A person falling into a black hole and being stretched as he approaches the horizon of the black hole. Credit: Leo Rodriguez and Shanshan Rodriguez, CC BY-ND
To solve the mysteries of black holes, a human simply has to venture into them. However, there is a rather complicated catch: a human can only do this if the respective black hole is supermassive and isolated, and if the person entering the black hole does not expect to report the findings to anyone in the entire universe.
We are both physicists studying black holes, albeit from a very safe distance. Black holes are among the most common astrophysical objects in our universe. These intriguing objects seem to be an essential ingredient in the evolution of the universe, of the Big Bang up to the present day. They likely influenced the formation of human life in our own galaxy.
Two types of black holes
The universe is dotted with a huge zoo with different types of black holes.
They can vary in size and be electrically charged, just as electrons or protons are in atoms. Some black holes are actually spinning. There are two types of black holes that are relevant to our discussion. The first does not rotate, is electrically neutral – that is, not positively or negatively charged – and has the mass of our sun. The second type is a supermassive black hole, with a mass of millions to billions of times that of our sun.
In addition to the mass difference between these two types of black holes, what also distinguishes them is the distance from their center to their “event horizon” – a measure called radial distance. The event horizon of a black hole is the point of no return. Anything that passes this point will be swallowed up by the black hole and disappear from our known universe forever.
The distance from the center of mass of a black hole to where gravity is too strong to overcome is called the event horizon. Credit: Leo and Shanshan, CC BY-ND
On the event horizon, the black hole’s gravity is so powerful that no amount of mechanical force can overcome or counteract it. Even light, the fastest moving thing in our universe, cannot escape – hence the term ‘black hole’.
The radial dimension of the event horizon depends on the mass of the respective black hole and is the key for a person to survive if they fall into it. For a black hole with a mass of our sun (one solar mass), the event horizon has a radius of just under 2 miles.
The supermassive black hole in the middle of us Milky Way Milky Way Galaxy, on the other hand, has a mass of about 4 million solar masses, and it has an event horizon with a radius of 7.3 million miles or 17 solar rays.
So a person falling into a star-sized black hole will get much, much closer to the center of the black hole before crossing the event horizon, rather than falling into a supermassive black hole.
This implies, due to its proximity to the center of the black hole, that the pull of the black hole on a person will differ from head to toe by a factor of 1,000 billion times, depending on which one is leading the free fall. In other words, if the person falls first, the force of gravity at their feet will be exponentially greater as they approach the event horizon of a black hole of a star mass, compared to the jerk of the black hole at their head.
The person would experience spaghetti and likely not survive if stretched into a long, thin noodle-like shape.
As the person approaches the event horizon of a black hole the size of the sun, the large difference in gravity between the person’s head and toes causes the person to stretch into a very long noodle, hence the term ‘spaghettification’. Credit: Leo and Shanshan Rodriguez, CC BY-ND
Now, a person falling into a supermassive black hole would reach the event horizon much farther from the central source of gravity, meaning the difference in gravity between head and toe is close to zero. Thus, the person would pass through the event horizon unaffected, not be stretched into a long, thin noodle, survive, and float painlessly along the horizon of the black hole.
A person falling into a supermassive black hole would likely survive. Credit: Leo and Shanshan Rodriguez, CC BY-ND
Other Considerations
Most of the black holes that we observe in the universe are surrounded by very hot discs of material, usually consisting of gas and dust or other objects such as stars and planets that got too close to the horizon and fell into the black hole. These discs are called accretion discs and are very hot and turbulent. They are certainly not hospitable and would make traveling to the black hole extremely dangerous.
To enter it safely, you should find a supermassive black hole that is completely isolated and does not feed on surrounding material, gas, and or even stars.
Now, if a person found an isolated supermassive black hole suitable for scientific research and decided to venture into it, anything observed or measured inside the black hole would remain within the black hole’s event horizon.
If you consider that nothing can escape gravity beyond the event horizon, the falling person would not be able to send information about their findings beyond this horizon. Their journey and findings would be forever lost to the rest of the entire universe. But they would enjoy the adventure, as long as they survived… maybe….
Written by Leo Rodriguez, Assistant Professor of Physics, Grinnell College and Shanshan Rodriguez, Assistant Professor of Physics, Grinnell College.
Originally published on The Conversation.