Black holes have long been associated with inconsistencies and limitations in known physics, particularly regarding how black holes interact with their environment. One such interaction involves tidal forces, the gravitational effects exerted by nearby objects, and is commonly used to investigate the internal structure of celestial bodies.
For decades, scientists have relied on a quantity known as the tidal Love number to explain how objects deform under these forces. While planets, stars, and even neutron stars exhibit measurable responses, black holes stand apart at a fixed value of zero, suggesting no deformation at all.
Zero value rule that defines a black hole
The concept of Love numbers dates back to 1909, when British mathematician Augustus Edward Hough Love introduced the concept to study how the Earth is deformed by the gravitational pull of the moon and sun. The same framework is now being applied to a wide range of astrophysics.
According to a study published in Physical Review Dblack holes consistently exhibit vanishing tidal numbers within the bounds of general relativity. This means that it does not respond conservatively to static tidal fields, unlike other small objects that typically exhibit non-zero values.
The study points out that this behavior is in clear contrast to neutron stars and similar dense objects, as well as black holes located in more complex environments. Situations involving surrounding matter, modified gravitational theories, or higher dimensional models already suggest exceptions where the love number can deviate from zero.
Transition from boson field to fermion field
The new study approaches the problem from a different angle by focusing on the fermion field rather than the bosonic source that is usually considered. Traditionally, the love number is derived using bosonic perturbations such as gravitational waves, electromagnetic fields, and scalar fields.
In this case, the researchers examined a Kerr black hole, a rotating, uncharged black hole described by Einstein’s theory of relativity, through the lens of a fermion source, like a massless Dirac field. This field is often compared to particles like neutrinos in quantum field theory.

The difference, the authors say, lies in a mathematical symmetry known as ladder symmetry. These symmetries force a zero solution to bosonic perturbations. On the other hand, the fermion field has its lowest multipole moment explained in the study “normal septic solution”
Towards a fermion-like concept of “hair”
This deviation from the zero-value law is what makes a black hole what physicists sayhair,“A term used to describe additional observable properties beyond mass, charge, and angular momentum. In this context, research suggests the existence of fermion hairs.”
This idea is conceptually similar to electroweak Hare, a theoretical framework involving clouds of W and Z bosons from which black holes can extract energy and angular momentum. Therefore, the presence of a fermion field could introduce a new structural layer around the black hole.
According to reports popular mechanismsthe authors emphasize that their discovery highlights the unique role of fermions in the possibility of circumventing established theorems. They wrote about this work:Opens new directions for investigating interactions between fundamental fields, black hole structure, and strong gravitational phenomenology” and points out new avenues for investigating some of the universe’s most mysterious objects.
Although these results, if confirmed, would not completely overturn existing physics, they would complicate a picture once thought to be settled and add yet another layer to the enduring mystery of black holes.
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