From a mathematical point of view, time is a dimension, much like the usual height-width-depth ones we deal with. At the microscopic scale, many processes are reversible - eg. they work just as well one way as in reverse. Use energy to split a water molecule into hydrogen and oxygen? Combine hydrogen & oxygen, and get just as much energy back.
At our level, though, we only see time move one way - known as the 'arrow of time'. It's related to entropy - the nature of systems to wind down. If you start with a glass of ice, it absorbs heat & becomes water - but a glass of water never emits enough heat to become a glass of ice. It takes more energy to chill the water to ice than is released by it (check the back of a fridge sometime; it's warm, and that's both the heat removed from the inside, plus more heat to make it happen).
Yes, relative time depends on your relative velocity compared to each other (there's no universal standard to measure against. No justice, just us!). Objects moving with the same velocity will experience the same rate of time. Objects moving much faster than others will see time passing more rapidly on those objects; those objects see time passing more slowly on the fast mover. You need speeds at large fractions of C to see much effect, but we know it's happening every time you use your phone - the in-phone GPS has to calculate that the rate of time on the orbiting satellites is a teeny bit slower than the rate of time down here, because they're moving much faster. But it's a tiny fractional effect.
Why do we only experience time going one way? No-one knows. Maybe conciousness is related to entropy - in the same way a twig in a river can only see the banks going past. But from a scientific point of view, it's just another dimension.
Now the "many worlds" hypothesis - splitting timelines - ties into quantum theory. In short, every time there's a choice at some level - eg. an atom emits an electron, that could go in X direction - all of these happen, just all in a separate, new timeline. So flip a coin, cause two complete new universes! And your conciousness ends up in both, but the one reading this doesn't have any way to know that the other one exists.
Some sci-fi writers have used this to get around the grandfather paradox in time travel. Go back in time & kill your grandfather, so you'll never be born? Fine! Just in this new shiny timeline. Technically quantum physics doesn't prevent this. But it does prevent you from ever going back to your original timeline. But the odds on this are low - there are other good reasons why time travel is unlikely to be possible.
(And if someone did sideslip from the steampunk universe into this one, odds are they'd be taken into custody as a raving loony, and put into an asylum for care. Assuming he/she ended up in a 1st world country. Imagine being a time-traveller appearing in Soviet Russia, with no papers, and a ridiculous story ...)
Just remember,
1) Quantum makes sense proportionally to inebriation
2) Friends don't let friends break causuality.
W.
