Lesson 3: Simultaneity
We can start examining the consequences of Einstein’s two postulates by first looking at time. In Newtonian mechanics, there is a universal time scale for all observers. If a series of events took 1 sec according to one observer, they would also last 1 sec according to an observer moving with respect to the first.
According to Einstein though, time interval measurements depend on the reference frame in which they are made. For example, two events that are simultaneous in one frame of reference are in general not simultaneous in a second frame moving with respect to the first. Thus we consider simultaneity to be a concept that is not absolute.
The central concept of this chapter is to understand that simultaneity is not an absolute concept but a relative concept.
- Understanding that any frame of reference in which the law of inertia holds is referred to as an inertial frame of reference.
- Knowing that a noninertial frame is one that is accelerating relative to an inertial frame.
- Being able to conceptualize the idea that there is no such thing as an absolute inertial frame of reference or absolute velocity; all of these concepts, including simultaneity are relative.
Click on the button below for a simulation that can be shown to introduce the relativity of simultaneity:
Below is a video that can be used to show that simultaneity is not an absolute concept; meaning that two different observers will not see an event happen at the same time.
The following video concisely explains the relativity of simultaneity using 3-D graphics.
In order to convey to the class that simultaneity is not an absolute concept a teacher can do the following:
One way to consolidate the lesson would be to ask the class to consolidate the lesson by explaining the relativity of simultaneity in their own words.
- Hirsch, A., Stewart, C., Martindale, D, & Barry, M. (2011). Nelson Physics 12. Toronto: Nelson Thomson Learning