WTF is the speed of gravity

[KrazyHorse]

Originally posted by Sagacious Dolphin
I once asked my school teacher if there was a magneto-gravitational force. A counterpart to gravity such as magnetic forces are to electical ones. He didn't have a clue what I was talking about - he was just a biology teacher trying to impersonate a physics teacher. .

The first thing all teachers should say when explaining EM:

"Magnetism does not exist. It's all electicity with some terms added in for rel. corrections"

[KrazyHorse]

Originally posted by SlowwHand
http://www.ldolphin.org/vanFlandern/gravityspeed.html

That site appears to be bollocks.

[dunk]

My high school physics teacher was a math major.

He once told us that a light year was, in fact, a measure of time.

Very cool experiment. Fascinating stuff.

[dunk]

Originally posted by Frogger


That site appears to be bollocks.

Worked for me.

I'll try to explain it.

The speed of gravity is the speed at which the force propagates through space.

Let's assume that the sun's mass is constantly changing.

The Earth is 8 light minutes from the sun. In other words, it takes light 8 minutes to reach the Earth from the sun.

Now I'll say the sun's mass is 10 kg at 8:00 AM. Bogus, I know, but i don't feel like using huge numbers. That means at 8:08 AM, the Earth experiences the sun's gravity as if it were 10 kg. When, in fact, the mass of the sun at 8:08 AM may be 9.9 kg.

This assumes that the speed of gravity is equal to the speed of light.

[KrazyHorse]

Thanks. I'm a grad student in physics.

What I meant was that the author of the site Slowwhand posted has some "interesting" viewpoints regarding the speed of propagation of gravity.

[TheStinger]

Originally posted by dunk


Worked for me.

I'll try to explain it.

The speed of gravity is the speed at which the force propagates through space.

Let's assume that the sun's mass is constantly changing.

The Earth is 8 light minutes from the sun. In other words, it takes light 8 minutes to reach the Earth from the sun.

Now I'll say the sun's mass is 10 kg at 8:00 AM. Bogus, I know, but i don't feel like using huge numbers. That means at 8:08 AM, the Earth experiences the sun's gravity as if it were 10 kg. When, in fact, the mass of the sun at 8:08 AM may be 9.9 kg.

This assumes that the speed of gravity is equal to the speed of light.

Thankyou.

[TheStinger]

I really like physics but my maths can't keep up

[Lefty Scaevola]

Re: Re: WTF is the speed of gravity

Originally posted by Frogger
The speed at which the force of gravity propagates.
Both GR and standard quant field theory would predict that it equals the speed o light.

Do they theorize it alwys to be the speed of light in a vaccum, or is it suposed to be slowed passing through other media as light is so slowed?

[Phil_de_geezer]

I can't for the life of me figure out how they are going to test this.

Astronomers hope to be able to measure this (light) displacement - which depends on the speed of gravity.

Surely this measures the strength of the field that the light is passing through?

[dunk]

That's what I think Phil. If the speed of gravity is greater than the speed of light, then the displacement will be visible sooner than if it is equal or less than.

[Phil_de_geezer]

If the speed of gravity is instantaneous, does it mean that the light will be bent sooner, therefore producing a greater displacement? That doesn't make much sense to me because Jupiter is constantly 'bending' light because it is constantly exerting a gravitational force.

Surely the only way to properly test it is to create a mass and then see how long it takes for it to attract something. Or am i completely nuts?

[devilmunchkin]

i didn't think gravity had a set speed since it was always accelerating till the object collided with something

[MrWhereItsAt]

Stellar explosions and other such cosmic events could potentially create gravity waves (if such things exist). Perhaps they aim to measure the speed of these somehow? If the waves had the speed of light, the light from the event would appear at the same moment as the gravity wave was detected, else there would be a time difference.

Is this part of the research into gravitational waves at Australian National University in Canberra, by any chance (being another Physics grad I need to take an interest in these things )?

[Lemmy]

i didn't think gravity had a set speed since it was always accelerating till the object collided with something

that would be the speed of an object under influence of gravity (in a vacuum).
The speed of gravity is...well, the speed of gravity

[Ned]

Interesting article on a number of points.

Point 1. The speed of light relative to a moving observer is not constant. Because matter is a wave, the clocks change to make it appear constant.

Point 2. If gravity has a speed, it is much, much greater than the speed of light.

Point 3. The postulate than nothing can propagate faster than the speed of light is false.