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Physics

Storms in a vacuum

Energy in empty space could destroy stars and even reveal the nature of dark matter

032-037_Vacuo_220-01Vacuum is not synonymous with nothing, at least for physicists.  They claim that even seemingly empty space still contains some form of energy that fluctuates constantly, like the small waves that wrinkle the surface of a lake on a windy day.  Although the energy in a vacuum and its oscillations are usually too subtle to be noticed, except at microscopic scales, a team of theoretical physicists in São Paulo believes these oscillations can be amplified to astronomical scales, to the point of destroying entire stars.  This is a consequence of the effect discovered four years ago by physicist Daniel Vanzella and his then doctoral student William Lima, who are currently carrying out a series of complicated calculations to try to understand, in detail, how this vacuum energy can influence the fate of the densest stars in the Universe, called neutron stars.

In 2010 Vanzella, Lima and the physicist George Matsas realized that, under certain conditions, the huge gravitational force of neutron stars would be able to perturb energy fluctuations in a vacuum.  They concluded that this kind of storm of energy would not last more than a second, but its violence would be enough to destroy the star that produced it.  The most recent calculations, however, suggest that if the effect they call awakening of the vacuum really exists, its consequences for neutron stars could vary much more than the physicists imagined.  It may be that the stars explode, leaving a black hole or nothing in their place.  Or they could survive, despite suffering a drastic reduction in mass and energy.  In their most recent article, published in the journal Physical Review D, the researchers clarify which analyses and tests still need to be done to determine the final consequences of this awakening of the vacuum.  “We want to know all of the possible outcomes for these stars,” states Vanzella, a researcher at the Institute of Physics at the University of São Paulo in São Carlos (IFSC-USP).

It may seem a mere theoretical curiosity, but determining the existence and intensity of the awakening of the vacuum should help us better understand the nature of dark matter, an invisible type of matter that permeates all space and is perceptible only due to the effect of its gravitational force on the movement of galaxies.  No one knows what dark matter is made of, only that it must consist of subatomic particles that are yet to be discovered.  Matsas and his colleagues already know that the awakening of the vacuum can only occur if at least some components of dark matter have some special properties.  “Depending on the properties of dark matter, neutron stars with mass and radius within a given range should not exist,” explains Matsas, a researcher at the Institute for Theoretical Physics, São Paulo State University (IFT-Unesp).

Particles and waves
Even physicists agree that the term “vacuum energy” sounds unusual.  But its existence has already been proven, directly and indirectly, in the last century.  Researchers in nanotechnology, for example, must take into account what is called the Casimir effect, a force that can attract or repel the microscopic metal parts they manipulate in environments from which air has been removed.  These forces, they believe, arise from differences in the vacuum energy in the space between the parts and surrounding them.

The theory that explains the origin of this energy is the same that describes what happens in experiments in particle accelerators, in which subatomic particles are forced to collide.  At the LHC, the largest particle accelerator ever built, for example, two beams of protons traveling at nearly the speed of light collide. After the collision, the protons disappear and new particles appear in their place, seemingly out of nowhere. This is not just because the protons break apart into more fundamental subatomic particles. This also occurs because the particles that existed are annihilated and new particles are created from their energy. The destruction and the creation of particles is possible because, according to the Special Theory of Relativity formulated by Albert Einstein in 1905, the mass of particles can be converted into energy and vice versa.

032-037_Vacuo_220-02Imagem: infographics: ana paula campos / illustration: fabio otuboBut relativity does not explain everything. Over the last century it also became clear that elementary particles obey the laws of quantum mechanics: they do not behave like solid, well-defined points. They are more like hybrid objects that sometimes behave like points, and sometimes propagate through space as if they were waves. The principal law that these particle-waves obey is the uncertainty principle, according to which the more precisely we know the position of a particle, the less we know about its momentum, and the opposite is also true. “That’s what makes quantum mechanics so extraordinary,” says Matsas. “We can no longer explain particles so simply.”

“Special relativity and quantum mechanics now lead us to understand that elementary particles arise from more fundamental entities, called quantum fields,” explains the physicist. One way to visualize these fields is to think of ripples in a pond. Similar to how each point on the surface of a pond can oscillate up and down, every point in the Universe is associated with a quantum field whose energy fluctuates constantly. According to this way of understanding the Cosmos, elementary particles are actually a special type of fluctuation of these fields—energy vibrations recorded by physicists’ detectors as particles. “If it’s any consolation,” says Matsas, “I also find it difficult to visualize these fields.”

Matsas also explains that the uncertainty principle prevents us from knowing—at the same time and with infinite precision—the properties of the field at each point in the Universe and how fast these properties change. One consequence of this quantum uncertainty is that even a region of empty space containing no particles has an energy that fluctuates around a value close to zero. There are several types of quantum fields permeating the Universe, and each known elementary particle, such as electrons, quarks, photons and neutrinos, arises from a specific type of field. The way in which a particle influences another depends on how the fields that gave rise to them are connected.

In an attempt to explain dark matter, many hypotheses assume the existence of free quantum fields. Called “free” because they are not thought to be connected to other fields, free fields give rise to elementary particles that barely interact with each other. A consequence of this property is that the particles generated by free fields can be here right now, crossing the Earth or even our bodies, without our being able to detect them directly. Since free fields would only be connected to the rest of the Universe via gravitational forces, neutron stars became the natural laboratory for testing these ideas. This is because they are the densest objects known, with the exception of black holes. “By trying to observe the awakening of the vacuum effect in stars, we can determine whether or not free fields exist,” says Matsas.

Curved space
The gravitational force is the only fundamental interaction that physicists cannot explain using quantum fields. “It is best described as one of the results of the deformation of the geometry of space-time in a region surrounding a mass or energy, such as near a star,” explains Vanzella, of IFSC-USP. The more concentrated the energy of a star, for example, the more intense the spatiotemporal deformation caused by it. In extreme cases, the deformation can make the star itself collapse, leaving a highly distorted region of empty space in its place—the famous black holes.

Matsas and Vanzella are experts in calculating how the quantum fields that give rise to elementary particles are affected by the spatiotemporal deformation of a star or a black hole. In their studies, they apply the same combination of general relativity and quantum field theory that physicist Stephen Hawking used in 1974 to discover that black holes emit elementary particles.

032-037_Vacuo_220-03Imagem: umar mohideen / uc riversideIn 2010, Vanzella and his then PhD student William Lima, currently working as a postdoctoral researcher at the IFT, published two articles in the journal Physical Review Letters, the second with Matsas, demonstrating how space-time curvature could, in certain circumstances, amplify the fluctuations of quantum field energy in a vacuum.

This might happen during the contraction of a neutron star. With a mass comparable to that of the Sun, but consisting of neutrons squished into a sphere about 20 kilometers in diameter, neutron stars originate from the death of a bigger star in an explosive event called a supernova. When a star with a mass about 10 times the mass of our Sun exhausts its nuclear fuel, its outer layers explode while its nucleus undergoes an implosion. The result of this event is the emergence of a black hole or a neutron star in the center of the region where the prior star had been located. Because they are so small, neutron stars are difficult to observe—they are often studied through their radio wave and X-ray emissions.

Lima, Vanzella and Matsas found that, if a neutron star shrinks to a certain diameter, its gravity begins to cause disruptions in space that would feed an exponential growth in the fluctuations of the quantum field vacuum energy. This means that, even if the total energy of the field is always roughly equal to zero, some points in space momentarily accumulate large quantities of positive energy, while others accumulate similar amounts of negative energy.

The situation is almost unimaginable. Imagine if small ripples on a lake suddenly began to fluctuate up and down frantically, to increasing heights and depths. “In milliseconds, the energy density of these fluctuations would be large enough to bend space-time more than the star itself,” Vanzella explains. “This growth, however, could not continue forever and something would need to happen to the star and the field to reestablish the space-time curvature.”

After the storm
To know exactly what could happen to stars and quantum fields, the researchers must deal with the equations of general relativity combined with those of quantum field theory, which are almost impossible to solve directly. The theorists are approaching the challenge slowly and in parts, using general physics principles and estimates, which reveal the details of the problem little by little.

In 2012, the physicist André Landulfo, currently at the Federal University of the ABC, joined the team to show that no matter what happens to the neutron star at the end of the process, a good portion of its energy is transferred to the quantum field, creating new particles. “When a fluctuation has grown tremendously during the unstable phase, the field will not return to the vacuum state when the system reestablishes itself” explains Vanzella. “The field will produce a lot of particles in the end.”

These new elementary particles would be invisible to telescopes, but would extract an immense amount of energy from the neutron star—or what remains of it. And this energy loss could have observable consequences.

“It’s an interesting possibility,” says the Italian physicist Paolo Pani, of the Lisbon Technical University, Portugal, who studies the relationship between alternative physics theories and astrophysical observations. Pani would be interested in seeing the result of including the awakening of the vacuum effect in simulations of supernova explosions. “These simulations would be important to understanding if the effect could explain gamma-ray bursts,” says Pani.

Matsas and Vanzella, however, point out that the results of their calculations can already be compared with observations even without performing sophisticated astrophysical simulations. “We can immediately discard the possibility of certain free fields that are still theoretically discussed,” explains Vanzella. “If, for example, we observe stars with a certain mass-radius ratio that should have been destroyed by the awakening of the vacuum effect of a certain field, this can only be because this field does not exist.”

The team’s latest article, written by the physicist Raissa Mendes, who is completing her PhD thesis under Matsas’ supervision, was also published in Physical Review D. In the article, the group determined that one can use the results of studies on the instability of stars and black holes performed by other researchers since the 1970s to find out what effect the awakening of the quantum vacuum would have on neutron stars.

From these calculations made by other researchers, the team determined that neutron stars can, in some cases, survive the awakening of the vacuum. According to Vanzella, this effect only occurs when one of the terms in the equation that determines how the free field interacts with space-time curvature takes on certain values. “The effect occurs for certain ranges of values of this term, some positive and some negative,” says the physicist. “Calculations performed by other researchers suggest that, for negative values, the creation of particles would be enough to interrupt the growth of vacuum energy and the star would survive.”

At the moment, Vanzella and the physicist Raphael Santarelli, who is a post-doctoral researcher working with Vanzella in São Carlos, are analyzing the case when this term takes on positive values. Preliminary results suggest that the star would be destroyed. “What we already know is that the creation of particles will not be enough to reestablish the system,” Vanzella says. “Something else needs to happen, perhaps the formation of a black hole.” In 2010, Matsas bet a crate of wine that a neutron star would always be destroyed by the awakening of the vacuum. “Now,” he says, “I seems I win half the box and lose the other half.”

Project
Physics in curved space-times (nº 2007/55449-1); Grant Mechanism Thematic Project; Principal investigator George Emanuel Avraam Matsas (IFT-Unesp); Investment R$ 181,501.15 (FAPESP).

Scientific articles
MENDES, R.F.P. et al. Quantum versus classical instability of scalar fields in curved backgrounds. Physical Review D. V. 89, p. 047503. Feb. 24, 2014.
LANDULFO, A.G.S. et al. Particle creation due to tachyonic instability in relativistic stars. Physical Review D. V. 86, p. 104025. 2012.