Black holes, in simple words, are huge bodies which have such a strong gravitational field that even light cannot escape from inside it. Wormholes can be visualized as a tunnel with two ends, both at different points in the spacetime axes.

BLACK HOLES: The idea of a body from which even light could not escape was proposed by astronomical pioneer and English clergyman John Michell in a letter published in November 1784. In 1915, Albert Einstein proposed the General Theory of Relativity, having proved earlier that gravity affects the motion of light. Later, the Schwarzschild radius (named after Karl Schwarzschild) was calculated, which is a value at which some of the terms of the Einstein field equations becomes infinite, and the collapsing star became a singularity. In 1931, S. Chandrasekhar calculated, using special relativity, the limiting mass at which the star collapses into a singularity. This limiting mass, now called the Chandrasekhar limit, is equal to 1.4 solar masses (1.4 times the mass of the sun).

Black holes are generally formed as a result of gravitational collapse, which occurs when a body’s internal pressure is not enough to support its gravitational field. In most stars, this happens when all of the star’s hydrogen has been converted to helium and other heavier elements. If a star has an initial mass less than 5 solar mass, it turns into a black dwarf. However, if the mass is above 5 solar mass, it can either turn into a neutron star or a black hole.

It is supposed that a black hole has only 3 physical properties: mass, charge, and angular momentum. The simplest static black holes have mass but neither electric charge nor angular momentum. At the center of a black hole lies a gravitational singularity at which the spacetime curvature becomes infinite. A striking feature of black holes is the formation of the event horizon. It is a boundary in spacetime through which matter and light can pass from outside towards the inside of the black hole. Nothing can escape from inside the event horizon.

WORMHOLES: If space is imagined to be a 2D plane, a wormhole would be something like a hole on the surface, leading into a cylindrical tube, and emerging as a similar hole from another point on the surface. In case of an actual wormhole, instead of circular holes on a 2D plane, the entry and exit points could be visualized as spheres in 3D space.

The equations of the theory of general relativity have valid solutions that contain wormholes. According to general relativity, the gravitational collapse of a sufficiently compact mass forms a singular Schwarzschild black hole. But, if the Einstein-Cartan-Sciama-Kibble theory of gravity is considered, it forms a regular Schwarzschild wormhole, also called an Einstein – Rosen bridge.

Wormholes connect two points in spacetime which means that they would, in principle, allow travel in space and time. In 1988, Morris, Thorne and Yurtsever worked out how to convert a wormhole traversing space into one traversing time by accelerating one of its two mouths. However, according to general relativity, it would not be possible to use a wormhole to travel back to a time earlier than when the wormhole was first converted into such a time machine.

Wormholes that can be traversed in both directions are called traversable wormholes. Non – traversable wormholes include Schwarzschild, Lorentzian, and Euclidean wormholes.