The Leftovers of Nothing - Zero Point Energy
NOTHINGS ain't what they used to be. By using his air pump -- one of the high
points of seventeenth-century technology -- to remove all the air from a
cavity, Sir Robert Boyle made it clear to restoration England what a vacuum
was. It was what was left when you took everything away: emptiness. In the
early twentieth century, quantum mechanics made everything more complicated.
A vacuum is still what is left over when everything is taken away; but that
no longer means that it is emptiness. The non-empty vacuum plays a
fundamental role in the way physicists think about matter.
Descendants of Boyle's air pump now produce vacuums that are, to all intents
and purposes, completely free of matter. But they can never be completely
free of energy. According to quantum theory, it is impossible to remove all
the energy from any system. As in a tin of sardines, there is always a
little bit in the corner that you cannot get out. The magnitude of this
"zero-point energy" is tiny; as far as everyday uses go, it can be ignored.
Nobody can measure the zero-point jiggling of a pendulum caused by the mote
of energy remaining in the system when nothing else is left. But not all
such effects are negligible. Electromagnetic fields also have zero-point
energies. In the vacuum, every electromagnetic mode--that is, every way in
which an electromagnetic field could vibrate, if there was one there--has
its zero-point energy. The energy for each mode is tiny, but there are an
awful lot of modes. Adding them together reveals a vacuum crammed with
It is surprisingly hard to find evidence of this sea of energy--largely
because the level of the energy is the lowest that can be reached. There is
no lower level with which it can be compared. Like sea-level for land maps,
the vacuum energy is the reference point above which all else is measured.
Zero-point effects do turn up, though, when matter and vacuum interact. The
first to be recorded was the atomic Lamb shift. Atoms are surrounded by
electrons which can have various different levels of energy. When an
electron moves from a higher level to a lower one, it emits a burst of light
at a particular wavelength: a photon. The wavelength can be predicted
precisely from theory. In some cases, though, the wavelength observed is
different from that predicted. The difference turns out to be exactly what
one would expect from the effects of lots of tiny electromagnetic fields
working on the electrons--the effect of the vacuum field.
Not only is the wavelength of the photon dependent on vacuum effects, so is
the fact that it appeared at all. There are two ways for an electron to
unburden itself of a photon and come down from a higher energy level. If the
electron is hit by a photon of the right wavelength, it will be knocked
down, and there will be two photons where there was one before. That is
stimulated emission, the principle behind the laser. Alternatively you can
wait for the electron to jump down on its own, giving up its photon by
spontaneous emission. When the vacuum energy is taken into account, the
distinction between these two breaks down. Spontaneous emission can be seen
as stimulated emission, with the zero-point energy of the vacuum providing
the stimulation. So the emission of light does not depend just on the
atom--it depends on the way that the atom and the vacuum interact. By
changing the vacuum, you can change the way the atom emits light.
A vacuum between two sheets of metal is not the same as one that is
unconstrained. Some of the modes of the electromagnetic field are
suppressed--the modes which represent waves in the field that are too big to
fit into the cavity. By changing the size of the cavity, you can lose
certain modes. Groups of scientists around the world have built cavities
that rule out certain modes of vacuum energy, and thus stop atoms from
emitting photons at various wavelengths. Using a related technique, they
have designed and built cavities that enhance the radiation by allowing the
atom to "see" more modes of the vacuum radiation than it would if there was
no cavity. The results of such experiments allow scientists to explore
otherwise inaccessible areas of quantum electrodynamics, the theory of
An intriguing theoretical point about the way that atoms interact with
vacuum has been made by Dr Hal Puthoff of the Institute for Advanced Studies
in Austin, Texas. For every atom there is an energy level below which the
electrons cannot sink. Dr Puthoff suggests that this is because, at the low
energy levels, electrons cannot lose energy any faster than they pick it up
from a vacuum. It is the vacuum energy that buoys them up, stopping them
from losing all their energy and collapsing into the atomic nucleus. That
means that the vacuum underpins the stability of every atom--and thus of
almost all matter in the universe.
Force from nowhere Vacuum zero-point energies can explain effects on a
larger scale as well. The vacuum energy exerts a pressure on everything.
Normally, this pressure has little effect, since it comes from all
directions at once and almost cancels out. But if two atoms are reasonably
close to each other, each will shield the other from some of the pressure.
There will be slightly less pressure from the direction of the neighbouring
atom than there is from every other direction--so the atoms will tend to
This is the Van der Waals force. Though it is weak, it is strong enough to
hold atoms and molecules together in gases and liquids. There are other ways
to describe Van der Waals forces, in terms of the way the electrons jitter
around the atoms, but they also depend on the vacuum; they just come at it
in a different way.
An analogous force can be measured between parallel metal plates which are
placed close together--say a few thousandths of a millimetre apart. Because
the distance between the plates limits the wavelengths available for the
zero-point energy, there are fewer modes available in the vacuum between the
plates than in the vacuum outside. So the pressure from outside is greater,
and becomes greater still as the plates are pulled together and yet more
modes are ruled out. This "Casimir effect" may prove an obstacle for people
who want to build machinery ever smaller, since it will tend to stick
On the other hand, it may be an opportunity. Dr Robert Forward, a physicist
who is always ready to speculate on the outlandish--from antimatter-driven
spaceships to life on the surfaces of collapsed stars--has suggested a
simple, impractical machine that could remove energy from the vacuum using
the Casimir effect. It is farfetched, but getting the Casimir effect to do
useful work by holding things together is theoretically possible.
There are further reaches to vacuum energy ideas which are controversial,
but still intriguing. Over many years, Dr Timothy Boyer of the City
University of New York has tried to show that many of the results of quantum
physics can be achieved using none of its assumptions, provided that
zero-point energy is allowed. Dr Puthoff has recently revived an idea mooted
by Dr Andrei Sakharov in the 1960s that gravity itself can be explained by
vacuum effects, more or less as a very long-range version of the Van der
Waals force between atoms and molecules. That goes against the grain of
modern theory, but some broad-minded colleagues see it as an intriguing
And there is the question of the other sorts of energy in the vacuum.
Interest has focused on the residual electromagnetic fields because there is
a successful theory with which to discuss them. But there are other types of
field--those associated with the nuclear forces--that are less well known.
The way that quarks are bundled together in nuclei may have to do with
vacuum pressure. There may still be a lot of mileage for physicists in
thinking about nothing at all.