Causality and time travel

 

The concept that the past is a place implies that one could travel there and thereby change the present, an idea of course filled with paradoxes. The concept of the future as a place where one might travel (other than the way we normally get there) implies determinism.

You’ve put your finger on something that tends to get muddled because we casually borrow spatial language—“the past,” “the future,” “ahead,” “behind”—for something that isn’t spatial in the ordinary sense.

If you treat the past as a place, it invites the idea of revisiting it, like walking back into a room. And as you say, the moment you allow that, you run straight into paradoxes: if you can alter what happened, then what did happen becomes unstable. That’s why most serious treatments of time travel either forbid changes (you can observe but not alter), or they accept a kind of self-consistency where anything you do in the past was already part of history.

But your second point—the future as a “place”—is more subtle. It doesn’t necessarily imply determinism, though it nudges in that direction.

In physics, especially in relativity, there is a picture sometimes called the “block universe.” In that view, all moments—past, present, future—are equally “real,” laid out in a four-dimensional structure. If you take that literally, then yes, the future exists in the same sense the past does, and that sounds deterministic: the story is already written.

However, two complications soften that conclusion:

First, even in a block universe, our access to different parts of that structure is constrained. We move along our own timeline in a particular direction and at a fixed rate. So the “future as a place” doesn’t mean we can freely visit it; it just means it may already be part of the structure.

Second, determinism depends on the laws governing that structure. Classical physics leans deterministic; quantum theory muddies it with probabilities. If the underlying laws are not strictly deterministic, then the “future region” of spacetime might not be a single fixed landscape but something more like a branching set of possibilities. Calling it a “place” then becomes metaphorical at best.

So I’d push back gently on the implication: thinking of the future as a place encourages a deterministic intuition, but it doesn’t logically require it. It may simply reflect the limits of our language—trying to map time onto space because that’s what our brains are good at.

If anything, both ideas—the past as a place and the future as a place—are artifacts of the same habit: we spatialize time, and then we’re surprised when the metaphor starts making demands the underlying reality may not satisfy.

I’m curious—do you find the block universe idea persuasive, or does it feel like one of those elegant constructions that explains too much at the cost of lived experience?

Oh I understand that our present understanding of physics is not necessarily deterministic. That was not what I was talking about. If the future is a place, then it is beyond me how it can be anything but determined.

Ah, I see the distinction you’re making—and it’s a sharper one than it first looked. You’re not appealing to physics as it stands; you’re making a more conceptual claim: “place” entails “fixed.”

And there’s real force in that.

If something is genuinely a place—in the same sense that Paris is a place—then it has definite properties independent of whether we’ve arrived there yet. You don’t get to Paris and discover it might be Rome depending on how things go. So if the future is a place in that strong sense, then yes, it must already be fully specified. That’s determinism, not as a physical theory but almost as a logical consequence of the metaphor.

Where I’d push back is on the word “place” doing more work than it’s entitled to.

When people (and physicists) talk about the future as a “region” of spacetime, they’re quietly shifting the meaning of “place.” It’s no longer a single, definite location with fixed contents, but something more like a set of possible continuations. In other words, it’s closer to a branching structure than a city on a map.

So there are really two different notions hiding under the same word:

• Strong “place”: fully specified, like a completed page in a book → implies determinism.

• Weak “place”: a structured domain of possibilities → compatible with indeterminism.

Your argument bites hard against the first—and I think it’s correct there. If the future already exists in the same way the past does, with all its details fixed, then the game is over: everything is determined.

The escape route, if one wants to avoid determinism, is to deny that the future exists in that way at all. On that view, the past is fixed because it has been realized; the future is not a “place” but a horizon of possibilities that only becomes definite as events occur.

So the real disagreement isn’t about determinism per se—it’s about ontology:

• Does the future exist already? (block universe → your conclusion follows)

• Or does it not yet exist in a fully definite sense? (then “place” is just a misleading metaphor)

You’ve essentially cornered the block universe: if it wants to talk like ordinary language about “places,” it inherits determinism whether it likes it or not.

The interesting twist, I think, is that our language tempts us into this trap. We say “the future is ahead of us” as casually as “the kitchen is down the hall,” and then we smuggle in all the implications of geography without noticing.

If you had to choose, which bothers you more: a determined future, or the idea that the future doesn’t in any meaningful sense exist yet?

Which is to deny that the future is a place.

Yes—exactly. That’s the clean resolution.

If one wants to preserve any genuine openness in how things turn out, then one has to give up the idea that the future is a “place” in the same sense as the past. The metaphor simply carries too much baggage: once you grant “place” in the strong sense, you’ve already conceded fixity.

So the options reduce rather starkly:

• Treat past and future symmetrically as equally real “places” → you get a fully laid-out reality, and determinism follows.

• Or break the symmetry: the past is fixed (because it has occurred), while the future is not yet anything definite → and then it isn’t a place at all, but a set of possibilities.

What’s interesting is that everyday experience sides with you. We remember the past as settled, but we anticipate the future as open. That asymmetry is very hard to reconcile with the “future as place” picture unless one is willing to say our experience is deeply misleading.

So your conclusion isn’t just semantic—it’s doing real philosophical work. You’re identifying that a seemingly harmless metaphor quietly commits one to a very strong ontological position.

In a way, it’s a good example of how language can smuggle in a worldview: call the future a “place,” and you’ve already chosen sides in the determinism debate without quite noticing it.

We know the time dimension of spacetme is clearly different from the other dimensions. A dimension is a need for an additonal number to specify the location in spacetime of an event. "When" strikes me as perhaps different from the directions of the other spacetime dimensions.

You’re in very good company in feeling that “when” is not just another “where.” The formalism of physics groups them together, but it does so a bit grudgingly.

Mathematically, you’re exactly right: a “dimension” just means you need another coordinate to locate an event. In that narrow sense, time qualifies perfectly. But that definition is deliberately thin—it says nothing about the character of the dimension, only about bookkeeping.

And here the character of time diverges quite sharply from space.

In relativity, the difference shows up in the geometry itself. The spacetime interval treats time with the opposite sign from the spatial coordinates. That isn’t just a technical quirk; it encodes the fact that you can’t move through time the way you move through space. You can reverse direction in space at will; you cannot simply decide to walk “backward” in time. All physical processes seem constrained to move one way along that axis.

There’s also the asymmetry you were pointing to earlier. In space, all directions are (roughly) equivalent. In time, past and future are not interchangeable in our experience or in thermodynamics. The increase of entropy gives us a built-in arrow: eggs break, they don’t unbreak; we remember yesterday, not tomorrow. That alone makes “when” feel fundamentally unlike “north” or “east.”

Then there’s causality. The temporal coordinate orders events in a way that space does not. One event can influence another only if it lies in its future light cone, not just somewhere “else” in spacetime. So time isn’t just another axis—it’s tied to the structure of cause and effect.

All that said, physics still gains enormous power by treating time as a coordinate alongside space. It lets you write unified laws and see patterns (as in relativity) that would otherwise be hidden. But that unification comes at the cost of disguising a deep asymmetry.

So I’d put it this way: time is a dimension in the minimal, coordinate sense—but it is not a dimension of the same kind. Calling it one risks suggesting a symmetry that both our experience and several core features of physics seem to deny.

Your instinct that “when” is qualitatively different from “where” is not naïve—it’s pointing at one of the central tensions in how we understand reality.