"How do you pick up the threads of an old life?  How do you go on, when in your heart you begin to understand there is no going back?  There are some things time cannot mend.  Some hurts that go too deep that have taken hold."  -LOTR

Though it seems obvious that we proceed linearly through time, physicists don't really have a good scientific explanation for this imprint.  According to most laws of physics, things should be temporally symmetric.  So what is time?  I suppose in part it is the arrow of entropy, pointing constantly from order to disorder.  It dictates the states of our universe.  Right now I am typing this, and a moment ago I was typing the LOTR quote.  And somehow, then, the universe was a more ordered place than it is now.

But of course there are all sorts of confusing things to think about with time, such as Einstein's twin paradox, and the speed of light being a barrier for total energy (spacial + temporal),  and wormholes, and multiple universes, and those confusing things about going back in time and killing your father.  It seems like some argument against free will.  If time travel exists, then my free will is limited.  Because, at least in one continuous world, if I go back in time before I was born, I cannot kill my mother.  I don't have that choice.

There is something about the nature of long-term-potentiation of excitatory post synaptic potentials in the brain and working, short term, and long term memory in its various locations (hippocampus, frontal cortex maybe?) that makes memories of the past but not of the future, even if physically the ordering of time seems a bit ambiguous.  Something about me traps me in this entity that no one can characterize, this weird next of seconds and the decay of atoms and slowly turning gears.

To me, at least, time is the most incomprehensible thing that we humans can think about, and yet we have managed to make it so integral in our daily lives.  In linguistics we learned that they thought they had found a tribe in some rainforest somewhere that had no language for discussing the future or the past I think.  They originally thought that this tribe had no concept of the future or past, but they've concluded now that they just didn't have a way to articulate their concepts of time.  Is it possible for a human being, isolated from everything, to be unaware of time?  Doesn't he or she sense something changing?  Change, derivatives... those somehow define our lives and yet they mean nothing, really, I mean, semantically, what does it mean for time to have passed?  We have all of these physical by-products we can reference (like atoms decaying), but that seems more like an effect that happens because time passes rather than the actual entity of the passage of time.  Who knows what that is?

What makes us so dependent on time?