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Why telescopes before event horizon telescope were not possible to take such picture of black hole that EHT taked?

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    $\begingroup$ Obligatory XKCD. Note the relative size of the event horizon of the giant black hole in the center or our galaxy. M87 is much farther away. $\endgroup$ – mmesser314 Jun 18 at 0:53
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Because, from our vantage point, it's very, very, very, very, very tiny.

The black hole is huge in absolute terms, but the distance from it to us is vastly huger: the horizon diameter (defined as twice the Schwarzschild radius) is about 36,000 Gm, about 3 times that of Pluto's orbit; and due to the gravitational effects its apparent size is magnified further to that of a black spherical object of diameter of around 94,000 Gm emplaced at the same distance. But the distance to the galaxy is about $5.0 \times 10^{14}\ \mathrm{Gm}$ - far larger, by ten orders of magnitude.

A simple trigonometric calculation shows that the apparent size is then

$$\text{apparent size} \approx \frac{94\thinspace000\ \mathrm{Gm}}{5.0 \times 10^{14}\ \mathrm{Gm}} \approx 0.2\ \mathrm{nrad}$$

that is nanoradians - for comparison, a bacterium held at arm's length (about 1 m) may occupy 1000 nrad or more in your own eye's field of view. So the black hole appears, to an observer on Earth, on the order of 5,000 times smaller than a bacterium held at arm's length.

There is a common misconception that the chief purpose of "ordinary" telescopes is to magnify - this is not so much the case: their real purpose is to brighten, that is, to gather up light and form an image that is brighter than what the human eye can see on its own. That's why they're so big. In fact, most of the astronomical objects one sees those pretty pictures of are actually not phenomenally "small" from our point of view, but rather they are only modestly small with their surface brightnesses very dim, which renders them invisible to the eye.

This black hole, on the other hand, DOES require massive magnification - on the order of a billion times, as you can see from above, making it more than a typical electron microscope, only as a telescope - to see. And that is very, very hard to do.

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  • $\begingroup$ Nice answer, but awkward choice of units.... Gigameters? Nanoradian? They may be strict SI compliant but unintuitive. Why not derived from lightyears and degrees/arcminuts/arcseconds which are way more familiar in astronomical context? $\endgroup$ – Thriveth Jun 18 at 18:42

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