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Author Topic: Does gravity ever vary?  (Read 3409 times)

Jim Firth

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Does gravity ever vary?
« on: 01/10/2011 12:30:02 »
Jim Firth  asked the Naked Scientists:
   
1) Excluding stars, star matter, etc, is gravity finite based on mass?

In particular I'm curious to know if another planet roughly the size of Earth must have the same gravitational properties. If not, what are the variables to be considered.

2.) Does the gravitation property ever change, (increase/decrease) and has Earth's gravity ever been different?

Jim Firth
Langley, BC Canada

What do you think?
« Last Edit: 01/10/2011 12:30:02 by _system »


 

Offline CZARCAR

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Does gravity ever vary?
« Reply #1 on: 01/10/2011 16:23:25 »
Planet density must be a factor?
 

Offline yor_on

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Does gravity ever vary?
« Reply #2 on: 01/10/2011 18:19:00 »
Are you wondering if gravity is a property of mass?

Maybe, at least it is coupled to mass. And, as far as physics can guess, most probably coupled to the idea of 'energy' too. That means that 'energy', like radiation is a prime example of, can express itself gravitationally. But then we have the idea of a 'momentum' too?

What happens with the momentum when I slow a photon in a BEC? Does it transfer into the BEC? What happens when I allow the 'stopped photon' to continue its propagation? From where does it gain its momentum?
 

Offline yor_on

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Does gravity ever vary?
« Reply #3 on: 01/10/2011 22:17:49 »
I have, of lately, come to consider it as being a 'still game' :)

And that is a very weird idea, but if we allow ourselves to imagine how that would be, then the question arise why we can't see it that way. Also it makes 'simplicity becoming complexity' into something different than when normally looked at, at least for me.

Because in a universe without 'moving parts' the linear causality chain we observe falls short as a explanation, and we will have to find new reasons for why we see that chain.


If you think of propagation you also think of motion, and motion seems very frame related to me. It may always be consistent from a observers frame, but when treated conceptually, using several observers description, you will need Lorentz transformations to, still conceptually, 'bind it' together.

But, in reality, whatever that is, there is no such thing as a 'conceptual distance'. There the distance you measure will be the one you have to complete, even if it changes with relative motion/acceleration.

When it comes to gravity and motion we have two cases. One is 'uniform motion' like a 'free fall' in where the gravity observable only can be the one you have 'intrinsically', well sort of, coupled to the matter you consist of, and then accelerations, in where the gravity observed is a, very local, direct consequence of you getting 'displaced' in time, eh, also sort of :), in your acceleration.

Both of those are consequences of you 'moving' in time, but they express themselves very differently.
==

It is also so that if you imagine a drum spinning in space, you will create a 'artificial' gravity, if you're inside that drum, even if it spins uniformly. Why that is is explained through the geometry of it, in that it constantly 'accelerates' you from your 'preferred direction', which is the geodesic you would take at any point of that drums 'dematerializing' in space, leaving you to your 'free fall' through space.

Even though each point of that drum is moving uniformly, they still create a gravity. So, it's not only accelerations in a 'straight line' that can make a 'gravity', it also has to do with the geometry of your situation.

Then we have mass, proper invariant restmass :)

That's what we normally consider the reason for a gravity.
« Last Edit: 01/10/2011 22:52:38 by yor_on »
 

Offline CliffordK

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Does gravity ever vary?
« Reply #4 on: 03/10/2011 06:12:42 »
If you look at the Grace Geoid Map of the Earth's gravitational field from space:



One of the first things one notes is that the gravitational field is not at all uniform.

Actually, there may be a couple of fallacies of measuring Earth's gravitational field from Space.  First of all, it ignores the elevation change differences from being on top of mountains and down in valleys. 

So, rather than showing as high gravity, the mountains might actually have lower gravity due to elevation and the greater distance from the center of Earth.  Earth also has a bulge in the middle, and more centrifugal force at the equator than the poles, tending to fling objects off of Earth, or reducing their weight due to gravity near the equator, but less so at the poles.

One of the fundamental tasks of the Grace Project is to measure changes in gravity over time.  It is sensitive enough to detect changes in the gravitational pull on Earth based on the annual freeze/thaw cycles in the polar region. as well as mass loss due to the gradual receding of glaciers.
 

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Does gravity ever vary?
« Reply #4 on: 03/10/2011 06:12:42 »

 

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