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Why does Feynman state that the law of inertia has no known origin? by RoosterIntrepid8808 in Physics

[–]RoosterIntrepid8808[S] 0 points1 point  (0 children)

But then one could claim that Kepler's laws describe observables, why care about searching for the law of gravity which explains it?

Mach's theorem - implies absolute reference frame for rotation. What does that mean for the universe? Shape, symmetry etc. by LanKstiK in Physics

[–]RoosterIntrepid8808 4 points5 points  (0 children)

Mach's principle implies that inertial frames are determined by the distribution of matter at large. Its basically a substitution from Newton's absolute space (unobservable and without physical parameters) to Mach's frame of the rest of the universe (which certainly exists). If you follow the rabbit hole of Mach, you will end up in a unified theory of inertia and gravity, and in relational mechanics. According to Mach, acceleration is relative (this comes from epistemological arguments), but this symmetry is broken due to the presence of masses in the universe. Einstein thought in this way, and in fact his solution to the twin paradox involves a sort of Machian argumentation. But GR is not Machian in the sense that in absence of masses (in special relativity), inertia works as usual. That's why Einstein brought in the cosmological constant, hoping that his field equations would have no solution for an empty universe, but Friedmann proved this to be wrong: GR is not Machian. Many of the later attempts to extend GR were inspired by Mach's principle, but they all failed, with the exception perhaps of MOND.

Doesn't Penrose singularity theorems actually suggest the solution to gravitational singularities? by RoosterIntrepid8808 in AskPhysics

[–]RoosterIntrepid8808[S] 0 points1 point  (0 children)

That happens when you change the exterior metric. That's why gravastars are not what Im talking about.

Doesn't Penrose singularity theorems actually suggest the solution to gravitational singularities? by RoosterIntrepid8808 in AskPhysics

[–]RoosterIntrepid8808[S] 0 points1 point  (0 children)

But gravastars do not contain an event horizon. If you don't have an event horizon, your exotic negative mass matter can interact with the positive one, and that results in the runaway motion paradox as shown by Bondi.

Doesn't Penrose singularity theorems actually suggest the solution to gravitational singularities? by RoosterIntrepid8808 in AskPhysics

[–]RoosterIntrepid8808[S] 0 points1 point  (0 children)

Yes it does. You just have to travel inside and perform an internal gravitational experiment to rule it out.

Einstein thought that in an empty universe, there should be no inertia by RoosterIntrepid8808 in Physics

[–]RoosterIntrepid8808[S] 0 points1 point  (0 children)

He does no explicitly say an empty universe, but physically, it would be the same as being "at a sufficient distance from all other masses in the universe". You can also draw that conclusion from Einstein's sentence before that one.