Is artificial gravity scientifically possible?

[Skeptic Ginger]

Where are you and Steve001 getting the idea that we don't understand gravity?

Where did you read me saying that?

I said harnessing dark matter.

 

[Roboramma]

Where did you read me saying that?

Well, Steve001 said: "The nature of gravity is unknown and the question you've asked is open." and you seemed to be agreeing with him. Maybe I misunderstood.

I said harnessing dark matter.

Dark matter's behavior and interaction with respect to gravity is no different to normal matter. Dark matter is only special in the sense that it doesn't interact with the electromagnetic force (hence "dark"). Energy causes spacetime to curve, and dark matter, ordinary matter, light, all of these things have energy.

You did suggest that your idea could be similar to the idea of having some extremely dense matter as a source of gravity. I agree that there's nothing in the laws of physics that would prevent that, but it's not really artificial gravity, it's just normal gravity. We have gravity on the earth because there's a huge mass beneath our feet. The problem with doing that on a spacecraft is that you'd need a similarly huge mass, and if you want to move your spacecraft you don't want to have to move a mass comparable to the mass of the earth.

I guess I'm exaggerating slightly: You wouldn't need something quite on the order of the mass of the earth, because the distance from the center of mass would be much less. But let's say that you concentrated your mass into something 1 meter in thickness*. The radius of the earth is, what, 6,000,000 meters? Gravity is proportional the the square of the distance, so if we're 6 million times closer, the force should be 3.6x10¹³ times greater, meaning we need 1/3.6x10¹³ of the earth's mass to produce 1 g.

One thirty six trillionth sounds like a small fraction, but as a fraction of the mass of the earth it's still an enormous mass. Whether you're supplying that mass with ordinary matter or dark matter, or with theprestige's idea of taking a diffuse energy source and concentrating it (maybe you're firing lasers through tubes in the floors) when and where you need it, you're still having to deal with that enormous amount of mass (or energy).

*never mind how you actually accomplish that. It's an engineering issue.

 

[Skeptic Ginger]

Well, Steve001 said: "The nature of gravity is unknown and the question you've asked is open." and you seemed to be agreeing with him. Maybe I misunderstood.

Yeah, sorry about that. I quoted him because the post he answered gave me an idea. Not your fault, I shouldn't have quoted anyone.

Dark matter's behavior and interaction with respect to gravity is no different to normal matter. Dark matter is only special in the sense that it doesn't interact with the electromagnetic force (hence "dark"). Energy causes spacetime to curve, and dark matter, ordinary matter, light, all of these things have energy.

You did suggest that your idea could be similar to the idea of having some extremely dense matter as a source of gravity. I agree that there's nothing in the laws of physics that would prevent that, but it's not really artificial gravity, it's just normal gravity. We have gravity on the earth because there's a huge mass beneath our feet. The problem with doing that on a spacecraft is that you'd need a similarly huge mass, and if you want to move your spacecraft you don't want to have to move a mass comparable to the mass of the earth.

I guess I'm exaggerating slightly: You wouldn't need something quite on the order of the mass of the earth, because the distance from the center of mass would be much less. But let's say that you concentrated your mass into something 1 meter in thickness*. The radius of the earth is, what, 6,000,000 meters? Gravity is proportional the the square of the distance, so if we're 6 million times closer, the force should be 3.6x10¹³ times greater, meaning we need 1/3.6x10¹³ of the earth's mass to produce 1 g.

One thirty six trillionth sounds like a small fraction, but as a fraction of the mass of the earth it's still an enormous mass. Whether you're supplying that mass with ordinary matter or dark matter, or with theprestige's idea of taking a diffuse energy source and concentrating it (maybe you're firing lasers through tubes in the floors) when and where you need it, you're still having to deal with that enormous amount of mass (or energy).

*never mind how you actually accomplish that. It's an engineering issue.

I was talking about sci-fi idea a few hundred years or more in the future. That means speculating about elements of dark matter we don't yet know about: like maybe something to do with extra dimensions quantum theory suggests.

You have more details obviously, but I was speculating about some things one can imagine might fit physics. Like how hyperspace is a valid concept even though we can't fold space-time yet.

 

[smartcooky]

Have you never gravitied from 0 to 40 between the lights on Sauchiehall street?

No, but in my younger days, I pub-crawled the length of it.

Does that count?

 

[Lothian]

No, but in my younger days, I pub-crawled the length of it.

Does that count?

Certainly. I can verify that pub-crawling generally tends to have a big effect on gravity making walking quite tricky.

 

[P.J. Denyer]

I was thinking along the same lines, what if you had a piece of hyper-dense matter located someplace around the central core of the craft that would in essence, attract everything loose towards that central point?

The other problem with this is inorder to have appreciable gravity your craft has to haul around the mass of a small planetoid, minimum. That's going to give your slick, powerful space craft the acceleration, braking and handling of a 1950's Cadillac running on one cylinder with the trunk and backseat filled with lead weights.

ETA: Ninja'd by Puppycow.

 

[P.J. Denyer]

The best sci-fi has this covered.

You accelerate at up to 1 G until you're half way to your destination.

Everyone straps down and the ship flips over end to end.

Then you decelerate at the same rate until you arrive at your destination.

This way your ship doesn't need moving parts that are destined to fail, and really horrible interfaces between moving parts and non moving parts. It also means you don't have to have the universe spinning around outside your windows. AND it means that your ship can have common 'floor' surfaces that are always in the right place.

One of the details I loved about "Rendezvous with Rama" was that the height of the 'cliffs' on either side of the central sea allowed them to calculate the maximum acceleration and deceleration of the craft. Of course that's assuming it doesn't have a second burst once the water is frozen.

 

[3point14]

You do see the problem with that though, right? If you have enough hyper-dense matter to create significant gravity, your spaceship is going to weigh as much as a small planet, and it will be much harder to maneuver and accelerate or decelerate.

Good point. It would probably massively reduce the DeltaV of such a craft...

 

[RecoveringYuppy]

Where are you and Steve001 getting the idea that we don't understand gravity?

Even if SG didn't mean what she appeared to there are still an unusual number of people expressing the idea that we don't understand gravity. We've understood it well enough theoretically since the time of Newton. And we've been producing artificial gravity (without realizing it) since prehistoric times (such as slings described in the David vs. Goliath story).

 

[RecoveringYuppy]

That is a good point actually. For example if a person throws a ball vertically upwards it will not come down in the same spot, like it would on earth. It will start having a horizontal vector from the thrower's point of view. A ball thrown at an angle would have a strange path. It could also produce motion sickness if a person is going up or down.
[... snipped something I'll come back to...]
The frustrating thing is that I tried to do a google search and though found a bit on the Coriolis effect, not much about the strange effects it would have on humans in a spacecraft.

The things you mentioned are also true to a smaller degree on Earth. It's just a matter of degree. And these affects and others were studied to a great degree back in the 70s.

How much you experience these affects on a space craft is a matter of it's rotation rate and radius. Studies in the 70s pretty came to the conclusion that 1 RPM was the highest rotation we could live well with. To get 1G at that rate you need a radius of half a mile.

The space station shown in 2001 A Space Oddysey was producing 1/6G (Moon gravity) if I recall correctly. Scaling that up to produce 1G means that style of station would be a torus a mile in diameter. You would have a 3 mile circumference so that would be the longest you could walk to get back to your starting point. It would be large enough to accommodate 10,000 people in an urban setting. If the occupants were prepared to accept something like aircraft carrier living conditions it might accommodate 50,000.

It would mass something like 20 million tons and most of that would be radiation shielding. It would require about 2 million tons of refined materials such as aluminum, titanium, and glass. If spread over 5 to 10 years that's not a large amount of refining industrial speaking. By Earth standards that would be a small mine on the moon and a small refinery/smelter somewhere in space.

And, this may be hard to believe, but it's not out of the question that we could launch 2 million tons of refined materials from Earth over that material and just get the slag for radiation shielding from the Moon or passing asteroid. It would require twice daily launches of the largest rocket ever designed for five years. Approximately 4,000 launches. Committing to 4,000 launches would be a game changer, it would make it possible for designers to invest in an assembly line approach. It could potentially put us at the point that fuel is the dominant cost of a launch which is very far from where we are now.

Not to mention the fact that a person's feet would have greater gravity than the person's head.

This is negligible in a large station. And it can also be put to advantage. The 1G area can be reserved for people to live in. Farming and industry could be in cheaper low gravity sections of the station. Low and zero G recreation and training areas are possible and accessible by a simple elevator ride.

 

[Giordano]

That is a good point actually. For example if a person throws a ball vertically upwards it will not come down in the same spot, like it would on earth. It will start having a horizontal vector from the thrower's point of view. A ball thrown at an angle would have a strange path. It could also produce motion sickness if a person is going up or down.

Not to mention the fact that a person's feet would have greater gravity than the person's head.

The frustrating thing is that I tried to do a google search and though found a bit on the Coriolis effect, not much about the strange effects it would have on humans in a spacecraft.

What you describe is exactly what I was thinking about as "strange effects." Essentially that our expectations of the path of moving objects, and probably our inner ear sensing of our own movements, will likely be different from those on Earth. I was only trying to point this out very generally and I thank you for clarifying my post. The precise effects of course depend on the details of the spinning environment and on the specifics of the moving objects.

 

[Giordano]

The things you mentioned are also true to a smaller degree on Earth. It's just a matter of degree. And these affects and others were studied to a great degree back in the 70s.

How much you experience these affects on a space craft is a matter of it's rotation rate and radius. Studies in the 70s pretty came to the conclusion that 1 RPM was the highest rotation we could live well with. To get 1G at that rate you need a radius of half a mile.

The space station shown in 2001 A Space Oddysey was producing 1/6G (Moon gravity) if I recall correctly. Scaling that up to produce 1G means that style of station would be a torus a mile in diameter. You would have a 3 mile circumference so that would be the longest you could walk to get back to your starting point. It would be large enough to accommodate 10,000 people in an urban setting. If the occupants were prepared to accept something like aircraft carrier living conditions it might accommodate 50,000.

It would mass something like 20 million tons and most of that would be radiation shielding. It would require about 2 million tons of refined materials such as aluminum, titanium, and glass. If spread over 5 to 10 years that's not a large amount of refining industrial speaking. By Earth standards that would be a small mine on the moon and a small refinery/smelter somewhere in space.

And, this may be hard to believe, but it's not out of the question that we could launch 2 million tons of refined materials from Earth over that material and just get the slag for radiation shielding from the Moon or passing asteroid. It would require twice daily launches of the largest rocket ever designed for five years. Approximately 4,000 launches. Committing to 4,000 launches would be a game changer, it would make it possible for designers to invest in an assembly line approach. It could potentially put us at the point that fuel is the dominant cost of a launch which is very far from where we are now.
[snip]

Don't stop there: Ringworld!

 

[PhantomWolf]

I can't think of any top tier SF that does this.

The Expanse

Completely impractical. Take whatever propulsion technology you're planning on using and consider how much fuel you'll need to carry to maintain constant acceleration over the time span of your trip. Don't forget to use the rocket equation.

The Expanse gets around this with their Epstein Drive

 

[Armitage72]

The Expanse



The Expanse gets around this with their Epstein Drive

The Battletech universe also uses it, but only for interplanetary travel. For most inhabited star systems, it's 3-4 days at 1 G acceleration (with a ridiculously efficient fusion torch drive), a flip-over, and then 3-4 days of 1 G acceleration in the opposite direction to stop. Interstellar travel uses nearly immobile ships that carry the transports as they jump through hyperspace 30 light-years at a time, with a week to recharge after each jump, and maintaining a significant distance from a system's star.

 

[HansMustermann]

TBH, that's why I keep mentioning that I favour just hollowing out an asteroid and giving it a spin, whenever this kind of idea about living in space comes about. We can easily find one larger than a mile across, and it would probably cost a lot less to send a few robots ahead to dig tunnels into one, than to send up all the materials to make a pretty torus in the sky.

On the other hand for interstellar travel, I'm surprised they don't use the centrifuge idea more often. The Enterprise D for example is already half a mile in size, so it could have half a g or so by just spinning the saucer. Because, yeah, you can also put the saucer sideways to minimize drag instead of moving the ship along the torus axis.

 

[RecoveringYuppy]

Do you think you can find an asteroid a mile across that won't fall apart if you spin it enough to get 1G?

 

[HansMustermann]

A 1 mile across one? Almost certainly not. A 600 mile across one like Ceres on the other hand? Well, I'm thinking it would probably hold together. We're talking 1/600 RPM at that point, or one spin every 10 hours.

 

[RecoveringYuppy]

Why the heck would you think that? And I think you forgot to account for Ceres existing gravity?

ETA: Every description of Ceres' mantle that I can find suggests rotating Ceres at that rate would simply rip it apart. And I can't think of anyway to get it to spin that fast without destroying it in the process before the spin destroys it.

 

[Mike!]

You do see the problem with that though, right? If you have enough hyper-dense matter to create significant gravity, your spaceship is going to weigh as much as a small planet, and it will be much harder to maneuver and accelerate or decelerate.

I did give it some thought, not much mind you, but, it went along the lines of, does it matter what the craft/ship weighs in space all that much? Since I'm not smart enough to know, I let it go at that.
What if the hyperdence matter was made in ultra thin sheets at the sweet spot between thick enough to generate a gravitational field but thin enough to be manageable? Since it seems mass is the key, and not size. Right?
I think there's a marketing slogan in there someplace... "It's hyperdence, yet ultra thin design..."

 

[phunk]

A 1 mile across one? Almost certainly not. A 600 mile across one like Ceres on the other hand? Well, I'm thinking it would probably hold together. We're talking 1/600 RPM at that point, or one spin every 10 hours.

Forget the rotation rate, rotation isn't what tears it apart, the "artificial gravity" being in the opposite direction is. Whatever you rotate has to be a single solid chunk of metal strong enough to hold it's own weight being pulled outward. Any asteroid that is a "rubble pile" is instantly out of the question.