Brief Answers to the Big Questions

by Stephen Hawking

To see where a particle is, one has to shine light on it. But by Planck’s work one can’t use an arbitrarily small amount of light. One has to use at least one quantum. This will disturb the particle and change its speed in a way that can’t be predicted. To measure the position of the particle accurately, you will have to use light of short wavelength, like ultra-violet, X-rays or gamma rays. But again, by Planck’s work, quanta of these forms of light have higher energies than those of visible light. So they will disturb the speed of the particle more. It is a no-win situation: the more accurately you try to measure the position of the particle, the less accurately you can know the speed, and vice versa. This is summed up in the Uncertainty Principle that Heisenberg formulated; the uncertainty in the position of a particle times the uncertainty in its speed is always greater than a quantity called Planck’s constant, divided by twice the mass of the particle.