Fantasy scenarios: what if instead of the moon there was a black hole?

Without the moon, it would perhaps be a less inspiring world, and certainly darker at night, under only the dim light of the stars. Artists and lovers would have no moonlight to contemplate, amateur astronomers would find themselves without their most familiar target, while Galileo and Newton would not have had the natural laboratory that helped inspire their great scientific intuitions.. No Apollo missions, no eclipses of the Sun, much less of the Moon.

This, in broad terms, is the scenario we would find ourselves in if, instead of the Moon, there was a black hole. A surprising picture, which may be worth reflecting on to rediscover the relationship with our satellite and to compare ourselves more closely with the most mysterious objects in the universe.

All concentrated in the center

In the collective imagination, the idea of ​​a black hole evokes a gigantic cosmic monster capable of swallowing stars and planets – and this is what actually happens in many parts of the cosmos. In our case, however, we could rest assured. In fact, let’s imagine that the “lunar” black hole has the same mass as our satellite.

This assumption has two important consequences. First of all, such a black hole would be very small, with a radius of just a tenth of a millimeter. To be clear, less than a grain of sand. Secondly, to a first approximation its orbit would remain unchanged. It would move, in fact, like the center of mass of the Moon, that ideal point which precisely defines the movement of our satellite in space. In practice, it is as if the entire Moon were concentrated in that point, so the black hole would move around us on the same orbit, without swallowing us. And it would be so small we wouldn’t even see it.

Let us then enter into the specifics of this object. «A black hole is characterized by three essential properties», explains Luciano Rezzolla, director of the Institute of Theoretical Physics at Goethe Universität in Frankfurt and among the protagonists of the first photo of a supermassive black hole: «mass, rotation (“spin”) and electric charge».

As far as we know, the electric charge is usually zero, because there are no known electrically charged celestial bodies in the universe. So mass and spin remain. Let’s imagine then that they are like those of the Moon, and see what would change.

Biological clock

One of the main differences, as we imagined at the beginning, would be the absence of moonlight

. The nights would therefore be much darker and, especially outside the cities, there would likely be a greater use of artificial lighting, with a consequent increase in energy consumption. Many living species would be affected, at least in comparison with those we know.

«The disappearance of moonlight would certainly have an impact on those animals that use it as a synchronizer of their biological clocks», comments Cristiano Bertolucci, professor of zoology at the University of Ferrara. «Among the most sensitive are marine organisms, such as those of the genus Platynereisannelids that possess photoreceptors specialized in the perception of the weak moonlight that regulates their physiology. Corals also synchronize the release of eggs and sperm with the phases of the Moon, so that millions of gametes are released at the same time, increasing the chances of fertilization. Without this astronomical signal, playback problems could arise.”. The balance between nocturnal predators and prey would also change.

«Unlike birds of prey such as the barn owl, which mainly use their hearing, a nocturnal bird such as the nightjar, which captures insects in flight, prolongs the time dedicated to hunting during the nights of the full moon, while with the new moon it reduces its activity. Without the moonlight he would probably have to change his eating strategies”, clarifies Bertolucci.

«The case of bats is different, as thanks to echolocation they would be able to orient themselves and hunt even in total darkness». If the Moon had never existed, the biologist concludes, life would presumably have developed anyway, but would have found completely different evolutionary balances.

Sudden flashes

From an astronomical point of view, the Moon also has an important stabilizing effect on the Earth’s rotation axis. If it were not there at all, the climate and seasons would be altered, with very marked variations. However, if in its place there was a black hole of the same mass, little or nothing would change. And the same applies to a first approximation for the tides, which would continue to regularly mark the rhythm of the oceans. For astronomers, however, a black hole Moon would be an authentic ghost: an invisible dot that does not reflect the light of the Sun..

According to General Relativity, in reality, a black hole strongly distorts space-time in its vicinity and deflects the light coming from the celestial bodies behind it, behaving like a lens (the so-called “gravitational lens”).

Thanks to this phenomenon, astronomers are able to “see” otherwise invisible objects, but in our case the effect would be confined to tiny scales, less than a millimetre, and it would be difficult to measure it from Earth (while getting closer, yes, would be dangerous).

. “The only clue to its presence would be if a small celestial body passed too close to it: falling towards the black hole, the matter would heat up to the point of emitting a brief flash of radiation,” says Rezzolla..

Slingshot effect

Without a surface to land on, the old Apollo missions (with the whole race to the Moon in the 1960s and 1970s) would not have happened, just as today there would be no Artemis program to return man to our satellite. But perhaps, on the other hand, we would be more projected towards the conquest of Mars. In their planetary missions, in fact, the probes use the so-called “slingshot effect”: with the right trajectory, passing very close to a celestial body, they exploit its gravity by “stealing” a small part of its orbital energy and thus increasing its speed without consuming fuel..

«The slingshot effect depends on the mass», explains Giacomo Tommei, professor of mathematical physics at the University of Pisa. «If instead of the Moon there were a black hole with the same mass, the effect would be practically identical. But we could get much closer (the Moon has a radius of 1,737 km) and receive a greater push”. However, getting too close would mean entering a region where the gravitational field changes violently even within a few meters. The front part of the probe would be attracted more intensely than the rear part, subjecting the structure to enormous tensions. «Less than a hundred kilometers away from the black hole, we would begin to risk the so-called “spaghettification”», observes Tommei: the object would begin to lengthen until it disintegrates.

A whole different story

In our culture, however, well before the arrival of the probes, the Moon had already been a precious observation point for astronomers and scientists. «The Babylonians learned to predict eclipses with surprising precision; Galileo observed mountains and craters there, demonstrating that celestial bodies were not perfect spheres; while Newton realized that the same force capable of making an apple fall also guided the motion of our satellite around the Earth, laying the foundations of the theory of universal gravitation”, comments Tommei. «In short, the Moon was one of the first great laboratories of physics and allowed us to compare observations and mathematical models, helping us understand how gravity works».

If there had always been a tiny black hole in its place, many of these observations would have been impossible, or at least delayed in time.

. The tides would have appeared much more mysterious. The ancients would have seen the sea advance and retreat regularly without being able to connect the phenomenon to anything visible in the sky.

The history of science would probably have followed a different path, with insights destined to arrive in roundabout ways. Not to mention the impact on our culture. Without the Moon many classics of poetry would never have been born, the serenades of musicians and the infinite metaphors that have inspired us all for millennia. The most profound change caused by that tiny invisible black hole, in short, would have concerned the way in which humanity has learned to look at the sky.