All worlds in the Altris Crystal Sphere generally obey the same laws of physics as does our world in reality. This means what goes up generally comes back down, for every action there is an opposing reaction, can’t breathe under water and so on. So without the aid of magic or some other means, if it cannot be done in reality, then it more than likely cannot be done here, with some exception listed below.
Crystal SpheresA crystal sphere holds a realm or solar system. All planets stars, comets, asteroids, and all other things in that realm are contained inside this sphere. The shell of the sphere is impossible to break, but there are portals that allow travelers to move outside the sphere into something called The Phlogiston. This is a highly volatile gas that exists between the crystal spheres.
Travel between planetsTravel between planets in the same crystal sphere is possible using air ships with magical propulsion. Leaving an atmosphere the ship provides the gravity needed to take an atmosphere with it that can sustain a crew for several days (depending on the size of the ship, and the size of the crew.) Falling or being thrown off a ship, the body’s gravity will also take air with it, though this will only last for about an hour, and unless the character has some sort of magical means of movement, he/she will drift until picked up by something or someone. These concepts can be found in the 2nd edition Advance Dungeons and Dragons Spelljammer rule set.
Planes of Existance
Our world resides in a plane of existence known as the Prime Material Plane. This prime plane is made up of four primary elements (earth, fire, water, air) and two energies (positive and negative). These elemental planes are called the inner planes, and the location where all of the merge is the prime material plane. The area between the primary elemental planes are called para-elemental planes, and those that border the energy planes are the quasi-elemental planes.
The Ethereal Plane
The space between the inner planes and the prime material plane is call the Ethereal Plane. This plane comes into direct contact with every point on the prime material plane and every point on all of the inner planes. It is through the ethereal plane that people can travel to the inner planes.
The ethereal plane is broken down into the deep ethereal and the border ethereal. The border ethereal is what comes into contact with all the planes it borders. The deep ethereal is a vast emptiness with small pocket planes throughout, including what is known as demi-planes.