The usual answer to the twin paradox is that the twin who undergoes acceleration is the one who finds the other has aged more, because the trajectory of the 'travelling twin' does not amount to a single inertial frame - the travelling twin has accelerated while the stationary twin has not. But isn't acceleration a relative phenomenon? If the universe were empty apart from the two twins, what does it mean to say that one and not the other is accelerating and how could it be decided which was which?
(Inevitably someone will say this is a duplicate - apologies if so but I can't find it)
Edit: thank you for those thoughtful responses - on reflection I was asking two questions and the twin paradox is about velocity not acceleration. I think the interesting question is how we distinguish the velocities or accelerations of two objects, which you need to do in order to calculate the proper time elapsed for each. It can't be enough to say that the accelerating one 'feels it' - that is true only insofar as the force is transmitted from one part of the object to another (such as from the rocketeer's back against the seat to her inner ear, or from the body of the accelerometer on the floor of the rocket to its detector component). Maybe it relates to Mach's conjecture about absolute rotation, ie that you have to make reference to the distribution of matter in the wider universe in order for the distinction of moving vs not moving to have meaning. If so there is a sense in which acceleration is relative rather than absolute. Anyway thank you.