# The Naked Scientists Forum

### Author Topic: Does gravity "travel" at the speed of light and how is it propagated?  (Read 10091 times)

#### ...lets split up...

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##### Does gravity "travel" at the speed of light and how is it propagated?
« on: 16/10/2008 12:32:40 »
I was just wondering. Let's say the entire universe consisted of a rock in space, one rock just floating around, and then 'Poof!!', there's another peice of matter making it a total of two objects in the universe. It's a flashlight with batteries and it's switched on and pointing at the rock.

Does the force of gravity between them instantly happen or does the light hit the rock first? Is gravity instant or is there a speed to it?
« Last Edit: 16/10/2008 16:51:13 by chris »

#### LeeE

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##### Re: Does gravity "travel" at the speed of light and how is it propagated?
« Reply #1 on: 16/10/2008 16:05:40 »
Gravity appears to propagate at the speed of light - the effects of gravity would first become apparent at the same time the light hit the rock.

#### lightarrow

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##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #2 on: 17/10/2008 17:23:08 »
I was just wondering. Let's say the entire universe consisted of a rock in space, one rock just floating around, and then 'Poof!!', there's another peice of matter making it a total of two objects in the universe. It's a flashlight with batteries and it's switched on and pointing at the rock.

Does the force of gravity between them instantly happen or does the light hit the rock first? Is gravity instant or is there a speed to it?

The gravity is already present in the point the flashlight appears, so obviously the light arrives after. But if now the rock moves, the variation of the gravitational field propagates with light's speed, as LeeE wrote.
« Last Edit: 17/10/2008 17:24:41 by lightarrow »

#### lyner

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##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #3 on: 17/10/2008 22:32:59 »
by the Mach principle, the sudden introduction of a large mass into the system is a bit of a problem. If it is introduced by some sort of energy transform then it would involve it's own huge flash and would take time. This would imply a low pass filtered step change. When would you start to notice the new arrival by its gravitational field.

Is this scenario valid?

« Last Edit: 18/10/2008 23:58:44 by sophiecentaur »

#### ...lets split up...

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##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #4 on: 18/10/2008 11:15:49 »
Thanks, i needed the answer for something but i can't remember why. Will post when i do.

But on a more sci-fi note, if we had a perfect way of measuring gravity between any two bodies in the universe, and there were bodies beyond the limit to our universe (speed of light since big bang, i think?), then we would be able to sense them with gravity if they weren't moving (as i understand by the post of 'lightarrow' earlier).

I know it's pretty much impossible, i just like the idea.

#### lightarrow

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##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #5 on: 19/10/2008 12:36:36 »
by the Mach principle, the sudden introduction of a large mass into the system is a bit of a problem. If it is introduced by some sort of energy transform then it would involve it's own huge flash and would take time. This would imply a low pass filtered step change. When would you start to notice the new arrival by its gravitational field.

Is this scenario valid?

Sorry, can you explain things more clearly? Thank you.

#### lyner

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##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #6 on: 20/10/2008 23:01:18 »
My thoughts are a bit of a jumble but doesn't Mach say that your mass and gravitational field rely on the existence of the rest of the Universe? That would involve some strange effect if another object were to be suddenly introduced into this universe.
I suggest that this additional object would have to result from the conversion of a lot of energy - some sort of mini-bang. The rate of energy to mass conversion would govern the rate at which the new mass could be created and the rate at which the gravitational fields could change (the time profile of the step change).

I find these 'what if' questions very unsatisfactory because one can't tell how much of 'reality' needs to be part of the scenario and how much  fantasy.

#### lightarrow

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##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #7 on: 21/10/2008 08:26:04 »
My thoughts are a bit of a jumble but doesn't Mach say that your mass and gravitational field rely on the existence of the rest of the Universe? That would involve some strange effect if another object were to be suddenly introduced into this universe.
That's interesting. Furthermore, it means that an object's (inertial) mass would not depend only on the number of protons/neutrons/electrons ecc. in the object, so even a single electron's mass, and so its rest energy, would depend on the presence of other objects. Very strange. Maybe equivalence principle, that is inertial mass = gravitational mass hold only in a universe with a fixed number of objects? Because in that case it could be postulated that it's a particle gravitational mass which doesn't vary.
Quote
I suggest that this additional object would have to result from the conversion of a lot of energy - some sort of mini-bang. The rate of energy to mass conversion would govern the rate at which the new mass could be created and the rate at which the gravitational fields could change (the time profile of the step change).

I find these 'what if' questions very unsatisfactory because one can't tell how much of 'reality' needs to be part of the scenario and how much  fantasy.
Yes, but it's interesting to speculate about it...

#### lyner

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##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #8 on: 21/10/2008 10:44:08 »
Please can I be excused now and go back to Newton's Laws? My brain has started to ache.

#### lightarrow

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##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #9 on: 21/10/2008 16:35:37 »
Please can I be excused now and go back to Newton's Laws? My brain has started to ache.
Ok, don't want to be responsible for a useful brain damage...

#### ...lets split up...

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##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #10 on: 21/10/2008 17:57:03 »
i too have lost cabin pressure...

#### ...lets split up...

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##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #11 on: 21/10/2008 18:12:43 »
Management just sent a message asking me to pose the title of my posts as questions, i couldn't understand why they kept changing them, but they sounded better in the end so i thought "gee, these guys are so nice"

little did i know it was bugging them
« Last Edit: 21/10/2008 18:14:19 by ...lets split up... »

#### The Naked Scientists Forum

##### Does gravity "travel" at the speed of light and how is it propagated?
« Reply #11 on: 21/10/2008 18:12:43 »