How fast is the volume of human knowledge increasing?

Question:

How fast is the volume of human knowledge increasing?

A friend of mine told me that human knowledge doubles every thirty years. Is there any research or publlished material on the subject?

Best Answer - Chosen by Asker: IT is not; IT IS SHRINKING
This site is proof! Er could be, or could be he is spewing horse manure. number of reality tv shows doubles every year though so we are obviously racing ahead as a species. humans constructed pyramids thousands of years ago. The more you learn, the more you know that you don't know anything. So yes, the number of answers increases constantly, but so does the number of unsolved questions and in a greater rate.
Also, don't forget the negative impact of obscurantism, nurtured by some religions (among which ours).