In a swing, the back-and-forth linear momentum comes from the tension in the wires, while the energy comes from your legs. No conservation laws are broken.

It is not possible to ride an unanchored (and non-couterbalanced) swing in a vacuum.

What special properties does an atmosphere confer to a swing?

You need “downwards” acceleration for a swing. The vacuum bit isn’t important, it’s the gravity. They probably meant 0g rather than vacuum.

It slows you down :)

The vacuum part was an emphasis on requiring a counterpart to change momentum.

In an atmosphere you could flap your wings, or use electric fans, and argue what it means to ride a swing, but it would be besides the point, as in an atmosphere there is no need for propellant-less engines from the article.

Maybe they meant in 0g?

none, but centers of gravity do

If you can move levers, it's possible to rate your body. Maybe the inertia can be used as an anchor? Or you could use a sail.

A swing works by moving you back and forth relative to the Earth (and moving the Earth back and forth slightly as well, in the opposite direction, more or less.) The you + Earth system is not actually going anywhere. :-)

So you need to leave something behind in order to go somewhere? But it would only be relative to the thing I left behind? But then there is some "special" relativity that applies everywhere?

> So you need to leave something behind in order to go somewhere?

As a definition, no. But no one has been able to figure out reactionless propulsion; as far as anyone can tell, the only way to accelerate in space is to throw something in the opposite direction (matter or light).

If you tie a rope to a hammer, then spin it around as fast as you can like a hammer thrower, and drop it. Wouldn't the breaking force propel you in the same direction your let go of the hammer?

If you're in zero g and not on a surface with friction to push against, you're not going to be able to do what a hammer thrower can do standing on Earth.

But, if you provide some momentum to an object like a hammer and release it, you will move. That goes back to "you need to leave something behind in order to go somewhere."

A swing would need gravity or some other driving forces for momentum.

riding bikes in 0g is also quite difficult, or so i hear.