OT: Gravity, Dark Energy, Universal expansion

[Suicide Junkie]

Raapys said:
I thought they'd just about concluded that even gravity was limited to c?

There are a number of observations that do indicate that:
http://en.wikipedia.org/wiki/Speed_o...measurement.3F


As MCT mentioned, however, the earth accelerates towards where the sun *is*, not where it was 8 minutes ago.
Otherwise the earth's orbit would be quite unstable, and we'd all go spinning off into space.

So, there seems to be a relativistic effect that mostly cancels out the lag for slowly moving things.

[Raapys]

Hmm, I don't get it. Why would the earth's orbit be unstable? I mean, the sun travels at a constant speed, doesn't it? And the gravitational pull is continous, even if there is 'lag'. So the worst thing I can picture is that the earth will be travelling a tiny bit closer to the sun on one side and a little further behind on the other.

[Suicide Junkie]

Equal and opposite reactions.

If the sun was perfectly still there wouldn't be a problem. But earth (mostly jupiter, due to mass&distance) pull on the sun too.

If the sun's gravity appears to be coming from where it was 8 minutes ago, that would be slightly in front of us, rather than directly opposite the center of mass.
Accelerating forwards would spin us out into deep space.

[Baron Munchausen]

No, the earth must be moving at the same rate as the sun in order to even be in orbit. So there is no danger of 'slippage' of that orbit due to the sun's movement over time. Remember that (as far as we know) the earth formed from the same cloud of gas and dust as the sun. It has had the same basic motion/momentum from the beginning. And we orbit the center of mass of the whole solar system, not the sun itself. (Yes, that center is probably about 6 centimeters from the center of the sun... )While it's true that they have not yet figured out how to prove that gravity propagates at the speed of light, your supposed 'insight' is a misunderstanding of orbital mechanics, not any sort of proof that it propagates faster than light.

[Suicide Junkie]

Its not the angular speed, but a phase shift that I was talking about.

Measurements of Earth's acceleration show that we are accelerating towards the Sun's current position (to the precision of the instruments), not its position 8 minutes ago.
If the acceleration isn't directly in line with the center of mass, then there will be an an angular acceleration component, which is bad news.

PS:
The center of mass is not 6 cm from the center of the sun, but about 500km according to some quick math.
1 AU makes for a very long lever

[Baron Munchausen]

The current position of the sun according to what measurement? If it's the visible position of the sun, it's towards where the sun was 8 minutes ago. Have they somehow measured that the earth is moving relative to where the sun is -- 8 minutes travel from its visible position? That would be big news if so, because it would be proof that gravity propagates faster than light. I'm pretty sure I would have heard about this.

And I didn't do any math about where the system's center of gravity is, I was just tossing out a number. The planets do have about 98 percent of the system's angular momentum, but the sun has 99.5 percent of the system's mass.

[Suicide Junkie]

I thought that was obvious given the context.

Basically the Earth's acceleration is not in the same direction as the incident light from the sun.

[capnq]

Raapys said: Why would the earth's orbit be unstable?

Because chaos theory gets involved, which I barely understand myself above the buzzword level. This astronomy abstract mentions chaotic orbits, but I'm already lost by the end of the first sentence.

[douglas]

That falls under the "no such inertial reference frame exists" clause, and the speed of gravity does matter for such systems. Technically the Sun is accelerating so no inertial reference frame has it perfectly at rest either, but the difference is small enough on the scale of the solar system that it hardly matters.

[Atrocities]

Even more to consider Link