The Physics of Causality

Kate Becker at FQXi:

Our lives are full of experiences that, like cause and effect, only run one way. The irreversibility of time, and of life, is an essential part of the experience of being human. But, incongruously, it is not an essential part of physics. In fact, the laws of physics don’t care at all which way time goes. Spin the clock backward and the equations still work out just fine. “The laws of physics at the fundamental level don’t distinguish between the past and the future,” says Sean Carroll, a theoretical physicist at Caltech. “So how do you reconcile the time symmetric laws of physics with the world in which we live?”

Moreover, we are not passively carried along by time’s river: We make decisions every day in an effort to actively cause the effects we want. Your neighbor’s kid chose to throw his ball recklessly close to your window, after all. Your toddler reasoned that his toy duck would enjoy a swim down the toilet. Deciding, then, is the living fulcrum of cause and effect. Yet physics has no language to describe this essentially human experience. “Physics and math departments, we write equations. And then there are humanities departments, where people talk about emotions and feelings,” says Carlo Rovelli, a theorist at the University of Aix-Marseille in France. “These things should not be separated. The world is one, and we should find the way to articulate the relationship between these fields.”

Now, with the support of two independent FQXi grants, Carroll and Rovelli are taking different approaches to untangling the messy knot that links the physics of cause and effect to our perceptions.

More here.