Imagine driving a car fitted with a GPS navigation system that glitches every time you look at the speedometer. For quantum particles, this is a reality: the better you know a particle’s speed, the less certain you are of its position.
This bizarre trade-off arises from Heisenberg’s uncertainty principle, which was first laid out by German physicist Werner Heisenberg in 1927. It states that any effort to reduce the error in your measurement of a particle’s momentum (that is, its mass multiplied by its velocity) will increase the error in your measurement of its position. The two error sizes, when multiplied together, will always exceed a certain value.
It’s a concept that makes no sense on a human scale, but that’s quantum mechanics for you. Particles governed by quantum physics, such as the electron, don’t really exist in a specific place. Instead, they are more like clouds of probabilities, with a varying chance of appearing in a certain place, which gives them a wave-like nature.
As you repeatedly measure an electron, you can build up certainty about its position. But the act of measurement involves interacting with the electron, which will cause it to change its momentum, increasing your uncertainty about that value.
Advertisement
We don’t normally see this behaviour in everyday objects because our measurements are so imprecise when looked at on a quantum scale, but the uncertainty principle underpins the wave-particle duality at the heart of quantum mechanics.
If all that leaves your scratching your head, you are not alone – Albert Einstein strongly objected to the uncertainty principle, saying it could not possibly reflect reality, and spent many years arguing with Heisenberg and others. In 1954, a year before Einstein died, Heisenberg had his final meeting with the great physicist. Einstein still disagreed with him. “I don’t like your kind of physics,” he said. Jacob Aron
