A Forty Year Journey

TL;DR

This paper reviews four decades of evidence confirming the existence of supermassive black holes, discusses recent measurements and future prospects for testing general relativity and the no-hair theorem at the Galactic Center.

Contribution

It provides a comprehensive overview of the progress in black hole research and outlines future observational strategies to test fundamental physics.

Findings

Mass measurements of SgrA* have improved by a factor of 10^6.

Density estimates of the Galactic Center black hole have increased by 10^18.

Future telescopes will enable tests of general relativity and the no-hair theorem.

Abstract

I try to describe the stepwise progress in proving that massive black holes do exist in the Universe. As compared to forty years ago, measurements have pushed the 'size' of the 4 million solar mass concentration in the Galactic Center downward by almost 10^6, and its density up by 10^18. Looking ahead toward the future, the question is probably no longer whether SgrA* must be a MBH, but rather whether GR is correct on the scales of the event horizon, whether space-time is described by the Kerr metric and whether the 'no hair theorem' holds. Further improvements of the VLT interferometer GRAVITY (to GRAVITY+) and the next generation 25-40m telescopes (the ESO-ELT, the TMT and the GMT) promise further progress. A test of the no hair theorem in the Galactic Center might come from combining the stellar dynamics with EHT measurements of the photon ring of SgrA*.