Exploring Black Holes

A black hole is, as the name suggests, a "black hole." The etymology is said to have come from a casual remark made by the American physicist John Wheeler in the 1960s. To put it simply, a black hole is a celestial object from which even light cannot escape due to its own gravity.

To put it simply, it's a state where an incredibly massive star can no longer support itself against its own gravity and collapses inward, becoming ultra-high density. When that happens, even light cannot escape from within a certain distance from the center point. If light cannot escape, it means all information cannot escape. That is a black hole.

That's right. Currently, it is understood as "a region where spacetime is cut off as a result of the distortion of spacetime caused by gravity in terms of 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, there was a person in the late 1700s named John Michell who calculated a region from which light could not escape as a result of Newtonian mechanics. At that time, the name "black hole" didn't exist yet.

Black holes come in huge and small sizes. I understand that what you recently discovered, Professor Oka, is a medium-sized black hole 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 because it cannot support its 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 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. And since quite a bit is lost during the process of stellar 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.

What is one solar mass in kilograms?

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

About 60 candidate objects for 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 galaxies and elliptical galaxies, and it is said that there are black holes at their centers ranging from a million to tens of billions of times the mass of the Sun.

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

For example, if you drop something, it breaks. In the case of gas, this becomes heat. Since objects with heat are hot, they radiate infrared, visible light, ultraviolet, X-rays, and so on. If we interpret it as seeing that, the consistency is very good. Therefore, it is currently believed that there are such massive black holes at the centers of galaxies.

So the black hole itself is invisible, but the things around it are glowing and visible.

Exactly.

Quasars are very bright stars, but they are extremely far away. To appear bright from such a distance, they must emit many times more light than a galaxy. So, when explaining where that light comes from, it's easier to explain if you bring in things like black holes or accretion disks, which are rotating disk-shaped gases, as the energy source.

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

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

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

That's exciting.

For example, the central core of the Milky Way has 4 million solar masses, but the radius of the event horizon, the region surrounding the black hole, is 0.1 astronomical units. It's 0.08 something.

One-tenth of the Earth-Sun distance. 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 surroundings are large.

Yes. The black hole itself is dark and invisible, but from what is happening in the vicinity, we are forced to infer that a black hole is there.

What you call the "three-piece set" are 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 that, you have to keep dropping things in. In other words, the things to be dropped in must always be right next to the black hole.

The 60 objects called black hole candidates in the Milky Way are all what we call close binary systems, where there is a normal star very close by. It orbits the black hole and constantly drops surface gas into it.

Most of the black holes that have been found are actually binary systems, aren't they?

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

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

Moreover, there is a prediction that there are actually many such things. Although there are 60 black hole candidates found currently, there is a prediction that there should actually be over 100 million.

We understand that stellar-mass black holes are the final form of stars, but we don't know how supermassive black holes were formed. However, several scenarios are being considered.

For example, in the early universe, a very large mass of gas became a black hole as it was. Or, stars formed in the early stages underwent massive explosions, leaving black holes at their cores, and those things grew. Among the growth theories, there is a theory that they grow by sucking in gas, and another theory that black holes grow by merging with each other.

Your theory is the latter, right?

Yes. My observation results 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 core but the entire galaxy.

For example, in the Milky Way, there is a round, bulging structure in the center called the bulge. There is observational data showing that the weight of that bulge and the central black hole are proportional in a relatively good 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 are merged to steadily grow the massive black hole at the galactic center.

Mergers might not occur with black holes alone, but may involve the stars that were their hosts—in other words, star clusters. A scenario was proposed in the early 2000s where if star clusters also merge at the core, the member stars of those clusters contribute to the formation of the bulge, while the black holes merge, making the core larger and larger.

Is there one supermassive black hole in each galaxy?

Yes. When you plot black hole mass and 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 any galaxy grew in that way.

However, an intermediate process is necessary for that, but this theory was completely missing the intermediate process. There are massive black holes from millions to tens of billions of solar masses. And there are also things from a few solar masses to about 20 solar masses. But there was nothing in between in our Milky Way.

And that is the intermediate-mass black hole.

I am the one claiming that (laughs). I believe that intermediate-mass black holes appear as an intermediate stage where smaller ones merge in the process of creating supermassive black holes.

What is the origin of those?

First, stars are often born in the form of star clusters. A well-known one is the Pleiades star cluster, also known as "Subaru." That one isn't very dense, but there are things called globular clusters where tens of thousands of stars are tightly gathered in a sphere.

The centers of such things have a very high stellar density, and the possibility of stars merging with each other has been pointed out. When stars merge, they become very heavy stars, and very heavy stars explode quickly and leave behind black holes. Heavy things, in what is called a self-gravitating system, tend to gather more and more at the center.

So they accumulate.

Light things are flung outward, and heavy things accumulate more and more at the center. Then, the heavy things at the center repeatedly merge. It is said that intermediate-mass black holes then 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, X-ray objects represented by Cygnus X-1 are fixed in everyone's minds as a stereotype.

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

That means the temperature is high.

They aren't emitted unless it's around 10 million degrees Celsius. That's why "Cygnus X-1," which was first called a black hole candidate, was found by an X-ray astronomy satellite.

However, gravitational waves were recently detected. Since that is an observation of black holes merging, we have actually seen black holes merging and growing. If that is repeated, what is created is an intermediate-mass black hole.

It was only recently that gravitational waves were observed, right?

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

They are all very far away. Because the range we are looking at is so wide, 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 we've only just started being able to detect them in these last two years. So, it's also that only the extremely large-scale ones are being seen, right?

That's right. They have to be large-scale and detected several times a year to be recognized. It means they couldn't be found unless we searched a range as wide as about 1.3 billion light-years.

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

Yes. Actually, it was on New Year's Day in 2016 that I published the 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 slightly.

However, about two weeks after the press release for the first paper, the LIGO gravitational wave announcement happened. So, our announcement was overshadowed (laughs).

Ah, I see. So 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 the so-called event horizon, but that's just an invisible boundary from which nothing can emerge—like a "do not enter" line. You can't conceive of someone living 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 going 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 it was boring (laughs).

As the next stage after a neutron star, "quark stars" are hypothesized.

Is that so?

Yes, if the density becomes even higher than that of 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.

So they can avoid becoming black holes?

Yes. For now, the surface maintains a size large enough to exist in 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 stand.

So there's still more to it.

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 if there's something with physical dimensions inside or not.

So if you keep making it smaller, you can create a high-density star without it becoming a black hole.

Which means it has a surface.

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

That's more interesting. I shouldn't just say whatever I want, 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 you could shrink the Sun 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 shrunk it to a radius of 9 millimeters.

About the size of a one-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 space faster than the speed of light.

We have some idea of how an entrance might be formed, but the exit is the difficult part. How would one even create it?

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

Theoretically, that was proposed very early on. However, it hasn't been observed, and we don't really know how to create 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 theoretical framework. Wormholes connecting black holes and white holes have also been considered, but according to Kip Thorne, they aren't very stable. Also, there's the theory that black holes themselves evaporate.

So they disappear. This is Dr. Hawking's theory again, right?

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

There were people saying it was dangerous because CERN (European Organization for Nuclear Research) might be able to create them in an accelerator.

I think academic study at a university is more interesting when it's somewhat detached from the practical world, but I think people in the business world are interested in whether black hole research is actually useful for anything.

I get asked that a lot. But in those cases, I can only smile and answer, "It's not useful for anything at all" (laughs).

But everyone uses location data 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 data would be off by several to over ten kilometers.

Whether it's general relativity or special relativity, there are many instances 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 isn't it like doing what people did during the Age of Discovery when they made world maps, just on a massive scale? Asking where we are, what kind of place we live in, and how it was made—these are fundamental human interests. I feel like you're mapping out human knowledge.

For example, there's the idea that if we could do accretion disk power generation, it would be amazing.

I think effectively utilizing nuclear fusion at the center of stars 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 in terms of energy utilization. For example, if there were something like a micro black hole battery (laughs).

I can't even imagine that. It would be terrifying.

You wouldn't want to put something like that nearby?

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 "just 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 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. "Makes you happy"—that's a nice way to put it.

Earlier, we talked about the growth of galaxies and the relationship with the bulge. The reason the Andromeda Galaxy has such a beautiful shape might also be related to the black hole at its center, right?

At the moment, the consensus is that it's not related.

It's not related?

Even if you have something with 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 of an effect. The overall structure seems to be determined by things like the total angular momentum, the rotation, the amount of gas, and so on.

What about dark matter?

Dark matter as well.

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

It seems there is this "dark matter" in the universe that we cannot recognize and that doesn't interact with observations 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 speed of rotation, 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 ten times more of it than the visible mass like stars and gas.

So, you can estimate the weight of a galaxy using a certain method, but when you look at the results, even if you add up everything we know, there shouldn't be that much mass, so something must be hidden—that's dark matter.

Yes. Black holes and neutron stars were also candidates for it. That hasn't been completely ruled out, but there are some observational facts that make it look unlikely. So, for now, the candidates for dark matter are some unknown particles that have mass but don't interact in other ways.

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

It would cause a lot of problems if it didn't. 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 form of energy.

Since the 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, Matsumoto-san, how do you teach 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. Generally, it's often said that high school astronomy classes are all about calculations, which makes students dislike the subject. However, when results are derived through actual experiences like observations, and the goal of what we are about to understand becomes clear, they really get hooked.

But 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 reactions like, "I'll try looking for it tonight." So, I make an effort to provide information and have them experience it when there's something they can observe directly.

Lately, there are so many beautiful astronomical photos. Don't people feel a bit disappointed when they 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 mind. Knowledge allows you to see what is invisible. Isn't that the case with the radio waves you work with, Oka-san?

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

Visualizing how the gas is moving in your head (laughs).

Yes. But not everyone is going to become an astronomer, so I think it's enough 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, imaging technology has advanced so much that you can easily photograph quite faint celestial objects like the Orion Nebula or globular clusters right in the middle of the Hiyoshi area.

We also do astronomical observations where I stay overnight at the school with the students, and we have quite a lot of fun chatting away.

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

There is a team doing meteors, and they are currently using radio observation for them. You only need to buy one receiver. That way, you can record them even if it's raining or during the day, so you can tell how many meteors are passing through. Though it only tells you the number.

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 the type to be moved by Saturn. When I was little, I had a vague desire to become 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 went from vaguely space, to vaguely radio waves, to vaguely the center of the Milky Way, and before I knew it, I was into 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.

We look forward to your continued success, Oka-san.