06-09-2002 17:27									#1
	WTF is the speed of gravity Lose 30 kilos (of popups)

Apparently boffins are going to test for it.

http://news.bbc.co.uk/1/hi/sci/tech/2238452.stm

Could somenone explain in simple terms if possible what it is.

I read a brief history of time and understood the title.

	06-09-2002 17:40									#2
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Uh... they're going to see if gravity has the speed of light. If it's faster, then, well, Einstein's theory needs to be reworked - if it's slower, the same.

I've always wondered what the speed of gravity is. Does gravity have to be necessarily a particle or wave based force (which can be blocked in one way or the other) if it does have speed? What if gravity is instant and effects immediately?

	06-09-2002 17:52									#3
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is it how quickly an object exrts its gravitational force and another object.

	06-09-2002 17:53									#4
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On Earth at Sea Level it's 9.8 meters/per second

	06-09-2002 17:55									#5
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quote: Originally posted by Sava On Earth at Sea Level it's 9.8 meters/per second

IIRC that is the acceleration of an object due to gravity i think thats different to the speed of gravity

	06-09-2002 18:01									#6
	Re: WTF is the speed of gravity Support Apolyton, buy GURPS/ Alpha Centauri

quote: Originally posted by TheStinger Apparently boffins are going to test for it. http://news.bbc.co.uk/1/hi/sci/tech/2238452.stm Could somenone explain in simple terms if possible what it is. I read a brief history of time and understood the title.

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.

	06-09-2002 18:03									#7
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Where are the Poly scientists when you need them? Frogger? Edit: Speak of the devil...

quote: IIRC that is the acceleration of an object due to gravity i think thats different to the speed of gravity

Yes, and it should be 9.8 m/s^2 .

	06-09-2002 18:03									#8
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I asked a teacher about the pseed of gravity when I was 16. He didn't know squat, because his formal training was as an elec. engineer.

	06-09-2002 18:05									#9
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I remember my maths teacher when i was 15 talking about light cones, considering half the class couldn't do basic trig, this was a bit brave.

	06-09-2002 18:06									#10
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I once measured the acc. due to gravity at sea level to within 0.0001% using a Kater's pendulum...

	06-09-2002 18:07									#11
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quote: Originally posted by TheStinger I remember my maths teacher when i was 15 talking about light cones, considering half the class couldn't do basic trig, this was a bit brave.

He would have been braver had he discussed 4-vectors or the stress-energy tensor...

	06-09-2002 18:09									#12
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quote: Originally posted by Frogger He would have been braver had he discussed 4-vectors or the stress-energy tensor...

He certianly would have lost me

	06-09-2002 18:21									#13
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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. .

	06-09-2002 18:24									#14
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Yeah, we had a biology teacher doing physics when i was 12/13 she was rubbish.

She would give you high marks for drawing the equipment well though. Wouldn't know about the laws of motion if they smacked her in the face

	06-09-2002 18:25									#15
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http://www.ldolphin.org/vanFlandern/gravityspeed.html

	06-09-2002 18:33									#16
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quote: 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"

	06-09-2002 18:36									#17
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quote: Originally posted by SlowwHand http://www.ldolphin.org/vanFlandern/gravityspeed.html

That site appears to be bollocks.

	06-09-2002 18:48									#18
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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.

	06-09-2002 18:53									#19
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quote: 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.

	06-09-2002 18:56									#20
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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.

	06-09-2002 18:56									#21
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quote: 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.

	06-09-2002 18:57									#22
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I really like physics but my maths can't keep up

	06-09-2002 19:30									#23
	Re: Re: WTF is the speed of gravity Remove this text

quote: 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?

	06-09-2002 19:53									#24
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I can't for the life of me figure out how they are going to test this.

quote: 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?

	06-09-2002 19:57									#25
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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.

	06-09-2002 20:10									#26
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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?

	06-09-2002 22:18									#27
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i didn't think gravity had a set speed since it was always accelerating till the object collided with something

	06-09-2002 22:33									#28
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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 )?

	06-09-2002 22:41									#29
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quote: 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

	07-09-2002 03:54									#30
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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.