When news broke out that Christopher Nolan was making a sci-fi movie based on space travel called Interstellar, I was eager to see how Nolan handles the grand canvas of the universe with his creative prowess. After watching the movie I am happy that it had all the essential Nolan ingredients - an audacious and improbable core, cerebral screenplay, witty dialogues, mesmerizing visuals and all connected using an emotional chord. But as a science enthusiast, the most heartening thing for me was to watch a lot of real world science littered all through the movie and the clever manner in which Nolan had exploited them for his screenplay. Right from Newtonian mechanics to Einstein's relativity to the Theory of Everything, the movie was a rollicking journey through science. Though there were some outrageuous speculative science towards the end, well thats the creative liberty provided by movie making. Now lets take a look at some of the real world science that was dealt in the movie.
Rocket Propulsion
Just before entering the black hole the AI robot TARS soberly utters, "Newton's Third Law : Can't get anywhere without leaving something behind." It is not only a witty comment that captures the emotional mood of the scene, but also suggests the physics underlying rocket propulsion. A rocket engine is similar to the internal combustion engine in cars, wherein fuel is ignited at high temperatures in a combustion chamber (Rockets carry their own oxidizer in the combustion chamber as the outer space is almost vacuum). In a car engine the combustion energy is used to drive a piston which in turn powers the vehicle. But in rockets the high pressure exhaust produced due to the combustion is accelerated to supersonic speeds using a convergent-divergent nozzle (remember Bernoulli's principle) before being ejected out. According to Newton's Second Law, this acceleration implies a net force acting on the exhaust and the reaction force to this is what provides the thrust that propels the rocket upwards.
Weightlessness
When the Endurance crew is launched into space, they start floating the moment they enter into orbit. Those who have seen videos or pictures of astronauts aboard ISS (International Space Station) would have noted that they too float. This is wrongly attributed to the absence of gravity and hence referred to as 'zero gravity'. But ISS orbits the earth at a height of 400km where the gravity is only marginally lesser than that on earth's surface. So why do astronauts aboard an orbiting spacecraft float or experience weightlessness ? To understand this lets consider a scenario back on earth where you are inside a lift in the tenth floor of a building. Now if you wanted to reach the first floor, the lift will start from rest and accelerate at some rate 'a' until it reaches the first floor. Lets assume that you were floating during this entire downward journey. This means you would have been accelerating at a rate 'g' which is the acceleration due to earth's gravity. Now if g > a then you would have hit the bottom of the lift and if g < a you would have hit the top of the lift and there is no way you could be floating. So if you had to be floating then both yourselves and the lift should be accelerating at the same rate. This can happen under some extraordinary circumstances when the lift cable snaps and both you and the lift are hurtling down at the same rate 'g'. Similarly when a satellite or spacecraft is in orbit around a celestial body (like the earth), the only force acting on it is the gravitational pull of the celestial body. And that explains the floating astronauts on orbiting vessels.
Artificial gravity
Immediately after they enter orbit, Cooper initiates a spin sequence for the spacecraft which induces some sort of artificial gravity that grounds the floating astronauts. What causes this artifical gravity? When the spacecraft starts spinning there is a centrifugal force that provides a downward force keeping the astronauts grounded. This is similar to the outward force that we experience when making a sharp bend while driving in hill stations or while riding the merry-go-round.
Black hole
One of the central characters (literally!) in the movie is a black hole called Gargantuan. Black holes are formed when white dwarf stars with mass exceeding the 'Chandrashekar Limit' have exhausted the nuclear fuel at their core. During the normal life of a star, there is a tug of war between its own gravity pulling it in and the energy produced by the nuclear reaction at the core which is pushing it out. But with time as the nuclear fuel gets exhausted, the star collapses under its own gravitational pull forming black holes . Black holes have so much of mass compressed into a very small volume that they exert an incredible gravitational pull in its vicinity. There is a region surrounding the black hole called 'event horizon' in which the gravitational pull is so enormous that even light cannot escape. And that is one of the reason that black holes are yet to be physically observed, though it is predicted that there is a super massive black hole at the centre of our own Milky Way galaxy.
Theory of Relativity
After dealing with time in a non-linear fashion in Memento, Nolan explores the concept of time dilation in this movie. When Romilly says that spending one hour on Miller's planet is equal to seven years back on earth, it sounds bewildering but is actually based on the works on one brilliant German physicist who goes by the name of Albert Einstein. Between 1905 and 1916 Einstein proposed two theories that shook our notion of space and time. The first among this was the Special Theory of Relativity which dealt a blow to the notion that time is absolute. According to this theory the flow of time is different for people who are travelling at constant velocities but in relative motion. More specifically, time runs slower for those moving at a relatively higher speed. Thats why Cooper says to his young daughter that he might be the same age as her when he returns as he might be travelling at the speed of light. The second theory is called General Theory of Relativity which deals with effect of gravity on space and time. When Newton proposed the laws of universal gravitation, it was a monumental scientific achievement because it was the first time the physical world was described in the language of mathematics. Though the law had been verified to a very high degree of accuracy, Einstein was uncomfortable with certain aspects of it and hence proposed the General Theory of Relativity. One of the key predictions of this theory which is relevant to the movie is that time runs slower in a stronger gravitational field and vice-versa. And that explains why time runs so slow on Miller's planet which is near to the black hole that exerts a strong gravitational field. (The Special theory of relativity and General Theory of Relativity aren't just theoretical predictions but have real world applications. The clocks in GPS satellites run slower than that on earth as these satellites orbit the earth at very high speeds of 14,000 km/hr. Also since they orbit the earth at heights of 20,000km, they run further slower as the earth's gravitational field is lower compared to that on surface of earth. These effects have been taken into consideration when designing the GPS satellites.)
Tidal force
When the Endurance crew land on Miller's planet, they encounter tides which are the size of mountains. These monster tides weren't randomly placed there to give a kick to the screenplay, but were caused due to the large tidal force exerted by the enormous gravitational pull of Gargantuan. It is similar to the tides caused in the earth's water surface by the moon's gravity.
Grand Unified Theory
The holy grail of theoretical physics currently is the formulation of a theory which unifies all the four fundamental forces of nature - strong force (present inside the nucleus of an atom holding together the protons), weak force (responsible for radioactive decay), electromagnetic force and gravitational force. The biggest hurdle towards this grand unification is the incompatibility between General Theory of Relativity and Quantum mechanics. General theory of relativity works well at the large scale like stars, galaxies et.al. whereas Quantum mechanics works well at the atomic scale. In our day to day life we never encounter massive objects at the microscopic level and so this discontinuity in the laws of nature is not a problem. But at the centre of the black hole there is a huge mass that has been compressed to the atomic scale and hence it is essential that the two theories reconcile with each other. So I presume that is the quantum data that Cooper transmits to his daughter which she uses to solve the gravity conundrum.
