Modern Physics in a nutshell: The Special Theory of Relativity (Einstein 1905): Time slows down at speeds close to the speed of light (c) Fantastic as it sounds, this is a well-observed effect, e.g., in cosmic rays. Unfortunately, it is also why we can't figure out any way to travel faster than light, not a welcome prospect for anyone interested in flying to the stars. The General Theory of Relativity (Einstein 1915): Time slows down in a gravitational field, which also curves space. (The notion that space can curve isn't hard to understand: the surface of Earth is round.) Fantastic as it sounds, this is also a well-observed effect, e.g. gravitational lensing (stars shift position) during an eclipse; astronauts age slower than they would have on Earth. "Once we eliminate the impossible, whatever is left, however improbable, must be the solution." -- Sherlock Holes (actually, Sir Arthur Conan Doyle) "Common sense is a set of prejudices one acquires before age 16." -- Einstein "Nothing is too wonderful to be true, if it is consistent with the laws of physics." -- Michael Faraday - A black hole is called a hole because it really is a hole, in space, and in time. Time stops in a black hole, and the curvature of space becomes infinite, so it's a hole. - Since nothing (no force at all) holds a black hole up against gravity, all the mass in concentrated into a mathematical point in the center, the singularity. (Recall the concept of escape velocity: the escape velocity of Earth is 11 km/s. If a rocket goes slower than this, it won't break free of Earth's gravity.) - Around this is the event horizon, inside which the escape velocity becomes greater than the speed of light. Nothing, therefore, can escape from a black hole, not even light: that's why it's black. (Misconception: black holes don't suck, no more than any other gravity field does. Black holes are different from other gravitating objects only in that they're bottomless.) Quantum Mechanics (Bohr, Heisenberg, Schroedinger, et al. 1900-1930): On the scale of atoms or smaller, reality becomes ruled by probability and randomness. It's not possible to calculate what any one atom will do, just what many are likely to do. (Einstein was wrong when he said "God doesn't play dice with the Universe." Niels Bohr, who was sitting next to him, said "Stop telling God what to do!") Why are we so sure that black holes are real, and that we have found them? 1) "We know that neutron stars exist, and black holes are pretty close!" This isn't a strong argument. Just because it's plausible doesn't mean it must exist. 2) To hold up against collapse, a neutron star with a mass of greater than 3 suns would have to be made of material that's infinitely strong (which isn't easy to do!). Indeed, nuclear theory predicts how strong neutron star material should be: a realistic neutron star should collapse, if it had a mass greater than 2.7 +/- 0.3 suns. Can we find a compact object with a mass we can prove is greater than 3 suns? We measure mass of stars (and black holes) with Kepler's Third Law, from the strength of gravity as they orbit each other. Recall that: m = a^3/P^2 (m = mass, in solar masses; a = semi-major axis, or orbital separation, in AU; P = orbital period, in years). This is done by getting spectra of the binary star system, all around the orbit, and measuring how much the stars' gravity pull each other around by measuring the Doppler shift. This is similar to how we detect planets around other stars, by measuring how their gravity pulls their parent stars around. The compact star in the binary star system Cygnus X-1 for many years (1971-1992) was hailed as the most likely black hole candidate, but the case wasn't decisive. Mass estimates were in the range of 10 suns, but they were only estimates. The problem was we couldn't measure the other properties of the system (in particular the orbital separation, or a), well enough to be sure. 3) V404 Cygni: In 1994, Jorge Casares, Phil Charles, and Tim Naylor (Oxford) got spectra to measure the masses of both stars in V404 Cygni, a binary system in which there had been a nova eruption. They found that one star is a K giant. The other, which emits X-rays, is compact, and has a mass of 6.0 +/- 0.4 sunsdefinitely a black hole. And there was much rejoicing There are now over a dozen similar black holes in binary star systems known. Still, this is only indirect evidence. Can we show black holes exist more directly? 4) Event horizons (not) seen: In 1997, Ramesh Narayan and Mike Garcia (Harvard) showed that binary star systems containing black holes are systematically fainter than similar star systems containing neutron stars. Why? Because they have something black in them 5) Spin effects: a NASA spacecraft, called Rossi X-ray Timing Explorer (RXTE), has seen the effect of spacetime being dragged around a rotating black hole. As gas falls into a black hole, waves in the gas get longer in wavelength: in exactly the way predicted by Einstein's theory of relativity, because of time slowing down (see adjoining article, from Mercury magazine). 6) Galaxy centers have spectral lines that are distorted in ways exactly like that predicted if they contained supermassive black holes, with masses of millions to billions of suns. Prediction: X-rays from the black holes in the centers of galaxies will be polarized. No present spacecraft can detect polarized X-rays, but ASTRO-E2 is being designed in Japan to do this. It is scheduled for launch in 2005. How can we not wish them luck?