# Exploiting a hard boundary between two levels of gravity [closed]

This question has to do with a "planet" existing under very different physics to ours. Gravity as we understand it doesn't exist in this universe.

Suppose you have a spherical region of space. Within this region, all matter is pulled towards the center with a constant force, say of 1 G. Outside the sharp line defining this region, there is no gravity.

How could you best exploit such an anomaly? Suppose a planet had congealed within the gravity sphere and a civilization has developed on the surface. What kinds of experiments could their space program perform on the boundary?

In general, if you had access to a sharp boundary line like this with different apparent gravitational forces on each side, what kind of physics-breaking machines could you build?

• 1 G is not a force, it's an acceleration.
– L.Dutch
Commented Sep 24, 2020 at 2:57
• Isn't that what the edit function is for?
– rek
Commented Sep 24, 2020 at 3:09
• @rek, I am not in the OP's mind to read if they meant constant acceleration or constant force
– L.Dutch
Commented Sep 24, 2020 at 3:10
• what kind of physics-breaking machines could you build Without knowing the detailed physics of your world, there is no way to answer this. We cannot apply our physical laws to something when your gravitational field does match our physical laws for gravity. Commented Sep 24, 2020 at 3:22
• @Ash: Imagine a waterless water wheel placed exactly at the boundary, with one half of the wheel inside the constant gravitational acceleration zone, and the other half outside it. Commented Sep 24, 2020 at 6:26

So I am imagining a space-time fabric which rather than smoothly bending like rubber when subjected to mass, bends along sharp borders like sheet metal in a break or press. If the mass is concentrated enough in one area, the bent edge around its distortion might approximate a right angle, leading from a gravity-less plane into a high gravity well with a very abrupt transition.

In such a universe, random tiny objects floating weightlessly across the flat plane between the planets, would eventually fall over the edge of the first gravity well they encounter straight on. Such objects, once lost into a well, would never return again and their added mass might further distort space leading to either a deepening of the well with no change in diameter diameter or expansion of the well, pulling a larger circle of space-time fabric down into the well with new right angle cliffs all around. That secondary type of collision might lead to a nested stack of increasing depth, something like an inverted wedding cake.

The problem with such a universe is that the gravity gradient which allows for orbiting bodies would not exist. Planets would not orbit stars. Moons would not orbit planets.

There would be no creative exploits of such an arrangement of physical laws because those physical laws would never allow creative sentient life to evolve in the first place.

• Why right angles? The description had me imagining something more like a cone. Commented Sep 24, 2020 at 6:38
• @CAEJones if its a cone then you dont have uniform 1G inside the sphere but some gradient going from 0 at the edgeto 1G at the bottom of the cone, this does make the physics a lot nicer
– jk.
Commented Sep 24, 2020 at 7:12
• There could be orbits inside the gravity well. Commented Sep 24, 2020 at 12:37

As discribed then passing the boundary you go from 0G to 1G instantaneously, this mean you have infinite jerk this is probably very bad for your back,

To avoid vehicle passengers' losing control over body motion and getting injured, it is necessary to limit the exposure to both the maximum force (acceleration) and maximum jerk, since time is needed to adjust muscle tension and adapt to even limited stress changes. Sudden changes in acceleration can cause injuries such as whiplash.

### Eternal, free power, via an orbital Piezoelectric generator.

Some materials change electrical properties in response to deformation, caused often by a mis-match in external forces. A truck weigh bridge is an example (truck pushes down, earth pushes up, change in resistance gives truck weight), but another example is a hammer hitting a crystal being able to provide thousands of volts - enough voltage for the spark in a cigarette lighter.

A ring around the planet of such materials (Crystals, ceramics, semiconductors, polymers there's many in the link that'd need to be individually pro/conned), just on the border of the 1G / 0G threshold, will provide free, eternal power (at the cost of a little station keeping to keep the piezoelectric material crossing the threshold)

• I don't think such an orbit is possible as per Henry's answer also I suspect the infinite jerk would destroy the Piezoelectrics, but I'm not certain
– jk.
Commented Sep 24, 2020 at 8:53

## Making use of the boundary effect

Imagine if you will, a cylinder. In this cylinder there is a Spring, and a weight "above" this spring.

Lower this cylinder into the gravity field, and the weight will be pulled down to compress the spring.

Raise it back out of the field, and the spring will push the weight back up.

Converting a cylinder's movement into usable energy is something we're doing all the time, so I'll omit that step ;)

Note that if you don't want a spring that wears out, you can simply have a gas get compressed by the weight. A spring is simply easier to visualize.

## Getting your experiment to the boundary and staying there

Here's the hard part. We have three options here:

### Anchored on the ground

If the border isn't too far away from the ground, we might be able to just build a tower! After we're done building, we don't need to spend more energy on keeping it upright. Nice!

### Floating above the border in gravity-less space

This is less ideal, and probably not doable. The bit of pull our entire assembly experiences every time we dip the cylinder into the gravity field and pull it back out will probably cost us more energy than we gain, especially if we have to convert it into thrust somehow.

### In Orbit - this means flying very fast IN the gravity field.

Orbital mechanics 101: you stay in orbit if you go sideways just fast enough to miss the ground. Go a bit slower and you start getting near the ground, go a bit faster and you will slowly go higher. This does not cost any energy if you don't get slowed down by atmosphere (so I hope the border is very high up).

By orbiting just under the border you can push the cylinder out of the field above you to let the spring do its thing, and pull it back towards you to let gravity compress the spring again.

• Note that there are no stable closed orbits in a world where gravity is a constant central force. Commented Sep 24, 2020 at 11:51
• @AlexP can you expand on that a bit? The wall of formulas in the wiki page you linked is a bit too dense for me to look for the exact bit of information I need, I'm afraid. I was under the impression that - simplified - if the centripetal force equals the pull from gravity, it would result in a stable, circular orbit? So if we solve for mg = mv^2/r and speed up to the required v for the r at which we're just under the border it should work - was my assumption. Please tell me where this is wrong, so I can learn :) Commented Sep 24, 2020 at 12:15
• I think it's circular but not stable, if you are slightly faster you end up escaping if you are slightly slower you end up spiralling into the planet, in effect you can orbit but you have to spend energy station keeping to stay there
– jk.
Commented Sep 24, 2020 at 12:35
• @jk. Oh, I've been thinking "stable" wrong then, right! Thanks for the clarification. Important to know, but as long as the weird engine I described produces more energy than needed for station keeping we should be fine. Though obviously this makes staying in orbit a lot more complicated than it is in our universe. Commented Sep 24, 2020 at 12:38
• @Syndic: Is this easier on the eyes? Basically, there are two and only two types of central forces which produce closed orbits: $F \propto r$ and $F \propto 1/r^2$. In particular, the constant force field of the question cannot produce closed orbits. Commented Sep 24, 2020 at 13:56