OT: Gravity, Dark Energy, Universal expansion - Page 6

Suicide Junkie · #51 January 9th, 2008, 03:25 PM

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 · #52 January 9th, 2008, 06:56 PM

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 · #53 January 9th, 2008, 07:37 PM

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 · #54 January 9th, 2008, 08:26 PM

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 · #55 January 9th, 2008, 08:55 PM

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.

narf poit chez BOOM · #56 January 9th, 2008, 10:17 PM

It wouldn't be that hard to predict the suns' position in eight minutes.

So. If gravity is found to travel at c and the earth orbits the suns' actual position...Could gravity be 'connected' to the object it came from?

Raapys · #57 January 9th, 2008, 11:18 PM

I'm confused. So there's both experiments that show that gravity is indeed travelling at the speed of light( or close to it ), and actual observations of the opposite?

Then that would indicate, as narf says, that there's some sort of connection to the object it came from. Perhaps gravity 'predicts' where the sun is going to be at the moment it reaches earth?

So if something was to suddenly push the sun out of its original and predictable path, it might take a while before gravity would catch up and start pulling us in the right direction again.

narf poit chez BOOM · #58 January 10th, 2008, 02:31 AM

That's what people are saying. I have little knowledge of the situation - Just trying to make sense of it.

capnq · #59 January 10th, 2008, 10:22 AM

Quote:

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 · #60 January 10th, 2008, 10:41 AM

One of the most basic assumptions of relativity is that all inertial reference frames are equally valid. Simply choose your inertial reference frame to be one where the sun is motionless, and you can trivially show that the speed of gravity makes virtually no difference to the influence of the sun on any of its planets. Working out how this is compensated for in other reference frames is a bit more work, but the conclusion is guaranteed unless relativity is wrong.

For the speed of gravity to have a significant effect on gravitational interactions, both masses must be large enough to have significant effects on each other so that both have significant acceleration and no inertial reference frame exists where either body is reasonably close to at rest.