Wandering Black Holes

Out in space, violence exists on an unimaginable scale. In a recent article published on the BBC website titled 'Rogue black holes prowl Milky Way', astronomers and scientists are reported to believe "there should be hundreds of intermediate-sized black holes wandering invisibly" around our galaxy. Well, that's the theory. No-one has yet proved that black holes actually exist.

Black hole theory

In 1995 the American physicist Ted Bunn published a Black Holes FAQ which explains to the non-scientist what a black hole is, how big they are, what would happen if you fell into one, whether they could suck in the entire Universe, and how a white hole is just the opposite.

Ted's article is worth a read, but as every schoolboy (and girl) should already know, a black hole comes into existence when a massive star burns itself out and implodes into a 'singularity' with so much gravity that not even light can escape. So they can't be seen. According to the theory though, we should be able to see their effect on surrounding matter. We might also be able to see where one exists, because its blackness would obscure bright stars passing behind, and its gravity would even 'bend' light coming to us from those stars.

The BBC 'black holes' article refers to a meeting of the American Astronomical Society, when it emerged that in addition to the small black holes and supermassive black holes we've all known about for years, there's now an intermediate type with a mass equivalent to a few thousand Suns. These are the ones that wander, but scientists say we've no need to worry. They're dangerous only if you get to within about 100 kilometres of their 'event horizon' - the distance from the singularity at which light can no longer escape.

The Earth is presumed 'safe' because of the vast distances between objects in space and the probability that our dying Sun will expand to kill our planet before one of these wandering black holes collides with us. So, in theory, it could happen. The experts are just saying it's unlikely, just as it's unlikely we'll be hit by a huge meteorite or stray asteroid. But if it does happen, it's going to be unimaginably violent.

Intermediate (middleweight) black holes

Astronomers say that small black holes have about as much mass as the Sun, and supermassive ones equal the mass of billions of Suns. The intermediate-mass type could equate to about 1000 Suns. But as Ted Bunn explains, mass is not the same as size. A black hole with the mass of the Sun would have a radius of about 3 kilometres and a supermassive one at the center of a galaxy would have a radius of 3 million kilometres, about four times the size of the Sun.

The bad news:

Let's assume this intermediate-mass wandering type of black hole is a modest 60 kilometres across. That's not very big, but they're formed by 'mergers' between black holes of different masses, after which they shoot away at speeds of thousands of kilometres per second. So… they're 'wandering' at very high speed in straight lines. A speed of 4000 kilometres per second equates to 9 million miles per hour - about one seventy-fifth of the speed of light - and if the object has a mass of 1000 Suns it possesses energy on a truly grand scale compared to our slow-moving lightweight planet. Furthermore, as it travels along, the black hole gobbles up anything in the path of its event horizon.

With these properties, I would guess that if such a black hole hit the Earth it would smash right through and burst out on the other side, leaving the whole planet (minus some material) wobbling like a jelly and punctured with two nasty 60 kilometre diameter holes spewing super-heated fire from inside the core. Monster tidal waves fifty miles high would quickly wash over much of the land, and lines of enormous volcanic eruptions would break out with massive force along big new cracks all over the Earth's surface. Every building on the planet would already be completely destroyed, and soon after, the atmosphere would be filled with a thick dark radioactive mist obscuring almost all the light of the sun for thousands or millions of years. The rest can only be imagined, since I'm not a scientist and I could be wrong about this cataclysmic sequence of events. Perhaps all that would happen is that the Earth would explode like a plum hit by a bullet.

The good news:

Not everyone in the scientific community believes in the existence of black holes of any kind. An eminent physicist from Holland, Martinus Veltman (who shared the 1999 Nobel Prize in physics), is reported as saying "I do doubt the existence of black holes. However, astronomers are sold on black holes, and talking to them I feel like Don Quixote fighting windmills." Dutch scientists, as might be expected, tend to be more circumspect than their excitable North American counterparts.

And then there is the matter of…

Gravastars!

A problem with black hole theory - the mathematical model for their existence - is that the singularity at the centre must have infinite pressure, and the concept of anything being 'infinite' is alien to many physicists. In 2002, astrophysical researchers Emil Mottola and Pawel Mazur presented a new theory to the American Physical Society. They redefined the theoretical black hole as a bubble-like spherical vacuum surrounded by a flexible but virtually indestructible shell, which they called 'gravastar' from gra(vitational) va(cuum) star.

Compared to a black hole, being inside a gravastar may not be such a bad thing. Mottola and Mazur have suggested our Universe may be the interior of one. You'd probably need to understand quantum physics to be able to make any real sense of this, but they've calculated that a gravastar's interior would be a gravitational version of something called the Bose-Einstein condensate, stopping its shell from collapsing. This 'condensate' would be formed from space-time: Einstein's unified four-dimensional fabric of the Universe.

Ah. We're back to good old Mr Einstein again, and the possibility that there's no such thing as a wandering black hole. In recent centuries, of course, each successive generation of humans is proudly aware that its scientific knowledge is greater than ever before. We look back with amusement, for example, at the age in which intelligent academics like Fromundus believed the Earth was immobile at the centre of a Godly Universe. No doubt future generations will do the same with ours.

So let's return to simple fact that the concept of black holes is nothing more than a mathematical model, without the checks and balances of scientific experiment and direct observation. Phenomena that seem to fit the model aren't evidence of their existence. Beware of astronomers on the BBC website saying that one thing is 'characteristic' of another and describing imagined events as if they really took place; this is known as science fiction.

Posted Monday, January 14th, 2008 at 11:52. Comments welcome.