There IS NO theory in which the universe appears the same, regardless
of inertial frame (considering the properties of light). S.R. is a good
try, but it won't work because you will get into trouble as soon as you
start accelerating.

There is a twin-experiment possible with only one acceleration, namely
that of the traveling clock at the far destination, to reverse speed
for the back journey. As soon as the traveling clock arrives back at
the stationary clock, it is discovered that the stationary clock is
time T ahead instead of time T behind, contrary to what was to be
expected, based upon the principles of S.R. The only cause can be the
one acceleration in the journey of the traveling clock. Because only
during this acceleration the conditions of S.R. were broken, it is that
during this acceleration the stationary clock overtook/caught-up the
time difference of no less than 2 * T, as seen from the traveling
clock. However, the length of T is dependent on the length of the
journey and not on the length or intensity of the acceleration. If the
traveling clock were to travel ten times as far, the SAME acceleration
to return would have a ten-fold overtake/catch-up as NECESSARY
consequence to ensure consistence of clocks upon arrival.

This weird overtake/catch-up behaviour is against symmetries in nature
that are far more basic than the symmetry that is "gained" by S.R.
Moreover: it is against what everyone knows of nature. The necessary
overtake/catch-up behaviour can be detected! The stationary clock can
send pulses at regular times by radio or light signals. They will be
received by the traveling clock, obeying the rules of Doppler and S.R.,
as long as there is no acceleration. As soon as the traveling clock
starts accelerating (decelerating/accelerating if you will) to return
back, there should come in a sudden burst of clock pulses of the
stationary clock. If the journey was longer, the burst will be stronger
and more frantic, to catch-up the "lost" time. Every object in the
neighborhood of the traveling clock, that did not this long journey,
but that is subjected to the same acceleration, receives the same
"burst" in clock pulses from the stationary clock. The "burst" can be
arbitrarily intense, because there is no theoretical limit on the
length of the journey. The universe should have if it were a "memory"
for the time-loss that has to be compensated during the acceleration.

Of course the universe has no such memory. S.R. leads to unacceptable
consequences. We know the universe does not work this way, therefore
S.R. is not a valid mathematical model for the reality of the universe.
It's a pitty for mister Einstein. He is one of the biggest jokers that
science has known. He has misled the world for more than 100 years (101
if I am well informed) with a fallacy that is so convoluted that
everyone thought it was brilliant.

However, the error can be exposed with less than 100 lines of plain
english text, as shown above. What distinguishes me from all the
parrots on this forum (the majority) that "believe" in S.R.? I have the
courage to use my common sense and to draw conclusions. You have all
been misled... unfortunately... And by sticking to theories that are
impossible you make yourself immortally laughable.