FEYNMAN LECTURES ON PHYSICS: The Theory of Gravitation

Planetary Motion “one of the most far-reaching generalizations of the human mind”  Story begins with the discovery that the planets go around the sun  Copernicus re-discovered this  Tycho Brahe decided to measure the planets positions (15 th century)  Kepler studied Brahe work and came up with a set of laws

Kepler’s laws  Found each planet goes round sun in an ellipse with sun at focus  Planets move faster when nearer the sun  The relationship between the orbit size and the period of any planet can be given by T α a 3/2

Development of Dynamics “An eclipse is not just an oval, but is a very specific and precise curve that can be obtained by using two tacks”  People believed that Planets kept going round as angels were pushing them.  Galileo was studying why the planets orbit  A body will keep moving until a force is acted upon it (inertia)  The force needed to keep a planet in orbit acts towards the sun not tangentially

Newton’s law of Gravitation “It falls away from the straight line that it would pursue”  Newton noticed the law of areas is because the force is acting towards the sun  Newton showed from Kepler’s third law and the fact that the force is toward the sun to show that there is a force which acts between to objects and it is inversely proportional to the distance  Newton generalised the force of gravity

Newton’s law of Gravitation “It falls away from the straight line that it would pursue”  Newton compared ball falling on Earth to moon falling in space to show the validity of inverse square law  Objects fall 16 feet in one second whether horizontal or vertical release  If you shoot a bullet quick enough the height in one second will be the same as when it left as the earth is curved  The law of Gravitation explains the tides

Universal Gravitation  We can deduce that the planets, sun and moon should be nearly spheres  Explained the moons of Jupiter (and the light)  Explained the orbit of Uranus and found planet (Neptune)  Explained the motion of double stars around one another  Works out to distances as far as Galaxies

Cavendish Experiment The experiment is called – “weighing the Earth’”  Cavendish designed an experiment to weight the Earth  Relates the attraction between the masses to the torsion in the wire and the oscillatory movement  Found that G = 6.67 ( )  Used G to calculate the weight of the Earth

What is Gravity?  There are many theories of what gravity is not just how it works  One such theory was an ether  No relation of gravity to the other forces like electromagnetic  Behaviour of forces is very similar (unified theory)  Ratio between forces is massive

Gravity and Relativity “In spite of all the excitement it caused Newton’s law of Gravitation is not correct”  Einstein modified Newton’s laws of Gravitation  Added in the effects of relativity to make Einstein’s law of gravitation  Since light has energy and energy is mass, then light can be deflected by an object  Challenge to quantify gravity to use quantum mechanics