IS SPACE just space? Or is it filled with some sort of mysterious, intangible
substance? The ancient Greeks believed so, and so did scientists in the 19th
century. Yet by the early part of the 20th century, the idea had been
discredited and seemed to have gone for good.
Now, however, quantum physics is casting new light on this murky subject.
Some of the ideas that fell from favour are creeping back into modern thought,
giving rise to the notion of a quantum ether.
This surprising revival is affording new insights into the nature of motion
through space, the deep interconnectedness of the Universe, and the possibility
of time travel. Ingenious new experiments may even allow us to detect the
quantum ether in the lab, or harness it for technological purposes.
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If so, we鈥檒l have answered a question that has troubled philosophers and
scientists for millennia. In the 5th century BC, Leucippus and Democritus
concluded that the physical universe was made of tiny
particles鈥攁toms鈥攎oving in a void. Impossible, countered the
followers of Parmenides. A void implies nothingness, and if two atoms were
separated by nothing, then they would not be separated at all, they would be
touching. So space cannot exist unless it is filled with something, a substance
they called the plenum.
If the plenum exists, it must be quite unlike normal matter. For example,
Isaac Newton鈥檚 laws of motion state that a body moving through empty space with
no forces acting on it will go on moving in the same way. So the plenum cannot
exert a frictional drag鈥攊ndeed, if it did, the Earth would slow down in
its orbit and spiral in towards the Sun.
Nevertheless, Newton himself was convinced that space was some kind of
substance. He noted that any body rotating in a vacuum鈥攁 planet spinning
in space, for example鈥攅xperiences a centrifugal force. The Earth bulges
slightly at the equator as a result. But truly empty space has no landmarks
against which to gauge rotation. So, thought Newton, there must be something
invisible lurking there to provide a frame of reference. This something,
reacting back on the rotating body, creates the centrifugal force.
The 17th century German philosopher Gottfried Leibniz disagreed. He believed
that all motion is relative, so rotation can only be gauged by reference to
distant matter in the Universe. We know the Earth is spinning because we see the
stars go round. Take away the rest of the Universe, Leibniz said, and there
would be no way to tell if the Earth was rotating, and hence no centrifugal
force.
The belief that space is filled with some strange, tenuous stuff was
bolstered in the 19th century. Michael Faraday and James Clerk Maxwell
considered electric and magnetic fields to be stresses in some invisible
material medium, which became known as the luminiferous ether. Maxwell believed
electromagnetic waves such as light to be vibrations in the ether. And the idea
that we are surrounded and interpenetrated by a sort of ghostly jelly appealed
to the spiritualists of the day, who concocted the notion that we each have an
etheric body as well as a material one.
But when Albert Michelson and Edward Morley tried to measure how fast the
Earth is moving through the ether, by comparing the speed of light signals going
in different directions, the answer they got was zero.
An explanation came from Albert Einstein: the ether simply doesn鈥檛 exist, and
Earth鈥檚 motion can be considered only relative to other material bodies, not to
space itself. In fact, no experiment can determine a body鈥檚 speed through space,
since uniform motion is purely relative, he said.
Sounds OK so far, but there was one complication: acceleration. If you are in
an aeroplane flying steadily, you can鈥檛 tell that you鈥檙e moving relative to the
ground unless you look out of the window, just as Einstein asserted. You can
pour a drink and sip it as comfortably as if you were at rest in your living
room. But if the plane surges ahead or slows suddenly, you notice at once
because your drink slops about. So although uniform motion is relative,
acceleration appears to be absolute: you can detect it without reference to
other bodies.
Einstein wanted to explain this inertial effect鈥攚hat we might commonly
call g-forces鈥攗sing the ideas of the Austrian philosopher Ernst Mach. Like
Leibniz, Mach believed that all motion is relative, including acceleration.
According to Mach, the slopping of your drink in the lurching aeroplane is
attributable to the influence of all the matter in the Universe鈥攁n idea
that became known as Mach鈥檚 principle. Einstein warmed to the idea that the
gravitational field of the rest of the Universe might explain centrifugal and
other inertial forces resulting from acceleration.
However, when in 1915 Einstein finished formulating his general theory of
relativity 鈥攁 theory of space, time and gravitation鈥攈e was
disappointed to find that it did not incorporate Mach鈥檚 principle. Indeed,
mathematician Kurt G枚del showed in 1948 that one solution to Einstein鈥檚
equations describes a universe in a state of absolute rotation鈥攕omething
that is impossible if rotation can only be relative to distant matter. So if
acceleration is not defined as relative to distant matter, what is it relative
to? Some new version of the ether?
In 1976 I began investigating what quantum mechanics might have to say.
According to quantum field theory, the vacuum has some strange properties.
Heisenberg鈥檚 uncertainty principle implies that even in empty space, subatomic
particles such as electrons and photons are constantly popping into being from
nowhere, then fading away again almost immediately. This means that the quantum
vacuum is a seething frolic of evanescent 鈥渧irtual particles鈥�.
Although these particles lack the permanence of normal matter, they can still
have a physical influence. For example, a pair of mirrors arranged facing one
another extremely close together will feel a tiny force of attraction, even in a
perfect vacuum, because of the way the set-up affects the behaviour of the
virtual photons. This has been confirmed in many experiments.
So clearly the quantum vacuum resembles the ether, in the sense that there鈥檚
more there than just nothing. But what exactly is the new version of the ether
like? You might think that a real particle such as an electron moving in this
sea of virtual particles would have to batter its way through, losing energy and
slowing down as it goes. Not so. Like the ether of old, the quantum vacuum
exerts no frictional drag on a particle with constant velocity.
But it鈥檚 a different story with acceleration. The quantum vacuum does affect
accelerating particles. For example, an electron circling an atom is jostled by
virtual photons from the vacuum, leading to a slight but measurable shift in its
energy.
And according to my 1976 calculations, an observer accelerating through empty
space should see themselves surrounded by electromagnetic radiation, like that
from a hot object. The stronger the acceleration, the hotter the radiation.
Later that year, William Unruh at the University of British Columbia reached
a similar conclusion by considering how the quantum vacuum might affect an
accelerating particle detector. Unruh鈥檚 method was readily adaptable to
rotational acceleration, and calculations revealed that a rotating detector in a
vacuum would also see radiation. Could this heat radiation be the ether
glowing?
To find out for sure, we would have to actually observe the radiation.
However, the effect is tiny: to register a temperature of just 1 kelvin requires
an acceleration of about 1021 g. Accelerating a physicist so severely is hardly
a practical proposition. But maybe we could subject a subatomic particle to such
violence. Last month, Daniel Vanzella and George Matsas of the State University
in S茫o Paulo, caused a stir by pointing out that if the radiation effect
exists, it could cause a proton to do something that would never happen
otherwise. A rapidly accelerated proton would absorb energy from the surrounding
radiation and turn into a neutron, creating a positron neutrino in the process.
But achieving such enormous accelerations is extremely difficult, even with a
proton.
So is there a gentler way? In the 1970s, Stephen Fulling and I, then working
at King鈥檚 College London, investigated how the quantum vacuum would be disturbed
by a moving mirror. We found that, as with a moving particle, there was no
effect if the mirror moves at a constant velocity. Somewhat to our puzzlement,
the same turned out to be true for a uniformly accelerating mirror. However, a
mirror that changes its acceleration鈥攂y wiggling back and forth,
say鈥攅xcites the quantum vacuum and creates real photons. It might be
possible to amplify this moving-mirror radiation by using a resonant cavity with
vibrating walls. Marc-Thierry Jaekel, Astrid Lambrecht and Serge Reynaud of the
University of Paris, Jussieu, described such an experiment earlier this year.
They showed that the resonant oscillations not only amplify the radiation, they
mean that it is emitted in sharply peaked bursts, helping to make it
distinctive. The unsolved problem is how to shake the cavity violently enough
while keeping it very cold, so that heat radiation doesn鈥檛 swamp the still faint
signal.
There could be a way to feel the ether more directly. Theory predicts that
the quantum vacuum behaves in some ways like a viscous fluid. According to
general relativity, a gravitational field is just a distortion of the geometry
of space-time. And it turns out that bending space puts a strain on the quantum
ether. If this strain changes with time, you get friction. Leonard Parker
discovered in the late 1960s that an expanding or contracting Universe would
create particles out of a pure vacuum. In effect, the stretching of space
jiggles up some of the virtual particles and turns them into real particles.
At about the same time, Unruh and Alexei Starobinskii of Moscow University
predicted a similar effect near black holes. They showed that if a black hole
(which is actually just highly warped empty space) rotates, it emits quantum
particles and glows. The quantum ether provides a neat way to explain this. As
the hole rotates, it drags the ether around with it. The dragging effect is
fiercer closer to the hole, so the ether is sheared, which heats it and makes it
glow. Unfortunately the glow is so faint that no readily foreseeable telescope
will be able to capture it.
Luckily, you don鈥檛 need a black hole to observe ether friction. In 1997, John
Pendry of Imperial College, London, showed that a mirror sliding sideways
parallel to another mirror facing it should experience friction even in a
vacuum, because the virtual photons sandwiched between the parallel plates would
heat up the mirror surfaces. This heat energy can come only from the kinetic
energy of the plates, which would therefore be slowed down.
The same would apply to a single atom moving near a metal surface. So in
theory, an atom dropped down the exact centre of a vertical metal pipe should
reach a terminal velocity as it ploughs through the viscous quantum vacuum, just
like a ball bearing dropped into oil. With advances in cold-atom optics, such an
experiment might be feasible in the near future.
Yet even if we could detect the quantum ether as dramatically as this, all
the effects I have described so far are weak. None of them has a powerful
influence on the Universe, so you might think the quantum ether is just a minor
curiosity. But some physicists think the very opposite is true.
Bernard Haisch of the California Institute for Physics and Astrophysics in
Palo Alto and his colleagues have calculated the effect of the quantum vacuum on
an accelerating charged particle, and claim that it mimics the effect of mass
(快猫短视频, 3 February, p 22).
This, says Haisch, is the true origin of inertia,
and solves the old conundrum about acceleration and relative
motion. Put bluntly, your drink slops when an aircraft lurches because the
quantum vacuum pushes against the accelerating atoms. Although few scientists
have so far accepted this claim, the possibility is tantalising.
And there is a curious pointer to something deeper. Quantum physics is famed
for its 鈥渘on-locality鈥�: the fact that it is not possible to characterise the
physical situation at a point in space without reference to the state of the
system in the wider surroundings. The quantum vacuum is no exception, since its
state is defined across all of space. This enables it to 鈥渇eel鈥� the structure of
the entire Universe, and thereby to link the global and the local in precisely
the manner that Mach had in mind. This nonlocality hints at a possible
connection between local physics and distant matter in the Universe 鈥攁
connection that could be mediated by the quantum ether. Among other things, it
could explain why we share an absolute frame of acceleration with the distant
stars.
This is not the ether of Maxwell. Rather than being the medium that transmits
light, it is made of light鈥攙irtual photons鈥攁nd other virtual
particles. Nor is it the plenum. The Greek philosophers鈥� original argument
against the void has lost much of its force, because physicists today have
little difficulty imagining the concept of empty space. But now they question
whether space itself is truly fundamental. Perhaps space as we know it is a
special configuration of a deeper quantum entity, the properties of which we can
only guess at. Far from abhorring a vacuum, nature may have worked very hard to
create one.
COULD we tap the quantum ether as a power source? The first consideration is
how much energy it contains. Calculating it using quantum field theory, you get
an enormous energy density鈥攁bout 10110 joules per cubic centimetre.
That may sound like a wealth of free energy waiting to be mined, but unfortunately it
can鈥檛 be true. Vacuum energy has an antigravitational effect鈥攊t pushes
space apart鈥攁nd that much antigravity would be catastrophic.
Astronomers do believe that some kind of dark energy is slowly speeding up
the Universe鈥檚 expansion. If the quantum vacuum is responsible, then it would
have to have an energy density of no more than a few joules per cubic
kilometre鈥攁 pretty poor energy source. What鈥檚 more, to get at this energy
you need a sink region of even lower energy into which the energy can flow. So
unless you can reduce the vacuum energy in a region of space, you can鈥檛 extract
what is there.
But we could yet find a more exotic use for the vacuum. Gravitational fields
modify the energy of the ether, and can sometimes make it negative. Some
astrophysicists have speculated about using negative-energy ether to build a
wormhole in space. Wormholes are hypothetical short cuts through space-time
between two widely separated points, and they have become famous as potential
time machines. According to general relativity, by traversing a wormhole and
returning through normal space at high speed, an astronaut could get home before
he or she left.
Calculations by Kip Thorne and his colleagues at the California Institute of
Technology showed that a wormhole would soon collapse under its own gravity
unless shored up by some exotic material with substantial negative
energy鈥攕uch as suitably modified ether.
However, visiting the past in this manner paves the way for all sorts of
troubling paradoxes, such as killing your own grandfather before he had any
children, thereby negating your own existence. Many physicists are deeply
unhappy about such paradoxes, and believe that nature will forbid travel
backwards in time. Stephen Hawking proposed a 鈥渃hronology protection hypothesis鈥�
which says that if you try to make a time machine, something will stop you.
But what might that something be? The answer could be the quantum ether
itself. All those virtual particles swarming in the vacuum would get caught up
in the time vortex around a wormhole. This would severely modify the structure
of the quantum ether, enormously boosting its energy near the wormhole. It
remains unclear whether the intense gravity associated with this seething energy
would wreck the wormhole and prevent time travel. Maybe a clever enough cosmic
engineer could harness negative ether energy to stabilise the wormhole鈥檚
interior, while preventing the ether energy swirling around the wormhole from
escalating out of control.