The direct detection of gravitational waves is one of those events long-expected by physicists that are nevertheless a significant occurrence which excites the world scientific community as if something unexpected has happened. The event that happened on September 14, 2015 was not kept in utmost secrecy; however, everybody was waiting for the press conference that took place on February 11, 2016
(the link can be found, e.g., here ) and, most importantly, the paper. Many rumors leaked ahead of the conference, and the fact that something (that is, a signal corresponding to gravitational waves) was discovered by LIGO (Laser Interferometer Gravitational-Wave Observatory) was discussed on the sidelines. The details of the observations and data analysis by the LIGO collaboration (over 1000 people) became available on the arXiv server on February 12, 2016. These papers, including the technical ones, are listed below:
At the same time, papers by authors who, apparently, expected this day knowing about the LIGO results, appeared on arXiv:
We can note the FERMI collaboration preprint (V. Connaughton, E. Burns, A. Goldstein, Fermi GBM Observations of LIGO Gravitational Wave event GW150914. ) which was not posted to arXiv at the time [ the paper appeared on arXiv on February 15, 2016: arXiv:1602.03920. ]. Also, several important popular-science comments from physicists and astrophysicists appeared on the RuNet during the first three days, and some even earlier (we list only a few of them here):
We should point out straight away that
the attempts to register and investigate experimentally the gravitational
waves, which were predicted in Albert Einstein's general theory of relativity,
were started in the 60's by Joseph Weber, a professor of physics at the
University of Maryland (USA). Despite the pessimistic
results of the experiments, the research was continued in many countries,
The construction of LIGO began in
the 90's (by the initiative of Rainer Weiss, Ronald Drever,
and Kip Thorne). Observations were conducted with LIGO in 2002-2010, with no
results. In September 2015, during the calibration measurements for the
modernized "Advanced LIGO" project, a signal was detected with a significance
above 5.1 sigma, corresponding to the one expected for the merger of two
massive compact objects.
Here we need to show several figures available from the LIGO website.
Fig.1. Scheme of the LIGO laser interferometer.
Fig.2. Photo of the gravitational wave detector in
Fig.3. The signal of the event GW150914, registered by the two LIGO detectors, interpreted as a result of a gravitational wave (which developed as a result of a merger of two black holes) passing through the interferometer. Right: data from the facility in
Based on the analysis results, the
collaboration obtained a set of physical quantities: the masses of the compact objects (black
holes) - the pair M1=29(+/-4)Msun and M1=36(+5,-4)Msun before the merger and M=62(+/-4)Msun
after the merger; the mass that converted to gravitational waves, Mgw=3.0(+/-0.5)Msun (with a
factor of c^2); the estimate of the redshift z_gw=0.09(+0.03,-0.04), and even the metallicity in the
neighborhood of GW150914 of the order of Z~0.5Zsun. In addition, the region in
the sky where the signal came from was determined (although with poor
accuracy). Also, the graviton mass limit was obtained as <1.2x10^(-22)eV.
A third-party researcher, after
reviewing the data presented in Fig.3, would naturally have the following
questions: 1) how do we know the registered signal is not a noise fluctiation? 2) why does the paper
talk about black holes? 3) how were the physical
parameters obtained from the presented recordings?
1) The registered signal with the decreasing frequency and increasing amplitude strictly corresponds to the theoretically predicted gravitational wave signal from merging compact objects. The "frequency-time" dependence (Fig.4) is a distinctive characteristic of such a signal.
Fig.4. The gravitational wave signal as a frequency-time dependence. Top:
2) The type of the time dependence of frequency characterizes the distance between the two merging compact objects. If we can estimate the masses of the objects, then the frequency and its rate of change give us the distance between the objects and thus impose a limit on their size (see (Physical Review Letters 116, 061102 (2016), arXiv:1602.03837) for details).
3) The determination of the physical parameters is related to point 2) above. A simple estimation is given in the popular article by B. Stern in the "Troitsky Variant" (we quote the part which is very useful for the understanding of the physics of the process):
"How would one, looking at Fig.
[here, Fig.3], estimate the mass of the merging black
holes and the distances to them? One should estimate the rotation period of the
merging objects at the last moment. Looking at the figure we see that the
distance between the last few peaks is about ten times less than the distance
between the marks, i.e. about 5 milliseconds, which is roughly the rotation half-period
of a still strongly deformed black hole. What is the linear rotation velocity
of its surface? Comparable to the velocity of light, but
smaller, about a third (Kerr black hole limit) - regardless of size. The
half-circle of rotation will then be approximately
precise computational approach, the estimates of the objects' masses (and the
distances between them) are obtained as a result of parameterization of the
masses of the system components and their merger process (taking into account
their rotation axes) and constructing the likelihood function to determine the
most probable parameter values and the corresponding significance levels. The
estimates are given in Properties of the binary black hole
arXiv:1602.03840. In this approach, the masses and rotation
components of the objects are free parameters. Furthermore, considering the
fact that the observed signal has a cosmological redshift
z, it (the signal) can be matched by a mass scale varying by the same factor.
The mass selected through the likelihood function is then itself a function, m(z)=(1+z)m_real ,
with an unspecified redshift that can be varied when
minimizing the functional constructed from the likelihood function. The
additional parameter (redshift) can thus be
determined, and therefore, we can also determine the luminosity distance).
Figs.5 and 6 show several likelihood functions from (arXiv:1602.03840). The presence of massive black holes at z~0.1
implies a relatively weak stellar wind in the vicinity of GW150914 and an
approximately 1/2 Solar metallicity (arXiv:1602.03846).
Fig.5. Posteriori likelihood functions for the masses of the compact objects - system components m1_source and m2_source, where m2_source <= m1_source.
Fig.6. Posteriori likelihood functions for the pairs of determined parameters. Left: for the luminosity distance D_L to the object and the inclination of the system theta_JN. Right: for the mass and spin of the merged object.
The 7 ms registration delay between the two LIGO instruments and the different orientation of the arms of the installations made it possible to estimate the direction in the sky where the signal may have come from (Fig.7).
Fig.7. Approximate direction of the signal. The purple contours show the 90% probability zone.
after the registration, the LIGO team appealed to the SWIFT team to search for
a signal in other spectral regions, but the latter did not find any signals in
this sky area either in the X-ray, the hard X-ray, or the UV ranges (arXiv:1602.03868). Simultaneously, the FERMI satellite saw a faint transient source
(gamma-ray burst) with an amplitude of over 50 keV 0.4
s after the GW150914 event with a false alarm probability level 0.0022 (see arXiv:1602.03920). The location of the gamma-ray burst does not contradict the GW150914
Let us also note among the February 12, 2014 arXiv preprints the following works: the paper about massive pulsars with a stable rotation used to search for and determine the parameters of the passing gravitational waves (arXiv:1602.03640); proposals to observe massive resonators based on red giants that exhibit asteroseismic activity (arXiv:1602.03667), and also (arXiv:1602.03883), from the group of David Spergel [one of the leaders of the NASA WMAP space mission, which targeted the cosmic microwave background], dedicated to the information that can be obtained from gravitational wave burst event counts. Note that the latter paper includes congratulations to the LIGO team in the comments, and the abstract has the [vague] phrase "the distribution of the candidate black holes binary systems observed by Advanced LIGO".
conclude, we can note that the formal (indirect) discovery of gravitational waves
was already marked by a Nobel prize, received by the American radio astronomer
Joseph Taylor and his student Russell Hulse for the
detection in 1974 and study of a pair of neutron stars orbiting each other, one
of which is the pulsar PSR B1913+16. Moreover, from the decrease of the period
of the binary, it was possible to verify the General Theory of Relativity,
which describes gravitational radiation losses. After 2014, when the discovery
of the primordial gravitational waves based on BICEP2 measurements of the
B-mode of CMB polarization did not take place, it would seem that there should
be nothing surprising in the direct detection of gravitational waves. However,
the strong signal GW150914 has shown that a new window has really been opened
into deep space. Apparently, a new, powerful, competitive field in astronomy,
related to both the new technologies and the possibility to reach the most
secret places in the Universe, was demonstrated to the international science
February 14, 2016.
(Go to popular content).