# Mrs Brown on relativity

 I briefly watched a few moments of Mrs Brown’s Boys Christmas special 2016; a comedy program on the BBC. Agnes Brown’s grandson Bono asked his grandmother about Einstein’s theory of relativity. The joke is that this is far to complicated for an old Irish Granny to answer.

This made me think. Einstein first published on special relativity in 1905 and the field equations for general relativity were published in 1916. So humanity has had knowledge of Einstein’s relativity for 100 years now, yet Mrs Brown was unable to say anything!

Not that we should expect everyone to have a detailed mathematical knowledge of Einstein’s relativity, but should just about everyone be able to say something?

Special relativity
Special relativity is based on two postulates – that then lead to a consistent mathematical description in terms of differential geometry, but we should ignore that for now – that can be paraphrased as follows

1.  The laws of physics are the same for all non-accelerating observers.
2.  All  non-accelerating observers measure the speed of light to be the same,  irrespective of the source of light.

Even more pedagogically, all observers that are not experiencing a net force are equally as good’ as far as determining the laws of physics are concerned, and on top of that, light waves travel at a fixed speed as measured by these privileged observers.

General relativity
This is a bit harder to paraphrase, but basically we have three key features

1. All observers (not just the non-accelerating ones) are equally as good for determining the laws of physics.
2. Space-time is curved and the force of gravity is the bending’ of space and time.
3. In `small enough’ regions, the non-gravitational laws of physics reduce to that of special relativity.

Again, all this can be made mathematically precise.

Are we expecting too much?
I think it is too much to expect any real knowledge of Einstein’s relativity from the general public. So, we should not ask for this, but only a vague idea of the ideas from most people.

From my own perspective it is all differential geometry 🙂