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Author Topic: What ever is gravity and how does it work?  (Read 3160 times)

Offline Lynda

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What ever is gravity and how does it work?
« on: 20/03/2011 20:06:46 »
I realise gravity is a means of attracting objects to each other - we stay put on this planet due to gravity - so does most other things.    What I would like to know firstly is what causes it?

I would also like to know how objects of huge distances away can attract each other.  We have planets orbiting the sun (with moons orbiting the planets), solar systems rotating in a galaxy, galaxies rotating in a galaxy cluster, galaxy clusters rotating around each other.   How does that work?



 

Offline wolfekeeper

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What ever is gravity and how does it work?
« Reply #1 on: 21/03/2011 00:29:27 »
The theory that describes gravity is "general relativity" (GR), in GR gravity is due to curved space-time.

Ok, exactly what that means comes in the form of a weird equation, but as a fairly good analogy, imagine you've got a rubber sheet, and you put ball bearings on it. The rubber sheet will curve and the ball bearings will roll together until they touch.

That's kinda like how space is around mass, but the mass generates the curvature directly, and it's all in 3D rather than 2D, and at 90 degrees to space isn't thin air, it's the time axes, and that's why they moved together, because the time axis points inwards around masses, so things progressively move together even without them doing anything.

I hope that's not too weird and confusing, but then again, GR is weird and confusing!

And the answer to your question is- galaxies and so forth move together because they curve space for each other, and so the time axis points inwards.
« Last Edit: 21/03/2011 00:31:27 by wolfekeeper »
 

Offline Bill S

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What ever is gravity and how does it work?
« Reply #2 on: 22/03/2011 18:10:44 »
Lynda, perhaps you were also asking where the energy for all this comes from. 

There are various lines of reasoning that maintain that no energy is needed. I'm not entirely happy with this, so have done quite a lot of thinking.  However, I'm not a scientist, so the point I have reached is not "main stream". 

If, standing on the surface of the Earth, you pick up a stone you impart gravitational potential energy to it, which takes it back to the ground when you let go.  I think the Big Bang may have imparted similar energy to the entire Universe.  If this is so, then every particle, and quantum of energy, has enough gravitational potential to bring it back into contact with every other.  GR shows us that the way back is via distortions of spacetime.

As I said, this is not main stream thinking, so a large pinch of salt may be needed to go with it.  :P
 

Offline yor_on

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What ever is gravity and how does it work?
« Reply #3 on: 22/03/2011 18:29:11 »
Think of the universe Lynda. How do you see it? In time, right?

Everything we do, everything we observe we do through the 'arrow of time'. It points only one way for us, and all 'forces' we know of we describe through times arrow. What Einstein did was to unite time with the other three properties we have, length, width and height. The result we name as SpaceTime.

When you want to tell someone how to go to a place you more often than not include a time estimate of some kind in your description, don't you? :) In General Relativity you always include 'time' in your description. 'SpaceTime' has those four macroscopic property's and they interact with each other, so you push one and the other 'deforms', like 'speed' will contract space for the one speeding near light (Lorentz contraction).

When it comes to 'gravity' Einstein looked at acceleration, wondering if the effect you felt in a constant non-changing acceleration at one gravity could be the same as a 'real' gravity on Earth. He defined the equivalence between those two roughly as, when being in a enclosed room without windows, you would not be able to devise any experiment differing between being on Earth and the accelerating rocket. The idea is that if you can't find a way to differ between those two places in any experiment they should, or have too, be the same (ignoring tidal forces).

And that made gravity something of a mystery, it's one of the fundamental four forces but there is no way, yet, of proving it as a result of 'quanta'. Quantum Mechanics builds on the idea of 'discrete quanta' creating all we know, so if QM want to make a unified theory of everything we need a way to incorporate gravity in that idea too. And that's what the LHC. tries to find proof for now, looking for Higgs particles and gravitons, amongst other things.

Gravity according to Einstein, is no 'force', At least not in GR, but an expression of matter (and energy) interacting with space under the auspices of our 'arrow of time'. So 'matter tells space how to bend, and space tells matter how to move.' as I think Feynman expressed it. It's like a 'field' in some way, controlled by matter/energy as observed in time, but there is 'nothing' really there. In a way very similar to 'time' that also can exist without 'really' being 'there'. And as far as I know it is not a 'energy' even though gravitational waves may exist. But it is a matter of definition, and I lean to Einsteins idea of it as a 'SpaceTime' property, more than defined as 'particles' having a usable 'energy'. So in my world a 'gravity wave' should be a (local) deformation of SpaceTime, propagating at 'c', as nothing inside SpaceTime can move faster. A little like a bubble in melted glass can move inside the material, or like a shock wave can transfer inside a jello when you hit it.

But there are other ways to view it too.

So, what is gravity?
A good Q.

« Last Edit: 22/03/2011 18:33:20 by yor_on »
 

Offline Ron Hughes

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What ever is gravity and how does it work?
« Reply #4 on: 22/03/2011 20:35:19 »
It is interesting that the arrow of time points in only one direction and that the force of gravity points only in one direction. Think their might be a connection? If I could explain why a clock slows as it falls into a gravity well I could probably explain how gravity works.
« Last Edit: 22/03/2011 20:57:56 by Ron Hughes »
 

Offline yor_on

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What ever is gravity and how does it work?
« Reply #5 on: 22/03/2011 22:15:33 »
If you mean that there is a direct coupling between time and gravity Ron?
It's a interesting thought although 'times arrow', as experienced by anyone measuring it inside his own frame of reference, never differs. And you can't really measure that speeding rockets 'time' as all measurements you can do must be done inside your own frame of reference. That's also relativity's explanation to why you need to get that twin back to Earth before being able to prove a time dilation relative the twin that never leaved Earth.

So inside your own frame time always 'ticks' the same, according to the theory of relativity. It's only when comparing frames there becomes temporal discrepancy's as I understands it, but that is also one of the most confusing things I know :) How to see 'frames of reference', where they start and stop. Electrons relative neutrons for example? do they share the same 'frame of reference'? They should, as they make a atom, well presumably? On the other hand, different mass and motion should create different 'frames'? 
« Last Edit: 22/03/2011 22:17:29 by yor_on »
 

Offline Ron Hughes

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What ever is gravity and how does it work?
« Reply #6 on: 23/03/2011 19:28:02 »
I'm saying that if you know the time dilation anywhere in the Universe you should be able to calculate the gravitational potential at that point.
 

Offline yor_on

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What ever is gravity and how does it work?
« Reply #7 on: 23/03/2011 21:29:37 »
As seen from the frame observing right?
That's another werdie with it.

How the he* will I decide if my frame is 'zero time'. I mean, sure I can call it inertial and define it as uniformly moving? But? If all uniform frames can have a different time dilation, and they should, shouldn't they? Then mine too will have one. The question about arbitrarily defining mine frame as 'zero' has a Janus face in that even though we can't define a 'zero' anything in SpaceTime, we still know that all frames must have a different 'time dilation'. You see how I think? Somewhere it has to be true that there is a 'objective' truth in that even though we can't finalize that 'time dilation' it is clear from our reasoning around uniformly moving frames that they all must be different relative the 'time' observed from any other frame.

So all frames must have a undefined time dilation as I see it. The best we can do is to decide  a frame that we measure ourselves against. That we don't notice it, is this amazing thing of it only being measurable between 'frames of reference', and only proved when returning to your point of origin as the twin in the twin experiment.

So there is, there has to be, some objective measurement not related to frames if you reverse the logic I used. CMB (cosmic microwave background?) or something, because otherwise I shouldn't be able to see that it is so, as I think of it. But it's probably not so simple as CMB, could be SpaceTime as a whole that is the referee, the 'jello' as I call it, all of those 'dimensions' together? Or ?

It's weird.
==

The other way around it is to define 'time' as some sort of illusion, but that one I'm not quite ready for :)
==

The third possibility is to decide that all frames is unique, and that they all contain SpaceTime, uniquely represented for you. That solves the problem in that we then only have one reference, ever. That is your own unique frame of reference and everything we define will be defined from ourselves. The problem then becomes how we can communicate through those different frames, and why we never notice their 'differences'? That one we can solve using radiation aka light.

It's the constant defining how frames communicate.
Well, weird as it is :) Maybe it works?
anyway (looks around, slightly confused as he tries to remember what he just said :)

Philosophy ahoy :)
« Last Edit: 23/03/2011 21:44:15 by yor_on »
 

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What ever is gravity and how does it work?
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