Black Holes Unveiled: From Sagittarius A* to Wormholes and the Future of Observational Astronomy

Overview

New Scientist takes you on a journey into black holes, from the Milky Way’s Sagittarius A* to exotic ideas like fuzz balls and gravastars. The video explains how black holes are defined by mass, spin, and electric charge, and how we detect them through stellar orbits, gravitational waves, and the first images captured by the Event Horizon Telescope. It surveys the diverse family of black holes—stellar-mass, supermassive, and intermediate-mass—and explores the twin physics puzzles of singularities and information. It then introduces speculative concepts such as wormholes and primordial black holes and previews future missions to sharpen observations of space-time around black holes.

Introduction

Black holes sit at the intersection of our best theories and their limits. At the center of the Milky Way lies Sagittarius A*, a supermassive black hole about 15 million miles wide with a mass of roughly 4 million suns. Yet the hole itself is invisible; we infer its presence from the light whirling around it and the way space-time twists as it accretes matter.

Black Hole Basics

Every black hole can be described by three numbers: mass, rotation, and electric charge. There are three well-supported types: stellar-mass black holes (millions in the galaxy, formed when massive stars collapse after supernovae), supermassive black holes (found in the centers of most galaxies, including our own), and intermediate-mass black holes (the missing link, with roughly 1 to 300 solar masses). Stellar-mass holes are typically about 30 kilometers across the event horizon, while Sagittarius A* is millions of times larger in mass and size. Detection methods have evolved from observing stellar orbits around invisible masses in the 1990s to watching gravitational waves from black hole mergers with LIGO, and finally capturing the first direct image of a black hole with the EHT in 2019.

Missing Link and Primordial Black Holes

Astrophysicists expect intermediate-mass black holes to bridge the gap between stellar-mass and supermassive holes, potentially forming in dense star clusters and helping seed galaxies. Another tantalizing possibility is primordial black holes formed in the early universe right after the Big Bang; Hawking radiation would cause the smallest ones to evaporate, but they could still be a dark matter candidate if they exist in the right mass range. The video discusses how gamma rays from evaporating primordial black holes are a key search channel.

The Twin Puzzles: Singularity and Information

Two central puzzles haunt physics: the singularity at a black hole’s core and the information paradox. General relativity predicts a singularity of infinite density, while quantum mechanics forbids information loss. To address these conflicts, theorists have proposed regular black holes with alternative internal structures such as fuzz balls, gravastars, and knee stars, and even speculative ideas like wormholes that could connect to white holes or other universes. The video explains how these ideas aim to preserve information while avoiding true infinities.

The Exotic Possibilities

Fuzz balls arise from string theory, where a black hole might be a dense tangle of strings and branes, removing the event horizon and singularity in a way that stores information. Gravastars replace the singularity with a bubble of repulsive vacuum energy and a thin shell of dense matter, while knee stars describe nested gravistars. Wormholes could, in theory, connect distant regions of space-time, but maintaining their stability requires exotic matter with negative energy density. NASA notes that real black holes are not wormholes, but the concept remains an intriguing possibility in theoretical physics.

The Next Frontier

The Event Horizon Telescope is a landmark achievement, and there is talk of expanding the observatory into space to capture even more detailed photon rings around black holes. A proposed mission, the Black Hole Explorer, could launch around 2031 to push our understanding of space-time near horizons and test competing theories about gravity, quantum effects, and information preservation. The video emphasizes that black holes continue to be a proving ground for fundamental physics and a driver of scientific curiosity.