Do black holes really exist?

Why most physicists accept that black holes are real

The strongest evidence comes from watching how things move. At the center of our galaxy, stars orbit an invisible object that is extremely heavy, and the only known way to pack that much mass into such a small space is a black hole [3]. This is indirect evidence, but it is consistent across many observations.

A second line of evidence comes from gravitational waves. The LIGO/Virgo/KAGRA detectors have observed merging objects whose masses fall in a range not usually associated with ordinary stellar remnants, and these observations are consistent with black hole mergers [5]. The paper reviewing this evidence argues that primordial black holes—black holes formed in the early universe—could explain these mergers along with several other puzzling observations, including high-redshift dwarf galaxies and the heating of the Galactic disk [5].

What a black hole is, and why the inside is problematic

A black hole is a region where gravity is so strong that nothing, not even light, can escape. In classical general relativity, the theory that describes gravity, a black hole contains a singularity—a point where density and curvature become infinite and the known laws of physics break down [1]. As Hawking argued, all known laws of physics must break down at the singularity, which makes it an important question whether this singularity can disappear in a quantum description of spacetime [1].

One 2024 paper quantizes the black hole interior and finds that the wave function is regular and well behaved around the singularity, which suggests that regular black holes—black holes without a singularity—may exist in quantum gravity [1]. This is a theoretical suggestion, not an observation, and it does not tell us whether the black holes we observe in the sky are regular or singular.

The definition problem: what counts as a black hole?

Part of the difficulty is that even the definition of a black hole is not settled. One 2022 paper compares definitions based on event horizons and apparent horizons and presents a non-singular model of a black hole with a closed apparent horizon [2]. This shows that there are different ways to define the boundary of a black hole, and the choice of definition can affect what we conclude about their existence.

The paper also proposes a massive thin shell model for describing particle creation in black holes and demonstrates that for black holes much larger than the Planck mass, the backreaction of the particle creation region on the black hole parameters is negligibly small [2]. This is a technical result about how black holes evaporate, but it matters for the existence question because it shows that black holes can be described consistently without immediately destroying themselves.

A minority view: gravity may not always attract

Not everyone is convinced. One 2021 paper argues that general relativity would lead to black holes if gravitation is always attractive, but that experiments show a reduction in weight as temperature increases, which the author says demonstrates the existence of repulsive gravitational force [3]. The author concludes that gravity is not always attractive and that general relativity may need to be extended to a five-dimensional theory, which would require rejustifying the existence of black holes [3].

This is a minority position and the paper does not provide a direct observation of a black hole being ruled out. The mainstream view remains that the invisible massive objects we observe are black holes, but the existence question is still debated at the level of fundamental theory [1][3].

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