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May 02, 2019 Thursday 02:41:06 PM IST

Hunters of black holes

Creative Living

The April 10th of 2019 brought apleasant surpriseto the world of science and technology! The Event Horizon Telescope (EHT) made the first ever snap shot of  M87 black hole, which is 55 million light-years away from the earth! A feather in the cap of the towering genius of Albert Einstein, who predicted for the first time the presence of black holes in the cosmos! A further bolster to his century-old (1915) theory of general relativity!

Einstein was an iconoclast, who dared to think different.

Already in 1687, Sir Isaac Newton produced an interesting explanation to the question as to what pulled an apple to the center of earth, while keeping the earth in motion around the sun. According to him, it was due to the force of gravity between the apple and earth or that between earth and sun. The force of gravitydepends on the mass of the bodies and the distance between them. He also formulated three laws of motion based on this conjecture. Newton, however, failed to answer the question:Whyis there such a force of attraction between two bodies? Newton claimed that gravity is an innate force of matter, which acts over a distance.

However, Einstein was not prepared to buy that argument of action over a distance.He explored deep into the questionand arrived at a more reasonable explanation to the existence of gravitational force between massive bodies.He got clues to the answer from the theories of English philosopher David Hume, as he later acknowledges. According to Hume’s epistemology (science of knowing), any idea naturally evokes an apparent reality. Thus, the idea of motion entails the concept of space and time, irrespective of whether there really is such a thing as ‘motion’. Based on this notion, Einstein deviseda new approach to describe motion. It deviated drastically from the conceptsof Newton, who described motion based on an absolute frame of reference, the three-dimensional space, as defined by Euclidean geometry. In contrast, Einstein conceived frames of reference that move relative to each other, causing apparent ‘motion’. In 1905, Einstein formulated the Special Theory of Relativity for inertial frames of references, or those frames of references that moved with constant velocity or at rest relative to one another. In 1915, hepublished the General Theory of Relativity, which could be applied to both inertial or acceleratingframes of references.

The Theory of Relativity

The Theory of Relativity is founded on two fundamental assumptions:

1.  The laws of physics are the same to all observers, irrespective of the frames of reference they adopt. Thisassumption goes against the concept of fixed or preferred frames of reference, as conceivedby Newton.

2.The speed of light does not depend on the relative motion between the source of light and the observer. In other words, the velocity of light is not relative, rather has an absolute value of299,792,458 m/s in vacuum. This is because electromagnetic waves require no medium to propagate, as different from that of mechanical waves, which necessarilyrequire a medium for propagation. Prior to Einstein, people believed that electromagnetic waves always require a medium to pass through, giving birth to a host of‘ether theories’ in physics, definingcertain strange space-filling substances or fields, which would allow transmission of electromagnetic waves in vacuum. (It should be noted here that, as electromagnetic waves pass through certain medium, their speeds will be dependent on the refractive index of that medium and this speedwill necessarily be smaller than its absolute velocity.)

These two assumptions generated a host of mind-boggling concepts such as the following:

1.  Dilation of time: Time does not pass at the same rate for everyone. An observer, who moves faster measures time passing slower than a stationary observer!

2.Contraction of length:  A fast-moving object appears shorter in the direction of motion, relative to a slow-moving one.

3.Equivalence of mass and energy: Mass and energy are different manifestations of the same thing. Energy is equivalent to the mass times the square of velocity or E = mc2.

4.Increase of mass: As a result of equivalence between mass and energy, a fast-moving object appears to have increased its mass relative to a slow-moving one. This is because, the increase in velocity corresponds to the increase in the kinetic energy of the object, which is equivalent to the increased mass.

5. Matter cannot travel faster than light: As an object reaches the absolute velocity, i.e. the velocity of light, it would have assumed infinite values for its mass, which will require infinite energy to further move it. This is practically impossible.

However, the most revolutionary concept emerged was that of space-time continuum.In the classical framework, an event is defined both by its spatial and temporal positions separately. Here, the phenomena like time dilation and length contraction, etc.will be observed. However, in a space-time framework as proposed by Hermann Minkowski(1864–1909),events will keep the same distance among them. In fact, Minkowski had predicted that “space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality” (from the keynote address at the 80th Assembly of German Natural Scientists and Physicians on September 21, 1908).

Concept of space-time continuum led Einstein to a totally new picture of force of gravitation.He considered gravitational and inertial mass of a body to be equivalent in a space-time continuum and concluded that the gravitational force is a fictitious or a pseudo-force. Thus, the force experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference. Accordingly, gravity is due to massive objects warping space-time continuum into gravity wellsjust like a steel ball placed over a rubber sheet. Planets choose the shortest elliptical paths along the gravity well of the sunas the mostenergy efficient paths.Thus, Einstein proposed a radically new theory of gravitation, which significantly diverged from that of Isaac Newton.

Naturally, Einstein’s theoryrequired evidences to establish itself. Einstein himself suggested three evidences for validity of his theory and more evidences followed in the course of the century. The theory is taken into account today in the design of any modern communication system to account for relativistic nature of the communicated data. Recent detection of gravitational waves, which won the Nobel Prize in 2016, is another milestone in the development of proofs to the Relativity Theory of Einstein. The fascination of the theory was such that the scientific world was rather obsessed to posit novel evidences to this grand old theory for a complete century.

Black hole photography

Black holes are formed by merging of many stars that collapses into each other to form an extremely massive spot in the space. Black hole mass is so dense that not even light can escape its gravitational attraction. The Schwarzschild radius (derived by Karl Schwarzschild in 1916), which is alternatively called gravitational radius,characterizes a black hole. Any object with a physical radius smaller than its Schwarzschild radius will be transformed into a black hole. For Sun with mass 2.0 × 1030 kg, the Schwarzschild radius is 3 km. In other words, if the entire mass of the sun is compressed into a sphere of radius 3 km, it will become a black hole. Similarly, if earth with a mass of 6.0 × 1024 kg, is compressed into a sphere of radius 8.7 mm, it will also become a black hole!

Even light cannot escape the gravitational attraction of a black hole. This makes it impossible to detect black holes.An event horizon is the point near to a black hole, at which the light sent to it will disappear. No communication with a black hole is possible. That makes black hole photography an extreme challenge, which was won by humankind on April 10, 2019! It was accomplished based on a clue given by Stephen Hawking, the British Physicist.

Stephen Hawking had shown that black holes emit huge jets of plasma as their immense gravity pulls in streams of matter into its core. When matter approaches a black hole's event horizon, it forms an orbiting disk. Matter in this orbiting disk experiencesfriction, which warms up the disk. It is just as we warm our hands on a cold day by rubbing them together. The closer the matter, the greater the friction. Thus, matter closer to the event horizon glows brilliantly bright with the heat of hundreds of Suns. This light could be photographed and along with it, the ‘silhouette’ of the black hole! A brilliant idea to take snapshots of a black hole!

But that was not an easy task to take photograph of an objectthousands of trillions of kilometers away. It was just like trying to count the dimples on a golf ball in Los Angelesfrom New York or imaging an orange on the moon. To achieve this feat, scientists developed an Event Horizon Telescope (EHT) merging eight radio telescopesstationed round the globe, namely, ALMA, APEX, the IRAM 30-meter telescope, the James Clerk Maxwell Telescope, the Large Millimeter Telescope Alfonso Serrano, the Submillimeter Array, the Submillimeter Telescope, and the South Pole Telescope. In other words, they turned Earth into one single giant telescope!They arranged atomic clocksin order to synchronize their photoshoot. They were so accurate that they lost just one second per hundred million years!

Every shot at the black hole produced huge amounts of data, which could not be transferred through the existing internet. Petabytes of raw data from the telescopes were physically transported and combined by highly specialized supercomputers hosted by the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory.

The report of the first successful photo shoot of the black hole at the heart of the Galaxy M87gotpublishedin theApril 2019 issue of Astrophysical Journal Letters. The photograph consisted of a bright ring of material surrounding a dark centre. Scientists also determined that the M87 black hole is 6.5 billion times the mass of the Sun and 40 billion km across, that's larger than Neptune's 200-year orbit of the sun.

On announcing this spectacle,SheperdDoeleman, the project director of the EHT project said: "This is a remarkable achievement ... We've exposed parts of the universe that we thought were invisible to us.”

The next target of EHT team is to take a photograph of the black hole Sagittarius A*, which is situated at the centre of our own galaxy, the Milky Way. The Milky Way's black hole was too challenging to image accurately due to rapid variability in light output. Hopefully, more telescopes will be added to the EHT's array soon, to get ever clearer images of these fascinating objects.

Facets of Einsteinian mind

The event once again revealed the enormous influence that Albert Einsteinexerts on the creative minds of the day. Einstein keeps on inspiring millions of scientists for over a centuryinviting them to hunt for black holes, which he postulated already in 1915! What made up Einstein’s mind? Let us explore.

1.      Life-long learning: Einstein’s mind kept on meditating over the contemporary developments in different fields of inquiry. He was well-equipped to understand the nuances of what happened in the frontiers of knowledge of various disciplines.

2.    Preparedness to relinquish unwanted assumptions: He was prepared to relinquish the assumptions of his time regarding the space and time and dared to postulate a space-time continuum, which radically changed the description of the universe and its gravitation.

3.    Ability to integrate different disciplines: As reported by Einstein, he received the first trigger to think beyond space and time, from the 18th century philosopher, David Hume. Einstein’s mind could integrate knowledge generated by seemingly unrelated disciplines, to producea totally new concept.

4.    Courage to embrace the radically new: Einstein dared to think differently. He also dared to propose new theories even in the face of lack of immediate experimental evidences. He touched new realities through his ‘thought experiments’, which engaged millions of creative minds to hunt for evidences for the theories.

Einstein keeps motivating the hunters of black holes!

Dr. Varghese Panthalookaran

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