Overnight, scientists will give the world its first actual glimpse at a black hole – and it's a big deal. University of Auckland astrophysicist Dr JJ Eldridge tells Jamie Morton why.

What's all of the excitement about?

Over the last few years radio astronomers have built the Event Horizon Telescope.

It's a radio telescope made by linking together radio telescopes all over the planet so we have a telescope almost the size of the Earth.

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This means we can make very accurate and high-resolution radio images of objects in space.

The excitement is because they have observed two black holes, the one at the centre of our own galaxy, and the other more massive black hole at the centre of the galaxy M87.

Why have we not been been able to see a black hole until now?

Black holes are really small. They are the densest objects we know of so in most cases it is really difficult to resolve them in an image.

The black hole at the centre of our galaxy is about five million times the mass of our sun but only 20 times the radius of our sun, and it's 24,000 light years away, so it's very, very
small.

By having a radio telescope that is the size of the Earth it now has the resolution to see
the black hole and get the image so it's taken a long time to build, design and operate the telescope to get the data and also a lot of time to combine the observations of different telescopes to make it effectively an image from one telescope.

So all together it's taken a lot of work, effort and time to get to the point where we could do this.

What might these "supermassive" black holes actually look like - and what are they physically capable of?

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The black holes will probably be mostly spherical and so we won't be able to see the objects themselves but if there is material like gas around the black hole that will be very hot and we'll be able to see the radio emission it gives off.

Our hope is that we can see the event horizon - the surface from which nothing, not even light, can escape the black hole - as a shadow amongst the material orbiting around the
black hole.

Supermassive black holes are interesting as they are in some ways linked to the evolution of galaxies.

They are at the centre of nearly every massive galaxy we know of.

One interesting thing about the supermassive black holes is that tidal forces near them are actually quite small.

For example, if an astronaut attempted to go near the event horizon of a black hole with the mass of the sun the tidal forces would tear them apart, but in the supermassive black holes these are a lot weaker so someone could enter the black hole quite easily.

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Of course what is inside the event horizon, we don't know.

Note this was in the film Interstellar! Also what we might see might be a very low resolution version of the black hole depicted in that film.

It's worth noting a lot movie or artist's impression of black holes are wrong, as they don't take the extreme warping and lensing of light near the event horizon into account.

Why does this feat represent a breakthrough for astrophysics - and where might we be able to go from here?

Being able to resolve the event horizon of a black hole is a big deal.

The nature of the event horizon is where all our uncertainty in how to mix general relativity with the rest of physics, especially quantum mechanics collide.

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What we see there, or don't see, will constrain how physics works in the most extreme location in the universe today.

In just the past few years, we've seen the first detection of gravitational waves and the discovery of more waves that resulted from the collision of neutron stars. What are the biggest questions that are still waiting to be answered and that may be in the near future?

For the black holes it is the nature of the event horizon, and for neutron stars the related quantity is the radius of the neutron stars.

Both these things sound like quite a simple measurement "how big is something" but for the black holes they'll help us understand gravity better, while for neutron stars they'll allow us to understand how the densest material in the universe is held together.