# Thread: Gravity in a handful of dust?

1. The wisemen says that gravity is a very weak force, I was wondering if there's a limit before gravity kicks in, a "critical mass" so to speak.

2.

3. A single atom has gravity. Any mass has gravity. There is no limit in either direction, it's just a matter of how small the gravitational force is.

4. No there is no such threshold.

Any two masses, however small, at any distance, however large, attract each other with a nonzero force you can easily compute if only you know their masses and distance. Even if they are two electrons in two different gallaxies.

In a handful of dust, any two particles of dust attract each other too. It's not noticeable, and I think it's not even measurable, because the forces are so unimaginably small. They are, for example, much smaller than the attraction those grains get from the Earth (assuming you are on Earth with your handful of dust), or even the moon, or the random impact of air particles that imparts Brownian motion on the dust - but they are not zero.

If you take bigger masses - grains of salt, grains of poppy seed, grains of wheat, peas, plums, apples, melons, cows, houses, cities, continents, planets - the attraction will grow accordingly, and at some point it will become noticeable. Allow me to stress this word: noticeable. Not "nonzero". It was nonzero right from the start. And where that "point" is when you notice the attraction depends only on A) the precision of your observations and B) how much or how little you are being distracted by other forces acting on your masses.

Hope this helps,
Leszek.

5. @ Leszek Luchowski
Sorry it doesn't help, now that you look at gas it kinda bumps around and kinda repulses itself, and with the new hydrogene-salttaps you can compress a large amount in a small area.

So this repulsion force kinda imo contradicts gravity, the same with magnetism as it can both repuse and attract.

Besides, if you have some dust in space, it doesn't seem to gather itself nicely into easily collectable piles, but scatter as it had no gravity.

6. Originally Posted by HexHammer
@ Leszek Luchowski
Sorry it doesn't help, now that you look at gas it kinda bumps around and kinda repulses itself, and with the new hydrogene-salttaps you can compress a large amount in a small area.

So this repulsion force kinda imo contradicts gravity, the same with magnetism as it can both repuse and attract.

Besides, if you have some dust in space, it doesn't seem to gather itself nicely into easily collectable piles, but scatter as it had no gravity.
except for, you know, stars and black holes and such

7. Originally Posted by Arcane_Mathematician
Besides, if you have some dust in space, it doesn't seem to gather itself nicely into easily collectable piles, but scatter as it had no gravity. except for, you know, stars and black holes and such
Maybe I wasn't clear enough .."handful of dust" it scantly compares to big scale things like those you just mentioned.

8. Originally Posted by HexHammer
Originally Posted by Arcane_Mathematician
Besides, if you have some dust in space, it doesn't seem to gather itself nicely into easily collectable piles, but scatter as it had no gravity. except for, you know, stars and black holes and such
Maybe I wasn't clear enough .."handful of dust" it scantly compares to big scale things like those you just mentioned.
Originally Posted by HexHammer
Besides, if you have some dust in space, it doesn't seem to gather itself nicely into easily collectable piles, but scatter as it had no gravity.
Right... it will compact. It just takes more time. The repulsion isn't exactly a constant force, it's only a contact force in this situation. But stars and planets and asteroids and such all start out as just a bunch of dust. the process happens at all scales, the only difference between big and small scale is how much pressure there is, how much force there us.

9. Originally Posted by HexHammer
@ Leszek Luchowski
Sorry it doesn't help, now that you look at gas it kinda bumps around and kinda repulses itself, and with the new hydrogene-salttaps you can compress a large amount in a small area.

So this repulsion force kinda imo contradicts gravity, the same with magnetism as it can both repuse and attract.

Besides, if you have some dust in space, it doesn't seem to gather itself nicely into easily collectable piles, but scatter as it had no gravity.
There is nothing in the action of gases that contradicts gravity.

But gravity is not particularly important in the model that you propose. The action of gas molecules is dominated by the electromagnetic force, and in particular the repulsion between the electron clouds of molecules. In that setting the electromagnetic force is MUCH greater than the gravitational force.

10. Originally Posted by Arcane_Mathematician
the only difference between big and small scale is how much pressure there is, how much force there us.
And how much time there is..?

11. Originally Posted by Arcane_Mathematician
it will compact. It just takes more time.
Seems to contradict Susskind's statemen 16:04 Einstein's General Theory of Relativity | Lecture 1

12. Originally Posted by HexHammer
Originally Posted by Arcane_Mathematician
it will compact. It just takes more time.
Seems to contradict Susskind's statemen 16:04 Einstein's General Theory of Relativity | Lecture 1
You have totally misunderstood Susskind's point. He is simply describing a freely falling reference frame. The subject is the equivalence principle of general relativity and not the gravitational effect of the dust particles on other dust particles.

That part of the lecture has nothing to do with the agglomeration of dust in free space under the influence of their mutual gravitation.

13. Originally Posted by DrRocket
[You have totally misunderstood Susskind's point. He is simply describing a freely falling reference frame. The subject is the equivalence principle of general relativity and not the gravitational effect of the dust particles on other dust particles.

That part of the lecture has nothing to do with the agglomeration of dust in free space under the influence of their mutual gravitation.
AHA!

Hmmm, but then it must be far outside of galaxies? Since everything in galaxies are subject to pull of gravity, dark energy and centrifuge force? ..etc?

14. I reckon that any energy-system in space that can be defined as a component of matter possesses spin and will exhibit forces of both attraction and repulsion.

Imagine a sphere spinning under water; it will force water away from it along the equatorial plane and water will be drawn in along the polar-axis.

It pushes; it pulls.

Whereas space itself has only the tendency to push.

15. Originally Posted by HexHammer
Originally Posted by DrRocket
[You have totally misunderstood Susskind's point. He is simply describing a freely falling reference frame. The subject is the equivalence principle of general relativity and not the gravitational effect of the dust particles on other dust particles.

That part of the lecture has nothing to do with the agglomeration of dust in free space under the influence of their mutual gravitation.
AHA!

Hmmm, but then it must be far outside of galaxies? Since everything in galaxies are subject to pull of gravity, dark energy and centrifuge force? ..etc?
Sussking is not trying to illustrate the physical process associated with anything in particular. He is dong a thought experiment in flat space in order to illustrate the equivalence priinciple of general relativity. It is a pedagogical thing and not a discussion of a cosmological phenomena.

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