Gravity”s Final Victory
Black holes that have a mass of about 10 times the mass of our Sun are known to exist.We believe they come from the collapse of massive starsthat fail to produce supernova explosions.Stars more massive than about 20-30 Msun may produce a neutron star core that is larger than 2-3 Msun. At about this mass, neutron degeneracy pressure would fail and nothing can stop itsgravitational collapse. Hydrostatic equilibrium cannot be maintainedand the neutron star collapses.Core would collapse into a singularity, and object withzero radiusinfinite density
The Ultimate Extreme ObjectGravity is so strong that nothing, not even light, can escape.Infalling matter is shredded by powerful tides and crushed to infinite density.Escape speed exceeds the speed of light.Becomes a Black Hole:”Black” because they neither emit nor reflect light.”Hole” because nothing entering can ever escape.
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Light cannot escape from a Black Hole if it comes from a radiuscloser than the Schwarzschild Radius, RS tothe singularity:(Graphic by R. Pogge)Where M = Mass of the Black HoleA black hole with a mass of 1 Msun wouldhave a Schwarzschild Radius of RS=3 km.Compare this with a typical 0.6 Msun White Dwarf, whichwould have a radius of about 1 Rearth (6370km), anda 1.4 Msun neutron star, which would have a radius of about 10km.
Comparison of a 1.5 Msun Black Hole and Neutron Star with Island of Manhattan for scale.(Graphic by R. Pogge)RS is named for German physicist Karl Schwarzschild who in1916 was one of the first people to explore the implications ofEinstein”s then-new General Theory of Relativity, the modern theory ofGravity.
The Event Horizon
RS defines the “Event Horizon” surroundingthe black hole”s singularity:Events occurring inside RS are invisible to the outside universe.Anything closer to the singularity than RS can never leave the black holeThe Event Horizon hides the singularity from the outside universe.The Event Horizon marks the “Point of No Return” for objects falling into a Black Hole.
Gravity around Black Holes
Far away from a black hole:Gravity is the same as that of star of the same mass.Close to a black hole:R S, there are no stable orbits – all matter gets sucked in.At R = 1.5 RS, photons would orbit in a circle!
Journey to a Black Hole: A Thought Experiment
Two observers: Jack & JillJack, in a spacesuit, is falling into a black hole. He is carrying a low-power laser beacon that flashes a beam of blue light once a second.Jill is orbiting the black hole in a starship at a safe distance away in a stable circular orbit. She watches Jack fall in by monitoring the incoming flashes from his laser beacon.
(Graphic by R. Pogge)From Jack”s point of view:He sees the ship getting further away.He flashes his blue laser at Jill once a second by his watch.From Jill”s point of view:Each laser flash take longer to arrive than the lastEach laser flash become redder and fainter than the one before it.
Near the Event Horizon…Jack Sees:His blue laser flash every second by his watchThe outside world looks oddly distorted (positions of stars have changed since he started).Jill Sees:Jack”s laser flashing about once every hour.The laser flashes are now shifted to radio wavelengths, andthe flashes are getting fainter with each flash.Down the hole…Jill Sees:One last flash from Jack”s laser after a long delay (months?)The last flash is very faint and at very long radio wavelengths.She never sees another flash from Jack…Jack Sees:The universe appear to vanish as he crosses the event horizonHe gets shredded by strong tides near the singularity and crushed to infinite density.Moral:The powerful gravity of a black hole warps space and time around it:Time appears to stand still at the event horizon as seen by a distant observer.Time flows as it always does as seen by an infalling astronaut.Light emerging from near the black hole is Gravitationally Redshifted to longer (red) wavelengths.Take a VirtualTrip to a Black Hole or Neutron Star. Pictures & movies byrelativist Robert Nemiroff at the Michigan Technical University.
Seeing what cannot be seen…
Question:If black holes are black, how can we hope to see them?Answer:Look for the effects of their gravity on their surroundings.Look for stars orbiting around an unseen massive objectLook for X-rays emitted by gas that is superheated as it falls into a black hole.
Bright, variable X-ray sources identified by X-ray observatory satellites:Spectroscopic binary with only one set of spectral lines – the second object is invisible.Gas from the visible star is dumped on the companion, heats up, and emits X-rays.Estimate the mass of the unseen companion from the parameters of itsorbit.A black hole candidate, conservatively, would be a system in which the mass of the unseen companion was larger than 3 Msun, the more massive the better.
Black Hole Candidates
A number of X-ray binaries have been found with unseen companions withMasses > 3 Msun, too big for a Neutron Star.Currently 20 confirmed black hole candidates in our Galaxy:First was Cygnus X-1: M = 7-13 MsunLargest is GRS1915+105: M = 10-18 MsunMost are in the range 4-10 MsunEstimated to be as many as 1 Billion stellar-mass black holes in ourGalaxy, which points out how very hard it is to find something that doesnot emit any radiation of its own.
Black Holes are not totally Black!
“Classical” General Relativity says:Black Holes are totally blackCan only grow in mass and sizeLast forever (nothing gets out once inside)But, General Relativity does not include the effects of Quantum Mechanics.
Evaporating Black Holes
Black Holes evaporate slowly by emitting subatomic particles and photons via”Hawking Radiation”:Very cold thermal radiation (Temperatures of ~10 nanoKelvin)Bigger Black holes are colderThe smaller the mass, the hotter the black hole, andso the faster the evaporation.For black holes in the real universe, the evaporation rate is VERY slow:A 3 Msun black hole would require about 1063 years to completely evaporate.This is about 1053 times the present age of the Universe.Probably unimportant today, but it could be an important process inthe distant future of the Universe.