# The Naked Scientists Forum

### Author Topic: Does gravity lag behind objects?  (Read 2650 times)

#### chris

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##### Does gravity lag behind objects?
« on: 22/02/2010 10:30:03 »

I understand that gravity is thought to propagate at the speed of light.

Consider two large bodies moving initially at the same high speed and
parallel to one another, close enough to exert a gravitational
attraction and far enough away from other bodies that they can be
considered isolated.

Now there will be a gravitational attraction between the two bodies drawing them together. But surely the direction of the vectors of attraction would lag slightly behind the actual position of the bodies if gravitational attraction has finite speed.

This would over a long period of time result in an overall slowing down
of the bodies.

-------------------------------------------------------------------

You could say that as the bodies moved closer together they lose
potential energy and gain kinetic energy. When they finally hit, the
energy becomes mostly heat and possibly some angular momentum. But you should still end up with a very large composite object moving at the same speed as the original ones.

Assuming gravity does have a finite speed, surely there are implications
for orbital systems as well....perhaps we would incorrectly estimate the
amount of mass in a far-off system as a result of this effect?

------------------------------------------------------------------

The above seems counter intuitive so have I made a mistake, what have I missed or is it just true that this is a known problem?

Thanks

Chris

What do you think?
« Last Edit: 22/02/2010 10:30:03 by _system »

#### LeeE

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• Posts: 3382
##### Does gravity lag behind objects?
« Reply #1 on: 22/02/2010 23:06:24 »
An interesting question

The first thing that comes to mind though, is that the two bodies share the same inertial frame of reference, so as far as they're concerned, they're both stationary and thus will simply move towards each other.

The second thing is that gravity doesn't radiate, like light from a light bulb.  When you turn a light on light starts radiating away from the light bulb and lights up the room, and when you turn it off the room becomes dark again but with gravity, as soon as there is any mass present there'll be a gravitational field: if gravity worked like light then just the presence of the light bulb would light the room and there'd be no turning it on or off.

Gravity waves are believed to propagate at the speed of light, but gravity waves are changes in gravity, not just the presence of gravity.  If you could teleport a lump of matter into some 'empty' space then a gravity wave would be produced when the matter suddenly appeared but you wouldn't get any more gravity waves until the mass changed e.g. when you teleported it away again.

Having said all that, a distant observer would see things a bit differently.  They would observe a gravity wave as the two masses zipped past them, but they'd also see a few other phenomena too, including the possibility that both masses, if traveling fast enough, might have apparently collapsed into black holes due to foreshortening increasing their apparent density, combined with relativistic mass increase.  They might see some relativistic frame dragging effects too.

#### yor_on

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##### Does gravity lag behind objects?
« Reply #2 on: 23/02/2010 04:48:59 »
Depends? don't it?
Not what LeeE wrote, but you can have different scenarios.

Like both objects accelerating uniformly at the same velocity.
Then you would also have a frame at rest with each other, but the gravity well would now be 'displaced' behind them (the rockets), and any vectors drawn would point to a place behind your combined frame depending on acceleration, as i understands it that is.

You can also imagine a non-uniform acceleration done by you both simultaneously with the same velocity. You would still be at rest relative each other and the gravity well would still be 'displaced'. The point being that SpaceTime would look different depending on choice of acceleration according to the equivalence principle, and some math:)

If you're traveling in a free fall it becomes very tricky to me. Then no matter your relative velocity (you comparing yourself to some other frame) you would still only be attracted by your common mass together, and the vectors would point directly to you, not lagging behind you any more. But then there also will be the question of 'relative mass' aka 'momentum' aka 'potential energy' that have a relevance to how fast you might be 'attracted' to each other.

As I see it there will be no difference, no matter the 'velocity' of your free fall, for how fast you will 'drift together' by gravitational forces, as long as you are at rest relative each other (being in the same frame of reference)

And so the question of 'potential energy' is resolved, (for me that is:) and the same goes for your 'momentum' and 'relative mass'. But I might be wrong there.

A very nice question Chris.
« Last Edit: 23/02/2010 04:50:45 by yor_on »

#### yor_on

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##### Does gravity lag behind objects?
« Reply #3 on: 25/02/2010 02:51:36 »
Thinking of your question "Now there will be a gravitational attraction between the two bodies drawing them together. But surely the direction of the vectors of attraction would lag slightly behind the actual position of the bodies if gravitational attraction has finite speed."

It's a good one, but if there is a field and that field contain objects 'at rest' with each other there is a 'balance', maybe? I'm not saying that it is so, but maybe those objects would be seen as laying in a 'form' of some sort, and only needing to 'update' their information as something disturbed the equivalence/equilibrium?

Can you see how I think :) But that would also mean that any frame, wherein you have objects at rest versus each other, in its own way also would have an 'gravitational entanglement' of sorts. But it would be a efficient way of saving on 'information/energy' if it would be so, as the only 'updates' needed would be when something changed, disturbing/breaking the equilibrium.

Looking at it that way you might also say that when it comes to being 'at rest', whether you're uniformly or non-uniformly accelerating, there would also be that field only needing to be updated when something changed between the objects. But it would lead to the conclusion that there is a enormous difference between accelerating and uniform motion.

No matter your speed, as I see it, all free falling objects in SpaceTime would then have the same 'frame' if so, no matter their velocity. But when we look at uniformly accelerating, we would find an infinite fold of frames, each one containing objects (rockets) at rest versus each other but all at a different (uniform) acceleration, but it would be as valid for non-uniform acceleration too as I see it as long as the objects kept to the exact same velocity simultaneously. Also it would mean that a frame would be shared 'instantly' with all other objects having the same velocity in SpaceTime.

Or maybe this relation would break down with acceleration?
It's just a thought, but no weirder than 'ordinary entanglements' to me :)
And as valid as there would be no information sent until one object broke out of the frame, and the information would have to obey 'c' so for objects to far away that information might never reach you.
==

In fact you would then get a very 'Copernican view' of gravity as all information of what is happening would lay 'dormant' in every point in SpaceTime, updating all other points only as needed only when changing. It suits my 'spiderweb concept' as well as inertia, also it seems very 'information/energy conserving'. I think I almost might be correct even  but i don't see how to test for it?:)
==

And the reason why your rockets would drift together:)
They're invariant mass, and therefore they will bend the geodesics of SpaceTime between them. That would mean that the 'web' I'm thinking of is a 'whole' but that invariant mass in its own way will act as 'attractors'. And as for uniform acceleration the vectors would be slightly angled due to two concepts working together, acceleration and invariant mass.
==

And then from there you could define an idea perhaps where..
Heh, I'll save that one to my essay :)
Yep.

« Last Edit: 25/02/2010 03:58:43 by yor_on »

#### yor_on

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##### Does gravity lag behind objects?
« Reply #4 on: 25/02/2010 20:06:13 »
Yep it would introduce 'far action'. don't really know about that :) I'll have to ponder some more here. But doesn't 'entanglement' already do that , even if from an 'quantum level'. and we don't really know what 'gravity' is, do we?

#### Robro

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##### Does gravity lag behind objects?
« Reply #5 on: 26/02/2010 06:55:57 »
Hi, I tend to think that gravity is a word to describe the effect of EM fields on photons and so I do not think that there would be an instant effect between the two bodies. Being that, suppose two large massive bodies (1 Sol mass) were in space traveling parallel to one another at a distance of 1 AU. If one were to suddenly vanish, the gravitational effect would be experienced by the remaining body until all EM radiation from the vanished body passed by. Then the remaining body would be free of the former gravitational effect.

#### yor_on

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##### Does gravity lag behind objects?
« Reply #6 on: 26/02/2010 07:02:58 »
If you read the proposal you will find that I agree with you, even though I don't know if gravity is any 'energy' we ever is going to manipulate as easy as we do EM.

The 'far effect' is not in any way free from that propagation, you can see it as you freeze something, to change the 'mold' there will be needed new information, and that information will obey 'c'.
==

You see, my view builds on emergences, and in that view you will have different relations, and as I expect 'global relations', depending on your frame, with the possible exception of acceleration that might introduce a local effect only :)

You can see it this way, accelerate and you momentarily change relation with SpaceTime, In fact, you're traveling in time relative the rest of the universe, it being in a 'free fall'. Change planet (Invariant mass) to a neutron star and as long as you survive you will do the same. Both seems to manipulate time, but all free falling bodies (uniform motion), if it now is a planet or a neutron star, still belong to that same frame of 'uniform motion' where no energy is spent, as far as we know.

Normal particles don't seem to need any energy being what they are either? ('virtual particles' is another thing though, you could call them 'frame dependent' as they might be real for an uniformly accelerating 'Rindler' observer, who in that motto can be seen to spend energy in exchange for observing that phenomena) They (normal 'constantly existing' particles) can 'spend energy' though (radioactivity). But acceleration and all other 'interactions' will spend energy, create shortlived new particles and transform 'work', if we look at it entropically , into 'work done'.

Wow, that was a long one :)
And probably I will have to correct it somewhere too ::))

Acceleration and invariant mass seems to represent two opposite poles to me, both having an effect on the time rate you will observe 'thingies' to have outside your own frame of reference. So when I look on it, I think that the way we might change times arrow is by acceleration (uniform, most probably) whereas invariant mass seems to hold a sharp line between itself and human survival.

Weird, ain't it.
« Last Edit: 26/02/2010 07:47:10 by yor_on »

#### The Naked Scientists Forum

##### Does gravity lag behind objects?
« Reply #6 on: 26/02/2010 07:02:58 »