Abstract
On February 11, 2016, the LIGO collaboration announced the discovery of gravitational radiation due to the merger of two black holes. As the event was observed on September 14, 2015, its official designation is GW150914. This discovery represents a major scientific breakthrough for physics, astrophysics, and cosmology. Probably even more important, on October 16, 2017, the LIGO/Virgo collaboration announced, together with a large number of other experiments, the first coincident observation of GWs and electromagnetic radiation. These observations are connected with a collision of two neutron stars 40 Mpc away from Earth, producing almost simultaneously both gravitational radiation (GW170817) and a short gamma ray burst (GRB170817A). The electromagnetic observations in the following days revealed signatures of recently synthesized material, including gold and platinum, solving a decades-long mystery concerning where about half of all elements heavier than iron are produced. The purpose of this chapter is to explain key features of the observed gravitational radiation in terms of introductory physics. Gravitational waves carry a form of radiant energy that the current generation of laser interferometers was finally able to detect. We use data on figures reported in the discovery papers to make estimates of the astrophysical parameters. Simple arguments based on Newtonian gravity, dimensional analysis and analogies with electromagnetic waves are employed. Key parameters obtained in this way (masses of merging objects, distances, emitted energy) are compared with the parameters reported in the discovery papers, in which they were extracted by fitting data to templates generated by numerical relativity. In the near future, networks of interferometers will help researchers to determine the locations of sources in the sky and trigger ”traditional” astronomical observations and neutrino telescopes for the study of high-energy processes in the Universe. Combining observations in this way is the basis of multimessenger astrophysics.
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Notes
- 1.
The LIGO Collaboration has published the GW’s discovery paper (Abbott et al. 2016) on PRL!
- 2.
In the electromagnetic theory, the Lorenz gauge condition (or Lorenz gauge) is a partial gauge fixing of the four-vector potential. The condition is that ∂ μ A μ = 0. In ordinary vector notation and SI units, the gauge condition is written as \( \nabla \cdot {\mathbf {A}}+{\frac {1}{c^{2}}}{\frac {\partial \varphi }{\partial t}}=0\). This does not completely determine the gauge: one can still make a gauge transformation A μ → A μ + ∂ μ f, where f is a scalar function satisfying ∂ μ ∂ μ f = 0.
- 3.
As the standard range of human audible frequencies is from 20 to 20,000 Hz, the signal of the passage of a GW can be transduced to a sound audible by human ears. There are different examples on the educational resources webpages of the experiments, https://www.ligo.caltech.edu/. However, remember that this is just a didactic and sociological trick, and that GWs are not detected by acoustic devices.
- 4.
This section can be skipped in the early reading steps.
- 5.
Circular orbits are used for simplicity, but careful analysis shows that even if the orbits were initially elliptical, emission of GWs would quickly produce circular orbits.
- 6.
Note this: minutes. This means that GW interferometers could, under favorable circumstances, pre-alert satellites and earth-based observatories!
- 7.
The GCN circulars for GRB170817A/GW170817 follow-up are available at the GCN website: https://gcn.gsfc.nasa.gov/other/G288732.gcn3.
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Spurio, M. (2018). Basics on the Observations of Gravitational Waves. In: Probes of Multimessenger Astrophysics. Astronomy and Astrophysics Library. Springer, Cham. https://doi.org/10.1007/978-3-319-96854-4_13
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