So, DO All Things Float In Space, Or Not?

Incredibly, I cannot find an answer to this anywhere.

The experts say that planets etc don't actually float in space, but instead continuously 'freefall' around the object they're orbiting and so on.

What about in deep open space, with no objects around and no gravitational pull from anything? What would happen to you if you got out of your ship? Would you immediately freefall into infinite space at significant velocity? Or float? What if you teleported an Earth-sized planet into said space? Are you saying it would literally just immediately freefall forever into the infinite void? And which direction would it freefall, for that matter?

This is such a profound question because, I assure you, the overwhelming majority of people think things simply float in space. And I don't blame them whatsoever.

• Things don't "float" in space. It's just the law of inertia. They wont move unless there's a force. But to get to the heart of your intuition. Yes, if you popped earth in the middle of space it would just "float" there, in the sense of not go anywhere unless it had an initial velocity. "Free Fall" is a dangerous term, because nothing would fall if it were just the earth (unless you were on it of course, you'd fall). But it wouldn't fall. It would just sit there. Lonely. With no friends. – InertialObserver Aug 23 '18 at 4:27
• @InertialObserver So, the natural, default state of all objects in space is indeed to float (unless acted on by another force)? – White Prime Aug 23 '18 at 4:30
• Exactly. That's newtons first law. A body at rest will stay at rest unless acted on by an external force. A body in motion will stay in motion (constant velocity) unless acted on by an external force. The second sentence isn't so relevant though if you're not planning on giving anything a velocity. – InertialObserver Aug 23 '18 at 4:31
• "no gravitational pull from anything" is not possible, as gravity extends to infinity. – hdhondt Aug 23 '18 at 6:30
• @hdhondt Well, that aside, my question is simple: Do all objects float in space? Is floating the default for planets, people etc in space? Or do things fall through just as they would in air? – White Prime Aug 23 '18 at 6:48

This is a good question. What do physicists mean by the term "free fall"? A physicist would say that an object is in free fall if the only force acting on it is the force of gravity. So if you throw a basketball in the air, ignoring air resistance, while the basketball is traveling in its familiar arc (first going up before it comes back down again) it is in free fall for the entire time.

Notice that "free fall" doesn't mean that something is falling. I cannot stress this enough. Even while the basketball is traveling UP, it is in free fall! Free fall has nothing to do with the velocity at which something is traveling. It only has to do with the acceleration of the object.

The basketball travels at multiple different velocities along its arc, but the entire time it is accelerating downward at $9.8 m/s^2$. In fact, all objects in free fall accelerate downward at $9.8 m/s^2$ on the surface of the earth. The reason that you, sitting in a chair right now, are not accelerating is because you are NOT in free fall. You chair is pushing up against your bottom with enough force to keep you stationary.

If you we to go very far away from the earth, the force of its gravity would grow weaker. Therefore, if you were to travel extremely far away in outer space, very far from all other massive bodies, such that the force of gravity on you was negligible, you would indeed be in free fall. However, the forces of gravity acting on you would be minuscule (basically $0$). So yes, you would be in "free fall," but you wouldn't have any appreciable acceleration.

Having said that, if you were far out in outer space, you could still travel at some velocity. By Newton's first law, an object with no forces acting on it will travel in a straight line with a constant velocity. That is indeed what would happen in outer space.

Now what about the International Space Station (ISS)? It is orbiting around the earth. While it is a little further away from the earth than we are, it is still pretty close, and the acceleration due to gravity on it is far from negligible.

Some people say that ISS is in free fall. Why do they say that? Well, there are no forces acting on it other than gravity! So by definition it is in free fall!

But if earth's gravity is always acting on the ISS, why doesn't it ever fall to the ground? The reason is that it is traveling very fast.

Isaac Newton himself conducted his own little thought experiment. He imagined what would happen if you mounted a cannon very high up and shot a cannonball out of it. (This is called Newton's Cannonball.)

If the cannonball was shot at normal speeds, it would fall back to earth. However, as he visualized, if you shot it fast enough, it would begin orbiting earth! Therefore, he realized that things orbit each other due to gravity! This is why the earth orbits the sun (the earth is in free fall around the sun) and why the ISS orbits the earth (the ISS is in free fall around the earth).

Your Question is very interesting, especially because I have also already thought about how objects would move in deep space, so when they are maybe not even inside a galaxy. Well, I think here it is helpful tot hink about space in terms of general relativity. That way, we can see open space as a region, where there is not necessarily a lot of spacetime curvature, vaguely speaking. So when you consider a star that got flung out of a galaxy because of its orbit in a binary star system, then the star would probably move linearly as long as no forces are acting on it; as long as it doesn’t encounter any regions where spacetime is curved a lot.