Lateral Thoughts: Cormac O’Raifeartaigh
E is for energy
“Energy, power, 101 FM!” This radio jingle is driving me
to distraction. Why are DJs convinced that energy and
power are the same thing? Is there some history to this? I
do wish the jingle writers would consult an encyclopedia
before going public.
Ah, energy. Such a little word, so often misunderstood,
yet such a wonderfully useful concept – and surely the
most ubiquitous quantity in physics. “Energy is the capacity to do work” we tell hapless students in their introductory physics courses. It is probably the first abstract
concept they encounter, but most find it intuitively easy to
understand – as in “I hadn’t the energy to get out of bed”.
Then they learn of kinetic and potential energy, and of
the law of conservation of energy, before going on to calculate the landing velocity of boys falling off cliffs. These
are simple ideas but deep (my favourite type). The conservation of energy, for example, arises from the translational symmetry of time, which is an example of Noether’s
theorem – not something that generally arises in a first
physics course.
As the student progresses in physics, the ubiquity of
energy becomes apparent. From mechanics to heat, from
optics to electrostatics, energy just keeps on turning up.
The history of how energy spread through physics is an
interesting one; apparently the word energeia was first
used by Aristotle, but the concept of energy in the modern sense had to wait the late 1600s, when Leibniz defined
vis viva as the mass of an object multiplied by its velocity
squared. And it was not until the 19th century that scientists discovered that heat is simply a form of energy – a discovery that paved the way for the laws of thermodynamics
and all that followed.
The ubiquity of energy continues into modern physics.
Consider special relativity. For all its startling predictions
of time dilation and length contraction, it is the discovery
of the equivalence of mass and energy that is the theory’s
crowning achievement. Mass is a hidden form of potential energy, no less. Even the sacred law of conservation
of energy becomes a law of conservation of mass–energy.
This brings us to E = mc2. Much has been written on
the fame of this equation, but for my money, it is all in the
E. Anyone can guess that E stands for energy, so it is easy
to grasp the excitement of an equation that predicts a
whole new form of energy (especially if the equation is
accompanied by a picture of an atomic bomb). That said,
friends tell me the humble c2 also gives a certain pizzazz
to the equation. Incidentally, “ee is mc two” is how the
equation is pronounced by the younger generation,
apparently. Sigh.
Energy plays an even more important role in the general theory of relativity, since the curvature of space–time
on one side of the equations is related to the density of
matter–energy on the other. (Or something like that.)
Sounds completely mad until you remember that effects
such as time dilation in a gravitational field have been verified beyond doubt.
And what about quantum theory, that other pillar of
modern physics? It is not long before one encounters the
Schrödinger equation – the starting point for almost any
problem in quantum physics. Written as Hψ = Eψ, this is
just another energy equation. While students struggle with
the true meaning of the wavefunction ψ (don’t we all), that
Chris Sattlberger/Science Photo Library
Any surfer will
tell you that
tides are a
damn sight
more reliable
than wind
or wave
energy operator H makes life a good deal simpler – thanks
to the Irish physicist William Rowan Hamilton.
Actually, my favourite sort of energy turns up in the
quantum world: the energy of the vacuum. According to
the Heisenberg uncertainty principle (ΔEΔt ≥ h– /2), particles can borrow energy to come into existence as long as
they are quick enough about it – a pretty neat trick, if a bit
short-lived. This vacuum energy is particularly important
in cosmology, as it is thought to be responsible for the current acceleration of the expansion of the universe (the so-called dark energy).
Come to think of it, just about all of Big Bang cosmology
can be stated succinctly in terms of energy: an ultra-hot
and ultra-dense universe gradually cooling and expanding is surely one great conversion of potential to kinetic
energy, from beginning to end. How about that for a brief
history of time?
That was a quick ramble through physics, but of course
the concept of energy is used throughout science. For the
public, the most familiar example is surely renewable energy. Plenty has been written in this issue on the potential of renewables to simultaneously address the twin
challenges of energy supply and carbon emissions. I am
no expert, but speaking as a surfer, my money is on tidal
energy: if you have a moon and oceans, then you have a
highly periodic and reliable free lunch, no? Any surfer
will tell you that tides are a damn sight more reliable than
wind or wave.
But wait, wait! Why do we speak of wave power but tidal
energy No wonder our friends in the media are confused.
Time for some standardization, people. I suggest we stick
with energy...and while we are at it, let’s change that jingle
to “Wave energy, tidal energy, 101 FM”.
Cormac O’Raifeartaigh lectures in physics at Waterford Institute
of Technology, Ireland, and is the author of the blog Antimatter,
e-mail coraifeartaigh@wit.ie
