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by Paul Davies 24 November 2011, New
Scientist http://www.newscientist.com/article/mg21228390.800-nothingness-the-turbulent-life-of-empty-space.html
Physicists struggled to ditch the ether and accept the void -
until quantum theory refilled the vacuum with unimaginable energy
"NATURE abhors a vacuum." This sentiment, which first
popped up in Greek philosophy some 2500 years ago, continues to
excite debate among scientists and philosophers. The concept of a
true void, apart from inducing a queasy feeling, strikes many people
as preposterous or even meaningless. If two bodies are separated by
nothing, should they not be in contact? How can "emptiness"
keep things apart, or have properties such as size or boundaries?
While we continue to struggle with such notions, our idea of the
vacuum has moved on. Empty space is richer than a mere absence of
things - and it plays an indispensable part in much of modern
physics.
Even among the ancient Greeks, the void divided loyalties. One
influential line of thought, first apparent in the work of the
philosopher Parmenides in the 5th century BC and today most commonly
associated with Aristotle, held that empty space is really filled
with an invisible medium. Proponents of the rival atomic theory,
among them Leucippus and Democritus, disagreed. In their view, the
cosmos consisted of a limitless void populated by tiny indestructible
particles, or atoms, that came together in various combinations to
form material objects.
Such metaphysical debates remained standard fare among
philosophers into the Middle Ages and beyond. The rise of modern
science in the 17th century did little to settle them. Englishman
Isaac Newton,
like Aristotle, thought that the space between bodies must be filled
with a medium, albeit of an unusual sort. It must be invisible, but
also frictionless, as Earth ploughs through it on its way round the
sun without meeting any resistance.
Newton appealed to this substance as a reference frame for his laws of
motion. They predicted, for example, that a spinning planet such as
Earth would experience a centrifugal force that would make it bulge
at the equator. This effect provided physical proof of the body's
rotation, yet such a rotation, and thus the existence of a force,
only made sense if there were some absolute frame of rest, a
stationary viewpoint against which to compare the motion. This, said Newton, was the
invisible medium that filled space.
His German rival Gottfried Leibniz disagreed. He maintained that
all motion, including rotation, was only to be judged relative to
other bodies in the universe - the distant stars, for example. An
observer on a merry-go-round in deep space would see the stars going
round and at the same time feel a centrifugal force. According to
Leibniz, if the stars were to vanish, so would the force; there was
no need for an invisible medium in between.
Leibniz's position was argued forcefully in the 19th century by
the German engineer and philosopher Ernst Mach, he of the Mach
numbers used in aircraft speed. He proposed that centrifugal forces
and related mechanical effects are caused by the gravitational action
of distant matter in the universe. Albert Einstein was strongly
influenced by Mach's ideas in formulating his theory of relativity,
and was disappointed to find that Mach's principle didn't in fact
emerge from it. In Einstein's theory, for example, a spinning black
hole is predicted to have a bulging waist even when no other object
exists.
During the 19th century, the nature of empty space began to
occupy the thoughts of physicists in a new context: the mystery of
how one charged body feels the pull of another, or how two magnets
sense each other's presence. The chemist and physicist Michael
Faraday's explanation was that charged or magnetic bodies created
regions of influence - fields - around them, which other bodies
experienced as a force.
But what, exactly, are these fields? One way physicists of the
time liked to explain them was by invoking an invisible medium
filling all of space, just as Newton
had. Electric and magnetic fields can be described as strains in this
medium, like those introduced to a block of rubber when you twist it.
The medium became known as the luminiferous aether, or just the
ether, and it had an enormous influence on 19th-century science. It
was also popular with spiritualists, who liked its ghostliness, and
invented obscure notions of "etheric bodies" said to
survive death. When James Clerk Maxwell unified electricity and
magnetism in the 1860s, it provided a natural habitat for the rather
ghostly electromagnetic waves his theory predicted - things like
radio waves and light.
So far, so good. Soon after Maxwell published his theory,
however, the old problem of relative motion resurfaced. Even if our
planet feels no friction as it slides through the ether, any movement
relative to it should still produce measurable effects. Most notably,
the speed of light should depend on the speed and direction of
Earth's motion. Attempts to detect this experimentally by comparing
the speed of light beams in different directions failed to find any
effect.
Einstein came to the
rescue. His special theory of relativity, published in 1905, suggests
that a body's motion must always be judged relative to another body,
and never to space itself, or space-filling invisible stuff. Electric
and magnetic fields exist, but no longer as strains in any
space-filling medium. Their strength and direction, and the forces
they exert, change with the motion of the observer such that the
speed of light is always measured to be the same, independently of
how the observer moves. So the ether is an unnecessary complication.
While it is true to say that a region of space pervaded by an
electric or magnetic field is not empty, the will-o'-the-wisp
"stuff" it contains is a far cry from what we normally
think of as matter. Fields might possess energy and exert pressure,
but they are not made up of anything more substantial.
A decade or so on, however, a new twist cast the problem of empty
space in a different light. It emerged from the theory of quantum
mechanics. At the level of atoms, the clockwork predictability of the
classical, Newtonian universe broke down, to be replaced by a strange
alternative set of rules. A particle such as an electron, for
example, does not move from A to B along a precisely defined
trajectory. At any given moment its position and motion will be, to a
degree, uncertain.
What's true for an electron is true for all physical entities,
including fields. An electric field, for instance, fluctuates in
intensity and direction as a result of quantum uncertainty, even if
the field is zero overall. Imagine a box containing no electric
charges - in fact containing nothing but a vacuum - and made of metal
so that no electric field can penetrate from the outside. According
to quantum mechanics, there will still be an irreducible electric
field inside the box, surging sometimes this way, sometimes that.
Overall, these fluctuations average out to zero, so a crude
measurement may not detect any electrical activity. A careful
atomic-level measurement, on the other hand, will.
We now encounter an important point. Although the field strength
of the fluctuations averages to zero, the energy does not, because an
electric field's energy is independent of its direction. So how much
energy resides in an empty box of a given size? Quick calculations on
the basis of quantum theory lead to an apparently nonsensical
conclusion: there is no limit. The vacuum is not empty. In fact, it
contains an infinite amount of energy.
Physicists have found a way around this conundrum, but only by
asking a different question. If you have two metal boxes of different
size or shape, what is the difference in their quantum vacuum energy?
The answer, it turns out, is tiny. But not so tiny that the
difference cannot be measured in the lab, proving once and for all
that the quantum fluctuations are real, and not just a crazy
theoretical prediction.
So the modern conception of the vacuum is one of a seething
ferment of quantum-field activity, with waves surging randomly this
way and that. In quantum mechanics, waves also have characteristics
of particles, so the quantum vacuum is often depicted as a sea of short-lived
particles - photons for the electromagnetic field, gravitons for the
gravitational field, and so on - popping out of nowhere and then
disappearing again. Wave or particle, what one gets is a picture of
the vacuum that is reminiscent, in some respects, of the ether. It
does not provide a special frame of rest against which bodies may be
said to move, but it does fill all of space and have measurable
physical properties such as energy density and pressure.
One of the most studied aspects of the quantum vacuum is its
gravitational action. Out there in the cosmos there is a lot of
space, all of it presumably chock-full of quantum-vacuum
fluctuations. All those particles popping in and out of existence
must weigh something. Perhaps that mass is enough to contribute to
the total gravitating power of the universe; perhaps, indeed, enough
to overwhelm the gravity of ordinary
matter.
Finding the answer is a demanding task. We must account not just
for electromagnetic fields, but all fields in nature - and we cannot
be sure we have all of these pinned down yet. One general result can
be readily deduced, however. In the event that the pressure of the
quantum vacuum is negative (a negative pressure is a tension), the
gravitational effect is also negative. That is, negative-pressure
quantum-vacuum fluctuations serve to create a repulsive, or
anti-gravitating, force.
Einstein had predicted that empty space would have such an
anti-gravitational effect in 1917, before quantum mechanics. He
couldn't put a number on the strength of the force, though, and later
abandoned the idea. But it never completely went away. Back-of-the-envelope
calculations today suggest that the quantum vacuum pressure should
indeed be negative in a space that has the geometry of our universe.
Sure enough, about 15 years ago evidence began to accumulate from
observations of far-off supernovae that a huge anti-gravitational
force is causing the entire universe to expand faster and faster. The
invisible quantum vacuum "ether" that is presumably at
least partially responsible has recently been restyled as "dark
energy". The work that led us to this discovery garnered
astrophysicists Saul Perlmutter, Adam Riess and Brian Schmitt this year's Nobel prize in physics.
While quantum mechanics gives us a way to begin the calculation,
a proper understanding of dark energy's strength and properties will
probably require new physics, perhaps coming from string theory or
some other attempt to bring all the fundamental forces of nature -
including gravity, the perennial outsider - under one umbrella.
One thing is clear, however. The notion that space is a mere void
with no physical properties is no longer tenable. Nature may abhor an
absolute vacuum, but it embraces the quantum vacuum with relish. This
is no semantic quibble. Depending on how dark energy works, the
universe may continue on a runaway expansion, culminating in a
universe of dark emptiness in which matter and radiation are diluted
to infinitesimal levels, or it might crush in on itself in a "big
crunch". The fate of the universe, it seems, lies in the
properties of the vacuum.
Paul Davies is director of the Beyond Center for Fundamental Concepts in
Science at Arizona State University in Tempe
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