What are black holes? How are they formed? What are they made of? Where do they go? These are some of the common questions people have when thinking or talking about the great devourers of the Universe.
In the following article, we'll take a quick look at these curious cosmic phenomena. We have tried to address some of the more common questions we've found about black holes. As such this should be considered akin to an FAQ.
Black Hole Definition
Black holes are generally defined as "a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because [the] matter has been squeezed into a tiny space." - NASA.
As light is unable to escape the holes' gravity it appears completely black - hence the name. They can, however, be 'seen' with some special 'jiggery-pockery' of data collected from a wide range of telescopes (more on this later).
How Are Black Holes Made And What Causes A Black Hole?
How black holes' form depends on their type and origin.
Current theory suggests that small black holes (some as small as an atom but with the mass of a large mountain) probably formed in the earliest seconds of the universe.
Stellar black holes (about the mass of 20 of our suns plus) are created when massive stars collapse in on themselves. This process warps space-time during their death throws.
This happens during supernova events when massive stars explode incredibly violently.
Supermassive black holes (Roughly 1 million of our Sun's mass plus) are thought to form as the galaxy they inhabit is formed.
A black holes' event horizon is its outermost boundary. This is the point at which the gravitational force precisely overcomes light's ability to escape the black hole's pull.
It is the literal point of no return - you cannot escape once you pass it.
At least that was the traditional view. The venerable Professor Stephen Hawking, during his life, was adamant that the definition of a black hole should be changed.
In his mind, he believed that event horizons, as they are traditionally understood, don't actually exist at all. They are, in fact, "apparent horizons" at the edge of black holes, where quantum mechanics goes crazy.
Here, like everywhere else, virtual particles pop in and out of existence causing the horizon to fluctuate, making it more of a flickering, growing and shrinking mess.
These "apparent horizons" are also a point where quantum effects create streams of hot particles that radiate back out into the Universe - so-called Hawking's radiation. It is thought that this will eventually cause the black hole to radiate away all its mass and disappear.
What Is A Singularity?
A singularity or gravitational singularity is a point at the very center of a black hole. It is a one-dimensional point that contains enormous amounts of mass in an infinitely small space.
Here gravity and density become infinite, space-time curves infinitely and the laws of known physics are thought to no longer apply.
Kip Thorne, the eminent American physicist, describes it as "the point where all laws of physics break down".
What Does A Black Hole Look Like?
As light cannot escape once past the black holes' event horizon they can't actually be 'seen' in a traditional sense. We can, however, infer their existence from their effects on other bodies in space (like Suns and gas clouds) we can see.
It might soon be possible to detect the boundary of the event horizon around the black hole - rather the Hawking's radiation emanating from it.
What Would Happen To You If You Fell Into A Black Hole?
So long as it's a supermassive black hole you wouldn't feel anything you'd actually be in freefall (what Einstein once called his "happiest thought"). You'd exist and then inevitably you wouldn't, your mass added to the ever hungry bulk of the black hole.
For an observer, however, its a very different story.
As you approach the event horizon you will appear to immediately accelerate, stretch and distort obscenely.
Interestingly you will appear to move in slow motion the closer you get to the horizon until you freeze (as if on pause).
Now for the fun bit, as you remain there motionless you will also begin to stretch across the surface of the horizon and as you start to heat up, you would also appear to become redder and redder.
You would then begin to slowly obliterate as you stretch across the curved space-time of the black hole. Time would appear to stop and the fire of Hawking's radiation will likely appear to engulf you.
Finally, you'd be reduced to ash before your remains would appear to plunge into the absolute darkness of the black hole proper.
A spectacular scene, to some gruesome, but one you would never see.
For smaller black holes you undergo a process commonly termed "spaghettification". This is a very different, and somewhat more disturbing, story.
What Is At The Center Of A Black Hole?
At the center of a black hole is something called a gravitational singularity, or singularity for short. This is where gravity and density are infinite and space-time extends into infinity.
This is the final destination for anything that strays too close to the black hole and disappears over the event horizon.
Just what the physics is like at this point in the black hole no-one can say for sure.
What Is The Closest Black Hole To Earth?
The closest black holes yet discovered to Earth are several thousand light years away from us. At this distance, these black holes will have no discernable effect on our planet or its environment.
To date, the nearest black hole, called V616 Monocerotosis, is 3,000 light years away and has a mass around 9-13 times that of our life-giving Sun. The next closest is Cygnus X-1 (about 6,000 light years away with a mass of 15 suns).
Next up is GRO J0422 + 32, which is actually one of the smallest yet 'discovered' and is roughly 7,800 light years away.
As far as we know the nearest supermassive black hole, Sgr A, to us sits in the middle of our home galaxy - The Milky Way. This monster is roughly 27,000 light years away from us.
You can 'find' it in the approximate direction of the Sagittarius constellation. Its enormous gravitational pull is currently busy tearing nearby stars to pieces, adding their mass to its own.
Our galaxy's supermassive black hole is estimated to be several million times (approx 4.1 million times to be precise) the mass of our sun. But don't worry its enormous distance from us doesn't directly affect our solar system - at least yet.
It is thought that in about 4 Billion years our galaxy will collide with our neighbor galaxy Andromeda. When this happens stars, and their respective black holes will be mixed together into a new blended galaxy.
Stars will start to interfere with each other's orbits likely sending some into the waiting, and always hungry, jaws of the two supermassive black holes. This will likely ring the death knell for many stars, and their child planets.
Don't worry just yet, the Sun will likely reach the end of its life long before this happens.
How Long Does It Take For A Black Hole To Die?
The lifespan of a black hole varies depending on its mass. You can only really know by running quantum field theory calculations in strongly curved space to find out - which is complex, to say the least.
As a general rule mass loss from Hawking's radiation occurs at different rates relative to the 'size' of the black hole. Interestingly lower mass black holes lose their mass quicker than larger ones.
This is because the curvature they create in space is more intense around their events horizons. But even so, it takes a very, very long time indeed.
By way of example, it is estimated it would take 10^67 years for a black hole with the Sun's mass to completely evaporate. For the larger black holes in the Universe, it would take an unbelievable 10^100 years.
These figures are much longer than the estimated age of our Universe, 13.8 times 10^9 years, but it's not forever. That means at the end of days, when all stars and planets have long since perished, black holes will dominate and eventually disappear themselves.
How Many Black Holes Are There In The Universe?
How long is a piece of string? How many grains of sand are there on a beach? How many stars are there in the Galaxy? These questions are nigh on impossible to answer.
The same is true for the number of black holes in the Universe, there are so many you couldn't ever hope to count them.
Even if we tried to count them we would never get the right answer as a large part of the Universe will be obscured from our view, forever. If such an attempt was made we would first need to limit our count to "our Universe" or what is more correctly called the "Observable Universe".
We can, however, make some educated guesses.
Stellar-mass black holes form from the supernovae of massive stars. Our Milky Way alone contains around 100 Billion stars and roughly one in every thousand stars is big enough to create a black hole when it dies.
This should mean that there might be as many as 100 million stellar-scale black holes in our galaxy. But this number is increasing with every second that passes.
New-stellar mass type black holes are thought to form once every second or so.
If we are talking about supermassive black holes these tend to lurk at the center of galaxies. In our local region of space, there are about 100 Billion supermassive black holes or thereabouts.
How Is It Possible To Detect A Black Hole?
Given the nature of these celestial phenomena, it's not actually possible to directly observe them with telescopes that rely on x-rays, light or any other form of EM radiation.
Rather, to find or detect them requires a bit of lateral thinking. They can be inferred by their gravitational impact on other nearby matter and objects.
A classic example would be if the black hole passes through an interstellar cloud. This event will draw matter inward towards the maw of the black hole in a process known as accretion.
Stars can also be deflected from their 'normal' motion if they pass near a black hole or, of course, can be torn apart.
In the latter scenario, the star's matter is accelerated as it moves towards the black hole and it emits x-rays into space.
"Recent discoveries offer some tantalizing evidence that black holes have a dramatic influence on the neighborhoods around them - emitting powerful gamma-ray bursts, devouring nearby stars, and spurring the growth of new stars in some areas while stalling it in others." - NASA.
You can also 'see' the perimeter of space that is close to the black holes' event horizon through something called the "lensing effect' or gravitational lensing.
You can also attempt to observe the black hole's Hawking's Radiation.
Can You Destroy A Black Hole?
As we've seen above you don't need to (if you could possibly live long enough) just wait for them to destroy themselves. But it might be theoretically possible to destroy a black hole artificially.
It turns out that black holes might actually have an Achilles heel - their event horizons. Physicists have theorized that if we could feed it and/or charge we might be able to reverse its inherent inequality.
This would, in turn, cause the black hole to dissipate and might just reveal its central singularity for all to see. But physicists admit they have no idea what the actual consequences of doing this would be.
What Would Happen If Two Black Holes Collide?
Black holes are naught but mass so if two (of equal mass) were to collide the result would be one new double-sized black hole. But the event would be incredibly violent.
Such an event would release enormous amounts of energy and send long ranging ripples in the very fabric of space-time - so-called gravitational ripples.
Although once the subject of science fiction and science theory astrophysics appear to have actually been able to detect or 'observed' just such an event occurring.
Do Black Holes Eventually Collapse?
The answer to this depends on your meaning behind the use of the term 'collapse'.
If by collapse the questioner means an end to the black hole then yes they do. Black holes can exist for a very long time but they are not 'immortal'.
Although they do die out over time it's not because they 'collapse' in the traditional sense of the word.
Black holes, namely their event horizons, become their very downfall. After they have consumed all matter around them that is possible they eventually evaporate as its energy and mass is sapped over time via Hawking's radiation.
If, however, we consider the meaning of collapse literally then the answer is very different indeed. Black holes are, in effect, the very definition of collapse.
They are the near-eternal process of collapse triggered by some of the mightest collapsing events in the history of time and space.
In this sense, black holes can do nothing other than collapse.
Do Black Holes Die?
Yes, they do, eventually. It takes a very long time indeed.
The process is a very slow one and requires the black hole to be starved of fresh matter from other celestial bodies nearby.
What Is A Black Hole Made Of?
Put simply we cannot really be sure. Black holes are by definition regions of spacetime where extreme gravitational forces prevent anything, including light from escaping.
In this sense rather than being made from something, they are, in fact, a result of the absence of visible stuff. It is known that the matter that falls into, and never leaves, these curious objects is no different from everything outside of them but that's about as precise as we can really be.
Once past the event horizon, as matter 'goes down the rabbit hole, the more and more our understanding of what's going on in there completely falls apart.
"Thanks to General Relativity, we think we understand what happens in this extreme gravity and, with the help of Quantum Mechanics, we can make an intelligent estimate as to what happens at smaller, microscopic scales. But if the two theories are combined – like they would be at the center of a black hole – they break down, leaving us with no idea as to what’s going on!" - spaceanswers.
This would require the grandly titled "Theory of Everything" realized to be able to tell us what the physics 'looks like' down there. Until such time we are at a loss to really say for sure.
What Is On The Other Side Of A Black Hole?
Are they gateways to other Universes? Perhaps they form wormholes we can use to quickly circumnavigate the vastness of space? These and many other theories exist for what could possibly be on the other side of a black hole, but the reality is, somewhat disappointing.
These massive black, ever hungry, forever collapsing parts of space are more of a final stop than a route to somewhere where even light ends its days.
Although we can't be entirely sure what's going on beyond the event horizon most physicists agree that you'd go absolutely nowhere. Crossing the point of no return would simply mean anything consumed by the black hole simply becomes part of it.
They are a literal road to nowhere. Objects that fall into the black hole are torn apart and incorporated into the greater mass of the entity until they end up within the singularity.
Their sacrifice will lead to the black hole becoming that little bit bigger and stronger. All that and rather than finding a nirvana of some sort, all that awaits you is disassembly and death.
Who Are The Top Scientists Whose Work Was About Black Holes And What Did They Discover/Claim?
The following 11 are some of the most important scientists whose labors helped forge our modern understanding of black holes.
1. John Michell
Year of Main Discovery: 1783
Description: John Michell was an English natural philosopher and geologist who was born in 1724. He wrote a letter to Henry Cavendish in which he postulates the idea of mass so large even light could not escape its pull.
2. Pierre-Simon Laplace
Year of Main Discovery: 1796
3. Albert Einstein
Year of Main Discovery: 1915
Description: Albert, a German-American theoretical physicist and all-around 'badass' developed his theory of general relativity. This followed his demonstration that light can be influenced by gravity.
4. Karl Schwarzschild
Year of Main Discovery: 1916
Description: Karl, a German physicist, was the first to provide a modern solution of general relativity that could be used to characterize a black hole.
5. Arthur Eddington
Year of Main Discovery: 1924
Description: Arthur, a British Astrophysicist, noted that the singularity in Einstein's work could disappear after the coordinates were altered.
6. Robert Oppenheimer
Year of Main Discovery: 1939
Description: One of the pre-eminent physicists of all time, Robert predicted that neutron stars in excess of 3 solar masses would likely collapse to form black holes.
7. David Finkelstein
Year of Main Discovery: 1958
Description: David, an American physicist, recognized that the Schwarzschild surface as actually an event horizon. He was also able to extend the Schwarzschild solution for the future of observers falling into a black hole.
8. Roy Kerr
Year of Main Discovery: 1963
Description: Roy, a New Zealand mathematician, derived the exact solution of a rotating black hole.
9. Ezra Newman
Year of Main Discovery: 1965
Description: Ezra, an American physicist, managed to discover the axisymmetric solution for a black hole that is both rotating and electrically charged.
10. James Bardeen
Year of Main Discovery: 1970's
Description: James, an American physicist, with 'some' help from Jacob Bekenstein,, Brandon Carter, and the later great Stephen Hawking, led to the formulation of black hole thermodynamics.
11. Stephen Hawking
Year of Main Discovery: 1974
Description: Stephen, the sadly missed British theoretical physicist and cosmologist, showed that black holes are not actually entirely 'black'. He postulated that small amounts of thermal radiation, called Hawking's radiation, is emitted by black holes.