Black Holes: What They Are and What They're Not

Japan Forward

4 days ago

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Almost everyone has heard the term "black hole" — it's one of the most recognizable concepts in modern science. But with that familiarity comes a lot of misunderstanding. While some misconceptions are too technical to unpack without advanced knowledge, this article focuses on several common ones about black holes that can be explained relatively clearly.
People often describe a black hole as "a hole in space-time." Even experts sometimes use this phrase, but it's just a metaphor.
In reality, a black hole has a surprisingly simple structure. It consists of only two parts: the singularity, where all the black hole's mass is compressed into a single point, and the event horizon that surrounds it. The event horizon isn't a physical substance like a membrane or mist. No matter how closely you look, there's nothing that resembles a surface.
A black hole isn't literally a hole or a vortex, and it's not a traditional celestial object. It's better understood as a region of space-time with extreme properties.
One of the most striking features is that beyond the event horizon, space behaves like time. It "flows" only inward toward the singularity, just as time only moves forward for us. This one-way flow is what gives the black hole its "hole-like" reputation.
So, the metaphor of a "hole in space-time" likely comes from this defining feature: a region of severely distorted space-time from which nothing can return.
If "observing a black hole" means directly detecting radiation from the singularity or the event horizon, then this idea is mostly correct. Hawking radiation, the thermodynamic radiation of black holes, is too weak to be detected for the foreseeable future, so it can be ignored in this discussion.
In practice, though, observing a black hole usually means finding evidence of its presence through indirect methods. In that sense, there are several reliable ways to do it.
The most common method is to observe electromagnetic radiation, such as strong X-rays or radio waves. The black hole itself does not emit radiation, but it pulls in a large amount of matter, usually gas or dust.
As the material spirals inward, it heats up due to friction and compression, producing intense radiation. While other cosmic objects can also emit radiation, the extreme brightness and compactness of the source often point to a black hole.
In the case of supermassive black holes, we can even map the surrounding radiation in enough detail to image the black hole's "shadow." The first image of this kind was captured by the Event Horizon Telescope (EHT), a global network of radio observatories. In April 2017, the EHT imaged the supermassive black hole at the center of the galaxy M87 in the Virgo constellation. The image was released to the public on April 10, 2019.
To observe a black hole this way, there must be nearby matter to interact with. But since space is mostly empty, black holes with visible material around them are relatively rare. That's why many remain hidden from direct observation.
Fortunately, newer indirect methods have made it possible to detect more of these hidden black holes. One is gravitational lensing, where a black hole bends the light from more distant stars. Another is the detection of gravitational waves, which are ripples in space-time produced when black holes collide.
These techniques have opened exciting new paths in astrophysics, helping scientists better understand black holes and the structure of the universe.
The idea that black holes are dangerous probably comes mainly from science fiction. However, in reality, black holes don't indiscriminately suck in or tear apart everything nearby.
It's true that black holes have incredibly strong gravity, but that's mainly because their mass is packed into an extremely small space. In fact, their compactness allows matter, and even light, to get much closer to the center than with other objects of the same mass. Stars or planets have physical surfaces or atmospheres that prevent such close approach. (©Sankei)
In fact, if the Sun were suddenly replaced by a black hole of the same mass, Earth and the other planets would continue orbiting just as they do now. We'd lose sunlight, which would be catastrophic for life, but Earth wouldn't be pulled in or torn apart.
Whether something falls into a black hole depends on how close it is and whether it can change its speed or direction. As long as it stays outside the event horizon — the point of no return — it can still escape. That's why we can observe light and matter swirling just outside black holes.
There's also a common idea that anything near a black hole gets stretched and ripped apart, a process nicknamed "spaghettification." This effect is real, but it mostly applies to smaller black holes. In those cases, tidal forces — differences in gravity across an object — become extreme just a few hundred kilometers from the center. A person or spacecraft getting too close would be torn apart long before reaching the event horizon.
However, for supermassive black holes, which are millions of times the mass of the Sun, you wouldn't be torn apart or feel any discomfort even near the event horizon. In fact, you might not notice anything unusual at all as you cross that boundary.
The reason for this big difference lies in the gravitational field around the black hole. For ordinary celestial bodies ike Earth, the difference in gravity over such a small distance is too weak to notice. In fact, even over a small distance, like from your toes to your head, there is a slight difference in gravitational strength.
But near a black hole, where gravity grows stronger the closer you get to the center, the more significant this difference becomes. The varying strength of gravitational pull across an object can become so extreme that it stretches and tears the object apart. Again, the distance from the black hole at which these extreme forces occur depends on the black hole's mass. (©Laura A Whitlock, Kara C Granger & Jane D Mahon)
The distance from the singularity to the event horizon, called the Schwarzschild radius, is also determined by the black hole's mass. The Schwarzschild radius grows much more rapidly than the distance at which extreme tidal forces begin to emerge.
Because of this difference, the larger the black hole, the safer it is to approach — up to a point. Once you cross the event horizon, there's no coming back. And the deeper you go, the stronger the tidal forces become. Eventually, even in a supermassive black hole, those forces would tear you apart before you reached the center.
Larger objects like stars don't fare any better. Even supermassive black holes can shred them before they reach the event horizon. So, if you're planning a trip near a black hole, leave the stars behind — and whatever you do, don't fall in.
Because black holes are often described as objects in space, it's easy to picture them having a solid, dark surface. But as explained earlier, a black hole isn't really a celestial object. It's more accurate to think of it as a region of space-time with extreme properties.
As mentioned before, you can actually get quite close to a large black hole without immediately being affected. But even up close, you wouldn't see a wall, a membrane, or a swirl of darkness.
The event horizon — the point of no return — has no visible surface and gives no physical warning. If you crossed it, you wouldn't feel anything special. No bump, no jolt, no sudden shift. In fact, you might not realize you've passed it at all. But once you do, escape becomes impossible. You'd be on a one-way path toward the singularity.
If black hole tourism ever becomes a thing, it's safe to assume there'd be clear warnings posted: "Do Not Enter: Black Hole Ahead ." The gravity would already be distorting your view of space around you, but without a visible marker, you wouldn't be able to tell where the event horizon actually is.
The rumor that particle accelerators could create black holes and destroy the Earth began during the construction of CERN's Large Hadron Collider (LHC). The idea even shows up in some science fiction stories, so you may have heard it before. But given that Earth is still intact, we can safely say this fear is unfounded. A particle accelerator at CERN. (©Maximilien Brice)
The concern arose because the LHC is capable of producing particle collisions at extremely high energies. Furthermore, some theoretical models also propose the existence of "extra dimensions" beyond the four we experience. If these extra dimensions exist and are larger than expected, it's theoretically possible — though extremely unlikely — that tiny black holes could form in these collisions.
Furthermore, this scenario depends on several optimistic assumptions. First, we don't yet know if extra dimensions exist. And even if they do, the conditions needed to produce black holes are probably not met by the LHC. More importantly, if the LHC could create black holes, nature would already have done so.
That's because cosmic rays, which are high-energy particles from space, routinely strike Earth's atmosphere with far more energy than the LHC can generate. These natural particle collisions have been happening for 4.6 billion years, all over the planet. If high-energy collisions could destroy Earth, it would have happened long ago.
Even in the unlikely event that the LHC did create a tiny black hole, it wouldn't be dangerous. According to theory, it would vanish almost instantly due to a process called Hawking radiation. Even if Hawking radiation turned out not to occur, any black hole produced would be traveling so fast that it would escape Earth's gravity and fly off into space.
And if, against all odds, such a black hole somehow stayed trapped by Earth's gravity, it would be smaller than an atom and would absorb almost nothing as it orbited through the planet. By the time it finally settled at Earth's core, millions or billions of years later, the Sun would have reached the end of its life, likely engulfing or incinerating the Earth long before any black hole could do serious harm. NASA Science Editorial Team. （Aug 13, 2019） "Shedding Light on Black Holes". NASA
Sara Rigby. （Mar 30, 2021） "7 black hole 'facts' that aren't true". BBC Science Focus.
Amanda Bauer & Christopher A Onken. "Black hole truths, myths and mysteries." Australian Academy of Science.
Author: The Sankei Shimbun
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