A team of researchers have claimed to have recently detected a telltale signature of stimulated Hawking radiation from a post-merger black hole. If the researchers’ analysis of gravitational wave data is correct, then they may have found the first evidence of Planck-scale quantum structure at the event horizon of a black hole (quantum horizons). The key signature of a non-classical horizon is an echo signal in the gravitational waves that are detected after the primary merger event of a binary black hole system. The evidence is tentative, but nevertheless tantalizing. Such research is pivotal to advancing our understanding of quantum effects in strong gravity, where novel aspects of the theory of quantum gravity may be hard at work, as exemplified in the remarkable research The Origin of Mass and the Nature of Gravity, in which physicist Nassim Haramein with his colleagues Dr. Olivier Alirol and Dr. Cyprien Guermonprez have demonstrated that the mass-energy of Hawking radiation from a baryonic-scale mini black hole exactly produces the observed rest-mass energy of the proton, demonstrating that the proton rest-mass is the result of quantum vacuum fluctuations of the electromagnetic field in strongly curved spacetime.

The analysis of gravitational wave data for an echo signature—providing the first tentative empirical evidence of quantum horizons and Hawking radiation—in conjunction with recent observation of Unruh radiation from accelerating electrons, is a significant potential confirmation of quantum gravitational predictions. In the unified physics solutions of Haramein *et al*., we see how Hawking radiation and associated non-zero energy density of the quantum vacuum are integral to understanding the source of mass and force originating from quantum vacuum fluctuations in curved spacetime. These latest findings may represent a major advancement in unified theory—at the interface between general relativity, thermodynamics, and quantum information—as Unruh-Hawking radiation is no longer “only theoretical”, but instead has been observed in experiments from accelerated electron temperature spectra to gravitational waves; even being observed and measured in “table-top” analog black hole systems.

## Scientists Detect Unruh-Hawking Radiation in the Lab and In Space

Gravitational waves and corresponding gravitational wave astronomy—the latter encompassing the detection and analysis of gravitational waves—are currently one of the most revealing signals from black holes because such signals are sensitive to the entire spacetime around the massive compact horizon-bound objects and are our best probe of strong-field regions like event horizons. Electromagnetic processes in the vicinities of black holes can only probe the region outside of the photosphere, which is the region around a black hole where light rays become trapped in orbit. So, we cannot see pass the photosphere with electromagnetic signals to, for example, observe and study the event horizon of a black hole (interestingly, contrary to the key characterizing term of “black hole”, outside the photosphere some supermassive black holes are the brightest objects in the known universe). Since gravitational waves are signals from the strong-field regions of black holes, they offer us the capability to investigate and probe these elusive highly curved spacetime regions that are normally electromagnetically hidden behind the “frozen” sphere of light around the horizon area.

Such highly curved spacetime regions, like the event horizon, are critically important to current development and understanding of a theory of quantum gravity and unified physics, as *quantum field theory in curved spacetime* models predict particle emissions from the quantum vacuum by the highly curved spacetime geometry of black hole event horizons, what is known as Hawking radiation. The fascinating phenomenon of Hawking radiation was proposed over 45 years ago by Stephen W. Hawking [1]. Using a semi-classical approximation, i.e. considering quantum fields around a classical geometry, Hawking realized that the event horizon of a black hole in the Schwarzschild metric should emit massless particles. Black holes are not entirely black. By the equivalence principle, Hawking radiation is the same phenomenon as Unruh radiation, that results in thermal radiation from the quantum vacuum as a result of acceleration.

Intriguingly, there has been recent evidence and direct observation of Unruh-Hawking radiation at the particle level, as reported in our ISF article *Unruh-Hawking Radiation Observed in accelerating electrons *[2,3]. The detection and verification of Unruh-Hawking radiation, at the particle level in accelerating electrons and at the astronomical scale in post-merger black holes, is a highly significant verification of theories of quantum field effects in curved spacetimes and corroborating to studies like *The Origin of Mass and the Nature of Gravity*, by Haramein *et al. *[4]*, *where it is demonstrated that Hawking-like radiation is what generates the observed rest-mass of the proton, revealing that not only are black holes quantum systems, but some of our most quintessential quantum systems are micro black holes.

In highly corroborative conjunction with the observation of Unruh-Hawking radiation at the particle level, there is a report of analysis performed on gravitational wave data from the Laser Interferometer Gravitational-Wave Observatory (LIGO) that tentatively indicates detection of quantum effects at the event horizon of a post-merger black hole. The quantum effects in question are a form of amplified particle emission from the quantum vacuum at the black hole event horizon, referred to as *stimulated Hawking radiation*. The signal, which the research team claims to have detected, is a long theorized tell-tale sign of Planck-scale quantum structure around black holes (or massively compact exotic objects, like putative gravastars and boson stars), which will alter the way gravitational waves interact with the horizon area, such as reflection of the radiation resulting in *gravitational wave echoes *[5].

## Echoes from the Abyss

_{Figure 1. }_{Image at Left: schematic representation of how gravitational wave echoes are physically formed following a black hole merger. Gravitational waves are reflected within a cavity between a membrane / firewall, which is Planck-scale quantum structure at the event horizon, and an angular momentum barrier. Image at Right: a typical gravitational-wave signal generated by a small star falling into a massive compact object with a horizon (top) and with a quantum or absent horizon (bottom). In the latter case, “echoes” of gravitational waves appear at late time and provide a smoking gun for putative quantum structure at the membrane (or a wormhole). Image and image description adapted from Cardoso, Pani – CERN Courier, 2017. }

A typical gravitational wave signal for a black hole merger looks like the top right signal (labeled ‘black hole’) in Figure 1. During the merger, as the photosphere is transited a burst of radiation is emitted and a sequence of pulses dubbed “quasinormal ringing” follow, which are determined by the characteristic modes of the black holes. If, however, there is quantum / Planck-scale structure at the event horizon of a black hole, or if the object is a novel class of highly curved spacetimes like a wormhole (Figure 2) or exotic star with a compact surface, then the signal will initially look the same but be followed by lower amplitude “echoes” that are generated by gravitational waves that “ricochet” between the photosphere and the surface / horizon of the object (depiction at left in Figure 1), generating gravitational wave echoes that will have signal like that represented in the bottom right graph of Figure 1.

_{Figure 2. Illustration of a dynamical process involving a compact horizonless object. A point particle plunges radially (red dashed curve) in a wormhole spacetime and emerges in another “universe”. The black curve denotes the wormhole’s throat, the two gray curves are the light rings. When the particle crosses each of these curves, it excites characteristic modes which are trapped between the light-ring potential wells. This is illustrated graphically in Figure 3 depicting the different signatures generated between a black hole GW signal (with a horizon) and that described here for a wormhole. Image and image description adapted from [5].}

The “quasinormal ringdown” signal following binary coalescence is often taken as conclusive proof for the formation of an event horizon after the merger, however universal ringdown waveforms can also form from the light rings of a wormhole, rather than horizons, and only precision observations of the late-time ringdown signal can distinguish the two possible case scenarios (Figure 3). Scattering of gravitational waves of wormholes therefore will produce tell-tale echo signatures for potential detection and identification of naturally forming Einstein-Rosen bridges [6].

_{Figure 3. Gravitational wave echo signals distinguish a massively curved spacetime object without a horizon (a wormhole) from a classical black hole (with a smooth event horizon). Figure reproduced from [4]. Referring back to Figure 2, when a particle crosses the light-ring potential wells, depicted in figure 2 by the red dotted line, it excites characteristic modes which are trapped between each curve (the grey rings in figure 2), resulting in the signal depicted graphically here in figure 3 by the red curve, with a characteristic echo that is a clear signature of the characteristic modes of a wormhole. }

## First Measurement of Stimulated Hawking Radiation from Black Holes

A research team led by Niayesh Afshordi, a professor of astrophysics at the University of Waterloo, analyzed data from one of the most massive binary black hole merger events observed to date, referred to as GW190521, and found compelling (though not conclusive) evidence of stimulated Hawking radiation [7]. Stimulated Hawking radiation of the GW190521 coalescence event was expected to form post-merger gravitational echoes due to partial reflection off Planckian quantum structure of the horizon. Stimulated Hawking radiation is a form of light amplification by stimulated emission of radiation (LASER), where the radiation is of the Unruh-Hawking variety (see our previous ISF article Analog White Hole-Black Hole Pair Demonstrates How Event Horizons are Tunable Factories of Quantum Entanglement to learn more about observation and measurements of stimulated quantum entangled Hawking radiation). Stimulated Hawking radiation leading to black hole lasing makes the relatively faint Hawking radiation observable. This amplified form of Hawking radiation has been observed in analog gravity systems [8], and now Afshordi’s team reports the first detection of it in a real black hole system via the detection of a tell-tale echo signal in LIGO/Virgo gravitational wave data, what has been called a smoking gun for quantum microstructure of black hole horizons.

Statistical analysis of the results cannot confirm beyond a doubt that the signal is true or an extremely unlikely artifact, so although the team has a >90% confidence interval in the result of the analysis it remains inconclusive. In physics, indisputable evidence is generally required to be at what is called the 5-sigma level (≥ 5σ; a 0.00006% chance the data is a fluctuation), a level the team did not see in their results. So, at the level of signal-to-noise ratio of the study the team maintains that evidence for the co-localization of the echoes and the main event in the sky remains inconclusive.

While such a result is none-the-less highly promising, irrefutable evidence of stimulated Hawking radiation from astronomical quantum black holes will require greater levels of gravitational wave detector sensitivity. Indeed, a team of researchers at the University of Tokyo and Kyoto University, Takahiro Tanaka *et al*., have performed an analysis of data from several coalescence events, including the GW190521 merger event, and reported null detection of any corresponding gravitational wave echo signals that would indicate deviation of spacetime microstructure from a classical horizon [9]. The team reports that although they did not find any significant echo signals, the limited number of events analyzed still gives large error regions and does not rule out such signals. So, although some of the initial results are compelling, it is as of yet inconclusive, and high-sensitivity gravitational wave detectors, like LISA, may be required to conclusively verify gravitational wave echo signals.

## Quantum Black Holes

In the 1960s, eminent physicist John Archibald Wheeler expressed the fact that black holes are lacking any observable features beyond their total mass, spin and charge with the phrase ‘black holes have no hair.’ This is known as the no-hair theorem (interestingly, the 3 properties that describe an elementary particle are its mass, spin, and charge—almost like particles were little micro-black holes). Detection of Planck-scale structure at the event horizon would indicate that at the quantum level the gravitational field is encoding information about what is inside the black hole, and trans-horizon entanglement of Hawking radiation would mean that information inside the volume of the black hole is accessible outside the horizon. Which would mean that, using the colloquial physics’ parlance, black holes do have hair [10]. As such, gravitational wave echoes and the latest studies indicating the detection of Hawking radiation can potentially conclusively answer questions about *the information loss paradox* and whether or not black holes have hair. Incidentally, Hawking himself had come to the conclusion that information must transmit across the event horizon of black holes, making them more like “grey holes”—or the Black Whole famously described by Haramein—something we discussed back in 2014 in our article Stephen Hawking Goes Grey.

If Hawking radiation is entangled across the trans-horizon area, as many leading theories and analog gravity empirical evidence suggest it should be, then the internal state of black holes (internal to the horizon) are not invariably inaccessible, and black holes have “quantum hair”. Indeed, studies have established the existence of ubiquitous quantum hair due to gravitational effects [quantum hair from gravity].

Beyond the information loss paradox (which we have discussed the significance of in previous articles like An Eventful Horizon), the question of Black hole hair and Planck-scale quantized spacetime architecture at the event horizon has direct relationship to understanding the origin of mass in the universe. In the quantum gravity and holographic mass solution of Haramein [11], we see how the entropy-information relationship structure between the volume and the surface of a black hole, tiled with Planck-scale quantum harmonic resonators of the electromagnetic vacuum (curving spacetime into gravitationally-bound discrete regions called Planck Spherical Units) results in screening of the Planck energy density of the quantum vacuum to the observed rest mass. Importantly, the holographic solution applies equally to astronomical black holes as it does to microscopic primordial black holes, the latter of which we conventionally refer to as particles. Therefore, the latest measurements of the gravitational wave echo signatures support the postulation of Planck-scale quantum structure at the event horizon as well as Hawking radiation, which we will now see has direct implications at the particle scale.

## Hawking Radiation at the Particle Level

The figure (derived from the article Linking Waves to Particles) depicts how by utilizing gravitational waves to investigate strong-field gravity—regions that are normally hidden from EM processes behind a photosphere—we may be able to gleam insights into particle physics. This is formulated within the context of how hypothetical particles like axions or scalar bosons (even possibly gravitons) may interact in the ergosphere of black holes, generating effects like superradiant resonances (see our article Ionization of Black Hole Atoms to learn more about this effect), which can potentially be detected in gravitational wave signatures. However, gravitational wave signals that reveal the quantum structure of black holes and what was previously only hypothetical behavior of quantum fields in curved spacetime geometries, like the latest analysis by Afshordi’s team giving initial observational evidence of Hawking radiation, can verify key processes at work in particle physics where the term quantum black holes take on a complete meaning because particles of the quantum domain are micro-black holes.

Therefore, as in studies like The Origin of Mass and the Nature of Gravity, by Haramein *et al*., where following evaluation of the dynamics of vacuum fluctuations in strongly curved spacetime geometry it is analytically demonstrated how the mass-energy of Hawking radiation from a proton-scale black hole generates fundamental properties like the rest-mass of the proton, we see that the results of this calculation are strongly supported by observational evidence. Such that together with the stunning precision of the resulting calculation—with a near exact equivalence between the Hawking-like radiation from quantum vacuum fluctuations near the proton Compton horizon with the rest-mass of the proton (Figure 4)—there is justifiable high confidence that the model is correct. Certainly, it can no longer be discounted on the basis that Hawking radiation is a hypothetical mechanism, as there are a number of empirical studies with strong evidence, from analog gravity systems to the aforementioned accelerating electrons [see previous citation, reference number 3] to observational astronomy (via gravitational waves), that give strong indication of the reality of the effect and advances Hawking’s prediction beyond the stage of hypothesis to verified effect (note: although credit for the vacuum thermalization effect in a strong gravitational field is accorded to Hawking, physicist Yakov Zel’dovich was the first to computationally discover that black holes will generate electromagnetic radiation due to interaction with vacuum fluctuations [12]).

**Figure 4. The Hawking radiation of a proton**. The equation reveals how the mass-energy (ε) of the Hawking temperature (T_{λ}) of the proton core black hole (beyond the proton Compton radius λ_{p}) on the proton charge radius (r_{p}) surface (A_{p}) with quantum vacuum fluctuations over the characteristic time τ_{p} is equivalent to the proton rest mass energy (m_{p})—following the well-known equivalency of E = mc^{2}.

By applying the Stefan-Boltzmann law for black body radiation (the left side of the equation in Figure 4 relating total energy emission ε to temperature T and surface area A) the Hawking temperature is obtained at the Compton horizon (λ_{p}). By considering the space between the Compton horizon and the charge radius surface (A_{p} in Figure 4) filled with vacuum fluctuations acting as a superfluid there is an isothermal process (in which the temperature remains constant) between the two surfaces and Hawking radiation energy is transferred—via a black body radiation mechanism—from the core black hole Compton horizon to the proton charge radius where the quantum vacuum radiation temperature is the proton rest mass-energy.

While it may be jarring to many who are accustomed to considering particle physics and black holes as two disparate domains of the universe (a major conceptual hurdle to bringing the science community to a unified physics model) and may have just become accustomed to considering the possibility of quantum field effects at general relativistic scales like vacuum thermalization at event horizons (more generally Killing horizons, including the Rindler horizon), the recent observation of Unruh-Hawking radiation by electrons now requires the consideration of strong-gravity influences on quantum fields at the particle scale, which we see from studies like that by Haramein *et al.* are not just restricted to extreme accelerations but the relativistic-equivalent strong gravity spacetime curvature from the quantum vacuum energy density. A unified physics requires understanding quantum field effects in curved spacetime, from the quantum to the cosmological scale, and resulting behavior of the unified field, such as Unruh-Hawking radiation, are critically important to understanding fundamental properties of the universe like the origin of mass and the nature of gravity.

## References

[1] Hawking SW. 1974. Black hole explosions? Nature 248, 30 ( 10.1038/248030a0)

[2] W. Brown. “Unruh-Hawking Radiation Observed in Accelerating Electrons.” The International Space Federation. 2024. Accessed: Feb. 14, 2024. [Online]. Available: https://www.spacefed.com/blog/unruh-hawking-radiation-observed-in-accelerating-electrons

[3] G. Gregori, G. Marocco, S. Sarkar, R. Bingham, and C. Wang, “Measuring Unruh radiation from accelerated electrons.” arXiv, Apr. 26, 2023. Accessed: Feb. 09, 2024. [Online]. Available: http://arxiv.org/abs/2301.06772

[4] N. Haramein, C. Guermonprez, and O. Alirol, “The Origin of Mass and the Nature of Gravity,” Sep. 2023, doi: 10.5281/zenodo.8381114.

[5] V. Cardoso, E. Franzin, and P. Pani, Is the gravitational wave ringdown a probe of the event horizon?, Phys. Rev. Lett. 116, 171101 (2016). Preprint available on arXiv at: https://arxiv.org/pdf/1602.07309.pdf

[6] A. A. Kirillov, E. P. Savelova, and O. M. Lecian, “Scattering of GWs on wormholes: foreshadow and afterglow/echoes from binary merges.” arXiv, Sep. 27, 2020. Accessed: Feb. 14, 2024. [Online]. Available: http://arxiv.org/abs/2003.13127

[7] J. Abedi, L. F. L. Micchi, and N. Afshordi, “GW190521: Search for Echoes due to Stimulated Hawking Radiation from Black Holes,” *Phys. Rev. D*, vol. 108, no. 4, p. 044047, Aug. 2023, doi: 10.1103/PhysRevD.108.044047.

[8] J. Steinhauer, “Confirmation of stimulated Hawking radiation, but not of black hole lasing,” *Phys. Rev. D*, vol. 106, no. 10, p. 102007, Nov. 2022, doi: 10.1103/PhysRevD.106.102007.

[9] N. Uchikata, T. Narikawa, H. Nakano, N. Sago, H. Tagoshi, and T. Tanaka, “Searching for gravitational wave echoes from black hole binary events in the third observing run of LIGO, Virgo, and KAGRA collaborations.” arXiv, Sep. 04, 2023. Accessed: Feb. 14, 2024. [Online]. Available: http://arxiv.org/abs/2309.01894

[10] X. Calmet, R. Casadio, S. D. H. Hsu, and F. Kuipers, “Quantum Hair from Gravity,” *Phys. Rev. Lett.*, vol. 128, no. 11, p. 111301, Mar. 2022, doi: 10.1103/PhysRevLett.128.111301.

[11] Haramein, N. (2012). Quantum Gravity and the Holographic Mass, * Physical Review & Research International*, ISSN: 2231-1815, Page 270-292

[12] Zel’Dovich, Y.B.: Generation of Waves by a Rotating Body. Soviet Journal of Experimental and Theoretical Physics Letters 14, 180 (1971)