Let's Talk Physics

A stochasticly updated blog about interesting topics in Physics & Astronomy

How it’s All Going to End: The Wandering Black hole (Part 4)

Ok, a little introduction to this article. Firstly, the probability of what I’m going to talk about is so small, it’s pretty much guaranteed  to never happen. However, it’s going to lead me to some very interesting parts of astrophysics, which is why I’m going to chat about it. Secondly, it’s an incredibly hard topic to talk about without equations popping up all over the place, so I ask for your forgiveness if this gets heavy quickly. Trust me, stick with it, because what I’m about to talk about is one of the most beautiful mathematical curiosities to have come out of the 20th century and also one of the scariest objects in the known Universe – it’s time to talk about black holes.

In 1915, Einstein published his theory on general relativity. The end result was a group of 10 equations – Einsteins field equations. Astrophysicists immediately began searching for solutions to these equations, and a few months after Einstein published his theory, Karl Scharzschild found a solution to the equations which specified a singularity in space – a region where some of the terms in Einstein’s equations went to infinity (which in physics, is normally very, very bad). In this case, the singularity was a region of infinite density in an infinitesimally small region. A region where, if you went past its threshold, you could never come back. In fact, not even light can escape from it. Which makes detecting these monsters quite a challenge.

First, let’s see how a black hole is formed. When a star finally reaches the end of its life cycle, and has no more fuel to burn (be it Hydrogen, Helium, Oxygen or heavier elements) several things happen, depending on the size of the star. Stars like our sun simply shed their outer layers and leave behind a core of spinning hot electrons. This core, which has a mass similar to that of our Sun but is only as big as the Earth, is incredibly bright and hot, but will cool down over billions of years to become a brown dwarf. So this is obviously not what we’re looking for.

Ok, so let’s take a larger star. Say one that gets around to burning oxygen in its core. When this one finally goes pop, it gives a hell of a show and goes supernovae. When it runs out of fuel in its core, everything collapses inward, gets halted towards the center of the star, and sends a shockwave backwards that blows the outer shells of the star off in a tremendous explosion. The aftermath of a supernovae can be seen in the image below.

The supernovae SN1987a as seen through the Hubble Space Telescope

The supernovae SN1987a as seen through the Hubble Space Telescope

What gets left behind after the supernovae of a semi-large star is a tiny star composed entirely of neutrons, which also has an incredibly powerful magnetic field, and can also spin very rapidly (these are called pulsars, and I will be talking about them in another post). But again, this isn’t what we’re looking for.

Ok, let’s take an extreme example. Imagine a red supergiant, as mentioned in the end of the post on Size Matters. When this star goes pop, what happens inside?  Well, for the previous 2 examples, the collapse of the layers of the star towards the center is eventually halted by some sort of physical process – for white dwarfs, it’s electron degeneracy, for neutron stars it is neutron degeneracy. In this third example, nothing halts the collapse. The entire star collapses into a single point in space. This is what we’re looking for.

So, why are black holes so hard to spot? Mainly because they don’t emit any light, and any light that strays too close (too close here being a distance called the Schwarzschild radius) will be sucked in and will never be let go. This means that a black hole that forms from the death of a star is pretty much undetectable. We have detected some black holes though, because fortunately, if you have a binary system (like Algol) and one of the stars in the system turns into a black hole, then the black hole has a terrible habit of eating the other star. As matter falls from the companion star into the black hole, it gets really, really hot and emits light. We can detect this light, and map out the disk around the black hole. So we can indirectly detect the black hole.

An artists impression of a star feeding a blackhole

How about black holes that aren’t in binary systems? Well, unless we’re looking at a region of space and a black hole just so happens to wander past and block the light, then we’ve got no hope. Which is scary, because from calculations of how old the Universe is and how many of these monsters should have been born in this time, there are a lot more black holes out there than we’ve detected.

Before continuing, I want to describe what a black hole might look like to an observer who was close to it. A black hole emits no light, and absorbs all of the light around it, after it is born. Before and during birth, light behaves as normal. Imagine you are an indestructible astronaut, and you’re looking at a super giant just as it begins to collapse. The star loses its outer shells through shockwaves from the core, but the inner shells fly towards the center at tremendous speeds. Then, suddenly, the shells seem to get to a certain radius and stop. But the light starts to get more red (remember that red light is less energetic than blue light). So, all you’re left with is this image of a fading star on a surface of…well…nothing?

Homer being sucked into a black hole

Homer being sucked into a black hole

This is known as the event horizon. This region where light seems to freeze. Actually, it isn’t light that freezes, it passes through this region with no problems. No, it’s actually time that is stopping. To someone outside, the star is collapsing forever, because for someone observing this object, time stops here. But what happens after the event horizon? Well, let’s go take a look. Now, you (the adventurous indestructible astronaut) are going to venture into the event horizon, and I’m going to stand outside and watch you. What I see is you going towards this (now black, due to the light from the star having faded) event horizon, and I will never see you actually reach it. Essentially, you become painted upon this surface for ever (thought the light that is emitted will continually become more red shifted, and eventually my eyes wouldn’t see you unless I wear infrared goggles). But what about you? You would approach this surface of blackness, pass through it and keep going into an empty region of space. To you, nothing really has changed (because you are indestructible, if you weren’t you’d be dead). However, what is ahead of you is something no one has ever witnessed before – the singularity. You see, every black hole in the Universe is surrounded by an event horizon which stops outside observers from seeing inside, instead we see images of what’s gone inside painted on the event horizon. This is known as the Cosmological Censorship Law.

So, now on to how one of these beasts might destroy us. Well, as I said, the chances of this are so, so tiny we can say it’ll never happen, but I’ll say it anyway. We think there are black holes in the Universe which are about the size of the Earth. They aren’t orbiting other stars, so we can’t see any light from them eating other stars. They died a long, long time ago and so the image of the star painted on its event horizon is too hard to detect. And, we think, they could just be wandering around the Universe. And who knows, maybe some day, one of these wandering beasts will cross the path of the Earth, long before our sun expands, and it’ll simply eat us. Once we pass the event horizon, there would be no hope for us.  How would it end? Well, imagine that scene from Star Trek, except with the black hole beside the planet, pulling to shreds and devouring it.

Now multiply that by 1,000. Yup, that would hurt.


About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

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This entry was posted on February 18, 2013 by in Astrophysics and tagged , , , , , , , , .
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