Black holes

Subject: Science and tech

Context:

Royal Swedish Academy of Sciences decided to award one half of the 2020 Nobel Prize in physics to Roger Penrose and the other half jointly to Reinhard Genzel and Andrea Ghez for furthering the understanding of black holes, the most “enigmatic” objects in the universe.

Concept:

• A black hole is formed when stars collapse and can be defined as a space in the universe with an escape velocity so strong that even light cannot escape it.
• Escape velocity is the speed at which an object must travel to override a planet or an object’s gravitational force.
• Since light cannot get out, black holes are invisible and can only be tracked with the help of a space telescope or other special tools.
• And the reason light cannot escape is mainly that the gravity inside a black hole is very strong as a result of a lot of matter being squeezed into a small space.
• It acts like an ideal black body reflecting no light. It continues to grow, by absorbing mass from its surroundings.

Black hole merger

• Gravitational waves, postulated by Albert Einstein 100 years ago but discovered only in 2015 do not produce any sound on their own.
• These are just ripples created in the fabric of space-time by moving celestial objects just like a moving boat produces ripples in water.
• But when converted into audio signals, these can produce signature sounds that can reveal the origin of the gravitational waves.
• The gravitational wave detected on September 14, 2015, is now known to have been produced by the merger of two black holes about 1.3 billion years ago.
• Scientists already knew the kind of sound that gravitational waves emanating from such events were likely to produce.
• As two such dense and massive objects, black holes or neutron stars, are about to merge, they start rotating around each other at almost the speed of light. The merger takes place within a fraction of a second.
• The gravitational waves released in this last bit, when converted into audio signals, produce sound that is within audible range of human beings.

LIGO

• The LIGO Scientific Collaboration (LSC) is a group of scientists focused on the direct detection of gravitational waves, using them to explore the fundamental physics of gravity, and developing the emerging field of gravitational wave science as a tool of astronomical discovery.
• The LSC works toward this goal through research on, and development of techniques for, gravitational wave detection; and the development, commissioning and exploitation of gravitational wave detectors.
• The project operates three gravitational-wave (GW) detectors. Two are at Hanford, Washington, north-western US, and one is at Livingston in Louisiana, south-eastern US.
• The proposed LIGO India project aims to move one advanced LIGO detector from Hanford to India.

VIRGO

• Virgo is a giant laser interferometer designed to detect gravitational waves.
• Virgo has been designed and built by a collaboration of  the French Centre National de la Recherche Scientifique (CNRS) and the Italian Istituto Nazionale di Fisica Nucleare (INFN)
• It is now operated and improved in Cascina, a small town near Pisa on the site of the European Gravitational Observatory (EGO), by an international collaboration of scientists from France, Italy, the Netherlands, Poland, and Hungary.

Working:

• It consists of two 3-kilometre-long arms, which house the various machinery required to form a laser interferometer.
• A beam-splitter divides a laser beam into two equal components, which are subsequently sent into the two interferometer arms.
• In each arm, a two-mirror Fabry-Perot resonant cavity extends the optical length. This is because of multiple reflections that occur within each cavity and which consequently amplify the tiny distance variation caused by a gravitational wave.
• The two beams of laser light that return from the two arms are recombined out of phase so that, in principle, no light reaches the so-called ‘dark fringe’ of the detector. Any variation caused by an alteration in the distance between the mirrors, produces a very small shift in phase between the beams and, thus, a variation of the intensity of the light, which is proportional to the wave’s amplitude.