Twins who go walkabout with equal and opposite velocities age at the same rate despite their relative velocity, but both age slower than their launchpad. If the twins are of the same mass or if they expel rocket propellant into free space to get moving, the distinguishing features of the launchpad are:

1. It's worldline has no kinks
2. The time axis of its reference frame is parallel to the centre of momentum.

To drive the 2nd point home, throw a clock at a wall and then throw the wall at the clock from the same distance. The elapsed time on the clock on impact is different in each case even if the relative velocity is the same.

The upshot is, the reference frame in which the time axis is parallel to the centre of momentum has the fastest clock. Why isn't that covered in undergraduate courses?

Mike Gale wrote:

> Twins who go walkabout with equal and opposite velocities age at the
> same rate despite their relative velocity, but both age slower than
> their launchpad. If the twins are of the same mass or if they expel
> rocket propellant into free space to get moving, the distinguishing
> features of the launchpad are:

no, how I see it, it depends on their path along the spacetime. Somehow
the spacetime keeps track on what they are doing.

not crazy.

US promises NOT to charge Americans to fly out of Afghanistan after
report of ‘$2,000 repatriation fee’ triggers outrage
https://on.rt.com/bex0

Anti-war activists mock CNN as it cries over $1 trillion worth of
‘desperately needed’ minerals left behind in Afghanistan
https://on.rt.com/bew7

meanwhile

Russia to pump 5.6 billion cubic meters of natural gas through Nord
Stream 2 pipeline in 2021 – Gazprom https://on.rt.com/bev9

and

Taliban’s return to power in Afghanistan is ‘moral victory of Islamism
over the West’ – top Merkel ally

On Friday, August 20, 2021 at 2:44:23 AM UTC-6, Cash Bornhoft wrote:
> Mike Gale wrote:
>
> > Twins who go walkabout with equal and opposite velocities age at the
> > same rate despite their relative velocity, but both age slower than
> > their launchpad. If the twins are of the same mass or if they expel
> > rocket propellant into free space to get moving, the distinguishing
> > features of the launchpad are:
> no, how I see it, it depends on their path along the spacetime. Somehow
> the spacetime keeps track on what they are doing.
>
> not crazy.
>
> US promises NOT to charge Americans to fly out of Afghanistan after
> report of ‘$2,000 repatriation fee’ triggers outrage
> https://on.rt.com/bex0
>
> Anti-war activists mock CNN as it cries over $1 trillion worth of
> ‘desperately needed’ minerals left behind in Afghanistan
> https://on.rt.com/bew7
>
> meanwhile
>
> Russia to pump 5.6 billion cubic meters of natural gas through Nord
> Stream 2 pipeline in 2021 – Gazprom https://on.rt.com/bev9
>
> and
>
> Taliban’s return to power in Afghanistan is ‘moral victory of Islamism
> over the West’ – top Merkel ally
You're talking about feature #1 - worldlines with no kinks, which means no contact forces. A kink means that you've deviated from the centre of momentum, which requires work, which implies a force that pins you to your seat. That's how spacetime keeps track of what you're doing.

On Sunday, August 22, 2021 at 9:18:03 PM UTC-6, Mike Gale wrote:
> On Friday, August 20, 2021 at 2:44:23 AM UTC-6, Cash Bornhoft wrote:
> > Mike Gale wrote:
> >
> > > Twins who go walkabout with equal and opposite velocities age at the
> > > same rate despite their relative velocity, but both age slower than
> > > their launchpad. If the twins are of the same mass or if they expel
> > > rocket propellant into free space to get moving, the distinguishing
> > > features of the launchpad are:
> > no, how I see it, it depends on their path along the spacetime. Somehow
> > the spacetime keeps track on what they are doing.
> >
> > not crazy.
> >
> > US promises NOT to charge Americans to fly out of Afghanistan after
> > report of ‘$2,000 repatriation fee’ triggers outrage
> > https://on.rt.com/bex0
> >
> > Anti-war activists mock CNN as it cries over $1 trillion worth of
> > ‘desperately needed’ minerals left behind in Afghanistan
> > https://on.rt.com/bew7
> >
> > meanwhile
> >
> > Russia to pump 5.6 billion cubic meters of natural gas through Nord
> > Stream 2 pipeline in 2021 – Gazprom https://on.rt.com/bev9
> >
> > and
> >
> > Taliban’s return to power in Afghanistan is ‘moral victory of Islamism
> > over the West’ – top Merkel ally
> You're talking about feature #1 - worldlines with no kinks, which means no contact forces. A kink means that you've deviated from the centre of momentum, which requires work, which implies a force that pins you to your seat.. That's how spacetime keeps track of what you're doing.
.... conservation of momentum.