Einstein's Special Relativity

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.

Jacaranda Physics 2, 2nd edition © John Wiley & Sons Australia, Ltd 2004 1 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

Jacaranda Physics 2, 2nd edition © John Wiley & Sons Australia, Ltd 2004 2 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 Performed an experiment based on Maxwell’s work that suggested Morley 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.