Synopsis

Science often proceeds by taking small steps that together build a general understanding. Here, we outline how experiments on the earth along with observations of our universe can lead to an amazing understanding of the natural world.

The outline below shows how simple experiments on the earth's surface can be used to detect a massive black hole at the center of our galaxy, the Milky Way.

1. Establish Newton's Theory of Gravity by measuring forces between hanging masses here on earth.
2. Use Newton's Theory of Gravity to determine the mass of the earth and check if it is consistent with what we know about earth's composition.
3. Predict the orbital shape and period of the moon based on the earth's mass. It is found that the calculated period is consistent with the measured one and the shape of the orbit is accurately predicted (ellipse with the earth at the ellipse focus). This evidence suggests that gravity acts on lunar distances.
4. From the orbits of the planets, the mass of the sun is determined. Every planetary orbit gives the same solar mass (with the sun at the focus of every ellipse), so gravity appears to work even on these larger scales. Furthermore, the density of the sun that is determined from its mass is consistent with what we know of the sun's composition from independent spectroscopic measurements.
5. The orbital properties of all bodies in the solar system obey Newton's Theory of Gravity with impeccable precision. This includes comets, asteroids, moons, satellites, and space ships.
6. Mercury's orbit is found to deviate ever so slightly from Newton's predictions. This irks physicists until Einstein formulates the General Theory of Relativity in 1918 which fully accounts for the small deviation. It turns out that Newton's Theory of Gravity is a special case of General Relativity, which predicts the possibility of the existence of black holes.
7. Telescopes that view infrared light are able to penetrate the dust that obscures the center of the Milky Way to visible light to see stars at our galaxy's center.
8. Astronomers measure the orbits of these stars over more than a decade. As predicted by Newton, the orbits of the stars are perfect ellipses. The foci of these ellipses all coincide with an invisible object.
9. Using the orbital data, the calculated mass of the dark object is almost 4 million solar masses.
10. Some of the stars get very close to the dark object, so an upper limit of the object's size is determined from the distance of closest approach.
11. The dark object's mass and density fall in the range predicted by general relativity.

The infrared observations of stellar orbits as described above does not prove the existence of a galactic black hole; but provides strong evidence. There are other independent measurements that all point to a black hole (see below). When the pieces of the puzzle are assembled, the picture that emerges places a huge black hole at the center of our galaxy.

Incidentally, such massive black holes are found at the centers of other galaxies and even globular clusters. Since evidence of smaller black holes are routinely "observed" in binary star systems, it becomes clear that black holes are out there.

This journey illustrates something fundamental about nature, and about the breadth of physical laws. Richard Freeman said it best, "Nature uses only the longest threads to weave her patterns, so that each small piece of her fabric reveals the organization of the entire tapestry."

A video that shows the motions of the stars around the central black hole can be found at the web site of the Max-Planck Intitut fur extraterrestrische Physik. The data covers over a decade of observations. See firsthand how scientists have been able to get a detailed physical picture of the galactic center's black-hole.