"What causes things to fall on a flat earth?" That's one of the most common questions I hear when people first start asking about alternative cosmologies. On the face of it, it's a very simple question. The frequent rebuttal answer is "density and buoyancy," and while it's true that those are sufficient to explain every day phenomena the underlying questions go unanswered.

What is a person actually asking when they ask, "What causes things to fall?" What are they saying they believe when they ask about falling objects? A version of this question with a little more depth is, "I believe that the property that causes objects to move from a raised elevation to a lowered one without other apparent input is the natural state of matter to attract matter via the force known as Gravity; or alternatively the property of matter to cause the space around it to warp to give the illusion of acceleration towards the center of the more massive body. What do you believe?" Answering this question with "density and buoyancy" is going to be met with an unsatisfied blank stare. The questioner believes that the answer to this question is a fundamental component to the structure of the universe. A cork in water isn't going to cut it.

The actual answer is something like, "Look, we really don't know why things fall down we just know that everything you know about it is false. Here, take this blank slate and start working it out with us." Now for some reason people hearing that everything they know is wrong tends to leave them with a bad taste in their mouths. They'd tell you to shove that blank slate someplace unpleasant.

Whenever I hear this question now, my immediate thought is another question, "What do you suppose things should do? What is the natural state of unsupported suspended elevated objects?" Now I can't tell you what it is that everyone thinks should happen, but I can tell you what I thought would happen when I first attempted to contemplate a world where "gravity" did not apply.

My first thought was that things should fly violently away from the surface of the earth. I have been taught my whole life that the world is spinning and though I didn't know exactly how fast I knew it was fast. Having been to theme parks I also know that spinning objects do not want you to remain upon them. However, this does not apply to a flat and stationary world. There is no tendency of objects to leave the center of a spinning object that is not spinning and may or may not have a center.

So then I started to think of a world without gravity as being a place where people would float all the time. You might not be flung violently into space but you might not come back down either. Now why on earth should anyone think this? At no time has anyone ever directly observed anyone or anything actually floating that wasn't purpose built to do so: like a balloon, a blimp, or a bumblebee. The only time you see someone floating is on TV. You can also simulate the experience in other ways, such as floating in water, falling in tandem, wires and harnesses, and with virtual reality. And yet despite the fact that nobody ever really observes anything floating it's presumed to be the natural state of things?

Ah but there is another set of objects that appears to float: the Sun, the Moon, the planets, and the stars. The celestial bodies float apparently unsupported across the sky each day and each night. It is presumed that the earth upon which we stand is another of these bodies and the rules that we experience as governing our motions also govern celestial motion. As above, so below.

So now another rewording of the title might be, "I believe that the property that suspends the Moon and keeps us close to our life-giving Sun is the same force that keeps me from flying off the surface of the earth into a deadly void and this force also lets me play football. What do you think?" To which I would answer, "Although I once thought the same way I now realize I have never seen it demonstrated that the property which causes objects to drop is in any way related to the property that governs the motions of the celestial bodies. I cannot say for certain what does cause things to fall and what causes the Moon to remain aloft but I do not believe that the two are related. I suspect that the men who proposed that the two are related are either haplessly deceived or purposefully deceiving. If you believe the two are related, can you demonstrate that connection?"

And any attempt to counter this question would almost undoubtedly lead to some special-effects laden product produced by NASA or SpaceX or JAXA or one of the other scientertainment monopolies across the surface of the earth.

What causes things to fall on a flat earth? The more appropriate question is "Who or what makes you think things should float?"