# Homework Help: (x1, ct1) = (25m, 25m) - What do the 'm' represent? (SR)

1. Aug 11, 2012

### seasponges

I'm currently completing a special relativity assignment, and whenever coordinates are referred to they're represented as (x1, ct1) = (25m, 25m) or (6m, 2m). There are no units given (other than the 'm', and I highly doubt they're using minutes for ct). If someone could clear this up for me I'd be very grateful.

Cheers!

2. Aug 11, 2012

### arildno

"m" as in Minkowski coordinates, perhaps?

ct has dimensions of length, btw.

Note that unless you work with the full 4-vector, it would be extremely easy for students to misunderstand the notation without the "m", falsely believing the time coordinate was a regular space coordinate.

Last edited: Aug 11, 2012
3. Aug 11, 2012

### Sybren

M for meter perhaps, since these have the dimension of length.

4. Aug 11, 2012

Just to clarify, it is for "meters."

Spatial units are typically denoted in m, and I'm sure you've seen x represent a spatial coordinate.

ct is also meters, because c is the speed of light and c=3*108 meters/second [m/s] except it is multiplied by time. So you have a velocity (c) times time.
(meters/seconds)*seconds=meters because the seconds cancel.

5. Aug 12, 2012

### seasponges

Ha, it didn't occur to me ct was in meters (very new to SR). Thanks!