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