The word "vacuum" usually conjures up absolute emptiness - outer space without planets, gas or light. However, quantum theory holds that the vacuum is never truly empty. Heisenberg's uncertainty principle, which forbids simultaneously measuring an object's position and velocity with precision, in quantum mechanics extends to the vacuum as well. It follows that the vacuum cannot be at rest - energy fluctuations occur in it constantly.
Recently physicists from the University of Cambridge (UK) presented preliminary results from an experiment in which they managed to directly record such fluctuations (random changes) in a laboratory-created quantum vacuum. Admittedly, scientists had observed manifestations of the quantum vacuum before - for example, its effect on atoms in the atmosphere is visible in the auroras.
The quantum vacuum is not merely a mathematical abstraction. According to theory, its energy fluctuations create the conditions for the gradual decrease of the mass of black holes. Modern cosmologists also believe that vacuum fluctuations played a role in the formation of galaxies in the first moments of the Universe's life.
One way to see the quantum vacuum directly is to bombard it with powerful lasers. The strong electric fields of laser pulses interact with particles that appear and disappear in the vacuum. The result is a birefringence effect: the vacuum begins to transmit light with different polarizations differently - roughly the same way polarized sunglasses block glare. Such an experiment should test quantum mechanics' predictions about the structure of the vacuum.
The effect of the quantum vacuum on atoms was first measured back in the mid-20th century. Electrons in atoms and molecules can occupy different energy levels. The lowest is the ground state, the others are excited states. An electron cannot remain in an excited state for long: the vacuum "pushes" it down, and light of a particular color is emitted in the transition. This is exactly how spectra arise - sets of colored lines. For oxygen and nitrogen in the atmosphere, these transitions produce the green, red and violet hues of the auroras.
In 1947 Willis Lamb precisely measured the shift of energy levels in the hydrogen atom caused by vacuum fluctuations. For this work, known as the Lamb shift, he later received the Nobel Prize in Physics. Today scientists working on quantum computers and quantum simulators must take these effects into account. A quantum computer uses qubits - pairs of energy levels analogous to the 0 and 1 of an ordinary bit. A quantum simulator makes some quantum particles, such as ultracold atoms, reproduce the behavior of another system, say a superconductor. In all these cases, calculations must include the influence of the vacuum.
Another consequence of the quantum vacuum is the evaporation of black holes. Near the event horizon, the curvature of space-time changes the distribution of particles in the vacuum, and the black hole gradually loses mass, as if dissolving into the surrounding space. This is the only known mechanism that allows a black hole to shrink.
Cosmologists also suggest that fluctuations of the quantum vacuum in the early Universe created tiny differences in the density of matter. Later, under the influence of gravity, these inhomogeneities led to the formation of bound clusters of galaxies.
From the structure of elementary particles to the fate of black holes and the large-scale structure of the Universe - the quantum vacuum turns out to be at the center of attention. It interacts with everything that exists, and its effects, already observed and predicted, promise new exciting experiments.