Black holes are defined by their event horizon, which is an imaginary line drawn through space and time. There are no warning bells, no klaxons, no sign or signal or evidence of its existence. But it’s there. And crossing it means simply this: you can never turn around.
The simplicity of black holes is known as the no-hair theorem. The theorem suggests that black holes have no distinguishing characteristics other than mass, spin, and charge. This means that two black holes with the same mass, electric charge, and spin rate would be identical.
The term “no-hair” was coined half as a joke. Evidence of any black-hole hair (a property other than mass or spin) would be a revolutionary smoking gun for new physics.
Black holes are extremely dense pockets of matter with such incredible mass and miniscule volume that they drastically warp the fabric of space-time. They are nearly undetectable unless they are feeding on space stuff or tugging on nearby stars.
Black holes have helped us test Einstein’s theory of general relativity, which describes how mass, space, and time are related to one another. Scientists think they can tell us much more about these and other essential rules of the universe.
Black holes have several unique properties:
- Event horizon A boundary in space-time where matter and light can only pass inward towards the black hole. Nothing, not even light, can escape from inside the event horizon.
- Gravitational force Black holes are the only objects in the universe that can trap light by gravitational force.
- Supermassive black holes These black holes live at the centers of nearly every galaxy and range in mass from 100,000 to billions of times the mass of the sun.
- Spin Black holes spin without the physical rotation of matter, just a twisted spacetime structure.
- Charge Real black holes are unlikely to be charged. If a black hole did somehow become charged, it would quickly neutralize itself by accreting charge of the opposite sign.
Here are some more interesting facts about black holes:
- Stellar-mass black holes These black holes can be created when two neutron stars merge.
- Time distortion Black holes can distort time. From the viewpoint of an observer outside the black hole, time stops near the event horizon, as light cannot escape it.
- Event horizon The event horizon of a black hole is the point of no return. Anything that passes this point will be swallowed by the black hole and forever vanish from our known universe.
- Gravity For a one-solar-mass black hole, the gravity at the event horizon is about 1.6 trillion g’s. This is 1.6 trillion times the gravitational acceleration we experience at the surface of the Earth.
- Spin Black holes spin without the physical rotation of matter, just a twisted spacetime structure. This means there is an upper limit to this spin due to the inherent properties of space and time.
Astronomers classify black holes into three types based on their mass:
- Stellar-mass black holes: These black holes are formed when a star collapses gravitationally. They have masses ranging from 5 to several tens of solar masses. The process is observed as a hypernova explosion or as a gamma ray burst.
- Intermediate-mass black holes: These black holes have distinct characteristics and behaviors.
- Supermassive black holes: These black holes live at the centers of nearly every galaxy and range in mass from 100,000 to billions of times the mass of the sun. The ultra massive black hole in the galaxy cluster Abell 1201 has a mass of 30 billion suns.
The mass ranges that define each group are approximate, and scientists are always reassessing where the boundaries should be set
Stellar black holes are the most common type of black hole. They are formed when stars that are at least 10 to 20 times as massive as the sun develop into black holes at the end of their lives. There are 40 quintillion stellar-mass black holes in the observable universe, making up approximately 1% of all normal matter.
Supermassive black holes are also very common, with nearly every large galaxy having one. Their mass is proportional to the mass of the host galaxy. For example, a galaxy twice as massive as another would have a black hole that is also twice as massive. However, more massive black holes are rare, with only fewer than several dozen having been discovered to date
A typical black hole weighs between 3 and 10 solar masses. The average stellar-mass black hole has a mass of 10 to 100 solar masses. The supermassive black hole at the center of the Milky Way, Sagittarius A*, has a mass of 4.3 million solar masses.
The material around a black hole is visible. Material falling into a black hole forms a disk, similar to a whirlpool in a bathtub drain.
Intermediate-mass black holes (IMBHs) are the rarest type of black hole. They are mid-size black holes that range from 100 to 100,000 times the sun’s mass. They are very difficult to locate.
There are several reasons why IMBHs are so rare, including:
- There are no common physical mechanisms in the universe that can collapse matter down to an IMBH.
- There have been several promising candidates, but no IMBHs have been definitively confirmed to exist.
Other rare types of black holes include:
- Micro black holes: Also called mini black holes or quantum mechanical black holes, these are hypothetical tiny black holes that are smaller than stellar mass.
- Primordial black holes: These are around 1 billion times the mass of the sun.
- Ultra-massive black holes: These sit in the center of galaxies and have more than five billion solar masses. The largest black hole we know of is Phoenix A, an UMBH with up to 100 billion solar masses.
Here are some other interesting black holes:
- TON 618: A very luminous quasar that’s thought to be over 40 billion solar masses
- SDSSJ0927+2943: The first known rogue black hole, which may be approximately 600 million times the mass of the sun
- M87* : The first black hole image captured by the NSF-supported Event Horizon Telescope in 2019
- Sgr A* : The black hole at the center of the Milky Way, which is 4 million times more massive than the sun
- Phoenix A: A black hole that’s 100 billion times the mass of the sun
- UHZ1: A black hole that’s 13.2 billion years old
- Cygnus X-1: The first black hole discovered in the 1960’s, which is 10 times more massive than the sun
Black holes are mysterious because:
- We can’t observe them directly Black holes are invisible to our eyes because nothing can escape them, not even light. Astronomers must “observe” black holes indirectly by studying how they affect their surroundings.
- Their origins are mysterious The basic formation process is understood, but one mystery is that black holes appear to exist on two radically different size scales.
- Their extreme gravity is mysterious Their extreme gravity is so strong that not even light can escape.
- The information paradox is mysterious According to the laws of physics, information cannot be destroyed. However, if an object falls into a black hole, its information appears to be lost forever.
- Astronomers don’t know what goes on inside black holes Even before you reach the event horizon, you would be “spaghettified” by the black hole’s tidal forces
Here are some other unsolved mysteries about black holes:
- The nature of their singularity: Scientists believe that the laws of physics break down at the singularity.
- How supermassive black holes form: Nobody knows how supermassive black holes form.
- The missing supermassive black hole: The Hubble Space Telescope and the Chandra X-ray Observatory have not found the presumed black hole in A2261-BCG.
- The link between a black hole and the stars it anchors: Supermassive black holes are connected, through gravity, to hundreds of billions of stars
It’s unlikely that we live inside a black hole:
- Black holes are too far away: Black holes are too far away to pull in any matter from our solar system.
- Black holes are finite: Black holes don’t have infinite density at their singularity/center.
- Black holes have singularities: In a black hole, all matter is compressed into a singularity, so there is no space for humans to live.
- Black holes have an event horizon: Time stops at the event horizon.
- Black holes have absolute zero temperature: If time stops, motion can not exist at the event horizon and the temperature is at absolute zero.
However, it’s possible that our universe was born from a black hole. The interior solution for a black hole is mathematically similar to that of a closed finite universe. However, our current universe looks to be infinite
A black hole is a region in space where the force of gravity is so strong that light cannot escape. The strong gravity occurs because matter has been pressed into a tiny space. This compression can take place at the end of a star’s life.
Black holes are extremely dense points in space that create deep gravity sinks. They are created when giant stars collapse, and perhaps by other methods that are still unknown.
Black holes are invisible because no light can get out. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.
The simplest kind of black hole is a Schwarzschild black hole, which is a black hole with mass, but with no electric charge, and no spin
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