Black holes

Source: https://iopscience.iop.org/journal/2041-8205

This week, the first photograph of a black hole was released. Although, technically, it’s a visualization of a whole load of data, not an actual photo. The difference isn’t that important. You can see the result above. It looks a bit like a blurry donut. So what’s up with this?

General relativity

Over a hundred years ago, Albert Einstein published his general theory of relativity. This theory relates space, time, and momentum. Basically, space and time curve because of high gravity and large relative speeds.

One of the implications of this theory is that a sufficiently large mass could curve spacetime to ‘infinity’. This is what has become known as a black hole.

The birth of black holes

Stars are giant nuclear fusion reactors that meld elements into heavier elements. The fusion reaction starts with hydrogen, converting it to helium, and onwards. Massive stars become onions of different types of fusion reactions, with heavier elements being fused deeper in the core.

At some point, this reaction can no longer sustain itself. Part of the star collapses in on itself, while outer layers are shed. In larger stars, this process is sudden and violent, causing a supernova.

If the star is large enough, the collapse crushes enough mass together to cause a black hole to form.

Light and black holes

The most notable feature, and the one for which black holes get their name, is that the mass inside is so large that even light cannot escape its gravitational pull.

This has some profound implications. Nothing in the universe can go faster than light. As a consequence, if light cannot escape the black hole, nothing can. So if something goes into a black hole, it doesn’t come out again – well, except through Hawking Radiation, but I’m not going to explain that here.

Event horizon

Since gravity lessens with distance, light passing beyond a certain range can escape the gravity of a black hole. You can look at this as golf balls going past the end hole. If the golf ball gets close enough it will fall in, but beyond a certain range it will pass by. The range at which light can not escape a black hole is called the event horizon.

The thing about the event horizon is that anything that goes closer to the black hole than this range, will not be coming out. Since nothing, including light, can escape, it is physically impossible to see inside the event horizon. That is why it’s called that. We cannot see any events past this horizon.

The picture above

Since nothing can escape a black hole, what is that picture above? Well, as you can see, it’s a donut with a dark center. That dark center is where we cannot see. Around the black hole, rings of energy form that we can see.

The picture was generated by combining radiation data from a number of radio telescopes on and around the Earth. The black hole depicted, is a supermassive black hole at the center of the Messier 87 galaxy. It has a mass several billions of times larger than our sun. The scale of the thing is hard to wrap your mind around.

A final word on time

As I’ve explained, spacetime curves to infinity around black holes. The implication is that light cannot escape. Since the speed of light is constant (theory of relativity), the implication is that the passage of time inside a black hole is very different to that outside. The closer you get to a black hole, the more time contracts. To an outside observer, the moment something passes the event horizon, the flow of time for it effectively stops.

What this means for time inside the event horizon is unknown. We can’t look inside, and so, we just don’t know. Given that gravity will tear everything apart that gets close, we won’t be sending a probe in any time soon to find out, either. It does provide interesting options for telling time travel stories, of course.

Conclusion

Black holes are what the name says: holes that we can’t see inside. They are massive, and intriguing, and now we have a picture of what they look like. Or, more accurately, what it looks like around them.

Author: Martin Stellinga

I'm a science fiction and fantasy writer from the Netherlands

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