Transcript 52 - Boston University Physics

the fates of collapsed star cores???
INITIAL MASS
units of solar mass
DEATH THROES
COLLAPSED CORE
mass
object
radius
PRESSURE
SOURCE
~>2
red giant, nova
< 1.4 m๏ white dwarf* earth electron degeneracy
white dwarf supernova I
~6000 km
a carbon bomb
~>3
massive supernova II 1.4-3 m๏ neutron star** ~20 km neutron degeneracy
iron bomb
Boston
~10
massive supernova II
big bang
black hole radius  m
Schwarzschild
3 km
30 km
mini black hole 1 cm
~>3 m๏
1 m๏
10 m๏
mearth
none
*1.4 m๏ is Chandrasekhar mass, electrons travelling at c
**“Landau” mass, neutrons going at c
melectron/mneutron = 1/2000 = rneutron star/rwhite dwarf
is the density of a black hole infinite?
escape from earth, 11 km/sec (here to Rt. 128 in a sec); from moon, 2.4 km/sec
Escape Velocity from Black Hole = c
Initial Kinetic Energy
= Final Gravitational Potential Energy
½ mass  (velocityescape)2 =
G mass M
r
shrink earth to radius = 1 cm radius (keeping mass the same),
light could not escape an earth mass marble
shrink our sun to 3 km radius, escape velocity = c…
no escape from the BU campus-sized black hole
…r is the Schwarzschild radius…the event horizon
…cannot know what is inside…info cannot escape
If our Sun shrank to a black hole,
we feel nothing…gravity different
only near & inside event horizon
charges & spin carried in,
but otherwise we have no further
contact with matter that disappears
orbits inside same as outside it,
circles, ellipses, hyperbola
Black holes don’t suck!
only stretch very local space
since so small,
hardest thing to fall into
tidal forces near event horizon
…lethal stretch to humans
if ~ < 3 Schwarzschild radii
from 3 MSun black hole, 5 km
tidal forces gentler near a supermassive black hole
…due to much bigger radius
Do black holes last forever?
Virtual Particles near a Black Hole
Particles pairs produced near black holes…
if one member of a virtual pair falls into the black hole
energy to transform virtual particle to real particle
comes out of black hole’s mass
Hawking Radiation
his conjecture: this real particle production
would cause black holes to “evaporate” over time
only “orphaned” subatomic particles left over