The speed of light travelling through air has been
slowed down for the first time, breaking what was thought to be a
constant physical measurement.
A team of physicists at the University
of Glasgow sent photons through a mask to change their shape
and then raced an altered photon against an unaltered one. Over a
distance of one metre the team observed that the altered photon was
slowed by up to 20 wavelengths, demonstrating for the first time
that light can be slowed in free space.
"The results give us a new way to think about the
properties of light," said professor Miles Padgett from the
University of Glasgow's optics group. The research was carried out
in conjunction with Heriot-Watt University with the findings being
published in the journal Science Express.
The speed of light in free space is 186,282 miles per
second, which until now was thought to be a constant. While
light slows down when passing through water or glass it returns to
the speed of light when it comes out the other side. In this
experiment the speed of a photon remains slightly slower.
As light behaves both like a wave and a particle it
is possible to change the shape of an individual photon as if it
were a wave and then race the two particles of light.
To explain how the experiment worked the researchers
compared the behaviour of a beam of light to that of a team of
cyclists. While the team of cyclists travels at a constant speed,
the individual cyclists will all be travelling at their own speeds
as they swap positions.
The same applies to a beam of light, which has a
number of different velocities. In this analogy the group of
cyclists is the beam of light travelling at the speed of light, but
each cyclist is an individual photon with its own speed.
"The delay we've introduced to the structured beam is
small, measured at several micrometres over a propagation distance
of one metre, but it is significant," said Daniel Giovannini, one
of the lead authors of the paper.
Don't
miss
Importantly the experiment is only applicable over
short distances. The researchers explained that the effect was
biggest when the lenses used to create the beam of light were large
and the distance travelled was small. This means our fundamental
understanding of how the universe works remains unaltered, but our
understanding of light has changed.
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Scottish scientists have slowed the speed of light
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