Everywhere in space-time, pairs of “virtual” particles are constantly arising and mutually annihilating.
Virtual particles are indeed real particles. Quantum theory predicts that every particle spends some time as a combination of other particles in all possible ways. These predictions are very well understood and tested.
Quantum mechanics allows, and indeed requires, temporary violations of conservation of energy, so one particle can become a pair of heavier particles (the so-called virtual particles), which quickly rejoin into the original particle as if they had never been there. If that were all that occurred we would still be confident that it was a real effect because it is an intrinsic part of quantum mechanics, which is extremely well tested, and is a complete and tightly woven theory--if any part of it were wrong the whole structure would collapse.
But while the virtual particles are briefly part of our world they can interact with other particles, and that leads to a number of tests of the quantum-mechanical predictions about virtual particles. The first test was understood in the late 1940s.
In a hydrogen atom an electron and a proton are bound together by photons (the quanta of the electromagnetic field). Every photon will spend some time as a virtual electron plus its antiparticle, the virtual positron, since this is allowed by quantum mechanics as described above. The hydrogen atom has two energy levels that coincidentally seem to have the same energy. But when the atom is in one of those levels it interacts differently with the virtual electron and positron than when it is in the other, so their energies are shifted a tiny bit because of those interactions. That shift was measured by Willis Lamb and the Lamb shift was born, for which a Nobel Prize was eventually awarded.
Quarks are particles much like electrons, but different in that they also interact via the strong force. Two of the lighter quarks, the so-called "up" and "down" quarks, bind together to make up protons and neutrons. The "top" quark is the heaviest of the six types of quarks. In the early 1990s it had been predicted to exist but had not been directly seen in any experiment. At the LEP collider at the European particle physics laboratory CERN, millions of Z bosons--the particles that mediate neutral weak interactions--were produced and their mass was very accurately measured. The Standard Model of particle physics predicts the mass of the Z boson, but the measured value differed a little. This small difference could be explained in terms of the time the Z spent as a virtual top quark if such a top quark had a certain mass. When the top quark mass was directly measured a few years later at the Tevatron collider at Fermi National Accelerator Laboratory near Chicago, the value agreed with that obtained from the virtual particle analysis, providing a dramatic test of our understanding of virtual particles.
Another very good test some readers may want to look up is the Casimir effect, where forces between metal plates in empty space are modified by the presence of virtual particles.
Thus virtual particles are indeed real and have observable effects that physicists have devised ways of measuring. Their properties and consequences are well established and well understood consequences of quantum mechanics.
Stephen Hawking realized that when these pairs arise straddling the horizon of a black hole, one virtual particle will get sucked in while its partner escapes, preventing their mutual destruction. The escaped particle becomes real, stealing the energy needed for the upgrade from the black hole’s gravitational field. Meanwhile the in-falling particle acquires negative energy, lowering the energy of the black hole.
Thus, one radiated particle at a time, the black hole blinks out of existence, ultimately leaving no trace: Hawking’s calculation indicated that the radiation is “thermal,” consisting of a featureless, random spread of energies that encodes no details about the collapsed star that formed the black hole, or about anything else of interest that might have fallen in.
According to quantum mechanics, the probabilities of all possible states of particles in the universe must respect “unitarity,” evolving in such a way that the universe’s past states can in principle always be uniquely determined by rewinding from its present state. But if information is lost when a black hole evaporates into a featureless gas of Hawking radiation, then the universe’s past can’t be gleaned from the present, and quantum mechanics breaks down.
Or perhaps Hawking erred.
To do his calculation, he made a key assumption: that space-time is smooth and continuous at the horizon of a black hole, as described by general relativity. Physicists believe that this is an approximation; zoom in far enough on Einstein’s space-time continuum, and a more fundamental, quantum form of gravity emerges. But whereas quantum gravity surely becomes important near a black hole’s super-dense center, known as its “singularity,” Hawking assumed that he could gloss over this short-distance physics in his description of quantum fluctuations at the horizon, where gravity is comparatively mild. According to general relativity, the slope of space-time is gentle enough at the horizon of a typical supermassive black hole (like those at the centers of many galaxies) that an astronaut floating past it wouldn’t even notice.
In 1981,
Unruh discovered that Hawking’s approximation scheme can also be applied to fluids. Like space-time, fluids appear continuous on large scales even though deep down they’re made of discrete atoms. Unruh showed that, just as pairs of particles fluctuate in and out of space-time, vibrations called “phonons,” the quantum units of sound, should surface throughout fluids. And when pairs of phonons arise near the sonic horizon of a sonic black hole, they should get wrenched apart and rendered permanent, producing the sonic analogue of Hawking radiation.
This is the phenomenon that Jeff Steinhauer reported in August in
Nature Physics, after toiling over his experiment since 2009 — “exclusively, all day every day,” he said. He created an exotic fluid called a “Bose-Einstein condensate” out of super-cooled rubidium atoms. He then got it flowing, and zapped the fluid partway along its flow path with a laser, accelerating it to a supersonic speed and creating a sonic horizon. Finally, Steinhauer measured quantum entanglement between pairs of phonons on either side of this horizon, consistent with sonic Hawking radiation.
The finding confirms that the fluid approximation works in the case of sonic black holes. “The question is, how related are the approximations?” said
Stephan Hartmann, a philosopher of physics at Ludwig Maximilian University in Munich, Germany. If sonic black holes serve as a true analogue, then Hawking’s approximation is correct, the event horizon is an uneventful place, and information gets destroyed in black holes, meaning that the probabilistic rules of quantum mechanics must be replaced by a more fundamental framework. If Hawking’s approximation is wrong, then sonic black holes are not good proxies for black holes, and quantum gravity might somehow encode black hole histories in their radiation, preserving information as black holes evaporate.
Unruh believes Hawking’s approximation is correct. In 2005, he and Ralf Schützhold of the University of Duisburg-Essen in Germany showed that Hawking radiation consistently came out as a robust theoretical prediction in both sonic black holes and actual black holes, no matter what theoretical assumptions they made about the details of the short-distance physics. The small-scale properties of space-time or fluids never affected the outcome of the calculation, suggesting that Hawking’s approximation wasn’t glossing over anything important. Unruh interprets this to mean that effects from quantum gravity aren’t capable of modifying Hawking radiation and rescuing information. In his opinion, Steinhauer’s result adds to the evidence that “this thermal radiation is a really robust phenomenon,” and thus, that “information is lost.”
However, most quantum gravity researchers believe that information is preserved — including Hawking, who switched camps in the 2000s. From their perspective, an analogue to Hawking radiation in sonic black holes says nothing about true black holes because the two are categorically different; whereas the fluid approximation is accurate in the case of sonic black holes, space-time must not be approximately smooth at black hole event horizons. Somehow, quantum gravity modifies horizons — and it must do so in an extreme way, to get around Unruh and Schutzhold’s argument about the robustness of Hawking radiation. “We are in the situation where something big has to give,” Bousso said. “But we still don’t know exactly what to replace general relativity with at the horizon.”
Some thought experiments suggest that black holes might be empty shells that carry all their information plastered on their horizons and project it outward to the rest of the universe like holograms. In that case, falling into a black hole would be less like a fish plunging over a waterfall and more like a bug going splat on a window.
In the majority opinion, the comparison with sonic black holes only reinforces how strange black holes and the theory of quantum gravity must be. Harlow, who takes this view, sees sonic black holes not as black hole analogues, but more like computer simulations that are running the wrong equations. If you were to simulate the equations of quantum gravity, “then I do expect you to find the right answer,” he said. “Currently, I don’t know which equations to give you.”
See:
https://www.scientificamerican.com/article/are-virtual-particles-rea/
See:
https://www.quantamagazine.org/what-sonic-black-holes-say-about-real-ones-20161108/
Detecting new particles around black holes with gravitational waves
by Staff Writers
Amsterdam. Netherlands (SPX)
Jun 08, 2022
Clouds of ultralight particles can form around rotating black holes. A team of physicists from the University of Amsterdam and Harvard University now show that these clouds would leave a characteristic imprint on the gravitational waves emitted by binary black holes.
Black holes are generally thought to swallow all forms of matter and energy surrounding them. It has long been known, however, that they can also shed some of their mass through a process called superradiance. While this phenomenon is known to occur, it is only effective if new, so far unobserved particles with very low mass exist in nature, as predicted by several theories beyond the Standard Model of particle physics.
Ionizing gravitational atoms
When mass is extracted from a black hole via superradiance, it forms a large cloud around the black hole, creating a so-called gravitational atom. Despite the immensely larger size of a gravitational atom, the comparison with sub-microscopic atoms is accurate because of the similarity of the black hole plus its cloud with the familiar structure of ordinary atoms, where clouds of electrons surround a core of protons and neutrons.
In a publication that appeared in Physical Review Letters this week, a team consisting of UvA physicists Daniel Baumann, Gianfranco Bertone, and Giovanni Maria Tomaselli, and Harvard University physicist John Stout, suggest that the analogy between ordinary and gravitational atoms runs deeper than just the similarity in structure. They claim that the resemblance can in fact be exploited to discover new particles with upcoming gravitational wave interferometers.
In the new work, the researchers studied the gravitational equivalent of the so-called 'photoelectric effect'. In this well-known process, which for example is exploited in solar cells to produce an electric current, ordinary electrons absorb the energy of incident particles of light and are thereby ejected from a material - the atoms 'ionize'.
In the gravitational analogue, when the gravitational atom is part of a binary system of two heavy objects, it gets perturbed by the presence of the massive companion, which could be a second black hole or a neutron star. Just as the electrons in the photoelectric effect absorb the energy of the incident light, the cloud of ultralight particles can absorb the orbital energy of the companion, so that some of the cloud gets ejected from the gravitational atom.
Finding new particles
The team demonstrated that this process may dramatically alter the evolution of such binary systems, significantly reducing the time required for the components to merge with each other. Moreover, the ionization of the gravitational atom is enhanced at very specific distances between the binary black holes, which leads to sharp features in the gravitational waves that we detect from such mergers.
Future gravitational wave interferometers - machines similar to the LIGO and Virgo detectors that over the past few years have shown us the first gravitational waves from black holes - could observe these effects. Finding the predicted features from gravitational atoms would provide distinctive evidence for the existence of new ultralight particles.
See:
https://www.spacedaily.com/reports/...black_holes_with_gravitational_waves_999.html
In the realist particle narrative, virtual particles pop up when observable particles get close together. They are emitted from one particle and absorbed by another, but they disappear before they can be measured. They transfer force between ordinary particles, giving them motion and life. For every different type of elementary particle (quark, photon, electron, etc.), there are also virtual quarks, virtual photons, and so on.
In the opposing narrative, virtual particles are
not real and show up only in the mathematical theories and equations of quantum physics, which describe the particle world. The equations are correct, the doubters recognize, predicting all sorts of things like the peculiar magnetic properties of electrons and muons.
But the entities called virtual particles are just parts of the math, it is claimed. Virtual particles have never been and cannot be directly observed, by their mathematical definition. They supposedly pop up only during fleeting particle interactions.
Hawking radiation describes hypothetical particles formed by a black hole 's boundary. This radiation implies black holes have temperatures that are inversely proportional to their mass. Putting it another way, the smaller a black hole is, the hotter it should glow.
Hartmann352