Special Relativity

The laws of physics must be the same in all inertial reference frames

The speed of light in vacuum has the same value in all inertial frames, regardless of the velocity of the observer or the velocity of the

source emitting the light

Einstein’s Principle of Relativity

d v mirror

An observer inside a moving train views a ray of light as follows:

An observer outside a moving train, on the ground, at rest views a ray of light as follows:

d

c =

v

Changing your frame of reference will create an observable change in momentum and energy, but they are both still conserved.

Invariant mass energy

Not affected by reference frame

Energy and momentum are not conserved separately, but as a combination

Einstein’s mass-energy equation is invariant and does not depend on you reference frame

Momentum & Energy

There is energy in mass Consider a person weighing 100 kg:

A few kilograms of fuel of a nuclear source is worth a few tons of fuel of a chemical source

Momentum & Energy

Invariant Distance The 2 points have to be points in time and in

space

ct

x An object sitting at rest,

whose position (x) stays the same

x

ct

An object with some velocity, v

Θ = 45º when the object is moving at the speed of light

A real object with mass cannot travel at the speed of light

Light has no mass So, a real object cannot go

beyond 45º from the y axis, in either direction

θ

Elsewhere

FUTURE

PAST

ELSEWHERE

ELSEWHERE

Strict Causality

X100 ly

Planet X is 100 ly away. t= If a signal were sent from earth, at the speed of

light, it would take Planet X 100 years to be able to recognize the signal

C=3xm/s t

ct

ct’

If an object, ct’, is moving and another object, ct is at rest, relative to ct’, it looks like ct is moving

Tilted Axis ct’

x’

θ Distance between 2

points

The speed of light is the same for all axis

x=ct

1. The pole in the barn effect

The pole and barn both see each other as being less than 100 m by the gamma factor.

At some instance the doors of the barn could be closed and opened, trapping the pole in the barn.The pole sees the doors of the barn close one at a time, and open one at a time

Consequences

v100 m 100 m

2. Twin Paradox

A B

Twin B Leaves on a rocket to mars.

Twin A sees twin B as moving close to the speed of light; he sees twin B as 21 and himself as 25.

When the rocket turns around, twin 2 sees himself as 25 and twin A as 21.

20 years old

20 years old

Once things accelerate, they are no longer in the inertial reference frame.General Relativity deals with accelerated reference frames.

General Relativitya=9.8m/ g=9.8m/

A person in an elevator ascending at 9.8m/ would feel no different than a person in an elevator at rest.

They could not determine whether the force they were feeling was from the acceleration of the elevator or the force due to gravity.

A person standing on an island wants to know how fast a canon ball is coming toward them. Classical version: v1 + v2 = v’Relativistic version: If the canon were a ‘photon torpedo’ from the starship enterprise moving at the speed of light:

Adding Velocities

v1

v2

Momentum & Energy:

Adding Velocities:

Special Relativity: Time Dilation:

Length Contraction:

Equations to Know