To be honest, gravity is probably the only reason we are on this planet. We wouldn’t have existed if there was no gravity in the first place, but imagine the case where the gravity from Earth suddenly vanishes for humans, and not for any other body like our Moon. What would happen? Well, on Equator, we are travelling at 1674.4 km/h around the Earth, which generates a lot of inertia (~400,000,000 kgm^2). So if suddenly the gravity vanishes, as our thought experiment, we would fly away in straight lines away from the earth like a slingshot, until the gravity from other celestial body starts attracting us. Maybe we can reach Moon because of this, or if Sun’s gravity overpowers that of Moon, we would get attracted to the Sun at an ever increasing velocity. In short, we all would be dead.

**But what exactly is gravity?**

Newton described gravity as a force between 2 objects with mass, which is proportional to their individual masses and inversely proportional to the square of the distance between them. That is, more massive the objects, the stronger the pull, and greater the distance between the objects, lesser the pull. Newton described gravity to be ever existing, that is, travelling with infinite speed.

Einstein had a different view of gravity. At the age of 26, he postulated that speed of light is the ultimate speed of anything travelling through space (derivation of Special theory of Relativity).

**So, is Newton wrong? Yes and No.**

Einstein’s General theory of relativity indicates that Gravity is caused by the bending of Space-Time fabric by a body of mass. Space-time is a 4-Dimensional mathematical model combining 3 spatial dimension and 1 temporal (time) dimension. It is impossible to imagine a 4-D continuum, but a simpler analogy can be to imagine this as a ball of steel kept on thin sheet of rubber, like the one in the image! Any other ball approaching the bigger ball will feel the pull towards it, and will eventually collide with it. You can imagine the bigger ball to be the Sun and the smaller ball orbiting around the Sun as the Earth. If you remember, Gravitation is the weakest of all the 4 forces in the ‘Theory of Everything’ blog post.

Image courtesy: ‘Clear Science’

**So was Newton completely wrong?
**No. Newton’s law of gravitation is so accurate that for most scenario’s, his theory stands true for general consideration and calculations. Only when the mass, and/ or density of object is very high and objects are rotating very close to it, do we need to consider the Einstein’s General Theory of Relativity.

This video is an excellent representation of gravitation as we know it, how planetary systems form, how the Moon rotates around the Earth and how interplanetary orbits are created, just like the Indian Space Research Organisation’s (ISRO’s) Mars Orbiter Mission ‘Mangalyaan’, using the gravitation force of the Sun.

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