There are many theories in particle physics that cannot be tested in experiments. Does this make it unscientific? This debate, featuring one of our favourite theoretical physicists, David Tong, explores the question.
The holy grail for 21st century physics is to produce a unified theory of everything that can describe the world at every level, from the tiniest particles to the largest galaxies. Currently the strongest contender for such a theory is something called M-theory. So what is this supposed mother of all theories all about?