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# What is the Heisenberg uncertainty principle?

### Transcript

One of the most bewildering developments of modern physics is the Heisenberg Uncertainty Principle. On today's Moment of Science we'll learn what this is, and try to clear up some of the uncertainty.

Heisenberg's Principle deals with the tiny particles of matter that make up atoms. It says that if you measure a particle's exact location, there is no way you can determine its speed. Likewise, the more you know about a particle's speed, the more uncertain it's location becomes. You can either know the particle's speed or location exactly, not both.

This seems to run against common sense. It's like saying that because you know your car is going exactly fifty miles an hour, it's impossible to say which city you're driving in. Or, once you learn you're at a certain corner in Indianapolis, you can no longer say whether you're parked or racing the Indy 500. Here's one way to understand it: You usually find out where your car is by looking for it. Light from the sun bounces off the car and into your eyes, and you learn--ah ha! It's over there! The same is true of atomic particles. The way to see them is to bounce light off them. This causes a problem though. Light itself is made of tiny particles called photons. Bouncing one of these off another particle might tell you where that particle is, but it will also give it a considerable shove, changing its speed in the process.

It's a little bit like trying to find a moving car by rolling huge boulders across the road. A crash will tell you exactly where the car is--or rather, was--but it will certainly change the car's speed in the process.

Once you learn you're at a certain corner in Indianapolis, you can no longer say whether you're parked or racing the Indy 500.

One of the most bewildering developments of modern physics is the Heisenberg Uncertainty Principle. On today's Moment of Science we'll learn what this is, and try to clear up some of the uncertainty.

Heisenberg's Principle deals with the tiny particles of matter that make up atoms. It says that if you measure a particle's exact location, there is no way you can determine its speed. Likewise, the more you know about a particle's speed, the more uncertain it's location becomes. You can either know the particle's speed or location exactly, not both.

This seems to run against common sense. It's like saying that because you know your car is going exactly fifty miles an hour, it's impossible to say which city you're driving in. Or, once you learn you're at a certain corner in Indianapolis, you can no longer say whether you're parked or racing the Indy 500. Here's one way to understand it: You usually find out where your car is by looking for it. Light from the sun bounces off the car and into your eyes, and you learn--ah ha! It's over there! The same is true of atomic particles. The way to see them is to bounce light off them. This causes a problem though. Light itself is made of tiny particles called photons. Bouncing one of these off another particle might tell you where that particle is, but it will also give it a considerable shove, changing its speed in the process.

It's a little bit like trying to find a moving car by rolling huge boulders across the road. A crash will tell you exactly where the car is--or rather, was--but it will certainly change the car's speed in the process.