I was reading an old post of Sabine H. which attracted a large number of comments, including a zillion from philosopher Tim Maudlin, whose article was the subject of the post. One statement of his was something like "virtual particles aren't real. They are just convenient fictions in a mathematical calculation."
It turns out that the subject is somewhat controversial even among physicists. Many seem to insist that virtual particles are quite real, and demonstrated by such phenomena as the Lamb shift and the Casimir effect. Others take the more moderate tact of saying something like "it depends on what you mean by real."