How the quantum vacuum perturbs atoms, shrinks black holes and formed our galaxies

From the microscopic structure of particles and atoms, to the fate of black holes and the large-scale structure of the universe, the quantum ‘vacuum’ plays a role.

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Cosmologists suggest that fluctuations of the quantum vacuum in the first moments of the universe played a role in the formation of galaxy clusters spanning the cosmos. Pictured here is the spiral galaxy NGC 2090, located in the constellation Columba. (ESA/Webb, NASA & CSA, A. Leroy)The word “vacuum” may conjure the image of empty space. Maybe you think of a region far outside the solar system without planets, debris, gas clouds or light.

According to quantum theory, however, a vacuum is never truly empty. Quantum mechanics extends the Heisenberg uncertainty principle — which states that we cannot measure the position and the velocity of an object exactly at the same time — to a vacuum. The result is that a vacuum can never be still.

Instead of an empty space, we can imagine the vacuum as a dynamic object, with regular fluctuations in energy.

Physicists at the University of Cambridge in the United Kingdom recently presented preliminary research showing direct imaging of these fluctuations in a lab-made quantum vacuum. Physicists have “seen” the quantum vacuum many times before, however. And we can all see the effects of the quantum vacuum on atoms in the atmosphere — through the gorgeous light displays of the aurora borealis.

The energy fluctuations of the quantum vacuum also create conditions for black holes to shrink. And modern cosmologists believe they played a role in the creation of galaxies, their impact felt in the earliest moments of our universe.

Impacts on real objects

The idea of a “quantum vacuum” that isn’t truly empty is more than an academic interpretation of equations. The most striking feature of the vacuum is the way it impacts the structure and dynamics of real objects.

For instance, there is an ongoing effort geared at observing the quantum vacuum directly in an experiment using high-intensity laser beams. The strong electric fields from the laser pulses interact with the particles fluctuating in and out of the vacuum and, as a result, experience an effect called “birefringence.”

The birefringent vacuum treats light of different polarizations differently, similar to how polarizing sunglasses preferentially block light from specific glare-producing sources.

This complex and interesting experiment will test the predictions of quantum mechanics on the structure of the vacuum through direct observation.

Changing colours of light

This will not be the first time scientists have “seen” the quantum vacuum at work. By the mid-20th century, quantum mechanics had been put to test after test and was proving capable of predicting and describing the internal structure of molecules, atoms and nuclei. For theory and experiments to agree, however, scientists had to account for the impact of the quantum vacuum.

In many cases, theoretical predictions were tested against the spectra of these objects found through experiments. For example, the electrons in oxygen atoms and nitrogen molecules have specific energy levels. The lowest energy level is the “ground” state, and the rest are “excited” states. Electrons can’t hang out in excited states forever. The quantum vacuum shakes and perturbs them until they fall back to the ground state. Transitions between levels are accompanied by emission of light with specific colours, dubbed the spectra.

In the case of oxygen atoms and nitrogen molecules, this spectrum takes the form of dazzling display of greens, reds and violets in the aurora borealis.

One of the specific predictions of quantum mechanics for the electron in the hydrogen atom is that the lower energy levels are particularly sensitive to quantum vacuum fluctuations. As the energy levels shift slightly, this changes the colour of the light that can be absorbed or emitted by the atom.

This effect in hydrogen was precisely measured in 1947 by Willis Lamb who eventually won the Nobel prize in physics for what is now called the Lamb shift.

Modern physics experiments

The impact of vacuum fluctuations on the structure of particles and their interaction with the environment has since played an important part in modern physics experiments. The subfield of physics that connects the structure of atoms and molecules to the light they interact with is called atomic, molecular and optical (AMO) physics.

I did my PhD in this area of physics, and those of us working in the lab rely on accurate calculation of spectra in order to use lasers to control quantum systems in neutral atom or trapped ion quantum computers and quantum simulators.

Quantum computers harness “qubits,” a pair of energy levels analogous to the 0 and 1 making up a classical bit, to perform complex calculations. Quantum simulators, meanwhile, subject quantum particles like ultracold atoms to environments that make them emulate the behaviour of another system, such as a superconductor.

Physicists must also take into account the direct impact of vacuum fluctuations on the basic properties of fundamental particles, such as the way in which electron-like particles called “muons” interact with magnetic fields, in order to describe what’s observed in experiments.

Shrinking black holes

A further interesting prediction of the quantum vacuum is how it behaves near extreme astrophysical objects, such as in the vicinity of a black hole.

The first picture of a black hole was captured at the centre of the galaxy M87, using the Event Horizon Telescope.
(Event Horizon Telescope Collaboration)

The strong gravitational acceleration toward the black hole generated by space-time curvature modifies the number and distribution of particles occupying the vacuum. This causes the black hole to “evaporate” into the surrounding empty space.

This is the only known mechanism by which black holes shrink and their contents, in some complex manner, escape from behind the event horizon. And this is enabled by the quantum vacuum.

Formation of galaxies

Modern cosmologists have taken these well-described microscopic effects and worked to glean information about the earliest moments in cosmic history.

They suggest that fluctuations of the quantum vacuum in the first moments of the universe “seeded” the small differences in the density of matter that eventually, under the influence of gravitational attraction, formed the cobwebs of interconnected galaxy clusters spanning the cosmos.

From the microscopic structure of fundamental particles and atoms to the fate of black holes and the large-scale structure of the universe, the “nothing” of the quantum vacuum takes centre stage.

The dynamic quantum vacuum interacts with everything and has important effects, both observed and predicted, offering a fascinating history to reflect on and an exciting range of future experiments to look forward to.
Jared Rivera receives funding from The Natural Sciences and Engineering Research Council of Canada (NSERC).

Tags: Academic Journalism, The Conversation, The Conversation Academic News