Astronomical Concepts – Week 7

This week – special relativity.


We started with the two main axioms of special relativity:

  1. The laws of physics are the same in all inertial frames of reference

  2. The speed of light is constant (299,792,458 m/s)

Tonight we had to imagine we were floating through space in a spaceship. Are we stationary or are we moving? Are the stars stationary or are they moving? Relative to us of course. With the window blinds down on our spacecraft we have no way of knowing if we are moving or not, there is no experiment we can do to find out.

However, an observer on another spacecraft has a different point of view. On our spacecraft if we bounce a ball we see it go down and then up in a straight line. If an observer on another spacecraft could see us bounce a ball as we drift past them in our spacecraft, and they are stationary they get a different point of view of the ball.


The definition of ‘now’

We accept that certain events that happen can be simultaneous. But how can we determine that two events that happen in different locations are simultaneous?

On Earth we carry clocks and they are synchronised, we accept that two events that happen happened at the same time. What we have to accept is that clocks, time, is influenced by motion. Einstein realised that space contracts and time dilates through motion. A moving clock runs more slowly as its velocity increases, until, at the speed of it stops running all together. So having clocks that run at different rates leads to strange effects – simultaneity is relative. Whether or not two events are simultaneous depend on your frame of reference.


So, one person’s definition of time is not the same as another’s. Also, the faster you travel the slower you age. At speeds close to c the effects are huge, at smaller speeds less so.

Further, the faster you move the more you contract. Close to c the amount of contraction is great, at slower speeds less so, tiny amounts. An observer at rest relative to the moving object would observe the moving object to be shorter in length of motion. As the object increases in speed and gets close to c the object would appear much shorter.


Special relativity is a work of pure genius by Albert Einstein. Our session tonight was an introduction, a mere taster of the theory and we were only able to scratch the surface. I loved the session, the concepts are so interesting, if a little hard to get your head around. Much more information about special relativity can be found here.

So, the only true constant is the speed of light. The faster you travel the more time slows down for you and the more you contract.

With the invention of atomic clocks we can now measure time to billionths of a second and can be accurate to within one second over 3.7 billion years. Einstein said that realising gravity and acceleration were the same thing was “the happiest thought of my life”.