What is a black hole?
A black hole is an astronomical body so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. The boundary of no escape is called the event horizon. In general relativity, a black hole's event horizon seals an object’s fate but produces no locally detectable change when crossed. In many ways, a black hole acts like an ideal. Black body as it reflects no light.
What does it look like?
Black holes themselves are invisible; they emit virtually no light and cannot be seen directly. But we have developed several ways to find them anyway.
If material is falling into a black hole, it travels at such high speeds that it gets hot and glows very brightly, and we can detect that. Scientists hope to use this method to learn a lot more about how and what black holes are.
We can find black holes by watching the movements of visible objects around them. For example, a black hole's gravity is so strong that nearby stars will orbit around it, so we can look for stars behaving strangely around a patch of space.
How do black holes form?
One way to make a black hole is to have a massive star collapse at the end of its life. Prof Subramanyan Chandrasekhar was the first to calculate that when a massive star burns up all its fuel, it will collapse. The idea was ridiculed at first, but other scientists calculated that the star continues forever to fall inward toward its center, thus creating what we call a black hole.
In a finding ripped from interstellar scientists say humans can indeed explore black holes, firsthand. A human can do this only 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. Grinnell College physicists explained,
That’s because of special physics found in supermassive black holes, resulting in a combination of gravity and event horizon that wouldn’t instantaneously pull the human being into a very dead piece of spaghetti.
Its gravitational pull could cause sphagettification, essentially stretching a person into an infinitely long and infinitely thin noodle.
But if a small black hole were to pass through a person. It could send a fatal supersonic shockwave akin to a gunshot wound, destroying tissue and pulling interconnected cells apart as it makes its way through the body.
Then again, a person could die from a black hole’s radiation, which would feel like a fireball burning from the inside out.
The 1990s transformed into a boon for black hole discoveries. It soon became clear that several type of black holes exists, or must exist. The most abundant type must be stellar black holes, like Cygnus X-1, that form from the deaths of massive stars. These stellar black holes must exist in the millions, even in our Milky Way galaxy. But they are very hard to find. Detecting involves favourable circumstances and their involvement with other luminous objects, as in a binary system. Soon, however, commenced an explosion of discoveries of another abundant type, the supermassive black hole. These form at the supermassive blackholes. These form at the centre of most galaxies, and they range from millions to billions of times the mass of the sun. The Hubble space telescope uncovered the evidence of countless supermassive blackholes. And we know that one Sagittarius A* exists in the center of our own Milky Way.
Based on their understanding of the early cosmos, astronomers also believe that primordial blackholes emerged soon after the Big Bang. Some 13.8 billion years ago, these are the tiny black holes that may have been the size of an atom. The smallest primordial black holes have doubtless evaporated, but the largest ones may still exist.
And in recent times, evidence has emerged for a fourth class, so-called intermediate mass black holes. These objects range from hundreds to hundreds of thousands of solar masses, and evidence for a small number of these objects is emerging. They may form in unusual cosmic areas, such as environments crowded with stars from mergers of stellar blackholes or from some other phenomenon.
Stretching time
When space is stretched, it’s time. A clock that is near a massive object will tick more slowly than one that is near a much less massive object. A clock near a black hole will tick very slowly compared to one on Earth. One year near a black hole could mean 80 years on Earth, as you have seen illustrated in the movie Interstellar.
In this way, a blackholes can be used to travel to the future. If you want to jump into the future on Earth. Simply fly near a black hole and then return to Earth.
If you get close enough to the centre of the blackhole, your clock will tick more slowly, but you should still be able to escape, so long as you don’t cross the event horizon.
Loops in time
What about the past? This is where things get truly interesting. A black hole bends time so much that it can wrap back on itself.
Imagine taking a white sheet of paper and joining the two ends to form a loop. That’s what a black hole seems to do in time.
This creates a natural Time Machine. If you could somehow get into the loop, which physicists call a closed time-like curve, you would find yourself on a trajectory through space that starts in the future and ends in the past. Inside the loop, you would also find that case and effect get hard to untangle. Things that are in the past cause things to happen in the future, which in turn cause things to happen in the past.
References
- science.nasa.gov
- profound.physics.com