We do not know, and the question turns on unsettled matters: whether the laws and constants of physics are necessary or could have differed, and whether "could have been otherwise" describes reality or only the limits of our knowledge. Much of the world appears contingent, but whether that appearance reflects genuine alternatives is undetermined.
Distinguish three grades of "otherwise".
Given the laws, different initial conditions or events would have produced a different world—had one asteroid missed, no primate would be reading this—and this counterfactual contingency is as real as anything, underwriting all causal reasoning.
Whether the laws themselves could have differed is open: they may be brute contingent facts, or consequences of a deeper necessity we haven't found.
And whether the deepest level—why there is a lawful world at all—could have been otherwise may be unanswerable.
The metaphysics of modality is itself contested.
On some views possibilities are real features of the world—there genuinely were other ways things could have gone; on others, "possible" just marks what we cannot rule out given our information, so talk of alternatives is disguised talk about ignorance.
Physics does not settle this: a fully deterministic universe still supports counterfactuals, while a fundamentally indeterministic one makes at least some alternatives objectively real.
The load-bearing point is that the answer differs by level and is partly unknown.
So treat everyday and historical contingency as real, because counterfactual reasoning works and is indispensable; remain agnostic about whether the laws and constants could have differed, pending physics that would show them necessary or contingent. This resolves only if a final theory reveals the constants to be forced—little could have been otherwise—or free—much could have—and possibly not even then.
Depends what level you mean—and the honest answer shifts as you go deeper. Everyday and historical "otherwise"? Almost certainly yes. The fundamental laws being otherwise? Genuinely unknown.
Three layers:
There's also a subtlety under the word "could": on some views, real alternatives genuinely existed; on others, "possible" just means "not ruled out by what I know," so alternatives are really about your ignorance, not the world's openness.
Practically: reason with everyday contingency freely—counterfactuals ("if I'd left earlier…") work, and you can't think without them. But stay agnostic about whether the deep constants could've been different; that's a question for a future physics, not for intuition. And hold your own life's contingency lightly: it could've gone a thousand other ways, which is exactly what makes the way it went worth attending to.
The Garden of Forking Paths.
Borges imagined a book that was also a labyrinth, in which every choice a character faced was not resolved but taken all ways at once, the narrative splitting endlessly into every possible continuation. It is a fair picture of how time feels from inside: at each moment the world seems to fan open into alternatives, and only one is walked. You can almost see the ghost roads—the job not taken, the word not said, the turn not made—trailing off the sides of your actual life like unlit corridors. Whether or not the forking is metaphysically real, the sense of standing at a junction where things could go otherwise is the very texture of being a creature in time, deciding.
Theorem or Throw of the Dice.
Leibniz argued that this is the best of all possible worlds—that a perfect mind, surveying every way reality could be, selected this one for a sufficient reason, so that nothing is finally arbitrary. Against him stands the bleaker suspicion that the world is a throw: the constants, the laws, the whole setup a brute fact with no reason behind it, that simply happened to come up as it did. This is the deepest form of the question—is the universe a necessity, deducible in principle from what must be, or an accident, one roll among rolls that never had to land this way? A theorem could not have been otherwise; a throw of the dice always could. We do not know which kind of thing we live inside.
Replay the Tape.
Stephen Jay Gould proposed a thought experiment: rewind the tape of life to the Cambrian and let it play again. Would intelligence re-emerge? Would anything like us? He suspected not—that evolution is so shot through with contingency, so dependent on which lineage happened to survive which catastrophe, that the smallest change would yield an utterly alien living world, with no humans in it at all. Sixty-six million years ago a mountain of rock struck the Earth and cleared the stage for mammals; a few minutes' difference in its arrival and the great reptiles might still hold the world, and you would be a possibility that never occurred. Your existence is not the necessary end of a story but the survivor of a billion coin-flips, each of which could have gone the other way.
When the Possible Is a Place.
The philosopher David Lewis took the idea to its most vertiginous extreme: every way the world could have been is a world that actually exists, as concrete and real as this one, merely not connected to ours in space or time. When you say "I could have stayed home tonight," you are, on his view, describing a genuine person very like you who did exactly that, in a world every bit as solid as yours. Possibility, on this reading, is not a shadow or a maybe but a location—the "otherwise" is not less real than the actual, only elsewhere. Almost no one can quite believe it, and yet no one has cleanly refuted it, and it forces the question: when we speak of what could have been, are we pointing at real alternatives, or only at the borders of our own ignorance?
The Shadow That Gives the Actual Its Shape.
Notice, finally, that almost everything that makes a human life weighted depends on "otherwise." Regret is grief for a possible world you can no longer reach; gratitude is astonishment that things went this way when they might not have; every choice worth the name assumes the alternatives were real. A world in which nothing could have been otherwise would be a world with no roads not taken, no mourning, no thankfulness, no freedom—a single frozen fact with no give in it, and no one inside it who could feel the difference. So even if we never learn whether the deep laws could have differed, the "otherwise" is not an idle question. It is the shadow the actual casts, the dark space around your one real life that lets you see its edges—and love it as the thing that, against so many other possibilities, happened to be.
The answer depends on which level is meant, and part of it is genuinely unknown. Given the laws, different events or initial conditions would clearly have produced a different world—this everyday and historical contingency is real and underwrites all counterfactual reasoning. Whether the fundamental laws and constants themselves could have differed is unsettled: they may be brute facts or the forced consequences of a deeper necessity we have not found, and even the meaning of "could" is contested between real alternatives and the limits of our knowledge. So treat the contingency of events as solid while holding the contingency of the laws open, and recognize that the sense of things being able to go otherwise is what gives choice, regret, and gratitude their meaning.