BBC Science The Theory of (nearly) Everything 2019

MEASURING THE SPEED OF LIGHT

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NEED TO KNOW

by FRANK CLOSE (@closefrank)

Frank is Emeritus Professor of physics

at the University of Oxford.

Five key scientific

terms to help you

understand light

ABERRATION

OF LIGHT

An optical phenomenon

whereby a star appears to

move about its true position.

It is a result of the f inite speed

of light and the motion

of the Earth.

ATOMIC CLOCK

This is the most accurate way of

measuring time we have. It uses

the frequency of microwave

signals that electrons in atoms

emit when they change

energy levels.

CAVITY RESONATOR

A hollow conductor blocked at

both ends, along which an

electromagnetic wave can

travel and be reflected back and

forth. A resonator of the correct

leng th will amplif y a w ave of

a given frequency.

ELECTRIC

PERMITTIVITY

An elec tric charge gives rise to

an electric field. The resistance

to forming this electric field is

known as the electric

permittivity. It can be

determined using capacitors,

which are devices for storing

electric charge.

MAGNETIC

PERMEABILITY

The measure of how easily a

substance, including empty

space, becomes magnetised.

The product of magnetic

permeability and electric

permittivity equals the inverse

of the square of the speed

of light.

This enables the speed of light to

be calculated to a n accu racy of 20

parts per trillion.

Modern descendents of the

Michelson-Morley technique use a

laser beam whose frequency is known

precisely. After the beam is split into

two paths and then recombined, the

interference pattern can be decoded to

determine the wavelength of the light.

The speed is then the product of this

wavelength and the frequency. In

1972, this led to a precision in the

measurement of the speed of better

than four parts per billion.

Today, advanced lasers and the

measurement of time intervals using

atomic clocks provide the most

accurate value of 299,792,458m/s, with

an uncertainty of just one metre per

second. The second can be defined

precisely using atomic clocks and the

remaining uncertainty in the speed of

light is due the accuracy of defining a

metre. As such, since 1983 it has been

agreed to ‘fix’ the speed of light at the

above value and to define the metre so

that there are exactly 299,792,458 of

them in the distance that light travels

in a vacuum in one second. So today,

instead of measuring the speed of light

relative to t he space-time of t he

Universe, as physicists struggled to do

for centuries, we use the speed of light

determine the latter.

An atomic clock at

the UK’s National

Physical Laboratory

in Teddington