Diameter of a black hole
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Diameter of a Black Hole: Insights from Recent Research
Black Hole Singularity and Physical Dimensions
A black hole is traditionally understood as a point of singularity where mass is infinitely dense and spacetime curvature becomes infinite. This singularity is a one-dimensional point where the known laws of physics cease to apply. However, recent theories propose that a black hole singularity might collapse to a finite, non-zero diameter. This new perspective suggests a balance between the principles of general relativity and quantum mechanics, allowing physics to remain valid within this finite singularity.
Event Horizon and Black Hole Shadow
The Event Horizon Telescope (EHT) has provided groundbreaking images of the supermassive black hole M87*, revealing an asymmetric bright emission ring. This ring has a diameter of approximately 42 ± 3 microarcseconds (μas), which aligns with the shadow of a Kerr black hole as predicted by general relativity . The shadow diameter is a crucial observable for estimating black hole parameters and testing various theories of gravity, including those that propose non-Kerr black holes .
Photon Rings and Interferometric Measurements
Black hole images are predicted to display a series of lensed "photon rings" that encode information about the spacetime geometry. Although these rings have not yet been resolved observationally, future space missions aim to capture the first and possibly the second photon rings of supermassive black holes like M87* and Sgr A*. The first photon ring, in particular, has a larger width-to-diameter ratio and can be measured in visibility space, providing a precise angle-dependent diameter that can be used to test the Kerr metric and strong-field general relativity.
Volume Inside a Black Hole
The three-dimensional volume inside a spherical black hole can be defined by extending the concept of volume from flat spacetime to a curved spacetime. As a black hole collapses, its internal volume grows over time, eventually reaching a large asymptotic form. This finding has significant implications for discussions on the information paradox and the nature of black holes.
Conclusion
Recent research has significantly advanced our understanding of black hole diameters, from the finite dimensions of singularities to the observable shadows and photon rings. The EHT's observations of M87* have provided critical data, supporting the Kerr black hole model while also opening avenues for testing alternative theories of gravity. Future observations and theoretical developments will continue to refine our knowledge of these enigmatic cosmic objects.
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