A.3 Galilean transformation

The Galilean transformation describes coordinate system transformations in nonrelativistic Newtonian physics. This note explains these transformation rules. Essentially the same analysis also applies to Lorentz transformations between observers using arbitrarily chosen coordinate systems. The small difference will be indicated.

Consider two observers A' and B' that are in inertial motion. In
other words, they do not experience accelerating forces. The two
observers move with a relative velocity of magnitude

Observer B' determines time using a clock that indicates a time

The question is now, what is the relationship between the times and positions that these two observers attach to arbitrary events.

To answer this, it is convenient to introduce two additional observers
A and B. Observer A is at rest compared to observer A'. However, she
takes her zero of time and the origin of her coordinate system from
observer B'. In particular, the location and time that A associates
with her origin at time zero is also the origin at time zero for
observer B':

The other additional observer, B, is at rest compared to B'. Like
observer A, observer B uses the same origin and zero of time as
observer B':

Observer B orients her coordinate system like A does.

That makes the relationship between A and B just like A and B as
discussed for the Lorentz transform, figure 1.2.
However, the classical Galilean transformation is much simpler than
the Lorentz transformation. It is

The question still is how to relate the times and locations that observer A' attaches to events to those that observer B' does. To answer that, it is convenient to do it in stages. First relate the times and locations that A' attaches to events to the ones that A does. Then use the formulae above to relate the times and locations that A attaches to events to the ones that B does. Or, if you want the relativistic transformation, at this stage use the Lorentz transformation (1.6). Finally, relate the times and locations that B attaches to events to the ones that B' does.

Consider then now the relationship between the times and locations
that A' attaches to events and the ones that A does. Since observer A
and A' are at rest relative to each other, they agree about
differences in time between events. However, A' uses a different zero
for time. Therefore, the relation between the times used by the
two observers is

Here

To specify the location of events, both observers A' and A use
Cartesian coordinate systems. Since the two observers are at rest
compared to each other, they agree on distances between locations.
However, their coordinate systems have different origins. And they
are also oriented under different angles. That makes the unit vectors

The above equations can be used to find the coordinates of A in terms of those of A. To do so, you will need to know the components of the unit vectors used by A' in terms of those used by A. In other words, you need to know the dot products in

Then these relations allow you to sum the

Note also that if you know these dot products, you also know the ones
for the inverse transformation, from A to A'. For example,

(In terms of linear algebra, the dot products form a 3

The relationship between observers B and B' is a simplified version of the one between observers A and A'. It is simpler because B and B' use the same zero of time and the same origin. Therefore the formulae can be obtained from the ones given above by replacing A' and A by B and B' and dropping the terms related to time and origin shifts.