Creating a beliavable virtual world would be very difficult. Look at the world around you -It's extremely complex. Strangely enough, when you dream, you can see such complex things. This is one idea I was thinking of giving to sony who are working on VR systems; There is a part of the brain that differentiates between what is real and what is not, or part-S of the brain. (I got this info from newspaper so it's not that specific) They should find a way to stop this part of the brain, or alternatively put the brain in a 'dream state' so that basic graphics can be shown but the brain (I'm guessing) will do the rest of the work to make it seem real.