Episode #125 of the Stack Overflow podcast is here. We talk Tilde Club and mechanical keyboards. Listen now
78

I was surprised too when I first heard they were trying to image M87's black hole. The short answer is because it's really, really big. It is 1500 times bigger (diameter) than our Sagittarius A*, and 2100 times farther away. This makes its apparent size about 70% of that of Sgr A*, which they are also attempting to image. A cursory search of wikipedia's ...


54

You have already got some good answers, but I'll just try to provide one more intuitive solution on why the event horizons will never separate again if overlapping each other: First, imagine a speck of dust that comes inside the EH of a black hole. I believe we'll agree this speck can never escape the black hole, because nothing can come back from behind the ...


52

If the event horizons ever touch and become one continuous surface, their fate is sealed - the two black holes will merge all the way in. They can never separate again, no matter what. There are several possible ways to explain it, with varying degrees of rigorousness. An intuitive explanation is that escape velocity at the event horizon equals the speed ...


42

It is not true that "objects float around" in the solar system. Perhaps you have seen video from the space station, and you can see things floating. This is not because there is no gravity, but because everything in the space station going at the same speed in the same direction. This makes it look as if things are floating. In fact the space station and ...


40

There was a mention of Sagittarius A* during the Q+A portion of the press conference; the team indicated that they hope to produce an image sometime in the future (although they were careful to make no promises, and they're not assuming they'll be successful). That said, I'm not wholly surprised that we ended up seeing M87, rather than Sgr A*, for a couple ...


33

There are a few criteria necessary to see a black hole with the Event Horizon Telescope. They are, in importance: Active Feeding: you need a thick accretion disk with lots of matter accreting onto the black hole. M87 fits this criteria, and is a glut, consuming about 90 Earth masses a day. Apparent size. Even though it is 53 million light-years away, M87 ...


33

To help with James K's excellent answer, a visual representation might help. Let's look at a thought experiment - Newton's Cannonball. Let's say you have a cannon, high enough that it's being held above Earth's atmosphere. You fire it, and it falls to Earth a little ways away ("D" in the below diagram). You fire another one with more power so it's moving ...


27

We have reasonably good measurements of the mass of Sagittarius A*, thanks to measurements of the movements of stars like S0-2 over several decades. It's been well-established that the mass of the central object is $M\approx4\times10^6M_{\odot}$; this alone is fairly good evidence for a supermassive black hole, and we can constrain the size of the object ...


26

Presumbably we rotate beacuse of the BH. No. The galaxy is being held in one piece due to its own total gravity. The black hole is only a small fraction of that. Basically, the BH doesn't matter. When the black hole dies in our galaxy The BH will probably be the last thing left of our galaxy at the end. And even then it will take some incredibly long ...


22

The problem with trying to form a black hole with dark matter is that dark matter can only weakly interact (if at all) with normal matter and itself, other than by gravity. This poses a problem. To get dark matter concentrated enough to form a black hole requires it to increase its (negative) gravitational binding energy without at the same time increasing ...


22

Answer: Not much The Milky Way's central black hole (BH) masses about 5 million suns, while the galaxy masses 100 billion to a trillion suns. Consequently, the central BH is pretty much irrelevant to the dynamics of stellar orbits except very close to the center. But what do you mean by "the black hole dies"? Do you mean evaporates through Hawking ...


21

Not at all a dumb question. As you have heard, it is true that time is affected by gravity. The stronger the gravitational field, the slower time passes. If you're far from any gravitating matter, time passes "normally". But to answer your question, we must specify what is meant by "the black holes's time" (let's call the black hole $\mathrm{BH}_\mathrm{Sgr\...


21

It is quite correct that a black hole has so much mass that light cannot escape from a region around the black hole. The edge of this region is called the event horizon. If you cross an event horizon you are never coming back. That applies equally to light, and matter. Around the black hole there may be matter in orbit. Since the Black hole has such strong ...


21

I've found an explanation in Dutch here by Heino Falcke, one of the EHT founders. Translation: Hard to photograph It was easiest to take a picture of M87. "It is very difficult to photograph the black hole in our Milky Way, because the material around it moves very fast: the vortex rotates around its axis in 20 minutes. Compare it to a toddler who ...


19

Ok, gotta quote XKCD on this. This is not how space works: This is: Gravity in low Earth orbit is almost as strong as gravity on the surface. The Space Station hasn't escaped Earth's gravity at all; it's experiencing about 90% the pull that we feel on the surface. To avoid falling back into the atmosphere, you have to go sideways really, really ...


18

As Ingolifs says, Sgr A* and M87* are the obvious candidates. At the press conference, Heino Falcke explained why they got a picture of M87* first: But it would take some more time because Sagittarius A Star is 1000 times faster and smaller. Its like a toddler who is moving constantly. In comparison, M87 is much slower, like a big bear. — The ...


16

Stellar mass black holes form from the collapse of massive stars at the end of their lives. You can then find them scattered throughout galaxies, just like you find massive stars. They typically have a mass a few times the mass of the sun. Supermassive black holes are found at the centers of galaxies. They typically have mass of millions of Suns. Recently ...


16

The main problem is angular momentum. In order for two gravitationally bound objects to merge (whether black holes, supermassive black holes, planets, stars, etc.), they must shed enough angular momentum for their orbital separation to become small enough. Average orbital separation (semi-major axis) is determined entirely from the angular momentum of the ...


15

Nothing "escapes" a BH - in the sense that a signal originating inside the event horizon remains forever inside. If something is observed moving away from the BH, then it was generated outside the event horizon. If it was generated inside, it would never be observed at all, forever and ever. Gravity itself does not "escape" a BH - and neither does "not ...


15

No, it would not, because it operates in the visible spectrum and the EHT is an array of radio telescopes. For the "very long baseline interferometry" technique to work, all the telescopes have to be operating at the same wavelength, because combining the signals involves measuring exactly how well the peaks and troughs of the radio waves from the different ...


13

Absolutely nothing left. The time for stellar black holes to evaporate is said to exceed the proton half life. How much more the galactic black holes. And by the way, this time is currently increasing as even stellar black holes are currently growing from the cosmic background radiation alone. The universe must pass through the intermediate phase of black ...


13

Yes, there are galaxies with two supermassive black holes in the center, see for instance 4C +37.11 Most likely such galaxies are formed by collision and merger of two galaxies, and their cores have not yet merged. Source


12

When it comes to accretion disks, nothing is coming out of the black hole. That's just orbiting matter, though it is swirled around a bit by frame dragging. Even at high gravity, the ability to orbit around a massive body still exists. The gravitational force is already being "used up" to cause the orbiting (it accounts for the centripetal force), so there ...


12

The light comes from well outside the black hole event horizon. Matter cannot fall into a black hole without first losing most of its angular momentum (otherwise it would just continue to orbit the black hole). This is accomplished by the outward transfer of angular momentum by viscosity (and other means) in an accretion disc surrounding the black hole. As ...


12

Under General Relativity (GR) alone, a Black Hole's (BH's) event horizon is a point of no return -- anything that passes through the event horizon is lost and gone forever, and nothing comes out. Hence, under GR alone, BHs are utterly black and don't have a temperature at all. This is why the absorption of radiation (or anything else) by a BH doesn't raise ...


12

Question: How are these images obtained? Later in the video the narrator says they took the images using ESO's VLT. 03:40 [Narrator] 14.​ Making these measurements pushed the power of ESO’s Very Large Telescope to the limits. (Source: ESO transscript) Over the whole observation period multiple telescopes and imaging instruments were used. Early ...


12

Another quick note - They are trying to get a photo of Sag. A*: From Space.com The project has been scrutinizing two black holes — the M87 behemoth, which harbors about 6.5 billion times the mass of Earth's sun, and our own Milky Way galaxy's central black hole, known as Sagittarius A*. This latter object, while still a supermassive black hole, is a runt ...


9

The mass of a black hole is not infinite. In fact, if a black hole is created that is big enough to survive evaporation, its mass will be its starting mass, plus any mass swallowed up, minus the radiation that leaves it. Which is why you hear phrases such as "A black hole with ten times the mass of our sun" or in the case of a supermassive black hole, "......


9

No, the galactic magnetic field is very weak, about 0.1nT. It is able to bend the trajectory of highly-energetic charged particles and also to align dust grains across the magnetic field. However, is too weak to affect the rotation of a galaxy. Although the origin of galactic magnetic field is not clear yet, the supermassive black holes do not ...


9

As pointed out by Rob Jeffries, forming a black hole (BH) from dark matter (DM) is impossible (unless there is a [hypothetical] interaction by which dark-matter can lose energy that evades all detection). Accreting DM into an existing BH is still unlikely (since DM cannot lose its excess energy and angular momentum as easily as gas), but not impossible and a ...


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