# The Naked Scientists Forum

### Author Topic: How do we know that gravity is an attractive force?  (Read 8386 times)

#### Aidan

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##### How do we know that gravity is an attractive force?
« on: 27/02/2008 23:43:44 »
Just wondering how do we know for sure that gravity is an attractive force and not a repulsive force that repels matter and is part of the space vacuum and time is actually linked to matter and not space.
Would the universe be any different. [?]
« Last Edit: 04/03/2008 23:18:39 by chris »

#### DoctorBeaver

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##### Re: How do we know that gravity is an attractive force?
« Reply #1 on: 28/02/2008 23:30:58 »
We can see that gravity is an attractive force by the fact that objects are drawn together (in the absence of any repulsive force). Were gravity a repulsive force then the direction of the objects would be random or, if the repulsive force is equal from all directions, they would remain stationary.

I think time dilation at relativistic speeds rules out time being linked to matter. However, 1 of our resident illustrious & highly knowledgable physicists can probably shed more light on that. In fact, they could probably shed more light on both parts of your question.

#### lightarrow

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##### Re: How do we know that gravity is an attractive force?
« Reply #2 on: 29/02/2008 13:33:34 »
Just wondering how do we know for sure that gravity is an attractive force and not a repulsive force that repels matter and is part of the space vacuum and time is actually linked to matter and not space.
Would the universe be any different. [?]

If you mean that gravity could be a force between void's virtual energy/particles, more than between normal energy/matter, than I think it could be a repulsive force. This should happen if void's energy density would be greater than matter's energy density.
Not sure of all this however.
The interesting thing, apart from the high void's energy density, would be that gravitational force would be similar to coulombian force in beeing repulsive for two masses of the same kind, that is both positive (we don't know negative masses).
Interesting idea! a +1!
« Last Edit: 29/02/2008 13:40:53 by lightarrow »

#### DoctorBeaver

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##### Re: How do we know that gravity is an attractive force?
« Reply #3 on: 29/02/2008 15:40:02 »
hmmmph!  [:(!]

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##### Re: How do we know that gravity is an attractive force?
« Reply #4 on: 29/02/2008 17:20:25 »
if the repulsive force is equal from all directions, they would remain stationary.

If gravity was a repulsive force, or an external force and equal in all directions then maybe an atoms electrons could absorb (in a sense) some of that incoming force and perhaps this is what keeps atoms together and gives electrons their momentum. It would then mean that an external Gravitational compression or force would then be the binding force and not any attraction from an (as yet) unfound and undetected gravity force in the atom.

If you had 2 masses close together they would still look like they are attracting but in the space between the masses the gravity pressure would be less than that on the outside. It would then act a bit like the casimir effect but on a larger scale. The observations would be exactly the same and it would still look like both masses are attracting each other.

However, that would mean one less force in the make up of the atom, I think!

#### lightarrow

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##### Re: How do we know that gravity is an attractive force?
« Reply #5 on: 29/02/2008 20:06:14 »
hmmmph!  [:(!]

It's because you don't like the idea?

#### DoctorBeaver

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##### Re: How do we know that gravity is an attractive force?
« Reply #6 on: 29/02/2008 21:22:59 »
hmmmph!  [:(!]

It's because you don't like the idea?

No. Because you're so damned clever!

#### DoctorBeaver

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##### Re: How do we know that gravity is an attractive force?
« Reply #7 on: 29/02/2008 21:25:40 »
if the repulsive force is equal from all directions, they would remain stationary.

If gravity was a repulsive force, or an external force and equal in all directions then maybe an atoms electrons could absorb (in a sense) some of that incoming force and perhaps this is what keeps atoms together and gives electrons their momentum. It would then mean that an external Gravitational compression or force would then be the binding force and not any attraction from an (as yet) unfound and undetected gravity force in the atom.

It is electromagnetism and the weak force that keep an atom and its components together, not an undetected gravity source

#### Soul Surfer

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##### Re: How do we know that gravity is an attractive force?
« Reply #8 on: 01/03/2008 15:06:09 »
It has been suggested that one way of looking at the gravitational force is like a sort of casimir effect where the presence of matter restricts the vacuum states available in the vicinity of the matter and therefore creates an effective attractive force between the masses.

The net results are the same under normal conditions so most of the universe would look just the same however there is one critical difference.  (it would of course have to look like that because if it didn't it must be wrong because it doesn't predict the universe to be the way we measure it)

To come back to the critical difference in extreme conditions.  The inverse square law sets no limit to how strong the force may be provided you can get dense enough material.  a "differential" force would have a limit where there is the presence of so much matter that it excludes all possibilities in the space and creates what might be called a true vacuum as opposed to the quantum mechanical vacuum of normal space.

#### Aidan

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##### Re: How do we know that gravity is an attractive force?
« Reply #9 on: 03/03/2008 23:35:23 »
Thank's for all your replies on my Gravity Question.
I asked the question because its 325 years since Newton published his theory of gravity 93 years since Einstein published General relativity and since then an awful lot of not much in the way of progress so there comes a point when maybe a different approach has to be taken or ruled out as may be the case.
String theory seems very nice but seeing as it can be made to explain almost everything and predict nothing.
So maybe we should check our Foundations and Signposts just to make sure we haven't mistaken a mirror image for reality.

#### DoctorBeaver

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##### Re: How do we know that gravity is an attractive force?
« Reply #10 on: 04/03/2008 08:05:45 »
There has been quite a lot of progress, much of it refining our knowledge and confirming relativity & QM.

However, the problem is that new theories outstrip the technological means to verify them. We need to probe distances and energy levels that are beyond our capabilities. We have probed around the weak scale (250MeV - 1TeV) but to probe the Planck scale (where gravity is theoretically strong again) we need 10 thousand trillion times the energy of the current largest particle accelerator. That is obviously way beyond reach.

But there is hope. The LHC at CERN will be online soon and that will probe even greater energies. Although it won't get anywhere near the Planck scale, some theories of gravity have detectable consequences that could be found.

These experiments at CERN could help explain the huge disparity in mass/energy/distance between the Weak scale & the Planck scale (known as The Hierarchy Problem) which has baffled physicists for many years and casts some doubt on the Standard Model. It is even suggested that we may find evidence of higher dimensions.