Kallog. I was not looking for a situation that could be equated to "no acceleration at all", but in this scenario both twins have experienced acceleration. I don't have the maths to work out which would have the greater effect, so let's try a different tack. If the travelling twin keeps going in a straight line at constant speed, at the point at which he is 10ly from Earth, is he younger than the other twin? There may be no way he could know that for sure, but the age difference should be there, unless we are saying that the age difference exists only if it can be directly observed.

As far as the photon is concerned, I am not saying that "when it starts out it 'thinks' it's already at B"; I am saying that it is at A and B, in its own F of R, in a sort of infinite "present". There is no need for someone to put the Earth in the way. The photon would have to be aware of the presence of the Earth at the outset, and would have no time in which to be aware. I think I am beginning to answer my own question, so I shall stop rabbiting, and do some thinking.

There never was nothing.