The first direct observation of gravitational waves was made on 14 September 2015 and was announced by the
LIGO
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large ...
and
Virgo
Virgo may refer to:
*Virgo (astrology), the sixth astrological sign of the zodiac
* Virgo (constellation), a constellation
*Virgo Cluster, a cluster of galaxies in the constellation Virgo
*Virgo Stellar Stream, remains of a dwarf galaxy
* Virgo Su ...
collaborations on 11 February 2016.
Previously,
gravitational waves had been inferred only indirectly, via their effect on the timing of
pulsars
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward E ...
in
binary star systems. The
waveform
In electronics, acoustics, and related fields, the waveform of a signal is the shape of its graph as a function of time, independent of its time and magnitude scales and of any displacement in time.David Crecraft, David Gorham, ''Electro ...
, detected by both LIGO observatories,
matched the predictions of
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
for a
gravitational wave emanating from the
inward spiral and
merger of a
pair of black holes of around 36 and 29
solar masses and the subsequent "ringdown" of the single resulting black hole. The signal was named GW150914 (from ''gravitational wave'' and the date of observation 2015-09-14).
It was also the first observation of a binary black hole merger, demonstrating both the existence of binary
stellar-mass black hole
A stellar black hole (or stellar-mass black hole) is a black hole formed by the gravitational collapse of a star. They have masses ranging from about 5 to several tens of solar masses. The process is observed as a hypernova explosion or as a g ...
systems and the fact that such mergers could occur within the current
age of the universe
In physical cosmology, the age of the universe is the time elapsed since the Big Bang. Astronomers have derived two different measurements of the age of the universe:
a measurement based on direct observations of an early state of the universe, ...
.
This first direct observation was reported around the world as a remarkable accomplishment for many reasons. Efforts to directly prove the existence of such waves had been ongoing for over fifty years, and the waves are so minuscule that
Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
himself doubted that they could ever be detected.
The waves given off by the cataclysmic merger of GW150914 reached Earth as a ripple in
spacetime
In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differ ...
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.
The energy released by the binary as it spiralled together and merged was immense, with the energy of ''c''
2 solar masses ( joules or
foes) in total radiated as gravitational waves, reaching a peak emission rate in its final few milliseconds of about
watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...
s – a level greater than the combined
power
Power most often refers to:
* Power (physics), meaning "rate of doing work"
** Engine power, the power put out by an engine
** Electric power
* Power (social and political), the ability to influence people or events
** Abusive power
Power may a ...
of all light radiated by all the stars in the
observable universe
The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these ob ...
.
The observation confirms the last remaining directly undetected prediction of
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
and corroborates its predictions of space-time distortion in the context of large scale cosmic events (known as
strong field tests). It was also heralded as inaugurating a new era of
gravitational-wave astronomy
Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves (minute distortions of spacetime predicted by Albert Einstein's theory of general relativity) to collect observational data about ...
, which will enable observations of violent astrophysical events that were not previously possible and potentially allow the direct observation of the very earliest
history of the universe
History (derived ) is the systematic study and the documentation of the human activity. The time period of event before the invention of writing systems is considered prehistory. "History" is an umbrella term comprising past events as well ...
.
[CNN quoting Prof. Martin Hendry (University of Glasgow, LIGO)](_blank)
– ''"Detecting gravitational waves will help us to probe the most extreme corners of the cosmos – the event horizon of a black hole, the innermost heart of a supernova, the internal structure of a neutron star: regions that are completely inaccessible to electromagnetic telescopes."'' On 15 June 2016, two more detections of gravitational waves, made in late 2015, were announced.
Eight more observations were made in 2017, including
GW170817
GW 170817 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017, originating from the shell elliptical galaxy . The signal was produced by the last minutes of a binary pair of neutron stars' insp ...
, the first observed merger of binary
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...
s, which was also observed in
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) li ...
.
Gravitational waves
Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
originally predicted the existence of gravitational waves in 1916, on the basis of his theory of
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
. General relativity interprets
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
as a consequence of distortions in
space-time, caused by
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
. Therefore, Einstein also predicted that events in the cosmos would cause "ripples" in space-time – distortions of space-time itself – which would spread outward, although they would be so minuscule that they would be nearly impossible to detect by any technology foreseen at that time.
It was also predicted that objects moving in an orbit would lose energy for this reason (a consequence of the law of
conservation of energy), as some energy would be given off as gravitational waves, although this would be insignificantly small in all but the most extreme cases.
One case where gravitational waves would be strongest is during the final moments of the merger of two
compact objects such as
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...
s or
black holes. Over a span of millions of years,
binary neutron stars, and
binary black holes lose energy, largely through gravitational waves, and as a result, they
spiral in towards each other. At the very end of this process, the two objects will reach extreme velocities, and in the final fraction of a second of their merger a substantial amount of their mass would theoretically be converted into gravitational energy, and travel outward as gravitational waves,
allowing a greater than usual chance for detection. However, since little was known about the number of compact binaries in the universe and reaching that final stage can be very slow, there was little certainty as to how often such events might happen.
Observation
Gravitational waves can be detected indirectly – by observing celestial phenomena caused by gravitational waves – or more directly by means of instruments such as the Earth-based
LIGO
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large ...
or the planned space-based
LISA Lisa or LISA may refer to:
People
People with the mononym
* Lisa Lisa (born 1967), American actress and lead singer of the Cult Jam
* Lisa (Japanese musician, born 1974), stylized "LISA", Japanese singer and producer
* Lisa Komine (born 1978), J ...
instrument.
Indirect observation
Evidence of gravitational waves was first deduced in 1974 through the motion of the double neutron star system
PSR B1913+16
PSR may refer to:
Organizations
* Pacific School of Religion, Berkeley, California, US
* Palestinian Center for Policy and Survey Research
* Physicians for Social Responsibility, US
;Political parties:
* Revolutionary Socialist Party (Portugal) ( ...
, in which one of the stars is a
pulsar that emits electro-magnetic pulses at radio frequencies at precise, regular intervals as it rotates.
Russell Hulse
Russell Alan Hulse (born November 28, 1950) is an American physicist and winner of the Nobel Prize in Physics, shared with his thesis advisor Joseph Hooton Taylor Jr., "''for the discovery of a new type of pulsar, a discovery that has opened up n ...
and
Joseph Taylor, who discovered the stars, also showed that over time, the frequency of pulses shortened, and that the stars were gradually spiralling towards each other with an energy loss that agreed closely with the predicted energy that would be radiated by gravitational waves. For this work, Hulse and Taylor were awarded the
Nobel Prize in Physics
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
in 1993. Further observations of this pulsar and others in multiple systems (such as the
double pulsar system
PSR J0737-3039
PSR may refer to:
Organizations
* Pacific School of Religion, Berkeley, California, US
* Palestinian Center for Policy and Survey Research
* Physicians for Social Responsibility, US
;Political parties:
* Revolutionary Socialist Party (Portugal) ( ...
) also agree with General Relativity to high precision.
Direct observation
![Northern leg of LIGO interferometer on Hanford Reservation](https://upload.wikimedia.org/wikipedia/commons/4/43/Northern_leg_of_LIGO_interferometer_on_Hanford_Reservation.JPG)
Direct observation of gravitational waves was not possible for the many decades after they were predicted due to the minuscule effect that would need to be detected and separated from the background of vibrations present everywhere on Earth. A technique called
interferometry was suggested in the 1960s and eventually technology developed sufficiently for this technique to become feasible.
In the present approach used by LIGO, a
laser beam
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
is split and the two halves are recombined after travelling different paths. Changes to the length of the paths or the time taken for the two split beams, caused by the effect of passing gravitational waves, to reach the point where they recombine are revealed as "
beats". Such a technique is extremely sensitive to tiny changes in the distance or time taken to traverse the two paths. In theory, an interferometer with arms about 4 km long would be capable of revealing the change of space-time – a tiny fraction of the size of a single
proton – as a gravitational wave of sufficient strength passed through Earth from elsewhere. This effect would be perceptible only to other interferometers of a similar size, such as the
Virgo
Virgo may refer to:
*Virgo (astrology), the sixth astrological sign of the zodiac
* Virgo (constellation), a constellation
*Virgo Cluster, a cluster of galaxies in the constellation Virgo
*Virgo Stellar Stream, remains of a dwarf galaxy
* Virgo Su ...
,
GEO 600
GEO600 is a gravitational wave detector located near Sarstedt, a town 20 km to the south of Hanover, Germany. It is designed and operated by scientists from the Max Planck Institute for Gravitational Physics, Max Planck Institute of Quantum Opt ...
and planned
KAGRA
The Kamioka Gravitational Wave Detector (KAGRA), is a large interferometer designed to detect gravitational waves predicted by the general theory of relativity. KAGRA is a Michelson interferometer that is isolated from external disturbances: its m ...
and
INDIGO
Indigo is a deep color close to the color wheel blue (a primary color in the RGB color space), as well as to some variants of ultramarine, based on the ancient dye of the same name. The word "indigo" comes from the Latin word ''indicum'', m ...
detectors. In practice at least two interferometers would be needed because any gravitational wave would be detected at both of these but other kinds of disturbance would generally not be present at both. This technique allows the sought-after signal to be distinguished from
noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
. This project was eventually founded in 1992 as the
Laser Interferometer Gravitational-Wave Observatory (LIGO). The original instruments were upgraded between 2010 and 2015 (to Advanced LIGO), giving an increase of around 10 times their original sensitivity.
LIGO operates two
gravitational-wave observatories in unison, located apart: the LIGO Livingston Observatory () in
Livingston, Louisiana
Livingston is the parish seat of Livingston Parish, Louisiana, United States. The population was 1,769 at the 2010 census.
Livingston hosts one of the two LIGO gravitational wave detector sites, the other one being located in Hanford, Washin ...
, and the LIGO Hanford Observatory, on the
DOE Hanford Site () near
Richland, Washington
Richland () is a city in Benton County, Washington, United States. It is located in southeastern Washington at the confluence of the Yakima and the Columbia Rivers. As of the 2020 census, the city's population was 60,560. Along with the nearby c ...
. The tiny shifts in the length of their arms are continually compared and significant patterns which appear to arise synchronously are followed up to determine whether a gravitational wave may have been detected or if some other cause was responsible.
Initial LIGO operations between 2002 and 2010 did not detect any statistically significant events that could be confirmed as gravitational waves. This was followed by a multi-year shut-down while the detectors were replaced by much improved "Advanced LIGO" versions. In February 2015, the two advanced detectors were brought into engineering mode, in which the instruments are operating fully for the purpose of testing and confirming they are functioning correctly before being used for research, with formal science observations due to begin on 18 September 2015.
Throughout the development and initial observations by LIGO, several "blind injections" of fake gravitational wave signals were introduced to test the ability of the researchers to identify such signals. To protect the efficacy of blind injections, only four LIGO scientists knew when such injections occurred, and that information was revealed only after a signal had been thoroughly analyzed by researchers.
On 14 September 2015, while LIGO was running in engineering mode but without any blind data injections, the instrument reported a possible gravitational wave detection. The detected event was given the name GW150914.
GW150914 event
Event detection
GW150914 was detected by the LIGO detectors in
Hanford, Washington
Hanford was a small agricultural community in Benton County, Washington, United States. It and White Bluffs were depopulated in 1943 in order to make room for the nuclear production facility known as the Hanford Site. The town was located in what ...
state, and
Livingston, Louisiana
Livingston is the parish seat of Livingston Parish, Louisiana, United States. The population was 1,769 at the 2010 census.
Livingston hosts one of the two LIGO gravitational wave detector sites, the other one being located in Hanford, Washin ...
, USA, at 09:50:45
UTC on 14 September 2015.
The LIGO detectors were operating in "engineering mode", meaning that they were operating fully but had not yet begun a formal "research" phase (which was due to commence three days later on 18 September), so initially there was a question as to whether the signals had been real detections or simulated data for testing purposes before it was ascertained that they were not tests.
The
chirp signal lasted over 0.2 seconds, and increased in frequency and amplitude in about 8 cycles from 35 Hz to 250 Hz.
The signal is in the
audible range
Hearing range describes the range of frequencies that can be heard by humans or other animals, though it can also refer to the range of levels. The human range is commonly given as 20 to 20,000 Hz, although there is considerable variatio ...
and has been described as resembling the
"chirp" of a bird;
astrophysicists and other interested parties the world over excitedly responded by imitating the signal on
social media
Social media are interactive media technologies that facilitate the creation and sharing of information, ideas, interests, and other forms of expression through virtual communities and networks. While challenges to the definition of ''social medi ...
upon the announcement of the discovery.
(The frequency increases because each
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
is noticeably faster than the one before during the final moments before merging.)
The trigger that indicated a possible detection was reported within three minutes of acquisition of the signal, using rapid ('online') search methods that provide a quick, initial analysis of the data from the detectors.
After the initial automatic alert at 09:54 UTC, a sequence of internal emails confirmed that no scheduled or unscheduled injections had been made, and that the data looked clean.
After this, the rest of the collaborating team was quickly made aware of the tentative detection and its parameters.
More detailed statistical analysis of the signal, and of 16 days of surrounding data from 12 September to 20 October 2015, identified GW150914 as a real event, with an estimated significance of at least
5.1 sigma or a
confidence level
In frequentist statistics, a confidence interval (CI) is a range of estimates for an unknown parameter. A confidence interval is computed at a designated ''confidence level''; the 95% confidence level is most common, but other levels, such as 9 ...
of 99.99994%. Corresponding wave peaks were seen at Livingston seven milliseconds before they arrived at Hanford. Gravitational waves propagate at the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
, and the disparity is consistent with the light travel time between the two sites.
The waves had traveled at the speed of light for more than a billion years.
At the time of the event, the
Virgo gravitational wave detector (near Pisa, Italy) was offline and undergoing an upgrade; had it been online it would likely have been sensitive enough to also detect the signal, which would have greatly improved the positioning of the event.
GEO600
GEO600 is a gravitational wave detector located near Sarstedt, a town 20 km to the south of Hanover, Germany. It is designed and operated by scientists from the Max Planck Institute for Gravitational Physics, Max Planck Institute of Quantum Opti ...
(near
Hannover
Hanover (; german: Hannover ; nds, Hannober) is the capital and largest city of the German state of Lower Saxony. Its 535,932 (2021) inhabitants make it the 13th-largest city in Germany as well as the fourth-largest city in Northern Germany ...
, Germany) was not sensitive enough to detect the signal.
Consequently, neither of those detectors was able to confirm the signal measured by the LIGO detectors.
Astrophysical origin
![MergingBlackHoles V2](https://upload.wikimedia.org/wikipedia/commons/c/c4/MergingBlackHoles_V2.jpg)
The event happened at a
luminosity distance Luminosity distance ''DL'' is defined in terms of the relationship between the absolute magnitude ''M'' and apparent magnitude ''m'' of an astronomical object.
: M = m - 5 \log_\!\,
which gives:
: D_L = 10^
where ''DL'' is measured in parsecs. F ...
of
megaparsec
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
s
(determined by the amplitude of the signal),
or billion
light years, corresponding to a cosmological
redshift of (90%
credible interval
In Bayesian statistics, a credible interval is an interval within which an unobserved parameter value falls with a particular probability. It is an interval in the domain of a posterior probability distribution or a predictive distribution. T ...
s). Analysis of the signal along with the inferred redshift suggested that it was produced by the merger of two
black holes with masses of times and times the mass of the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
(in the source frame), resulting in a post-merger black hole of solar masses.
The
mass–energy of the missing
solar masses was radiated away in the form of gravitational waves.
During the final 20 milliseconds of the merger, the power of the radiated gravitational waves peaked at about or 526
dBm
DBM or dbm may refer to:
Science and technology
* dBm, a unit for power measurement
* DBM (computing), family of key-value database engines including dbm, ndbm, gdbm, and Berkeley DB
* Database Manager (DBM), a component of 1987's ''Extended Edi ...
– 50 times greater than the combined power of all light radiated by all the stars in the
observable universe
The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these ob ...
.
Across the 0.2-second duration of the detectable signal, the relative tangential (orbiting) velocity of the black holes increased from 30% to 60% of the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
. The orbital frequency of 75 Hz (half the gravitational wave frequency) means that the objects were orbiting each other at a distance of only 350 km by the time they merged. The
phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
changes to the signal's
polarization allowed calculation of the objects' orbital frequency, and taken together with the
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
and pattern of the signal, allowed calculation of their masses and therefore their extreme final velocities and orbital separation (distance apart) when they merged. That information showed that the objects had to be black holes, as any other kind of known objects with these masses would have been physically larger and therefore merged before that point, or would not have reached such velocities in such a small orbit. The highest observed neutron star mass is two solar masses, with a conservative
upper limit for the mass of a stable neutron star of three solar masses, so that a pair of neutron stars would not have had sufficient mass to account for the merger (unless exotic alternatives exist, for example,
boson stars),
while a black hole-
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...
pair would have merged sooner, resulting in a final orbital frequency that was not so high.
The decay of the waveform after it peaked was consistent with the damped oscillations of a black hole as it relaxed to a final merged configuration.
Although the inspiral motion of compact binaries can be described well from
post-Newtonian calculations, the strong gravitational field merger stage can only be solved in full generality by large-scale
numerical relativity
Numerical relativity is one of the branches of general relativity that uses numerical methods and algorithms to solve and analyze problems. To this end, supercomputers are often employed to study black holes, gravitational waves, neutron stars a ...
simulations.
In the improved model and analysis, the post-merger object is found to be a
rotating Kerr black hole with a spin parameter of ,
i.e. one with 2/3 of the
maximum possible angular momentum for its mass.
The two stars which formed the two black holes were likely formed about 2 billion years after the
Big Bang with masses of between 40 and 100 times the mass of the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
.
Location in the sky
Gravitational wave instruments are whole-sky monitors with little ability to resolve signals spatially. A network of such instruments is needed to locate the source in the sky through
triangulation. With only the two LIGO instruments in observational mode, GW150914's source location could only be confined to an arc on the sky. This was done via analysis of the ms time-delay, along with amplitude and phase consistency across both detectors. This analysis produced a credible region of 150 deg
2 with a probability of 50% or 610 deg
2 with a probability of 90% located mainly in the
Southern Celestial Hemisphere
The southern celestial hemisphere, also called the Southern Sky, is the southern half of the celestial sphere; that is, it lies south of the celestial equator. This arbitrary sphere, on which seemingly fixed stars form constellations, appea ...
,
in the rough direction of (but much farther than) the
Magellanic Clouds
The Magellanic Clouds (''Magellanic system'' or ''Nubeculae Magellani'') are two irregular dwarf galaxies in the southern celestial hemisphere. Orbiting the Milky Way galaxy, these satellite galaxies are members of the Local Group. Because bo ...
.
For
comparison, the area of the constellation Orion is 594 deg
2.
Coincident gamma-ray observation
The
Fermi Gamma-ray Space Telescope
The Fermi Gamma-ray Space Telescope (FGST, also FGRST), formerly called the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is ...
reported that its Gamma-Ray Burst Monitor (GBM) instrument detected a weak
gamma-ray burst
In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies. They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten millise ...
above 50 keV, starting 0.4 seconds after the LIGO event and with a positional uncertainty region overlapping that of the LIGO observation. The Fermi team calculated the odds of such an event being the result of a coincidence or noise at 0.22%.
However a gamma ray burst would not have been expected, and observations from the
INTEGRAL
In mathematics, an integral assigns numbers to functions in a way that describes displacement, area, volume, and other concepts that arise by combining infinitesimal data. The process of finding integrals is called integration. Along wit ...
telescope's all-sky SPI-ACS instrument indicated that any energy emission in gamma-rays and hard X-rays from the event was less than one millionth of the energy emitted as gravitational waves, which "excludes the possibility that the event is associated with substantial gamma-ray radiation, directed towards the observer". If the signal observed by the Fermi GBM was genuinely astrophysical, INTEGRAL would have indicated a clear detection at a significance of 15 sigma above background radiation.
The
AGILE space telescope also did not detect a gamma-ray counterpart of the event.
A follow-up analysis by an independent group, released in June 2016, developed a different statistical approach to estimate the spectrum of the gamma-ray transient. It concluded that Fermi GBM's data did not show evidence of a gamma ray burst, and was either background radiation or an Earth albedo transient on a 1-second timescale.
A rebuttal of this follow-up analysis, however, pointed out that the independent group misrepresented the analysis of the original Fermi GBM Team paper and therefore misconstrued the results of the original analysis. The rebuttal reaffirmed that the false coincidence probability is calculated empirically and is not refuted by the independent analysis.
Black hole mergers of the type thought to have produced the gravitational wave event are not expected to produce gamma-ray bursts, as stellar-mass black hole binaries are not expected to have large amounts of orbiting matter.
Avi Loeb
Abraham "Avi" Loeb ( he, אברהם (אבי) לייב; born February 26, 1962) is an Israeli-American theoretical physicist who works on astrophysics and cosmology. Loeb is the Frank B. Baird Jr. Professor of Science at Harvard University. He h ...
has theorised that if a massive star is rapidly rotating, the centrifugal force produced during its collapse will lead to the formation of a rotating bar that breaks into two dense clumps of matter with a dumbbell configuration that becomes a black hole binary, and at the end of the star's collapse it triggers a gamma-ray burst.
Loeb suggests that the 0.4 second delay is the time it took the gamma-ray burst to cross the star, relative to the gravitational waves.
Other follow-up observations
The reconstructed source area was targeted by follow-up observations covering
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmi ...
,
optical
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
,
near infra-red,
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
, and
gamma-ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically sh ...
wavelengths along with searches for coincident
neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s.
However, because LIGO had not yet started its science run, notice to other telescopes was delayed.
The
ANTARES
Antares is the brightest star in the constellation of Scorpius. It has the Bayer designation α Scorpii, which is Latinised to Alpha Scorpii. Often referred to as "the heart of the scorpion", Antares is flanked by σ Scorpii and τ ...
telescope detected no neutrino candidates within ±500 seconds of GW150914. The
IceCube Neutrino Observatory
The IceCube Neutrino Observatory (or simply IceCube) is a neutrino observatory constructed at the Amundsen–Scott South Pole Station in Antarctica. The project is a recognized CERN experiment (RE10).
Its thousands of sensors are located under ...
detected three neutrino candidates within ±500 seconds of GW150914. One event was found in the southern sky and two in the northern sky. This was consistent with the expectation of background detection levels. None of the candidates were compatible with the 90% confidence area of the merger event.
Although no neutrinos were detected, the lack of such observations provided a limit on neutrino emission from this type of gravitational wave event.
Observations by the
Swift Gamma-Ray Burst Mission
Neil Gehrels Swift Observatory, previously called the Swift Gamma-Ray Burst Explorer, is a NASA three-telescope space observatory for studying gamma-ray bursts (GRBs) and monitoring the afterglow in X-ray, and UV/Visible light at the location o ...
of nearby galaxies in the region of the detection, two days after the event, did not detect any new X-ray, optical or ultraviolet sources.
Announcement
The announcement of the detection was made on 11 February 2016
at a news conference in Washington, D.C. by
David Reitze
David Howard Reitze (born 6 January 1961) is an American laser physicist who is Professor of Physics at the University of Florida and served as the scientific spokesman of the Laser Interferometer Gravitational-Wave Observatory (LIGO) experimen ...
, the executive director of LIGO,
with a panel comprising
Gabriela González
Gabriela Ines González, (born 24 February 1965) is a professor of physics and astronomy at the Louisiana State University and was the spokesperson for the LIGO Scientific Collaboration from March 2011 until March 2017.
Biography
Gabriela Gonzá ...
,
Rainer Weiss
Rainer "Rai" Weiss ( , ; born September 29, 1932) is an American physicist, known for his contributions in gravitational physics and astrophysics. He is a professor of physics emeritus at MIT and an adjunct professor at LSU. He is best known ...
and
Kip Thorne
Kip Stephen Thorne (born June 1, 1940) is an American theoretical physicist known for his contributions in gravitational physics and astrophysics. A longtime friend and colleague of Stephen Hawking and Carl Sagan, he was the Richard P. F ...
, of LIGO, and
France A. Córdova, the director of
NSF
NSF may stand for:
Political organizations
*National Socialist Front, a Swedish National Socialist party
*NS-Frauenschaft, the women's wing of the former German Nazi party
*National Students Federation, a leftist Pakistani students' political gr ...
.
Barry Barish
Barry Clark Barish (born January 27, 1936) is an American experimental physicist and Nobel Laureate. He is a Linde Professor of Physics, emeritus at California Institute of Technology and a leading expert on gravitational waves.
In 2017, Bar ...
delivered the first presentation on this discovery to a scientific audience simultaneously with the public announcement.
The initial announcement paper was published during the news conference in ''
Physical Review Letters
''Physical Review Letters'' (''PRL''), established in 1958, is a peer-reviewed, scientific journal that is published 52 times per year by the American Physical Society. As also confirmed by various measurement standards, which include the ''Journa ...
'',
with further papers either published shortly afterwards
or immediately available in
preprint form.
Awards and recognition
In May 2016, the full collaboration, and in particular
Ronald Drever
Ronald William Prest Drever (26 October 1931 – 7 March 2017) was a Scottish experimental physicist. He was a professor emeritus at the California Institute of Technology, co-founded the LIGO project, and was a co-inventor of the Pound–Drever ...
,
Kip Thorne
Kip Stephen Thorne (born June 1, 1940) is an American theoretical physicist known for his contributions in gravitational physics and astrophysics. A longtime friend and colleague of Stephen Hawking and Carl Sagan, he was the Richard P. F ...
, and
Rainer Weiss
Rainer "Rai" Weiss ( , ; born September 29, 1932) is an American physicist, known for his contributions in gravitational physics and astrophysics. He is a professor of physics emeritus at MIT and an adjunct professor at LSU. He is best known ...
, received the
Special Breakthrough Prize in Fundamental Physics for the observation of gravitational waves. Drever, Thorne, Weiss, and the LIGO discovery team also received the
Gruber Prize in Cosmology
The Gruber Prize in Cosmology, established in 2000, is one of three prestigious international awards worth US$500,000 made by the Gruber Foundation, a non-profit organization based at Yale University in New Haven, Connecticut.
Since 2001, the G ...
. Drever, Thorne, and Weiss were also awarded the 2016
Shaw Prize in Astronomy and the 2016
Kavli Prize
The Kavli Prize was established in 2005 as a joint venture of the Norwegian Academy of Science and Letters, the Norwegian Ministry of Education and Research, and the Kavli Foundation. It honors, supports, and recognizes scientists for outstan ...
in Astrophysics. Barish was awarded the 2016
Enrico Fermi Prize
The Enrico Fermi Prize, first awarded in 2001, is given by the Italian Physical Society (Società Italiana di Fisica). It is a yearly award of €30,000 honoring one or more Members of the Society who have "particularly honoured physics with thei ...
from the
Italian Physical Society The (SIF) or Italian Physical Society was founded in 1897 and is a non-profit organization whose aim is to promote, encourage, protect the study and the progress of physics in Italy and in the world.
It is associated with the journal series '' Nuo ...
(Società Italiana di Fisica). In January 2017, LIGO spokesperson
Gabriela González
Gabriela Ines González, (born 24 February 1965) is a professor of physics and astronomy at the Louisiana State University and was the spokesperson for the LIGO Scientific Collaboration from March 2011 until March 2017.
Biography
Gabriela Gonzá ...
and the LIGO team were awarded the 2017
Bruno Rossi Prize
The Bruno Rossi Prize is awarded annually by the High Energy Astrophysics division of the American Astronomical Society "for a significant contribution to High Energy Astrophysics, with particular emphasis on recent, original work". Named after as ...
.
The 2017
Nobel Prize in Physics
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
was awarded to Rainer Weiss, Barry Barish and Kip Thorne "for decisive contributions to the LIGO detector and the observation of gravitational waves".
Implications
The observation was heralded as inaugurating a revolutionary era of
gravitational-wave astronomy
Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves (minute distortions of spacetime predicted by Albert Einstein's theory of general relativity) to collect observational data about ...
.
Prior to this detection, astrophysicists and cosmologists were able to make observations based upon
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) li ...
(including visible light, X-rays, microwave, radio waves, gamma rays) and particle-like entities (
cosmic ray
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s,
stellar winds,
neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s, and so on). These have significant limitations – light and other radiation may not be emitted by many kinds of objects, and can also be obscured or hidden behind other objects. Objects such as galaxies and nebulae can also absorb, re-emit, or modify light generated within or behind them, and
compact star
In astronomy, the term compact star (or compact object) refers collectively to white dwarfs, neutron stars, and black holes. It would grow to include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects ha ...
s or
exotic star
An exotic star is a hypothetical compact star composed of exotic matter (something not made of electrons, protons, neutrons or muons), and balanced against gravitational collapse by degeneracy pressure or other quantum properties. Exotic stars incl ...
s may contain material which is dark and radio silent, and as a result there is little evidence of their presence other than through their gravitational interactions.
Expectations for detection of future binary merger events
On 15 June 2016, the
LIGO
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large ...
group announced an observation of another gravitational wave signal, named
GW151226
GW151226 was a gravitational wave signal detected by the LIGO observatory on 25 December 2015 local time (26 Dec 2015 UTC). On 15 June 2016, the LIGO and Virgo collaborations announced that they had verified the signal, making it the second such ...
. The Advanced LIGO was predicted to detect five more black hole mergers like GW150914 in its next observing campaign from November 2016 until August 2017 (it turned out to be
seven
7 is a number, numeral, and glyph.
7 or seven may also refer to:
* AD 7, the seventh year of the AD era
* 7 BC, the seventh year before the AD era
* The month of
July
Music Artists
* Seven (Swiss singer) (born 1978), a Swiss recording artist ...
), and then 40 binary star mergers each year, in addition to an unknown number of more exotic gravitational wave sources, some of which may not be anticipated by current theory.
Planned upgrades are expected to double the
signal-to-noise
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal (electrical engineering), signal to the level of background Noise (signal processing), noise. SNR is defined as the ratio ...
ratio, expanding the volume of space in which events like GW150914 can be detected by a factor of ten. Additionally, Advanced Virgo,
KAGRA
The Kamioka Gravitational Wave Detector (KAGRA), is a large interferometer designed to detect gravitational waves predicted by the general theory of relativity. KAGRA is a Michelson interferometer that is isolated from external disturbances: its m ...
, and a possible third LIGO detector in India will extend the network and significantly improve the position reconstruction and parameter estimation of sources.
Laser Interferometer Space Antenna
The Laser Interferometer Space Antenna (LISA) is a proposed space probe to detect and accurately measure gravitational waves—tiny ripples in the fabric of spacetime—from astronomical sources. LISA would be the first dedicated space-based gr ...
(LISA) is a proposed space based observation mission to detect gravitational waves. With the proposed sensitivity range of LISA, merging binaries like GW150914 would be detectable about 1000 years before they merge, providing for a class of previously unknown sources for this observatory if they exist within about 10 megaparsecs.
LISA Pathfinder
LISA Pathfinder, formerly Small Missions for Advanced Research in Technology-2 (SMART-2), was an ESA spacecraft that was launched on 3 December 2015 on board Vega flight VV06. The mission tested technologies needed for the Laser Interferometer S ...
, LISA's technology development mission, was launched in December 2015 and it demonstrated that the LISA mission is feasible.
A current model predicts LIGO will detect approximately 1000 black hole mergers per year after it reaches full sensitivity planned for 2020.
Lessons for stellar evolution and astrophysics
The masses of the two pre-merger black holes provide information about
stellar evolution. Both black holes were more massive than previously discovered
stellar-mass black holes, which were inferred from
X-ray binary
X-ray binaries are a class of binary stars that are luminous in X-rays.
The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', whi ...
observations. This implies that the
stellar winds from their progenitor stars must have been relatively weak, and therefore that the
metallicity (mass fraction of chemical elements heavier than hydrogen and helium) must have been less than about half the solar value.
The fact that the pre-merger black holes were present in a
binary star system, as well as the fact that the system was compact enough to merge within the age of the universe, constrains either binary star evolution or
dynamical formation scenarios, depending on how the black hole binary was formed. A significant number of black holes must receive low
natal kicks (the velocity a black hole gains at its formation in a
core-collapse supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
event), otherwise the black hole forming in a binary star system would be ejected and an event like GW would be prevented.
The survival of such binaries, through common envelope phases of high rotation in massive progenitor stars, may be necessary for their survival. The majority of the latest black hole model predictions comply with these added constraints.
The discovery of the GW merger event increases the lower limit on the rate of such events, and rules out certain theoretical models that predicted very low rates of less than 1 Gpc
−3yr
−1 (one event per cubic gigaparsec per year).
Analysis resulted in lowering the previous upper limit rate on events like GW150914 from ~140 Gpc
−3yr
−1 to Gpc
−3yr
−1.
Impact on future cosmological observation
Measurement of the waveform and amplitude of the gravitational waves from a black hole merger event makes accurate determination of its distance possible. The accumulation of black hole merger data from cosmologically distant events may help to create more precise models of the history of the expansion of the universe and the nature of the
dark energy
In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univ ...
that influences it.
The
earliest universe is opaque since the cosmos was so energetic then that most matter was ionized and photons were scattered by free electrons. However, this opacity would not affect gravitational waves from that time, so if they occurred at levels strong enough to be detected at this distance, it would allow a window to observe the cosmos beyond the current
visible universe
The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these obj ...
. Gravitational-wave astronomy therefore may some day allow direct observation of the earliest
history of the universe
History (derived ) is the systematic study and the documentation of the human activity. The time period of event before the invention of writing systems is considered prehistory. "History" is an umbrella term comprising past events as well ...
.
Tests of general relativity
The inferred fundamental properties, mass and spin, of the post-merger black hole were consistent with those of the two pre-merger black holes, following the predictions of general relativity.
This is the first test of general relativity in the very
strong-field regime.
No evidence could be established against the predictions of general relativity.
The opportunity was limited in this signal to investigate the more complex general relativity interactions, such as tails produced by interactions between the gravitational wave and curved space-time background. Although a moderately strong signal, it is much smaller than that produced by binary-pulsar systems. In the future stronger signals, in conjunction with more sensitive detectors, could be used to explore the intricate interactions of gravitational waves as well as to improve the constraints on deviations from general relativity.
Speed of gravitational waves and limit on possible mass of graviton
The speed of gravitational waves (''v
g'') is predicted by general relativity to be the speed of light (''c'').
The extent of any deviation from this relationship can be parameterized in terms of the mass of the hypothetical
graviton
In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction. There is no complete quantum field theory of gravitons due to an outstanding mathem ...
. The graviton is the name given to an
elementary particle
In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions ( quarks, leptons, a ...
that would act as the
force carrier In quantum field theory, a force carrier, also known as messenger particle or intermediate particle, is a type of particle that gives rise to forces between other particles. These particles serve as the quanta of a particular kind of physical fi ...
for gravity, in
quantum theories about gravity. It is expected to be massless if, as it appears, gravitation has an infinite range. (This is because the more massive a
gauge boson is, the shorter is the range of the associated force; as with the infinite range of
electromagnetism
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...
, which is due to the massless
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
, the infinite range of gravity implies that any associated force-carrying particle would also be massless.) If the graviton were not massless, gravitational waves would propagate below lightspeed, with lower frequencies (''ƒ'') being slower than higher frequencies, leading to dispersion of the waves from the merger event.
No such dispersion was observed.
The observations of the inspiral slightly improve (lower) the upper limit on the mass of the graviton from Solar System observations to , corresponding to or a
Compton wavelength
The Compton wavelength is a quantum mechanical property of a particle. The Compton wavelength of a particle is equal to the wavelength of a photon whose energy is the same as the rest energy of that particle (see mass–energy equivalence). It was ...
(''λ
g'') of greater than km, roughly 1 light-year.
Using the lowest observed frequency of 35 Hz, this translates to a lower limit on ''v
g'' such that the upper limit on 1-''v
g'' /''c'' is ~ .
See also
*
Gravitational-wave astronomy
Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves (minute distortions of spacetime predicted by Albert Einstein's theory of general relativity) to collect observational data about ...
*
Gravitational-wave observatory
A gravitational-wave detector (used in a gravitational-wave observatory) is any device designed to measure tiny distortions of spacetime called gravitational waves. Since the 1960s, various kinds of gravitational-wave detectors have been built ...
*
List of gravitational wave observations
This is a list of observed/candidate gravitational wave events. Direct observation of gravitational waves, which commenced with the detection of an event by LIGO in 2015, constitutes part of gravitational wave astronomy. LIGO has played a role i ...
Notes
References
Further reading
*
External links
GW150914 data releaseby the LIGO Open Science Center
Gravitational wave modelling of GW150914 by the
Max Planck Institute for Gravitational Physics
The Max Planck Institute for Gravitational Physics (Albert Einstein Institute) is a Max Planck Institute whose research is aimed at investigating Einstein's theory of relativity and beyond: Mathematics, quantum gravity, astrophysical relativity, a ...
*
Video: GW150914 discovery press conference (71:29)by the
National Science Foundation
The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National ...
(11 February 2016)
Video: "The hunters – the detection of gravitational waves" (11:47)by the Max Planck Institute for Gravitational Physics (22 February 2016)
by
Dennis Overbye
Dennis Overbye (born June 2, 1944, in Seattle, Washington) is a science writer specializing in physics and cosmology and is the cosmic affairs correspondent for '' The New York Times''.
Biography
Overbye received his B.S. in physics from M.I.T. ...
, ''
The New York Times
''The New York Times'' (''the Times'', ''NYT'', or the Gray Lady) is a daily newspaper based in New York City with a worldwide readership reported in 2020 to comprise a declining 840,000 paid print subscribers, and a growing 6 million paid d ...
'' (11 February 2016)
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