A ball is sitting motionless on a dome. According to standard physics, it should stay put indefinitely, at least until the ball or the dome is perturbed. Suddenly, it begins sliding down the dome without any external force. How is that possible?
While such a scenario violates our intuitive notion that every physical effect must have a preceding cause, philosopher of science John D. Norton at the University of Pittsburgh, shows it's possible within classical Newtonian mechanics. Most surprisingly, there's no way to predict when this motion will occur or in which direction the mass will travel.
"No cause determines when the mass will spontaneously accelerate or the direction of its motion," Norton explains. "The physical conditions on the dome are the same for all times prior to the moment of excitation and are the same in all directions on the surface."
This isn't some mathematical trick that violates Newton's laws. The system fully complies with Newton's Laws of Motion at every instant: "At t=T [the moment motion begins], there is no force and the mass is unaccelerated. At any t>T, there is a nonzero force and the mass is accelerated accordingly." The key insight is that there is no actual "first moment" of motion—any candidate moment is preceded by an earlier one where the mass is already moving.
While this example may seem artificial, it reveals that even classical physics — usually considered thoroughly understood—can still surprise us with behaviors that defy our intuitive notions of cause and effect.
Previously:
• Nobel laureate occasionally hangs out on street corners, answering physics questions
• Actual textbook physics problem
• This physics professor is becoming a TikTok star for her cool demonstrations
• Physicists find evidence of 'negative time' in photons
• 3 Things the Higgs Boson can teach you about physics
