LIGO detects most massive black hole merger to date

Nature of Black Hole Mergers

• Consensus: two black holes merge into a single, more massive black hole; mass, spin and charge combine, with some energy radiated away as gravitational waves.
• Mass determines horizon “size”; larger black holes are less dense on average (radius ∝ mass).
• Commenters debate “consume vs merge”; better analogy is two droplets joining or two tears in fabric fusing into one.
• Event horizons are described as geometric boundaries, not physical surfaces; crossing is defined by escape velocity reaching c.

Shape, Spin, and Horizons

• Non-rotating (Schwarzschild) black holes have spherical horizons; rotating (Kerr) black holes have oblate horizons and additional structures like ergospheres and Cauchy horizons.
• During mergers, the horizon can be highly distorted (“peanut-shaped”) but must relax to a smooth spherical/oblate shape; GR doesn’t allow permanent “lumpy” horizons.
• Some discussion on how to even define “volume,” “density,” or “shape” in curved spacetime; several people flag this as conceptually tricky.

Time Dilation and What We Can See

• From a distant observer’s frame, infalling matter (or another black hole) appears to slow and “freeze” at the horizon, redshifting into invisibility.
• This leads to confusion about whether black holes “really” form or merge; multiple comments stress that what happens inside the horizon, or at the singularity, is fundamentally inaccessible.
• Numerical simulations deliberately treat the interior as untrustworthy; errors are “trapped” inside the horizon while the exterior evolution is modeled accurately.

Energy Release and Gravitational Waves

• The merger into a 225-solar-mass black hole implies ~15 solar masses converted to energy, mostly as gravitational waves.
• Commenters quantify this as more instantaneous power than all stars in the observable universe combined, comparable to tens of thousands of Sun lifetimes released in seconds.
• Gravitational waves are incredibly weak by the time they reach Earth (strain ~10⁻²⁰), illustrating both the stiffness of spacetime and the huge energy at the source.

Thought Experiments on Collisions

• Head-on, high-speed collisions: kinetic energy largely ends up in the final black hole’s mass, minus what escapes as gravitational waves; momentum and energy conservation still hold.
• Grazing encounters could, in principle, briefly share apparent horizons without forming a single global horizon, but once a true shared horizon forms, separation is impossible.

Cosmological Analogies

• Some discussion on whether a black hole with the mass of the (observable) universe would be about the size of the universe, and whether the early universe “was” a black hole; participants highlight unresolved and unclear aspects here.

Detectors, Funding, and Networks

• Multiple comments worry about proposed U.S. funding cuts to NSF and LIGO, including risk of shutting one U.S. interferometer.
• Triangulation and sky localization currently rely on a small global network (LIGO sites, Virgo, KAGRA, GEO600); losing a LIGO site would significantly degrade localization.
• LISA (the planned space-based detector) is led by ESA; some concern is expressed about NASA’s role and U.S. budget decisions, but ESA’s core mission is moving forward.

Usefulness and Spin-Offs

• Direct “practical uses” are unclear; commenters emphasize that fundamental experiments often pay off via enabling technologies: ultra-stable lasers, precision metrology, isolation systems, advanced detectors, and software pipelines.
• Gravitational-wave astronomy may probe the very early universe, beyond the photon-based cosmic microwave background, potentially informing new physics.

Awe, Scale, and the ‘Chirp’

• Many express a sense of existential smallness and awe at energies and scales involved.
• The audible “chirp” from the signal, if up-shifted into hearing range, corresponds to massive black holes orbiting hundreds–thousands of times per second; listeners find it eerie and “insane.”