Exploring Black Holes

The name "black hole" suggests a "black hole" in space. The term was reportedly coined in the 1960s by American physicist John Wheeler. Simply put, a black hole is a celestial object whose gravity is so strong that not even light can escape from it.

To put it simply, a massive star becomes unable to support its own gravity and collapses inward, reaching a state of ultra-high density. When this happens, not even light can escape from within a certain distance of the center. Since light cannot escape, no information can escape either. That is a black hole.

That's right. Today, it is understood as a region where spacetime is cut off as a result of the distortion of spacetime by gravity, according to general relativity. However, the concept of a region from which light cannot escape doesn't actually require general relativity. It's possible even in Newtonian mechanics.

In fact, in the late 1700s, a man named John Michell calculated the existence of a region from which light could not escape based on Newtonian mechanics. At that time, the name "black hole" didn't exist yet.

Black holes come in both giant and small sizes. I understand that the one you recently discovered, Professor Oka, is a medium-sized one called an intermediate-mass black hole.

When a star more than about 30 times heavier than the Sun undergoes a supernova explosion and can no longer maintain its shape against gravity, it becomes a black hole. These are called stellar-mass black holes, and their formation process is theoretically well understood.

Those are the smaller ones.

However, the mass of the black hole formed at that time cannot exceed the weight of the original star. So far, no stars heavier than 200 solar masses (200 times the Sun) have been found, so stars heavier than that cannot be created. Furthermore, since a significant amount of mass is lost during the star's evolution, it seems that black holes formed this way are at most around 20 solar masses.

Our Sun is too small to become a black hole.

It's not even in the running.

How many kilograms is one solar mass?

It's 2 × 10^30 kilograms. We just covered that in high school class (laughs).

About 60 candidate stellar-mass black holes have been found in the Local Group of galaxies, including our Milky Way, but even the largest is only 16 solar masses.

On the other hand, it is said that supermassive black holes exist at the centers of galaxies. There are many galaxies in the universe, such as spiral and elliptical galaxies, and it is believed that their centers contain black holes ranging from a million to tens of billions of times the mass of the Sun.

For example, there are celestial objects called quasars, which used to be called quasi-stellar objects. These are the prime examples, and it has become clear that there are extremely bright point-like objects at the centers of many galaxies. To explain the release of this vast amount of energy, the theory of gravitational energy release when matter falls into a black hole has become the leading explanation.

For instance, if you drop an object, it breaks. In the case of gas, this becomes heat. Hot objects radiate infrared, visible light, ultraviolet, and X-rays. If we interpret what we see as that, it's very consistent. Therefore, it is currently believed that such giant black holes exist at the centers of galaxies.

So, while the black hole itself is invisible, the objects around it are glowing and visible.

Exactly.

Quasars are very bright stars, but they are extremely far away. To appear so bright from such a distance, they must emit many times more light than a whole galaxy. To explain where that light comes from, it's easier to use the concept of energy from a black hole or an accretion disk—a rotating disk of gas.

There is one promising candidate for the supermassive black hole at the center of the Milky Way, which is an observable radio source called Sagittarius A* (A-star). It is very faint.

However, the motion of bright stars around it has been observed. The radio source itself is faint and invisible, but the stars are moving in a way that requires something very heavy to be there. You can see them in elliptical orbits, like planets orbiting the Sun, which is called Keplerian motion.

There are over 90 stars whose orbits have been observed, and a star called S2 is predicted to pass closest to the object suspected to be the black hole this year. We might see relativistic effects there.

That's exciting.

For example, the nucleus of the Milky Way has a mass of 4 million solar masses, but the radius of the event horizon—the region surrounding the black hole—is only 0.1 astronomical units. It's about 0.08 something.

One-tenth the distance between the Earth and the Sun. 15 million kilometers. The mass is incredible, but the size is actually quite small.

That's right.

And that's just the radius of the event horizon.

The black hole itself might be even smaller. In short, even if the black hole itself is small, the surrounding area is large.

Yes. The black hole itself is dark and invisible, but we are forced to infer its presence from what is happening in its vicinity.

What you call the "three-piece set" consists of the black hole, the accretion disk, and the jet.

If there is an accretion disk, it means something is being pulled in. However, to create one, you have to keep feeding it matter. In other words, the matter being pulled in must always be right next to the black hole.

The 60 black hole candidates in the Milky Way are all part of what we call close binary systems, where an ordinary star is very close by. It orbits the black hole and its surface gas is constantly being pulled into the black hole.

So most of the black holes that have been found are actually in binary systems.

But what I found is a black hole candidate that is not in a binary system.

So it's very significant because you found a "stray black hole" that isn't in a binary system.

Moreover, there are predictions that there are actually many such objects. While there are only 60 black hole candidates found so far, it's predicted that there should actually be over 100 million.

We know that stellar-mass black holes are the final state of stars, but we don't know how supermassive black holes were formed. However, several scenarios have been proposed.

For example, a massive cloud of gas in the early universe might have become a black hole directly. Or, stars formed in the early universe might have undergone massive explosions, leaving black holes at their cores, which then grew. Among the growth theories, there is the theory that they grow by sucking in gas, and the theory that they grow by merging with other black holes.

Your theory is the latter, right?

Yes. My observations support the scenario where black holes grow by merging with each other. In that scenario, the growth of the central black hole affects not just the nucleus but the entire galaxy.

For example, in the Milky Way, there is a rounded, bulging structure in the center called the bulge. There is observational data showing that the mass of the bulge and the mass of the central black hole are proportional in a fairly consistent relationship. Therefore, it is thought that the bulge and the black hole evolved together. Evolving together means that both grew. That's where the merger theory comes in. It's a scenario where black holes merge to continuously grow the massive black hole at the galactic center.

Mergers might not happen with black holes alone, but could involve the stars that were their hosts—in other words, star clusters. A scenario was proposed in the early 2000s where star clusters also merge at the nucleus; the member stars of the cluster contribute to the formation of the bulge, while the black holes merge, causing the nucleus to grow larger and larger.

Is there one supermassive black hole in every galaxy?

Yes. When you plot black hole mass against bulge mass for various galaxies, a correlation is shown that falls neatly on a line. So, it's thought that the black hole at the center of every galaxy grew in that way.

However, that requires an intermediate stage of growth, and this theory was missing that intermediate stage entirely. We have massive black holes ranging from millions to tens of billions of solar masses. And we have ones ranging from a few to about 20 solar masses. But there were absolutely no intermediate ones in our Milky Way.

And those are the intermediate-mass black holes.

I'm the one arguing for that (laughs). I believe intermediate-mass black holes appear as an intermediate stage in the process of smaller ones merging to create supermassive black holes.

What is the origin of those?

First of all, stars are often born in the form of star clusters. A well-known one is the Pleiades, or "Subaru." That one isn't very dense, but there are things called globular clusters where tens of thousands of stars are tightly packed together.

The centers of such clusters have a very high stellar density, and the possibility of stars merging has been pointed out. When stars merge, they become very heavy, and very heavy stars soon explode, leaving behind black holes. Heavy objects, in what is called a self-gravitating system, tend to gather toward the center.

So they accumulate.

Lighter objects are flung outward, while heavy ones accumulate at the center. Then, the heavy objects at the center repeatedly merge. It is said that this causes an intermediate-mass black hole to grow within the star cluster.

Are there black holes inside globular clusters too?

Since about 10 years ago, several theories have emerged suggesting there might be black holes in globular clusters. However, there are also strong opposing opinions because no X-ray sources have been found. When people think of black hole candidates, the fixed idea in everyone's mind is X-ray objects like Cygnus X-1.

X-rays aren't emitted unless the energy is quite high, right?

Meaning the temperature is high.

They aren't emitted unless the temperature is around 10 million degrees Celsius. That's why Cygnus X-1, the first object to be called a black hole candidate, was found by an X-ray astronomy satellite.

However, gravitational waves were recently detected. Since those are observations of black holes merging, we've actually seen black holes merging and growing. If that process repeats, what you get is an intermediate-mass black hole.

Gravitational waves were only observed recently, right?

Two years ago. Since they've been found one after another right after the detectors started operating, it seems they really are everywhere. The sight of black holes merging to form intermediate-mass black holes is being confirmed several times a year.

They are all very far away. Because the range we're looking at is so vast, the first black hole merger event that caused gravitational waves was 1.3 billion light-years away.

Gravitational wave detectors like LIGO and Virgo were only recently completed and have just started detecting things in the last two years. So, it's also possible that we're only seeing the extremely large-scale events.

That's right. Because the scale is so large, you can't recognize them unless they are detected many times a year. It means they couldn't be found without searching a range as wide as 1.3 billion light-years.

So this announcement provides a tailwind for you, Professor Oka.

Yes. Actually, it was New Year's Day in 2016 when I published a paper in The Astrophysical Journal Letters suggesting that an intermediate-mass black hole might be located here. The second one was the recent paper in Nature Astronomy in September 2017.

The first paper was a "suspicion," with the title starting with "Signature of," and the second paper ended with the word "candidate." The degree of certainty has increased a bit.

However, about two weeks after we issued a press release for the first paper, the announcement of LIGO's gravitational waves came out. So, our announcement was overshadowed (laughs).

Ah, I see. That's what happened.

Even so, it was picked up quite a bit by overseas media and featured reasonably well. This year's paper gained an even greater reputation abroad. That's probably because the reach of Nature Astronomy was so powerful.

From a sci-fi writer's perspective, black holes are actually boring (laughs). That's because they have no surface. There is what's called the event horizon, but that's just an invisible boundary where things stop coming out—like a "no-entry" line saying "do not go beyond this point." So, you can't conceive of a story where someone lives on the surface (laughs).

A long time ago, Hal Clement wrote a novel called "Mission of Gravity," which imagined a planet with about 700G. But because it was rotating, the poles were 700G while the equator was about 3G, and the story involved people entering from there and interacting with the inhabitants.

Around 1980, Robert Forward wrote a work ("Dragon's Egg") about special intelligent life forms living on a neutron star with a surface gravity of 70 billion G. But you can't do that with a black hole, so I thought they were boring (laughs).

As the next stage after a neutron star, something called a quark star is hypothesized.

Is that so?

Yes, if the density becomes even higher than a slushy mass of neutrons, it's thought there might be stars that are a slushy mass of quarks.

It means they could exist in the universe in the state of a quark star before becoming a black hole, but this hasn't been confirmed yet.

Can they avoid becoming a black hole?

Yes. For now, the surface maintains a size large enough to be exposed to the universe as a quark star. However, there is a lot of uncertainty in that area, so theoretical calculations aren't perfect yet. So, neutron degeneracy pressure isn't necessarily the final resort.

So there's still more beyond that.

Even at higher densities, there's a possibility it can be supported. However, if it becomes too small, whether it's supported or not, the moment the event horizon is exposed, it's already a black hole. At that point, it doesn't matter whether there's something with physical dimensions inside it or not.

If you keep making it smaller and smaller, you can create a high-density star without it becoming a black hole, right?

Which means it has a surface.

It means the surface exists without being hidden by the event horizon.

That's more interesting. I shouldn't speak so carelessly, though (laughs). On the other hand, the Sun won't even go that far; its final stage will probably be a small star called a white dwarf.

It maintains its size because it's supported by electron degeneracy pressure, but it's said that if for some reason even the Sun were shrunk to a radius of about 3 kilometers, it could become a black hole. In the case of the Earth, it would become one if you made its radius 9 millimeters.

About the size of a 1-yen coin? (laughs)

On the other hand, recently there's talk of mini black holes or micro black holes. In Japanese sci-fi, there's a story where someone puts a micro black hole in their ochazuke and eats it. They say it feels tingly (laughs).

Also, in Star Wars-style movies, black holes, white holes, and wormholes are often set up as tools for traveling through the universe faster than the speed of light.

I kind of understand how to make an entrance, but making an exit is difficult. I wonder how one would create it.

The idea behind white holes is that they can't just keep sucking things in; since things go in, they must be coming out somewhere else (laughs).

Theoretically, that was proposed very early on. However, they haven't been observed, and we don't really know how to make one. You can make a black hole by crushing a star, but no one knows how to make a white hole yet.

The possibility of their existence cannot be denied. It's allowed within the framework of the theory. Wormholes connecting black holes and white holes have also been considered, but according to Kip Thorne, they seem to be quite unstable. Also, there's the theory that black holes themselves evaporate.

That they disappear. This is again about Dr. Hawking, right?

Micro black holes, like those thought to be created in particle accelerators, are so small that they are said to evaporate immediately.

Some people were saying it's dangerous because CERN (European Organization for Nuclear Research) might be able to create them with an accelerator.

I think university academics are more interesting when they are as detached from the practical world as possible, but I think people in the business world are interested in whether black hole research is useful for anything.

I get asked that a lot. But in those cases, I have no choice but to smile and answer, "It's not useful for anything" (laughs).

But everyone uses location info on their smartphones, and you couldn't do that without relativity. Satellites orbit at quite high speeds, so if you don't apply corrections based on relativity, GPS location info would be off by several to over ten kilometers.

Whether it's general relativity or special relativity, there are many cases where fundamental physics is useful, but astronomy is about things like "there's a black hole at the center of a galaxy hundreds of millions of light-years away" (laughs).

But aren't you doing the same kind of thing people did when they made world maps during the Age of Discovery, just on a massive scale? Wanting to know where we are, what kind of place we live in, and how it was made is a fundamental human interest. I feel like you're mapping the knowledge of humanity.

For example, talking about how amazing it would be if we could do accretion disk power generation.

I think effectively utilizing nuclear fusion at the center of a star would be safer and more stable. There's a possibility of that being realized in the near future. Black holes are quite difficult to control.

Actually, compared to nuclear fusion, black holes are supposedly one step more efficient at utilizing energy. For example, if there were something like a micro black hole battery (laughs).

I can't quite imagine that. I mean, it's scary.

You wouldn't want to put something like that near you?

No. If I said there was a black hole in my lab, no one would come near it (laughs). But I think the fact that "it's interesting" is everything.

Rather than forcing a justification, it's "I'm researching it because I want to know."

Exactly. I want to know, and you want to know too, right? Everyone probably has that kind of interest to some degree. When you find something like "Oh, there's that thing over there," it makes you a little happy.

Yes. "Making you happy"—that's nice.

Earlier, the growth of galaxies and the relationship with the bulge came up, but the reason the Andromeda Galaxy has such a beautiful shape might also be related to the black hole at its center.

As of now, the consensus is that it's not related.

It's not related?

Even if there's something about 4 million solar masses inside something as massive as Andromeda or the Milky Way, which are about 1 trillion solar masses, it doesn't have much impact. It seems the overall structure is determined by things like the total angular momentum, the rotation, and the amount of gas.

What about dark matter?

Dark matter is the same.

Dark matter, that's a good term (laughs).

It seems there is something called dark matter in the universe—dark material that we cannot recognize and that doesn't involve observation at all.

We know this because most galaxies, including the Milky Way, are rotating. By rotating, they can resist the tendency to collapse gravitationally. This allows them to maintain a certain state of equilibrium. This means that from the rotation speed, you can calculate how much mass is inside that galaxy.

When you calculate it, the numbers don't add up unless there is a significant amount of invisible mass accompanying the galaxy over a wide area. That is what's called dark matter. It seems there's about 10 times more of it than the visible mass like stars and gas.

You can estimate the weight of a galaxy using certain methods, but when you look at the results, even if you add up everything known, there shouldn't be that much mass, so something is hidden—that's dark matter, right?

Yes. Black holes and neutron stars were also candidates for it. That hasn't been completely ruled out, but there are observational facts that make it look unlikely. So, for now, the candidate for dark matter is some strange particle that has mass but doesn't interact with anything else.

We'd be in trouble if it didn't exist, wouldn't we?

We'd be in trouble in many ways. The ratio of dark matter in the early universe has also been calculated; dark matter must account for 23% of the total, and 70% must be dark energy, which is another mysterious dark force.

Since baryonic matter (heavy particles) like protons, neutrons, and electrons that make us up only accounts for 4% of the entire universe, we'd be in trouble without dark matter and dark energy.

By the way, Mr. Matsumoto, how do you go about teaching the appeal of astronomy to high school students?

Since it's Earth Science, we cover everything from the solid earth to meteorology, but I feel like students react more directly to astronomy compared to geology or meteorology. It used to be said that high school astronomy classes were all about calculations and just made people hate astronomy, but when results are derived through actual experiences like observations, and the goal of what we are about to understand becomes clear, they really bite.

However, the number of astronomy fans has decreased. The core enthusiasts have all become old men (laughs). Even among young people, if I bring up topics like a supermoon or the Geminid meteor shower, I get quite a few reactions like, "Then I'll try looking tonight," so I make an effort to provide information and have them experience it when there's an opportunity for direct experience.

Lately, there are so many beautiful astronomical photographs. Isn't there a bit of a feeling of disappointment when you actually look through a telescope?

When you look through a telescope, it's actually quite underwhelming (laughs). But as you get used to it, you can supplement it in your brain. Knowledge makes the invisible visible. Isn't it the same with the radio waves that you work with, Professor Oka?

When you're as obsessed as I am, you can roughly tell from the arrangement of the spectrum what kind of scene it would be in your head, but if everyone became like that, the world would be over (laughs).

With gas moving like this in your head (laughs).

Yes. But not everyone is going to become an astronomer, so I think it's fine if they experience it, enjoy it, and find happiness in it.

I was also moved when I first saw Saturn's rings through a telescope. It was just a small diamond shape, but I thought, "Oh, I see it!"

There is a 15cm refracting telescope on the roof of Keio High School as well. Nowadays, the progress in photography technology is amazing, and you can easily capture quite faint celestial objects like the Orion Nebula or globular clusters even in the middle of the Hiyoshi town area.

I also do astronomical observations by staying overnight at the school with the students, and we have quite a bit of fun chatting away.

For something like a meteor shower, you could lie down and count them.

There is also a team doing meteors, and currently, they are doing radio observations of meteors. You only need to buy one receiver. That way, you can record them even if it rains or during the day, so you can tell how many meteors are falling. Though you only get the count.

I see. Professor Oka, have you also been interested in celestial bodies since you were a child?

Yes. I had a small astronomical telescope bought for me, and I was one of those people moved by Saturn. When I was little, I had a vague idea that I wanted to be a scientist. Ever since I saw a large shooting star in junior high school, I thought, "Okay, I'll do space."

It started from a shooting star?

It somehow became space, then somehow radio waves, then somehow the center of the Milky Way, and before I knew it, it's become black holes.

The emergence of radio astronomy is a tremendous step forward for humanity, isn't it?

Several Nobel Prizes have come out of radio astronomy, after all.

I want to continue to expect great things from Professor Oka's activities.