Can we make black holes reveal themselves in echoes of light?

When it comes to making sense of our universe, the importance of black holes is hard to understate. Scientists know that a black hole exerts a strong gravitational pull, so much so that any object that gets closer to its centre beyond a point can never get back out. The effects of black holes on their surroundings include the release of a tremendous amount of energy. These effects are crucial to determine the structures of the galaxies they occupy and how the stars around them evolved over time.

A study published in the Astrophysical Journal Letters on November 7 is notable in this wider context. It was carried out by astrophysicists from the Institute for Advanced Study in Princeton, New Jersey, led by George Wong of the School of Natural Sciences at Princeton University. In their study, the researchers presented a new method to measure the properties of black holes by using the effects they have on light flowing around them.

Signatures in the light

When light passes around a very heavy object, like a black hole, its path bends. As a result some parts of the light may take a direct route to the viewer while others may pass around the black hole a few times before getting back on its original path. In this way, light emitted by a distant source in the cosmos may reach the earth at different instances, depending on its interactions with black holes on the way. When two beams of light emitted by the same source reach the earth at different points, the beam to arrive second will be an echo of the beam that arrived first. This phenomenon is thus called a light echo.

The manner and extent to which light circles around a black hole depends on the black hole’s mass and radius. If the black hole is spinning (a.k.a. a Kerr black hole), it will also depend on the object’s angular momentum. Thus, according to the study, scientists can use light echoes as a new and independent way to the masses and spins of black holes.

In general, the task of measuring a black hole’s mass and spin is quite tedious because all the matter, hot gases, and the radiation swirling around the object complicate observations and make signals harder to extract from the noise. Light, fortunately, is affected differently and light echoes could offer a better signal-to-noise ratio.

Lenses and rings

An object that bends light is called a lens. Black holes do this by the sheer strength of their gravity, thus the phenomenon is called gravitational lensing. Scientists theorised long ago that gravitational lensing could create light echoes but they have not been directly measured so far. To get around this issue, the new study proposes the use of a technique called long-baseline interferometry. The principle here is that the non-simultaneous arrival of two signals — like two light beams — could interfere with each other to create a new, unique signal.

To spot light echoes created by a black hole, one telescope could be placed on the earth and the other in space. While the number of instruments may seem modest, they will have to operate with supreme technical rigour.

The main motivation for the new study was the fact that some of the supermassive black holes in the centre of the Milky Way and the nearby M87 galaxies have been found to have bright rings of light at a frequency of 230 GHz around them. The structure of these rings is influenced by astrophysical forces and the spacetime geometry of black holes, and scientists have been keen to study them in detail using very long baseline interferometry techniques. One particular aspiration is to trace the black hole’s shadow on these rings to understand spacetime around the black holes.

Independent of colour

The analysis in the new study essentially focused on a black hole at the centre of the M87 galaxy — an appealing object of study for light echoes since it’s quite large in the sky. But the results are also applicable to other black holes. The baseline in ‘long baseline interferometry’ refers to the distance between the two telescopes that receive the light. According to the study, it should be at least 40 Gλ, where Gλ is a unit of measurement that refers to the telescopes’ ability to collect signals at a specific frequency.

The Princeton team also carried out preliminary high-resolution simulations to test the credibility of their technique. For this, team members collected several thousand instantaneous images of light travelling around the M87 black hole, located nearly 55 million lightyears away, using the Event Horizon Telescope. Then they estimated the time beams of light took to travel from the near end of the black hole to its far end, which, according to their idea, would depend on the black hole’s mass and angular momentum, after adjusting for the angle at which the telescope was viewing it. From this simulated data, the team inferred the echo delay.

Albert Einstein’s general theory of relativity also anticipated the phenomenon of light echoes. In particular the theory predicts the echoes will be achromatic, meaning light of all frequencies should be able to form echoes. (Since G_λ is inversely proportional to the frequency, building a telescope to detect the echoes is a separate headache.) Thus any approach to detect light echoes at multiple frequencies at the same time could provide a good test of the new technique. A positive result will also be yet another confirmation that the general theory of relativity provides an accurate description of black holes.

Qudsia Gani is an assistant professor in the Department of Physics, Government Degree College Pattan, Baramulla.