Algodoo

Imagine I'm in a box that is on Earth and you are in a box that's accelerating in space, assuming no information (ie matter, light, etc) can get in, there would be NO WAY to tell if you are in a box that is in space, or in a Box that is just sitting around on Earth. Here I use a green rectangle as the ground, which is accelerating upwards and some objects that "fall" to the ground. By the way, this is called "Einstein's Principle of Equivalence". If you don't think that the box is accelerating upwards, I left the clouds on so you see that the ground IS accelerating upwards.