Transcript General Relativity

Physics 11 Advanced
Special to General

Special Relativity is all about inertial
reference frames.

Example of an Inertial reference frame:
Deep Space
○ No Earth
○ Far from sources
of gravity
○ Very lonely place
where we like to
do physics
Properties of Non-Inertial Frames
No forces acting on it
 Constant velocity
 If our space-physicist let go of an object
without exerting a force on it, the object
just floats
 Boring

Making it Non-interial

What happens if this person is in a room
in space that has a rocket on it?
○ A “rocket room” if you will

What happens we
they let go of the
open bottle?
Making it Non-interial

What happens if this person is in a room
in space that has a rocket on it?
○ A “rocket room” if you will

What happens we
they let go of the
open bottle?



No force on bottle
We are pushed up
We perceive Object
“falling”
The Equivalence Principle

Einstein’s radical idea was to say the
following:
“A homogeneous gravitational field is
completely equivalent to a uniformly
accelerated reference frame”

Sure, but what does this mean?
 The physics in a gravitational field should be
the same as the physics of an accelerating
reference frame.
These people have no Idea…
Non-inertial Reference Frame

What we perceive is a sort of “artificial”
gravity

Is it “artificial”?
Newton’s Gravity

Newton gave us a model of gravity
between two objects that states the
force of gravity depends on the mass of
the two objects
…That’s fine. (More on this in Grade 12)
but consider the following….
Newton vs. Einstein

Newton states that an object must have
mass to be affected by gravity

Einstein disagrees.
Einstein Says…
…because of the equivalence principle,
the force of gravity is just the ground
acclerating towards us
 If we are accelerating,
how do we see a pulse
of light?
 Einstein says
MASSLESS light is
affected by gravity

So who is Right?

Turns out that light is bent by
gravitational fields
○ Einstein 1: Newton 0
Effect known as
“Gravitational Lensing”
 First observed during a solar
eclipse in 1905
 Shift in apparent position
of a star in the sky

Gravitational Lensing

1905 eclipse

Deep space Galaxies
Light and Space-time

It is important to note here, that light still
travels in a straight line in space-time.

It appears to bend because space-time
is warped
Light and Gravity

Consider light as it leaves a massive
body (the sun)
 GR tells us that time warps/bends as you approach a
massive body
 SR tells us that the speed of light must be constant

So like a rocket on earth, light needs to
escape the pull of gravity
 Decrease energy of photons, lower frequency
E  hf
 Changing space-time should changes frequency and
wavelength of light

c  f
Everything suggests a frequency
change of light
Black Holes
If space-time is warped enough, light
emitted from a body will not escape the
field.
 This is a ‘Black hole’
 At the very edge of the black hole,
space-time is warped just enough that
light orbits the mass.
 This radius tells us how large the ‘black’
part of the black hole is.

○ “Schwarzschild Radius”
More Alice and Bob
Problems
○ A 656nm photon is emitted from a star and
observed on earth to be red shifted by 15nm.
How fast is the star moving away from the
earth?
○ Two musicians play a note of 440Hz; one
musician is at rest while the other is moving to
the right. If a beat frequency of 5Hz is heard,
how fast is the second musician moving wrt to
the first?