Exploring the Retarded Wave Solution of Maxwell's Equations for Gravitation

[paige turner]

TL;DR Summary

physics dept says that when you bounce a ball off the wall of the space station, the ball is actually pushing against reverse-time gravity waves from every object in the future universe.

Supposedly, the retarded wave solution to Maxwell's equations applies to gravitation as well as electrodynamics.

The space station doesn't fly off into the distance because every object in the universe (at whatever distance) focuses gravity through the mass of the station. Every object on the station's null cone pushes back.

please evaluate and let me know if this is true because I'm not smart enough to tell. if it is true, it's important. I didn't write it. I barely understand it. I don't have anything to do with it.

https://physics.fullerton.edu/~jimw/general/inertia/index.htm

 

[Vanadium 50]

physics dept says

Actually, he's a historian. Really, Paige, you need to do your homework first.

What is the point of this? If you didn't understand the source, while do you think you'll understand others' critiques of the source?

 

[PeterDonis]

Supposedly, the retarded wave solution to Maxwell's equations applies to gravitation as well as electrodynamics.

That can't possibly be true since the correct field equation for gravity is the Einstein Field Equation, not Maxwell's Equations.

The space station doesn't fly off into the distance because every object in the universe (at whatever distance) focuses gravity through the mass of the station. Every object on the station's null cone pushes back.

This is a speculation that, however attractive it sounds to some people, has never led to anything useful.

https://physics.fullerton.edu/~jimw/general/inertia/index.htm

This is not a valid reference; it's one person's opinions about the speculation referred to above, not a textbook or peer-reviewed paper.

 

[Ibix]

Actually, he's a historian.

A historian who has confused Lorenz and Lorentz? That's not a good sign...

OP: My understanding of this is that there's a lot of stuff there that nobody will argue with. The bit about explaining the stability of free fall orbits with a finite propagation speed for gravitational field changes is surprisingly complicated. But the conclusion that inertia is caused by distant gravitational fields is a bit more of a stretch. Those who are convinced of the Machian nature of GR are very convinced, and everyone else is rather on the fence.

Personally, it feels to me like trying to use GR in a Machian way is a bit odd. You can use it in a model universe that's empty of all but a single small spherical source of gravity and get predictions of orbits that work well in the real world. That seems inconsistent with the idea that free fall is picked out by distant masses. I don't claim to be an expert on Mach's principle, though. Perhaps there's an answer to that - although I suspect the argument will disappear rapidly into philosophy more than empirical science.

 

[Vanadium 50]

A lot of time could be spent trying to find merit in the speculations of a historian. I am not willing to spend that time before it's published in a peer-revieewed journal. That's the point of being a "peer" - we all spend time on a small fraction of papers so our colleagues don't waste their time.

 

[PeterDonis]

Those who are convinced of the Machian nature of GR are very convinced, and everyone else is rather on the fence.

It's not even that simple. In the "subtleties" section of the web page that the OP linked to, there are three options given. The article then dismisses option 2, that some "local field" is responsible for inertia, as the "least likely" explanation, without even bothering to mention that this is the explanation that standard GR gives. The "local field" is the metric: the spacetime geometry. The standard GR interpretation is that the spacetime geometry is what "tells" matter which states of motion, i.e., worldlines, are free fall and which are not, and how much acceleration (i.e., geometric path curvature) is required to have a given non-free-fall worldline. And the spacetime geometry is determined by the distribution of stress-energy via the Einstein Field Equation. This includes the distribution of stress-energy of "distant matter", because spacetime curvature can propagate. The spacetime geometry at any given event is determined by the distribution of stress-energy in the past light cone of that event. This is perfectly "local".

Even Wheeler, who wrote a whole textbook with Cuifolini entitled "Gravitation and Inertia" to argue for a Machian view of GR, did not argue for the kind of "Machian" view that the web page the OP linked to is arguing for. Wheeler simply pointed out, in the commonly quoted phrase, that "spacetime tells matter how to move, and matter tells spacetime how to curve", and then elaborated on that along the lines I described briefly above. So it's not even that there's one "Machian" view with some people who are proponents and others who are skeptical. There are at least two "Machian" views, one of which is perfectly compatible with standard GR, the other of which...isn't.

 

[paige turner]

thank you.

this is the kind of answer I was looking for, not yelling at me for asking the question.

but I'm wondering how local curvature stops the space station from flying off when you throw a tennis ball at it. What does gravitation have to do with that?

you can conduct the experiment in a universe with nothing but the space station and a tennis ball, and it still wouldn't fly backwards. it would still have inertial mass.

 

[paige turner]

>, you need to do your homework first.

i know a great deal more than i let on.

What is the point of this?

to trigger a discussion of mach's principle.

i tried to do it without posting, "hey guys, whaddya think of mach's principle?"

but everyone wants to either ridicule or be angry at me for asking the question. I don't care. I'm autistic. but that's BULL dang.

i should go somewhere serious. I just want to know what's going on around me, and what's "around me" is the universe. with normal people, everyone is ego–involved.

If you didn't understand the source, why do you think you'll understand others' critiques of the source?

[points at the above sentence] sophistry. bah!

Another chance for you:

If you didn't understand the source, why do you think you'll understand others' critiques of the source?

others' critiques probably don't include tensor calculus that I'm too stupid to understand. their critiques would probably be in words.

> he's a historian.

this was published on the physics dept site because he's qualified to teach physics too. it's been there for many years, ever since I first read it.

anyway, you should judge what it is, not who wrote it.

 

[Vanadium 50]

i know a great deal more than i let on.

Perhaps then you should let on.

 

[PeterDonis]

this is the kind of answer I was looking for, not yelling at me for asking the question.

If you try to obfuscate your actual question instead of asking it directly, you have to expect that the responses you get won't necessarily address your actual question.

to trigger a discussion of mach's principle.

i tried to do it without posting, "hey guys, whaddya think of mach's principle?"

Why in the world would you do that? Why not just ask: Does General Relativity incorporate Mach's principle? Or something to that effect.

but everyone wants to either ridicule or be angry at me for asking the question.

The responses you got were not ridicule or anger. They were a signal to you that you did not do a good job of conveying what your actual question was. Just asking it straight out would have been better.

i know a great deal more than i let on.

So far I have not seen any evidence that this is actually true.

 

[PeterDonis]

anyway, you should judge what it is, not who wrote it.

You already got a response that did that: my post #3 (last paragraph).

 

[PeterDonis]

I'm wondering how local curvature stops the space station from flying off when you throw a tennis ball at it.

I don't understand the scenario you are proposing here. Where are you throwing the tennis ball from?

Generally, the local spacetime curvature is what determines the space station's trajectory.

What does gravitation have to do with that?

"Gravitation", i.e., the local spacetime curvature, determines the space station's trajectory. The local spacetime curvature is determined by the distribution of matter and energy: the dominant source of that in the Earth's vicinity, of course, is the Earth.

you can conduct the experiment in a universe with nothing but the space station and a tennis ball, and it still wouldn't fly backwards. it would still have inertial mass.

How can you possibly know this? It is impossible to conduct such an experiment.

 

[PeterDonis]

Where are you throwing the tennis ball from?

Never mind, I see you describe that briefly in your OP. When you throw a tennis ball inside a freely falling space station, and it bounces off the space station's wall, the space station does recoil. However, the motion of the center of mass of the entire system--the space station, the tennis ball, and you--does not change. No internal interactions in a system can change the motion of the system's center of mass as a whole.

However, none of this has anything to do with gravity or spacetime curvature. You can ignore those things when doing experiments internal to the space station. This is the basic idea of the equivalence principle. Gravity and spacetime curvature only come into play when you ask what the trajectory of the center of mass of the system as a whole (the space station, the tennis ball, and you) is, and how the presence of massive objects like the Earth affects it.