Heh

And now for mine definition.

It has to do with what we call frames of reference. Sounds almost magical that one, doesn't it?

If you let a light bounce on that train between two fixed mirrors, the mirror facing furthest away (B) in the direction of the trains motion will see a red shifted light (photon) coming at it at the speed of, eh, light

'c' that is.

The other mirror (A) won't see that redshift as I think of it, now assuming that it magically could follow that 'lights propagation', but as the light bounce back from the second (B:s) mirror towards it (A) again it will find the light to be blue but coming at it at 'c' according to its ruler and its own 'wrist watch'.

So the energy of that light will blue (energetic), respective red (weak) shift depending on the mirror measuring relative the frame of that train.

Which could be one way of defining it, but in this particular case would be totally wrong as I see it. Both mirrors share a common fixed reference frame mounted, unmovingly, to the floor of that train, assuming the train to have a constant uniform motion (and ignoring gravity for this). To assume that it could red, relative blue, shift here would in fact introduce an idea of a 'absolute motion', which light then would 'use' as a measure, bouncing.

But motion, and 'energy' is always relative the observer, and when sharing the same 'frame of reference' (the train) there will be noting 'weird' to be seen.

If you instead assume that one mirror (B) move from the other mounted one (A), on the train then? Well, then the 'frame of reference' can't be the same for both and both will find a redshift.

So what frame of reference would be able to see that red, respective blue, shift from the mirrors light bouncing? Now, that's a question of geometry, and 'frames of reference'.

What would someone see if standing 'still', on the side of the bank, watching that train pass by?

Another thing that is really important to understand is that what an observer finds to differ always will be a definition from comparing 'frames of references'. 'Locally', using your wrist watch and your ruler nothing never change for you, no matter where you are or how fast you move. Your esteemed life span and length will always be the same according to those. What changes is the relation your 'local frame of reference' finds itself to have relative another frame of reference.

And if we look at a length contraction then the universe must 'shrink' in a acceleration for you. The problem here is where to define that local 'frame of reference', but very loosely speaking you might assume that your 'space craft' is one undifferentiated same 'frame of reference' for this. So the universe shrinks, but you stay the exact same.

And that is the truth,the whole truth and nothing but the truth, so help me, eh, Einstein?

Can you see what I'm getting on here? That the thing defining your universe will be your ruler and your clock. Assuming that there exist a 'conceptually same universe' in fact becomes a solely conceptual exercise, not the reality you experience.

So now the question becomes, what would you define as more 'real'?

What you expect from preconceptions, grounded since birth, or what experiments tells you?

To find repeatable experiments assume a 'same universe', or, it assume a 'locally same universe', then as defined relative 'your ruler' and 'your clock'. That locally same universe is defined through constants. And 'c' is the one creating both time dilation's and LorentzFitzgerald contractions. And 'c' is also the information carrier telling you what there is to 'observe'. It's your 'curtain of light', painting your 'reality' constantly.

And 'c' doesn't care about motion, as I see it.