Science is based on fact, right? Cold, unchanging, unarguable facts. Perhaps not, says physicist Tara Shears.

Tara is more inclined to follow the principles of the Anglo-Austrian philosopher, Karl Popper. He believed that human knowledge progresses through 'falsification'. A theory or idea shouldn't be described as scientific unless it could, in principle, be proven false.

Raised in a Vienna in thrall to Marxism and Freudianism, Popper bristled against these 'sciences' which could adapt and survive to prevailing political and social conditions. They could not be proven false and so they were not science. The ideas of Einstein, by contrast, could be tested scientifically and might one day be proven false.

An interesting principle certainly, but potentially demoralising for a scientist who could see her life's work dissolve in front of her eyes. Tara joins her colleagues at the Large Hadron Collider in Geneva to ponder the implications of Popper's work. She also meets Popper's former student, John Worrall and string theoretician David Tong.

This is part of a week of programmes asking how we can know anything at all.