# How much gravity should I have on my spaceship?

The starship 'Exciting Undertaking' has set off on a brand new five and a bit year expedition to explore the galaxy. To make such a starship possible, an artificial gravity generator had to be invented. The first model was delivered and tested and worked just fine, thank you.

When Exciting Undertaking is launched, at what level should we set the artificial gravity generation? Sure humans are used to 1$g$, but is that really the best gravity force to set it at?

Details:

• Only humans are on board these vessels.
• The gravity generator can be controlled differently in different spaces on the ship, as necessary. Gravity is provided by continuous plating, so each separate room or corridor must be kept at a constant gravity level.
• Gravity can be controlled to 0.01$g$ precision.
• The rate of change of the gravity generator is relatively slow due to the desire to limit structural stresses on the ship. Assume a rate of change of 0.1$g$ per minute.
• The gravity generator is a power hog, but with an anti-matter powered reactor, the cost of running the generators at any setting is not a major concern.
• How finely grained can the artificial gravity be controlled? If it could be controlled with enough precision it could be used to disable boarders and make moving cargo extremely easy. Is the force vector always along the normal of the deck or can it be arbitrarily oriented? Can it be reconfigured on the fly? How rapidly can it be reconfigured? How do you define best? Jan 2 '18 at 14:16
• Please define your "best". Also, if we are supposed to take into account other species, you probably should list them and tell us what are they used to and what are their "borders" - as we only have reliable data on humans. Jan 2 '18 at 14:20
• @sphennings As for best, whatever helps the ship most effectively complete its mission is the best. For details of missions, see here. Jan 2 '18 at 14:39
• Does the gravity generator require different amounts of power at different settings? For example, do higher gravity settings require more power? Does maintaining different settings across different parts of the ship require more power than having a uniform gravity across the entire ship? Jan 2 '18 at 14:49
• Is cost effectiveness a concern? Can the gravity generator create opposing forces to counteract the stresses that it creates? Jan 2 '18 at 15:06

Justin's on the right track, let's add to it.

• With perhaps a few exceptions, variation needs to be minimized or you'll need "gravity locks" (the equivalent of "air locks") all over the ship to help people transition from one gravity to another. If you don't, the stress on the cardiovascular system would be substantial.

• Crew members destined to go planet bound for anything other than minutes (or maybe an hour or two, depending on how friendly the environment is predicted to be) would need to "decompress" to that gravity level. In other words, if the ship operates at 0.8g all the time, but the planet the away team is about to visit (a completely unknown planet, you might encounter anything) is 1.2g, then your away team would need to live in an increasing gravity for a period of time to ensure they were ready physically to deal with any contingency on the planet. Otherwise (in this example), they risk sitting duck status because they're too weak to push against the planet's gravity.

Having said that, let's think about what would be "best" for the crew.

• Moving things around in 0g is a lot more painful than people might think. You still need some way to move around objects of heavier mass than yourself. Under normal conditions you'd use a pallet jack to compensate for the difference, but there's nowhere to put the wheels in 0g. That means thrusters in a 3D environment. Very chaotic.

• Other than exercise, I can't think of any reason why you'd want heavier "g" anywhere in the ship. But the gym is a good place for it as a bit more gravity will help with the cardio.

• Finally, you want an improvement in performance without a loss of precision (see 0g discussion, above).

I'm going to conclude (and yes, I'm pulling this judgement out of my left ear), that you want 0.9g everywhere on the ship other than specific locations, like the gym. This improves the efficiency of personal strength without losing basic traction and leverage.

• Medical may want 0.8g to relieve stress on the heart. However, medical is the one place where you'll want different rooms to have completely reconfigurable "g" values. It will have the greatest number of reasons to vary it.

• The gym, perhaps two or three rooms, 1.1g, 1.4g, 1.8g. Your hulking red shirts will be in the 1.8g room lifting weights all the time.

Keeping all the rooms (save a few) minimizes stresses on the ship and also minimizes the potential for harm if emergency maneuvers are required. A sharp turn in that 1.8g gym has a greater potential for harm than a sharp turn in the 0.6g surgery.

Summary: while specific room requirements may vary, and you'll need gradients to move between the rooms unless the delta-g is low, my belief is that 0.9g is the "best" overall value to improve strength without losing traction and leverage.

• That "decompression" scheme sounds wild. Astronauts just walked out of the space shuttle after months on the ISS at zero g. Visiting a planet with 1.4g needs a lot of beforehand exercise. And humans just don't work and walk safely at 1.8g. You'd have to design a new brand.
– Karl
Jan 2 '18 at 19:47
• @Karl, I thought about spacewalking, but there was nothing requiring strength and leverage (well, almost nothing) that wasn't in a completely controlled environment. How would the human body react if you beamed down to (for convenience) a 2g planet and suddenly found yourself running for your life? You'd feel twice as heavy. Granted, you should be in great shape to begin with, but it would have a toll. That's avoided (or, at least, ameliorated) with the "decompression" time acclimating to the planetary gravity. Seemed like something cool to add to a book 'cause no one's ever cared before. Jan 2 '18 at 20:58
• I strongly doubt that a human can adapt to 2g. A fifty kilogram person cannot just carry fifty extra kilograms. With 20kg worth of extra muscles perhaps, but then you're at 2x70=140kg total. Not to mention your circulation etc. Otoh 1.4 should be unproblematic for a healthy, sportive individual, although I'd definitely demand extra pay. You don't want those astronauts to just be there, but do actual work, right?
– Karl
Jan 2 '18 at 21:17
• @Karl The returning astronauts from the space shuttle DID go through a 'regravitation' procedure. The Russians seem much more open to showing this to the public than the Americans do. But there is a 'return-to-earth' protocol process that astronauts go through. It involves bed rest and remaining seated. I really like the foresight of the suggestion. Jan 2 '18 at 21:33
• @Karl, the comments are as fun as actually answering the qeustion. Working out the gazintas and gazoutas of how something that doesn't exist might work is fun, and it illustrates how nearly impossible it is for just one person to solve almost any problem in today's complex world. You insight into the gym is good. A 1.1 or 1.2g environment would help, but the 1.4 would have a high liklihood of substantial injury (just as powerlifters today do). Jan 2 '18 at 22:24

When you say 'Best' do you mean 'best for the crew', 'best for operation of the ship', 'best for maximum use of space', or 'best for the author, in case the book is made into a film'?

Star Trek had 'normal' artificial gravity simply because it was much easier to film the series in 'normal' gravity.

But as we now know from ISS, a lack of gravity is really, really useful for utilizing every available bit of space in the ship. With no 'up' or 'down', and no 'falling out of cupboards', then storage lockers can be put on all of the walls, complete surround storage. So in terms of space utilization efficiency, no gravity is best.

But in terms of housekeeping, when suspended particles never move, except in the air flow, keeping the central room space empty requires a bit of gravity to attract everything to a surface. Thus, for housekeeping it makes sense to have a surround gravity. That is, every surface in the space has some gravitational attraction sufficient to keep things on the surface.

In terms of human health, are the inhabitants ever expected to return on-planet? If they are permanent spacers, then humans can do just fine in point 2 or point 3 gravity. Mars, for instance, would need no gravity enhancement in the living spaces.

But since inertia is always the same with or without gravity, the lower the gravity the slower the spacemen could move. Accelerating too fast means that, when they hit solid wall, they do so with a painful thump. So they would move very slowly to limit acceleration, and therefore minimize inertia. Having all of the outside surfaces be the gravitational attraction would perhaps ease the problem, because there would always be 'counter gravitational forces' in every direction.

However, gyms and such exercise and sports spaces would definitely want high 'up-down' gravitational fields. When jogging, for instance, you want sufficient gravity to allow high acceleration without inertial run-away, and you want the track to be 'down'. You want sufficient gravity to allow good friction between boot and tack surface.

To maximize sleeping surface, a round tube where bunks would be placed all around the outside, with minimal gravity around the outside to keep the occupants from drifting. In naval ships,the bunks are stacked four and five high to maximize sleeping space. Imagine if they could utilize top and bottom surfaces? That is, crew members face each other when sleeping.

But eating quarters and dining rooms? Definitely things work better when the table is down and your mouth is up. Makes food much easier to cut and stab.

Office-type quarters would require a constant up-down gravity field. Typing is rather difficult when every key press sends the typist backwards. Writing is hard when an over-emphatic pen stroke sends the writer flying off to the side. Fidgeting in the chair can launch one into an unintended trajectory. And squirming in a chair? Forget it, unless something is 'sticking' you to it.

However, I can foresee that perhaps the propulsion system would work best in no gravity. 'Containment vessels' are much easier to control when there is no gravity pulling on the object to be contained. Place the substance in the general area, it does not move. Turn on the containment field around it. No need for supports before the containment field is turned on.

So the bottom line. If you are really after 'best', and the technology allows it, different gravity on different surfaces in different parts of the ship would be 'best overall'. Say in the storage areas, no gravity; in the gyms, high gravity on the playing surface (perhaps 'dial-a-gravity' to increase the intensity of the workout); in sleeping quarters, mild gravity around the walls; in transit hallways, no gravity but lots of handholds, padding, and inertial shock absorbers at the ends; in the dining room, low gravity on the table; and in office-type areas, low gravity in an up-down orientation to allow paper to remain still in slight breezes, and people to stay sitting even while fidgeting.

And how about 'dial-a-gravity' special surfaces throughout the ship for housekeeping? Turn them up, way up, all the garbage gets attracted to them. Just wipe the surface off, and return to normal gravity situation. No more clutter, no more dust.

• Okay, my last point just created in me the image of 'cleaner maintenance bots' traveling throughout the ship, with a high gravitational field around them, drawing everything into them. Like a vacuum cleaner on steroids. Jan 2 '18 at 15:58
• If they are permanent spacers, then humans can do just fine in point 2 or point 3 gravity.[1] Citation needed. Given the detrimental health effects observed on modern astronauts from mere weeks or months in micro-gravity, it seems like our bodies were designed to work under 1g, and don't tolerate large deviations from that particularly well. I'd bet that unless the gravity is set at or near 1g, your crew would start experiencing undesirable side effects. Jan 2 '18 at 20:16
• @ HopelessN00b These side effects are only an issue if you want to return to a high gravity environment. And they can be countered by exercising in a high gravity gym. There are no functions of the human body that are dependent on gravity, they tend to just atrophy in the absence of it. By your own reference, the lack of day-night cycling and work stress are the major contributors to ongoing health issues. Jan 2 '18 at 21:16
• Losing 1% of your bone mass a month, and degenerative weakening of your muscles, including your heart muscle, don't sound like issues that only matter in normal gravity. Maybe osteoporosis and muscle atrophy are more tolerable in micro-gravity, but I suspect that cardiovascular degeneration is going to be a big problem for people who want to continue living, even if they stay in micro-gravity. Of course, one of the other side effects is kidney stones, so maybe they'd rather have their hearts give out quickly than spend years passing kidney stones in micro-gravity. Jan 2 '18 at 21:26
• @JustinThyme, the human body is a carrier for a huge number of bacteria -- by cell count, a human can be considered a multi-species bacterial colony with a eukaryotic life form wrapped around it. Some of these bacteria are essential for life, most are neutral, some are harmful, and all are kept in check by the immune system. Even ignoring the possibility of mutation, a loss of immune function is going to result in long-term problems.
– Mark
Jan 3 '18 at 9:39

The best setting for gravity plates onboard the Exciting Undertaking would be a constant 99 g. The gravity plates capable such strong gravity would be installed in the ceiling of all habitable work spaces accessed by the human crew. The benefit lies in maximizing travel time across interstellar space. There will be a second set of gravity plates installed in the floors of all work spaces.

One of the less commented upon problems of interstellar space travel is the long acceleration times required to reach sufficiently high velocities to travel between stars within modest fractions of a human lifetime. Accelerating close to lightspeed provides the necessary velocity and comes with additional bonus of relativistic time dilation.

Obviously a mighty space craft like the Exciting Undertaking if it is powered by antimatter reactors will have no difficulty in being equipped with a propulsion system, doubtless powered by antimatter, capable of accelerations of up to one hundred gravities (100 g). At an acceleration of 100 g the Exciting Undertaking will attain near-lightspeed in roughly four days.

This will give it a brief acceleration phase, push it to sufficiently close to lightspeed to provide a reasonable amount of relativistic time dilation, and thereafter its antimatter engines will generate a constant 1 g acceleration up until it reaches the midpoint of its journey when it commence a long 1 g phase. Once the Exciting Undertaking is within two light days of its destination it can once more engage in a 100 g deceleration phase.

The 99 g gravity plates will compensate for excessive "gravity" in the acceleration and deceleration phases. The human crew will experience an apparent internal gravity of a comfortable 1 g during those acceleration and deceleration phases.

Please starships capable of constant 1 g acceleration will take of the order of one year plus to attain similar amounts of close to lightspeed velocities. The Exciting Undertaking, by contrast, using its 99 g gravity plates as acceleration compensators will do so in only four days. This will reduce its rest frame travel times by about one year. It will also spend more time experiencing relativistic time dilation. The human crew will have shorter travel times.

It is noted, more in sorrow than anger, that the OP has not specified whether the Exciting Undertaking is capable of faster-than-light travel. If it is a FTL vessel, then this cunning scheme for enhanced relativistic spaceflight by means of compensating gravity generators with high-gravity settings may have been in vain.

• That has such delightful potential to go horribly horribly wrong Jan 3 '18 at 11:42
• @Separatrix Oh yes, there is absolutely no way that this will ever cause your crew to be turned into puddles of fleshy goo on the ceiling. Yep, no chance at all. Jan 3 '18 at 12:48
• @Separatrix I'm glad you noticed. But fortunately the infallible technology installed in the Exciting Undertaking has it under control. Lowest possible tender, you say. Where does that enter into the equation? No way! Jan 4 '18 at 0:53
• Interesting. Your conjecture leads to some interesting physics. Time dilation is also mediated by gravity. Could these gravity plates be used to reverse the effects of time dilation due to speed? I am not about to do the math, but would accelerating to almost cee in four days cause the 'drive' portion of the ship to be at a much higher velocity than the far-away-from-drive section? The closer to the gravitational source, the slower time goes. But it has to be 'true' gravity. not apparent gravity due to acceleration. Make EVERY point in the ship a 'high gravity' source? Aug 28 '18 at 15:44
• And does artificial gravity cancel artificial gravity? Would a 100g source at one end and a 99g source at the other end cancel to 1g? Or would they both 'pull' each other with a net 199g? Would the time dilation effects due to gravity be equivalent to 1g, 100g, or 199g? Aug 28 '18 at 15:53

(The following assumes "proper" artificial gravity, i.e. that is, as long as you do not try to move the source, indistinguishable from real gravity. Not sth. which is directed, like the electric field in a capacitor.)

Basically, the graviational field of a point-like source decays with square of the distance, of an infinite rod linearly with distance, and of an infinite plane is constant. For a finite-size ship, that's still approximately true as long as you're significantly closer to the source than to the edge of the source.

You surely want the gravity gradient to be small on the length of a human body to avoid a lot of upturned stomachs. ;-) That rules out a point source, because you have to stay far away from it, and you have to make it dangerously strong. A rod-like source would still mean a huge inaccessible space in the center of your ship, and it's unclear to me how you would hold it, because it would still crumble any construction material in close distance.

So the most sensible, economic idea is probably a flat generator, with a pyramidal multi-story ship on one or both sides. Make the pyramid a bit higher to add rooms with lower gravity. The generator tiles in the outer area are turned up a bit, so you don't have to tilt the floor too much in that area.

That's also the big problem with adjustable gravity (unless you do it ship-wide): If your neighbour thinks he wants to sleep at $0.6g$ tonight instead of the regular $0.8g$, your glassware falls out of the cupboard, not to mention the danger of the same cupboard falling onto you.

As to the absolute $g$ value, that's totally in your hand. If the generator running costs are cheap, go with $9m/s^2$. Otherwise I'm sure humans would do fine with 5 or even 4. Maybe have a gym with higher gravity on the other side of the plane and lock your spacefarers in there for one hour on every weekday.

• But doesn't gravity always pull towards the 'center of gravity', no matter what the surface configuration or shape? Jan 2 '18 at 16:00
• @JustinThyme Not exactly. A planar generator looks to everyone on the surface like a (incredibly strong) point source located far away under the plane. The surface (outer, at the same radius) gravity on earth would be the same if all the mass was concentrated in a singularity at the center.
– Karl
Jan 2 '18 at 16:03
• All this assumes of course "regular" gravity, only it's not generated by a huge amount of mass . On USS Enterprise, they obvously have things working quite differently.
– Karl
Jan 2 '18 at 16:09
• Like, apparently, that single point source is somewhere out in the middle of space, 'below' the ship. Jan 2 '18 at 16:14
• Obviously a different inertial fame of reference. Jan 2 '18 at 19:45