## What's so Special about Relativity?

Special relativity is one of those fun topics I like to do when possible.  While I usually cover it the last week of school, this year I have decided to do it the week between the Easter break and the start of the student projects on modern physics.

I start in the usual way, by motivating the scientific conclusion that everybody measures the speed of light to be the same, no matter how they are moving.  This then leads to using the animation that shows that a moving clock ticks slow.  But rather than use the geometry of this situation to derive the time-dilation equation (a MEGO exercise most times), I use the interpretation of SR that everybody is moving through space-time at the speed of light.  If you are at zero speed in space, you are moving through time at the normal rate.  If you are moving through space at a non-zero speed, you are moving through time at a slower than normal rate.

This interpretation then leads to using this model.

This is a picture of a Geogebra dynamic model that I display to the class (available here) and that they can use on their iPads.  Students are given a worksheet which they use to calculate the time dilation for various speeds and also the speeds needed for various time dilations.  The next day comes time travel!

By not using the algebraic equations too soon, I have found more students engaged and  understanding SR. We then spend a couple of days discussing the possibility and feasibility of time travel.

P.S.  If you have not checked the program Geogebra, I suggest you do.  Learning how to use it is not too hard, and there are some examples (and more) you can use in your class.  Since it is FOSS, you share it with your students, embed things in webpages, and otherwise make static situations dynamic.  Below is the model embedded in this page (if you don't see the app, come back later.  I am having problems getting it to work on this page.  Go to this page to see it in action.).