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

Scanning electronic microscope Copyrighted image Icon Copyright: BBC article icon

Science, Maths & Technology 

Scanning electronic microscope

How the scanning electronic microscope allows us to see ultramicroscopic objects so clearly

Article
Light: Wave-particle duality Creative commons image Icon By Douglas Hofstadter [CC-BY-SA-3.0], via Wikimedia Commons under Creative-Commons license article icon

Science, Maths & Technology 

Light: Wave-particle duality

One of the most confusing concepts in physics, wave-particle duality is unlike anything we see in the ordinary world.

Article
Fundamental physics and the magnetic monopole Copyrighted image Icon Copyright: Dreamstime article icon

Science, Maths & Technology 

Fundamental physics and the magnetic monopole

David Milstead can't predict what might spin off from his research into magnetic monopoles but, just as splitting the atom led to the electronics revolution, he's sure that today's pure physics research will lead to tomorrow's everyday technology.

Article
Build an accurate clock challenge Copyrighted image Icon Copyright: Production team article icon

Science, Maths & Technology 

Build an accurate clock challenge

The science behind clocks, sundials and keeping time, part of the BBC/OU's programme website for Rough Science 2

Article
Accurate weighing balance Copyrighted image Icon Copyright: Production team article icon

Science, Maths & Technology 

Accurate weighing balance

Instructions for making your own accurate weighing balance, one of the scientists' challenges on the BBC/OU series Rough Science 3

Article
article icon

Science, Maths & Technology 

Battery Challenge Glossary

Some of the key terms associated with recharging a battery

Article
Kathy's Carriacou diary: Music Copyrighted image Icon Copyright: Production team article icon

Science, Maths & Technology 

Kathy's Carriacou diary: Music

Kathy Sykes's Science of Celebration diary, from the BBC/OU series Rough Science 2

Article
Kathy's Carriacou diary: Sun & sea Copyrighted image Icon Copyright: Production team article icon

Science, Maths & Technology 

Kathy's Carriacou diary: Sun & sea

Kathy Sykes's Sun and Sea diary, from the BBC/OU series Rough Science 2

Article
Teleportation: Talk the Talk Copyrighted image Icon Copyright: Jackq | Dreamstime.com article icon

Science, Maths & Technology 

Teleportation: Talk the Talk

How to sound like an expert and talk confidently about teleportation

Article