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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