The “Impossible” Black Hole Collision That Challenged Einstein. How Scientists Finally Explained It

In late 2023, an extraordinary gravitational-wave signal rippled across Earth’s most sensitive detectors. The event, named GW231123, represented one of the most powerful and puzzling black hole collisions ever recorded. What made it exceptional wasn’t only its enormous energy but the fact that the two black holes involved were, by all known laws of stellar evolution, not supposed to exist at all.

For months, physicists called them “forbidden” black holes. Their masses, spins, and origin defied theory, pushing Einstein’s general relativity to its limits and opening fierce debate about how such objects could form. Now, new simulations and theoretical work have finally produced a compelling explanation.

A Collision That Should Not Have Happened

The LIGO–Virgo–KAGRA collaboration detected two black holes of roughly 100 and 140 solar masses spiraling together before merging into a single monster black hole. These masses fall inside the well-known pair-instability mass gap a range in which theory says black holes cannot directly form from collapsing stars. Stars massive enough to produce such remnants are expected to explode completely, leaving no black hole behind.

Yet here they were: massive, spinning rapidly, and merging in a titanic clash that shook space itself.

The detection immediately baffled astrophysicists. If massive stars cannot create black holes in this mass range, then where did these objects come from?

Cracking the Mystery: Magnetic Fields in Stellar Collapse

A breakthrough emerged when astrophysicist Ore Gottlieb and colleagues at the Flatiron Institute ran some of the most sophisticated simulations ever performed of magnetized, rapidly rotating massive stars collapsing into black holes.

The key lying hidden in the physics: magnetic fields.

When a massive star collapses, tangled magnetic fields can launch powerful outflows, ejecting a significant fraction of the star’s mass before the black hole forms. This mass loss changes everything:

• More magnetic strength → more mass ejected → smaller final black hole
• Less magnetic strength → less mass ejected → larger final black hole

Suddenly, this provided a natural way to produce black holes inside the “forbidden” mass gap. The simulated mass–spin relationships matched the properties of the real objects observed in GW231123 almost perfectly.

In other words:
magnetically-driven mass loss can bypass the “impossible” barrier.

A Test of Relativity at the Edge of Physics

Scientists were astonished by how powerfully the event warped spacetime. The merger allowed researchers to probe general relativity under some of the strongest gravitational conditions ever observed.

As Gottlieb explained in the research announcement:

“Extreme events like GW231123 stretch general relativity to its breaking point.”

Yet even under this extreme stress, Einstein’s equations passed another major test.

Implications: New Types of Black Holes, and New Cosmic Histories

Solving this puzzle reshapes several areas of astrophysics:

1. A New Formation Channel

Extremely massive, magnetized, fast-rotating stars may create black holes previously thought impossible.

2. Support for “Hierarchical Mergers”

Some of the largest black holes may form from multiple generations of mergers, where smaller black holes collide repeatedly inside dense star clusters.

3. Insights Into Early-Universe Black Holes

These results may explain how the universe produced huge black holes only a few hundred million years after the Big Bang.

Conclusion

What was once labeled an “impossible” collision is now becoming one of the most enlightening discoveries in modern astrophysics. The GW231123 event forced scientists to revisit long-held assumptions, expand models of stellar collapse, and confront the limits of Einstein’s theory.

Today, it stands as a stunning reminder that the universe still hides surprises powerful enough to challenge our deepest understanding of physics and to reshape it.

Sources 

• Ore Gottlieb, Brian D. Metzger, Danat Issa, Sean E. Li, Mathieu Renzo & Maximiliano Isi, “Spinning into the Gap: Direct-Horizon Collapse as the Origin of GW231123 from End-to-End GRMHD Simulations”, The Astrophysical Journal Letters, 2025 (pre-print available on arXiv) (arXiv)
  Space.com article: “Scientists solve the mystery of ‘impossible’ merger of ‘forbidden’ black holes” (Space)
  ScienceDaily coverage: “Astronomers just solved the mystery of ‘impossible’ black holes” (ScienceDaily)
  Phys.org article: “‘Impossible’ merger of two massive black holes explained” (Phys.org)
  LIGO / LVK collaboration technical summary: “GW231123: The Most Massive Black Hole Binary” (ligo.org)