## Can artificial gravity be created in space? (Intermediate)

I would like to ask, in space, is possible to create artificial gravity? If so, how? And if not what are we missing to create this gravity?

Dave: In space, it is possible to create "artificial gravity" by spinning your spacecraft or space station. When the station spins, centrifugal force acts to pull the inhabitants to the outside. This process could be used to simulate gravity. It wouldn't be exactly the same, though, because large Coriolis forces would also be present, and things would fall in curves instead of straight lines.

I am reading about space colonies and I saw some pictures of what it would be like inside a cylinder shaped space station. There is land all around the inside, so if you were standing on land in it and looked up, you would be looking down on the people above you. How would the gravity work then and wouldn't the gravity of the people above you pull you "up?"

Lynn: As stated above, the method proposed for creating artificial gravity on a space station is to use a rotating system (like a rotating cylinder, torus, or sphere). Technically, rotation produces the same effect as gravity because it produces a force (called the centrifugal force) just like gravity produces a force. By adjusting certain parameters of a space station such as the radius and rotation rate, you can create a force on the outside walls that equals the force of gravity.

This is sort of like the amusement park ride where you get in a big cylinder with a lot of people and line up against the walls. Then they spin the cylinder, producing a force that makes you feel pressed up against the sides. Everyone becomes glued to the walls of the chamber, and then they drop the floor out. No one falls to the ground because they are being held to the edges by a force due to rotation. Another example would be swinging a bucket of water around over your head. The water doesn't fall out if you spin the bucket around fast enough.

In a rotating space station, people will be "stuck" to the outside too, but with a force equal to that of gravity so they will be able to walk around on the edges. The force will be the same all around the outside of the rotating cylinder, so depending on the design it could look like people are living on the ceiling!

The gravity of people around you will not make any noticeable difference. It is true that all objects which have mass exert a gravitational pull on other objects, but unless the mass is very large (like the earth) it has little effect. The people on the space station will not change the artificial gravity on the space station just like they do not effect the gravity while they are on Earth.

Can gravity be produced in a space city without rotating the whole city?

Ryan: As far as anyone knows, there is no way to produce gravity other than with mass. Things that have mass have a certain amount of gravity and will interact with other things that have mass. By rotating a city in space you would not create gravity, you would simulate it. Assuming your city was ring-shaped, and spinning fast enough, everything in it would feel a force pulling them outward, but it would be the centrifugal force, not gravity. For most purposes, it would act in a similar way, but it would not be identical. For example, if you dropped something from very "high" (close to the center of the ring) it would not hit the ground directly below it. As the falling object traveled toward the "ground" the ground would rotate underneath it. This is sometimes called the Coriolis force. If you read Arthur C. Clarke's novel "Rendezvous with Rama" he describes a city that rotates to simulate gravity, and even talks about what its weather might be like.

Can gravity be produced uniformly over a surface, from gravitons???

Ryan: Gravitons are theoretical particles that would carry the gravitational force, the same way that photons (light particles) can be thought to carry the electromagnetic force. Gravity is very weak compared to the other forces in the universe, so its force-carrying particles are very difficult to detect. Nobody has ever detected a graviton, and the only way that we know of to produce them is to have mass, as I described above. So the only way that we know of to produce gravity uniformly from a surface would be to make that surface have a lot of mass!

I can't see how spinning anything in space using centrifugal force could work. Don't you have to have weight to do that? Otherwise the floor would just slip beneath your feet. And if there was a wall attached to the floor, you would only be slapped by it. I only see it working if you had some sort of constant acceleration beneath you, pushing you in one direction to keep you upright.

Ryan All you need to feel centrifugal force is mass. A brick has the same mass on the surface of the earth as it does in deep space. Weight is the force that something feels due to gravity: so the brick would have a much larger weight near the earth's surface than it does in deep space. Now that we have that out of the way, let's talk about the centrifugal force.

I can see why you are confused: if I understand correctly you are picturing an astronaut floating inside a stationary space station and not touching anything. When the space station begins to rotate, it seems like it shouldn't do anything to the astronaut, right? Right! If there is nothing exerting a force on the astronaut, they will just float while the station rotates around them.

But a space station is not empty. There is air, and as you say, there are walls and other obstacles. As the station begins to rotate, the air inside will begin to move with it, so even without any other objects, a slight push from the surrounding air would make the astronaut start to move. Once the astronaut is moving, it is only a matter of time before they come into contact with the outer edge of the rotating space station (which is soon going to be the floor).

Now we need to think about how the centrifugal force works. There is another way to think about it that I will describe here, which makes it seem like it's not a real force at all! Ok, so what is a force? A force is something that causes a mass to accelerate. We defined mass up above, but what is acceleration? The simplest definition is that acceleration is a change in how fast something is going or the direction that it is going. So, our astronaut had been nudged by something inside the space station that is rotating with the station itself, and the astronaut is now drifting along in some direction. The laws of physics say that the astronaut will travel in a straight line (with no acceleration) until some new force acts. If you are an astronaut traveling in a straight line inside a round, rotating space station, eventually that straight line will intersect with the "floor". When it does, your body wants to keep traveling in a straight line, but the floor is in the way, so it gives you a nudge inward and in the direction that it is moving. The floor exerts a force, which causes acceleration.

You can probably see where this is going. As the astronaut gets bumped along by the rotating floor, eventually, they will be going at the same speed. But remember, acceleration is a change in speed or direction! If you watched from afar, the astronaut would appear to be traveling in a curved path, around and around the circular space station. To travel in a curved path, there must be some acceleration to keep that astronaut's path turning, and therefore there must be some force. Which direction is the force? Well, if you were the astronaut, you would now be standing on the floor of the rotating space station with your feet pointing outward from the axis of rotation. It would feel like the floor is pushing on your feet, so the force is actually inward, toward the central axis!

Confused yet? Ok, think of it this way. When you stand on the ground on the earth, which way is the force of gravity? Down. So why don't you fall in a straight line to the center of the Earth? Because the surface of the earth pushes up on your shoes with the exact same force but in the opposite direction! Same idea in the space station: the centrifugal "force" appears to push objects outward, but the strength of the space station provides an opposing "centripetal force" which pushes inward.

The end result is that it feels similar to walking on the surface of a planet (if the space station is spinning at the right speed).

Page last updated on June 22, 2015.

#### Ryan Anderson

Ryan is a research fellow at USGS in Flagstaff, AZ and is a member of the Curiosity ChemCam team. He also loves explaining all aspects of astronomy. Check out his blog!

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