Patrick sabu philip asked:
What is a black hole, how does it form and what happens when something is sucked in? Is it disintegrated into smaller dust to form nebula?
We put this question to Cambridge astronphysicist Zephyr Penoyre...
Zephyr - That is a very difficult question. What a black hole is, is an object that’s just so heavy that light itself can't escape from it.
Now, we’re often told that light isn’t affected by gravity. As far as things on Earth are true that is completely true. The amount of gravity, the amount of mass needed to actually affect light is huge. But black holes are remnants of massive stars that have collapsed into a very small space, and they're so dense, they're so heavy, that even light cannot escape.
What actually happens when something passes, what we call the event horizon – the distance away from the actual body which light can't escape from – is very, very unclear and we may never know because it’s impossible for anything that we send through, past that point, to ever come back because light - the fastest traveling thing - even that, cannot escape. Likely, it wouldn’t be a particularly pleasant process. Even as you approached it, you get stretched and stretched, and stretched.
Chris - Would you feel that though? Would you actually feel yourself being pulled?
Zephyr - You'd be pulled by potentially, depending on how fast you approach the black hole. You might be pulled kind of string-thin, you might be torn.
Chris - Over what timescale would it take as you felt yourself – because literally, you’re feeling a force on your legs. It would be like being put on a medieval rack then? Is that what you're saying? You would literally feel yourself being drawn out.
Zephyr - Yes. The quicker you go into the black hole, the better it is for you in the process of going in.
Chris - [Laughs] - That's one way of putting it...
Zephyr - The quicker you go in, the less stretching the time you're stretched.
Chris - So, a really agonising approach would be a really slow journey into the black hole.
Zephyr - If you spiralled around for years and years, you will slowly get longer and longer.
Chris - And then what would happen?
Zephyr - Then that is where it gets really unclear because there are things like absence of a black hole, it doesn’t seem like there's any force to stop the mass at the centre collapsing more and more. And so, you'd expect the density at the centre to go up and up, and up, faster and faster and faster, and become infinite. And that must be impossible, because if there was infinite density, it would have an infinite force and everything, everyone in universe will be sucked in.
So we’re very unclear on what exactly is stopping that happening in the centre of the black hole. There are theories even that time must slow down and stop at that centre, so that we can't reach this infinite density...
Entertaining as it may be to contemplate the exotic characteristics of black holes, what is supposed to happen to bodies falling into them, etc., the truth is that nobody would ever survive to provide empirical evidence to support the falling predictions; and evidence to support the other predictions is extremely indirect and inconclusive. At least a few physicists will admit that black holes are still very theoretical objects. By contrast, it is entirely within our technological reach to arrange to observe a test mass falling into an ORDINARY hole through an ORDINARY body of matter. Yet we've never done this. Galileo proposed the idea in 1632. Since the experiment involves the undisturbed and collision-free motion of two bodies of matter, the apparatus needed to carry it out may be called a Small Low-Energy Non-Collider. Instead of investing so much ink, bandwidth, and mental energy on predictions that can never be tested, on exotic objects that reside perhaps entirely on paper and in the mind, why don't we extend our exploration of gravity in the real physical world where we have not yet looked? Why don't we build and operate a Small Low-Energy Non-Collider? As scientists, are we not obliged to abide by the empirical spirit of Galileo, to make sure the standard prediction for his experiment is correct (or discover that it is not)? Richard Benish, Mon, 21st Dec 2015