### Segments in this Video

#### Measurement: Not as Simple as it Looks(01:25)

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Alan Davies explains that the task of measurement has posed challenges for man through the ages.

#### How Long is a Piece of String? About 32 cm.(01:04)

Alan Davies purchase a piece of string and asks the shopkeeper to measure it. The string measures 32 centimeters--simple enough. However, Alan is off to visit a professor of mathematics Marcus du Sautoy to learn why it's not that simple.

#### Measurement: The Problem of Standardization(02:15)

Man has been obsessed with measurement for thousands of yeas; measurement is the beginning of mathematics.

#### How Long is a Piece of String? About 9 inches(01:54)

The earliest recorded measurement was the Egyptian cubit, the length from the elbow to the tip of the middle finger. The problem of this system is the variation in sizes of people and their arms. The National Physics Laboratory is the next stop.

#### How Long is a Piece of String? 18 Fingers(01:50)

The National Physics Laboratory is home to objects that define the units we use to measure every day: the kilo, inch, meter, the yard. A 200 year-old meter bar is made from the most stable metal, a mixture of 90% platinum and 10% iridium.

#### Meter: Defined by the Speed of light(01:23)

A visit to the Length Bar Interferometry at the National Physics Laboratory illustrates the use of a laser beam to perform measurement. A meter is defined by light, how far light will travel in in 1/299792458 of a second.

#### How Long is a Piece of String? 319.442 mm.(02:03)

Physicists at the The National Physics Laboratory help host Alan Davies measure his string with a tracking laser. The result: 319 mm 490 micrometers

#### Measuring the Coastline of Cornwall(02:57)

Since a landmass has features at all scales, from hundreds of kilometers in size to tiny fractions of a millimeter and below, there is no obvious limit to the size of the smallest feature that should not be measured around

#### How Long is a Piece of String? Infinite(04:13)

The Koch Snowflake demonstrates the infinite nature of the piece of string. At every iteration, this fractal becomes 4/3 times longer. After an infinite number of iterations, this fractal will become infinitely long.

#### How Long is a Sunset(00:60)

Alan Davies and Mathematician Marcus du Sautoy enjoy the irony of attempting to measure what time the sun set--instead of just watching it..

#### How Long is a Piece of String? 1,000,000,000 Atoms(01:57)

Host Davies travels to the Simon Langton School for Boys to meet one of the country's best physics teachers, Becky Parker. She explains that to answer the question of measuring things on a tiny scale they must go on a mind-blowing journey.

#### Measurement and Quantum Mechanics: Atoms(02:50)

Physics: Physicist Becky Parker illustrates that most of an atom is nothing; when matter is collapsed to only electrons, protons and neutrons, the human race has the volume of a sugar cube.

#### Quantum Mechanics: Two Places at Once(03:11)

Physicist Becky Parker uses soccer balls to illustrate a fundamental tenet of quantum physics that explains how the smallest objects behave. She points to a central problem of string measurement: Where does the string end?

#### Evidence of Objects in Two Places at Once(02:35)

At Imperial College in London, special equipment makes visible individual particles. The slit experiment shows that a particle of light can interfere with itself, as if it traveled through both slits at once.

#### The Slit Experiment: Photon Version(02:10)

At Imperial College in London, the slit experiment is performed with a single proton with the same results as when a laser light is used. A physicist explains that working with this scale, the notion of position ceases to be a good idea.

#### Am I in Two Places at Once?(01:12)

After his trip to Imperial College's physics lab to measure his string, Alan Davies has even more questions.

#### How Long is a Piece of String? Alan Davies(02:46)

MIT professor of quantum physics Seth Lloyd explains Erwin Schrödinger's cat experiment.

#### Schrödinger's Cat: Dead or Alive?(01:01)

MIT professor of quantum physics Seth Lloyd explains that, from a quantum perspective, the cat is both dead and alive at the same time.

#### Observation and Quantum Physics(01:46)

Big things don't show up being two places at once because they interact with their environment. Each interaction is an observation that measures the cat as dead OR alive.

#### Questioning the Relevance of Quantum Mechanics(01:05)

Host Davies' mind is boggled. Does his string have a length? Does any of this matter? MIT professor of quantum physics Seth Lloyd offers to show that it does matter.

#### Quantum Mechanics: Depend on It(01:18)

At a greenhouse MIT professor of quantum physics Seth Lloyd demonstrates that all life on earth relies on quantum mechanics.

#### Photosynthesis and Quantum Physics: Energy Transfer(02:21)

A photon particle of light from the sun is absorbed in a gigantic molecule like a chlorophyll, and creates an excited electron, which needs to move from the side of the molecule where it was created to the other side to become energy.

#### How Long is a Piece of String? The End of the World(03:44)

Using the wave length of laser light is the most accurate way to measure length. MIT professor of quantum physics Seth Lloyd uses Einstein's theory, E=MC2, to explain how infinitely measuring a string this way could theoretically lead to a black hole.

#### Measurement: A Philosophical Journey(02:11)

Host Davies says that the fundamental building blocks of everything defy reality. We are only just scratching the surface of what's going on in the world. He returns to share his final solution to the problem with Professor Marcus du Sautoy.

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# How Long Is a Piece of String?

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### Description

In the spirit of a famous rhetorical question, this program explores the world of scientific measurement at the molecular and atomic level. Host Alan Davies tries to establish the length of a piece of household string—but what appears to be a simple task soon turns into a mind-bending voyage of discovery. After discussing the matter with leading mathematician Marcus du Sautoy, Davies learns that his segment of string may in fact be infinitely long. And when MIT professor of quantum physics Seth Lloyd gets involved, events take an even stranger turn. Not only do objects appear in many places at once, but reality itself becomes a nebulous distortion—in which the humble act of measuring string could, in theory, pull the world into a black hole. A BBC Production. (51 minutes)

Length: 52 minutes

Item#: BVL41686

ISBN: 978-1-61616-683-0