This is our final article on teaching special relativity in the first week of an introductory physics course.1–3 One of the profound changes in our view of the world was Einstein's discovery of the lack of simultaneity. He illustrated this result with a thought experiment in which we observe a railroad car passing by us. We see the two ends of the car struck simultaneously by lightning bolts, but to someone riding inside the car, the lightning strikes were not simultaneous. My difficulty with this thought experiment is that while doing calculations, I have to go back and forth between two imagined points of view. To avoid this, I actually perform an experiment that involves two simultaneous events. then all we have to imagine is how the experiment looks to someone moving by us. Not only does the order of the two events depend on the direction of motion of the observer, but we can demonstrate that if information could travel faster than the speed of light, we could get answers to questions that have not yet been thought of.

1.
Elisha
Huggins
, “
Special relativity in week one: 1) The principle of relativity
,”
Phys. Teach.
49
,
148
151
(
March 2011
).
2.
Elisha
Huggins
, “
Special relativity in week one: 2) All clocks run slow
,”
Phys. Teach.
49
,
220
221
(
April 2011
).
3.
Elisha
Huggins
, “
Special relativity in week one: 3) Introducing the Lorentz contraction
,”
Phys.Teach.
49
,
302
303
(
May 2011
).
4.
We introduced the use of the laser and beam splitter in
Huggins
, “
On teaching the lack of simultaneity
,”
Am. J. Phys.
43
,
599
600
(
July 1975
). There we also describe how to pull an uncontracted rocket ship in through a contracted barn door.
5.
A somewhat similar approach can be found in
Joseph A.
Spizuoco
, “
The simultaneity of relativistic pitchforks
,”
Phys. Teach.
31
,
552
554
(
Dec. 1993
). In that paper, our light pulse trigger signals are replaced by compressional pulses running down the tines of the pitchfork. We find that it complicates the mathematics to use trigger signals that travel at a speed less than the speed of light.
6.
In the paper by
Rachel E.
Scherr
,
Peter S.
Shaffer
, and
Stamatis
Vokos
, “
Student understanding of time in special relativity: Simultaneity and reference frames
,”
Am. J. Phys.
69
,
S24
S35
(
July 2001
), the authors find that many of the 800 students in several different level introductory physics courses had difficulty with a simultaneity question. The question involved the simultaneous eruption of Mt. Rainer and Mt. Hood as viewed from a high-speed spacecraft moving by. We suspect that most of the student confusion resulted from the reliance on teaching about frames of reference and coordinate transformations. If we had renamed our red and green flashbulbs Mt. Rainer and Mt. Hood, we think that few students would have had a problem with the question.