Questions about gravity

[Magical Realist]

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?

https://www.facebook.com/reel/414270421559139

 

[James R]

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.

 

[Magical Realist]

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?

 

[Seattle]

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.

 

[Magical Realist]

It's not a hole.

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/

 

[James R]

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

I'm glad you've 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.

 

[Janus58]

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".

 

[Magical Realist]

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?

 

[James R]

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

But, Magical Realist, I answered that question just recently for you in another thread. Did you read my answer? Have you forgotten? Here's the link:

https://www.sciforums.com/threads/d...up-to-make-the-math-work.166382/#post-3728113

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?

 

[Pinball1970]

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.

 

[Magical Realist]

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

 

[Seattle]

Angular momentum.

 

[Pinball1970]

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

 

[Seattle]

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.

 

[James R]

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.

 

[Fool]

Magical Realist, you are asking PhD questions. If you can produce theoretically connected and reliable answers to those, it would be worthy of a PhD. Einstein said, "I ask the simplest of questions, and then answer them."

The questions I ask are:

What is mass?
How does mass turn into energy/photons?
How does energy/photons turn into mass?
What is a photon?
How does a photon travel through nothing/space?
Why does electricity's velocity in a wire depend on the insulation not the conductor?

How does mass produce gravity?
How does an electron or proton produce it's field?


Nonsensical answers:

Energy manifests itself in a circular vortex associated with the diameter of massive particles, electron, proton, neutron. Quarks?

Through particle interaction, matter antimatter, the vortexes are released in a directionally straight path.

Photons/energy is twisted into an orbital vortex the size of the massive particles, during extreme events. Particle colliders and such.

A photon is a electromagnetic wave manifestation of the virtual particles in a vacuum. The aether colloquially.

Electricity/photons traveling in a wire use the insulations particles to support part of their electromagnetic wave, because they are traveling on the conductors surface. The conductor is opaque to photons, the insulation is transparent.

Electrons, protons and mass inflict a spooky action at a distance. IOW, I don't know.

I don't have a clue but ponder those things.

How can an electromagnetic wave travel where there are no electrons? Virtual particles, that appear and disappear while a photon travels by?

How does mass effect gravity which travels at the speed of light? Some kind of photonic effect, right? Photons in an extremely small vortex emitting gravitons? Negative wavelength? Infinite wavelength?

Newton described gravity as a pulling force. It is measurable. Why it pulls is completely unknown. Relativity models it as a warping of space. No longer rectilinear. It now is merely a mathematical construct, a set of descriptive equations. It predicts gravity's behavior, but doesn't tell what it is or how it unites with electrical theory.

Is dark matter just ephemeral virtual particles, being created and destroyed by traveling photons, or other fields? I don't have any answers or data for that question.

.

 

[axocanth]

Newton described gravity as a pulling force. It is measurable. Why it pulls is completely unknown. Relativity models it as a warping of space. No longer rectilinear. It now is merely a mathematical construct, a set of descriptive equations. It predicts gravity's behavior, but doesn't tell what it is or how it unites with electrical theory..

Like most any scientific theory, either can be given a literal realist interpretation (of one kind or another) or an antirealist instrumentalist interpretation (of one kind or another).

On a realist reading of Newtonian theory, then, gravity is an attractive force. That's what it is. Similarly, on a realist reading of relativity theory, gravity is the curvature of spacetime. That's what it is.

Both views are to be found among scientists. Evidently you take a realist view of Newtonian theory and an instrumentalist view of relativity theory.

 

[axocanth]

Gravity is the warping of spacetime . . .

. . . if you give Einstein's theory a literal, realist reading. Not everyone does.


For illustration, around the year 1900, scientists differed on the proper way to understand atomic theory. Some -- scientific realists -- took the theory to be asserting just what it appeared to be asserting: atoms are real. Others, most notably Ernst Mach, expressing an empiricist or antirealist view, insisted this was the wrong way to interpret the theory; atoms were a "useful fiction" and nothing more. The theory is not to be read literally.

This kind of divergence of views is still extremely common. E.g. is the Many Worlds theory of quantum mechanics to be read literally -- there really are many worlds? Or is it to be understood as merely a "mathematical construct"? - to borrow Fool's words from above.

 

[James R]

. . . if you give Einstein's theory a literal, realist reading. Not everyone does.

I was responding to Magical Realist's question. Context is relevant, axocanth. Moreover, it is usually impractical - if for no other reason than the amount of time it takes - to qualify every statement one makes to the n-th degree.

From a philosophical standpoint, one can always find another layer of questioning beneath the previous one. One must learn to judge how much information the learner is actually seeking, prior to embarking on a detailed multi-volume exposition of the ins and outs of philosophical hair splitting.

Don't you agree?

Bear in mind that this thread is in the Science section of the forum, not Philosophy.

 

[axocanth]

I was responding to Magical Realist's question. Context is relevant, axocanth. Moreover, it is usually impractical - if for no other reason than the amount of time it takes - to qualify every statement one makes to the n-th degree.

From a philosophical standpoint, one can always find another layer of questioning beneath the previous one. One must learn to judge how much information the learner is actually seeking, prior to embarking on a detailed multi-volume exposition of the ins and outs of philosophical hair splitting.

Don't you agree?

I felt there was some unclarity among other members' posts, i.e., they seemed to be laboring under the impression that the theory itself determines what is being asserted (i.e. does GR assert that gravity is the curvature of spacetime or not).

The theory no more decides this than the statement "Jimmy is a pig" decides whether a literal assertion is being made or not. We decide. We can choose to read it literally or metaphorically.

Edit: Indeed it is commonly held that Einstein himself made a kind of conversion from an earlier antirealist reading of his own theory, to a later realist reading.