4
$\begingroup$

While brainstorming for my story I thought of a character utilizing their gravity-controlling ability to "flip" their center of gravity and walking on the ceiling/street pole /side-of-skyscraper as if it were the ground. (less Spider-Man and more Inception). Essentially up would become down, and I'd like to know if that makes any sense. If not what would? Since I'm relatively new to writing, I'd like a more experienced writer's point of view on how such an ability should work.

$\endgroup$
9
  • 2
    $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Jun 29, 2023 at 8:02
  • 2
    $\begingroup$ H.G.Wells did something like this in his (rather silly) short story 'The Truth about Pyecraft'. The physics can't possibly work, but it is still a good read. $\endgroup$ Jun 29, 2023 at 10:33
  • 5
    $\begingroup$ Sanderson's Stormlight Archive series has some characters that can temporarily (until energy runs out) modify the 'down' direction for themselves or others. They can also do it fractionally, so that the weight of someone/something can be partially or completely negated without causing it to float away. It that what you are looking for? $\endgroup$ Jun 29, 2023 at 13:17
  • 3
    $\begingroup$ Hi Meek, I noticed you've accepted 3 different answers to this post so far. Generally, it's best to wait at least a few days before accepting an answer, as you can then select the one you consider best, while also allowing other people more time to provide answers of their own. $\endgroup$
    – M S
    Jun 29, 2023 at 15:50
  • 4
    $\begingroup$ You cannot flip the centre of gravity! It is a point and it has no direction. Flipping it will do absolutely nothing. You want to flip the direction of gravity rather. $\endgroup$ Jun 30, 2023 at 10:26

5 Answers 5

9
$\begingroup$

Fictional sense

From a fictional sense, anything can make sense. It's called suspension of disbelief. Where normal sunlight hitting an incredibly efficient 1m² solar panel can't run a computer, it can somehow power Superman to move faster than light, punch through walls, and fire miniature copies of himself (yes, really). It only has to make some fictional sense in the universe you create.

Flipping your own center of gravity might seem like a solution, but if we look at it technically, it falls apart. Your center of gravity is a dot, which can move, but that will not matter. Even if it flips compared to your body it might be around your waist at first and at your neck after, making you only top heavy.

But assuming you mean the gravitational pull, then still no. Let's surmise you flip your own pull of gravity, basically becoming a push. The Earth's gravitational pull hasn't changed and is much, much stronger. Just compare the mass of you to the Earth for a quick reference, as more mass has more gravity. You'll only succeed in becoming trivially lighter. It can be imagined as having a concrete slab twice your weight floating above you. Sure, it "negates" the pull of your own body toward the ground, but the Earth has so much gravity to spare it's negligible.

Let's say you flip gravity itself for just yourself. You could walk on ceilings, as the ceiling is directly opposite the Earth, but flipping it to the wall will be met by maybe some gentle floating. What is powering the gravity that pushes you into the wall? There is no Earth-sized object to push from or pull from.

That all being said, if you just say someone can decide what direction the available gravity pulls them in, no one will bat an eye. It makes fictional sense. You have the gravity of the Earth pulling you down, so now they decided it is going sideways to a wall instead. Are they in space? Different pull from some celestial bodies. As long as it makes just a lick of sense, no one will care.

$\endgroup$
4
  • 1
    $\begingroup$ I have not done the math, but i am pretty sure if you make a gravitational field above you with a pull of 9.81m/s² on a local scale (i.e the field is just a few meters across at most), that will rip you apart. If it is a point mass, you have just make a black hole. If it is a spherical mass, the Gravitational pull on the surface would be insane as well. But again, not sure if exactly $\endgroup$
    – ErikHall
    Jun 29, 2023 at 18:43
  • $\begingroup$ @ErikHall that would mean adding a whole Earth floating above your head. I'm talking about cancelling out the gravity that your own body makes. That is so insignificant we can disregard it compared to the Earth, so messing with it will result in a net zero. If you mess with the gravity of the Earth instead of your own you have bigger problems. $\endgroup$
    – Trioxidane
    Jun 30, 2023 at 6:40
  • 1
    $\begingroup$ It may be worth noting that we can ruin this effect if we try too hard to explain what's happening. As you explain, if the author just says someone can change which direction gravity pulls them, then fine, it's magic. It does whatever the author says it does. But if the author then continues from there to say the power works by flipping the character's center of gravity, now they've invited some portion of the audience to say "wait, what? that's not how center of gravity works at all." $\endgroup$ Jun 30, 2023 at 17:42
  • $\begingroup$ If gravity manipulation can somehow make your mass negative, then the earth's gravitational field would push you away. $\endgroup$ Jun 30, 2023 at 21:29
7
$\begingroup$

Either you modify the gravitational field locally so it points upwards.. but that will make anything caught in that area fall upwards.

Or you flip the mass of the hero to negative. Only the gravitational mass, otherwise things get REALLY funky.

In either case, walking on ceilings will be the least interesting effect by a large margin. In both cases he could go to space with no effort (dont forget the space suit though) and also function as a perpetuum mobile. You could get around that by stating he somehow takes energy for the flipping from some, very very large, reservoire wich eventually will get depleted if he does physical work with his ability.

Also in the second case his sh*t (when "flipped") will probably be the most valuable substance in the known universe.

I think the best way to deal with this if you "just" want ceiling-walking without all the physical consequences, is to don't overthink it and mostly ignore physics. Or make it a dream like inception, so logic and physics do not need to apply.

$\endgroup$
4
  • $\begingroup$ Thank you! For clarification's sake, my story is set in a hero world, so the ability would be used for traversal( i e "hopping" from building to building and shifting one Center of mass on the Fly)I will simply using ceiling crawling as an example. In practice the hero would be able to do this on a virtually anything. As for space travel with this, that does sound super cool. $\endgroup$
    – Meek '
    Jun 29, 2023 at 9:22
  • 1
    $\begingroup$ In most homes, wouldn't the sheetrock be so thin you'd anti-fall right through it? Commercial buildings aren't much better, in the US at least you see alot of ceiling tiles, and those aren't even fastened. $\endgroup$
    – John O
    Jun 29, 2023 at 18:10
  • 2
    $\begingroup$ Awesome answer. The hero having negative mass makes a lot more sense than having a "flipped centre of mass". However I think it could be fun if both phrasings were used in the story, by different characters, and one of the characters maybe being annoyed when people say "flipped centre of mass" and telling them "no!! that's not how this works!! this phrasing doesn't make sense!" but people are still going to call it that. $\endgroup$
    – Stef
    Jun 30, 2023 at 10:41
  • $\begingroup$ @JohnO Yeah, most public buildings have a very thin "fake ceiling" in every room, slightly below the true ceiling of the room, whose main purpose is hiding all the stuff that's on the ceiling, including pipes and wires. That thing would completely break if someone was to anti-walk on it, not to mention anti-fall from ground height. $\endgroup$
    – Stef
    Jun 30, 2023 at 10:43
5
$\begingroup$

I sense some confusion in your wording.

First of all, as explained in other answers, on a purely scientific base, gravity cannot be inverted. However, one can go with negative mass and have a repulsive gravity, which pushes up instead of pulling down, and sprinkle it with a good dose of suspension of disbelief. Wells' The first man in the Moon and its Cavorite is probably an egregious example for this.

But... you can't reverse one's center of gravity. By its definition

The center of mass is the unique point at the center of a distribution of mass in space that has the property that the weighted position vectors relative to this point sum to zero. In analogy to statistics, the center of mass is the mean location of a distribution of mass in space.

The center of mass of a distribution of positive masses is the same as that of the same distribution but with negative masses.

$\endgroup$
2
  • $\begingroup$ Well in this scenario, it's purely fictional, but I'd like to have a certain level of realism. The characters' ability would allow them to manipulate the Earth's gravitational pull. Specifically, they can reverse, increase or negate the effects of gravity on themselves or others by touch. Would it be feasible then? I apologize for my ignorance. $\endgroup$
    – Meek '
    Jun 29, 2023 at 9:50
  • 2
    $\begingroup$ @Meek just say "negative mass" and it'll make sense. See how magnets can have a positive or negative charge, and inverting the charge changes the direction of the force, from repulsion to attraction and vice versa? Gravity doesn't have an equivalent of that, all masses are positive numbers, but in fiction you can totally decide that the hero suddenly has "negative mass" and everyone will understand that he's now repulsed by gravity instead of being attracted. $\endgroup$
    – Stef
    Jun 30, 2023 at 10:47
3
$\begingroup$

This is a fun twist of science that the masters of science fiction take advantage of. The world does not obey our physical laws. We construct our physical laws to mirror the world around us. So if some characters can walk on the ceiling, the reason they can do so is because they can walk on the ceiling. There's nothing about centers of masses or gravities or anything like that. It's simply the thing that is done.

Now we do like to come up with the wordings for our laws. We want to have a law like "If one does XYZ, one's center of gravity is flipped." But the crucial thing is that we invent those wordings, not the world. They're phrases for our benefit as human beings. The world doesn't care.

And this leads to interesting directions because we get to consider who is coming up with these terms. As many of the answers have pointed out, the center of gravity is a point, with no orientation, so in the strictest physics-textbook sense, the center of gravity cannot "flip." You need an orientation to flip. So its unlikely that a group of people with a solid physics background would use that phrasing first. They'd try others.

For instance, many of the answers have suggested making the individual have "negative mass." Would that fit the story better? If a thing had negative mass, all forces would accelerate it in the "wrong" direction. This includes the object being pushed. If the object is pushed around in the "right" direction, "negative mass" would really have to mean "negative gravitational mass." There's "gravitational mass" and "inertial mass" and a standing curiosity in science is that, for some reason, gravitational mass and inertial mass are exactly the same. We'd be unlikely to give up that convenience.

We could say that the force of gravity is inverted. This would fit well with your description of gravity manipulations, and it's relatively easy to decouple from other effects. This is a likely phrasing.

Its possible that the phrase was invented by non-physics majors. They may have used the term "flip your center of gravity" just because it sounded good. They may use the phrasing because it "feels right." Maybe one of them is a skater and "flipping" fit well with their understanding of how skateboards move.

And its also possible that "flip your center of gravity" was indeed a physically accurate phrasing. Physicists facing such a gravity manipulation ability would certainly plumb the secrets of this ability. They may find that this is new physics and adapt new patterns to fit the predictions. For instance, they may find that a center of mass is less of a point and more of a dipole, which is an abstraction that has an orientation and thus can "flip" (dipoles show up in magnetics). Perhaps we have to recognize that there is a new spatial dimension within which a dipole can be flipped.

This is similar to what was done in the Saga of the Skolian Empire. In that saga, there is faster-than-light travel. They originally thought the world operated using our modern relativistic rules, where you cannot exceed the speed of light. However, what they found was that velocity was not just a real number, it was complex. "You cannot travel at the speed of light" became a singularity that you rotate around to get yourself into the faster-than-light regime.

Catherine Asaro builds a world full of physicists who explored this strange behavior for quite some time before deciding that complex numbers were the "correct" description of what was going on. But remember, the world never cared about what we called it. This use of math was simply what we used to make sense of the world.

$\endgroup$
2
  • $\begingroup$ I completely agree with all of your arguments about phrasing. However, being precise matters. If someone told me their power was specifically "walk on the ceiling", I'd immediately wonder: does this mean they can walk upside down with inverted gravity, or is there something special about ceilings? Could they walk upside down on a surface which is not a ceiling? I know that not everyone is bursting with questions like I am. And confusion can be fun too. Characters in the story might assume that the power works one way, but then in a later situation they're surprised because it doesn't. $\endgroup$
    – Stef
    Jun 30, 2023 at 10:51
  • $\begingroup$ And characters with different backgrounds would react differently. Maybe the hero can only walk upside down on ceilings, and not on other non-ceiling surfaces, and some characters find this totally logical, but a character with a more physics-oriented background would be surprised because they'd be trying to understand the hero's power in terms of reversed gravity, and that's not what it is. $\endgroup$
    – Stef
    Jun 30, 2023 at 10:53
2
$\begingroup$

It's not plausible that you could negate the force of gravity for one object and not other nearby objects. It would violate the equivalence principle, which guarantees that no property of any particular object affects how it moves under the influence of gravity. Even if you suppose that negative-mass objects are possible, general relativity predicts that they fall downward like everything else. (So does Newtonian gravity, for that matter.)

Reversing one's center of gravity doesn't make sense. From context, I think that when you say "center of gravity" you really mean mass.

$\endgroup$
2
  • 2
    $\begingroup$ 'Center of gravity' and 'center of mass' both mean the same thing, and neither makes sense in terms of 'reversing' it. $\endgroup$
    – Glen Yates
    Jun 29, 2023 at 21:49
  • 1
    $\begingroup$ @GlenYates O guess benrg meant "reversing mass" (i.e. have something of negative mass). Though that is completely speculative physics. $\endgroup$ Jun 30, 2023 at 0:23

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .