Why does gravity make everything round?

Question

I am studying ways to make an anti-gravity machine (!!). I believe it can be done, or theoretically be done using gravity itself as a "force" that pushes outwards instead of inwards. A similar question was asked here, which speaks about why objects in the universe are round, which as stated, is due to Gravity. I understand Gravity is the strongest, most consistent "force" in the universe and to traverse it we have to push against it - jets, propulsion, etc. Never the less, I am perplexed why gravity's forces make objects round - eventually or over time. Why can't it turn a planet into a cube for example? Why can't Gravity make objects flat, and perhaps round at the perimeter? If I can answer this question, I am sure I can find a way to "bend" Gravity for my invention.

Answer 1

I am studying ways to make an anti-gravity machine (!!). I believe it can be done, or theoretically be done using gravity itself as a "force" that pushes outwards instead of inwards.

It doesn't work that way. No one has found a repulsive version of gravity and there is no evidence this is even possible.

And in science, we go by the evidence.

A similar question was asked here, which speaks about why objects in the universe are round, which as stated, is due to Gravity.

Which the question's answer explains by the attractive nature of gravity. No evidence (that word again) of repulsion.

I understand Gravity is the strongest, most consistent "force" in the universe

You understand wrong.

It is considered the weakest of the fundamental forces. This Physics SE answer explains why.

It appears to be strong only because on average, over a large scale, matter is generally neutrally charged, whereas mass (and energy) just gets bigger the more space you have (a little simplistic, but that's the general idea).

As for "consistent", well all the forces behave consistently or we would not be able to model them with mathematics.

and to traverse it we have to push against it - jets, propulsion, etc.

More evidence for no repulsion !

Never the less, I am perplexed why gravity's forces make objects round - eventually or over time. Why can't it turn a planet into a cube for example? Why can't Gravity make objects flat, and perhaps round at the perimeter ?

Because these are unstable configurations. Even if they could form from natural processes (which is unlikely as I'll explain) the resulting forces and stresses (for large objects) would be too great to prevent collapse into a more stable shape.

Now smaller objects can form less rounded shapes - small asteroids can be all sorts of shapes. Only their relatively small mass (and hence small gravity) allows them to resist compression into "nicer" round shapes.

But larger objects have lots of gravity simply because the have lots of mass. The materials they are made of will not withstand compressive forces on them and they will form roughly round shapes, because that's the most resistive configuration.

So why more-or-less spherical ?

Because these larger objects tend to form in a way that builds up material equally all over the surface, so there is some tendency to be spherically symmetrical just from the way material accretes as it grows.

However once they get really big (planets, larger moons and so on) the forces on the materials will be so great that they will as a general rule, behave a little like fluids. And fluids will end up in that nice spherical shape as they lack the rigidity to resist compression. Again a little simplistic, but this gets really complicated quite quickly. They key here is to stop thinking of e.g. rocky planets as rigid objects. Maybe on a timescale of seconds they are, but on astronomical timescales they're not so rigid.

So for larger objects, more-or-less spherical is inevitable simply because they're large.

If I can answer this question, I am sure I can find a way to "bend" Gravity for my invention.

No anti-gravity to be seen here.

Really, learn the basics of physics if your interested and hopefully you'll find that not only is it interesting, but it can explain things without the need to invent other explanations. Usually. :-)

Answer 2

I understand Gravity is the strongest, most consistent "force" in the universe.

As noted in other answers, you understand incorrectly. Gravitation is the weakest of the four fundamental forces. Gravitation is such a minuscule force at the atomic level that it is hard to distinguish whether gravitation acts as predicted on antimatter (theoretically the same as how gravitation acts on matter).

Gravitation emerges as a powerful force at the level of stars and planets because the two strongest fundamental forces, the strong and weak interactions, are extremely short range, and because the electromagnetic force is limited by the fact that macro scale objects tend to be electrically neutral. Gravitation remains in play because it has infinite extent and because mass, at least theoretically, is never negative.

This, by the way, argues against your premise of creating antigravity. As far as we known, all objects (including antimatter) have a positive mass, and hence there is no such thing as antigravity.

Why does gravity make everything round?

Gravity does not make everything round. Small objects tend to have a fractal shape. Somewhat larger objects, where gravitation plays some role, tend to look like misshapen potatoes. It's only with sufficiently large objects, where gravitation plays a dominant role, that things begin to appear round.

An object needs to be a few hundred kilometers in diameter before self-gravitation can overcome the electromagnetic ability of smaller clumps of matter to maintain whatever non-spherical shape they happen to have. The size at which gravitation begins to make things "roundish" is called the potato radius.

The potato radius depends on an object's composition. Rock has a much higher yield strength than does ice. Icy objects that formed in the outer solar system tend to be "roundish" when their diameter exceed 400 kilometers, but potato shaped when they are less than this size. For stony objects, the relevant size between lumpy potato and more or less roundishness is about twice that of an icy object.

Why can't it turn a planet into a cube for example?

Because the potential energy due to self gravitation (and due to rotation in the case of a rotating object) at the surface of a cube is not uniform. This non-uniform energy distribution means the object has excess energy. This excess energy is minuscule for a cube the size of a die but is huge for a cube the size of a planet.

The mantles of terrestrial planets are solid, more or less. (It is highly erroneous to think of the Earth's mantle as molten. It isn't.) That "more or less" is important. A better description is that the mantles of the terrestrial planets are elastoplastic, or perhaps even viscoelastoplastic. They deform elastically in response to short-term stresses and strains, plastically in response to longer term stresses and strains, and can even flow over even longer periods of time. This means that a cube the size of a planet will eventually deform into something close to the entropically-favored spherical or oblate spheroid shape.

Gravitation and non-conservative interactions can shape very large and very diffuse objects into a shape that is markedly non-spherical. An interstellar gas cloud, for example, eventually collapses into a disk shaped object with a massive protostar in the center. Given the right conditions, an interstellar gas cloud initially collapses toward a spherical shape, but pancakes once enough material concentrates at the center of the cloud. Here volume is not anything close to a conserved quantity. Angular momentum is however close to a conserved quantity. This, coupled with collisions and gravitation, is what make such gas clouds collapse into circumstellar disks.

Answer 3

The reason gravity creates round objects is because the equation describing the force of gravity is the following.

$$F_G = -\frac{Gm_1m_2}{r^2}$$

where $G$ is the gravitational constant, $m_1$ and $m_2$ are two point masses which are gravitationally attracting one another, and $r$ is the distance between (the center of mass of) the two masses.

If you think about if for a minute, it should be abundantly clear that gravity causes objects to be round because gravity is only dependent on distance and not on any angle or orientation.

That means if I take a planet and throw additional mass onto it, gravity will cause that mass to be attracted to the center of mass of that planet. There's no other way in which gravity can act upon it. If everything is equally pulled towards the center, the only possible object you can form is a sphere.

Of course there are objects like asteroids which aren't round, but that's because the structural integrity of the rocks is actually stronger than gravity and gravity, while still trying to make the asteroid round, just can't overcome the strength in the rocks.

Answer 4

Never the less, I am perplexed why gravity's forces make objects round - eventually or over time. Why can't it turn a planet into a cube for example?

Think of an Earth-size cube as a planet with 8 HUGE mountains on it. In fact, these mountains would be so huge that the bottom of the mountains couldn't support the top of the mountains. As they collapse, the mountains would flow toward the centers of the cube faces. The end result would be almost a sphere.

It might make it easier to visualize if you consider a cubical planet made entirely of water. Clearly the tops of the "water mountains" would flow down into the "valleys".