I could have been born at any time, in any place in the universe - or not at all, for that matter. Why do I get to experience the universe in a unique way? What gives rise to this?
You were also born in 1988 in CT. This other you was born to different parents, had a different childhood, and has different memories. You think you are different from this other you (and you are!) because you have separate thoughts and memories. You remember your own life and not your other life because humans aren't telepathic.
In case it isn't yet clear, this other you is me. Hi, other me. We haven't talked before, I don't think.
In other words, if you were born in a different place at a different time in a different community... then that you wouldn't be you. It would be a different person. So it doesn't really make sense to ask "what if I had been born at a different time or place".
I like to entertain the idea that humans are the host of a species of sentient alien bacteria that lives in our guts and controls us through the vagus nerve and uses humans to build technology to advance to the point where they can build space ships to colonize other planets. Could make a good sci-fi book if anything...
-If you weren’t, you wouldn’t be asking this question. The only version of reality in which you’re able to ponder it is one with you in it. This is a narrower version of the weak anthropic principle.
-There are a multitude of universes or regions of one universe with an infinitude of you asking this question without bound.
-God or the universe inevitably leads to it. This is a form of the strong anthropic principle.
-Only universes and times which allow such questions to be asked can allow for people like you to ponder it. For most of history in which humans were around you’d almost certainly lack the leisure and education to have these thoughts. (WAP again)
-Everything capable of making this observation does so wherever and whenever they exist in the universe, and there is nothing interesting about the question or answer because it’s arbitrary. Although it is difficult to appreciate from each of their perspectives, the question itself is as ubiquitous as those asking it.
There is also the theories that consciousness is all that exists, e.g. "Donald Hoffman's Multimodal User Interface (MMUI) theory of reality and doctrine of Conscious Realism together posit that the reality humans perceive is like a simulation in which non-physical conscious agents interact in order to generate the dynamics we observe".
Unless you're getting into Cartesian dualism and souls, I'm genuinely confused.
But yes, if materialism is true, probably the question is empty. However, if some other philosophy of mind is true (there are many other options beyond just materialism and Cartesian dualism), maybe it means something. And, if one has this sense that it is profound rather than empty, that might give one a reason to prefer some other position in the philosophy of mind that treats that question more sympathetically.
Can you give some examples of the kind of options you're thinking of? I mean, I have a lot of time for functionalist accounts, and given Sufficiently Smart Technology (tm) I'm happy to allow for the possibility of something recognizably "me" being artificially duplicated in a different physical substrate. (After which we'd rapidly start diverging, and would need some new vocabulary to describe what's going on.)
But GGP's question was specifically about being born. I really struggle to imagine how someone born somewhere else, somewhen else, could be "me" with anything like the fidelity I think our intuition demands. And if you loosen your identity test to the point where it could, doesn't it then become useless as an individuation test, in that lots of people could be sufficiently like me all at the same time?
This is why I think it's tough to say "I - singular - could have been born in Brazil in 1873" without invoking something like a soul which acts as both an arbitrary GUID tag and as kind of mutex on embodiments.
Up front let me admit this is effectively a cheat.
BUT in a multiverse or infinite universe not only are there multiple “you’s“ there are in fact infinite identical versions of you. I grant that this is not taking the spirit of the conversation into account, and is untestable to boot, it is genuinely possible.
Note though that an infinite universe doesn't imply that everything imaginable occurs within it. There are an infinite number of even integers, but none of them is 3.
True, but in an infinite universe, everything with non-zero probability almost surely happens, an infinite number of times.
Since our own lives have non-zero probability of existing, a spatially infinite universe implies that almost surely there are an infinite number of exact copies of ourselves existing right now, and an infinite future implies that almost surely our current lives will be exactly repeated an infinite number of times in the future.
But when I say (or think) "I", am I saying or thinking of any specific one of those versions? How could I be? It seems, if there are infinite number of identical copies of me, when I say "I", I must mean all of them equally.
Though that still raises the question of why this particular set of living experiences and not others.
Also, if this sort of thing is the case, I think it would make being a moral person significantly more complex. You would need to not only make moral decisions, but create as many moral possible universes as possible and shut out the immoral universes from existence by not creating paths to them. Though I guess that would make me a highly immoral person at this point (causing pain and suffering in all the universes where I've died), though I do try to be moral in this universe as much as I can.
I'm not sure how easy it is even with idealism to make "Why am I I?" a meaningful question. But, if our choice is between viewing that question as a pointer to some profound mystery on the one hand, or viewing it as simply a case of muddle-headed confusion on the other, the allegiance of my heart is with the first choice rather than the second.
If preserving a sense of profound mystery is your goal then... honestly, that mindset is sufficiently alien to me that I can't really think of anything useful to say on the subject. I can understand and share it as an aesthetic choice - mysteries are cool! - but not as a philosophical one. Philosophy by definition is the love of knowledge, not the love of not-knowledge.
And if you want your mystery to be something you solve, rather than something you live with, I do think you should be able to explain what your mysterious question means.
Maybe the answer to the question then is not a proposition, but a liberation from that prison. If one is a religious believer, one might hope for that liberation after death. If one is a transhumanist (which is maybe just a different sort of religion), one might hope that future technologies like telepathy implants or mind uploading might make such a liberation possible.
We know that a human being can survive with large portions of their brain missing. Entire hemispheres in fact. You understand what it is to be the amalgamation of experiences from both hemispheres, but the hemispheres don't understand what it is to be that experience, they only know the experience of being a hemisphere.
If you want to go in that direction though, then this could help. As you approach c, length contraction in the direction of travel is extreme. In “theory” a photon doesn’t really travel at all, the universe is flattened like a pancake in the direction of its travel. There is no time for events to occur “at once” and no year passes on a lightlike curve. A photon is emitted and absorbed without a “subjective” interval, so talking about what such an interval might be missing that point.
In QED or other photon-containing QFTs like the Standard Model, it is not only a "starter", it is essentially the point of the thing. Surely the photon "sees" the electron in a Compton scattering?
Photons obey the spin statistics for massless gauge bosons, so they can pile up at one point in spacetime. Equivalently, null geodesics can intersect. There can be a spacetime-point occupied by a pair of photons while the rest of spacetime is filled with points where they are occupied by only one (or neither).
(Indeed, the Penrose theorem arises because null geodesics, and photons travelling on them, can collide and stay collided, so for this and other reasons there may be multiple points in spacetime occupied by both of our pair of photons.)
One must be able to understand that for a photon there may be an early/late distinction -- either it is in a gravitational singularity or it isn't; either it is occupying a point with the other of our pair of photons or it isn't; either it's scattering or isn't. Following the photon and thinking about what it experiences, we can't use proper time, since it will be identical (practically[1]) everywhere along its path; likewise, spacetime intervals for all photons travelling on null geodesics are zero. The need for this understanding increases with photon interactions with charged matter: a pair of photons from different quasars arriving at a pair of detectors here tells us something about the location in spacetime of the quasar and about the metric expansion of space. Using cosmological coordinates, for each of the pair there is a _time_ before which no point in space contains it, and a time when they arrive at a detector on or around Earth. Given that each of the pair is from a different quasar, generally the emission time will be different. Moreover, at emission the wavelength of the photon will be shorter, and it will be longer at later times, right up to detection.
Proper time (and proper length for wavelength) makes a mess of this in general, but we do not need to parameterize a non-timelike geodesic with proper time. Instead, for a null geodeseic, we use an affine parameter and consider its properties at each value of the affine parameter.
For each of our quasar photons, at different affine parameter values the photon has a different energy-momentum. For the Minkowski vacuum photons in the first paragraph above, for a given affine parameter value, one photon is in vacuum or it is at the same point as the other photon. For accuracy, when we are dealing with physical photons, the stress-energy-momentum tensor must encode this. As we get away from simple Minkowski vacuum, we want all the tensor-fields to be accurate.
We can recast your second paragraph's second sentence as taking the limit of the geodesic in which it becomes null, parameterizing by an un-rescaled extremized proper time. We can simply rescale so that we always have a different value at each point on the geodesic as we take this limit. Indeed, we can substitute any monotonic function of spacetime position on the geodesic and get a way to compare field-properties. For a timelike geodesic, proper time tends to be a "good" choice. The affine parameter, the unique monotonic function that satisfies the geodesic equation, tends to be a "good" choice for a null geodesic, in particular because it preserves the tangent vectors under parallel transport.
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[1] Photons generally do not move at c in a medium, and one can describe this in a variety of equivalent ways, some of which demote a photon from a null curve to a timelike one while interacting with the bulk of the medium. There are serious proposals to study laboratory ultra-slow light in the context of General Relativity. From a theoretical perspective, Gordon in 1923 outlined an https://en.wikipedia.org/wiki/Optical_metric in which light follows different geodesics from the null geodesics sourced by the metric tensor, and there are various approaches to extending this to media with nonnegligible dispersion. In this sort of approach, a photon or a pulse of classical light can have a pretty conventional proper time.
This all assumes no FTL or CTCs of course, with either you would lose causality and events separated by spacelike intervals could influence each other.
I think this needs qualification.
If I am stood directly behind Alice or Bill then it is possible I never see the interaction at all even though I am an observer, just as if I hold a two identical pencils together and look at them from the correct angle I observe only a single pencil.
The important thing is in all cases, observers "see" space-time interactions as the geodesic of the event projected on to their own observational frame.
Through that projection it is possible to lose degrees of freedom which can be space-time etc.
It's through this projection that HUP emerges since if in the observational frame the time dimension is limited, the projected time axis of the event tends toward zero.
But yes, you are sort of correct, two events can have the same metric, and one would not be able to distinguish which of them is responsible, if they are both capable of becoming present at the same speed.