Question: Presentism is the position that all that exists, exists in the present. Though one can speak of the past, and of events in the past, strictly speaking (in this position), there is no temporal event located there, that we are strictly referring to (as opposed to a memory of); and the same, or so for the future.

But what *is* present? 

That which is present right here, right now; I mean by this: this now, this time on the clock, on this table, this morning; and all that is *simultaneous* with this now.

This, taking into account a little physics (I mean special and not general relativity) is justified *locally*; for the concept of simultaneity is still possible then.

So I can talk about now - in this room, or a little further away - on this earth.

But for distant places, which will be moving in some way to where I am here now, the concept of simultaneity fails.

Thus does presentism fail? (I mean for this reason; and not for others - since there are others).

This can be fixed - I think - if we take into account how some thing, or place (say, a star or galaxy) is moving in relation to us (since the concept of simultaneity is not absolute, but relational).

In this sense, it seems we can fix up the notion of what is present to me now, not just here, in my room; or a little further - on this earth; but everywhere.

But is this correct?

Answer: 
I think that according to special relativity, there is no *now* somewhere else, regardless of the distance, at least as long as we are not talking of quantum mechanics scales, "now" is only "here".

This is how Feynman puts it: "Alpha Centauri 'now' is an idea or concept of our mind; it is not something that is really definable physically at the moment." but the point is that it does not matter if we are talking of another galaxy or another continent, or another city or another room.

the tip of a light cone is just a point in space-time and any event outside the light cone has no fact-of-the-matter time order relation to it, regardless of the distance.

it is actually a mind blowing property of space-time, that we do not appreciate enough.

naturally, we have a little complexity in that the brain is about 10-15 cm in diameter, a problem that can be exploited for an indeterminate amount of contemplation.

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you may be thinking now, "what is this guy talking about? didn't he hear of relative simultaneity?"

The planes of relative simultaneity are useful as mathematical abstractions but do not fit the bill of a *Now*. try to think about it while walking back and forth in your room, contemplating deeply what some hypothetical aliens are doing *right now* 2.5M light years away (on the axis of your march), but note that this remote *Now* of yours absurdly shifts back and forth days or even weeks each time you change direction. this problem is called the [Andromeda Paradox](https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument).

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as for presentism, it seems to me that things that have ceased to exist in your past light cone, like Socrates, do not exist (anymore), and things that will come into being in your future light cone, like the philosophers of the 31st century, do not exist (yet), and as for events outside your light cone, it is meaningless to ask if they have already happened or not. 

there is no such thing as a snapshot of existence in the universe.
"Simultaneity" happens all the time (so to speak.) For example a "fortuitous" encounter is the chance meeting of someone expected to meet in the future that does in fact happen but not according to any plan or "foresight." There is also "spontaneity" where you are "acting" in present time but not really "knowing" where you are going with whatever it is you're thinking or are uncertain as to the outcome. There is also the idea of "serendipity"... a chance meeting that seems to assure one's "future" for all time. These and many other "facts of living" prove that while presentism existsit is only a construct of the mind and not in fact how we as humans nor any other sentient being in fact exists. Birds build nests, whales sing songs...all with a "future purpose" in mind (having shelter, getting food, etc)

Humans are unique in explicitly doing away with time in order to discover "truths" however. The technical term I believe is called "instinct" or in sports "being in the zone" where you are not conscious of time when "acting." For example we have yet to discover any other living organism that attempts to build something like a Cathedral...which we know will require generations or "not in our time" to be completed. This is a unique form of "presentism" as it requires a type of plan that can be understood in the here and now AND in the future thus implying some type of "Master Plan" or "destiny." Of course if I take a well worn footpath and turn it into a road and then some type of highway or "interchange" this might seem to imply some greater intelligence on my part but in reality I'm just trodding the same path and merely expanding (or expounding) on the possibilities should I consider more doing the same. This "more of the same" is neither in the present nor having to do with "simultaneity" but simply projecting out "into the future" what is merely observed. Technically speaking such "thinking" is false as if everything is "just a roll of the dice" the "odds" are still the same...namely the number is the number observed not the odds of said number "going to be observed." 

One could argue and be right however if the dice are rolled and two sixes appear every time that "there is a problem with the dice." As Einstein famously said "God does not play dice with the Planets."