THE FUNDAMENTALS OF PHYSICS
5 now, which doesn’t leave very much
room for free will. This is sometimes
called ‘Newton’s Clockwork Universe’.
But according to quantum physics,
an electron is never located at a precise
place (because of its wave nature), and
it is never sure where it is going. This
is the ‘uncertainty principle’
discovered by Werner Heisenberg,
who found that there is a trade-off.
Quantum objects can either have a
relatively well-defined position and
a poorly defined direction, or a well-
defined direction and a poorly defined
position. But they can’t have both. It’s
the price of free will.
This ties in wit h a not her concept
that’s key to quantum physics:
probability. You can never say
precisely where a quantum entity
is or where it is going, but you can use
quantum physics rules to work out
probabilities, such as the probability
t hat a n elect ron will follow a cer tain
trajectory, or the probability that a
sa mple of radioactive material will
decay and spit out a particle within
a certain time.QWhat is a quantum?
A quantum is the smallest
amount of something that it is
possible to have. The smallest amount
of light you can have, for example, is a
particle called a photon. If you have a
bright light, there are many photons
streaming outwards. But as you turn
the light down, there are fewer and
fewer photons. Eventually, there are
so few photons that they can be
detected one at a time.
Astronomers see this happening
when t hey build up images of ver y
faint objects using long exposures of
charge-coupled devices (CCDs). When
atoms emit light, t hey do so by
rearranging their electrons to radiate
energy. Like a ball bouncing down a
staircase, the electron jumps from one
energy level to another inside the atom
a nd a photon is emitted. This jump is
known as a quantum leap.
A quantum leap is the smallest
cha nge it is possible to ma ke –
something to remember next time you
see the term used in advertising.Proof that light can be a wave or a particle
THE KEY EXPERIMENT
In the 18th century, debate raged
as to whether light was a wave
or a particle. But in 1803, English
scientist Thomas Young showed
that, when light is passed through
two slits onto a backboard, an
interference pattern appears. This
is similar to what’s seen when
two sets of similarly generated
waves collide in water (A). Light,
he deduced, must be a wave. In
the early 20th century, however,
Einstein and others demonstrated
that light can also be seen as a
stream of particles – photons.
This is where things get tricky.
When individual particles are sent
one at a time through a double
slit, as in Young’s experiment, theyshould ‘pile up’ in two bands
(B). Photons don’t, though: even
if you send photons through
the double slit individually, an
interference pattern is observed
(C). Just to complicate matters,
if you monitor which slit each
photon is going through, the
interference patterns are replaced
by two bands.
The same applies to other
fundamental particles, such
as electrons. If that sounds a
bit mind-blowing, welcome to
the world of quantum physics,
where ‘wave-particle duality’ is
commonplace and where the mere
act of observing can affect the
outcome of an experiment.WAVESCREEN WITH
TWO SLITSDETECTOR
SCREENPATTERNS
SEEN ON
SCREENA
Light acting
as a particleLight acting
as a waveSCREEN WITH TWO SLITSDETECTOR SCREENBC
PHOTONEXPECTED
PATTERNSOBSERVED
PATTERNSA