Congratulations to the EHT team!

Posted on April 11, 2019 by Bonald

Filed under: Uncategorized |

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6 Responses

  1. josh, on April 11, 2019 at 12:40 pm said:

    Hey, Bonald. Why is the black hole visible? What am I looking at exactly and why is this picture-taking a big deal that took a whole team of very smart people to accomplish?

  2. John Q. Public, on April 11, 2019 at 3:22 pm said:

  3. Bonald, on April 11, 2019 at 4:34 pm said:

    Hi Josh,

    It’s difficult because the horizon has an angular size of only about 20microarcseconds (about 5 billionths of a degree, unless my arithmetic is wrong). Nevertheless, the black hole at the center of M87 is, along with Sgr A* (which they also tried to image, but problems with obscuring gas turned out to be worse) the biggest black hole in the sky (meaning not biggest in actual radius, but in the angle taken up in our field of vision, which also depends on distance). That is to say, unlike ALMA taking images of protoplanetary disks and LIGO detecting gravitational waves from merging black hole binaries, this is not something that can become routine, not something we can do with other systems. To image even this black hole took a planet-wide array of telescopes. (Using interferometry, data from them can be combined, allowing us to do some of the things one could do with a planet-sized dish.)

    The reason one can see something from the neighborhood of M87’s black hole is that the black hole is accreting, i.e. gas is orbiting the black hole, heating up, glowing, and spiraling in. It’s the gas that one can see.

  4. josh, on April 12, 2019 at 3:01 pm said:

    Thank you for the reply. So I am looking at a kind of composite picture from a lot of photos, and not just a colorized output from a computer algorithm like I feared. This is actually what a black hole looks like. Neat.

  5. Bonald, on April 12, 2019 at 6:57 pm said:

    Hi Josh. That’s essentially right, with two caveats.

    1) The telescopes measure millimeter-wavelength radiation, which is somewhere in the microwave. This works better than visible light for various reasons, but it’s still real data.

    2) The total picture is a composite, but none of the components would have shown much. When they are combined with phase information intact (this is why the combination is an intensive calculation), radiation measured at different points on the Earth interferes constructively or destructively, and the much higher-resolution picture emerges. This process has been used in radio astronomy for a long time, and the principle is uncontroversial.

    It does rub me the wrong way this bragging I hear about the amazing computer analysis they did. Long baseline interferometry is an intensive and difficult process, but it’s supposed to be a computationally straightforward one. The more things hinge on clever data analysis, the less I trust the result. I am actually somewhat relieved that they failed on Sgr A*.

  6. c matt, on April 23, 2019 at 4:29 pm said:

    I see that image and the first thing that comes to mind is the Hand of God grasping the giant blurry doughnut just above a Starbuck’s Venti.

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