GUIDE TO LIFE ARCHIVE GUIDE TO LIFE ⇒

From my position of limited knowledge, I would say that there are two methods to explain black holes – using either the Newtonian or the Einsteinian concept of gravity. I know that, strictly speaking, Newtonian gravity isn't right, but it’s still useful, and, because it was the first way I began to understand anything about black holes, I would, even though I have acquired a limited understanding of relativity, still explain them to someone first by using the Newtonian model.

All you actually need to understand is, firstly, that objects all have a mass. Mass measures how much matter they contain and is not the same as weight, which depends on gravity. You could be different weights on different planets that have different gravities, but your mass would always the same. Secondly, everything with mass has gravity and the more massive an object, the greater its gravitational pull. Thirdly, light, along other forms of electromagnetic radiation like x-rays or microwaves, is both a wave and a particle, and because it is a particle is affected by gravity. Lastly, nothing can travel faster than the speed of light.

So then there's the concept of an escape velocity which is something that's talked about with spacecraft leaving the earth. When a spacecraft takes off, it's moving in the opposite direction to the earth's gravitational pull so there's the force of its acceleration one way and the force of gravity the other way so in order not to be pulled back down to earth it has to be going fast enough - and this is the escape velocity - i.e how fast you have to be going in order to escape. The more massive the object you're trying to escape from, the higher of course the escape velocity is, and so extrapolating from this, if an object is very, very massive it will have an escape velocity faster than the speed of light and when that happens then light tries to leave the object and it can't and what you have is a black hole because nothing, not even light, can escape. Now I'm hazy on the Einsteinian stuff, but according to that the space/time continuum is basically flat and light travels along it in straight lines and what happens when you get an object exerting a gravitational force is that it causes a distortion in the space/time continuum and bends it. It's like a piece of stretched cloth with a grid drawn on it and what happens when you put a ball in the middle - the cloth distorts and the grid lines bend. So what happens when a piece of light travels close to a source of gravity is that it gets deflected from its straight line path and is forced to climb up the slope of the dent made in the space/time continuum in order to escape. So what happens is that it climbs the slope and as it does so it loses energy. For not very massive objects this effect is negligible but with stars it's detectable. It's been measured that light coming from a star which travels past another star on its way to us has its path deflected by the second star so the first star appears to be in a different position from what it really is and maybe, but I’m not sure about this, the colour changes too as the light photon loses energy so it gets what's called red-shifted which means the wavelength gets longer so it goes towards the red end of the spectrum where the wavelengths are longer than at the blue end. I'm actually not sure about any of this stuff. You see if there's a black hole then the slope the light has to go up is so steep that it can never get to the top, but is this because the space/time continuum is actually so warped that it's bent back on itself so there's no way out or is it that the photon loses so much energy that it's red-shifted away to nothing - until the wavelength is so long it's not there any more, and then what happens? Or is it that the black hole doesn't let out any visible light as its too red-shifted but it does let out lots of very low energy radiation or is there some of that and then some of the light getting converted to mass, because of course Einstein’s E=mc2 theory is all about how mass and energy are the same. If it is a case of infinite warpage of the space/time continuum then how does anything get in? And if gravity is and exchange of particles like gravitons, which I've heard it is, then how do they get out - wouldn't the black hole just disappear from the universe to all intents and purposes, taking its mass with it?