 ## #2 Einstein’s Mass Velocity Relation

Albert Einstein, an absolute genius has given groundbreaking theories and one theory among them which was a revolution was the Theory of Relativity. It took 100 years to get strong proofs for this theory.

In 1905 Einstein proved that the laws of physics are the same for all non-accelerating observers and that the speed of light in a vacuum was independent of the motion of all observers. This was the Special Theory of Relativity. Einstein then spent 10 more years to include acceleration in his theory and published his General Theory of Relativity.

These theories had many revolutionary equations and relation. Let us see one such equation which is Einstein’s mass-velocity relation. Where,

• v is the magnitude of the velocity
• c is the speed of light
• m0 is the rest mass of the body
• m is the relativistic mass

Now, what is so special about this mass-velocity relation. Let us understand what does this equation mean. This equation tells us that if we travel with speeds approaching the speed of light then our mass will increase with speed. So, do you really gain mass? Do you become fat? No, let me tell you that we take inertial mass into consideration here. Inertial mass measures an object’s resistance to being accelerated by a force. Now, every body has rest mass which is m0 here in the formula.

If an object moves with some speed then the kinetic energy adds up to the rest mass and overall the inertial mass increases. This means that if an object approaches light speed then its inertial mass increases rapidly and accelerating it further becomes more and more difficult. If any object reaches light speed its inertial mass approaches infinite according to the above equation. Thus, Einstein stated that no object can travel faster than light speed.

## Einstein’s theory of relativity confirmed by first-ever black hole image

The first-ever image of a black hole released by the Event Horizon Telescope has once again proved Einstein’s general theory of relativity to be correct. Avery Broderick, of the University of Waterloo and Perimeter Institute, Canada, who was also in the EHT team mentioned in the press in Washington that the century-old theory of general relativity put forward by Einstein passed a crucial test, spanning from the horizon to the stars.

In his theory, Einstein gave the description of gravity as a geometric property of both space and time. To be specific, the spacetime curvature is related to the momentum and energy of the matter and radiation which are present. After he published the special theory of relativity, Einstein began work on how he can include gravity in his relativistic approach. It took him almost a decade to find a relativistic approach to the gravitational forces. And the culmination of his work are the Einstein field equations which he presented to the Prussian Academy of Science.

The relativity theory predicts that whenever the ratio of the mass of an object to its radius is very very large, it leads to the formation of the black hole. This is a region from which nothing can escape including light. It also predicts that each of these black holes has an event horizon which is almost circular and of a size which can be predicted by the mass of black holes. The event horizons are the boundaries which demarcate one region from the remaining spacetime. Image from Event Horizon Telescope revealing the gargantuan black hole at the heart of galaxy Messier 87 (Credits – Wikimedia Commons)

And this is exactly what we see in the images of the black hole released by EHT. It shows the silhouette of a black hole at the centre of M87, a giant galaxy which is 55 million light years away from Earth. The event horizon in the image is nearly circular and the calculated mass also matches due to the massive distance. The mass of the black hole is 6.5 billion times that of our sun which is pretty big number even by standards of supermassive black holes.

This is not the first time that Einstein’s theory has been successfully verified, but it has survived the challenge posed by many experiments in the past century. A very recent example is that the general relativity predicts that objects with a very large mass and travelling at a great speed generate ripples in space-time called gravitational waves. And they were confirmed in 2015 by the Laser Interferometer Gravitational Wave Observatory(LIGO) which detected the ripples generated between two black holes.

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