12/15/20 – Shell vs. Core

Spacetrawler, audio version For the blind or visually impaired, December 15, 2020.




It’s hard to even IMAGINE what “gravity” like that would feel.


  1. Rex Vivat

    I… honestly don’t see how it would feel any different than regular gravity? Being repulsed is pretty much the same as being attracted from the opposite side (which they also are, apparently). If they were being attracted and/or repulsed from different non-parallel directions at once, I could understand how that could feel weird, but this I’m not really getting at all.

    1. Erhannis

      Yeaaah, it seems any number of forces and gravities and repulsions all just add up to a force in a particular direction – and according to special relativity, gravity and forces are more or less indistinguishable when you can’t see what’s causing them.

      …Buuuut, the characters can clearly see that the panels are upside down, allowing them to distinguish gravity from other forces. ;P

      1. Pedantic, Ph.D.

        @Coyoty, that depends on what you mean by being pushed and pulled. If you’re talking about nonuniform applications of force, like someone pulling down just on your feet or pushing down just on your shoulders, you could absolutely tell the difference. One part of your body experiences a compressive downward force, another doesn’t, and the bits in between get stuck mediating the difference (or not, if it’s a “rip you apart” level of pull).

        If you’re talking about a uniform force acting simultaneously on all parts of your body (e.g. a gravitational field or some “repellitational field”, there’s no functional difference between being pushed and being pulled that you can detect. That’s because the only difference is an arbitrary semantic one that’s a product of your mental model and the language you’re using.

        All that said, your standard mass-concentration-based gravitational field is NOT uniform (just look up terms like “black hole” and “spaghettification”). It’s simply that we’re so small and have such non-finely-tuned internal pressure-sensation abilities that we’re not equipped to notice that non-uniformity. But, for real, your lower bits are being pulled on by gravity a smidge more than your upper bits, and your middle bits have a tiny inward force on them relative to the average force on your body. In a repelling field following the inverse square law, that would be reversed – your lower bits are being pushed on by repellence a smidge less than your upper bits, and your middle bits have a tiny outward force on them relative to the average force on your body.

        So, if you simply hypothesize that something in G.O.B. food enhances the sensitivity of humans’ awareness of intra-body force differences to a point where it’s just subliminally registering, then shifting to a region where the usual gravitational tidal forces were ‘backward’ could reasonably be noticeable at some level, and you could tell the difference between pulled against a floor by concentrated gravitational mass vs pushed against a floor by concentrated repellent mass.

        Though that would also likely make shipboard artificial gravity feel weird too.

    1. r4m0n

      Not quite. In the center of the shell, yes, the forces cancel. But where they are standing, they’ll feel the attraction pointing outside. Don’t forget the inverse square on distance, the opposite side of the shell exerts considerably less force on them.

      1. TB

        I used to think that, but no, a hollow sphere has no gravity at all inside it. Things just float around, like that core would. Basically, although the rest of the sphere interior is farther away, there’s a lot more of it. I never did get the math.

        Don’t feel bad. Larry Niven a similar mistake when he thought Ringworld was stable.

      2. Nope, I remember doing this exact integral calculation back working on my physics BA lo these many years ago. Assuming it’s a perfect sphere of perfectly uniform density, the gravity of everything “outside” of your position completely cancels out. So, if you were burrowing through a solid sphere with a radius of, say, 5,000 km: On the surface you’d feel a downward gravitational pull of a sphere of radius 5,000 km. 1,000 km down, you’d feel a downward gravitational pull of a sphere of radius 4,000 km. 2,000 km down, you’d feel the downward gravitational pull of a sphere of radius 3,000 km, etc.

        It would work exactly the same with a hollow sphere, except there would be nothing “inside” to exert a downward gravitational pull on you. You’d feel no pull at all.

    2. someone

      The repellium core may somehow screen them from the gravity of the crust on the other side. So they’d feel a local gravity anyway. Also, the rotation of the planet can offer them a measure of perceived gravity thanks to inertia (the “centrifugal force” effect).

    3. Doghouse

      I came here to say exactly that. Inside a spherical shell of uniform mass, the gravitational effect sums to zero at all points.

      It’s one of those fascinating little bits of trivia that science nerds such as myself can never forget once they’ve heard them.

  2. Pitgamer

    If you imagine that Chris’s strip is actually created by cameras that tag along with the characters, then this is simply a reference issue, like when you see footage of astronauts where the camera has them “upside down.” But, fortunately, Chris’s imagination is a lot more fun than “reality.”

    1. Keith

      That makes me smile. 🙂 Mind, I also use the tears of collectors to make my coffee. Do fun little things like use oxy/acetylene to weld cast steel and such (cast iron is way tougher as is bronze)

  3. Pete Rogan

    I would imagine the sensation of repulsion replacing gravity would be as pictured: Like standing in a room where all the furniture is on the floor above you and you need to keep from stepping on the lights on the ceiling. Deeply unnatural but also incapable of being resolved as a sensible reference frame.

    The more so because the mass of the shell is exerting a pull that feels as if it comes from the hollow center of the sphere, i.e., normal gravity. The middle ear is getting confused signals and the vestibular system can’t resolve them in a sensible way. Precisely what you might expect to feel encountering a place dominated by an antiphysical property, e.g. antigravity.

    Now if you really want to get confused, figure out what happens when the repellium is fully mined and the shell is left alone in space. Does it collapse? Fly apart? Draw a series of diagrams explaining the result, and be sure to show your work.

  4. Gregg Eshelman

    I recently read a SF book where artificial gravity hadn’t been figured out, but they did have a pusher field. It was installed in the overheads of ships and used to push everything toward the desk. But it acted like light, not gravity. So it could only act on the upper surfaces of stuff. Hold a hand out and it’d create a ‘shadow’ in the field so that anything under the hand would feel no force pushing it down, same is using a hand to shield something from light.

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