Why does gravity make everything round?

  • 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.

    Maybe you should start with Newtonian gravity... then invest a few years to a B.Sc. in physics, until you can discuss the foundations of general relativity. Could give your invention a bit more underpinning...

    The 'why round' question is indeed answered in the other question you mention. For the rest there is nothing answerable in your question - voting to close as duplicate. It seems like you want proof that gravity does not necessarily make objects round - to support your own theory.

    google "why are planets round" (no quotes) and "potato radius" (in quotes) for starters. While anti-gravity might be possible, we've seen no evidence of it so far. Using gravity to cause repulsion might be possible, but you'd be relying on other forces for the actual repulsion itself. Example: the Sun's gravity will cause it to go nova, exploding outwards, at some point, but that's ultimately a nuclear reaction, not a gravitational one.

    why are soap bubbles round, its the least surface area for a given volume, lowest energy state. It also distributes the gravity equally, if the earth ( will call it squerth, square earth ) was square gravity would be strongest across the corners ( diagonal through the center of squerth), the matter that makes up squerth is generally not strong enough to resist this ( think of it like sand ). So naturally everything falls to the lowest energy state. What is that entropy or something?

    1/r^2 is symmetric about a point.

    @ArtisticPhoenix - Yes, it's entropy (or something). An oblate spheroid whose surface has a uniform gravitational + centrifugal potential energy is the most favored state from the point of view of entropy. Whether an object can get to (or get close to) that entropically-favored state depends very much on size. Small objects can't get there. Large objects can get very close.

    Since our current knowledge of physics renders an "anti-gravity machine" impossible, just make it up *without* giving a detailed scientific explanation, same as they do for human teleportation, photon torpedos, traction beams, light sabres, "jumping to light speed", fighter spaceships that make noises like aeroplanes as they bank at impossible angles, etc.

    van de walls force also makes water round in space. same with soap bubbles, good illustration by artistic pheonix.

  • zephyr

    zephyr Correct answer

    4 years ago

    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.

    $F=-Gm_1m_2/r^2$ is valid only for objects with a spherical mass distribution.

    @DavidHammen That is true, I've amended to say I'm referring to point masses. I referenced the equation to imply that is the source of calculating gravitational forces. Sure, you can create some non-spherically symmetric gravitational force via a non-spherical mass distribution, but ultimately, I believe I my point still remains that at its core, gravity forms spherical objects because the attraction between any two (point) masses is not dependent on angle.

    @DavidHammen There might be a few steps of logic between the base equation of gravity and how a large, square planet (for example) would ultimately become spherical, but I don't believe my statement is wrong.

    A central force does not prohibit non-spherical objects. The discussion should be based on minimising the total energy and indeed, rotating stars and planets are not spherical despite obeying the same gravitational laws.

    @RobJeffries I never said you couldn't get non-spherical objects and in fact I touched upon an example of why you may still get non-spherical objects despite my explanation. And if you're going to describe your answer in terms of minimizing total energy, my response would be, what determines the minimum energy? The answer to that, fundamentally is that the force (and likewise the potential energy) only depends on distance and not orientation. If that wasn't true, gravity wouldn't make things spherical. There's a lot of concepts on top of that, but I believe my answer gets to the core.

    Your answer provides an obvious explanation for why the OP's "anti-gravity machine" is impossible (at least at the macro scale): for F to be positive i.e. repulsive, one of the masses would have to be negative.

    @Chappo See this related question/answer on that point.

    This made perfect sense to me. I am into maths anyway :) Thank you.

    This is not the correct answer, very much so.

    @DavidHammen We can agree to disagree. The question was specifically why does **gravity** make things round, not why are objects under the influence of gravity and other forces not round? I answer the question as asked.

License under CC-BY-SA with attribution

Content dated before 7/24/2021 11:53 AM

Tags used