Does Gravity Travel at the Speed of Light? (1998)

Reference to: http://knowyourmeme.com/memes/bill-oreilly-you-cant-explain-...

Here's another way to understand it.

The difficultly: Despite the propagation delay, gravity always points to where the object should be, instead of where it was when the gravity was "emitted".

The explanation: Just like the object itself is moving, the gravitational field (or electric/magnetic field) is ALSO moving! It moves at exactly the same speed, and in the same direction as the object which created it.

It's "disconnected" from the source - so if the source suddenly changes direction the field it emitted doesn't know about it, and continues to point to where it expected the object to be.

More technical:

Why wormholes are impossible, and why you can not create a single magnetic monopoles:

What about conservation of momentum? If an object is pulled to where the other object is expected to be, not where it is, then momentum exchange between the two of them would seem to not add up!

But it works because you can never create a gravitational or electric field from nothing. You can only move them around, so the momentum always catches up.

What about a magnetic field? You can create those from nothing - ah, but you can only create magnetic dipoles, with opposing fields, so again it works.

But, you can never create a magnetic monopole because you would suddenly have a magnetic "charge" where none existed before and the momentum would not add up. What you can do is create two monopoles, of opposite poles. (So this implies a conservation of magnetic pole, just like conservation of electric charge - assuming monopoles exist.)

What about wormholes? They have the same momentum problem - an object suddenly appears where none was before and it pulls on other objects. You could then move it away before it gets pulled in turn by those objects, and that would violate conservation of momentum. So you can't do that.

And, faster than light objects would have the exact same problem - they could move out of the area before they properly shared the momentum. So I suspect they can't exist either. (Unless there is some complicated math I didn't think of which "fixes" it.)

The linked article directly disagrees with this post (except the last paragraph, which itself disagrees with the rest of the post), however:

> The fact that gravitational damping is measured at all is a strong indication that the propagation speed of gravity is not infinite. If the calculational framework of general relativity is accepted, the damping can be used to calculate the speed, and the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%. (Measurements of at least one other binary pulsar system, PSR B1534+12, confirm this result, although so far with less precision.)

This was always my problem with RobotRollCall on Reddit - no one could challenge anything she wrote lest they be downvoted into oblivion. Furthermore, she never took criticism very well, and would assert authoritative dominance over the topic of knowledge (how dare you disagree with me, I read research papers!).

Her explanations were good, but they weren't ever very complete and while they were well written, they weren't always accurate.

I want to point out that Wolfram doesn't think the universe is a giant cellular automaton. He gives CA as examples of how easy it is to produce impermeable complexity. His more serious proposal is a graph-based system, though I haven't perused enough to know if that's also for the sake of example or if it's intended to be taken as a serious physical theory. Most likely it's an earnest attempt at a heuristic towards the goal of, in his terminology, "finding the program of the universe."