At some point everything will fall into black holes

Physics: How to Escape a Black Hole

An excited look at sensitive instruments

So far, the quantum halo scenarios cannot be derived from any physical theory that would harmonize quantum mechanics with gravity. They are just conceivable solutions to the problems we face and compatible with what we see. If they are true, they are only an approximate description of the reality. For a better picture, our ideas of space and time need a fundamental reorganization. Current work on the understanding of black holes is similar to the first attempts by Bohr and others to explain the physics of the atom. The early atomic models were also only roughly approximate and only gradually led to today's deep theoretical structure of quantum mechanics. Tinkering with the principle of locality may seem absurd at first, but the ideas of quantum mechanics seemed no less crazy at the time they were discovered.

In such a situation, physicists hope for experimental evidence that will guide the way to a new theory. Mankind now has two ways of observing black holes: on the one hand, the images of the EHT, on the other hand, facilities such as LIGO (Laser Interferometer Gravitational-Wave Observatory), which record gravitational waves from events in which black holes collide and merge. The waves carry valuable information about the properties and behavior of the objects that created them.

At first glance, it seems excessive to demand that EHT or LIGO detect the smallest deviations from Einstein's description of the black holes. Because physicists actually only expect noticeable differences to the theory of relativity when space-time is extremely bent. This happens near the center of a black hole; however, the curvatures near the horizon are quite weak. But the information paradox described makes it clear: Changes to the current laws of physics are not only necessary to describe phenomena deep in a black hole. Deviations from classic predictions are also evident in its outer areas. In the case of the black hole in M87, for example, the event horizon is many times the size of our solar system.

With today's data from EHT and LIGO, some particularly exotic, but fundamentally logically consistent constructions for black holes can be ruled out. If, for example, there were remains with a diameter slightly more than twice as large at the location of black holes, physicists would have found evidence of this in the data of both experiments. In the case of the EHT, the radiation on the basis of which the famous image was created comes from a region that is only about one and a half times as extensive as the event horizon. And with LIGO, part of the signal comes from an area where the colliding objects are at a similarly small distance. The measurements are still being analyzed in more detail, but apparently EHT and LIGO show very dark and very compact objects that look exactly as one should expect for black holes. More detailed studies of the signals may provide more clues about the quantum properties of black holes. Even if there aren't any surprising effects, that might discard some models.

Remnants with a strongly deviating diameter are already excluded. But what about scenarios that only differ in terms of their description very close to the horizon of the black hole? For concrete predictions, the theories of fuzzballs, fire walls and similar remains would need to be worked out in more detail. At least there are some initial clues. However, if these objects were hardly larger than the event horizon of a corresponding black hole, neither EHT nor LIGO observations could reveal any differences between the conceivable structures. But we may still have a chance.

In 2016, physicists Vitor Cardoso from the University of Lisbon, Edgardo Franzin from the University of Barcelona and Paolo Pani from the University of Rome predicted: If two colliding objects merge, the fused remnant could reflect gravitational waves on its surface - if there are any. These "echoes" could be found in the signal. However, it is an open question why such structures should be stable at all instead of simply collapsing into black holes under their own weight. Admittedly, this is a general problem for all scenarios in which an extensive remnant arises, but given the enormous forces involved in a collision, this would be particularly difficult to explain here.

If, on the other hand, we are dealing with only subtle changes in spacetime geometry in a wide area beyond the horizon, the prospects of experimental verification are better. For example, in the strong scenario I propose, the ripple of the quantum halo deflects the light that passes near the black hole. Over time, this could be reflected in the images of the EHT.

Detective search for clues on the way to reorganizing space and time

Together with the EHT scientist Dimitrios Psaltis, I calculated for the case of the black hole in the center of the Milky Way: The changes that could be seen in distortions in the vicinity of the object took place on a typical time scale of one hour. Since the EHT combines observations lasting several hours into an average, that would hardly be seen here. But the corresponding time for the black hole in the galaxy M87, which is more than 1000 times larger, would be in the range of dozens of days. So we may be able to see something if we extend the EHT observation time beyond the original project duration of one week. If that does not provide any indication, the more subtle weak quantum scenario would still be possible.

The latter is more difficult to test because of the tiny changes in spacetime geometry. According to preliminary studies, the absorption or reflection of gravitational waves could take place differently. This may lead to observable effects in gravitational wave signals. Or even more exotic physics comes into play.

In any case, all previous approaches based on quantum mechanics suggest that spacetime itself is not fundamental, but arises from a more fundamental mathematical structure. But we only have a chance of making measurements if we expand and improve both the EHT observatories and those for gravitational waves. Significantly longer observations are already planned for the EHT. In the case of gravitational waves, additional detectors in Japan and India, for example, will complement the existing facilities in the USA and Europe.

In addition, further theoretical efforts must be made in order to refine scenarios for the environment of black holes and to understand their origins and consequences. Just as what happens in atoms helped advance quantum mechanics theory, black holes are likely to provide crucial clues to spark the next conceptual revolution in physics.