The Parachute Cities fall endlessly, ploughing their way downwards through an infinite blue sky.

The Big Blue is a universe of sorts. It certainly has no bottom, but it does have a concept of ‘down’. Objects, airships, people and whole islands plummet through the occasional clouds, permanently lit and heated by suns that float through the sky in the far distance.

But the air doesn’t move. Everything has a terminal velocity caused by air resistance, yet (as the famous philosopher Aristocralopholes points out) if the universe is endless, has no bottom, and everything is accelerating downwards, the air too should be moving downwards. The fact the air doesn’t move means the cities aren’t in freefall, so the residents feel gravity.

The question is why the air is still, or at least why the residents of the Parachute Cities can’t observe it’s movement through the universe.


For the sake of clarity: in this particular universe gravity is uniform (but not necessarily 1g) the universe is infinite, and all points are filled with breathable air (save those filled by something else). Don’t worry about habitability of the universe or how objects come to be there, the focus of this question is by what mechanism the atmosphere can be prevented from falling at the same rate as everything else, leading to the phenomenon of terminal velocity and hence ‘gravity’.

  • 1
    $\begingroup$ Is the atmosphere of Earth falling? No, it isn't. But it shows a different phenomenon: air pressure varies with altitude. That's because gases have a built-in property which counteracts gravitation: they tend to expand to fill the available volume. This means that at any given point air is in equilibrium with the weight of the air above it. I'm afraid that this cannot be otherwise except in if the world of those Fallin' Cities some sort of Aristotelian physics applies, or at least the part with each element trying to find its just place in the world; having actual gravity won't work. $\endgroup$ – AlexP Feb 16 '18 at 23:34
  • $\begingroup$ @AlexP: I fear you’ve somewhat missed the point of the question. Air pressure on Earth arises because there is a surface on which the air can rest and gravity pulls the air down towards that surface. This universe has no bottom and ‘gravity’ is acting uniformly, in one direction, through the entirety of the universe. The entire point of this question is to establish what property (or set of properties) allows the air to remain stationary. An answer based on Aristotelian views of the mechanics of the universe is a valid (and interesting) answer for that question. $\endgroup$ – Joe Bloggs Feb 16 '18 at 23:46
  • $\begingroup$ Air pressure on Earth or any other planet absolutely does not require a hard surface on which to push. Imagine a column of air with a cross section of 1 sq. mile. Gravity acts uniformly downwards throughout the column. Imagine now a small 1 cubic inch voxel somewhere in this column. If the rest of the column were empty the air in this volume would expand to fill it all. But it's not empty. The air in each voxel is stationary, and the pressure equals the weight of the air above it. An universe with Aristotelian physics is hard to describe, because it's so different from what works in ours. $\endgroup$ – AlexP Feb 17 '18 at 0:01
  • $\begingroup$ @AlexP: if there is no surface at the base of this hypothetical column of air and uniform gravity then every voxel of air is in free-fall, not stationary. $\endgroup$ – Joe Bloggs Feb 17 '18 at 0:12
  • $\begingroup$ @AlexP: I never claimed this was an easy question to answer. $\endgroup$ – Joe Bloggs Feb 17 '18 at 0:13

This is not a question for an Einsteinian, for sure. Neither is this answer.

I picture this as a universe that is encased in an infinitely large torus, such that something would fall forever, while circling around. However, this image is not absolutely necessary for the solution.

In quantum field theory, it is implied that the fields occupy the universe, equally, and are equally distributed throughout. The fields never move in this universe. That is, they are static. Okay, hard to quantify when field theory is point-oriented, but nonetheless.

So the air molecules in your universe are just very large special bosons, which are captured within the 'net' of a corresponding field. This field, like every other field, fills your infinite universe and is static. In terms of the torus, it 'fills' the torus and defines its shape. Think in terms of this 'field' making the torus 'solid' or 'frozen', as an allegory. Everything moves in this 'solid' (field). The air molecule bosons can not move within this 'solid' stationary field, as there is no mechanism for their movement. They are a field unto themselves, independent of the other fields that are responsible for movement (gravity, mass, the Higgs boson, electromagnetism, the strong and weak fields, etc.) However, the independence is not absolute. These molecules also contain Higgs bosons on steroids, which therefore add 'mass' effects (inertia), and thus air resistance for terminal velocity.

However, the theory behind this would require a physics textbook much, much thicker than the one we have today. One in which Einstein was but a footnote. It posits a boson and a field which, frankly, we haven't even conjectured.

The fact that these bosons are uniformly distributed, explains why there is no increasing pressure. These bosons, as I stated, do not interact with gravity nor do the molecules contain any bosons that DO interact with gravity.


In this scenario, I envision your 'islands' to be like asteroids, which would have their own slight gravity, but the movement of everything around the torus would not be due to gravity, but some rotational force, like a particle in a toroidal magnetic field. Again, another boson and another field, apart from gravity.

  • $\begingroup$ I like this a lot- space full of particles that interact electromagnetically/strongly but are massless. $\endgroup$ – Dubukay Feb 17 '18 at 8:08
  • $\begingroup$ I’m a great fan of the rotating toroidal universe. Also has some interesting (and vaguely terrifying) implications for ventilation: if I’m understanding your idea correctly air densities should form standing waves inside a room, which is hella cool. $\endgroup$ – Joe Bloggs Feb 17 '18 at 9:58
  • $\begingroup$ This also becomes very flexible if we instead treat this field as something that interacts with specific molecules and holds them in place rather than the air being a special kind of particle in a similar vein to magnetism and ferrous materials. $\endgroup$ – Joe Bloggs Feb 17 '18 at 10:35
  • $\begingroup$ Since you are writing the physics textbook, both ideas are possible. I had not thought about the standing waves bit, but there is nothing inherently incompatible with the theory. And yes to the special kind of particle - if it were the air itself, then wind would not be possible. Fans would not work. The 'ferrous materials' allegory works. Perhaps maybe the opposite - everything ELSE has the bosons which react to the field, causing then to spin, but these bosons are not in particles that constitute air. Silica, for instance. $\endgroup$ – Justin Thyme Feb 17 '18 at 14:50
  • $\begingroup$ Silica contains the bosons necessary for spin, perhaps, but nitrogen, oxygen, carbon do not. But you still need the 'stationary' boson in air, so it does not gain momentum from everything else that is moving. Helium, for instance. I can't think of a reason why helium would have to move. It is not necessary for anything living, nor for most chemical reactions. It is inert no compounds of helium have ever been made Make your atmosphere say 50% helium with the stationary boson. $\endgroup$ – Justin Thyme Feb 17 '18 at 15:00

The air's gravitational and inertial masses are different.

You could have something similar to this, except the concept is more flexible for reasons I will explain more shortly (and of course it doesn't need to be frictionless since air resistance is an integral concern in your world).

First of all, since gravity is already different in your universe (where there's a downward gravitational pull without any mass-energy generating it), there's no need to keep gravitational and inertial masses identical. For anything. This could open up far-reaching consequences for all sorts of things, not necessarily just why the air and the falling objects are moving at different rates. At a bare minimum, the air alone could have zero total gravitational mass, and everything else could have its normal gravitational mass. Alternatively, all sorts of different elements/materials/forms of energy, including but not limited to air, could have varying or even negative gravitational mass, allowing all kinds of interesting effects à la The Edge Chronicles.

Secondly, if one decides not to throw out every single principle of general relativity, gravity and acceleration can still be equivalent. In this case, it doesn't even matter that the air's gravitational mass is exactly zero, simply that it differs from its inertial mass (or even that the ratio between the gravitational and inertial masses of air differs from the same ratio for the falling objects). This way, the air and everything else will fall at different accelerations, and if you observe from a reference frame in which the air is stationary, the air will be stationary, with everything else falling through it at a rate that may be adjusted by adjusting the strength of gravity. It can even be falling "upward", and no one would be able to tell the difference. (Just swap "up" and "down" and it's the exact same universe.) Another option is that the air is the only thing that has nonzero gravitational mass, and it is actually what's falling upward. All of these are (basically) entirely equivalent descriptions of the same reality — that is, one in which air falls at a different acceleration from everything else.

Decoupling inertial and gravitational mass could also partially solve the issue you identified in your previous question. The cities need not fall at 1 g; they could be made of a material with a much lower gravitational than inertial mass (measuring, of course, from a reference frame stationary with respect to the air, as might be standard in-universe), and hence fall slower and probably more stably. In order to experience gravity ordinarily, the same cannot apply to humans, but the terminal velocity for a city (to the best of my limited knowledge) is going to be higher than, say, a human-sized chunk of city material, simply due to higher volume-to-surface-area ratio. It could be that the terminal velocities of humans and cities are in the same general range, allowing humans to skydive between cities after all.

In all, substances with differing gravitational and inertial masses might be able to solve this problem, and possibly more.

  • 1
    $\begingroup$ I like. Never considered d coupling mass. Also has some interesting applications for air travel (found a material with low gravitational mass but high inertial mass? Sounds like something to make the bones of a Flying Fortress out of. $\endgroup$ – Joe Bloggs Feb 17 '18 at 9:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.