I am a physicist and a physics teacher and so of course am well aware of this popular lecture demonstration where a bowling ball or some other heavy object is hung from the ceiling of a lecture hall. A person then stands some distance away from the lowest point at which the ball is resting and brings the ball up to their nose with the rope kept taut. The ball is then released and it swings away from the person and returns, just like a pendulum. The point is whether the person will flinch when they see the ball come back towards their face. The law of conservation of energy predicts the ball will not rise higher than the initial release point and so will never hit the face, and the panel looks at how different scientists might respond to being in that situation.
I disagree with the engineer’s reasoning. I cannot think of any way that you can accidentally incorrectly hang the ball so it rises higher at the end of the cycle than at the beginning. To achieve that would require the deliberate insertion of some device at the suspension point to impart some energy to the system, like the way that we do with a pendulum held in one’s hand, where a slight wiggling motion by the hand can increase the amplitude of the oscillation.
So we would be safe on that score. As a physicist, you might think that I would be confident and not flinch. Actually, I never do this experiment and it is not because I do not trust the law of conservation of energy or worry about incorrect hanging. I think that flinching, as the biologist says, is a perfectly valid reaction and says nothing about one’s faith in the law. Why I do not do it is because the person might subtly and involuntarily move their head forward after releasing the ball and so get hit. I would not risk myself or my students to such a possibility.
But the physicist’s argument about trusting the law of conservation of energy is also worth considering. What does it say to ‘trust’ a law of physics? Does it mean that we are 100% confident it will not fail? The history of science has shown us that all laws have to be considered fallible. Philosophers of science have concluded that we cannot be absolutely sure of any law and that however many times a law has been sustained, it is no guarantee that it will not fail the next time. This is known as the problem of induction.
But we have no choice to trust the laws of science all the time because our entire lives and societies are based on them. When I fly on a plane, I am putting my trust that the laws of mechanics and electrodynamics and others that enable it to fly will not fail, even though I am not 100% sure of that. So why am I willing to get on a plane, risking my life, while not putting my face in front of a pendulum, even though in both cases I am placing my trust in laws of science that are not guaranteed to be true? It is because of a simple cost-benefit analysis. The benefits of plane flying more than compensate for the very slight chance of the laws failing. There is really no benefit in risking getting hit in the face by a bowling ball other than as a demonstration of my ability to control my flinching reflex. It is mostly an act of bravado.
So xkcd should put in an extra panel (or replace the engineer one) with a panel with a philosopher of science expressing the reason why it is not a good idea.
As you might have guessed, this discussion of why we can and need to trust the scientific consensus even if we are not certain that it is true is the heart of my book The Great Paradox of Science: Why its conclusions can be relied upon even though they cannot be proven. You should read it if you have not done so already!