What is gravity?

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Offline thedoc

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What is gravity?
« on: 24/08/2012 01:30:02 »
Brian Keaney  asked the Naked Scientists:
So what is gravity?

You describe it as a force causing everything to be attracted to everything else. Didn't Einstein describe it as the curvature of space-time? If that's right, how does Einstein 's explanation of gravity fit with your description of it as being the reason for the uneven distribution of matter in the universe?


What do you think?
« Last Edit: 24/08/2012 01:30:02 by _system »


Offline evan_au

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Re: What is gravity?
« Reply #1 on: 24/08/2012 12:47:34 »
According to Isaac Newton, gravity follows an inverse-square law, which means that it attracts close things more strongly than things far away. This causes matter to clump together in local masses that we call stars, planets and galaxies, with gaps between them (if not counteracted by high temperatures, supernovae, etc).

Einstein's description of curved space-time may be imagined as "dents" in a two-dimensional rubber sheet. When objects pass by each other, they are attracted into each other's "gravitational well". The classic inverse square law just describes how this curvature varies with distance; it does not contradict Einstein's description.

However, to actually merge two objects into one object, they must lose their relative kinetic energy and angular momentum. Exactly how this occurs is still a bit of a mystery, but infra-red observations with the upcoming James Webb space telescope may reveal some stellar systems where this is happening, and shed some light on the process.
« Last Edit: 24/08/2012 12:49:16 by evan_au »


Offline yor_on

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Re: What is gravity?
« Reply #2 on: 24/08/2012 12:56:19 »
I'm not sure what you mean by 'the uneven distribution of matter in the universe' Brian?

It depends on your perspective sort of. The space we see is fairly homogeneous and isotropic, meaning that it looks much the same from any point we might stand on, on Earth looking out. If you're thinking of the way matter clumps together into a solar system or galaxy then Evan have a answer :) As for how 'mass' as in matter once begun the main stream definition is particles clumping together exchanging their (relative  each other) momentum, kinetic energy into heat, fusing into bigger clumps of melted matter that then would attract even more particles as they 'dent' the 'space' around them.
« Last Edit: 24/08/2012 12:57:50 by yor_on »
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Offline Phractality

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Re: What is gravity?
« Reply #3 on: 24/08/2012 20:04:28 »
Gravity affects light, and we define space-time in terms of the trajectory of photons. There are ways to define space-time, so that a photon follows a curved path, but Minkowski space-time is more useful. The trajectory of a photon is the definition of a straight line in Minkowski space-time. In strong gravity, the trajectories of a pair of photons may be parallel for a time and intersect at another time. In flat space-time, parallel lines can't cross; in warped space-time they may. In flat space-time, the internal angles of a triangle must add up to 180, but in warped space-time, they generally don't.

Minkowski space-time is warped because gravity affects photons. The mainstream view is that gravity affects photons because space-time is warped. That amounts to sweeping the real question of gravity under the rug. Mainstream science has chosen to avoid asking the question of what gravity really is. Scientists who offer meaningful explanations of gravity tend to be shunned by the mainstream. Since this is the mainstream forum, and meaningful explanations of gravity are outside the mainstream, this is not the correct formum in which to look for the answer. Try the New Theories forum.
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