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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.
What-is-black-hole?
Black Hole


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.
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


In 1915, Albert Einstein Developed His Theory Of General Relativity, Having Earlier Shown That Gravity Does Influence Light's Motion. Only A Few Months Later, Karl Schwarzschild Found A Solution To The Einstein Field Equations, Which Describes The Gravitational Field Of A Point Mass And A Spherical Mass. A Few Months After Schwarzschild, Johannes Droste, A Student Of Hendrik Lorentz, Independently Gave The Same Solution For The Point Mass And Wrote More Extensively About Its Properties. This Solution Had A Peculiar Behavior At What Is Now Called The Schwarzschild Radius, Where It Became Singular, Meaning That Some Of The Terms In The Einstein Equations Became Infinite. The Nature Of This Surface Was Not Quite Understood At The Time. In 1924, Arthur Eddington Showed That The Singularity Disappeared After A Change Of Coordinates, Although It Took Until 1933 For Georges Lemaître To Realize That This Meant The Singularity At The Schwarzschild Radius Was A Non-Physical Coordinate Singularity. Arthur Eddington Did However Comment On The Possibility Of A Star With Mass Compressed To The Schwarzschild Radius In A 1926 Book, Noting That Einstein's Theory Allows Us To Rule Out Overly Large Densities For Visible Stars Like Betelgeuse Because "A Star Of 250 Million Km Radius Could Not Possibly Have So High A Density As The Sun. Firstly, The Force Of Gravitation Would Be So Great That Light Would-Be Unable To Escape From It, The Rays Falling Back To The Star-Like A Stone To The Earth. Secondly, The Redshift Of The Spectral Lines Would Be So Great That The Spectrum Would Be Shifted Out Of Existence. Thirdly, The Mass Would Produce So Much Curvature Of The Space-Time Metric That Space Would Close Up Around The Star, Leaving Us Outside ."
Albert-Einstein
Albert Einstein

In 1931, Subrahmanyan Chandrasekhar Calculated, Using Special Relativity, That A Non-Rotating Body Of Electron-Degenerate Matter Above A Certain Limiting Mass Has No Stable Solutions. His Arguments Were Opposed By Many Of His Contemporaries Like Eddington And Lev Landau, Who Argued That Some Yet Unknown Mechanism Would Stop The Collapse. They Were Partly Correct: A White Dwarf Slightly More Massive Than The Chandrasekhar Limit Will Collapse Into A Neutron Star, Which Is Itself Stable. But In 1939, Robert Oppenheimer And Others Predicted That Neutron Stars Above Another Limit Would Collapse Further For The Reasons Presented By Chandrasekhar And Concluded That No Law Of Physics Was Likely To Intervene And Stop At Least Some Stars From Collapsing To Black Holes. Their Original Calculations, Based On The Pauli Exclusion Principle, Gave It As; Subsequent Consideration Of Strong Force-Mediated Neutron-Neutron Repulsion Raised The Estimate To Approximately To. Observations Of The Neutron Star Merger GW170817, Which Is Thought To Have Generated A Black Hole Shortly Afterwards, Have Refined The TOV Limit Estimate To ~.

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


In 1958, David Finkelstein Identified The Schwarzschild Surface As An Event Horizon, "A Perfect Unidirectional Membrane: Causal Influences Can Cross It In Only One Direction". This Did Not Strictly Contradict Oppenheimer's Results But Extended Them To Include The Point Of View Of Infalling Observers. Finkelstein's Solution Extended The Schwarzschild Solution For The Future Of Observers Falling Into A Black Hole. A Complete Extension Had Already Been Found By Martin Kruskal, Who Was Urged To Publish It.

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


The No-Hair Conjecture Postulates That Once It Achieves A Stable Condition After Formation, A Black Hole Has Only Three Independent Physical Properties: Mass, Charge, And Angular Momentum; The Black Hole Is Otherwise Featureless. If The Conjecture Is True, Any Two Black Holes That Share The Same Values For These Properties, Or Parameters, Are Indistinguishable From One Another. The Degree To Which The Conjecture Is True For Real Black Holes Under The Laws Of Modern Physics Is Currently An Unsolved Problem.

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


The Simplest Static Black Holes Have Mass But Neither Electric Charge Nor Angular Momentum. These Black Holes Are Often Referred To As Schwarzschild Black Holes After Karl Schwarzschild Who Discovered This Solution In 1916. This Means That There Is No Observable Difference At A Distance Between The Gravitational Field Of Such A Black Hole And That Of Any Other Spherical Object Of The Same Mass. The Popular Notion Of A Black Hole "Sucking In Everything" In Its Surroundings Is Therefore Only Correct Near A Black Hole's Horizon; Far Away, The External Gravitational Field Is Identical To That Of Any Other Body Of The Same Mass.
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


The Defining Feature Of A Black Hole Is The Appearance Of An Event Horizon—A Boundary In Spacetime Through Which Matter And Light Can Only Pass Inward Towards The Mass Of The Black Hole. Nothing, Not Even Light, Can Escape From Inside The Event Horizon. The Event Horizon Is Referred To As Such Because If An Event Occurs Within The Boundary, Information From That Event Cannot Reach An Outside Observer, Making It Impossible To Determine If Such An Event Occurred.
How-Black-Hole-Works?
Black Hole
As Predicted By General Relativity, The Presence Of A Mass Deforms Spacetime In Such A Way That The Paths Taken By Particles Bend Towards The Mass. At The Event Horizon Of A Black Hole, This Deformation Becomes So Strong That There Are No Paths That Lead Away From The 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


At The Center Of A Black Hole, As Described By General Relativity, May Lie A Gravitational Singularity, A Region Where The Spacetime Curvature Becomes Infinite. For A Non-Rotating Black Hole, This Region Takes The Shape Of A Single Point And For A Rotating Black Hole, It Is Smeared Out To Form A Ring Singularity That Lies In The Plane Of Rotation. In Both Cases, The Singular Region Has Zero Volume. It Can Also Be Shown That The Singular Region Contains All The Mass Of The Black Hole Solution. The Singular Region Can Thus Be Thought Of As Having Infinite Density.

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


The Photon Sphere Is A Spherical Boundary Of Zero Thickness In Which Photons That Move On Tangents To That Sphere Would Be Trapped In A Circular Orbit About The Black Hole. For Non-Rotating Black Holes, The Photon Sphere Has A Radius 1.5 Times The Schwarzschild Radius. Their Orbits Would Be Dynamically Unstable, Hence Any Small Perturbation, Such As A Particle Of Infalling Matter, Would Cause Instability That Would Grow Over Time, Either Setting The Photon On An Outward Trajectory Causing It To Escape The Black Hole, Or On An Inward Spiral Where It Would Eventually Cross The Event Horizon.

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


In Newtonian Gravity, Test Particles Can Stably Orbit At Arbitrary Distances From A Central Object. In General Relativity, However, There Exists An Innermost Stable Circular Orbit, Inside Of Which, Any Infinitesimal Perturbations To A Circular Orbit Will Lead To Inspiral Into The Black Hole. The Location Of The ISCO Depends On The Spin Of The Black Hole, In The Case Of A Schwarzschild Black Hole Is And Decreases With Increasing Black Hole Spin For Particles Orbiting In The Same Direction As The Spin.

Formation And Evolution


Given The Bizarre Character Of Black Holes, It Was Long Questioned Whether Such Objects Could Actually Exist In Nature Or Whether They Were Merely Pathological Solutions To Einstein's Equations. Einstein Himself Wrongly Thought That Black Holes Would Not Form, Because He Held That The Angular Momentum Of Collapsing Particles Would Stabilize Their Motion At Some Radius. This Led The General Relativity Community To Dismiss All Results To The Contrary For Many Years. However, A Minority Of Relativists Continued To Contend That Black Holes Were Physical Objects, And By The End Of The 1960s, They Had Persuaded The Majority Of Researchers In The Field That There Is No Obstacle To The Formation Of An Event Horizon.

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


Gravitational Collapse Requires Great Density. In The Current Epoch Of The Universe These High Densities Are Only Found In Stars, But In The Early Universe Shortly After The Big Bang Densities Were Much Greater, Possibly Allowing For The Creation Of Black Holes. High-Density Alone Is Not Enough To Allow Black Hole Formation Since A Uniform Mass Distribution Will Not Allow The Mass To Bunch Up. In Order For Primordial Black Holes To Have Formed In Such A Dense Medium, There Must Have Been Initial Density Perturbations That Could Then Grow Under Their Own Gravity. Different Models For The Early Universe Vary Widely In Their Predictions Of The Scale Of These Fluctuations. Various Models Predict The Creation Of Primordial Black Holes Ranging In Size From A Planck Mass To Hundreds Of Thousands Of Solar Masses.

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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