Download highen_13_gravwaves - Mullard Space Science Laboratory

High energy Astrophysics
Mat Page
Mullard Space Science Lab, UCL
13. Gravitational waves
Slide 2
12. Gravitational Waves
• This lecture:
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What are they?
How are they made?
Detection: indirect methods
Detection: direct methods
Gravitational wave astronomy
Slide 3
What are gravitational waves?
• Waves in the gravitational field
• We are used to tides on the earth.
– differential gravitational forces due to the difference
in distance from the moon to the near and far parts
of the Earth.
• Imagine if the sun was a binary star rotating
rapidly and you have on the earth a test particle
that samples the gravitational field
– It would oscillate due to the changing gravity
– We can see that an oscillating body will produce a
gravity wave, somewhat analogous to the way an
oscillating charge produces an electromagnetic
wave
Slide 4
But gravitational waves are a little different
• Mass behaves different to charge.
• Momentum is conserved
– Proper treatment of gravitational radiation is
hideous – whole chapter of Misner, Thorne and
Wheeler.
– Whenever mass is accelerated one way, other
masses experience equal and opposite change
in momentum in the other direction
– Radiation does not depend on the dipole
moment, so it doesn’t just move things up and
down.
Slide 5
Before we get any further:
• Why are gravitational waves important?
• Predicted by Einstein’s general relativity
– Important, testable prediction of the theory.
• Has important astrophysical consequences:
– Can be a significant energy loss mechanism
– Can cause compact orbits to decay
• Completely different means of
astronomical observation
– Potentially even more revolutionary than
radio astronomy or X-ray astronomy!
Slide 6
Displacement of test particles
by a gravity wave
Slide 7
Energy of a gravity wave
• Just like electromagnetic waves,
gravitational radiation is able to cause
acceleration of external systems and so it
carries energy.
• If the separation between orbiting systems
is L, power emitted can be written:
PGW ~
c5
G
GM 5
c2L
( )
Slide 8
• Now there is a maximum: L cannot be smaller
than half the Schwarzschild radius
• So no pair of orbiting bodies can emit more
than 4x1052 W regardless of their mass
• For the Earth around the Sun, about 200W are
emitted – so our orbit should last another 1023
years before it decays by gravitational
radiation
• When dimensions of orbit approach the
Schwarzschild radius, gravitational radiation is
pretty serious.
Slide 9
So: some obvious predicted sources and
consequences of gravitational waves:
• Very compact binary systems lose energy
rapidly by gravitational radiation.
• If short g-ray bursts are caused by the
coalescence of neutron stars or black
holes, there should be a strong
gravitational wave signal as the orbit
rapidly decays.
Slide 10
Indirect detection
• Gravitational waves remove energy
from an orbiting system.
• If we find a system with well defined
orbital parameters that imply it should
be radiating gravitational radiation
– We can predict how fast the orbit should be
slowing due to gravitational radiation
– Measure and compare
Slide 11
Binary pulsar
• Binary system of two orbiting neutron stars,
one of which a pulsar, discovered by Joseph
Taylor and Russel Hulse in 1975.
– Orbital period 7.8 hours.
– Pulse period 0.059s
• Pulses combined with general relativity allow
very accurate determination of the binary
parameters.
• Orbital period change prediction in agreement
with measurement of rate of period change =
-2.1 + - 0.4 x 10-12
• Secured 1993 Nobel prize!
Slide 12
Ultracompact binaries
• The pulsar in the binary pulsar enabled accurate
determination of the orbital parameters, but this is
not the most extreme binary system in the galaxy.
• There are now (at least) 3 ultracompact binaries
known, in which both stars are white dwarfs in which
the orbital period is less than 10 minutes!
• All 3 were discovered as X-ray sources within the
last 15 years.
• Two of these were discovered by Gavin Ramsay of
MSSL and colleagues.
• They will make excellent test cases for gravitational
radiation.
Direct detection
Slide 13
• The principal observable effect of gravitational
waves is the slight fractional distortion in the
distance between two points.
• Called ‘gravitational strain’.
• For reference, strain expected from a maximally
emitting source 1kpc distant, the 1kHz strain is
only 10-14
• More realistic supernova producing a neutron star
a Mpc away may produce a strain more like 10-20
– Absurdly small – atomic nucleus added to a 1 AU
baseline
• Two types of detectors:
– Resonant bars
– Laser interferometers
Slide 14
Resonant bars
• Pioneered by J. Weber.
• Gravitational strain will make bars resonate, - like
making a bell ring (but very small amplitude).
• 1969 Weber published detection of bursts of
gravitational radiation – about 3 per day(!) using
aluminium cylinders about 2m long and resonant
frequency 1.6 kHz
• Levine and Garwin (1973) failed to detect any
gravity waves with similar apparatus – Weber was
not detecting gravity waves.
• Gravity waves have not been detected with
resonant bars…
Slide 15
Laser interferometers
• New gravitational wave experiments based
on laser interferometers.
• Laser light is transmitted along 2
perpendicular paths, reflected at each end
and recombined in the centre to produce
interference fringes.
• Movement of the fringes indicates relative
movement of the two arms.
Slide 16
Geo 600 (600m German/UK) in Hannover
Slide 17
Laser interferometer gravitationalwave observatory (LIGO)
Slide 18
LIGO Hanford interferometer
Slide 19
LIGO
• Two interferometers, Livingstone and
Hanford in US. More perhaps to be added.
• The arms are 4km long. The whole light path
is in vacuum (all 4km x2 at each station!)
• Two stations so that terrestrial interference
such as small scale seismic activity is not
confused with gravitational waves
Slide 20
Schematic of LIGO
Slide 21
One arm of the Livingstone
interferometer
Slide 22
The interferometer station at
Livingstone
Slide 23
So do we expect them to detect
gravitational waves?
• Yes!
• But maybe not as soon as the
gravitational wave people always claim!
• LIGO will only be sensitive to highfrequency gravitational waves.
• For slower gravitational waves (e.g.
from supermassive black holes
merging) the only option is space!
Slide 24
Laser interferometer space antenna
(LISA)
• Recently
selected to be
an ESA
mission
launching
2034.
• By going into
space can get
much longer
interferometer
LISA formation
Slide 25
Slide 26
We can really do gravitational wave
astronomy with LISA!
Slide 27
Some key points:
• Gravitational radiation is a phenomena predicted
from general relativity.
• (sort of) the gravitational equivalent of
electromagnetic radiation.
• Causes small oscillations in the fractional
displacement between points in space.
• Important energy loss mechanism in very close
binaries.
• Detected indirectly in the binary pulsar
• Laser interferometers should be capable of
detecting astronomical sources by gravitational
radiation
• Could be truly revolutionary new astronomy