am-tep

A very good suggestion.

Molecules in a gas move randomly because they undergo 'collisions' with each other.These collisions are nothing but repulsion of like charges. As the colliding molecules move further apart there is no force in gases which counters this repulsion.In liquids the cohesive forces(which are also em in nature) counter the repulsions to some extent.So gases try to occupy as much space as possible.But in spite of this spread, gravity does ultimately manage to hold the molecules together to some extent. If it was'nt so, the atmosphere would not stick to the Earth and niether would the oceans!

Really? Where does the pressure of a liquid or a gas come from? Electromagnetism. And to go a hundred metres up, the electromagnetic force needs to work against gravity. So the energy needs to be stored up. So you aren't comparing the actual field strengths.

The original question was beautiful. The answer is just as beautiful. Air gets it's 'power' from the electromagnetic force. In fact, gravity is an extremely weak force. Any ordinary table can stop a book from falling countering the gravity due to the entire earth in the process!

Photons do have mass.The rest mass of a photon is zero. Which means that the photon will cease to exist at rest. However, a photon can never be brought to rest. The photon is even influenced by a gravitational field. As you may be aware light bends around a dent in the spatial fabric. It responds to the presence of mass,that is to gravity. Thus a photon can easily impart momentum for it has momentum.

Why do photons have momentum? Well... As an example, consider the infra-red photons or even microwave radiation.EM-Radiation is capable of increasing the temperature of an object.Which means it can impart momentum to the molecules thereby changing their kinetic energies.Considering the fact that momentum is a conserved quantity, we are forced to conclude that radiation must possess momentum.