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

## 38 posts in this topic

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
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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
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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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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
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Scientists should define is vacuum energy less closer to big mass. If so then object , increasing space/time and increasing its energy , reduces vacuum energy ( falling in gravity).

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I never have quite bought into the rubber sheet analogy. It suggests to me that spacetime and a massive object are separate entities that interact with each other. It just seems to me more plausible that the massive object is a fluctuation of spacetime that has caused spacetime to contract around that fluctuation drawing in the surrounding spacetime making the area around the massive object or fluctuation, for lack of a better analogy, less dense causing any other massive object to alter it's course through spacetime.

Edited by Pugdaddy
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I never have quite bought into the rubber sheet analogy. It suggests to me that spacetime and a massive object are separate entities that interact with each other.

That is what the theory says.

If you have an alternative model, please feel free to post the mathematics and the evidence in the Speculations forum.

Edited by Strange
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I have read somewhre regarding this topic and I come across this information.

Gravity is a weak force. The two objects will have an attraction to each other. But it will be so weak in comparison to other factors and to the attraction to the ground that the attraction to each other will not be noticeable. There is a concerted effort to obscure our understanding of gravity. So we do not see much research on the attraction between small objects.

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I have read somewhre regarding this topic and I come across this information.

Gravity is a weak force. The two objects will have an attraction to each other. But it will be so weak in comparison to other factors and to the attraction to the ground that the attraction to each other will not be noticeable. There is a concerted effort to obscure our understanding of gravity. So we do not see much research on the attraction between small objects.

Gravity is weak at human scales - the usual example is that a fridge has enough attraction to the fridge door to overcome all the mass of the earth; however gravity does not come in two polarities that are opposite and cancel out and as such is always attractive and does not get shielded this means it works on cosmological scales where the other forces tend to be unimportant.

The force between two objects of a human scale is not noticeable in everyday life - but it is measureable Henry Cavendish was measuring the gravitational attraction of two objects in his lab over 200 years ago

There is no concerted effort to obscure our knowledge of gravity. There is ignorance and there is culpable ignorance - if you had the will you would be able to download for free (and legally) texts/lessons from the very basics on places like Khan, through tougher stuff on edx.org etc, and finally to graduate level with Feymann's courses and Sean Carrol's lecture notes on GR.

There are huge numbers of programs dealing with measuring gravity at the human level - G Newtons Gravitational Constant is not known to high precision so there is lots of work to try to lower the error margins on this very important constant

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So we do not see much research on the attraction between small objects.

What about all the direct measurements of G performed by Cavendish and many others?

What about the CERN Alpha project that aims to test the gravitational effect of anti-matter using a small number of (anti) hydrogen atoms. You don't get much smaller than that.

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What about the CERN Alpha project that aims to test the gravitational effect of anti-matter using a small number of (anti) hydrogen atoms. You don't get much smaller than that.

Perfect example - I was trying to think of a high profile one but couldn't get my mind off the Watt Balance at NIST and wasn't sure that was as relevant as I thought it was

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Good answer not sure I could have said it better myself.

Lets look at another basic definition.

Thanks for the new insight.

Gravity is weak at human scales - the usual example is that a fridge has enough attraction to the fridge door to overcome all the mass of the earth; however gravity does not come in two polarities that are opposite and cancel out and as such is always attractive and does not get shielded this means it works on cosmological scales where the other forces tend to be unimportant.

So can follow this information right?

The force of gravity is proportional to the size of the object (mass) and your position relative to it (distance from the center of gravity).

Here is the formula:

g=-GM/(r*r),

where, g is the acceleration of gravity,

G is the gravitational constant,

M is the mass of the object and

r is the radius or the distance of the observer from the center of gravity.

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Thanks for the new insight.

So can follow this information right?

The force of gravity is proportional to the size of the object (mass) and your position relative to it (distance from the center of gravity).

Here is the formula:

g=-GM/(r*r),

where, g is the acceleration of gravity,

G is the gravitational constant,

M is the mass of the object and

r is the radius or the distance of the observer from the center of gravity.

That is the acceleration due to gravity (as you say) - the force is proportional to your mass, the objects mass, and the distance between

$F = \frac{GMm}{r^2}$

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