How Can We Achieve The Sharpest-Ever Black Hole Image?
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
Credit: ESO
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TechTranscript
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.