Black holes and space

Answers to popular questions on black holes and space.

By using my knowledge of our solar system and the basic needs of living things, I can produce a reasoned argument on the likelihood of life existing elsewhere in the universe – SCN 3-06a.

By researching developments used to observe or explore space, I can illustrate how our knowledge of the universe has evolved over time – SCN 4-06a.

For almost 100 years we could predict what the answer would be, based on Einstein’s theories, but then in September 2015 for the very first time we were able to actually measure what happens when black holes collide, thanks to two incredibly sensitive instruments called LIGO that detected “gravitational waves” – the ripples in space and time that are produced when massive stars like black holes are orbiting each other really fast (a bit like the ripples produced on the surface of a pond when you throw a rock into the water).   So, the short answer to your question is, when two black holes collide you make an even bigger black hole, but in the process some of the mass of the two black holes you started with gets turned into gravitational-wave energy: it was that energy that LIGO detected, spreading out across the universe from two black holes that collided more than a billion light years away.  Since that first discovery, LIGO (and our partner detector Virgo, in Italy) have detected 50 pairs of black holes or neutron stars colliding. You can learn much more about gravitational waves and LIGO at our website at www.ligo.org and you might find this page useful too.

Well you might be surprised to learn that nothing ever gets sucked into a black hole, although that phrase is used a lot in books and videos about them.

Black holes, just like other stars – as well as planets and moons – can attract other matter towards them because of their gravity, and because black holes’ gravity is very intense that’s perhaps why we often read quite scary things about being “sucked” into them.  But compared with the Earth, even our Sun is really massive – and we don’t seem to worry about the Earth being sucked into the Sun!  That’s because the Earth is orbiting the Sun and our orbit around the Sun creates a force pushing away from the Sun that exactly balances the force of gravity trying to pull us towards the Sun.  That balance means our planet has been able to orbit the Sun quite happily for billions of years.

In principle, it would be similar if we were orbiting around a black hole: the forces due to gravity and due to the orbit would be nicely in balance and we could keep on orbiting without getting pulled in.

Where things get a bit trickier, however, is when you get close to a black hole.  In that case it would be harder to keep things in balance because there would be other forces – that we call tides – that become important.  You maybe know about tides on the Earth: these are due to the gravitational pull of the Moon on the Earth and cause the water in the Earth’s oceans on the side of the Earth that is closer to the Moon to rise up because that water is a bit closer to the Moon (so feels its gravity more strongly) than the water in other parts of the Earth.

In a black hole the tides are much stronger – so much so that if you were an astronaut quite close to a black hole then the gravity pulling on your feet would be much stronger than the gravity pulling on your head, and you’d get stretched out: what astronomers call (very technical term, here!) “spaghettification”.  So this tidal effect, if you were orbiting close to black hole, would cause your orbit to spiral inwards (a bit like the two black holes spiralling towards each other; see Question 1) – in other words the black hole’s gravity would pull you in.  You could still escape from the black hole provided you had enough fuel to speed up your spacecraft, but once you got sufficiently close to the black hole – inside what we call the Event Horizon – then even that wouldn’t work and there really would be no escape from the black hole’s clutches I’m afraid.

Now the question was asking about whether the Earth could get pulled into a black hole. Fortunately, since there aren’t any black holes in our Solar System and the Sun isn’t nearly massive enough to become a black hole, we’re pretty safe from any of this happening to our planet!

We don’t really know the answer (Aside that it is gravity pulling you into a black hole, and not sucking you in!)

According to Albert Einstein’s theories of gravity, at the centre of a black hole is what is called a singularity: an infinitely dense point in space and time where everything that passes inside the black hole’s event horizon must inevitably wind up.  So if you get pulled inside the event horizon of a black hole you’d get crushed out of existence at the singularity; doesn’t sound very pleasant, does it?…

Well, the complication is that scientists aren’t really sure if singularities could actually exist – in fact most think they probably can’t.  Instead we think that Einstein’s ideas about gravity break down and we need another, better theory to describe what is going on at the heart of a black hole.  The trouble is, we don’t have such a theory yet – i.e. no one has been able to come up with one!  We have some good ideas, and very clever scientists like Stephen Hawking worked on those for many years, but there are still many unanswered questions to solve before we could really say what goes on inside a black hole – and what would happen to an intrepid astronaut who ventures there.  Could they wind up travelling to another universe altogether, or perhaps take a “short cut” through a wormhole to a different place in our universe? Nobody really knows, but it’s fun trying to work it out!

To find out a bit more about scientists’ ideas, and about black holes in general, check out this webpage from NASA’s Space Place: https://spaceplace.nasa.gov/black-holes/en/.

The Milky Way is the combined light from billions of stars in our galaxy, so you might expect it to be really bright. Almost all of those stars are really far away from us, however, so we see the Milky Way instead as a kind of fuzzy background glow – not so different from e.g. how you would see the glow from the lights of a city like Glasgow on the horizon when you’re driving (say) along the M8 from Edinburgh at night. And because the light of the Milky Way is quite diffuse, it’s especially difficult to spot it at all when you’re close to a city – because all of those streetlights can drown it out completely. So the best way to see the Milky Way is to get somewhere really dark, far away from city lights, so the faint background glow from all of those far away stars can really shine through. And if you ever get the chance to see it from somewhere really dark – like one of the observatories in a very remote location where we put our biggest telescopes (see this image here, of the Milky Way seen above the Paranal Observatory in Chile, for example: https://www.eso.org/public/images/epod-cc-rf18284/) then it’s an amazing sight: a band of stars that stretches across the whole sky. Even then, however, the Milky Way would still be drowned out by e.g. the light of the full Moon.

The honest answer is “we don’t know”.  However, this question is related to some very deep ideas about the branch of science that we call “quantum physics” – which is incredibly successful at describing the behaviour of atoms and molecules and tiny particles like electrons and photons.  According to quantum physics predicting exactly how individual particles will behave isn’t possible, and their rules of behaviour are random and unpredictable: one way to interpret all of that is to imagine that every time a quantum event happens which has lots of possible outcomes, then all of those outcomes do occur – but in alternate universes.  These are fascinating ideas, and increasingly we find them woven in our favourite science fiction movies (like the alternate realities of the “Quantum Realm” in Avengers Endgame, for example) but we are a long, long way from knowing for sure whether those alternate realities exist or not.

One of the most important principles about how we do science is that we develop an idea, or theory, and we look for some way to test our theory by carrying out an experiment.  So a particular challenge for testing theories about alternate universes is how can we test if they exist if we can’t actually observe them?… It might seem like there is no way to get around that, but remarkably there are some ideas about how we might use observations of what the universe was like a long time ago to give us clues about whether or not those other possible universes exist.  Perhaps in the future we will be able to solve this particular puzzle, but there’s no sign of that happening any time soon, I’m afraid.

If we are going to have to find another star system to make our home in about 5 billion years, then fortunately we should have plenty of choice – because our Milky Way galaxy contains something like about 400 billion stars, and probably a sizeable fraction of those stars have planets.  The real problem, however, is that these stars are very far away!!   The Sun is about 150 million km from the Earth, which is already hundreds of times further away than the distance from the Earth to the Moon – i.e. the distance travelled by the Apollo astronauts like Neil Armstrong.   But even the nearest star beyond our Solar System is more than 250,000 times further away than the Sun.  This is so far away that measuring distance in miles or kilometres doesn’t really make much sense as the numbers start to get too big. So we use the light year – which is the distance that light travels in one year.  Light travels really fast: in fact according to Albert Einstein’s theories, nothing can travel faster than this – so it’s kind of like the cosmic speed limit!  Light travels 300,000 kilometres every second so just think how far light must be able to travel in a whole year.  In fact one light year is about 10 million million kilometres (you see what I mean about the numbers getting really big!) and the nearest star beyond our Solar System lies at a distance of about 4.3 light years.

The question was asking about how long it would take to get out of our galaxy, however.  Well, our galaxy is about 100,000 light years from one side to the other, but our solar system isn’t in the middle – but roughly about halfway out towards the edge. But our Milky Way is quite flat, like a pancake, so if we travelled away from the plane of the galaxy – i.e. away from the pancake, rather than “through” the pancake – then we could escape the galaxy more quickly.  But even if we had a spaceship that could travel almost at the speed of light, it would still take hundreds – or even thousands – of years.  There’s a little bit of a catch, however – again thanks to the theories of Albert Einstein.  If we could travel in a spaceship close to the speed of light, then time would run differently for us onboard than it would for people back on Earth. So it wouldn’t seem as if so much time had passed for us, maybe even just a few years – depending on how close to the speed of light our spaceship could travel – but it would still be thousands of years from the point of view of everyone back on Earth.