Pyknons: A Suggestion for Rebranding Black Holes

Abstract

‘Black hole’ is the denomination of the most extreme prediction of the General Theory of Relativity made popular by J. A. Wheeler in the late sixties of the twentieth century, having now entered widely into the collective imagination. Nonetheless, the term is somewhat misleading since there is nothing that tears apart in black holes, which, furthermore, are not even black. Thus, the new name pyknons, from the ancient Greek word for ‘compact; constricted; close-packed’, is proposed for them since it captures a key distinctive feature of theirs. In deference to the objects thus renamed, it also has the merit of introducing a greater compactness in the terms denoting them.

1. Introduction

The most intriguing prediction of the classical, i.e., non-quantum, gravitational theory of Einstein is that it allows the existence of unidirectional regions in spacetime, which are causally disconnected from the rest of it due to their extreme curvature. Not bounded by any material surface, they are the so-called ‘black holes’ [1,2,3], according to a quite fortunate but somehow misleading and, strictly speaking, incorrect naming popularized by the physicist J. A. Wheeler in the late sixties of the past century [4].

Initially regarded just as mere mathematical curiosities, if not even artifacts deprived of any physical meaning1, their actual existence in the real world started to gain credibility when a realistic physical mechanism able to unavoidably produce gravitationally, completely collapsed objects of stellar size was devised [6]. Originally limited just to the unphysical situation that saw only a non-rotating and perfectly spherical object completely collapse, such a fate was later proved to be an unexcapable outcome irrespectively of the departures from the spherically symmetry of the contracting body [7]. In this picture, a black hole forms when a star shrinks so much that it becomes smaller than a certain size proportional to its mass. In this respect, a black hole is an extremely compact object since the aforementioned threshold usually amounts to just a few kilometers for solar mass stars. Absent in the Newtonian gravitational theory, such a sort of characteristic gravitational radius represents a truly watershed for the motion of massive particle and light rays whose trajectories dramatically change according to whether they penetrate the spatial region delimited by it2. Indeed, after having trespassed it, they cannot come back anymore, doomed to proceed towards an ubiquitous spacetime singularity, which looms inexorably in their future everywhere [8,9,10].

Later, black holes of stellar size became the subject of intensive astrophysical research after the discovery of their first candidate Cyg X-1 [11]. The recent detection in dedicated Earth-based facilities of a kind of high-frequency gravitational waves that could only be emitted by inspiralling binary black holes of some tens of solar masses further strengthened our confidence in their actual existence [12,13].

A further fundamental step towards the acceptance of this theoretical concept as something physically real was made when extraordinarily heavy black holes, with masses ranging from a million to even tens of billions of solar masses [14], were hypothesized to explain extremely energetic phenomena occurring in several active galactic nuclei; for an overview, see, e.g., [15], and references therein. To this aim, it should be recalled that the need for extreme gravitational objects at the centers of galaxies in order to explain extragalactic radio sources was recognized for the first time by Ambartsumian in the mid-1950s [16,17]. In fact, Ambartsumian postulated the explosion of alleged superdense material objects also in later works [18], as reported in [19]; thus, he was not credited for the modern conception of an active galaxy. Lynden-Bell and Rees [20,21] interpreted it in terms of giant black holes surrounded by a magnetized accretion disk; see the recollection in [19]. Despite this, it is still unclear how such monsters could actually come into being [22]. Trust in their reality as physical objects was greatly enhanced after the discovery of the cluster of S stars revolving about a directly unseen, extremely compact region in the Galactic Centre at Sgr A^* [23]. Furthermore, the recently imaged black holes’ shadows cast on the electromagnetic emission from material accretion disks in the centers of both our galaxy [24] and M87 [25], after a long gestation [26,27,28], strongly supported the existence of such peculiar astrophysical massive compact objects. A further confirmation of their existence seems to have been recently obtained from the detection of very low-frequency gravitational wave background attributed to the merger of supermassive black hole binaries [29,30].

Quantum considerations led to the introduction of the, so far, hypothetical thermal emission of black holes [31,32,33] along with the concept of temperature and entropy for them [34,35]. One of the most relevant consequences of such quantum features of black holes is the information paradox and the many attempts to resolve it [36,37].

Remarkably, despite their ubiquity in an increasing number of areas in theoretical physics and astrophysics, there is still a lack of a universally accepted definition of black holes [38].

From the above summary considerations, it is clear that a black hole is, in fact, neither a ‘hole’ in something nor is it really ‘black’. Thus, in the next section, a different, more accurate name that unambiguously captures a distinctive feature of black holes, common to all scales and theoretical frameworks, is suggested, although, perhaps, less suggestive from the point of view of mass media. The property one is talking about here about is compactness in the sense of Buchdal’s theorem3 [40]. According to this theory, the compactness of a self-gravitating, isotropic, spherically symmetric, perfect fluid material body of mass M and radius R, defined as $\mathcal{C}:=GM/R{c}^2$, where G is the Newtonian constant of universal gravitation and c is the speed of light in a vacuum, satisfies the following condition: $\mathcal{C}\le 4/9\simeq 0.44$. On the other hand, a Schwarzschild black hole characterized by a Schwarzschild radius of $R_{•}=2GM/c^2$ has the largest possible compactness, amounting to ${\mathcal{C}}_{•}=1/2=0.5$.

2. A New Name for Black Holes

In principle, a better noun for denoting a black hole is pyknon. It may be thought to be derived from the neuter nominative singular of the ancient Greek adjective4 $\mathsf{\pi }\mathsf{\upsilon }\mathsf{\kappa }\mathsf{\nu }$ó$\varsigma$, $-\acute{\mathsf{\eta }}$, −ó$\mathsf{\nu }$ (pyknós, -h$\acute{\overline{e}}$, -ón), meaning, i.a., ‘close, compact; narrow, constricted; close-packed, crowded’. Thus, the plural form may be chosen as pyknons or, more formally, pykna. As a consequence, a supermassive black hole may be conveniently dubbed as megapyknon, with a considerable gain in conciseness. Indeed, while the noun ‘black hole’ consists of two distinct words for a total of nine characters, ‘pyknon’ is a single word formed by only six characters, to say nothing of the three distinct words forming ‘supermassive black hole’ for a staggering total of twenty-one characters, compared to just the ten characters that make up ‘megapyknon’. As a by-product, the points of closest and farthest approach may be dubbed in the traditional way as peripyknon and apopyknon, respectively, from the ancient Greek prepositions $\mathsf{\pi }\mathsf{ϵ}\mathsf{\rho }\acute{\mathsf{\iota }}$ (perí), meaning ‘around, about’, and ἀ$\mathsf{\pi }$ó (apó) and ‘from, away from, far from’.

Another possible choice may be pyknoma, from the nominative singular of the ancient Greek neuter noun5 $\mathsf{\pi }\acute{\mathsf{\upsilon }}\mathsf{\kappa }\mathsf{\nu }\mathsf{\omega }\mathsf{\mu }\mathsf{\alpha }$, $-\mathsf{\alpha }\mathsf{\tau }$o$\varsigma$, $\mathsf{\tau }$ó (pýknōma, -atos, tó), meaning, i.a., ‘dense mass, concentration’. In this case, the plural form may be pyknomas or, more formally, pyknomata. Even in this case, there would be a saving of characters compared to the traditional denominations, although less notable.

3. Summary and Conclusions

Since a black hole is neither a tear in anything nor black, the noun pyknon, from the ancient Greek word for ‘compact; constricted; close-packed’ is proposed for it. It captures an essential and unquestioned feature of such a general relativistic prediction that is found in any theoretical scheme and that is also common to every other possible alternative to it [41] for which wordy expressions like ‘alternative compact objects’ or ‘black hole mimickers’ are commonly used. Last but not least, the present suggestion would also lead to more concise terms, sparing a significant number of characters, especially as far as a supermassive black hole, now named megapyknon, are concerned: a compact name for the most compact objects existing in nature.