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I know low gravity can have negative effects on humans, and I would guess high gravity would also have negative effects. So would it be possible to increase or decrease a plant’s gravity to better suit human life for the purpose of colonization? Assuming real world science and our understanding of these matters.

As to the civilization that would be using this technology, here’s a few notes on their technology. This civilization has the ability to create traversable wormholes, as well as bioengineering and terraforming technologies. The technology they use to create traversable wormholes (portal engines) are fairly ubiquitous within their civilization, as they are incredibly technologically advanced. They’re colonizing worlds across the galaxy (not necessarily our own). I don’t know how they would relate to us time wise, but probably thousands of years into the future. As to their resources and the amount of time over which this colonization process would take place, they could spend hundreds-thousands of years in colonization efforts, I would hope though that they could produce habitable planets within a timescale of at least a couple hundred years. I was originally thinking that maybe their portal engine technology could share some technology in common with whatever method they use to adjust gravity, like maybe both of them use a similar type of technology built on the same concepts applied differently? They don’t have to both work off the same principles, but if that makes it easier, then they definitely can.

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    $\begingroup$ Depends on the rules of your fictional world. $\endgroup$
    – sphennings
    Apr 20, 2023 at 19:38
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    $\begingroup$ Could you expand on the technology level of the society (if they're colonising an extra solar-system planet, they're way ahead of us) and the available time and resources to achieve anything like this? $\endgroup$ Apr 20, 2023 at 19:43
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    $\begingroup$ Sure, you just need to add or remove enough quantities of mater to the planets mass to adjust it to the desired gravity .. if you could do it then compressing the whole planet so it's smaller but the same mass also works for increasing gravity and 'fluffing it up' so it's bigger with the same mass would work to reduce gravity .. just let me know if you ever find a plausible way to do any of that that's not magic ;) $\endgroup$
    – Pelinore
    Apr 20, 2023 at 19:47
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    $\begingroup$ Oh another way you can adjust the 'effective gravity' is to spin the planet on its access faster, centrifugal force will counteract gravity a bit that way, if you have sensitive enough scales you'll be able to see that on Earth you weigh marginally less on the equator than on the poles because of this effect .. this one actually has some plausible (if highly unlikely due to costs etc) tech you could use to do it, it's called a [Dyson Planetary Spin Motor] $\endgroup$
    – Pelinore
    Apr 20, 2023 at 19:59
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    $\begingroup$ @Kal Madda You don't have guess about higher gravity having negative effects. You can study centrifuge experiments on animals and humans during the last 70 years. Probably no non biologically modified humans would want to live on planets with surface gravities higher than 1.25 g or 1.50 g. $\endgroup$ Apr 21, 2023 at 4:50

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Use your wormhole-generator

At the point you're creating wormholes for interstellar travel, you've got a very high-tech (and probably dangerous) option: Tweak your wormhole engine a bit, or put it inside the planet. The reason this works is that wormholes look like black holes at a distance, and both black holes and planets follow the same inverse-distance-squared force, so provided you're on / near the planet's surface (or sufficiently far from the black hole / wormhole), it's pretty much impossible to tell the difference.

Basically, your wormhole engine creates that gravitational potential in two different locations, and stitches together the two nadirs. So to raise a planet's mass, put both ends of the wormhole inside the same planet, and tune the steepness of the "mouth" to the gravitational field you want. Of course, this assumes your generator can create planet-scale wormholes - which would take a lot more energy than something desired to merely transport an aircraft carrier. It also means you need to stabilize the wormhole against a much larger mass which is always present rather than briefly transiting it....

You could presumably siphon off excess mass by putting one end of the wormhole in the center of the planet and sucking out part of the high-density core, but you would probably want to be able to turn it on & off with very precise timing so you don't take out too much of the planet's mass.

Either way, expect large earthquakes on the surface as the planet's mass distribution is redistributed. Colonists (and their stuff) should arrive once everything has settled down.

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I can think of four basic ways to change the gravity you feel, but I don't know how to do one of them.

1. Adding or removing mass from the planet.

This means mining a lot of rock and launching it into space to reduce the planets mass and gravity or guiding lots of asteroids to collide with the planet to increase its mass and gravity, either option is going to be a long and expensive process over centuries if not thousands of years.

2. Changing the speed the planet spins on its axis.

One way you can adjust the 'effective gravity' you feel on its surface is to spin the planet on its axis faster, centrifugal force will counteract gravity a bit that way, if you have sensitive enough scales you'll be able to see that on Earth you weigh marginally less on the equator than on the poles because of this effect.

This one actually has some plausible (if highly unlikely due to costs etc) tech you could use to do it, it's called a [Dyson Planetary Spin Motor.]

But this is only good for reducing the effect of gravity and has greatest effect at the equator and no effect at the extreme poles.

3. Increasing or decreasing the planets overall density.

Squeezing the planet into a smaller ball will increase the surface gravity and doing the reverse will decrease it.

If you're having problems with that idea think of it this way, the effect of gravity reduces with distance and a planet is a pretty big thing .. so standing on its surface on one side of it puts you a rather a long way from a lot of its mass on the other side .. squash it into a smaller ball and you'll be closer to a lot more of its mass .. and vice versa.

But how do we squish it into a tighter ball or do the reverse? I have no idea, it's probably impossible & we can't, but if we could it would work.

4. Change your location in relation to the planets' mass.

Remember what I said earlier about the effect of gravity reducing with distance? further away from its source you'll feel less gravity.

At the top of a mountain, further from the Earths' centre, gravity is less.

Howsoever, this effect is too small to notice without instruments on top of even the highest mountain on Earth and you're already well past the death zone where you can't live without oxygen tanks at the top of Everest .. so it's doubtful it's going to be useful for your purposes.

4.1 Go deep instead.

Not only does the gravity you feel get less the further from the surface you go, it also gets less the deeper you go.

Because the deeper you go the more of the planet is above you pulling you the other way from the bit of it below you.

So you might consider having them living deep with sunlight piped in from the surface by fibre optics instead of trying to mess with the planet to change its gravity.

The big problem with this one is that the increasing heat as you go deeper will likely make it unlivable long before you get deep enough to have noticeable or useful effect on the gravity experienced, unless the core has cooled a lot more than Earths' has.

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    $\begingroup$ Kal's wormholes could be used to pump in/out mass at the core of the planet. Might be a fun element of the story, dealing with the long-term consequences of doing that. I should think it would take a honking long time (as in geological time spans) for the changes in internal pressure to equalize - after doing whatever damage they'd do to the crust. $\endgroup$
    – JBH
    Apr 21, 2023 at 2:37
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    $\begingroup$ @JBH 👍 Problem is if they need gate tech both ends of the connection then it won't work cos you couldn't get the gate tech into the core, everything would melt, if they're one way travel from journey origin same problem, if you can go both ways through them how do you stop the molten magma wrecking the gate as it comes through and it wouldn't come through because fluid flows downhill pulled by gravity and "downhill" through a gate with one point anchored in space and the other in a planet is to the planet .. you'd have to push or shove it through the gate and how do you do that from the core? $\endgroup$
    – Pelinore
    Apr 21, 2023 at 4:09
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    $\begingroup$ Ah, rats. That seemed like a fun solution to the problem. Dropping mass on the surface seems easy, but what needs to be added is mostly metal. Not a useful item on the surface in those quantities - and I'd hate to be the schlemiel who had to crowbar that much mass out of the gravity well. Might actually be cheaper to dig the gate to the core. O'course, now that I think about it... if you put the gate face-down on the surface and the companion on a high gravity planet... it might dig itself down. Probably take too much of the good stuff (the crust) with it, but maybe.... $\endgroup$
    – JBH
    Apr 21, 2023 at 4:11
  • $\begingroup$ regarding 4.1, perhaps a planet thats much colder on the surface, so the habitable temperature is reached way below? $\endgroup$
    – Eumel
    Apr 21, 2023 at 6:46
  • $\begingroup$ @Pelinore The portal engines only have to open a wormhole from one end, but the magma could potentially be a problem. I do think some kind of use of a black hole could be interesting, because it would use common technology to the portal engines (as I understand it wormholes are formed from two black holes). Perhaps a stable portal/blackhole could be created that would naturally stay open itself. Since it doesn’t need to be traversable, it could allow matter to be shared with “spagettification”. Maybe that could work? $\endgroup$
    – Kal Madda
    Apr 21, 2023 at 11:32
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Not without removing a large chunk of the planet.

Gravity, at least under current understandings of physics, is a function of mass. To put it another way, the only way to lighten a planet by 10% is to haul away 10% of its mass; and conversely, the only way to make a planet 20% heavier is to chuck enough comets and/or asteroids at it that the result has 20% more stuff (which is probably molten hot and pulverized after that bombardment).

So, not something you're going to want to do anywhere you want to actually live, unless your wormhole tech is really cheap (to the point where you can siphon that 10% mass off just by sticking it into the wormhole).

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There is one alternative to removing large quantities of mass, but it's not significantly simpler:

You could increase the planet's spin. That will reduce effective gravity along the equator, giving you a band of habitable area.

You get more habitable area by just removing excess mass, though, since that'll reduce gravity everywhere, so given that you can make wormholes to shove mass through... I'd probably just go with that. Then you can use the extra bits to build new planets!

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    $\begingroup$ There is a third way as well .. increasing or decreasing the entire planets density .. squeezing it into a smaller ball will increase the surface gravity and doing the reverse will decrease it .. but don't ask me how you'd do that though, because I haven't a clue ;) $\endgroup$
    – Pelinore
    Apr 20, 2023 at 22:14
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One way to increase the mass of a planet without increasing the radius, and thus increasing the density of the planet, and increasing the surface gravity and escape velocity more than by just piling matter onto the surface, is to insert extremely dense matter into the planet.

Blackholes which are stellar remnants have a minimum mass of several times the mass of the Sun. I strongly doubt that there could be rocky planet so large that increasing its mass by several solar masses would give it a surface gravity comfortable for humans. Anyway, without a lot of light elements like hydrogen and helium to fuse and produce energy to resist the black hole's gravity and turn the planet into an uninhabitable star, it will merely rapidly collapse into the black hole and make the black hole a little bit more massive.

Thus a much less massive black hole would be needed. Primordial black holes with a very wide mass range might possibly have been formed in the Big Bang. The smallest ones would have evaporated by now, but ones massive enough for the purpose of increasing the mass of a planet should have survived.

So the problem becomes one of locating a primordial black hole with a mass within the range calculated as acceptable for your terraforming project, moving it over presumably interstellar distances to the target planet, and then causing a collision between them at a slow enough rate that the blackhole becomes trapped inside the planet, or the planet become trapped around the black hole.

I imagine that calculating the rate at which the black hole will absorb the planet's mass would be a complicated problem in physics. If the black hole would swallow up a significant fraction of the planet in too short a time period, the project would not be worthwhile.

In my answer here,

Cloud formations on an ocean-covered moon

I discuss the possibility of retaining a breathable atmosphere on a lunar mass world.

IN part Seven of my answer I suggested inserting a primordial black hole inside a moon to make the total mass equal to that of Luna, Earth's moon, while having an escape velocity high enough to retain an atmosphere for geological eras of time.

And I calculated the escape velocity for various combinations of black hole mass and moon mass that total the mass of Luna, Earth's Moon.

Of course you are not restricted to a mass equal to that of Luna, Earth's Moon. So it should be easier for you to come up with a combination of an initially too low mass world with a blackhole of sufficient mass artificially inserted into it to give a high enough escape velocity.

Though of course I give no guarantee about how long the world will remain habitable with a black hole inside slowly swallowing it up.

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One solution would be to use dark matter. Since we don't know what dark matter consists of you can do what you want with it to increase a planets gravity (the suggestion of primordial black holes is a subset of this.

A dark matter planet for example would have gravity but you could pass freely through it. Super impose it over your small moon and you could increase its gravity.

For example, this mentions dark matter planets: https://www.space.com/dark-matter-planets-blown-up-detection

For making it lighter, we have dark energy which is uniformly distributed. It is less likely to clump but again since we don't know what it actually is you can do what you want with it. There may be a form that can lighten a planets pull.

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  • $\begingroup$ I was expecting the good ol' mass == gravity answer, but never thought about dark matter! $\endgroup$
    – Seggan
    Apr 21, 2023 at 17:17

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