image: astronvine
The sci-fi loving public and foundationally minded cosmologists alike are enamored of black holes. Black holes, as everyone knows, form from the crushed remains of certain burned-out stars -- those of a particular size whose gravitational collapse has outpaced their dwindling nuclear pressure, sending the leftover mass into a spiraling dive toward oblivion. (The mass is "leftover" because the first step in forming black holes is a supernova, where a huge portion of the star's bulk is ejected into space, leaving the core to fend for itself.) Down, down the stellar remnant goes, crunched ever tighter by the force of its own gravitation, possibly -- here the public and cosmologists differ in their degree of certainty -- terminating a zero-volume point called a "singularity."
Maybe. But even though "everyone knows" that singularities reside at the centers of black holes, the popular science version of this concept may have outpaced the actual science.
Singularities are philosophically unlikely objects -- if "object" is even the right term for one -- that seem to have more in common with the world of the mathematician than that of the experimental physicist. They are infinitely dense (singularities, that is) and have no volume at all, making them, in a certain sense, abstractions; much like mathematical points. Some would say, *too much* like mathematical points; or too much like "mathematical singularities," which my mathematician friends tell me is a bit of a catch-all phrase for places where an object is either ill-behaved or simply undefined. Even the infinite number of points "found" in a perfectly well-behaved line are, when you get down to it, a geometric idealization, a kind of mental shorthand for things that don't enjoy actual material existence.
But that's just what singularities do. They not only have material existence of their own, they have conspicuous observable effects, even powering the cores of active galactic nuclei. Like overacting children insisting that they are still there, these inaccessible, less-than-zero somethings warp spacetime severely, leading to the fabled contortions we all hope will one day allow for superluminal communication, or even time travel.
image: fernando
And yet the general public will also be forgiven for repeatedly raising basic objections to singularities, ones that are by no means to be dismissed as naive simply for being straightforward:
1. An object with zero volume is no object at all. (A bit more subtly: if a singularity has literally no volume, then it does not exist in space. This appears to be equivalent to saying it does not exist.)
2. An object cannot be infinitely dense unless it has always been so. (Again, more subtly: finite densities are on an asymptote approaching infinite density or, assuming spacetime to be flat, infinite rarefaction. But this is simply to say that any real object has a certain density.)
3. The Big Bang was supposed, in the SCM, to have begun with a singularity -- an infinitely hot point that expanded, and thus cooled, to the presently observed state of affairs. But how does something "cool down" from infinity? (If the cosmos was infinitely hot 14 billion years ago, it is infinitely hot today: upping the volume is merely dividing infinity by some positive integer.)
The hedge claim sometimes made by cosmologists that singularities "approach infinite density‚" or are "almost infinite" merely takes us back to point #2. "Approaching infinite" is finite.
So perhaps we have to wait for a satisfactory theory of quantum gravity really to understand singularities, in the meanwhile treating them as quasi-mathematical conveniences. This need be no barrier to research, of course; quantum mechanics works perfectly well without anyone, as Richard Feynman humorously noted, understanding any of it. But there's a foundational question of a significant order lying in the hearts of certain collapsed stars.
image: Heaven's Gate
However we feel about them, it turns out there's a big one in Messier 33.
The mother of all stellar black holes has been discovered this month by James Orosz of San Diego State University, California, and his team. The whopper weighs in at almost 16 times the mass of the sun, larger than any detected so far, and several times larger than many. Stellar black holes, we should hasten to add, are distinct from the supermassive holes at the centers of galaxies, which can be billions of times the solar mass.
What Orosz and his team found in the Messier galaxy seems to be a collapsing binary, with the huge star -- 70 solar masses! -- eclipsing the huge hole every few days. This is the "sweetheart" type of configuration that allows for precise X-ray measurements to be taken from Earth as matter is sucked from one body to the other, radiating as it goes.
Black hole enthusiasts of all stripes are thrilled by the find, showing again how foundational questions bring us together. Whatever singularities are, one of their most significant effects in the cosmos is the generation of delight.