite+1" energy and accelerate your way out, would you emerge after the heat death of the universe because it took so long? And before anyone says boo...yes, I am positing "infinite+1" energy only to make a point, not to say it is possible.
Or perhaps black holes are truly and utterly one way, and you would be expelled (and not in a healthy form) into a new big bang in another universe? In fact...what if your "infinite+1" energy were the catalyst that caused a big bang ...and this turned out to be how big bangs occur, when some phenom causes something to "escape" a black hole. Sounds like soft sci-fi fodder for a good story. I expect name credit and royalties, though...
DiogenesDue wrote:
Elroch wrote:
Seems to me that a key thing about black holes that is way too often ignored in popular discussions is that the event horizon of a black hole and anything inside it always lays in the future of all points outside the horizon. So "observing" the inside of a black hole would be observing the future. Most people seem to have the idea that the inside of a black hole is something that can exist "now", which is not so in any sense. This is even true of the classical black hole textbook Schwarzschild model - a spherically symmetrical solution of Einstein's field equations.
To observe the interior of a black hole requires going somewhere some of it is in the causal past. That means falling into it, crossing the horizon to a place infinitely far in the future of the outside universe. I believe it is accurate to say there is uncertainty about what this would be like. There is agreement about there being massive tidal forces for smaller black holes (before the horizon), but only weak tidal forces for supermassive black holes, but one hypothesis involves absolute obliteration in a firewall at the horizon while the classical viewpoint says crossing the horizon is uneventful except for the tidal forces!
With the curvature of spacetime that extreme I would assume that past/present/future are all mashed together,
This seems confused. You are discussing the relationship of points deep in the gravitational field (at least) while I was referring to the relationship between, say, us and the event horizon. While quantum gravity may disrupt time between points at the event horizon, all of those points still have a clear relationship to us - they are in the causal future. This means we can affect them (by falling in) but they cannot affect us. Can't get much more "black" than that!
but since existence inside a black hole is completely different and passing in and out is not really an option I am not sure that the distinction matters. If you could actually apply "infinite+1" energy and accelerate your way out, would you emerge after the heat death of the universe because it took so long? And before anyone says boo...yes, I am positing "infinite+1" energy only to make a point, not to say it is possible.
Or perhaps black holes are truly and utterly one way, and you would be expelled (and not in a healthy form) into a new big bang in another universe? In fact...what if your "infinite+1" energy were the catalyst that caused a big bang ...and this turned out to be how big bangs occur, when some phenom causes something to "escape" a black hole. Sounds like soft sci-fi fodder for a good story. I expect name credit and royalties, though...
I am not convinced there is anything beyond an event horizon (itself only classically forming after infinite time and requiring quantum gravity to understand properly).
A key point is that the kinetic energy of ANYTHING (eg a single radio wave photon) that falls into a black hole is infinite at the point it crosses the event horizon (by extrapolation, from the point of view of someone outside). However, this is compensated for by the gravitational time dilation, which is also infinite (in the limit) from any external location.
The idea that black holes may be associated with the birth of new Universes seems quite reasonable. The infinite (in the limit) kinetic energy and discussion of a stringball sounds very much like discussions of the very earliest stage of the Big Bang.
Elroch wrote:
This seems confused. You are discussing the relationship of points deep in the gravitational field (at least) while I was referring to the relationship between, say, us and the event horizon. While quantum gravity may disrupt time between points at the event horizon, all of those points still have a clear relationship to us - they are in the causal future. This means we can affect them (by falling in) but they cannot affect us. Can't get much more "black" than that!
[and]
I am not convinced there is anything beyond an event horizon (itself only classically forming after infinite time and requiring quantum gravity to understand properly).
A key point is that the kinetic energy of ANYTHING (eg a single radio wave photon) that falls into a black hole is infinite at the point it crosses the event horizon (by extrapolation, from the point of view of someone outside). However, this is compensated for by the gravitational time dilation, which is also infinite (in the limit) from any external location.
The idea that black holes may be associated with the birth of new Universes seems quite reasonable. The infinite (in the limit) kinetic energy and discussion of a stringball sounds very much like discussions of the very earliest stage of the Big Bang.
Yes, I was referring to what might be going on inside black holes in a more general way, because AFAIK there's still no consensus on whether crossing the event horizon necessarily results in being pulled to the center and torn asunder imminently or if it is possible to "hang out" just inside the event horizon for longer if you have the energy/means.
Do photons that get trapped go down in a spiraling orbit (and I am not talking about the theoretical photon sphere at 150% radius of the event horizon which is another phenom yet to be directly observed) that is significantly slower or faster depending on the angle at which they entered the black hole? Etc.
Obviously, some of this depends on how the interior of black holes are commonly arranged, so it's not 100% knowable, but perhaps mathematically predictable.
The photon sphere is a inference which is surely real, but not terribly important.
It is undoubtedly possible for a photon to orbit a large number of times before falling in, if its path was just short of what is needed to be in the photon sphere (i.e. just below horizontal at that radius). But I would guess falling in without a single orbit is the norm.
I reiterate that even the existence of a region beyond the event horizon is highly questionable. If such a region exists it is infinitely far in the future from here (i.e. there is no time you could wait that would be sufficient to stop it being in the future).
("The future" of a point in space time is all space time within its future light cone).
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Gravity turns out not to be about matter (or mass). It's about energy-momentum (a Lorentz-covariant quantity, the relativistic analog of a vector). Energy-momentum determines the curvature of space-time. (Mass is of course a form of energy that isn't moving, and when it is viewed from a moving frame it has momentum, but there are forms of energy without mass).
Explanations of why things happen in physics always move the question rather than being the last word. For example, the electric force can be viewed as being caused by the exchange of virtual photons. That mechanism requires more explanation.