# What are 'rods' and 'clocks in relativity?

Sul
Hey I need some help on this. What are 'rods' and 'clocks'. Someone told me it's a coordinate system. I don't understand it. Any help?

arcnets
Hi Sul,
probably you refer to a statement like "The length of a rod and the speed of a clock are relative" or the like. Am I on the right track?

Staff Emeritus
Gold Member
As used by Albert Einstein in referance to Relativity, A rod is something that measures length, a clock is something that measures time.

You use rods and clocks to establish a coordinate system.

Stranger
Hi Sul(that's what I'm called as well)...

Relativity uses the concepts of rods and clocks instead of meter or seconds because every person, according to reltivity, has his own time...time, length and mass are relative...thats why its more appropriate to use rods and clocks...

Stranger, I not sure you have it right there.
For example in one reference frame a second might be defined as so many cycles of a certain atom clock, in another reference frame it could be defined as the same number of cycles of the same design of atomic clock. The clocks moving relative to each other would just not agree with each other, but they could each still define a second as the same thing.

Stranger
meter or seconds

Thanks Dave...I know that I shouldn't have written this down...

Originally posted by Sul
Hey I need some help on this. What are 'rods' and 'clocks'. Someone told me it's a coordinate system. I don't understand it. Any help?

In relativity there are two principles

(1) The laws of physics are the same in all inertial frames of referance

(2) The speed of light is indendent of the source in an inertial frame of referance

When put together it can be shown that
(1) a moving clock runs slower than an identical clock which is at rest
(2) a moving rod is shorter than an identical rod which is at rest

The slowing of clocks implies that the slowing of the clock has to do with the slowing of time itself. And same with the shortening of the rod - space itself has contracted.

Hence special relativity is the theory of space and time i.e. Space-Time!

Then there's General Relativity which gets more complicated. It can be shown that in the mass can curve spacetime!

Pete

Eugene Shubert

Originally posted by Sul
What are 'rods' and 'clocks'. Someone told me it's a coordinate system.
A “rod” is just a ruler. A “clock” is an instrument that measures elapsed time. See http://www.everythingimportant.org/relativity to understand how rods and clocks are used to comprehend spacetime.