On 15 Sep, 14:05, Mike Williams kindly
replied to say:

Relativity.

Electromagnetic radiation always travels at the speed of light relative
to any observer. So radiation that set off from the big bang 13.7
billion years ago arrives at the speed of light.

This reflects my lack of understanding. My question is not really
about how fast it arrives but how does it arrive here? If I went off
into space and shone a beam of coherent light from my laser in the
diametrically opposite direction to where an observer was positioned,
then he/she wouldn't be able to detect it (would he?). Shouldn't all
the photons from the big bang are travelling away from us like the
galaxies?

In exactly the same way
that light from a star arrives at the speed of light, even if the star
is travelling away from us at half the speed of light.

I think I grasp this even though it doesn't make "common sense" to my
limited terrestially evolved brain. As I understand it, it's all in
the maths.

The speed of the em-radiation is constant, but the speed difference
between source and observer causes the wavelength to be shifted.

Doppler Shift, check.

In the
case of the cosmic background, the red shift is so great that it's gone
right though the visible wavelengths and down into the microwave region.

I'm up to speed on this as well, but I end up returning to the
question, why does it arrive here?

Thanks again,
Paul