The issue of what happens when the very small world meets the very large was first considered in the 1970s. That’s when British physicist Stephen Hawking began thinking about what happened to particles that experienced the unparalleled gravitational forces at the edge of a black hole, a place known as the event horizon. Anything slightly inside the event horizon will unavoidably fall into the black hole, whereas anything just outside it still has a chance to escape.
Hawking wanted to know what would happen to pairs of particles—a particle and its antiparticle partner—that spontaneously appeared at a black hole’s event horizon. These couplets emerge from the “empty” vacuum of space, and quantum mechanics tells us they constantly wink in and out of existence everywhere. As soon as a particle meets up with its antiparticle, they destroy each other in a fraction of a second, and the universe at large doesn’t notice their presence.
Hawking showed that if one of the partners appeared within the event horizon, however, it would fall into the black hole while its associate on the horizon’s other side would get flung outward with tremendous force. To conserve the total energy of the black hole and abide by a tenet of physics, the infalling particle must carry negative energy (and hence negative mass), and the launched one must have positive energy. In this way, black holes emit a type of energy now called Hawking radiation and, over time, this escaping positive energy depletes them, which causes them to evaporate.
About six years ago astrophysicist Heino Falcke of Radboud University in the Netherlands started thinking more deeply about the physics involved in these processes and whether the event horizon was a necessary component. In other words, could this same evaporation occur for other objects? “I asked a few experts and got very different answers,” he recalls.
Falcke enlisted the help of quantum physicist Michael Wondrak and mathematician Walter van Suijlekom, both at Radboud, to take another look at the issue. The trio decided to approach the topic from an atypical angle by using equations from a related phenomenon known as the Schwinger effect. This effect describes how charged particles and antiparticles get torn apart when they emerge from the vacuum in the presence of a powerful electromagnetic field. The process could be considered analogous to particle pairs experiencing strong gravitational forces at a black hole’s event horizon.
The researchers’ mathematical analysis showed how any object with mass—and not just a superheavy one such as that of a black hole—affects the pairs of particles and antiparticles that emerge from the vacuum of space. In more wavelike terms, these particles can be thought of as having a cloud of probability regarding where they might be located in space, says Tyler McMaken, a Ph.D. student who studies theoretical astrophysics at the University of Colorado Boulder. In the absence of any external forces, electromagnetic or gravitational, the clouds of both the particle and antiparticle will overlap, and they will annihilate each other. But if gravity tugs on one cloud more than the other, each will be shifted slightly. They won’t overlap and therefore won’t be annihilated. Instead they will produce radiation, much like a particle that gets flung from a black hole’s event horizon.
The team’s calculations, published on June 2 in Physical Review Letters, suggest that anything with gravity, meaning basically every object in the universe, will emit a Hawking-like radiation and evaporate. The equations indicate that the process will take trillions upon trillions of years, so it’s likely that you and your personal belongings will be long gone before this effect comes into play. But the long-lived remnants of dead stars such as white dwarfs and neutron stars—which have enormous mass—might have their life shortened if the phenomenon is real.
The analysis seems promising, says McMaken, who was not involved in the work. “This shows that there is definitively some effect where particles can be ripped apart just solely from gravitational forces in the vacuum,” he adds. McMaken and his colleagues have considered doing similar calculations, he says, so he’s pleased that scientists did a thorough check to see what happens in these situations.
But other researchers disagree. “Personally, I’d be kind of skeptical that all previous calculations are wrong” about what happens to particles near massive objects, says theoretical physicist Sabine Hossenfelder of the Munich Center for Mathematical Philosophy in Germany. She suspects that a more careful analysis will show that the particle-antiparticle pairs don’t actually radiate from massive non-black hole objects.
Current technology isn’t sensitive enough to detect this evaporative effect and prove the new claim one way or another. Falcke and his team suggest that experiments could focus on observing the Schwinger effect, which also remains theoretical at this point, to potentially bolster their own claims.
ABOUT THE AUTHOR(S)
Adam Mann is a journalist specializing in astronomy and physics. His work has appeared in National Geographic, the Wall Street Journal, Wired, and elsewhere. Credit: Nick Higgins
