"That's the greatest single scientific discovery of the twentieth century. You can't study anything without changing it." Since Galileo, scientists had adopted the view that they were objective observers of the natural world. That was implicit in every aspect of their behavior, even the way they wrote scientific papers, saying things like "It was observed ..." As if nobody had observed it. For three hundred years, that impersonal quality was the hallmark of science. Science was objective, and the observer had no influence on the results he or she described. This objectivity made science different from the humanities, or from religion--fields where the observer's point of view was integral, where the observer was inextricably mixed up in the results observed. But in the twentieth century, that difference had vanished. Scientific objectivity was gone, even at the most fundamental levels. Physicists now knew you couldn't even measure a single subatomic particle without affecting it totally. If you stuck your instruments in to measure a particle's position, you changed its velocity. If you measured its velocity, you changed its position. That basic truth became the Heisenberg uncertainty principle: that whatever you studied you also changed. In the end, it became clear that all scientists were participants in a participatory universe which did not allow anyone to be a mere observer."