#### Magical Realist

Valued Senior Member
Brian Greene rocks, giving an elegant demonstration and description of gravity on The Late Show. But I have a question: what is the force by which massive objects like Sun warp spacetime? It's gotta be some force it exerts on spacetime. But what is it? How can gravity precede the warping of spacetime?

what is the force by which massive objects like Sun warp spacetime?
The relevant processes all fall under the banner of "gravitation".

In the General Theory of Relativity, gravity is not a force, however. The 'mechanism' is that objects with mass warp the space and time around themselves. So, all that is required is the presence of mass/energy. Or, to put it another way, gravity is an inherent property related to mass. It's just something that mass does.
It's gotta be some force it exerts on spacetime.
Why does it "gotta"?
How can gravity precede the warping of spacetime?
Gravity is the warping of spacetime.

Gravity is the warping of spacetime.

So mass is what warps spacetime, resulting in gravity. But mass doesn't exert a force to do this? It just happens. Got it.

When a body's mass is so great that it results in a black hole, which eventually crushes that body out of existence, what is left to cause the remaining gravity? Iow how can a hole in spacetime have mass?

So mass is what warps spacetime, resulting in gravity. But mass doesn't exert a force to do this? It just happens. Got it.

When a body's mass is so great that it results in a black hole, which eventually crushes that body out of existence, what is left to cause the remaining gravity? Iow how can a hole in spacetime have mass?
It's not a hole.

So mass is what warps spacetime, resulting in gravity. But mass doesn't exert a force to do this? It just happens. Got it.

To give you another example, consider electricity. What is the relevant force? Well, there's an electric force that acts on anything that has electric charge. Why do some things have electric charge? Nobody knows. That's just the way the universe is. Electrical force is a property caused by charge, just as gravitational force (if you want to call it that) is a property caused by mass. Nobody knows why. It's just how our universe works. The task of science is to model the processes we observe in nature. These things are among the things we observe.

So, you're right. Things "just happen". Things are the way they are. Start getting used to living in this universe of ours. It is the way it is. You're stuck with it, like it or not, understand it or not.
When a body's mass is so great that it results in a black hole, which eventually crushes that body out of existence, what is left to cause the remaining gravity?
The mass that formed the hole.

By the way, nobody can say whether a body's mass is "crushed out of existence" when it falls into a black hole. After all, we can't go inside to look (or, at least, if we did go inside we couldn't transmit the news to anybody on the outside).
Iow how can a hole in spacetime have mass?
One way to think about it is that, from the outside, a black hole looks, gravitationally speaking, just like an ordinary lump of matter with a certain mass.

In other words, if you could somehow turn our Sun into a black hole of the same mass right now, the Earth's orbit wouldn't be affected. The Earth would keep right on orbiting the black hole, in the same orbit that it now orbits the Sun. There would be some other nasty side-effects, of course, but gravitationally, we here on Earth wouldn't notice any difference.
Wouldn't it rip a hole in spacetime?
There's no evidence that black holes rip holes in spacetime.
"While black holes are mysterious and exotic, they are also a key consequence of how gravity works: When a lot of mass gets compressed into a small enough space, the resulting object rips the very fabric of space and time, becoming what is called a singularity."--- https://science.nasa.gov/universe/10-questions-you-might-have-about-black-holes/
Eww. I don't know who wrote that article, but it's not correct.

A singularity is a kind of mathematical artifact in some equations. There's no evidence for physical singularities. There's no evidence that there's an actual physical singularity at the centre of a black hole.

This kind of pop-science FAQ stuff is not hard to find, unfortunately. It tends to give the general public misleading ideas about black holes. The people who write this sort of thing really ought to be more careful.

Wouldn't it rip a hole in spacetime?

"While black holes are mysterious and exotic, they are also a key consequence of how gravity works: When a lot of mass gets compressed into a small enough space, the resulting object rips the very fabric of space and time, becoming what is called a singularity."--- https://science.nasa.gov/universe/10-questions-you-might-have-about-black-holes/
The issue here is treating spacetime like it is a material substance. It isn't. Even when they use terms like the "fabric" of spacetime, it is meant more in the lines as when someone uses the term " the fabric of modern society" It refers to a more intangible concept. And spacetime "warping" really means " The rules governing geometry deviates from those of Euclidean geometry".

Things "just happen". Things are the way they are. Start getting used to living in this universe of ours. It is the way it is.

Kinda defeats the purpose of science doesn't it? I mean if everything that occurs is "just what happens" why do science at all?

Kinda defeats the purpose of science doesn't it? I mean if everything that occurs is "just what happens" why do science at all?

The TL;DR version for you, if you have a limited attention span, is that we do science in order to try to explain how our world works, which allows us to predict how it will work going forward. Moreover, it increases our ability to bring aspects of our world under control.

For example, the computer or other electronic device you are reading this on would not exist without science. If scientists hadn't nutted out how electricity and magnetism work, we'd have no harnessed electrical power, let alone any devices as complicated as modern digital computers.

Are you really unaware of any of the benefits that science has brought you? Are you not grateful that your life expectancy is probably at least twice as long as that of your great grandparents, due to science?

Kinda defeats the purpose of science doesn't it? I mean if everything that occurs is "just what happens" why do science at all?
Why questions are never any good. “How” questions give us meaningful answers and make predictions.

Newton treated gravity as a force but did not attempt to explain it, he was not able to.

Einstein gave us a much deeper explanation, but the geometry is not something you can explain in words.

One of the worst explanations for me ever in science is, "matter tells spacetime how to curve, and curved spacetime tells matter how to move".

Does that give an insight into vectors, tensors, Rieman geometry? No, zero.

GR and much of physics answers the how questions and gives results and prediction via the mathematics.

Using words only would be like be like describing chemical reactions without symbols and reaction equations.

What causes the flat rotational curve of a spiral galaxy? Why are all the spiraling stars in one plane?

Angular momentum.

What causes the flat rotational curve of a spiral galaxy? Why are all the spiraling stars in one plane?

As per Seattle for "one plane"but there are other shaped galaxies.

If I could tell you precisely why some galaxies have flat rotation curves I would be collecting the Nobel!

Have a look at the animation, our solar system has planets close to the sun and some further out, the further out the less the influence of the suns gravity with decreasing velocity.

We should expect the same for stars in a galaxy but that is not what happens, the velocities are closer to equal . Is there a lot of extra mass distributed that we cannot see? Dark Matter?

That is one solution, modified theories of gravity or a mixture of both are others but they still do not know.

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

As far as I know, most younger galaxies have "arms" and those that don't tend to be older galaxies that have merged with other galaxies and the arm structures (dust/gas lanes) have been disrupted and as you say, dark matter is the current theory for the dust/gas lane rotation speeds.

What causes the flat rotational curve of a spiral galaxy?
Dark matter. i.e. there's a lot more mass in the galaxy than we can see with our telescopes.

Another possible explanation is that that's no dark matter, but gravity doesn't quite work the way we currently think it does.

This is an active area of ongoing research.
Why are all the spiraling stars in one plane?
It's a side-effect due to the fact that the galaxy is rotating. Rotation tends to flatten spheres into discs. Think, for example, about how pizza bases are made from lumps of dough, traditionally. Even our own planet is a "squashed" sphere, with a smaller polar diameter than the equatorial diameter.

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