Physicists struggled to ditch the ether and accept the void – until quantum theory refilled the vacuum with unimaginable energy
Read more: “The nature of nothingness“
“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
- From issue 2839 of New Scientist magazine, page 50-52.
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