We found out about black holes behind yesterday's door. We're celebrating the eighth and ninth days of advent with all the questions you've ever wanted to ask about black holes. Back in 2015, before black holes had even been detected, we were lucky enough to ask cosmologist Pau Figueras all your questions about the physical reality of a black hole...

Why do we think they exist?

They are a prediction of the general theory of relativity. We think that compact dark objects, for which we do have evidence, should be black holes, [as] described by general relativity.

An artist's impression of gravitational waves caused by a pair of massive stars spiralling towards each other and finally colliding. Click here to see an animation of this process on the NASA website. Image: NASA/Dana Berry, Sky Works Digital.

Firstly, when massive stars run out of nuclear fuel there is no other force in nature which can withstand the pull of gravity. General relativity tells us that under such conditions any such object will collapse [under its own gravity] and form a black hole.

[Secondly], at the centre of most galaxies, in fact probably [at the centre of] all of them, there lurks a super-massive black hole weighing millions or even billions of solar masses. In particular at the centre of our Milky Way, we do have a compact dark object which weighs about four million solar masses. It's small, it doesn't emit light, and the only object in nature with such properties that we know of is a black hole.

So we haven't [directly] seen [black holes] because they are small, they are dark, [and] so they are very hard to see.

We have seen indirect evidence, for example when there is a star orbiting around the black hole, the black hole sucks gas from the stars. This gas gets very hot as it approaches the black hole and this emits x-rays, which we can observe. We haven't detected black holes directly yet, but this is likely to happen at the end of this year, or maybe next year, with the new gravitational wave detectors.

Update - black holes exist!

At the time of this interview in 2015, black holes still hadn't been directly observed. But Figueras was exactly correct with his prediction, they were first detected directly later that year and this was announced in early 2016. This hard evidence came from the first detection of gravitational waves from two black holes colliding over 1 billion lightyears away. You can find out more behind Door #4. At the announcement in 2016 Figueras told us: "It provides a direct detection (and hence confirms) two inevitable consequences of [Einstein's theory of gravity]: gravitational waves and black holes... It's the greatest discovery in experimental gravitational physics of the last hundred years."

This is an excellent question and the honest answer is that we don't know. Often infinity, in physical theories, just simply means that the theory that you use to describe the situation breaks down. So we think that infinities do not occur in nature.

It's just that in [the region of a black hole], curvature is very large but the region is also microscopic in size, so we know that, to describe the physics of these singularities, you would need a theory that is able to describe gravity – namely strong gravitational fields – and at the same time micro-physics – namely a theory of quantum gravity. There are some candidates [for such a theory] but we are still far in terms of understanding how to describe what happens in singularities.

What's remarkable is that the singularities are hidden inside black holes, namely they are covered by the horizon. In the region inside the horizon gravity is so strong that nothing can escape, so even if we do know how to describe the singularities, whatever happens there cannot influence what happens to the world outside. So in some sense, black holes conceal our ignorance in their interior.

What would happen if you passed over the horizon of a black hole?

What's important about the horizon is that it acts like a one-way membrane. It's a soft surface, so you can just cross it and you wouldn't even notice that you'd crossed the horizon.

It's not like the surface of a star where you just hit that and you burn. There is some debate about whether it is indeed a soft surface or not - it's not a properly settled issue - but it seems, according to general relativity alone, that it should be a soft surface. You can just cross it, and in principle if the black hole is large enough then you wouldn't feel anything until it's too late.

An artist's impression of a black hole. Image: Robert Hurt, NASA/JPL-Caltech.

If the black hole is small then the difference between the curvature [of spacetime] at your head and at your feet is so large that you get stretched, like spaghetti. It's like [a] tidal effect. But if the black hole is very big then the gravitational field is very uniform, even though it's strong, so the difference between gravity at your head and at your feet wouldn't be that large. You can approach the horizon of a very big black hole, such as the one at the centre of a galaxy, and not feel anything at all.

Close to the black hole, because gravity is strong, time runs slower for you. Even if you could stay there you wouldn't feel much, but if you compared your clock with [the clock of] an observer far from the black hole then you would see that you haven't aged that much whilst he or she has become much older. Other than that you'd be perfectly fine.

Now if you cross [the horizon], then general relativity tells us that once you are inside the black hole you are essentially doomed. You will eventually hit the singularity and you will be destroyed, so it's not something I would advise you to do!

You can find all Figueras' answers, including whether we could create tiny black holes on Earth, in the full article, and accompanying video and podcast! And you can find out all about the mathematics of a black hole behind tomorrow's door!

This year's advent calendar was inspired by our work on the documentary series, Universe Unravelled, which explores the work done by researchers at the Stephen Hawking Centre for Theoretical Cosmology and is available on discovery+. Return to the 2020 Plus Advent Calendar.