BBC iD

# How Long is a Piece of String?

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Duration: 1 hour

Alan Davies attempts to answer the proverbial question: how long is a piece of string? But what appears to be a simple task soon turns into a mind-bending voyage of discovery where nothing is as it seems.

An encounter with leading mathematician Marcus du Sautoy reveals that Alan's short length of string may in fact be infinitely long. When Alan attempts to measure his string at the atomic scale, events take an even stranger turn. Not only do objects appear in many places at once, but reality itself seems to be an illusion.

Ultimately, Alan finds that measuring his piece of string could - in theory at least - create a black hole, bringing about the end of the world.

## More episodes

See all episodes from Horizon

## Chapters

9 items
• ### How Long is a Piece of String?: Introduction

Alan Davies discovers that answering that question is much harder than he originally thought.

• ### Different Types of Measurement

Alan visits mathematician Marcus du Sautoy to help him find an answer. However, this creates more questions, which requires a trip to a place dedicated to measurement, the National Physics Laboratory.

• ### Infinitely Long String

Finding out the exact length of the string proves even more troublesome when Marcus proposes the string could be infinitely long, thanks to the theory of fractals.

• ### A Physics Approach

After leaving Marcus, Alan visits a physics teacher to see if measuring the string in atoms can help him find an answer. However, this means exploring Quantum Mechanics.

• ### Two Places at Once

Alan goes to a lab to explore the theory of Quantum Mechanics in more detail to see if his string can be in two places at once, confusing him even further.

Still no closer to an answer Alan is introduced to the theory that his string may have multiples lengths at the same time, all with the help of a stuffed cat.

• ### Proving Quantum Mechanics Works

Alan doesnâ€™t believe Quantum Mechanics is important on a basic scale, but photosynthesis and the ability to smell might just change his mind.

• ### End of the World

If Alan was to find the most accurate way of measuring his string he could theoretically create a black hole.

• ### Final Summary

Trying to find the solution to this question has been a philosophical journey for Alan, but he still visits Marcus du Sautoy to tell him his personal answer.

Presenter
Alan Davies
Participant
Marcus du Sautoy