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Where does the energy come from when two objects gravitate towards each other?


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#21 Jonas Taelman

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Posted 15 February 2017 - 02:08 PM

It's known that all objects with a certain amount of mass generate gravity. This is because of Einstein's relitivity theory. Look at it like this:

Get at towel or a blanket and stretch it on all four sides. Now put a poolball on the blanket. What you see is that the blanket is bent.

If you put a smaller ball on the blanket you will see that the smaller ball will roll over to the heavier poolball because the blanket is more bent at the poolball's side.

 

The same thing happens in space. All objects, with mass, bend space (and time). So the more mass the object has, the more gravity it will have.

On earth we consider to have 1G, if the earth had 2x the mass it has now it would be 2G of gravity.

 

When two bodies attract eachother, think about the blanket. Space is bent around both bodies and they just "roll" inside eachother's bent area.


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#22 Delta1212

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Posted 16 February 2017 - 12:08 PM

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#23 mistermack

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Posted 18 February 2017 - 05:23 PM

I've never liked the rubber sheet picture. I don't think it reflects the reality of 3D spacetime distortion.

 

To understand gravity you need to understand what mass is. And what energy is. Since we're not there yet, we have to make do with models that can represent what's happening accurately, so that we can calculate what will happen. 

But they are still just models.

 

My own mental picture is that a massive particle is just a huge amount of wave energy, rotating around itself in a tiny spherical ball. 

When it's in a gravitational field, spacetime is pulled out of it's natural symmetrical shape, and the ball becomes slightly unsymmetrical, in the direction of the object causing the field. A bit pear-shaped. So the wave isn't following the same curve everywhere any more. And it's the lack of symmetry that makes the whole thing want to accelerate in the direction of the mass causing the gravitational field.

 

That's my mental picture at the moment. It's sure to be bollocks, but it keeps me happy till I hear one I like better.


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#24 Eise

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Posted 19 February 2017 - 11:33 AM

I nearly never post youtube links, but this one is worth it:

 

 


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#25 mistermack

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Posted 19 February 2017 - 02:40 PM

I like that better than the rubber sheet. It's very good. But it's carefully phrased at the end. That's "HOW" gravity makes things fall. 

It describes it well, but it doesn't say "why" things fall. 

 

It sort of transfers the question. Instead of "why do things fall?" you now need to know, "why do things want to go "straight" when the orientation of local space is constantly changing? Why don't massive particles follow the orientation of the local space, as it changes with time?

 

So exactly the same question remains unanswered, it's just been transcribed.


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#26 Strange

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Posted 19 February 2017 - 04:17 PM

So exactly the same question remains unanswered, it's just been transcribed.

 

So GR explains why Newtonian gravity works. But why does GR work?

 

That question will always be unanswered. If someone comes up with a new theory (let's call it "X1")  that explains "why" general relativity works then someone will just ask: "But why does X1 work?"

 

Eventually, there might be a new theory  (let's call it "X2")  that explains why X1 works. At which point, someone will ask: "But why does X2 work?"

 

Repeat for Xi (for i = 3 to infinity).


It describes it well, but it doesn't say "why" things fall. 

 

Actually, I have now watched the video. And you are wrong. It does explain perfectly why things fall in curved space-time. 


Edited by Strange, 19 February 2017 - 04:12 PM.

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#27 StringJunky

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Posted 19 February 2017 - 04:19 PM

 

It sort of transfers the question. Instead of "why do things fall?" you now need to know, "why do things want to go "straight" when the orientation of local space is constantly changing? Why don't massive particles follow the orientation of the local space, as it changes with time?

Because they have Inertia, which is resistance to change in velocity or position when stationary which is a function of their mass and/or momentum.


Edited by StringJunky, 19 February 2017 - 04:22 PM.

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#28 mistermack

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Posted 19 February 2017 - 06:09 PM

Because they have Inertia, which is resistance to change in velocity or position when stationary which is a function of their mass and/or momentum.

Yes, I thought of that. But change in position relative to what? Obviously not the space around the object. 

Is there some fixed set of coordinates that an object is stationary in?

If space in a gravitational field is curving with time, what frame of reference is there that is independent of that?

If there was a master frame of reference, that everything moved relative to, that you could get a fix on, it would all be a lot easier to picture. 


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#29 StringJunky

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Posted 19 February 2017 - 06:46 PM

Yes, I thought of that. But change in position relative to what? Obviously not the space around the object. 

Is there some fixed set of coordinates that an object is stationary in?

If space in a gravitational field is curving with time, what frame of reference is there that is independent of that?

If there was a master frame of reference, that everything moved relative to, that you could get a fix on, it would all be a lot easier to picture. 

The idea of stationary is dependent on  the observer co-moving with an object i.e moving in the same direction with the same speed. Along with motion, it's always relative to something. If you picture yourself on a rock in space and there''s nothing around you except another rock coming towards you... or are you moving towards it?  There's no way to know because an absolute reference doesn't exist.


Edited by StringJunky, 19 February 2017 - 06:47 PM.

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