Black Holes have a certain aura about them. They are associated, in the minds of the general populace, with a certain mystique or ultra-mystery about them – terrifying objects that gobble up everything within range – the ultimate devourer, doomsday machine, berserker and weapon of mass destruction (if you could figure out how to manipulate one of course) all rolled into one. But Black Holes have other aspects about them that are equally fascinating, and not really all that weird, though some bits are weirder than others. But you don’t have to be a geek to come to terms with these concepts.
The aura of the Black Hole, even if not quite as dramatic as a doomsday device, is hardly less within the astronomical community, to quantum physicists, or relativists (scientists who special in general and/or special relativity). Though there’s little doubt today of their actual existence, a logical consequence of Einstein’s theories of relativity, Einstein himself refused to give credence to them. The well-ordered universe just wouldn’t actually create such monstrosities he believed. He wasn’t alone in that point of view, and as their theoretical certainty became ever stronger, scientists tried to find ever more unique ways to prevent them from forming – to no avail.
But are Black Holes really as strange and mysterious and deserving of their aura and status as unique astronomical objects?
Black Holes may have no hair, which is to say they lack the individuality of whatever formed them so if you’ve ‘seen’ one Black Hole you’ve ‘seen’ them all. Translated, a Black Hole made out of rusted automobiles will ‘look’ the same as one made out of star-stuff, as one made out of pure gold, silver and diamonds. But Black Holes do have (or could have) certain properties. All Black Holes most certainly have mass and therefore gravity; they certainly have size (a volume, an area, a circumference, etc.); they certainly have a shape (spherical). Black Holes (against all intuitive prediction) have a temperature (Hawking radiation). They can have spin (rotation), and they may have an overall electric charge. So what’s unique about that?
The property we most associate with Black Holes is gravity, a function of mass - the more mass, the more gravity. Associated with that concept is escape velocity – how fast do you need to go to escape an object’s gravity well never to return.
Now our moon has gravity and an associated escape velocity. Planet Earth has greater gravity and therefore a higher escape velocity (about seven miles per second). Planet Jupiter has an even greater gravitational field and thus you need even more oomph to escape. Our sun is another notch higher up, and so it goes. Keeping in mind that gravity is related not to something’s size, but to its mass, a White Dwarf star, while smaller than our sun, has greater gravity and therefore escape velocity. Then comes Neutron Stars (pulsars) and you really need some rocket power to get away from those babies!
However, there is a limit to velocity, escape or otherwise. That limit is the speed of light, or about 186,000 miles per second (roughly 300,000 kilometers/second). So what happens when there is so much mass, or so much gravity, that the escape velocity exceeds that of 186,000 miles per second? The quick answer is nothing – you can’t escape; nothing can escape – not even light. That’s pretty straight forward and you don’t even need a course in relativity to figure it out! The absence of light is darkness, so any object that has an escape velocity greater than that of light will be dark – in other words, a Black Hole. The only difference twixt a Black Hole and any other macro object is that a Black Hole’s escape velocity exceeds that of light. That’s it; end of differences.
If you can’t see a Black Hole, how could you know they actually (as opposed to theoretically) exist? Simple – Black Holes have gravity, and the gravity of Black Holes can influence matter we can see. So, if you see a star going too and fro in orbit around something you can’t see, then that something is probably a Black Hole. Matter (interstellar dust and gas) spiraling into, but just prior to entering a Black Hole can also give off a tell-tale electromagnetic signature.
Because of such intense gravity, individuality is squeezed out. Planet Earth has highs (mountain peaks) and lows (ocean troughs) and a slight equatorial bulge, but if it’s size were reduced (while retaining mass) to the extent that her gravity created a greater-than-light escape velocity, then Planet Earth would become a perfect sphere of super dense crushed matter. No peaks, no troughs, no bulge – no personality, or no hair!
Now objects tend to have a surface – an inside and an outside. In the case of Planet Earth, let’s call beneath the crust Earth’s inside; above the crust Earth’s exterior. The same goes for Black Holes. The inside center of a Black Hole is called a singularity. The ‘surface’ of a Black Hole is called the event horizon – it’s the purely mathematical line where the escape velocity goes from faster than light speed (event horizon and below) to a permitted escape velocity (event horizon and above). Earth’s usually quoted escape velocity is given to be at Earth’s solid surface or sea level. But even sat 100 miles above, there’s still as escape velocity, it’s just less than 100 miles further down. In like style, a Black Hole’s escape velocity decreases from the singularity outwards, but doesn’t become permissible (less than light speed) until the altitude of the event horizon is reached. Thus one can not see anything, any events that are below this mathematical event horizon because anything below can’t get out, including light. Finally, the distance between the singularity and the event horizon varies depending on the mass of the Black Hole.
It’s what’s below the event horizon that’s really of interest given that it can’t be seen; no information escapes to inform us or give us any real clues of the conditions beneath. One has to rely on physics’ theoretical equations to predict conditions – conditions that really can’t be verified by any direct observation.
Unfortunately, these equations, the equations of general relativity, break down when one approaches the singularity. That’s because in order to come to terms with what a singularity is like, one has to merge general relativity (gravity) with quantum physics (because the singularity is thought to be of a size within the realm of quantum phenomena), or produce a theory of quantum gravity. Alas, that has yet to be accomplished. So, understanding the physics inside a Black Hole is one of Mother Nature’s final frontiers!
For example, taken to their logical conclusions, physics’ equations (general relativity) dictate that a singularity must have zero volume and infinite density. Physicists are well aware that whenever ‘infinities’ pop up in their musings, something’s wrong and they need to go back to the drawing board (blackboard?) and refine things to a greater or lesser extent. Hopefully, a theory of quantum gravity will do that, but for the here and now, you’ll find texts which state that a singularity has zero volume and infinite density. That’s clearly a nonsense, for if one had infinity density, one must have infinite gravity as the greater the density an object has, the greater its gravitational attraction. Now even though gravity dilutes as it spreads throughout space and away from the object of its affection, any dilution of infinity is still infinity. Since Black Holes and associated singularities are thought to be common in the observable universe, there should be at least one that’s had time since the Big Bang to project its gravitational influence onto us – say the massive Black Hole singularity at the center of our Milky Way Galaxy, less than 50,000 light years away. Quite obviously we’re not being subjected to an infinite gravitational attraction towards our galactic center, which tends to put the kibosh on, and confirms the breakdown as to what the equations predict for a Black Hole’s singularity.
So, if a Black Hole’s singularity doesn’t have zero volume and therefore infinite density, then it must clearly have a finite volume and a finite density which has implications for the origin of our Universe since conventional wisdom associates the Big Bang event with a singularity (and if there were to ever be a Big Crunch event, that would have to end up as a singularity).
The logic goes something like this. A singularity must have a finite density because having an infinite density is ridiculous. A singularity must have a finite volume because any object that has mass can’t be dimensionless – that too would be ridiculous – and Black Holes certainly have mass since they have gravity. If the Black Hole continues to grow, then the singularity continues to add mass to it, and its density increases. But, eventually the density reaches some sort of maximum possible – it’s finite after all and can’t become infinite. So as matter continues to be added to the singularity, the volume or size of the singularity must grow – and grow – and grow – and grow. Eventually, the volume of the singularity must be such that it falls outside of the realm of quantum physics. Translated, in other words, not only is a singularity of greater than zero volume, it may not even be tiny. It could be massive – stellar sized; even galactic sized! That then does away with the absurdity that our entire universe started out as something less than atomic sized something akin to a tiny pinprick!
Now the other interesting thing is that gravity probably isn’t really a force like electromagnetism or the strong and weak nuclear forces and shouldn’t be lumped in with them (which physics texts do). Rather, gravity, according to general relativity, is rather a manifestation of space-time geometry. As the saying goes, ‘matter (gravity) tells space-time how to bend; bent space-time tells matter how to move’. That movement we interpret as gravity.
So, space-time near, around or in a Black Hole is about as bent, or warped, as you can get, or conversely, the local geometry is so extreme or curved that not even light can get beyond the Black Hole’s event horizon. The geometry creates a sort of well, so deep and so steep, that the velocity needed to escape is greater than special relativity allows. [Special relativity covers the speed of light; general relativity deals with gravity and space-time.]
What does the extreme warping of space-time mean – apart from making the Black Hole, black? Well, presumably if you distort space-time sufficiently, then you, in theory, can make shortcuts through space and/or time.
Let’s have an analogy. Say you take a balloon and mark out a North and South Pole on the surface. The distance between the two is either half the circumference of the balloon (if you go via the surface or normal space), or the diameter (if you tunnel through, call that hyperspace). Now squeeze the balloon such that the North and South Poles are forced close together; maybe even touching. While this doesn’t help reduce the traveling distance if you stay on the surface (normal space), the tunneling (hyperspace) distance in the now warped balloon is vastly reduced. If it took you a year say to tunnel from North to South in the standard balloon, then post warping it might take you only a week (or less). In fact, if the Poles were squeezed into direct contact, then you could travel via normal space from one to the other instantaneously – no need for hyperspace. Of course if you actually wished to travel from some other point on the balloon’s surface to some other point, the squeezing might not do you much good. In fact, the East – West distance has increased! So, the odds that the warping will be just right for your travel needs could be highly problematical. A local Black Hole warping that favors you traveling to Sirius quick smart is of little consequence if you wish to actually go to Alpha Centauri. But then as some old wise sage said, ‘life wasn’t meant to be easy’!
Now since space and time are intractably connected, points in time, like points in space, can be squeezed closer together. So the North and South Pole bits could easily have been a past and a future. Actually, because it’s really space-time, you probably have a combination of both. You don’t travel from 2000 AD Adelaide to 2000 AD Sydney in the wink of an eye; nor from 2000 AD Adelaide to 3000 AD Adelaide in that same wink, rather from (say) 2000 AD Adelaide to 3000 AD Sydney in an eye blink.
I’ve seen speculation that a Black Hole could warp space-time so greatly that it could ‘pinch’ itself off from our Universe and disappear entirely. Of course if it did so it could no longer have any influence within our cosmos. However, if something as massive as the Black Hole at the center of our Milky Way Galaxy isn’t enough to pinch space-time sufficiently to disappear, then perhaps it just doesn’t happen – or maybe it takes the mass of an entire universe to do it. Say one universe’s Big Crunch’s mother of all Black Holes plus singularity warps space-time so much that it becomes another universe’s Big Bang!
To be continued...
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