Linking string theory with observations, frozen stars shed new light on black holes and the clash between quantum mechanics and relativity.
In an attempt to solve some of the biggest paradoxes in black hole physics, a recent study has suggested that black holes may not be what they think they are — as described by Einstein’s general relativity. Instead, they propose they could actually be “frozen stars.”
“Frozen stars are a type of black hole mimicker: they are [still] ultra compact astrophysical objects [but they] are free of singularities and lack [an event] horizon,” explained Ramy Brustein of Ben-Gurion University, one of the study’s contributing scientists, in an email. “Yet they [display] many of the same observable properties, [such as strong gravitational fields].”
Frozen stars, which emerge from string theory — a framework in physics that aims to unify all fundamental forces and interactions between particles within a single quantum theory — represent a dramatic departure from the traditional concept of black holes.
Unlike black holes, which are primarily composed of empty space with a singularity of infinite density at their core, frozen stars consist of matter uniformly distributed throughout their interiors. This matter, thought to be composed of rigid strings under constant tension, generates immense pressure that stabilizes the structure of the frozen star and prevents the formation of a singularity.
This distinction is crucial, as infinities — such as the infinite density at a black hole’s core — indicates the breakdown of our understanding of nature, with many physicists arguing that no infinities should exist in physical reality.
Another key issue with black holes is the information paradox — also known as the Hawking information paradox — that arises from an apparent conflict between quantum mechanics and Einstein’s theory of general relativity.
Quantum mechanics demands that information be conserved, while general relativity seems to allow information to vanish inside black holes. This paradox is compounded by the existence of an event horizon — the boundary beyond which no information or signal can escape.
Near the event horizon, particle pairs are created from the vacuum, as famously described by Stephen Hawking. The resulting Hawking radiation causes black holes to gradually evaporate, intensifying the problem by leaving no clear mechanism for preserving information even inside black holes.
The frozen star model offers a compelling solution. By eliminating singularities and event horizons, frozen stars avoid the information paradox altogether. Instead of an event horizon, they feature a surface similar to that of neutron stars. The radiation emitted by frozen stars is analogous to the slow release of energy from an ordinary hot body, with information being preserved as the star changes state.
Making frozen stars spin
While the frozen star theory provides a resolution to many paradoxes, it initially had limitations. In earlier work, the frozen star model, developed by Brustein and his colleague A.J.M. Medved, applied only to non-rotating frozen stars.
This was done because including rotation significantly complicates the mathematics and physics of the system. However, this was also a significant drawback, as real black holes are often highly rotational due to the angular momentum of the collapsing matter that forms them (much like the rotation of the solar system).
In their latest study, published in Progress of Physics, Brustein and Medved made their model more realistic by now including rotating frozen stars. “We have realized a generalized model for a frozen star that can rotate at velocities up to the maximally allowed speed of rotation (essentially, the speed of light),” Medved explained.
This achievement is significant because previous attempts to incorporate rotation into black hole mimicker models typically dealt with low rotational speeds–this simplifies the calculations.
The new frozen star model not only reproduces the mass and angular momentum of a real rotating black hole but also mimics its external geometry. This geometry determines how matter moves in the vicinity of the object, making frozen stars nearly indistinguishable from black holes to external observers under most conditions.
Implications and future directions
The ability of frozen stars to mimic both static and rotating black holes has profound implications for both astrophysics and fundamental physics. By resolving long-standing paradoxes, the model challenges traditional notions of black holes. However, detecting frozen stars observationally remains a challenge.
“For frozen stars in equilibrium, there is no practical way to observationally discriminate between these objects and conventional black holes,” Brustein said. However, deviations from equilibrium — such as those occurring in binary mergers — could reveal differences due to the internal structure of frozen stars.
The researchers plan to analyze gravitational-wave data from observatories like LIGO and Virgo to search for signals unique to frozen stars. Such emissions during mergers could differ from those predicted for black holes, providing a potential observational test for the model.
“The main prospect for our work is to come up with formal calculations that can be tested within the scope of current and upcoming empirical data on gravitational-wave emissions,” Brustein said.
By bridging theoretical insights from string theory with experimental observations, the frozen star model opens new avenues for understanding the Universe’s most mysterious objects and resolving fundamental tensions between quantum mechanics and general relativity.
Reference: Ram Brustein and A.J.M. Medved, Sourcing the Kerr Geometry, Progress of Physics (2025). DOI: 10.1002/prop.202400256
Feature image credit: Placidplace on Pixabay
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