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9.2 Space: 4. Current and emerging understanding about time and space

Syllabus reference (October 2002 version)
4. Current and emerging understanding about time and space has been dependent upon earlier models of the transmission of light
Students learn to: Students:

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 11 June 10]

Prior Learning: HSC modules 9.2 (subsections 2 and 3)

Background: The theory of relativity came out of considerations about the way that light travels, but to do so physicists had to overcome a long held belief in a theory that eventually proved to be incorrect. The relativity effects considered here apply to circumstances that we do not normally experience — travelling at a significant fraction of the speed of light. Application of the theory suggests some tantalising possibilities for space travel.

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outline the features of the aether model for the transmission of light

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gather and process information to interpret the results of the Michelson-Morley experiment

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describe and evaluate the Michelson-Morley attempt to measure the relative velocity of the Earth through the aether

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discuss the role of the Michelson-Morley experiments in making determinations about competing theories

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perform an investigation to help distinguish between non-inertial and inertial frames of reference

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outline the nature of inertial frames of reference

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discuss the principle of relativity

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analyse and interpret some of Einstein’s thought experiments involving mirrors and trains and discuss the relationship between thought and reality

Imagine that you are sitting in a train facing forwards. The train is moving at the speed of light. You hold up a mirror in front of you, at arm’s length. Will you be able to see your reflection in the mirror?

The experiment could have one of two possible outcomes, each of which involves a dilemma for the scientific community of the time that believed in the aether model:

  • No, the reflection will not appear. This is the result predicted by the aether model, since light can only travel at a set speed (3 × 108 m s-1) through the aether. If the train is travelling at that speed then the light cannot catch the mirror to return as a reflection. Unfortunately, this violates the principle of relativity, which states that in an inertial frame of reference you cannot perform any experiment to tell that you are moving.

  • Yes, the reflection will be seen because, according to the principle of relativity, it would not be possible for the person in the train to do anything to detect the constant motion with which he or she is travelling. However, a person watching this from the side of the track should see the light from your face travelling at twice its normal speed!

Einstein decided that:

  • the reflection will be seen as normal, because he believed that the principle of relativity should always hold true

  • the person at the side of the track sees the light travelling normally. BUT, this means that time passes differently for you on the train and for the person at the side of the track

  • the aether mode; has nothing to do with it. Einstein described it as superfluous.
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describe the significance of Einstein’s assumption of the constancy of the speed of light

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identify that if c is constant then space and time become relative

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discuss the concept that length standards are defined in terms of time in contrast to the original metre standard

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analyse information to discuss the relationship between theory and the evidence supporting it, using Einstein’s predictions based on relativity that were made many years before evidence was available to support it

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explain qualitatively and quantitatively the consequence of special relativity in relation to:

  • the relativity of simultaneity
  • the equivalence between mass and energy
  • length contraction
  • time dilation
  • mass dilation

The relativity of simultaneity

The equivalence between mass and energy

Length contraction

Time dilation

Mass dilation

Web sites that demonstrate the consequences of relativity:

Relativity Tutorial (external website) Ned Wright, UCLA Astronomy Faculty, California, USA

C-ship: Relativistic ray traced images (external website) Fourmilab, Switzerland

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solve problems and analyse information using: E= mc-squared, equations for length contraction, time dilation and mass dilation

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discuss the implications of time dilation and length contraction for space travel

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