Neither I nor Laplace claimed to be in possession of such omniscience, so to that extent it's theoretical. However, it also applies to the dreaded "closed systems" to the extent you can find one.
However, it has turned out that there are real limits to how deterministically the universe will behave. The archetypal illustration is a certain apocryphal half-dead German cat.
Yes, but only in historical time. Maybe not so in "individual person time" (if I can put it that way). I mean, you had this epiphany when you were a teenager. Laplace didn't publish anything about whatever like epiphany he may have had until his fifties, in Mechaninque Celeste (in 5 volumes, appearing 1799-1805)....
Recently I came across an enormously engaging and surprising book, Suspended in Language: Niels Bohr's Life, Discoveries, and the Century He Shaped by Jim Ottaviani and Leland Purvis et. al. [2004]. I was scandalized at first to discover the entire piece is laid out as a cartoon. But it really is a superb book in terms of explicating the subject matter of its title. IMHO, a wondrous book for bright high school students, and others who might delight and have an interest in the magnificent achievements of 20th century science.
Anyhoot, that's where I got the Laplace. I left out the rejoinder to his statement:
[Question:] This "intelligence" of yours, would it be the author of the universe, who I note you left out of your book Mechanique Celeste?Poor ol' Schroedinger's cat. On my reading, VR, the cat isn't "half alive/half dead." The cat is both simultaneously alive and dead. In the language of quantum theory, it is in a superposed state, an analogy to the superposition of particle/wave of what we have come to describe as "matter."[Laplace replies:] hmph...I know what you're driving at, sire, but I have no need of this ... "God" hypothesis.
[Questioner reply:] Mon Dieu! [p. 282]
The analogy says that we don't know whether the cat is alive or dead until we go look. Absent observation, the problem is undecideable.
Fun stuff to think about! :^)
Thank you so much for writing, VR!