The geology of Mars is the scientific study of the surface, crust, and interior of the planet Mars.
It emphasizes the composition, structure, history, and physical
processes that shape the planet. It is analogous to the field of
terrestrial geology. In planetary science, the term geology is used in its broadest sense to mean the study of the solid parts of planets and moons. The term incorporates aspects of geophysics, geochemistry, mineralogy, geodesy, and cartography. A neologism, areology, from the Greek word Arēs (Mars), sometimes appears as a synonym for Mars's geology in the popular media and works of science fiction (e.g. Kim Stanley Robinson'sMars trilogy). The term areology is also used by the Areological Society.
The InSight lander mission is designed to study the deep interior of Mars. The mission landed on 26 November 2018. and deployed a sensitive seismometer to enable 3D structure mapping of the deep interior. On 25 October 2023, scientists, helped by information from InSight, reported that the planet Mars has a radioactivemagma ocean under its crust.
Global physiography
Mars
has a number of distinct, large-scale surface features that indicate
the types of geological processes that have operated on the planet over
time. This section introduces several of the larger physiographic
regions of Mars. Together, these regions illustrate how geologic
processes involving volcanism, tectonism, water, ice, and impacts have shaped the planet on a global scale.
Mars
Orbital Laser Altimeter (MOLA) colorized shaded-relief maps showing
elevations in the western and eastern hemispheres of Mars. (Left): The
western hemisphere is dominated by the Tharsis
region (red and brown). Tall volcanoes appear white. Valles Marineris
(blue) is the long gash-like feature to the right. (Right): Eastern
hemisphere shows the cratered highlands (yellow to red) with the Hellas basin
(deep blue/purple) at lower left. The Elysium province is at the upper
right edge. Areas north of the dichotomy boundary appear as shades of
blue on both maps.
The northern and southern hemispheres of Mars are strikingly different from each other in topography and physiography. This dichotomy
is a fundamental global geologic feature of the planet. The northern
part is an enormous topographic depression. About one-third of the
surface (mostly in the northern hemisphere) lies 3–6 km lower in
elevation than the southern two-thirds. This is a first-order relief
feature on par with the elevation difference between Earth's continents
and ocean basins. The dichotomy is also expressed in two other ways: as a difference in
impact crater density and crustal thickness between the two hemispheres. The hemisphere south of the dichotomy boundary (often called the
southern highlands or uplands) is very heavily cratered and ancient,
characterized by rugged surfaces that date back to the period of heavy bombardment.
In contrast, the lowlands north of the dichotomy boundary have few
large craters, are very smooth and flat, and have other features
indicating that extensive resurfacing has occurred since the southern
highlands formed. The third distinction between the two hemispheres is
in crustal thickness. Topographic and geophysical gravity data indicate
that the crust in the southern highlands has a maximum thickness of
about 58 km (36 mi), whereas the crust in the northern lowlands "peaks"
at around 32 km (20 mi) in thickness. The location of the dichotomy boundary varies in latitude across Mars
and depends on which of the three physical expressions of the dichotomy
is being considered.
The origin and age of the hemispheric dichotomy are still debated. Hypotheses of origin generally fall into two categories: one, the
dichotomy was produced by a mega-impact event or several large impacts
early in the planet's history (exogenic theories) or two, the dichotomy was produced by crustal thinning in the northern
hemisphere by mantle convection, overturning, or other chemical and
thermal processes in the planet's interior (endogenic theories). One endogenic model proposes an early episode of plate tectonics producing a thinner crust in the north, similar to what is occurring at spreading plate boundaries on Earth. Whatever its origin, the Martian dichotomy appears to be extremely old. A new theory based on the Southern Polar Giant Impact and validated by the discovery of twelve hemispherical alignments shows that exogenic theories appear to be stronger than endogenic theories and that Mars never had plate tectonics that could modify the dichotomy. Laser altimeters and radar-sounding
data from orbiting spacecraft have identified a large number of
basin-sized structures previously hidden in visual images. Called
quasi-circular depressions (QCDs), these features likely represent
derelict impact craters from the period of heavy bombardment that are
now covered by a veneer of younger deposits. Crater counting studies of
QCDs suggest that the underlying surface in the northern hemisphere is
at least as old as the oldest exposed crust in the southern highlands. The ancient age of the dichotomy places a significant constraint on theories of its origin.
Tharsis and Elysium volcanic provinces
The Tharsis region with main features annotated. The Tharsis Montes are the three aligned volcanoes at the center bottom. Olympus Mons sits off at the center left. The feature at the upper right is Alba Mons.
Straddling the dichotomy boundary in Mars's western hemisphere is a massive volcano-tectonic province known as the Tharsis
region or the Tharsis bulge. This immense, elevated structure is
thousands of kilometers in diameter and covers up to 25% of the planet's
surface. Averaging 7–10 km above datum (Martian "sea" level), Tharsis contains
the highest elevations on the planet and the largest known volcanoes in
the Solar System. Three enormous volcanoes, Ascraeus Mons, Pavonis Mons, and Arsia Mons (collectively known as the Tharsis Montes), sit aligned NE-SW along the crest of the bulge. The vast Alba Mons (formerly Alba Patera) occupies the northern part of the region. The huge shield volcanoOlympus Mons lies off the main bulge, at the western edge of the province. The extreme massiveness of Tharsis has placed tremendous stress on the planet's lithosphere. As a result, immense extensional fractures (grabens and rift valleys) radiate outward from Tharsis, extending halfway around the planet.
A smaller volcanic center lies several thousand kilometers west of Tharsis in Elysium. The Elysium volcanic complex is about 2,000 kilometers in diameter and consists of three main volcanoes, Elysium Mons, Hecates Tholus, and Albor Tholus.
The Elysium group of volcanoes is thought to be somewhat different from
the Tharsis Montes, in that development of the former involved both
lavas and pyroclastics.
Large impact basins
Several enormous, circular impact basins are present on Mars. The largest one that is readily visible is the Hellas basin
located in the southern hemisphere. It is the second largest confirmed
impact structure on the planet, centered at about 64°E longitude and
40°S latitude. The central part of the basin (Hellas Planitia) is
1,800 km in diameter and surrounded by a broad, heavily eroded annular rim structure characterized by closely spaced rugged irregular mountains (massifs), which probably represent uplifted, jostled blocks of old pre-basin crust. (See Anseris Mons,
for example.) Ancient, low-relief volcanic constructs (highland
paterae) are located on the northeastern and southwestern parts of the
rim. The basin floor contains thick, structurally complex sedimentary
deposits that have a long geologic history of deposition, erosion, and
internal deformation. The lowest elevations on the planet are located
within the Hellas basin, with some areas of the basin floor lying over
8 km below datum.
The two other large impact structures on the planet are the Argyre and Isidis
basins. Like Hellas, Argyre (800 km in diameter) is located in the
southern highlands and is surrounded by a broad ring of mountains. The
mountains in the southern portion of the rim, Charitum Montes, may have been eroded by valley glaciers and ice sheets at some point in Mars's history. The Isidis basin (roughly 1,000 km in diameter) lies on the dichotomy
boundary at about 87°E longitude. The northeastern part of the basin rim
has been eroded and is now buried by northern plains deposits, giving
the basin a semicircular outline. The northwestern rim of the basin is
characterized by arcuate grabens (Nili Fossae) that are circumferential to the basin. One additional large basin, Utopia,
is completely buried by northern plains deposits. Its outline is
clearly discernable only from altimetry data. All of the large basins on
Mars are extremely old, dating to the late heavy bombardment. They are
thought to be comparable in age to the Imbrium and Orientale basins on the Moon.
Equatorial canyon system
Viking Orbiter 1 view image of Valles Marineris.
Near the equator in the western hemisphere lies an immense system of
deep, interconnected canyons and troughs collectively known as the Valles Marineris.
The canyon system extends eastward from Tharsis for a length of over
4,000 km, nearly a quarter of the planet's circumference. If placed on
Earth, Valles Marineris would span the width of North America. In places, the canyons are up to 300 km wide and 10 km deep. Often compared to Earth's Grand Canyon,
the Valles Marineris has a very different origin than its tinier,
so-called counterpart on Earth. The Grand Canyon is largely a product of
water erosion. The Martian equatorial canyons were of tectonic origin,
i.e. they were formed mostly by faulting. They could be similar to the East African Rift valleys. The canyons represent the surface expression of a powerful extensional strain in the Martian crust, probably due to loading from the Tharsis bulge.
Chaotic terrain and outflow channels
The
terrain at the eastern end of the Valles Marineris grades into dense
jumbles of low rounded hills that seem to have formed by the collapse of
upland surfaces to form broad, rubble-filled hollows. Called chaotic terrain, these areas mark the heads of huge outflow channels that emerge full size from the chaotic terrain and empty (debouch) northward into Chryse Planitia. The presence of streamlined islands and other geomorphic features indicate that the channels were most likely formed by catastrophic releases of water from aquifers
or the melting of subsurface ice. However, these features could also be
formed by abundant volcanic lava flows coming from Tharsis. The channels, which include Ares, Shalbatana,
Simud, and Tiu Valles, are enormous by terrestrial standards, and the
flows that formed them correspondingly immense. For example, the peak
discharge required to carve the 28-km-wide Ares Vallis is estimated to
have been 14 million cubic metres (500 million cu ft) per second, over
ten thousand times the average discharge of the Mississippi River.
Mars Orbital Laser Altimeter (MOLA) derived image of Planum Boreum. Vertical exaggeration is extreme. Note that residual ice cap is only the thin veneer (shown in white) on top of the plateau.
The polar ice caps are well-known telescopic features of Mars, first identified by Christiaan Huygens in 1672. Since the 1960s, we have known that the seasonal caps (those seen in
the telescope to grow and wane seasonally) are composed of carbon
dioxide (CO2) ice that condenses out of the atmosphere as temperatures fall to 148 K, the frost point of CO2, during the polar wintertime. In the north, the CO2 ice completely dissipates (sublimes) in summer, leaving behind a residual cap of water (H2O) ice. At the south pole, a small residual cap of CO2 ice remains in summer.
Both residual ice caps overlie thick layered deposits of
interbedded ice and dust. In the north, the layered deposits form a
3 km-high, 1,000 km-diameter plateau called Planum Boreum. A similar kilometers-thick plateau, Planum Australe,
lies in the south. Both plana (the Latin plural of planum) are
sometimes treated as synonymous with the polar ice caps, but the
permanent ice (seen as the high albedo, white surfaces in images) forms
only a relatively thin mantle on top of the layered deposits. The
layered deposits probably represent alternating cycles of dust and ice
deposition caused by climate changes related to variations in the
planet's orbital parameters over time (see also Milankovitch cycles). The polar layered deposits are some of the youngest geologic units on Mars.
Mollweide
projection of albedo features on Mars from Hubble Space Telescope.
Bright ochre areas in left, center, and right are Tharsis, Arabia, and
Elysium, respectively. The dark region at top center left is Acidalia
Planitia. Syrtis Major is the dark area projecting upward in the center
right. Note orographic clouds over Olympus and Elysium Montes (left and right, respectively).
No topography is visible on Mars from Earth. The bright areas and dark markings seen through a telescope are albedo features. The bright, red-ochre
areas are locations where fine dust covers the surface. Bright areas
(excluding the polar caps and clouds) include Hellas, Tharsis, and Arabia Terra.
The dark gray markings represent areas that the wind has swept clean of
dust, leaving behind the lower layer of dark, rocky material. Dark
markings are most distinct in a broad belt from 0° to 40° S latitude.
However, the most prominent dark marking, Syrtis Major Planum, is in the northern hemisphere. The classical albedo feature, Mare Acidalium (Acidalia Planitia),
is another prominent dark area in the northern hemisphere. A third type
of area, intermediate in color and albedo, is also present and thought
to represent regions containing a mixture of the material from the
bright and dark areas.
Impact craters
Impact craters were first identified on Mars by the Mariner 4 spacecraft in 1965. Early observations showed that Martian craters were generally shallower
and smoother than lunar craters, indicating that Mars has a more active
history of erosion and deposition than the Moon.
In other aspects, Martian craters resemble lunar craters. Both are products of hypervelocity impacts
and show a progression of morphology types with increasing size.
Martian craters below about 7 km in diameter are called simple craters;
they are bowl-shaped with sharp raised rims and have depth/diameter
ratios of about 1/5. Martian craters change from simple to more complex types at diameters
of roughly 5 to 8 km. Complex craters have central peaks (or peak
complexes), relatively flat floors, and terracing or slumping along the
inner walls. Complex craters are shallower than simple craters in
proportion to their widths, with depth/diameter ratios ranging from 1/5
at the simple-to-complex transition diameter (~7 km) to about 1/30 for a
100-km diameter crater. Another transition occurs at crater diameters
of around 130 km as central peaks turn into concentric rings of hills to
form multi-ring basins.
Mars has the greatest diversity of impact crater types of any planet in the Solar System. This is partly because the presence of both rocky and volatile-rich
layers in the subsurface produces a range of morphologies even among
craters within the same size classes. Mars also has an atmosphere that
plays a role in ejecta emplacement and subsequent erosion. Moreover,
Mars has a rate of volcanic and tectonic activity low enough that
ancient, eroded craters are still preserved, yet high enough to have
resurfaced large areas, producing a diverse range of crater populations
of widely differing ages. Over 42,000 impact craters greater than 5 km
in diameter have been catalogued on Mars, and the number of smaller craters is probably innumerable. The density
of craters on Mars is highest in the southern hemisphere, south of the
dichotomy boundary. This is where most of the large craters and basins
are located.
Crater morphology provides information about the physical
structure and composition of the surface and subsurface at the time of
impact. For example, the size of central peaks in Martian craters is
larger than comparable craters on Mercury or the Moon. In addition, the central peaks of many large craters on Mars have pit
craters at their summits. Central pit craters are rare on the Moon but
are very common on Mars and the icy satellites of the outer Solar
System. Large central peaks and the abundance of pit craters probably
indicate the presence of near-surface ice at the time of impact. Polewards of 30 degrees of latitude, the form of older impact craters is rounded out ("softened") by acceleration of soil creep by ground ice.
The most notable difference between Martian craters and other
craters in the Solar System is the presence of lobate (fluidized) ejecta
blankets. Many craters at equatorial and mid-latitudes on Mars have
this form of ejecta morphology, which is thought to arise when the
impacting object melts ice in the subsurface. Liquid water in the
ejected material forms a muddy slurry that flows along the surface,
producing the characteristic lobe shapes. The crater Yuty is a good example of a rampart crater, which is so called because of the rampart-like edge to its ejecta blanket.
Martian craters are commonly classified by their ejecta. Craters with
one ejecta layer are called single-layer ejecta (SLE) craters. Craters
with two superposed ejecta blankets are called double-layer ejecta (DLE)
craters, and craters with more than two ejecta layers are called
multiple-layered ejecta (MLE) craters. These morphological differences
are thought to reflect compositional differences (i.e. interlayered ice,
rock, or water) in the subsurface at the time of impact.
Martian craters show a large diversity of preservational states, from
extremely fresh to old and eroded. Degraded and infilled impact craters
record variations in volcanic, fluvial, and eolian activity over geologic time. Pedestal craters are craters
with their ejecta sitting above the surrounding terrain to form raised
platforms. They occur because the crater's ejecta forms a resistant
layer so that the area nearest the crater erodes more slowly than the
rest of the region. Some pedestals were hundreds of meters above the
surrounding area, meaning that hundreds of meters of material were
eroded. Pedestal craters were first observed during the Mariner 9 mission in 1972.
Volcanic structures and landforms cover large parts of the Martian
surface. The most conspicuous volcanoes on Mars are located in Tharsis and Elysium.
Geologists think one of the reasons volcanoes on Mars were able to grow
so large is that Mars has fewer tectonic boundaries in comparison to
Earth. Lava from a stationary hot spot was able to accumulate at one location on the surface for many hundreds of millions of years.
Scientists have never recorded an active volcano eruption on the surface of Mars. Searches for thermal signatures and surface changes within the last decade have not yielded evidence for active volcanism.
On October 17, 2012, the Curiosity rover on the planet Mars at "Rocknest" performed the first X-ray diffraction analysis of Martian soil. The results from the rover's CheMin analyzer revealed the presence of several minerals, including feldspar, pyroxenes and olivine, and suggested that the Martian soil in the sample was similar to the "weathered basaltic soils" of Hawaiian volcanoes. In July 2015, the same rover identified tridymite
in a rock sample from Gale Crater, leading scientists to conclude that
silicic volcanism might have played a much more prevalent role in the
planet's volcanic history than previously thought.
Collection of spheres, each about 3 mm in diameter as seen by Opportunity rover
Flowing water appears to have been common on the surface of Mars at
various points in its history, and especially on ancient Mars. Many of these flows carved the surface, forming valley networks and producing sediment. This sediment has been redeposited in a wide variety of wet environments, including in alluvial fans, meandering channels, deltas, lakes, and perhaps even oceans.The processes of deposition and transportation are associated with
gravity. Due to gravity, related differences in water fluxes and flow
speeds, inferred from grain size distributions, Martian landscapes were
created by different environmental conditions. Nevertheless, there are other ways of estimating the amount of water on ancient Mars (see: Water on Mars). Groundwater has been implicated in the cementation of aeolian sediments and the formation and transport of a wide variety of sedimentary minerals including clays, sulphates and hematite.
When the surface has been dry, wind has been a major geomorphic agent. Wind driven sand bodies like megaripples and dunes are extremely common on the modern Martian surface, and Opportunity has documented abundant aeolian sandstones on its traverse. Ventifacts, like Jake Matijevic (rock), are another aeolian landform on the Martian Surface.
A wide variety of other sedimentological facies are also present locally on Mars, including glacial deposits, hot springs, dry mass movement deposits (especially landslides), and cryogenic and periglacial material, amongst many others. Evidence for ancient rivers, a lake, and dune fields have all been observed in the preserved strata by rovers at Meridiani Planum and Gale crater.
One
group of researchers proposed that some of the layers on Mars were
caused by groundwater rising to the surface in many places, especially
inside of craters. According to the theory, groundwater with dissolved
minerals came to the surface, in and later around craters, and helped to
form layers by adding minerals (especially sulfate) and cementing
sediments. This hypothesis is supported by a groundwater model and by
sulfates discovered in a wide area. At first, by examining surface materials with Opportunity Rover, scientists discovered that groundwater had repeatedly risen and deposited sulfates. Later studies with instruments on board the Mars Reconnaissance Orbiter showed that the same kinds of materials existed in a large area that included Arabia.
Interesting geomorphological features
Avalanches
On February 19, 2008, images obtained by the HiRISE camera on the Mars Reconnaissance Orbiter
showed a spectacular avalanche, in which debris thought to be
fine-grained ice, dust, and large blocks fell from a 700-metre
(2,300 ft) high cliff. Evidence of the avalanche included dust clouds
rising from the cliff afterwards. Such geological events are theorized to be the cause of geologic patterns known as slope streaks.
Possible caves
NASA scientists studying pictures from the Odyssey spacecraft have spotted what might be seven caves on the flanks of the Arsia Monsvolcano on Mars.
The pit entrances measure from 100 to 252 metres (328 to 827 ft) wide
and they are thought to be at least 73 to 96 metres (240 to 315 ft)
deep. See image below: the pits have been informally named (A) Dena, (B)
Chloe, (C) Wendy, (D) Annie, (E) Abby (left) and Nikki, and (F) Jeanne.
Dena's floor was observed and found to be 130 m deep. Further investigation suggested that these were not necessarily lava tube "skylights". Review of the images has resulted in yet more discoveries of deep pits. Recently, a global database (MGC3) of over 1,000 Martian cave candidates at Tharsis Montes has been developed by the USGS Astrogeology Science Center. In 2021, scientists are applying machine-learning algorithms to extend the MGC3 database across the entire surface of Mars.
It has been suggested that human explorers on Mars could use lava
tubes as shelters. The caves may be the only natural structures
offering protection from the micrometeoroids, UV radiation, solar flares, and high energy particles that bombard the planet's surface. These features may enhance preservation of biosignatures over long periods of time and make caves an attractive astrobiology target in the search for evidence of life beyond Earth.
Some areas of Mars show inverted relief, where features that were
once depressions, like streams, are now above the surface. It is
believed that materials like large rocks were deposited in low-lying
areas. Later, wind erosion removed much of the surface layers, but left
behind the more resistant deposits. Other ways of making inverted relief
might be lava flowing down a stream bed or materials being cemented by
minerals dissolved in water. On Earth, materials cemented by silica are
highly resistant to all kinds of erosional forces. Examples of inverted
channels on Earth are found in the Cedar Mountain Formation near Green River, Utah. Inverted relief in the shape of streams are further evidence of water flowing on the Martian surface in past times. Inverted relief in the form of stream channels suggests that the
climate was different—much wetter—when the inverted channels were
formed.
In an article published in 2010, a large group of scientists
endorsed the idea of searching for life in Miyamoto Crater because of
inverted stream channels and minerals that indicated the past presence
of water.
Gravitational waves transport energy as gravitational radiation, a form of radiant energy similar to electromagnetic radiation. Newton's law of universal gravitation, part of classical mechanics,
does not provide for their existence, instead asserting that gravity
has instantaneous effect everywhere. Gravitational waves therefore stand
as an important relativistic phenomenon that is absent from Newtonian
physics.
The first indirect evidence for the existence of gravitational waves came in 1974 from the observed orbital decay of the Hulse–Taylor binary pulsar, which matched the decay predicted by general relativity for energy lost to gravitational radiation. In 1993, Russell Alan Hulse and Joseph Hooton Taylor Jr. received the Nobel Prize in Physics for this discovery.
As a gravitational wave passes an observer, that observer will find spacetime distorted by the effects of strain.
Distances between objects increase and decrease rhythmically as the
wave passes, at a frequency equal to that of the wave. The magnitude of
this effect is inversely proportional to the distance (not distance squared) from the source.
Inspiraling binary neutron stars are predicted to be a powerful source of gravitational waves as they coalesce, due to the very large acceleration of their masses as they orbit
close to one another. However, due to the astronomical distances to
these sources, the effects when measured on Earth are predicted to be
very small, having strains of less than 1 part in 1020.
Scientists demonstrate the existence of these waves with highly-sensitive detectors at multiple observation sites. As of 2012, the LIGO and Virgo observatories were the most sensitive detectors, operating at resolutions of about one part in 5×1022. The Japanese detector KAGRA was completed in 2019; its first joint detection with LIGO and VIRGO was reported in 2021. Another European ground-based detector, the Einstein Telescope, is under development. A space-based observatory, the Laser Interferometer Space Antenna (LISA), is also being developed by the European Space Agency.
Linearly polarized gravitational wave
Gravitational waves do not strongly interact with matter in the way that electromagnetic radiation does.[1]: 33–34
This allows for the observation of events involving exotic objects in
the distant universe that cannot be observed with more traditional means
such as optical telescopes or radio telescopes; accordingly, gravitational wave astronomy gives new insights into the workings of the universe.
In particular, gravitational waves could be of interest to cosmologists
as they offer a possible way of observing the very early universe. This
is not possible with conventional astronomy, since before recombination the universe was opaque to electromagnetic radiation. Precise measurements of gravitational waves will also allow scientists
to test more thoroughly the general theory of relativity.
In principle, gravitational waves can exist at any frequency. Very low frequency waves can be detected using pulsar timing arrays.
In this technique, the timing of approximately 100 pulsars spread
widely across our galaxy is monitored over the course of years.
Detectable changes in the arrival time of their signals can result from
passing gravitational waves generated by merging supermassive black holes (SMBH) with wavelengths measured in light-years. These timing changes can be used to locate the source of the waves.
Using this technique, astronomers have discovered the 'hum' of various SMBH mergers occurring in the universe. Stephen Hawking and Werner Israel list different frequency bands for gravitational waves that could plausibly be detected, ranging from 10−7 Hz up to 1011 Hz.
The speed of gravitational waves in the general theory of relativity is equal to the speed of light in vacuum, c. Within the theory of special relativity, the constant c is not only about light; instead it is the highest possible speed for any interaction in nature. Formally, c is a conversion factor for changing the unit of time to the unit of space. This makes it the only speed which does not depend either on the motion of an observer or a source of light and/or gravity.
Thus, the speed of "light" is also the speed of gravitational
waves, and, further, the speed of any massless particle. Such particles
include the gluon (carrier of the strong force), the photons that make up light (hence carrier of electromagnetic force), and the hypothetical gravitons
(which are the presumptive field particles associated with gravity;
however, an understanding of the graviton, if any exist, requires an
as-yet unavailable theory of quantum gravity).
In August 2017, the LIGO and Virgo detectors received a gravitational wave signal, GW170817, at nearly the same time as gamma ray satellites and optical telescopes received signals from its source in galaxy NGC 4993, about 130 million light years away. This measurement constrained the experimental difference between the
speed of gravitational waves and light to be smaller than one part in 1015.
The possibility of gravitational waves and that those might travel at the speed of light was discussed in 1893 by Oliver Heaviside, using the analogy between the inverse-square law of gravitation and the electrostatic force. In 1905, Henri Poincaré
proposed gravitational waves, emanating from a body and propagating at
the speed of light, as being required by the Lorentz transformations and suggested that, in analogy to an accelerating electrical charge producing electromagnetic waves, accelerated masses in a relativistic field theory of gravity should produce gravitational waves.
In 1915 Einstein published his general theory of relativity,
a complete relativistic theory of gravitation. He conjectured, like
Poincaré, that the equation would produce gravitational waves, but, as
he mentions in a letter to Schwarzschild in February 1916, these could not be similar to electromagnetic waves. Electromagnetic
waves can be produced by dipole motion, requiring both a positive and a
negative charge. Gravitation has no equivalent to negative charge.
Einstein continued to work through the complexity of the equations of
general relativity to find an alternative wave model. The result was
published in June 1916, and there he came to the conclusion that the gravitational wave must
propagate with the speed of light, and there must, in fact, be three
types of gravitational waves dubbed longitudinal–longitudinal,
transverse–longitudinal, and transverse–transverse by Hermann Weyl.
However, the nature of Einstein's approximations led many (including Einstein himself) to doubt the result. In 1922, Arthur Eddington
showed that two of Einstein's types of waves were artifacts of the
coordinate system he used, and could be made to propagate at any speed
by choosing appropriate coordinates, leading Eddington to jest that they
"propagate at the speed of thought".
This also cast doubt on the physicality of the third
(transverse–transverse) type that Eddington showed always propagate at
the speed of light regardless of coordinate system. In 1936, Einstein and Nathan Rosen submitted a paper to Physical Review
in which they claimed gravitational waves could not exist in the full
general theory of relativity because any such solution of the field
equations would have a singularity. The journal sent their manuscript to
be reviewed by Howard P. Robertson,
who anonymously reported that the singularities in question were simply
the harmless coordinate singularities of the employed cylindrical
coordinates. Einstein, who was unfamiliar with the concept of peer
review, angrily withdrew the manuscript, never to publish in Physical Review again. Nonetheless, his assistant Leopold Infeld,
who had been in contact with Robertson, convinced Einstein that the
criticism was correct, and the paper was rewritten with the opposite
conclusion and published elsewhere. In 1956, Felix Pirani
remedied the confusion caused by the use of various coordinate systems
by rephrasing the gravitational waves in terms of the manifestly
observable Riemann curvature tensor.
At the time, Pirani's work was overshadowed by the community's
focus on a different question: whether gravitational waves could
transmit energy. This matter was settled by a thought experiment proposed by Richard Feynman during the first "GR" conference at Chapel Hill in 1957. In short, his argument known as the "sticky bead argument"
notes that if one takes a rod with beads then the effect of a passing
gravitational wave would be to move the beads along the rod; friction
would then produce heat, implying that the passing wave had done work. Shortly after, Hermann Bondi published a detailed version of the "sticky bead argument". This later led to a series of articles (1959 to 1989) by Bondi and Pirani that established the existence of plane wave solutions for gravitational waves.
Paul Dirac
further postulated the existence of gravitational waves, declaring them
to have "physical significance" in his 1959 lecture at the Lindau Meetings. Further, it was Dirac who predicted gravitational waves with a well-defined energy density in 1964.
After the Chapel Hill conference, Joseph Weber started designing and building the first gravitational wave detectors now known as Weber bars.
In 1969, Weber claimed to have detected the first gravitational waves,
and by 1970 he was "detecting" signals regularly from the Galactic Center;
however, the frequency of detection soon raised doubts on the validity
of his observations as the implied rate of energy loss of the Milky Way
would drain our galaxy of energy on a timescale much shorter than its
inferred age. These doubts were strengthened when, by the mid-1970s,
repeated experiments from other groups building their own Weber bars
across the globe failed to find any signals, and by the late 1970s
consensus was that Weber's results were spurious.
In the same period, the first indirect evidence of gravitational waves was discovered. In 1974, Russell Alan Hulse and Joseph Hooton Taylor, Jr. discovered the first binary pulsar, which earned them the 1993 Nobel Prize in Physics. Pulsar timing observations over the next decade showed a gradual decay
of the orbital period of the Hulse–Taylor pulsar that matched the loss
of energy and angular momentum in gravitational radiation predicted by
general relativity.
This indirect detection of gravitational waves motivated further
searches, despite Weber's discredited result. Some groups continued to
improve Weber's original concept, while others pursued the detection of
gravitational waves using laser interferometers. The idea of using a
laser interferometer for this seems to have been floated independently
by various people, including M.E. Gertsenshtein and V. I. Pustovoit in
1962, and Vladimir B. Braginskiĭ in 1966. The first prototypes were developed in the 1970s by Robert L. Forward and Rainer Weiss. In the decades that followed, ever more sensitive instruments were constructed, culminating in the construction of GEO600, LIGO, and Virgo.
After years of producing null results, improved detectors became operational in 2015. On 11 February 2016, the LIGO-Virgo collaborations announced the first observation of gravitational waves, from a signal (dubbed GW150914) detected at 09:50:45 GMT on 14 September 2015 of two black holes with masses of 29 and 36 solar masses
merging about 1.3 billion light-years away. During the final fraction
of a second of the merger, it released more than 50 times the power of all the stars in the observable universe combined. The signal increased in frequency from 35 to 250 Hz over 10 cycles (5
orbits) as it rose in strength for a period of 0.2 second. The mass of the new merged black hole was 62 solar masses. Energy
equivalent to three solar masses was emitted as gravitational waves. The signal was seen by both LIGO detectors in Livingston and Hanford,
with a time difference of 7 milliseconds due to the angle between the
two detectors and the source. The signal came from the Southern Celestial Hemisphere, in the rough direction of (but much farther away than) the Magellanic Clouds. The confidence level of this being an observation of gravitational waves was 99.99994%.
A year earlier, the BICEP2 collaboration claimed that they had detected the imprint of gravitational waves in the cosmic microwave background. However, they were later forced to retract this result.
In 2023, NANOGrav, EPTA, PPTA, InPTA, and CPTA announced that they found evidence of a gravitational wave background. North American Nanohertz Observatory for Gravitational Waves states, that they were created over cosmological time scales by supermassive black holes, identifying the distinctive Hellings-Downs curve in 15 years of radio observations of 67 pulsars. Similar results are published by European Pulsar Timing Array, who claimed a -significance. They expect that a -significance will be achieved by 2025 by combining the measurements of several collaborations.
Effects of passing
The effect of a plus-polarized gravitational wave on a ring of particlesThe effect of a cross-polarized gravitational wave on a ring of particles
Gravitational waves are constantly passing Earth;
however, even the strongest have a minuscule effect since their sources
are generally at a great distance. For example, the waves given off by
the cataclysmic final merger of GW150914 reached Earth after travelling over a billion light-years, as a ripple in spacetime that changed the length of a 4 km LIGO arm by a thousandth of the width of a proton, proportionally equivalent to changing the distance to the nearest star outside the Solar System by one hair's width. This tiny effect from even extreme gravitational waves makes them
observable on Earth only with the most sophisticated detectors.
The effects of a passing gravitational wave, in an extremely
exaggerated form, can be visualized by imagining a perfectly flat region
of spacetime
with a group of motionless test particles lying in a plane, e.g., the
surface of a computer screen. As a gravitational wave passes through the
particles along a line perpendicular to the plane of the particles,
i.e., following the observer's line of vision into the screen, the
particles will follow the distortion in spacetime, oscillating in a "cruciform"
manner, as shown in the animations. The area enclosed by the test
particles does not change and there is no motion along the direction of
propagation.
The oscillations depicted in the animation are exaggerated for
the purpose of discussion – in reality a gravitational wave has a very
small amplitude (as formulated in linearized gravity).
However, they help illustrate the kind of oscillations associated with
gravitational waves as produced by a pair of masses in a circular orbit. In this case the amplitude of the gravitational wave is constant, but its plane of polarization
changes or rotates at twice the orbital rate, so the time-varying
gravitational wave size, or 'periodic spacetime strain', exhibits a
variation as shown in the animation. If the orbit of the masses is elliptical then the gravitational wave's amplitude also varies with time according to Einstein's quadrupole formula.
As with other waves, there are a number of characteristics used to describe a gravitational wave:
Amplitude: Usually denoted h, this is the size of the wave – the fraction of stretching or squeezing in the animation. The amplitude shown here is roughly h = 0.5 (or 50%). Gravitational waves passing through the Earth are many sextillion times weaker than this – h ≈ 10−20.
Frequency: Usually denoted f,
this is the frequency with which the wave oscillates (1 divided by the
amount of time between two successive maximum stretches or squeezes)
Wavelength: Usually denoted λ, this is the distance along the wave between points of maximum stretch or squeeze.
Speed:
This is the speed at which a point on the wave (for example, a point of
maximum stretch or squeeze) travels. For gravitational waves with small
amplitudes, this wave speed is equal to the speed of light (c).
The speed, wavelength, and frequency of a gravitational wave are related by the equation c = λf, just like the equation for a light wave.
For example, the animations shown here oscillate roughly once every two
seconds. This would correspond to a frequency of 0.5 Hz, and a
wavelength of about 600 000 km, or 47 times the diameter of the Earth.
In the above example, it is assumed that the wave is linearly polarized with a "plus" polarization, written h+.
Polarization of a gravitational wave is just like polarization of a
light wave except that the polarizations of a gravitational wave are
45 degrees apart, as opposed to 90 degrees. In particular, in a "cross"-polarized gravitational wave, h×,
the effect on the test particles would be basically the same, but
rotated by 45 degrees, as shown in the second animation. Just as with
light polarization, the polarizations of gravitational waves may also be
expressed in terms of circularly polarized waves. Gravitational waves are polarized because of the nature of their source.
Sources
The gravitational wave spectrum with sources and detectors. Credit: NASA Goddard Space Flight Center
In general terms, gravitational waves are radiated by large, coherent
motions of immense mass, especially in regions where gravity is so
strong that Newtonian gravity begins to fail.
The effect does not occur in a purely spherically symmetric system. A simple example of this principle is a spinning dumbbell.
If the dumbbell spins around its axis of symmetry, it will not radiate
gravitational waves; if it tumbles end over end, as in the case of two
planets orbiting each other, it will radiate gravitational waves. The
heavier the dumbbell, and the faster it tumbles, the greater is the
gravitational radiation it will give off. In an extreme case, such as
when the two weights of the dumbbell are massive stars like neutron
stars or black holes, orbiting each other quickly, then significant
amounts of gravitational radiation would be given off.
Some more detailed examples:
Two objects orbiting each other, as a planet would orbit the Sun, will radiate.
A spinning non-axisymmetric planetoid – say with a large bump or dimple on the equator – will radiate.
A supernovawill radiate except in the unlikely event that the explosion is perfectly symmetric.
An isolated non-spinning solid object moving at a constant velocity will not radiate. This can be regarded as a consequence of the principle of conservation of linear momentum.
A spherically pulsating spherical star (non-zero monopole moment or mass, but zero quadrupole moment) will not radiate, in agreement with Birkhoff's theorem.
More technically, the second time derivative of the quadrupole moment (or the l-th time derivative of the l-th multipole moment) of an isolated system's stress–energy tensor
must be non-zero in order for it to emit gravitational radiation. This
is analogous to the changing dipole moment of charge or current that is
necessary for the emission of electromagnetic radiation.
Two stars of dissimilar mass are in circular orbits. Each revolves about their common center of mass (denoted by the small red cross) in a circle with the larger mass having the smaller orbit.Two stars of similar mass in circular orbits about their center of massTwo stars of similar mass in highly elliptical orbits about their center of mass
Gravitational waves carry energy away from their sources and, in the
case of orbiting bodies, this is associated with an in-spiral or
decrease in orbit. Imagine for example a simple system of two masses – such as the
Earth–Sun system – moving slowly compared to the speed of light in
circular orbits. Assume that these two masses orbit each other in a
circular orbit in the x–y plane. To a good approximation, the masses follow simple Keplerian orbits. However, such an orbit represents a changing quadrupole moment. That is, the system will give off gravitational waves.
In theory, the loss of energy through gravitational radiation could eventually drop the Earth into the Sun. However, the total energy of the Earth orbiting the Sun (kinetic energy + gravitational potential energy) is about 1.14×1036joules of which only 200 watts (joules per second) is lost through gravitational radiation, leading to a decay in the orbit by about 1×10−15 meters per day or roughly the diameter of a proton. At this rate, it would take the Earth approximately 3×1013 times more than the current age of the universe to spiral onto the Sun. This estimate overlooks the decrease in r over time, but the radius varies only slowly for most of the time and plunges at later stages, as with the initial radius and the total time needed to fully coalesce.
More generally, the rate of orbital decay can be approximated by
where r is the separation between the bodies, t time, G the gravitational constant, c the speed of light, and m1 and m2 the masses of the bodies. This leads to an expected time to merger of
Compact binaries
Compact stars like white dwarfs and neutron stars can be constituents of binaries. For example, a pair of solar mass neutron stars in a circular orbit at a separation of 1.89×108 m (189,000 km) has an orbital period of 1,000 seconds, and an expected lifetime of 1.30×1013 seconds or about 414,000 years. Such a system could be observed by LISA
if it were not too far away. A far greater number of white dwarf
binaries exist with orbital periods in this range. White dwarf binaries
have masses in the order of the Sun, and diameters in the order of the Earth. They cannot get much closer together than 10,000 km before they will merge and explode in a supernova
which would also end the emission of gravitational waves. Until then,
their gravitational radiation would be comparable to that of a neutron
star binary.
Artist's impression of merging neutron stars, a source of gravitational waves
When the orbit of a neutron star binary has decayed to 1.89×106
m (1890 km), its remaining lifetime is about 130,000 seconds or 36
hours. The orbital frequency will vary from 1 orbit per second at the
start, to 918 orbits per second when the orbit has shrunk to 20 km at
merger. The majority of gravitational radiation emitted will be at twice
the orbital frequency. Just before merger, the inspiral could be
observed by LIGO if such a binary were close enough. LIGO has only a few
minutes to observe this merger out of a total orbital lifetime that may
have been billions of years. In August 2017, LIGO and Virgo observed
the first binary neutron star inspiral in GW170817, and 70 observatories collaborated to detect the electromagnetic counterpart, a kilonova in the galaxy NGC 4993, 40 megaparsecs away, emitting a short gamma ray burst (GRB 170817A) seconds after the merger, followed by a longer optical transient (AT 2017gfo) powered by r-process
nuclei. Advanced LIGO detectors should be able to detect such events up
to 200 megaparsecs away; at this range, around 40 detections per year
would be expected.
Black hole binaries emit gravitational waves during their in-spiral, merger,
and ring-down phases. Hence, in the early 1990s the physics community
rallied around a concerted effort to predict the waveforms of
gravitational waves from these systems with the Binary Black Hole Grand Challenge Alliance. The largest amplitude of emission occurs during the merger phase, which
can be modeled with the techniques of numerical relativity. The first direct detection of gravitational waves, GW150914, came from the merger of two black holes.
A supernova is a transient astronomical event
that occurs during the last stellar evolutionary stages of a massive
star's life, whose dramatic and catastrophic destruction is marked by
one final titanic explosion. This explosion can happen in one of many
ways, but in all of them a significant proportion of the matter in the
star is blown away into the surrounding space at extremely high
velocities (up to 10% of the speed of light). Unless there is perfect
spherical symmetry in these explosions (i.e., unless matter is spewed
out evenly in all directions), there will be gravitational radiation
from the explosion. This is because gravitational waves are generated by a changing quadrupole moment,
which can happen only when there is asymmetrical movement of masses.
Since the exact mechanism by which supernovae take place is not fully
understood, it is not easy to model the gravitational radiation emitted
by them.
Spinning neutron stars
As
noted above, a mass distribution will emit gravitational radiation only
when there is spherically asymmetric motion among the masses. A spinning neutron star
will generally emit no gravitational radiation because neutron stars
are highly dense objects with a strong gravitational field that keeps
them almost perfectly spherical. In some cases, however, there might be
slight deformities on the surface called "mountains", which are bumps
extending no more than 10 centimeters (4 inches) above the surface, that make the spinning spherically asymmetric. This gives the star a
quadrupole moment that changes with time, and it will emit gravitational
waves until the deformities are smoothed out.
Primordial gravitational wave
Gravitational
waves from the early universe could provide a unique probe for
cosmology. Because these wave interact very weakly with matter they
would propagate freely from very early time when other signals are
trapped by the large density of energy. If this gravitational radiation
could be detected it would be gravitational wave background complementary to the cosmic microwave background data. However the existence of primordial gravitational waves can also be inferred from their effects.
Models of "slow-roll" cosmic inflation in the early universe predicts primordial gravitational waves that would impact the polarisation of the cosmic microwave background, creating a specific pattern of B-mode polarization.
Detection of this pattern would support the theory of inflation and
their strength can confirm and exclude different models of inflation. While claims that this characteristic pattern of B-mode polarization had been measured by BICEP2 instrument were later attributed to cosmic dust due to new results of the Planck experiment, subsequent reanalysis with compensation for foreground dust show limits in agreement with results from Lambda-CDM models.
Properties and behaviour
Energy, momentum, and angular momentum
Water waves, sound waves, and electromagnetic waves are able to carry energy, momentum, and angular momentum and by doing so they carry those away from the source. Gravitational waves perform the same function. Thus, for example, a
binary system loses angular momentum as the two orbiting objects spiral
towards each other – the angular momentum is radiated away by
gravitational waves.
The waves can also carry off linear momentum, a possibility that has some interesting implications for astrophysics. After two supermassive black holes coalesce, emission of linear
momentum can produce a "kick" with amplitude as large as 4000 km/s. This
is fast enough to eject the coalesced black hole completely from its
host galaxy. Even if the kick is too small to eject the black hole
completely, it can remove it temporarily from the nucleus of the galaxy,
after which it will oscillate about the center, eventually coming to
rest. A kicked black hole can also carry a star cluster with it, forming a hyper-compact stellar system. Or it may carry gas, allowing the recoiling black hole to appear temporarily as a "naked quasar".
The quasarSDSS J092712.65+294344.0 is thought to contain a recoiling supermassive black hole.
If such a particle exists, it is expected to be massless (because the gravitational force appears to have unlimited range) and must be a spin-2 boson.
It can be shown that any massless spin-2 field would give rise to a
force indistinguishable from gravitation, because a massless spin-2
field must couple to (interact with) the stress-energy tensor in the
same way that the gravitational field does; therefore if a massless
spin-2 particle were ever discovered, it would be likely to be the
graviton without further distinction from other massless spin-2
particles. Such a discovery would unite quantum theory with gravity.
Significance for study of the early universe
Due
to the weakness of the coupling of gravity to matter, gravitational
waves experience very little absorption or scattering, even as they
travel over astronomical distances. In particular, gravitational waves
are expected to be unaffected by the opacity of the very early universe.
In these early phases, space had not yet become "transparent", so
observations based upon light, radio waves, and other electromagnetic
radiation that far back into time are limited or unavailable. Therefore,
gravitational waves are expected in principle to have the potential to
provide a wealth of observational data about the very early universe.
Determining direction of travel
The
difficulty in directly detecting gravitational waves means it is also
difficult for a single detector to identify by itself the direction of a
source. Therefore, multiple detectors are used, both to distinguish
signals from other "noise" by confirming the signal is not of earthly
origin, and also to determine direction by means of triangulation. This technique uses the fact that the waves travel at the speed of light
and will reach different detectors at different times depending on
their source direction. Although the differences in arrival time may be
just a few milliseconds, this is sufficient to identify the direction of the origin of the wave with considerable precision.
Only in the case of GW170814
were three detectors operating at the time of the event, therefore, the
direction is precisely defined. The detection by all three instruments
led to a very accurate estimate of the position of the source, with a
90% credible region of just 60 deg2, a factor 20 more accurate than before.
Two-dimensional representation of gravitational waves generated by two neutron stars orbiting each other.
During the past century, astronomy
has been revolutionized by the use of new methods for observing the
universe. Astronomical observations were initially made using visible light. Galileo Galilei pioneered the use of telescopes to enhance these observations. However, visible light is only a small portion of the electromagnetic spectrum,
and not all objects in the distant universe shine strongly in this
particular band. More information may be found, for example, in radio
wavelengths. Using radio telescopes, astronomers have discovered pulsars and quasars, for example. Observations in the microwave band led to the detection of faint imprints of the Big Bang, a discovery Stephen Hawking called the "greatest discovery of the century, if not all time". Similar advances in observations using gamma rays, x-rays, ultraviolet light, and infrared light
have also brought new insights to astronomy. As each of these regions
of the spectrum has opened, new discoveries have been made that could
not have been made otherwise. The astronomy community hopes that the
same holds true of gravitational waves.
Gravitational waves have two important and unique properties.
First, there is no need for any type of matter to be present nearby in
order for the waves to be generated by a binary system of uncharged
black holes, which would emit no electromagnetic radiation. Second,
gravitational waves can pass through any intervening matter without
being scattered significantly. Whereas light from distant stars may be
blocked out by interstellar dust,
for example, gravitational waves will pass through essentially
unimpeded. These two features allow gravitational waves to carry
information about astronomical phenomena heretofore never observed by
humans.
The sources of gravitational waves described above are in the low-frequency end of the gravitational-wave spectrum (10−7 to 105 Hz). An astrophysical source at the high-frequency end of the gravitational-wave spectrum (above 105 Hz and probably 1010 Hz) generates relic gravitational waves that are theorized to be faint imprints of the Big Bang like the cosmic microwave background. At these high frequencies it is potentially possible that the sources may be "man made" that is, gravitational waves generated and detected in the laboratory.
A supermassive black hole, created from the merger of the black holes at the center of two merging galaxies detected by the Hubble Space Telescope, is theorized to have been ejected from the merger center by gravitational waves.
Now disproved evidence allegedly showing gravitational waves in the infant universe was found by the BICEP2radio telescope. The microscopic examination of the focal plane of the BICEP2 detector is shown here. In January 2015, however, the BICEP2 findings were confirmed to be the result of cosmic dust.
Indirect detection
Although
the waves from the Earth–Sun system are minuscule, astronomers can
point to other sources for which the radiation should be substantial.
One important example is the Hulse–Taylor binary – a pair of stars, one of which is a pulsar. The characteristics of their orbit can be deduced from the Doppler shifting of radio signals given off by the pulsar. Each of the stars is about 1.4 M☉ and the size of their orbits is about 1/75 of the Earth–Sun orbit,
just a few times larger than the diameter of our own Sun. The
combination of greater masses and smaller separation means that the
energy given off by the Hulse–Taylor binary will be far greater than the
energy given off by the Earth–Sun system – roughly 1022 times as much.
The information about the orbit can be used to predict how much
energy (and angular momentum) would be radiated in the form of
gravitational waves. As the binary system loses energy, the stars
gradually draw closer to each other, and the orbital period decreases.
The resulting trajectory of each star is an inspiral, a spiral with
decreasing radius. General relativity precisely describes these
trajectories; in particular, the energy radiated in gravitational waves
determines the rate of decrease in the period, defined as the time
interval between successive periastrons (points of closest approach of
the two stars). For the Hulse–Taylor pulsar, the predicted current
change in radius is about 3 mm per orbit, and the change in the 7.75 hr
period is about 2 seconds per year. Following a preliminary observation
showing an orbital energy loss consistent with gravitational waves, careful timing observations by Taylor and Joel Weisberg dramatically confirmed the predicted period decrease to within 10%. With the improved statistics of more than 30 years of timing data since
the pulsar's discovery, the observed change in the orbital period
currently matches the prediction from gravitational radiation assumed by
general relativity to within 0.2 percent. In 1993, spurred in part by this indirect detection of gravitational
waves, the Nobel Committee awarded the Nobel Prize in Physics to Hulse
and Taylor for "the discovery of a new type of pulsar, a discovery that
has opened up new possibilities for the study of gravitation". The lifetime of this binary system, from the present to merger is estimated to be a few hundred million years.
Inspirals are very important sources of gravitational waves. Any time two compact objects (white dwarfs, neutron stars, or black holes)
are in close orbits, they send out intense gravitational waves. As they
spiral closer to each other, these waves become more intense. At some
point they should become so intense that direct detection by their
effect on objects on Earth or in space is possible. This direct
detection is the goal of several large-scale experiments.
The only difficulty is that most systems like the Hulse–Taylor
binary are so far away. The amplitude of waves given off by the
Hulse–Taylor binary at Earth would be roughly h ≈ 10−26. There are some sources, however, that astrophysicists expect to find that produce much greater amplitudes of h ≈ 10−20. At least eight other binary pulsars have been discovered.
Difficulties
Gravitational
waves are not easily detectable. When they reach the Earth, they have a
small amplitude with strain approximately 10−21, meaning that an extremely sensitive detector is needed, and that other sources of noise can overwhelm the signal. Gravitational waves are expected to have frequencies 10−16 Hz < f < 104 Hz.
Ground-based detectors
A schematic diagram of a laser interferometer
Though the Hulse–Taylor observations were very important, they give only indirect evidence for gravitational waves. A more conclusive observation would be a direct
measurement of the effect of a passing gravitational wave, which could
also provide more information about the system that generated it. Any
such direct detection is complicated by the extraordinarily small
effect the waves would produce on a detector. The amplitude of a
spherical wave will fall off as the inverse of the distance from the
source (the 1/R term in the formulas for h above). Thus,
even waves from extreme systems like merging binary black holes die out
to very small amplitudes by the time they reach the Earth.
Astrophysicists expect that some gravitational waves passing the Earth
may be as large as h ≈ 10−20, but generally no bigger.
Resonant antennas
A simple device theorised to detect the expected wave motion is called a Weber bar –
a large, solid bar of metal isolated from outside vibrations. This
type of instrument was the first type of gravitational wave detector.
Strains in space due to an incident gravitational wave excite the bar's resonant frequency
and could thus be amplified to detectable levels. Conceivably, a nearby
supernova might be strong enough to be seen without resonant
amplification. With this instrument, Joseph Weber claimed to have detected daily signals of gravitational waves. His results, however, were contested in 1974 by physicists Richard Garwin and David Douglass. Modern forms of the Weber bar are still operated, cryogenically cooled, with superconducting quantum interference devices to detect vibration. Weber bars are not sensitive enough to detect anything but extremely powerful gravitational waves.
MiniGRAIL is a spherical gravitational wave antenna using this principle. It is based at Leiden University, consisting of an exactingly machined 1,150 kg sphere cryogenically cooled to 20 millikelvins. The spherical configuration allows for equal sensitivity in all
directions, and is somewhat experimentally simpler than larger linear
devices requiring high vacuum. Events are detected by measuring deformation of the detector sphere.
MiniGRAIL is highly sensitive in the 2–4 kHz range, suitable for
detecting gravitational waves from rotating neutron star instabilities
or small black hole mergers.
There are currently two detectors focused on the higher end of the gravitational wave spectrum (10−7 to 105 Hz): one at University of Birmingham, England, and the other at INFN Genoa, Italy. A third is under development at Chongqing University, China. The Birmingham detector measures changes in the polarization state of a microwave
beam circulating in a closed loop about one meter across. Both
detectors are expected to be sensitive to periodic spacetime strains of h ~ 2×10−13 /√Hz, given as an amplitude spectral density. The INFN Genoa detector is a resonant antenna consisting of two coupled spherical superconducting
harmonic oscillators a few centimeters in diameter. The oscillators are
designed to have (when uncoupled) almost equal resonant frequencies.
The system is currently expected to have a sensitivity to periodic
spacetime strains of h ~ 2×10−17 /√Hz, with an expectation to reach a sensitivity of h ~ 2×10−20 /√Hz.
The Chongqing University detector is planned to detect relic
high-frequency gravitational waves with the predicted typical parameters
≈1011 Hz (100 GHz) and h ≈10−30 to 10−32.
Simplified operation of a gravitational wave observatory
Figure 1:
A beamsplitter (green line) splits coherent light (from the white box)
into two beams which reflect off the mirrors (cyan oblongs); only one
outgoing and reflected beam in each arm is shown, and separated for
clarity. The reflected beams recombine and an interference pattern is
detected (purple circle).
Figure 2: A gravitational wave passing over the left arm (yellow) changes its length and thus the interference pattern.
A more sensitive class of detector uses a laser Michelson interferometer to measure gravitational-wave induced motion between separated 'free' masses. This allows the masses to be separated by large distances (increasing
the signal size); a further advantage is that it is sensitive to a wide
range of frequencies (not just those near a resonance as is the case for
Weber bars). After years of development ground-based interferometers
made the first detection of gravitational waves in 2015.
Currently, the most sensitive is LIGO – the Laser Interferometer Gravitational Wave Observatory. LIGO has three detectors: one in Livingston, Louisiana, one at the Hanford site in Richland, Washington and a third (formerly installed as a second detector at Hanford) that is planned to be moved to India. Each observatory has two light storage arms
that are 4 kilometers in length. These are at 90 degree angles to each
other, with the light passing through 1 m diameter vacuum tubes running
the entire 4 kilometers. A passing gravitational wave will slightly
stretch one arm as it shortens the other. This is the motion to which an
interferometer is most sensitive.
Even with such long arms, the strongest gravitational waves will
only change the distance between the ends of the arms by at most roughly
10−18 m. LIGO should be able to detect gravitational waves as small as h ~ 5×10−22. Upgrades to LIGO and Virgo should increase the sensitivity still further. Another highly sensitive interferometer, KAGRA, which is located in the Kamioka Observatory
in Japan, is in operation since February 2020. A key point is that a
tenfold increase in sensitivity (radius of 'reach') increases the volume
of space accessible to the instrument by one thousand times. This
increases the rate at which detectable signals might be seen from one
per tens of years of observation, to tens per year.
Interferometric detectors are limited at high frequencies by shot noise,
which occurs because the lasers produce photons randomly; one analogy
is to rainfall – the rate of rainfall, like the laser intensity, is
measurable, but the raindrops, like photons, fall at random times,
causing fluctuations around the average value. This leads to noise at
the output of the detector, much like radio static. In addition, for
sufficiently high laser power, the random momentum transferred to the
test masses by the laser photons shakes the mirrors, masking signals of
low frequencies. Thermal noise (e.g., Brownian motion)
is another limit to sensitivity. In addition to these 'stationary'
(constant) noise sources, all ground-based detectors are also limited at
low frequencies by seismic
noise and other forms of environmental vibration, and other
'non-stationary' noise sources; creaks in mechanical structures,
lightning or other large electrical disturbances, etc. may also create
noise masking an event or may even imitate an event. All of these must
be taken into account and excluded by analysis before detection may be
considered a true gravitational wave event.
The simplest gravitational waves are those with constant frequency.
The waves given off by a spinning, non-axisymmetric neutron star would
be approximately monochromatic: a pure tone in acoustics.
Unlike signals from supernovae or binary black holes, these signals
evolve little in amplitude or frequency over the period it would be
observed by ground-based detectors. However, there would be some change
in the measured signal, because of Doppler shifting
caused by the motion of the Earth. Despite the signals being simple,
detection is extremely computationally expensive, because of the long
stretches of data that must be analysed.
The Einstein@Home project is a distributed computing project similar to SETI@home
intended to detect this type of gravitational wave. By taking data from
LIGO and GEO, and sending it out in little pieces to thousands of
volunteers for parallel analysis on their home computers, Einstein@Home
can sift through the data far more quickly than would be possible
otherwise.
Space-based interferometers
Space-based interferometers, such as LISA and DECIGO,
are also being developed. LISA's design calls for three test masses
forming an equilateral triangle, with lasers from each spacecraft to
each other spacecraft forming two independent interferometers. LISA is
planned to occupy a solar orbit trailing the Earth, with each arm of the
triangle being 2.5 million kilometers. This puts the detector in an excellent vacuum far from Earth-based
sources of noise, though it will still be susceptible to heat, shot noise, and artifacts caused by cosmic rays and solar wind.
Plot of correlation between pulsars observed by NANOGrav vs angular separation between pulsars, compared with a theoretical Hellings-Downs model (dashed purple) and if there were no gravitational wave background (solid green)
Pulsars
are highly magnetized, rapidly rotating neutron stars. A pulsar emits
beams of radio waves that, like lighthouse beams, sweep through the sky
as the pulsar rotates. The signal from a pulsar can be detected by radio
telescopes as a series of regularly spaced pulses, essentially like the
ticks of a clock. GWs affect the time it takes the pulses to travel
from the pulsar to a telescope on Earth. A pulsar timing array uses millisecond pulsars
to seek out perturbations due to GWs in measurements of the time of
arrival of pulses to a telescope, in other words, to look for deviations
in the clock ticks. To detect GWs, pulsar timing arrays search for a distinct quadrupolar pattern of correlation and anti-correlation between the time of arrival of pulses from different pulsar pairs as a function of their angular separation in the sky. Although pulsar pulses travel through space for hundreds or thousands
of years to reach us, pulsar timing arrays are sensitive to
perturbations in their travel time of much less than a millionth of a
second.
The most likely source of GWs to which pulsar timing arrays are
sensitive are supermassive black hole binaries, which form from the
collision of galaxies. In addition to individual binary systems, pulsar timing arrays are
sensitive to a stochastic background of GWs made from the sum of GWs
from many galaxy mergers. Other potential signal sources include cosmic strings and the primordial background of GWs from cosmic inflation.
In June 2023, NANOGrav, EPTA, InPTA, PPTA, and CPTA published the first evidence for a stochastic gravitational wave background. In particular, they announced evidence for the Hellings-Downs curve, the tell-tale sign of the gravitational wave origin of the observed background. In December 2024, MPTA also published evidence for the gravitational wave background.
Primordial gravitational waves are gravitational waves observed in the cosmic microwave background. They were allegedly detected by the BICEP2
instrument, an announcement made on 17 March 2014, which was withdrawn
on 30 January 2015 ("the signal can be entirely attributed to dust in the Milky Way").
LIGO
measurement of the gravitational waves at the Hanford (left) and
Livingston (right) detectors, compared to the theoretical predicted
values.
On 11 February 2016, the LIGO collaboration announced the first observation of gravitational waves, from a signal detected at 09:50:45 GMT on 14 September 2015 of two black holes with masses of 29 and 36 solar masses
merging about 1.3 billion light-years away. During the final fraction
of a second of the merger, it released more than 50 times the power of all the stars in the observable universe combined. The signal increased in frequency from 35 to 250 Hz over 10 cycles (5
orbits) as it rose in strength for a period of 0.2 second. The mass of the new merged black hole was 62 solar masses. Energy
equivalent to three solar masses was emitted as gravitational waves. The signal was seen by both LIGO detectors in Livingston and Hanford,
with a time difference of 7 milliseconds due to the angle between the
two detectors and the source. The signal came from the Southern Celestial Hemisphere, in the rough direction of (but much farther away than) the Magellanic Clouds. The gravitational waves were observed in the region more than 5 sigma (in other words, 99.99997% chances of showing/getting the same result),
the probability of finding enough to have been assessed/considered as
the evidence/proof in an experiment of statistical physics.
On 16 October 2017, the LIGO and Virgo collaborations announced
the first-ever detection of gravitational waves originating from the
coalescence of a binary neutron star system. The observation of the GW170817
transient, which occurred on 17 August 2017, allowed for constraining
the masses of the neutron stars involved between 0.86 and 2.26 solar
masses. Further analysis allowed a greater restriction of the mass
values to the interval 1.17–1.60 solar masses, with the total system
mass measured to be 2.73–2.78 solar masses. The inclusion of the Virgo
detector in the observation effort allowed for an improvement of the
localization of the source by a factor of 10. This in turn facilitated
the electromagnetic follow-up of the event. The signal lasted about 100
seconds, much longer than the few seconds measured from binary black
holes. Also in contrast to the case of binary black hole mergers, binary
neutron star mergers were expected to yield an electromagnetic
counterpart, that is, a light signal associated with the event. A
gamma-ray burst (GRB 170817A) was detected by the Fermi Gamma-ray Space Telescope, occurring 1.7 seconds after the gravitational wave transient. The signal, originating near the galaxy NGC 4993, was associated with the neutron star merger. This was corroborated by the electromagnetic follow-up of the event (AT 2017gfo),
involving 70 telescopes and observatories and yielding observations
over a large region of the electromagnetic spectrum which further
confirmed the neutron star nature of the merged objects and the
associated kilonova.
In 2021, the detection of the first two neutron star-black hole
binaries by the LIGO and VIRGO detectors was published in the
Astrophysical Journal Letters, allowing to first set bounds on the
quantity of such systems. No neutron star-black hole binary had ever
been observed using conventional means before the gravitational
observation.
Microscopic sources
In 1964, L. Halpern
and B. Laurent theoretically proved that gravitational spin-2 electron
transitions are possible in atoms. Compared to electric and magnetic
transitions the emission probability is extremely low. Stimulated
emission was discussed for increasing the efficiency of the process. Due
to the lack of mirrors or resonators for gravitational waves, they
determined that a single pass GASER (a kind of laser emitting
gravitational waves) is practically unfeasible.
In 1998, the possibility of a different implementation of the
above theoretical analysis was proposed by Giorgio Fontana. The required
coherence for a practical GASER could be obtained by Cooper pairs in superconductors that are characterized by a macroscopic collective wave-function. Cuprate high temperature superconductors are characterized by the presence of s-wave and d-wave Cooper pairs. Transitions between s-wave and d-wave are gravitational
spin-2. Out of equilibrium conditions can be induced by injecting s-wave
Cooper pairs from a low temperature superconductor, for instance lead or niobium, which is pure s-wave, by means of a Josephson junction with high critical current. The amplification mechanism can be described as the effect of superradiance,
and 10 cubic centimeters of cuprate high temperature superconductor
seem sufficient for the mechanism to properly work. A detailed
description of the approach can be found in "High Temperature
Superconductors as Quantum Sources of Gravitational Waves: The HTSC
GASER". Chapter 3 of this book.
In Stanislaw Lem's 1986 novel Fiasco,
a "gravity gun" or "gracer" (gravity amplification by collimated
emission of resonance) is used to reshape a collapsar, so that the
protagonists can exploit the extreme relativistic effects and make an
interstellar journey.
In Greg Egan's 1997 novel Diaspora,
the analysis of a gravitational wave signal from the inspiral of a
nearby binary neutron star reveals that its collision and merger is
imminent, implying a large gamma-ray burst is going to impact the Earth.
In Liu Cixin's 2006 Remembrance of Earth's Past
series, gravitational waves are used as an interstellar broadcast
signal, which serves as a central plot point in the conflict between
civilizations within the galaxy.
Figure 2 for the geologic map of Mars
.svg)
