How Could We Achieve The Sharpest-Ever Black Hole Image?

Space.com

The Event Horizon Telescope collaboration managed to capture the 1st image of the Milky Way's black hole in what was a groundbreaking moment for astronomy. Now, they want to snap an even clearer view of the incredible object.  credit: ESO

Transcript

00:00Remember these?

00:04These black holes are so compact and so far away that in order to get these pictures,

00:09we needed a telescope the size of the Earth.

00:12The resulting images were among the sharpest views of the cosmos ever obtained.

00:17Despite them being groundbreaking, some people were disappointed, calling the images blurry.

00:25I personally was super excited that we finally got to see what black holes look like up close.

00:32I just wish we could go a little deeper.

00:36Well, my wish may be about to come true.

00:45Astronomers just announced that they succeeded in making the highest resolution observations ever obtained from the surface of the Earth.

00:53But if we already had a telescope the size of the Earth, how is that even possible?

00:58Let's take a step back.

01:00These images, taken by the Event Horizon Telescope, or EHT for short, show the biggest black holes in the sky.

01:07But they are still extremely compact, only about the size of a doughnut on the Moon as seen from Earth.

01:15And what we're seeing here are not actually the black holes themselves,

01:19but the material that is caught in a death spiral around them.

01:23And we're not observing visible light that we can see with our eyes,

01:27but in a part of the electromagnetic spectrum called the millimetre, more precisely at 1.3 millimetres.

01:35To detect millimetre waves, we need dish antennas.

01:40To detect millimetre waves, we need dish antennas.

01:44The larger these dishes, the smaller the patch of sky we observe, and the finer the level of detail we can see.

01:51We call the level of detail we can just about recognise angular resolution.

01:55So, if we want a more detailed image, the obvious solution is just to build bigger and bigger dishes.

02:02The problem is, at some point, this costs too much, requires too many materials, and just becomes physically impossible.

02:10Instead of building one impossibly colossal single telescope,

02:14we can use a special technique called interferometry to construct a giant virtual telescope consisting of several individual telescopes.

02:23The combined observation will have an angular resolution, or image detail,

02:28the same as one giant telescope with a diameter corresponding to the biggest distance between any two telescopes.

02:36Interferometry is also how ALMA, in which ESO is a partner, works.

02:41ALMA consists of 66 individual antennas spread across the Chajnantor Plain in the northern Chilean Atacama Desert,

02:49and it can make extremely detailed images of our cosmos.

02:55ALMA can reach angular resolutions down to 5 milli-arc-seconds,

03:00which is the same as seeing a 10-metre-long bus on the Moon.

03:04While ALMA was already a game-changer for millimetre observations,

03:08the EHT went a step further by using Very Long Baseline Interferometry, or VLBI for short.

03:15VLBI is a little bit like interferometry on steroids.

03:19Rather than just tens of kilometres, the individual telescopes can be spaced hundreds or even thousands of kilometres apart.

03:26The EHT combines simultaneous observations from telescopes around the world, including ALMA.

03:34And it then uses sophisticated computer processing to obtain these iconic images.

03:41In case you're wondering why not all modern telescopes are interferometers,

03:45well, this technique comes with some serious caveats.

03:49In particular, the data has gaps.

03:52This is a bit like a sentence where letters or even whole words are missing.

03:56In order to reconstruct the sentence, it helps to have some idea of the content beforehand.

04:02The same is true for interferometry, in particular VLBI,

04:06where the spatial gaps in the data can be extremely large.

04:10Image reconstruction in interferometry is an extremely complex process,

04:14and it would take too long to explain it here.

04:17But if you would like a separate video on this, please let us know in the comments, and we'll be happy to oblige.

04:23Given that the EHT is pretty much the size of our planet,

04:26there doesn't seem to be much room for improvement in terms of telescope diameter, bar going to space.

04:32So does that mean these are the sharpest images of black holes we'll ever get?

04:37In fact, the telescope diameter, or the maximum separation between individual telescopes and interferometry, is not the whole story.

04:44To get a sharper image, which means a smaller angular resolution,

04:48you can also observe at shorter wavelengths.

04:51So basically, you get a sharper image if you have a big telescope or small wavelengths.

04:57To illustrate this fact, you can check out these images of a planet-forming disk taken with ALMA.

05:02They were taken at slightly different wavelengths,

05:04and you can see that the one taken at shorter wavelengths is clearly sharper,

05:08which means that we can distinguish more detail.

05:11So if shorter wavelengths mean sharper images,

05:14why does the EHT observe in the millimetre rather than, say, in visible light?

05:20The answer lies in how VLBI works.

05:24The answer lies in how VLBI works.

05:27Radio, or millimetre waves, are stable and coherent over large distances, unlike visible light.

05:34And this means that even if we have our telescopes on different continents,

05:38we can be sure that we're recording the same wave.

05:41Also, since the telescopes are not physically connected,

05:44in order to combine the data, we need to first record the individual data streams and clock them accurately.

05:51For visible light, this is simply not possible because our clocks are not accurate enough.

05:56So the trick is to find the sweet spot,

05:59the shortest possible wavelength where digitisation of the signal is still possible.

06:04Currently, this lies in the submillimetre regime, so just shortward of the millimetre.

06:09This is why it's so exciting that they finally presented their groundbreaking observations at 0.87 millimetres.

06:17These are the highest angular resolution observations ever taken from the ground.

06:22The new EHT results constituted test observations, which means that we're not sharing any images just yet.

06:29But the team managed to achieve an impressive angular resolution of 19 microarcseconds.

06:37While that might not sound like much of an improvement on the 20 microarcseconds they had previously,

06:42it does serve as a proof of concept, showing us that we can push to shorter wavelengths

06:47and obtain even sharper images in the future.

06:50The thing is that during these test observations, the weather wasn't great at all of the sites,

06:55meaning that not all telescopes' data could be used.

06:58The EHT team estimate that with better observing conditions,

07:02they will be able to make images with an angular resolution 50% better than was possible before.

07:10And more detailed images means better science.

07:14In the future, we hope to be able to measure the U-shaped orbit of light around the black hole,

07:20and we hope to be able to see the launching point of their massive jets.

07:25We also hope to be able to test in multiple ways the predictions of general relativity.

07:31We may even be able to obtain images of other, smaller, fainter black holes beyond the two we already have.

07:38This is just the beginning.

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