Black Holes, Wormholes, and time travel
Black Holes
Black holes are regions of space where the pull of gravity is so strong that nothing, not even light, can escape. If light cannot escape from black holes, that means that they are invisible. The only way for scientists to identify a black hole is by studying stars to see if they are orbiting black holes. Scientists think that the smallest black holes can be the size of an atom, but have the mass of a mountain [13]. The other two types of black holes are stellar black holes and supermassive black holes. The mass of a stellar black hole can be up to twenty times more than the mass of the Sun, whereas a supermassive black hole can have the mass of more than one million Suns combined.
Scientists believe that the smallest black holes formed when the Universe began and that supermassive black holes formed at the same time as the galaxies in which they reside. Stellar black holes, on the other hand, form when a big star collapses in on itself, causing a “supernova.” Every galaxy contains a supermassive black hole at its center. Yes, even the Milky Way Galaxy. The black hole at the center of our galaxy is called “Sagittarius A” and has a mass of about four million Suns. But don’t worry! This black hole is too far away from Earth for anyone to worry about falling in.
Sagittarius A compared to the Sun
Even though the chances of any humans encountering a black hole in this lifetime are extremely, extremely slim, morbid curiosity cannot stop many from wondering, What would happen if I got sucked into a black hole?
"Spaghettification"
The outer boundary of a black hole is its “event horizon.” Once a human, or anything, passed that point, there would be no going back. A human immediately past the event horizon would mind-blowingly be able to see out of the black hole, but to anyone else watching, she’d be gone. The scientific term for what would physically happen to a human body in a black hole is “spaghettification.” For example, if this hypothetical black hole explorer were to dive headfirst into said black hole, she would feel the pull of gravity much more at the top of her head than at the tips of her toes. She would be stretched and stretched until she became a stream of subatomic particles [14].
On the bright side, supermassive black holes have less extreme surfaces, so if this explorer fell into a black hole the size of the Solar System, she would theoretically be able to see what Einstein was talking about: the curve of space-time. As she fell, she would approach the speed of light, which would cause time to slow down. Looking forward into the black hole, she would see everything that has ever fallen into it, and looking backwards, she would see everything that would fall into it [15]. In short, she would “...get to see the entire history of that spot in the universe simultaneously, from the Big Bang all the way into the distant future” [16]. We have now arrived at the topic of time travel.
Wormholes
The Wormhole Theory says that a theoretical passage through space-time could create shortcuts for journeys through space. The idea of a wormhole first came to be in 1916. Wormholes were thought of as time reversals of black holes. In 1935, Einstein and Nathan Rosen expanded this theory, proposing “bridges” that connect two different points in space-time, supposedly reducing travel time and distance. The good news is that Einstein mathematically predicted the existence of wormholes, but the bad news is that none has been found. The other bad news is that wormholes are predicted to be way too small—microscopically small. The other, other bad news is that wormholes are extremely unstable and collapse very quickly. The only way to stabilize them is to use “exotic” matter, but no one knows what exotic matter would do to humans. Even if scientists found a wormhole and found a way to stabilize it, no one knows what would happen to any human who might travel through it.
A model of "folded" space-time and a wormhole bridge
13. "What Is a Black Hole?," NASA, last modified August 21, 2018, accessed December 7, 2020, https://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-a-black-hole-k4.html.
14. Natalie Wolchover, "What Would Happen If You Fell into a Black Hole?," Live Science, last modified April 13, 2012, accessed December 7, 2020, https://www.livescience.com/19683-happen-fall-black-hole.html.
15. Wolchover, "What Would," Live Science.
16. Wolchover, "What Would," Live Science.