**I am a great admirer of James Clerk Maxwell.**

**And amongst his greatest achievements** was the prediction that waves in electric and magnetic fields should travel at the speed of light.

**He arrived at his prediction** by considering the observed strength of *static* electric magnetic fields.

**For example**, studies had established the strength of the force from a given amount of electric charge at a given distance.**This electrical force was characterised by a constant**called (for historical reasons)*the permittivity of free space*. It was given the symbol ε_{0}– the greek letter ‘epsilon’ with a subscript of zero. It was considered to represent in some way how ‘disturbed’ the space was around an electric charge.**Similarly**, studies had established the strength of the magnetic force from a given electric current at a given distance.**This magnetic force was characterised by a constant**called (for historical reasons)*the permeability of free space*. It was given the symbol μ_{0}– the greek letter ‘mu’ with a subscript of zero. It was considered to represent in some way how ‘disturbed’ the space was around an electric current.

**Maxwell analysed these static experiments** and predicted that there should be coupled waves in the electric and magnetic fields and that they would travel with a speed of:

**And when Maxwell calculated this number** he arrived at a number very close to the previously measured speed of light.

**He observed that this was unlikely to be a coincidence** and concluded that light was a wave in the electromagnetic field.

**I can still remember how I felt** when – aged 19 – I followed Maxwell’s footsteps and ‘discovered’ this connection: I was gob-struck!

**Other waves**

**This type of formula is typical of expressions** for the speed of waves. For example, the speed of a wave on a stretched wire or string is given by:

where *T* is the tension in the string and *m* is the mass per unit length of the string. So a wave will travel quickly when the string is taut and low mass.

**And in general we expect the speed of waves** to reflect how the medium in which the waves travel responds to a disturbance.

**Gravity waves**

**And that is why last years’ announcement** (LIGO, Popular Report) that gravity waves travel at the speed of light is so profoundly important.

**This discovery implies that there is a connection** between:

- electricity and magnetism – responsible for just about all the phenomena we experience around us – and…
- gravity – which is associated with space and time and mass.

**Alternatively, **it could indicate a connection between them both and something else we don’t know about.

**But the experimental fact of this connection astounds me** as much if not more than the connection that Maxwell made.

**And it makes me wonder** just what he would have to say about the discovery.

**Now I know this connection is not ‘new’: **I can remember being told that gravity waves *would* travel at the speed of light many years ago.

**But the discovery of the experimental fact** of the speeds of light and gravity being equal seems to me to be more profound than the mere expectation that it should be so.

**You can see more about the discovery** in the LIGO video below