BRIDGEMANART^ LIBRARY,^ SCIENCE^ PHOTOLIBRARY,^ GETTY1690
After Ole Rømer shows
that light travels at a
finite speed, fellow Dane
Christiaan Huygens
calculates this speed to
be around 220,000km/s.1862
French physicist Léon
Foucault uses rotating
mirrors to calculate
the speed of light at
299,796km/s.1865
James Maxwell shows light to be an
electromagnetic wave, enabling its speed
to be calculated from known properties
of space.1905
The concept that the speed of light is
universal, independent of the speed of the
source or of the observer, forms the basis of
the Special Theory of Relativity developed
by Albert Einstein (above).1972
A laser (below) is used to measure the
frequency of a particular spectral line
of a krypton atom. By combining this
information with the definition of the
metre, the speed of light in a vacuum is
measured as 299,792,458m/s.1983
Light speed is made absolute at the 17th
General Conference on Weights and
Measures. As a result, a metre is now
defined as 1/ 299,792,458th the distance
travelled by light in a vacuum in one second.5 magnetic field emerges, and vice
versa, over and over. The resistance or
‘stiffness’ of free space to the former is
called its electric permittivity, while
its resistance to the magnetic field is
called its magnetic permeability. In
Maxwell’s theory, the speed of light is
related to these quantities. The ease
with which the electric and magnetic
fields can oscillate back and forth
determines the speed at which the
electromagnetic wave travels. It turns
out that the product of these quantities
is proportional to the inverse of the
square of the speed of light.
So, in a sense, Kepler was right,
centuries ago. If space offered no
resistance – in Maxwell’s theory, if the
elect ric or magnetic ‘stiff ness’ were
zero – the speed of light would indeed
be infinite. But in reality, the electric
and magnetic ‘stiffness’ are not zero
a nd, when t heir values were inser ted
into Maxwell’s equations at the end of
the 19th century, they gave a value of
299,788k m/s, t hen t he most accu rate
estimate of the speed of light available.
In the USA in 1887, Albert
Michelson and Edward Morley
attempted to measure the speed of
Earth through the ‘ether’ (a medium
t hen believed to permeate all space)
by measuring the difference in the
speed of light in two perpendicular
directions. They used semi-
transparent mirrors, which deflected
light through 90° while also allowing
some to ca r r y on unhindered. By
reflecting the two beams back along
their paths and recombining them,
any difference in speed would show
up by the two waves being out of phase- a mismatch between their peaks and
troughs that would show up as a subtle
set of dark and light fringes, known as
an interference pattern.
Onwards to Einstein
Michelson and Morley’s set-up proved
highly sensitive and, to their surprise,demonstrated that the speed of light is
universal, independent of direction. In
turn, this led Albert Einstein to insist
that the ether does not exist (at least in
the form then believed) and to propose
his theory of Special Relativity in- Thus precise measurements of
the speed of light had led to profound
new insights into the nature of space
and time, courtesy of Einstein.
In particular, Einstein’s theory
implies that the speed of light in a
vacuum is natu re’s speed limit: no
object that has mass can ever attain
the speed of light in a vacuum, while
any particles that have no mass must
travel through a vacuum at this
universal speed. But light is slowed
when it passes through a transparent
medium, such as water or glass; it is
possible for particles, such as an
electron, to travel through the medium
faster than light, but still below the
absolute speed limit.
Before the invention of the laser,
independent measurements of the
frequency and wavelengths of
electromagnetic waves were made
in the 1950s using ‘cavity resonators’,
which gave a value of 299,792km/s
with an uncertainty of 3km/s. A
modern demonstration is to put a
chocolate bar in a microwave oven.
Remove the turntable so the specimen
is stationa r y a nd it will cook fastest at
the points where the waves are most
intense. The distance between two
successive spots is half the wavelength
of the microwaves. Multiply the
wavelength by the microwave
frequency (typically 2,450MHz, but
check with your manual) and the
speed of light results, though with less
accuracy than in the 1950s laboratory.
Modern large-length experiments
involve sending radio signals to
different spacecraft whose positions
in the Solar System have been
precisely calculated, allowing for
the gravity of the Sun and planets.
“Precise measurements of the speed
of light had led to profound new
insights into the nature of space and
time, courtesy of Einstein”
TIMELINE
THE FUNDAMENTALS OF PHYSICS