Liquid space

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There's so much going on in a vacuum that it's beginning to look like a substance in its own right. Paul Davies offers you a guided tour of the quantum ether

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.

If so, we'll 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-atoms-moving 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's 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-indeed, 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-a planet spinning in space, for example-experiences 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't exist, and Earth's motion can be considered only relative to other material bodies, not to space itself. In fact, no experiment can determine a body's 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't tell that you're 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-what we might commonly call g-forces-using 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-an idea that became known as Mach's 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 -a theory of space, time and gravitation-he was disappointed to find that it did not incorporate Mach's principle. Indeed, mathematician Kurt GÖdel showed in 1948 that one solution to Einstein's equations describes a universe in a state of absolute rotation-something 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's 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 "virtual particles".

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's 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's 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's 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 SÃo 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's 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-by wiggling back and forth, say-excites 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't 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't 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 (New Scientist, 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 "non-locality": 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 "feel" 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 -a 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-virtual photons-and other virtual particles. Nor is it the plenum. The Greek philosophers' 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.