OrionWorks wrote: > Jonse sez: > >> For those who haven't seen it: >> >> "The Speed of Gravity What the Experiments Say" >> >> Tom Van Flandern, Meta Research >> >> [as published in Physics Letters A 250:1-11 (1998)] >> >> http://metaresearch.org/cosmology/speed_of_gravity.asp >> >> hint: this is not a pdf file but gavity is pdf (pretty damn fast) >> > >>From the report: > > "How can black holes have gravity when nothing can get out because > escape speed is greater than the speed of light?" > > Always wondered about that conundrum.
The gravitational field doesn't "get out", it just "is out" -- it doesn't propagate, it just is. And in fact it predates the formation of the black hole -- the far field *does* *not* *change* when a black hole forms. Micro black holes, for instance, have exactly the same gravitational field the (small amount of) matter which went into their formation had, as long as we're farther away than the original radius of the pre-hole blob; the only difference is the radius of a black hole is vastly smaller than the radius of the original blob of pre-black-hole matter which formed it. Similarly, the field of an electron doesn't propagate, it just exists. Watch a stationary electron; how fast is its field "propagating"? Answer: It's not, just like the gravitational field of the Earth isn't "propagating", nor is the field of a black hole. (Quantum gravity may put a different "spin" on the picture, of course; anything I say about it comes from the classical GR picture.) Radiation propagates, but radiation results from a *change* in a field, typically due to acceleration of the object producing it. Gravitational radiation propagates at C (according to the standard theory -- nobody's detected it, and that includes Van Flandern, so its velocity certainly hasn't been measured). EMR propagates at C also, and that has been measured, of course. Since a static gravitational field doesn't propagate, it shows no aberration either. Similarly, the static field of an electron doesn't propagate, and it also shows no aberration. People occasionally point to the lack of aberration of the Sun's gravity as "evidence" for a high gravitational propagation speed, which really makes little sense. It's like pointing to the lack of aberration of an electron's field as evidence that an EM field propagates infinitely fast -- really, in both cases there's no propagation involved. Here's a classic "gedanken experiment" which illustrates what I'm talking about: Imagine two spaceships sitting a few light hours apart, stationary relative to each other. Their clocks are synchronized (they're stationary relative to each other, so that's easy enough to do). Now, someone a very very long distance away fires a negatively charged particle at one of the ships. The particle, traveling at constant velocity, moves along a line perpendicular to the line connecting the ships. The particle arrives at spaceship B at 3:00 sharp. Now, over on spaceship A, there is a sensitive electric field detector, which senses the field of the charged particle; an indicator points at the (moving!) location of the particle, by pointing in the direction its electric field *currently* points (*no* compensation for its being a moving target). At some point, the detector will point directly toward spaceship B; that's the moment when the folks on A detect the arrival of the particle at ship B. WHEN WILL THAT HAPPEN? Answer: 3:00 sharp, according to relativity theory. There is *NO* "propagation delay". That's what is meant by "lack of aberration" -- the field constantly points toward the CURRENT location of the particle (as long as it moves uniformly). Similarly, the gravitational field of the Sun always points directly at the Sun, rather than to a point where it "was recently" (as long as the Sun is moving uniformly). If anyone's curious I can go into more detail on this. Note particularly that we assumed the particle was traveling at uniform velocity -- if it accelerates, it radiates, and the picture gets more complicated. Don't ask me about virtual photons, tho, 'cause I don't know diddly about them. This is just classic field theory I'm talking about here (which matches experiment nicely AFAIK, until you get into the quantum realm). * * * I've encountered Van Flandern before in the physics news groups, and I wouldn't tend to spend a lot of time on his writing. Anyhow the blurb on Wiki pretty much sums it up: > Van Flandern is best known for his contention that certain features > on the surface of Mars are artificial sculptures of "faces" created > by extraterrestrial beings, that Mercury may be a former moon of > Venus, and that planets sometimes spontaneously explode. He's not an amateur; he's a professional (retired?) astronomer who worked at the Naval Research Laboratory for 20 years, for whatever that's worth.
