"Never look down to test the ground before taking your next step; only he who keeps his eye fixed on the far horizon will find the right road." -Dag Hammarskjold

One of the great discoveries of the past few decades was that of a supermassive black hole at the center of our Milky Way. No longer was it mere conjecture or unverified theory; observations in the X-ray, infrared, radio, and of stars orbiting a central, non-luminous point all indicate the presence of a 4 million solar mass black hole at a location known as Sagittarius A*.

At a distance of 26,000 light years, an object as small as this black hole's event horizon -- even at 23 million km in diameter -- would be unresolvable to a telescope the size of an entire country. But thanks to the very clever technique of very long baseline interferometry, the proposed Event Horizon Telescope has the capabilities, for the first time, of imaging a black hole's event horizon directly.

Find out how this is possible, and what our prospects for finding out are, on this week's Ask Ethan!

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Nice little article Ethan. IMHO black holes are really really interesting.

What would we actually see though? Just a black circle with a distorted starfield behind it?

"You see, there are certain wavelengths of light, particularly the radio and the X-ray, where either the black hole can become very bright momentarily, or the objects passing near the black hole could illuminate the event horizon by back-lighting it."

This gives the impression the event horizon itself would reflect light which is, I assume, not the case. What becomes bright is the accretion disk around it and the back-lighting would just show the outline, right? Also, would it look smaller than it really is because of the bending of light? Larger?

"What would we actually see though? Just a black circle with a distorted starfield behind it?"

And the accretion disc. What else would you expect to see? Elephants?

"This gives the impression the event horizon itself would reflect light "

Nope, backlighting means the light comes from behind. If it were to reflect from there, it would go away from you and not be visible.

@JollyJoker #2: What we _should_ actually see (keep in mind that this is at radio and microwavelengths, NOT visible light!) is a truly black (i.e., zero emission) region, surrounded by a bright, distorted ring corresponding to the accretion disk. The third figure on this page shows the expected results for two different models.

If the object at Sgr A* is really a black hole, then the event horizon neither reflects nor emits any radiation at all, at any observable wavelength. If we were reasonably close to it (a few hundred light years, for example), then it would look very much like a circular hole in the sky (check out "Bok globules" (https://en.wikipedia.org/wiki/Bok_globule) for a visual analogue), surrounding by a hot, glowing accretion disk.

Whether we observe any radiation from the disk of the hole, will allow us to set limits on alternative models or formulations of gravity. There are models where Sgr A* could be just a really dense physical object, in which case it would have a true _surface_ which could absorb, emit, and reflect radiation. The EHT, by resolving the disk, can help to either reject or confirm those models.

@Wow #3 I was thinking about "illuminating the event horizon" which gives the wrong impression, imo.

@Michael Kelsey #4 Thanks. There are a bunch of different gifs on the net showing quite different views of black holes

http://jila.colorado.edu/~ajsh/insidebh/lensearth_640x480.gif

https://media.giphy.com/media/frPNVXlCZZxU4/giphy.gif

Do we know anything about the orientation/size/brightness of Sagittarius A*'s accretion disk (compared to the size of the event horizon?) If the latter gif is a real simulation, it looks like the event horizon is very small compared to the bright accreting stuff around it. That's of course a stellar black hole instead of a galactic center one.

"@Wow #3 I was thinking about “illuminating the event horizon” which gives the wrong impression, imo."

Not when it says specifically and in black and white "backlit".

If you're going to pick words and ignore others, then the problem isn't the "confusing" sentence, it's you. Put the blame where it belongs and nobody will fault you for it.

Relativity tells us that an observer falling through the event horizon of a black hole won't notice anything unusual at that point. However, an observer away from the event horizon will see time slowing down for the falling astronaut, and will in fact observe the person falling closer and closer to the event horizon, but never actually passing it. In this case how do you get the black hole mergers that are meant to have created Sagitarius A* - the supermassive black hole at the centre of our galaxy? Surely from the perspective of an outside observer no two black holes can ever merge?

Think of two soft putty balls being smooshed together.

The sides that coalesce are distorted by the pressure of the other, and they look to merge. The constituents however remain separate and don't (appreciably) mix like two drops of water coalescing.

Another way to imagine it is that they are flat disks of flappy paper being pushed toward each other. They meet and the two pieces of paper ride up when they meet, pointing vertically upward at a greater and greater angle.

Now look at it from above.

Two circles that meet and merge.

But we know it's "really" just twisting out of the way.

I read articles but nowhere I have seen gravity structure or it's interaction with nuclei. I have seen endless coverage of observed effect of gravity. This has one out come. Lots of assumption. From them they have made a plausible picture and explanation. With this picture they explain the universe. Solar system creation. Now with the new technology they gather information and make it fit to the picture. then like a child satisfying smile after having forced the square shape into the round hole. publish the result and expect applause. like invited audience of game show every one give standing aviation, but I can not because I know they are wrong. if they knew gravity interaction with nuclei they would rewrite their science books. my information will not fit into current picture. That is why I have been able to give information and later confirmed by space crafts past 40 years. Latest being Rosetta, Pluto and Ceres. They cal it discovery I call it re-inventing the wheels. If you know any where I can find structure of gravity, photon, electron and nuclei please let me know. I need to compare it with mine. In 1980s I sent schematic diagram of photon to White house and prime ministers office, but mankind dose not have technology to observe it. MG1

"I read articles but nowhere I have seen gravity structure or it’s interaction with nuclei. "

Because the coulomb repulsion between two electrons is 36 orders of magnitude bigger than their gravitational attraction.

It's not even a rounding error on the biggest desk calculator.

Hell, the IEEE floating point values of the supercomputers which use 128-bit floats doesn't preserve that level of accuracy in the mantissa.

"This has one out come. Lots of assumption."

Yes, you certainly LOVE leaping to the conclusion, don't you?

"my information will not fit into current picture."

Ah, the lorn-filled cry of the lesser spotted crank looking for a mate.

"That is why I have been able to give information and later confirmed by space crafts past 40 year"

But unable to manage English grammar.

"I need to compare it with mine"

Yeah, good luck. You need a scheme yourself before you can make a comparison. You can't just pretend to have one.

"I sent schematic diagram of photon to White house and prime ministers office, but mankind dose not have technology to observe it."

No, every human, ear enough, has eyes and the Mk1 Eyeball has been able to discern diagrams ever since the first scribing tool was invented.

@Waterbergs #7 & Wow #8

It follows that if we could get a close look at a black hole, we could see all the things it "ate", including black hole mergers. That would handily answer some questions about how supermassive holes form!

Not really.

Any more than when looking at a galaxy a billion light years away tells us what's happening there now.

We'd have to infer from what we know about how things are to deduce anything. That's a little circular, though.

We would have to see one forming to be sure how they form.

@Wow Thanks for the analogies there. Does this mean the event horizon of the blackholes gets squashed out of spherical into some increasingly strange shape? As I understand it Sagitarius A* has a mass of about 4 million solar masses. If one assumes each blach hole contributed around 20 solar masses then this means 200 000 black holes merged to create it. It must be a total mess of "immiscible" event horizons.

"Does this mean the event horizon of the blackholes gets squashed out of spherical into some increasingly strange shape?"

Both yes and no.

In the same way as "Does a triangle with three straight sides get distorted into some strange shape if you draw it on a sphere".

"It must be a total mess of “immiscible” event horizons."

Don't extend analogies beyond their applicability.

They aren't putty balls. The event horizon is a geometrical construct that is a perfect (well, for non rotating black holes with little or no net charge) sphere PROSCRIBED ON A NON EUCLIDIAN SURFACE. When we attempt to visualise that sphere in non-euclidian space, we don't see a sphere any more.

And there were TWO analogies. Why pick that one? Because you can make it "weird" by extrapolating that one, but not the other?

It still IS a sphere, but our perspective is interpreting it differently.

The sun looks like a flat disk.

It IS a sphere.

But our perspective makes it look circular, not spherical.

Still puzzled by the "immiscible" event horizon thing. If one black hole will never fall into another from our persepctive then we have 200 000 odd objects of some shape or other packed together yet never quite touching - immiscible. No extention of analogies here, just some physics. It must have some sort of appearance, I'm just wondering what that is.

"Still puzzled by the “immiscible” event horizon thing."

Since you're the only one who thinks it exists and is even talking about it, feel free. But what do you expect anyone else to do about that?

Tell you what, go off, sit down, and be puzzled in your own time. If you work it out, then fine, if you don't, then that is also fine.

But what anyone can do about something you've invented yourself that you find puzzling when you've already decided not to take the advice of "stop thinking that such a thing exists" is just be puzzled at why you're telling anyone about it.

Personally, I only read the bit I quoted, since it is sufficient to tell me that there's nothing I can do about something you invented yourself to be puzzled about, therefore there's unlikely to be anything worth reading later on.

Whatever it was, I hope you're not puzzled about it.

@Waterbergs: I think your problem is what is called "reification" -- you have taken the name of something which is a _description_, and you have interpreted it as being a _thing_ (some sort of material object). That's just wrong, and once you get past that misinterpretation, then I think you'll be able to resolve your own "puzzlement."

An "event horizon" is not a thing. It describes a particular region of space near any dense mass.

It is the place where the radial coordinate of spacetime becomes purely timelike (that is, the direction pointing toward "r=0" also points toward "t->infinity". Depending on the mass distribution, what this place is shaped like can be very different.

Most objects have their event horizon locations buried deep within their interior, in a non-physical way. A few objects (the ones we call black holes) happen to have their event horizon locations exterior to themselves, in which case we can see their effects.

If you are really good at solving four-dimensional coupled partial differential equations (or have access to suitable software), then you can work out for yourself the shapes of event horizons for different distributions of mass. For a static, sufficiently dense mass distribution of any shape (sphere, cube, pyramid, whatever), it turns out that the event horizon is a spherically symmetric region around the center of mass of the distribution.

For more complicated systems, such as, let's say, a pair of dense masses near, but not too near, one another, the event horizon forms a dumbell shaped region. As the two masses get closer and closer together, they eventually get close enough to fall into the first category above, and the event horizon location becomes spherically symmetric.

For a non-static mass, such as a very dense sphere rotating very, very fast, the event horizon region can become toroidal (donut shaped) instead of spherical.

Thanks Michael, very helpful. OK, so the event horizon is "not a thing". However, the mass distribution is. What does that look like for a super massive black hole that has been formed from 200 000 smaller ones?

@Waterbergs #18: The mass distribution is irrelevant. See my fourth paragraph of #17 (ignore the spurious carriage-return), "For a static, sufficiently dense mass distribution of any shape (sphere, cube, pyramid, whatever), it turns out that the event horizon is a spherically symmetric region around the center of mass of the distribution."

Basically, once the constituents have gotten close enough together (which does *not* have to be within each other's event horizons), the event horizon of the whole system becomes spherical.

At that point, you can't know what the mass distribution inside is (that's part of what the event horizon does -- it hides the details).

He also took "think of soft putty" as real rather than analogy. He could well be back being "puzzled" about what you described and reifying something in your description erroneously.

Two balls of putty smooshing together shows how the result can be something bigger than either on their own. The two-circles-of-paper thing doesn't change size of the merger. However, the paper visual analogy shows the multi-dimensional topological changes of a merging BH system whilst the putty visualisation doesn't.

And he thinks (or claims anyway) that taking the putty idea, which IRL won't mix, means that the event horizons are imiscible,like they were putty (and, as you point out, real).

Quite why the paper analogy was dropped in favour of being "puzzled" is a question open to conjecture, since Waterberg isn't really forthcoming on that point.

Dear Michael. Thanks again for the helpful reply. However, I am really wanting to understand the dynamics as two black holes approach each other from a substantial distance. For example, if a test mass falls towards a black hole it will appear to asimtotically approach the event horizon of that black hole, but never pass it. The dynamics there seem clear, though please do correct me if I am wrong. If a black hole falls towards another black hole from a distance, then how do the dynamics compare with the former case? What does the equivalent time dilation look like? You have described the case where two black holes already have a joint dumbell shaped event horizon, but what precceded that, and how did time dilation fit into it?

Get a BSc or MSc in maths, then. You'll need it. And stop wasting everyone else's time.