Black holes Flashcards
(18 cards)
General Relativity and Time
Time passes more slowly in regions of
greater spacetime curvature
general relativty and time. Earth:
clocks tick (and particles decay)
slightly slower closer to the surface
general relativity and time: sun
time passes slightly slower compared
to the Earth closer to the Sun’s greater
mass
General relativity and time: Black hole
time stops altogether
Gravitational Redshift
* Longer time intervals
between ticks means
the
frequency decreases
Light traveling near a mass
experiences a
gravitational redshift
Curved Spacetime
Matter
curves spacetime around it:
* least paths are curved lines.
* more mass = greater spacetime curvature.
* closer = greater spacetime curvature
Close to a singularity:
Gravity so strong, spacetime so
curved that everything, even light, is
trapped
why is it called a black holes
“Black”: no light emitted
* “Hole”: nothing entering can escape
Black holes are formed by
supernovae from the most massive stars.
Schwarzschild Radius
Light cannot escape if it comes from
inside the Schwarzschild Radius. For
a nonrotating black hole,2
M = Mass of
black hole
For M = 1 MSun, RS 3 km
For M = 1 MEarth, RS 9 mm
For M = 1 MMoon, RS 0.1 mm
Event Horizon –
The “sphere of no return”
RS defines the Event Horizon:
* Events inside RS are
invisible to us.
* Nothing within RS can ever leave.
Gravity around Black Holes
Far from a black hole
Gravity is the same as for a star of
the same mass
Close to a black hole:
R < 3 RS, no stable orbits - all
matter gets sucked in.
* photon sphere: At R = 1.5 RS,
photons orbit in a circle
Evil space aliens zap the Sun with their
collapse-o-raytm and cause it to become a black hole.
What happens to the orbit of the Earth?
It would stay the same as before.
How to find a black hole?
Look for the effects of gravity:
* A star orbiting an unseen
massive companion.
* X-rays emitted by gas heated
to extreme temperatures as it
falls into the hole.
The easiest way to discover a black hole is to look for
a strong X-ray source too massive to be a neutron
star
Light which reaches us from a massive object will be
receive with it
frequency decreased.
