I am reading Steven Hawking’s new book (for the semi-ignorant masses like myself) The Grand Design. I am enjoying the book thoroughly. It is the first time I've seen Richard Feynman get the attention he deserves. About halfway through the book Hawking talks about a consequence of Heisenberg’s uncertainty principle, that there is no such thing as empty space. Empty space cannot be empty because then the both the value of the field, and the rate of change would be exactly zero and that cannot obtain. Although it seems to me that the problem Heisenberg uncertainty principle depends upon is having something to observe. So this argument seems a bit circular. But, there's a much easier way to show that space is not empty.
A little thought experiment would suggest that within the knowable universe, if Einstein was right, it’s pretty easy to decide space is not empty, in fact it is quite full, maybe completely full (whatever that means). Einstein suggested that energy and mass are forms of the same thing. The conservation of energy and the conservation of mass were both wrong, but combined are true.
Imagine you are in orbit around the earth. You look away from the sun. What do you see? Stars! You see billions of stars. In fact, if your eyes were more sensitive, perhaps more sensitive than the Hubble Space Telescope, perhaps you could see a couple hundred billion galaxies, each made up of 200 billion to 400 billion stars. That’s a lot of photons hitting your little eye all the time. Now imagine you move a few inches over to your right. What do you see? The same thing. If fact, wherever you move in space you will still be impacted by billions and billions of photons. Thus, space is not empty. Now, all I need is a photon sail and I'm off.