Notes on Relativity and Cosmology - Physics Department, UCSB

26 CHAPTER 1. SPACE, TIME, AND NEWTONIAN PHYSICS
PHY101 or PHY211. I suggest that you take the opportunity to reflect on this
at a deeper level than you may have done before.
‘Newtonian’ physics is the stuff embodied in the work of Isaac Newton 2 . Now,
there were a lot of developments in the 200 years between Newton and Einstein,
but an important conceptual framework remained unchanged. It is this framework
that we will refer to as Newtonian Physics and, in this sense, the term
can be applied to all physics up until the development of Relativity by Einstein.
Reviewing this framework will also give us an opportunity to discuss how people
came to believe in such a strange thing as the constancy of the speed of light
and why you should believe it too. (Note: so far I have given you no reason to
believe such an obviously ridiculous statement.)
⋆ Many people feel that Newtonian Physics is just a precise formulation of their
intuitive understanding of physics based on their life experiences. Granted, as
those of you who have taken PHY101 or 211 know, there are many subtleties.
But still, the basic rules of Newtonian physics ought to ‘make sense’ in the sense
of meshing with your intuition.
Oh, you should actively ponder the question“What does it mean for something
to ‘make sense’?” throughout this course....
1.1 Coordinate Systems
We’re going to be concerned with things like speed (e.g., speed of light), distance,
and time. As a result, coordinate systems will be very important.
How many of you have worked with coordinate systems?
Let me remind you that a coordinate system is a way of labeling points; say, on
a line. You need:
A zero
A positive direction
A scale of distance
+x
x=-1m
x=0 x=+1m
We’re going to stick with one-dimensional motion most of the time. Of course,
space is 3-dimensional, but 1 dimension is easier to draw and captures some of
the most important properties.
In this course, we’re interested in space and time:
2 For our purposes, the most important part of this work was actually done by Galileo.
However, we will also make use of the refinements added by Newton and the phrase ‘Galilean
Physics as refined by Newton’ is just to long to use.