Caption: A video of a computer simulation of a black hole binary undergoing a black hole merger and emitting gravitational waves. See also the Youtube version Warped Spacetime and Horizons of GW150914 | 1:13.

But show the classroom the GREATEST EVER black hole merger video (i.e., Gravitational waves: GW20150904: LIGO video 2016feb11 | 2:55), hereafter just referred to as the video.

The image-linked video is NOT as good (i.e., Video simulation of the changing gravitational field, and gravitational waves, during the final inspiral, merger and ringdown of GW150914 | 1:13). CLICK on the link just given or on the image and then click on the start button to play the linked video.

Features:

1. The video shows a black hole binary and its evolving gravitational field and the emitted gravitational waves during the final phase inspiral, merger, and ringdown.

   The emitted gravitational waves carry away kinetic energy of the orbital motion, and thus the orbital decay i.e., inspiral to merger.

2. The video is for GW150914: the first observation of gravitational waves and spans time from -75 s to 10 s with time zero being instant of strongest signal---or so it seems---which can be defined as the instant of merger.

3. The gravitational wave signal is shown in the lower panel with a blue segment eventually tracing the evolution of the signal.

4. Recall that the gravitational field in general relativity (GR) manifests itself as a curvature of space.

   Since curved 3-dimensional space is hard for humans to visualize, it is usually represented as curved 2-dimensional space which humans can visualize.

   In the video, the gravitational field (representative curved 2-dimensional space) is shown below the visualized black hole binary.

5. GW150914 was detected 2015 Sep14 (as its name implies) by LIGO and was announced 2016 Feb11.

   LIGO is a major US project dedicated to the detection gravitational waves from astrophysical sources.

   The likeliest source is the merger of compact remnants (excluding white dwarfs): i.e., black hole mergers, neutron star mergers, and black hole neutron star mergers.

   Another source is the collapse of a nearby massive star core in a core-collapse supernova. How nearby? Probably only one in the Local Group???? or we would already have a detection of such an event???.

   Only sources like these might produce gravitational waves strong enough to be detected with the current-state-of-the-art detectors.

6. Analysis of the data suggests that GW150914 occurred at cosmological redshift z = 0.093(33) (see Wikipedia: GW150914: Astrophysical origin).

   According to the Λ-CDM model of cosmology, the luminosity distance is 1.4(6) Gly ≅ 0.440(170) Gpc and the lookback time 1.4(6) Gyr which corresponds to cosmic time 12.4(6) Gyr after the Big Bang (see Wikipedia: Age of the observable universe = 13.797(23) Gyr (Planck 2018)).

   The analysis suggests that merging black hole binary pair had masses 35(4) M_☉ 30(4) M_☉. The merged black hole had mass 62(4) M_☉ (see Wikipedia: GW150914: Astrophysical origin).

   These black holes are stellar-mass black holes since their masses are below the ∼ 100--10**6 M_☉ range of intermediate-mass black holes and the range ∼> 10**6 M_☉ for supermassive black holes.

7. GW150914 and other gravitational wave events verify Albert Einstein's (1879--1955) 1936 prediction of gravitational waves based on general relativity (GR).

   The observation of gravitational waves is a major verification of GR to add to all the others.

   There remain competitor theories to general relativity, but none are favored over GR and some may be strongly disfavored or even ruled out---maybe by the current known gravitational wave events.

   However, since GR is NOT consistent quantum mechanics, almost everyone believes that there is quantum gravity theory whose macroscopic limit is GR (or a competitor). Thus, GR emerges from a lower-in-the-hierarchy emergent theory.

8. Actually, gravitational waves were verified indirectly by highly accurate/precise observations of binary pulsar PSR B1913+16 (which is a binary pulsar consisting of a pulsar and a non-pulsar neutron star) starting with discovery in 1974.

   PSR B1913+16 loses gravitational potential energy and mechanical energy exactly as predicted by GR via gravitational waves within uncertainty.

9. There is remarkable feature about gravitational waves. They travel through space with the energy-momentum tensor (the right-hand side) of Einstein field equations being zero---unless, of course, there is other stuff along the way---galaxies, dark energy, etc.---but that stuff is incidental to the gravitational waves---it just happens to be along the way and is NOT part of their nature. Somehow gravitational waves carry energy and momentum NOT in a density form (i.e., so much here, so much there), but in a nonlocal form: i.e., the energy and momentum are coded into the gravitational waves in a non-density form. See Roger Penrose's (1931--) The Road to Reality (2004, p. 467).

10. As well as verifying the gravitational waves prediction of GR, GW150914 and subsequent observed black hole mergers (see Wikipedia: List of gravitational wave observations) also verify the black hole prediction of GR.

    This is because the gravitational wave signals from the observed black hole mergers are exactly as predicted by GR within uncertainty.

    The predictions do have free parameters (e.g., the masses black hole axial angular momentum), but these are just due to the peculiarities of the black hole binary systems and NOT in GR itself.

11. Recall, the defining characteristic of a black hole (other than just as super compact gravitational source) is the existence of the event horizon---the surface from which and from below which light CANNOT escape.

    Since the gravitational wave signals are strong evidence for black holes, they are strong evidence for event horizons.

    After GW150914: the first observation of gravitational waves, there was only a little doubt of event horizons.

    Then in 2019, the EHT image of the M87 supermassive black hole (M87*) showed just what was expected from computer simulations for a supermassive black hole (SMBH) surrounded by an accretion disk. (see M87 Supermassive Black Hole First Image).

    After these pieces of evidence, the existence of event horizons is very certain.

    There are other theories for the observations, but they seem very improbable. In fact, one seldom absolutely rules out low-probability theories. But if their probability gets too low, they become uninteresting for people to consider any further unless they get revived by some astonishing new data---which occasionally happens. Almost all scientific verifications of theories allow that low-probability theories are NOT absolutely ruled out and this often goes without saying so.

12. However, one must say that there is also the black hole firewall paradox wherein GR and quantum mechanics give different answers as to what happens when an object crosses the event horizon.

    Until this paradox is resolved, there is a major uncertainty about the nature of event horizons.

13. The history of gravitational waves is interesting. The idea that they could exist goes back to before the theory of relativity (i.e., special and general). Albert Einstein (1879--1955) theorized about them inconclusively for some time in the context of general relativity (GR). Then in 1936, he and Nathan Rosen (1909--1995) wrote a scientific article showing that gravitational waves could NOT exist. However, the referee of the scientific article Howard P. Robertson (1903--1961) showed that the authors had blundered in their analysis and (at least Wikipedia implies this) if it were corrected, they would reach the opposite conclusion. The paper was corrected and published (see Wikipedia: Gravitational wave: History). Clearly, Robertson should get partial credit for the theoretical discovery of gravitational waves with Einstein and Rosen.

14. See Black hole keywords below (local link / general link: black_hole_keywords.html):
    EOF

15. Other relevant keywords: Λ-CDM model, age of the observable universe = 13.797(23) Gyr (Planck 2018), cosmic time, cosmological redshift z, cosmology, luminosity distance, etc.