Unit 3 Detailed Study 1
Einstein’s special relativity
Clarification of knowledge and skills — Chapter 6
Delete dot points 4 and 7 in the list of key ideas on page 130 and add
• Describe Maxwell’s prediction that the speed of light depends only on the electrical and
magnetic properties of the medium it is passing through and not on the speed of the source
or the speed of the medium.
• Contrast Maxwell’s prediction with the principles of Galilean relativity
Insert at end of point 4 page 137 under Electromagnetism
‘The speed of electromagnetic waves are dependent only on the electrical and magnetic
properties of the medium the wave is passing through.’
Insert into existing text three lines below the blue panel on page 138. The insertion is in italic.
… chief among these was the medium for these waves) in the vacuum of space, something
called the …
Insert a new paragraph above the last paragraph on page 138.
‘The luminiferous aether was thought to provide the vacuum of space with the electric and
magnetic properties that enabled electromagnetic waves to propagate. These properties are
different in air, water and other materials, causing the speed of light to change with the
medium. As a result, light travels more slowly in water than in vacuum. Maxwell predicted
that the speed of light was unaffected by the speed of the source of the light. In this way it
was a bit like sound. However, sound will be carried along faster if a wind is blowing in the
direction that the sound is propagating. Similarly sound travelling upwind will take longer to
reach its destination. Maxwell’s equations predicted the speed of electromagnetic waves to be
independent of the speed of the source, or the medium. A light shone into the wind would
travel at the same speed as a light pointing downwind. Similarly, a light shone underwater in
a flowing stream would travel slower than light travelling in air, due to the electric and
magnetic properties of the water, but the speed of light upstream would be equal to the speed
of the light pointing downstream. Light, according to Maxwell, moved at a speed relative to
the luminiferous aether. The motion of matter through the aether, either the matter of the
source, or the matter of a transparent medium that the light passes through, have no effect on
the speed of the light relative to the aether.
This idea of a light (and all electromagnetic effects) having a frame of reference that had
special influence was a contrast to Galilean relativity, where any inertial reference frame was
as good as any other for describing motion.’
Change the 8th dot point in the chapter Summary on page 149 to
Maxwell’s electromagnetism did not agree with Galilean relativity because it held that the
luminiferous aether was the reference frame that the laws of electromagnetism applied to
while Galilean relativity held that the laws of physics were the same in all inertial
reference frames.
•
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Insert an additional dot point after the 9th dot point (... light was probably an electromagnetic
wave) in the chapter Summary on page 149.
• Maxwell predicted that light travelled at a speed relative to the luminiferous aether that
was independent of the motion of the source or the medium, but depended on the electric
and magnetic properties of the medium.
Additions — Chapter 7
Change dot points 6 and 7 in the list of key ideas on page 152 to:
• Model mathematically time dilation, length contraction and mass increase with,
respectively, the equations t = t0γ, L = L0 / γ and m = m0 γ, where γ = …
• … values for time, length and mass for a range of situations …
The additions are shown in italics.
Add two more key ideas to the list on page 152
• explain the relationship between the relativistic mass of a body and the energy equivalent
according to Einstein’s equation E = mc2
• Explain the equivalence of work done to increased ‘mass energy’ according to Einstein’s
equation E = mc2
Insert a new paragraph above the last paragraph of page170.
‘This equivalence of mass and energy meant that the former laws of conservation of mass and
conservation of energy needed to be replaced by a single law which is referred to as
conservation of “mass energy” — a loss of mass can occur if an equal increase in other forms
of energy using E = mc2 balances the mass loss.’
Insert an additional sentence at the end of the first paragraph on page 171, immediately above
Sample Problem 7.5.
This mass m is often described as the relativistic mass as its value depends on the reference
frame of the observer.
Correction
Page 178: Remove the deltas to make the equation E = mc2 in Summary dot point 14 (the
fourth dot point in the right hand column.)
Addition
Add the following table at the end of page 177
Table 7.1 Major contributions to our understanding of relativity
Scientist
Contributions to understanding of relativity
Galileo
All inertial reference frames are equivalent; the laws of physics are
the same whatever the inertial reference frame of the observer.
Newton
The values for time, space and mass measurements do not depend
on the reference frame of the observer
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Maxwell
Electromagnetic waves travel at a rate that is independent of the
speed of the source or the material medium the waves are passing
through. The rate depends only on the electric and magnetic
properties of the medium and is relative to the hypothetical
luminiferous aether. This is in violation of Galilean relativity.
Michelson and
Morley
Performed an experiment based on Maxwell’s work that suggested
that the speed of the Earth through the luminiferous aether should
be measurable. No speed was detected.
Einstein
Declared the non-existence of luminiferous aether, stated that light
travelled at a rate that is independent of the speed of the observer
or source. He also restated Galileo’s relativity principle but
declared that it apply to all the laws of physics, including
electromagnetism. As a consequence, time, space and mass
measurements depend on the reference frame of the observer.
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© John Wiley & Sons Australia, Ltd 2004
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