A. Questions for Submission

3.1 Spacetime Diagrams

Label the following parts of the diagram below: (a) timelike worldlines; (b) spacelike curves; (c) lightlike curves; (d) inertial trajectory; (e) hypersurface of simultaneity for the inertial trajectory:

3.2 Visualising the Relativity of Simultaneity

Suppose an observer on a spaceship judges two flashes to be simultaneous, while an observer on Earth does not, as illustrated in the diagram below.

1. Draw a spacetime diagram illustrating the Earth observer's worldline at rest.
2. In the same diagram, draw the worldlines for the front and the back of the spaceship.
3. In the same diagram, draw a hypersurface of simultaneity for the ship, and indicate the locations of the two flashes on that hypersurface.
4. Use the diagram to explain the different judgements of the spaceship observer and the Earth observer about whether or not the flashes occurred simultaneously.

3.3 Presentism

1. Give a clear statement of the meaning of 'presentism'.
2. What problem does special relativity pose for presentism?
3. Does special relativity support eternalism instead?
4. Can you imagine any alternative view?

 

B. Further Discussion (No Submission)

Further views on presentism

• Katherine Brading (optional reading) argues that presentism is an empirical question, in that it must be settled by experiment. In your view, is this correct?
• John Norton (optional reading, "The Burning Fuse") argues that presentism and eternalism are equally problematic, suggesting a sense in which the entire debate is misguided. Is he correct?
• Have a look at Hinchcliff's paper (optional reading) and evaluate his "point present" model.

Spacetime and simultaneity

Consider two events E₁ and E₂ below. One event is the birth of a person in London, and the other is the birth of a person in Paris. The events are illustrated in each of the spacetime diagrams below.

1. Draw the worldline of an observer that judges the London and Paris births to occur at the same moment, in that E₁ and E₂are simultaneous. Then draw a hypersurface of events that A will judge to be simultaneous which passes through E₁. Explain how the diagram illustrates the two events occur simultaneously.
2. Draw the worldline of an observer that judges the London baby to be born before the Paris baby, so that they do not have the same birth-time, or possibly not even the same birthday.
3. Explain how the concept of `having the same birthday' is observer-relative. Is a lesson of relativity thus that 'everything is relative'?