Skip to content
Science, Maths & Technology

Uncertain principles

Updated Thursday 3rd August 2006

Get to grips with Heisenbergs uncertainty principle

Daniel Craig as Heisenberg from BBC TV's Copenhagen Copyrighted image Icon Copyright: BBC

Quantum mechanics does away with the deterministic view of the future. It is no longer cause and effect. So what can we know about the world? It was the belief of the classical physicists that it was possible to know the position and velocity of a particle as accurately as one would wish.

After all, if you are travelling at 30mph and you are sat on your bicycle then you know these things at the same time. But they and you are wrong. In quantum mechanics there is a fundamental limit to the accuracy that can be achieved, no matter how good the measuring device.

In 1927 Heisenberg made a startling discovery. Quantum theory implies a limitation on how accurately certain pairs of physical variables could be measured simultaneously. Using some of the matrix mechanics that had been proposed by Max Born, Heisenberg realised that position and momentum (the relationship between mass and velocity) were non-commutable; you could not precisely know them both at the same time.

Hence, there is no way of accurately locating the exact position of a sub-atomic particle unless you are willing to be uncertain about its momentum. But there is no way you can be certain about momentum without being uncertain about position. It is impossible to precisely measure them both at the same time.

So, you might ask, how can I drive my car at 50mph at 26 degrees latitude and 99 degrees longitude? Well, the value of the constant that determines the uncertainty about either position or momentum is so small (in fact, it is Planck’s constant: 6.62620 x 10-34 Js) that it has a negligible impact in this world. So, no need to worry too much about this particular uncertainty.

This article was first published in 1999

 

For further information, take a look at our frequently asked questions which may give you the support you need.

Have a question?

Other content you may like

The sea sounds experiment audio icon

Science, Maths & Technology 

The sea sounds experiment

David Sharp takes his microphone and shell into a recording studio to find out why you can hear the sounds of the sea when you hold a shell to your ear.

Audio
10 mins
article icon

Science, Maths & Technology 

Ice Challenge Glossary

Some of the key terms associated with making ice

Article
Measure the speed of light… with your microwave Copyrighted image Icon Copyright: The Open University video icon

Science, Maths & Technology 

Measure the speed of light… with your microwave

This video discovers an unconventional way to measure the speed of light using the microwave in your kitchen

Video
5 mins
article icon

Science, Maths & Technology 

Microscope Challenge Glossary

Some of the key terms associated with making a microscope

Article
What Makes A Sound Musical? Creative commons image Icon Jesse Kruger under Creative Commons BY-NC-SA 4.0 license article icon

Science, Maths & Technology 

What Makes A Sound Musical?

We look at the defining characteristics that make a sound musical

Article
Renewable energy Copyrighted image Icon Copyright: OU article icon

Science, Maths & Technology 

Renewable energy

What exactly is energy - where does it come from, and how do we store and release it?

Article
Inside the science - radioactivity Copyrighted image Icon Copyright: Production team article icon

Science, Maths & Technology 

Inside the science - radioactivity

Amongst the challenges for the Rough Scientists as they explore the mine is making a radioactivity detector. But what is the science behind radioactivity?

Article
Metal detector Copyrighted image Icon Copyright: Production team article icon

Science, Maths & Technology 

Metal detector

Outline of how to make a metal detector, one of the scientists' challenges on the BBC/OU series Rough Science 3

Article
Jonathan's Carriacou diary: Bugs and barometers Copyrighted image Icon Copyright: Production team article icon

Science, Maths & Technology 

Jonathan's Carriacou diary: Bugs and barometers

Jonathan Hare's Bugs and Barometers diary, from the BBC/OU series Rough Science 2

Article