# Study Note

**Topics:**Spacetime, General relativity, Special relativity

**Pages:**9 (3008 words)

**Published:**March 10, 2013

Lecture 1

* 1st law: A body remains at rest, or moves in a straight line at a constant speed, unless acted upon by a net outside force. Lecture 2

* Einstein does not care about ether at all resulted in quarrels with other scientists who believe light needs ether to travel. * Einstein said that the common sense of getting rid of Galilean transformation is wrong. * U=2C contradicts 2nd postulate but Einstein still states 2nd postulate is valid. * When light comes out from its source, it forgets about its source. * Einstein said that time is not special and should be treated with equal footing as space. * The gamma (Lorentz factor) term imposes the speed of light limit. * The Universe is made of events or world lines (loci).

* For the person holding the ball, it does not shrink but for another person, the ball shrinks * The idea of Correspondence Principle.

Lecture 3

* Einstein’s equation is general.

* M=gamma*M0. It is the gamma that changes with speed, not mass. * Total energy = Kinetic energy + rest energy.

* When people write E=MC2, they are referring to the rest energy. * Einstein unified Mass and Energy.

* Mass is not equivalent to energy or vice versa.

* AMU = atomic mass unit.

Lecture 4

* Inverse square law discovered by Newton

* Differentiation of Gravitation Potential produces Gravitational Force * Square of Pythagoras relationship terms

* Time is treated as equal footing in Einstein Relativity * The irritating negative sign.

* No one can visualize 4 dimensional space-time, not even Einstein * The g can be functions or cross terms instead of numbers * Einstein’s summation convention

* Einstein was unable to unify gravity and electromagnetism after making potentials that are located diagonal of the metric tensor and using complex numbers * EFE favoured an expanding universe but due to prejudice of the time, Einstein favoured a static Universe, hence he added the constant to suppress this expansion. His biggest blunder. * LHS of EFE tells us geometry. Space-time and matter/energy play dynamical role. * Gravitational force plays no role according to Einstein. Lecture 5

* Light is moving in space-time fabric without mass

* According to Paul Hewitt, gravity ‘pulls’ on the energy of light because energy is equivalent to mass. But actually there is no pulling, light is just following the space-time fabric. * Einstein’s GR verified by gravitational lensing – the Einstein’s Ring. * Paul Hewitt stated that an important message of Science is that not knowing is okay..(Page 18). Lecture 6

* Events that lie outside the light cone and hence, cannot decide which reference frame is correct. * When we measure something, we measure it simultaneously. * For the muons, distance shrinks. For the observer, the time must be dilated. * The reason for the extra terms in the generalised Pythagoras theorem – the beta and alpha terms – is so that we can describe the wavy, curved and non-orthogonal geometry. * The coefficient in front of Pythagoras relationship can be function just like what Einstein said. * The g terms in the metric tensor that form part of the general Pythagoras relationship are the solution to the EFE. * In Einstein, there is no attractive or repulsive force but geometry. * Geodesics which are the shortest path are not necessarily straight lines. * When people said 5 dimensions, they just add to the space. There is no time added. Ie 5+1. Lecture 7

* Open cluster: Stars gather under the influence of gravity. No particular shape, called open cluster. Pleiades M45 is known as the 7 sisters but actually there are more than 7 stars. Field is not very strong so do not need to use EFE. * Nebula: Stars are not always round dot. In Singapore, we call smoke ring. Fuzzy with the star in the middle. When tars die, contract instead of exploding. *...

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