Recently I had a debate with someone who was downright evangelical about the Block Universe (BU) being, absolutely, positively, the way things are. Because Special Relativity. In particular because of what SR says about simultaneity between inertial frames.
Up to that point I’d never given the BU a great deal of thought other than to file it under «Probably Not the Case» (for reasons I’ll get to). But during my morning walks I’ve turned it over in my mind, and after due consideration,… I still think it’s probably not the case.
I get why people feel SR seems to imply a BU, but I don’t see the necessity of that implication. In fact, it almost seems contrary to a basic tenant of SR, that “now” is strictly a local concept.
I’ll get to that, but first it should be clear that belief in the BU — the philosophy of eternalism — is just that, a philosophical belief. It’s an interpretation of physics as we understand it.
There is no way to prove or disprove it, so it amounts to being a metaphysical statement about the nature of reality.
There are a couple of key considerations:
Firstly, the idea of causal determinism, especially superdeterminism. If there is no freedom of choice at any level, then reality, at the least, amounts to a BU. If the future is predetermined, in effect, it already exists.
Secondly, SR seems to suggest that it’s possible to be simultaneous with an event in someone else’s future (according to their notion of simultaneous), so therefore the future must exist.
I have mixed feelings about causal determinism.
On the one hand, it does seem to be the way physics works. (And certainly a lot of physics does seem deterministic.) The reverse of determinism is reduction, and no one has yet shown where reduction fails.
On the other hand is our experience of choice and the idea that the right kinds of systems might be non-deterministic. (Certainly chaotic systems are unpredictable, which might be as good as it gets, but I think the jury is still out when it comes to the true nature of reality.)
In any event, even a superdetermined universe can evolve over time without requiring the future already exist, so there’s no necessity on that count.
(There is an ontological question about the reality of things that must happen even when they don’t seem to have happened yet. Think of the sensation of watching a car accident about to happen. Vivid and real!)
My impression of the BU belief is that it perceives the future (all of it) to exist in a concrete sense. It imagines all of reality as a kind of 4D Lucite spacetime block with embedded bits of matter tracing out 4D world lines.
Here’s my key problem with that:
That Lucite block contains a lot of structure — it’s the entire universe, not just in size, but in time. The matter traces involve a lot of interactions.
How and when were all those computed?
They had to be calculated by something. Those interactions follow patterns, so there must be underlying rules that govern them.
And all that structure must have taken time to calculate, so is there a separate spacetime for the Lucite block to be created? Time seems to need to flow in order to make a calculation of a fact you don’t know. (E.g. What’s two plus three? It takes time to calculate an answer.)
Or does everything — all that vastness of structure — just spring into existence all at once? How? According to what physics? According to God?
On this count alone the BU seems improbable.
Note that, in contrast, if time flows and the universe evolves, then the structure calculates itself as it goes. Each moment generates the next moment of structure.
A question I have is: Does the BU include the Big Bang as part of a larger universe, or does the BU only contain what the Big Bang created?
In other words: BB < BU or BB = BU?
One thing that’s never made much sense to me is the question:
Is time a coordinate or a dimension?
This confuses me, because if something is a coordinate, that means it’s a number along a dimension. And if something is a dimension, then it has coordinates along it.
I think I get what’s being asked, though: Is time something that flows “through” us and “passes,” or is it fixed like a spatial dimension and we, in some sense, move through it.
Perhaps it amounts to the same thing: time flows.
Even in the BU there is a sense of flow that comes from… apparently not time, but something. Another objection I have to the BU is that it needs to account for that sense of flow. (It can’t be tied to consciousness, because chemical reactions and orbits seem to flow without being observed.)
FWIW, I take time to be a fundamental property of reality, related to, but not at all the same as, space. In 4D spacetime, I think time stands alone.
This is obvious when we consider the spacetime interval:
Δs2 = Δx2 + Δy2 + Δz2 – Δt2
Time is taken as, in a sense, opposing space! Distances in 4D Minkowski space are not Pythagorean because time is not a spatial dimension!
Speaking of 4D spacetime, a key premise of the BU is that SR suggests the future exists. Specifically, that an event seen as simultaneous in one frame of reference is seen from another as in the future (or the past).
What I’d like to demonstrate is that a claim of simultaneity is virtual and can only be validated after the fact.
It’s similar to, while you’re driving over the river and through the woods to grandmother’s house, thinking that “right now she’s baking the turkey!”
Maybe she is, maybe she isn’t. You won’t know until you get there. (Maybe she’s been playing Cribbage with her friends all afternoon and thought you were bringing pizza.)
To make sense of this we need to take a close look at what we mean by simultaneity.
Take a look at diagram 1. (I apologize for the crude diagrams. I’m feeling pretty casual and lazy about the topic.)
I’m going to assume you are familiar with spacetime diagrams. If not there are resources on the web, or you can try my SR series where I explored both the diagrams and simultaneity in detail.
In diagram 1, the dark black vertical line (x=0) is the observer, Al, in his frame of reference.
At point A (x=0, t=-4) Al fires a laser (red line) which crosses space to point B (x=8, t=+4) and is reflected back across space to Al at point C (x=0, t=+12).
Al can then (and only then) subtract the departure time from the arrival time to see that the round trip took 16 clock ticks.
Since both legs of such a trip must be symmetrical, it takes light half that time (8 clock ticks) to go one way. That means point B is 8 space clicks away (because time clicks and space clicks are normalized in these diagrams).
More importantly, Al can tell that point B occurred at t=+4. That is, it was simultaneous with Al’s point D back when his clock read +4. (Currently it reads at least +12 or more.)
Crucially, D can only be deduced after point C.
Note that if Al knows where point B is, and (as in this case) point B is in his frame of reference, it’s possible to claim at point D that point B will turn out to have been simultaneous (once confirmed at point C).
That would be like predicting what grandma is doing before you get there. The point being, since she’s roughly in your frame of reference, that you can assume she’s doing something while you’re driving there. But you still can’t make any factual statements about it until you get there.
Now let’s consider that scenario with Barb, who passes by (from left to right) at 0.5 c.
Diagram 2 illustrates how this appears as rendered from Al’s point of view!
(To Barb things appear very similar to how they appeared to Al in diagram1, but the distance appears foreshortened and therefore the light takes less time to make the round trip.)
The dark vertical line is still Al. The dark green line is Barb’s world line as she passes Al.
The dotted lines (in both diagrams) are the lines of simultaneity as seen by Al. (In Barb’s frame, her dotted lines would be horizontal, and Al’s would be angled. To her, Al’s situation looks like diagram 2, except Al seems to be passing from right to left, so his lines are slanted the other way.)
As in the first scenario, once Barb reaches point C she can say that point B occurred simultaneously with the origin point. (Which I forgot to label in both diagrams! Oops!)
So it appears that Barb sees, as simultaneous with her t=0 line, something that Al sees as simultaneous with his t=+4 line.
Barb appears to almost see into Al’s future!
But does she really? She certainly can’t make any use of supposedly knowing Al’s future. In fact, she doesn’t know anything about that future (yet).
By definition, any spacetime event that occurs on your surface of simultaneity is outside your causal influence. That is, outside your light cone:
Which is why you don’t know what grandma was really up to all afternoon while you were driving.
Barb can’t know anything about Al’s supposed future events until the light from those events reaches her.
Just above I used the term surface of simultaneity rather than (line of).
In 2D spacetime diagrams (such as shown here), because there is only one spatial dimension (x), there is a line of simultaneity.
If we make 3D spacetime diagrams, there are two spatial dimensions (x and y), so there is a plane of simultaneity.
Which we can at least visualize (diagram 4).
In 4D spacetime, with three spatial dimensions, there is a 3D surface of simultaneity — a 3D volume that, in moving frames, seems angled in four dimensions. (Which we can’t visualize!)
The point is that this surface can only be probed remotely because, by definition, it’s outside your causality (your light cone).
You can’t make any statements about events in that surface until their light reaches you.
In a very real sense, that surface of simultaneity is virtual. As such, it doesn’t speak to the reality of “future” events you can’t say anything about until after they’ve happened.
SR teaches us that the concept of “now” is a strictly local concept with no real meaning over distance. Believers in the BU treat simultaneity as if it reifies the “now” throughout all of spacetime.
Not only do I think that doesn’t follow from SR. I think it’s contrary to it!
So, in summary:
- How and when was the BU structure calculated?
- What makes time appear to flow in the BU?
- Relative simultaneity is virtual; “now” is strictly local.
In short, the BU is BS! 😀
Stay unblocked, my friends!