###
**What Is Black Hole?**

A Black Hole Is A
Region Of Spacetime Exhibiting Gravitational Acceleration So Strong That
Nothing—No Particles Or Even Electromagnetic Radiation Such As Light—Can Escape
From It. The Theory Of General Relativity Predicts That A Sufficiently Compact
Mass Can Deform Spacetime To Form A Black Hole. The Boundary Of The Region From
Which No Escape Is Possible Is Called The Event Horizon. Although The Event
Horizon Has An Enormous Effect On The Fate And Circumstances Of An Object
Crossing It, No Locally Detectable Features Appear To Be Observed. Many
Ways, A Black Hole Acts Like An Ideal Black Body, As It Reflects No Light. Moreover,
Quantum Field Theory In Curved Spacetime Predicts That Event Horizons Emit Hawking
Radiation, With The Same Spectrum As A Black Body Of A Temperature Inversely
Proportional To Its Mass. This Temperature Is On The Order Of Billionths Of A
Kelvin For Black Holes Of Stellar Mass, Making It Essentially Impossible To
Observe.

Objects Whose Gravitational Fields Are Too Strong For Light To Escape Were First Considered In The 18th Century By John Michell And Pierre-Simon Laplace. On 10 April 2019, The First-Ever Direct Image Of A Black Hole And Its Vicinity Were Published, Following Observations Made By The Event Horizon Telescope In 2017 Of The Supermassive Black Hole In Messier 87's Galactic Center.

Black Hole |

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History

The Idea Of A Body So Massive That Even Light Could Not Escape Was Briefly
Proposed By Astronomical Pioneer And English Clergyman John Michell In A Letter
Published In November 1784. Michell's Simplistic Calculations Assumed That Such
A Body Might Have The Same Density As The Sun, And Concluded That Such A Body
Would Form When A Star's Diameter Exceeds The Sun's By A Factor Of 500 And The
Surface Escape Velocity Exceeds The Usual Speed Of Light. Michell Correctly
Noted That Such Supermassive But Non-Radiating Bodies Might Be Detectable
Through Their Gravitational Effects On Nearby Visible Bodies. Scholars Of The
Time Was Initially Excited By The Proposal That Giant But Invisible Stars
Might Be Hiding In Plain View, But Enthusiasm Dampened When The Wavelike Nature
Of Light Became Apparent In The Early Nineteenth Century.###

History

If Light Were A Wave Rather Than A "Corpuscle", It Is Unclear What,
If Any, Influence Gravity Would Have On Escaping Light Waves.

### General Relativity

Albert Einstein |

Oppenheimer And His Co-Authors Interpreted The Singularity At The Boundary Of The Schwarzschild Radius As Indicating That This Was The Boundary Of A Bubble In Which Time Stopped. This Is A Valid Point Of View For External Observers, But Not For Infalling Observers. Because Of This Property, The Collapsed Stars Were Called "Frozen Stars", Because An Outside Observer Would See The Surface Of The Star Frozen In Time At The Instant Where Its Collapse Takes It To The Schwarzschild Radius.

### Golden Age

These Results Came At The Beginning Of The Golden Age Of General Relativity, Which Was Marked By General Relativity And Black Holes Becoming Mainstream Subjects Of Research. This Process Was Helped By The Discovery Of Pulsars By Jocelyn Bell Burnell In 1967, Which, By 1969, Were Shown To Be Rapidly Rotating Neutron Stars. Until That Time, Neutron Stars, Like Black Holes, Were Regarded As Just Theoretical Curiosities; But The Discovery Of Pulsars Showed Their Physical Relevance And Spurred Further Interest In All Types Of Compact Objects That Might Be Formed By Gravitational Collapse.

In This Period More General Black Hole Solutions Were Found. In 1963, Roy Kerr Found The Exact Solution For A Rotating Black Hole. Two Years Later, Ezra Newman Found The Axisymmetric Solution For A Black Hole That Is Both Rotating And Electrically Charged. Through The Work Of Werner Israel, Brandon Carter, And David Robinson The No-Hair Theorem Emerged, Stating That A Stationary Black Hole Solution Is Completely Described By The Three Parameters Of The Kerr–Newman Metric: Mass, Angular Momentum, And Electric Charge. And Stephen Hawking Used Global Techniques To Prove That Singularities Appear Generically.

Work By James Bardeen, Jacob Bekenstein, Carter, And Hawking In The Early 1970's Led To The Formulation Of Black Hole Thermodynamics. These Laws Describe The Behaviour Of A Black Hole In Close Analogy To The Laws Of Thermodynamics By Relating Mass To Energy, Area To Entropy, And Surface Gravity To Temperature. The Analogy Was Completed When Hawking, In 1974, Showed That Quantum Field Theory Implies That Black Holes Should Radiate Like A Black Body With A Temperature Proportional To The Surface Gravity Of The Black Hole, Predicting The Effect Now Known As Hawking Radiation. And In The Early 20th Century, Physicists Used The Term "Gravitationally Collapsed Object". Science Writer Marcia Bartusiak Traces The Term "Black Hole" To Physicist Robert H. Dicke, Who In The Early 1960's Reportedly Compared The Phenomenon To The Black Hole Of Calcutta, Notorious As A Prison Where People Entered But Never Left Alive.

The Term "Black Hole" Was Used In Print By Life And Science News Magazines In 1963,

In December 1967, A Student Reportedly Suggested The Phrase "Black Hole" At A Lecture By John Wheeler; Leading Some To Credit Wheeler With Coining The Phrase.

### Properties And Structure

These Properties Are Special Because They Are Visible From Outside A Black Hole. For Example, A Charged Black Hole Repels Other Like Charges Just Like Any Other Charged Object. Similarly, The Total Mass Inside A Sphere Containing A Black Hole Can Be Found By Using The Gravitational Analog Of Gauss's Law, The ADM Mass, Far Away From The Black Hole. Likewise, The Angular Momentum Can Be Measured From Far Away Using Frame Dragging By The Gravitomagnetic Field.

When An Object Falls Into A Black Hole, Any Information About The Shape Of The Object Or Distribution Of Charge On It Is Evenly Distributed Along The Horizon Of The Black Hole And Is Lost To Outside Observers. The Behavior Of The Horizon In This Situation Is A Dissipative System That Is Closely Analogous To That Of A Conductive Stretchy Membrane With Friction And Electrical Resistance—The Membrane Paradigm. This Is Different From Other Field Theories Such As Electromagnetism, Which Do Not Have Any Friction Or Resistivity At The Microscopic Level, Because They Are Time-Reversible. Because Of A Black Hole Eventually Achieves A Stable State With Only Three Parameters, There Is No Way To Avoid Losing Information About The Initial Conditions: The Gravitational And Electric Fields Of A Black Hole Give Very Little Information About What Went In. The Information That Is Lost Includes Every Quantity That Cannot Be Measured Far Away From The Black Hole Horizon, Including Approximately Conserved Quantum Numbers Such As The Total Baryon Number And Lepton Number. This Behaviour Is So Puzzling That It Has Been Called The Black Hole Information Loss Paradox.

### Physical Properties

Solutions Describing More General Black Holes Also Exist. Non-Rotating Charged Black Holes Are Described By The Reissner–Nordström Metric, While The Kerr Metric Describes A Non-Charged Rotating Black Hole. The Most General Stationery Black Hole Solution Known Is The Kerr–Newman Metric, Which Describes A Black Hole With Both Charge And Angular Momentum.

While The Mass Of A Black Hole Can Take Any Positive Value, The Charge And Angular Momentum Are Constrained By The Mass. In Planck Units, The Total Electric Charge Q And The Total Angular Momentum J Are Expected To Satisfy.

For A Black Hole Of Mass M. Black Holes With The Minimum Possible Mass Satisfying This Inequality Are Called Extremal. Solutions Of Einstein's Equations That Violate This Inequality Exists, But They Do Not Possess An Event Horizon. These Solutions Have So-Called Naked Singularities That Can Be Observed From The Outside And Hence Are Deemed Unphysical. The Cosmic Censorship Hypothesis Rules The Formation Of Such Singularities When They Are Created Through The Gravitational Collapse Of Realistic Matter.

Due To The Relatively Large Strength Of The Electromagnetic Force, Black Holes Forming From The Collapse Of Stars Are Expected To Retain The Nearly Neutral Charge Of The Star. Rotation, However, Is Expected To Be A Universal Feature Of Compact Astrophysical Objects. The Black-Hole Candidate Binary X-Ray Source GRS 1915+105 Appears To Have An Angular Momentum Near The Maximum Allowed Value. That Uncharged Limit Is

Black Holes Are Commonly Classified According To Their Mass, Independent Of Angular Momentum, J. The Size Of A Black Hole, As Determined By The Radius Of The Event Horizon, Or Schwarzschild Radius, Is Proportional To The Mass M, Through

Where R Is The Schwarzschild Radius And M Is The Mass Of The Sun. A Black Hole With Nonzero Spin And/Or Electric Charge, The Radius Is Smaller, Until An Extremal Black Hole Could Have An Event Horizon Close To.

### Event Horizon

Black Hole |

To A Distant Observer, Clocks Near A Black Hole Would Appear To Tick More Slowly Than Those Further Away From The Black Hole. Due To This Effect, Known As Gravitational Time Dilation, An Object Falling Into A Black Hole Appears To Slow As It Approaches The Event Horizon, Taking An Infinite Time To Reach It. At The Same Time, All Processes On This Object Slow Down, From The Viewpoint Of A Fixed Outside Observer, Causing Any Light Emitted By The Object To Appear Redder and Dimmer, An Effect Known As Gravitational Redshift. Eventually, The Falling Object Fades Away Until It Can No Longer Be Seen. Typically This Process Happens Very Rapidly With An Object Disappearing From View Within Less Than A Second.

On The Other Hand, Indestructible Observers Falling Into A Black Hole Do Not Notice Any Of These Effects As They Cross The Event Horizon. According To Their Own Clocks, Which Appear To Them To Tick Normally, They Cross The Event Horizon After A Finite-Time Without Noting Any Singular Behavior; In Classical General Relativity, It Is Impossible To Determine The Location Of The Event Horizon From Local Observations, Due To Einstein's Equivalence Principle.

The Shape Of The Event Horizon Of A Black Hole Is Always Approximately Spherical. For Non-Rotating Black Holes, The Geometry Of The Event Horizon Is Precisely Spherical, While For Rotating Black Holes The Event Horizon Is Oblate.

### Singularity

Observers Falling Into A Schwarzschild Black Hole Cannot Avoid Being Carried Into The Singularity, Once They Cross The Event Horizon. They Can Prolong The Experience By Accelerating Away To Slow Their Descent, But Only Up To A Limit. When They Reach The Singularity, They Are Crushed To Infinite Density And Their Mass Is Added To The Total Of The Black Hole. Before That Happens, They Will Have Been Torn Apart By The Growing Tidal Forces In A Process Sometimes Referred To As Spaghettification Or The "Noodle Effect".

In The Case Of A Charged Or Rotating Black Hole, It Is Possible To Avoid The Singularity. Extending These Solutions As Far As Possible Reveals The Hypothetical Possibility Of Exiting The Black Hole Into A Different Spacetime With The Black Hole Acting As A Wormhole. The Possibility Of Traveling To Another Universe Is, However, Only Theoretical Since Any Perturbation Would Destroy This Possibility. It Also Appears To Be Possible To Follow Closed Timelike Curves Around The Kerr Singularity, Which Leads To Problems With Causality Like The Grandfather Paradox. It Is Expected That None Of These Peculiar Effects Would Survive In A Proper Quantum Treatment Of Rotating And Charged Black Holes.

The Appearance Of Singularities In General Relativity Is Commonly Perceived As Signalling The Breakdown Of The Theory. This Breakdown, However, Is Expected; It Occurs In A Situation Where Quantum Effects Should Describe These Actions, Due To The Extremely High Density And Therefore Particle Interactions. To Date, It Has Not Been Possible To Combine Quantum And Gravitational Effects Into A Single Theory, Although There Exist Attempts To Formulate Such A Theory Of Quantum Gravity. It Is Generally Expected That Such A Theory Will Not Feature Any Singularities.

### Photon Sphere

While Light Can Still Escape From The Photon Sphere, Any Light That Crosses The Photon Sphere On An Inbound Trajectory Is Captured By The Black Hole. Hence Any Light That Reaches An Outside Observer From The Photon Sphere Must Have Been Emitted By Objects Between The Photon Sphere And The Event Horizon.

Rotating Black Holes Are Surrounded By A Region Of Spacetime In Which It Is Impossible To Stand Still, Called The Ergosphere. This Is The Result Of A Process Known As Frame-Dragging General Relativity Predicts That Any Rotating Mass Will Tend To Slightly "Drag" Along The Spacetime Immediately Surrounding It. Any Object Near The Rotating Mass Will Tend To Start Moving In The Direction Of Rotation. For A Rotating Black Hole, This Effect Is So Strong Near The Event Horizon That An Object Would Have To Move Faster Than The Speed Of Light In The Opposite Direction To Just Standstill.

The Ergosphere Of A Black Hole Is A Volume Whose Inner Boundary Is The Black Hole's Oblate Spheroid Event Horizon And A Pumpkin-Shaped Outer Boundary, Which Coincides With The Event Horizon At The Poles But Noticeably Wider Around The Equator. The Outer Boundary Is Sometimes Called The Ergosurface. A Variation Of The Penrose Process In The Presence Of Strong Magnetic Fields, The Blandford–Znajek Process Is Considered A Likely Mechanism For The Enormous Luminosity And Relativistic Jets Of Quasars And Other Active Galactic Nuclei.

### Innermost Stable Circular Orbit

### Formation And Evolution

Penrose Demonstrated That Once An Event Horizon Forms, General Relativity Without Quantum Mechanics Requires That A Singularity Will Form Within. Conventional Black Holes Are Formed By Gravitational Collapse Of Heavy Objects Such As Stars, But They Can Also, In Theory, Be Formed By Other Processes.

The Collapse May Be Stopped By The Degeneracy Pressure Of The Star's Constituents, Allowing The Condensation Of Matter Into An Exotic Denser State. The Result Is One Of The Various Types Of Compact Stars. Which Type Forms Depends On The Mass Of The Remnant Of The Original Star Left After The Outer Layers Have Been Blown Away. Such Explosions And Pulsations Lead To Planetary Nebula. This Mass Can Be Substantially Less Than The Original Star. Remnants Exceeding Are Produced By Stars That Were Over Before The Collapse. It Has Further Been Suggested That Supermassive Black Holes With Typical Masses Of ~ Could Have Formed From The Direct Collapse Of Gas Clouds In The Young Universe. Some Candidates For Such Objects Have Been Found In Observations Of The Young Universe.

### Primordial Black Holes And The Big Bang

Despite The Early Universe Being Extremely Dense—Far Denser Than Is Usually
Required To Form A Black Hole—It Did Not Re-Collapse Into A Black Hole During
The Big Bang. Models For Gravitational Collapse Of Objects Of Relatively
Constant Size, Such As Stars, Do Not Necessarily Apply In The Same Way Too
Rapidly Expand Space Such As The Big Bang.

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