Is the Future Predestined? (Script) (Patreon)

Einstein’s special theory of relativity combines space and time into one dynamic, unified entity - spacetime. But if time is connected to space, could the universe be anything but deterministic? And does that mean that the future is predestined?

—-

In the last episode we saw that we could think of the unified spacetime in terms of the block universe - an a-temporal entity that sort of just exists. In a block universe of Einstein’s relativity, there’s no way to cleanly define the present, and so no way to cleanly separate the future from the past. The notion of the present is relative - to us it looks like a particular slice through the block universe. But different observes will slice the block at different angles that depend on their velocities. That means that for every observer, it’s possible to imagine another observer who lives in their definition of the present, but for whom your future is already the past. Or for whom your past is their future.

How does this give us a deterministic universe? Well, if we accept a unique reality  for those other observers in our present, and we accept that their perception of the present is as valid as ours, then all of the future is defined. That seems to support the philosophical stance of eternalism - the idea that all space and time exist from a sort of gods-eye view from outside of both space and time. And it suggests a deterministic universe, where every slice through the block universe is completely determined by every other slice, either behind it or ahead of it. They are connected by the unalterable laws of physics.

So here’s the big question - given that relativity is definitely a good description of space and time - what would it take for our future to remain undefined? It seems we have two options: Either the entire block universe is defined, past, present and future, or everything outside out past light cone is UNdefined - including other observers and the room around you in what you would normally think of as the present. And the only aspect of the present that “exists” is a vanishingly small patch of spacetime around your own brain.

In order to even try addressing which of these options is right, we need to take into account something we’ve totally ignored so far. Where relativity is the key to understanding the universe of the large or the very fast, we also need to understand the universe of the very tiny. We need to see what quantum mechanics says about determinism, and how it plays with relativity and the block universe.

According to quantum mechanics, physical systems - parts of the universe - evolve as “wavefunctions” - not as well-defined reality, but rather as distributions of probability that describe all possible states that chunk of the universe could be in were it to be observed. The wavefunction can be thought of as a state in which all physically possible realities intermingle. Countless extremely precise and frequently replicated experiments tell us that the universe really does exist in this indeterminate state in certain circumstances. The wavefunction is real. The big question is what causes the transition from this world of quantum indeterminacy to the solid, singular reality of the macroscopic world.

We’ve talked about this profound weirdness in many previous episodes, including how to interpret it. Exploring those interpretations again may help us understand the state of the universe beyond our immediate selves and the immediate moment.

One of the earliest and still most accepted interpretations of this weirdness is the so-called Copenhagen interpretation, which states that a quantum system is literally in a state of undefinedness until observed, at which point the wavefunction collapses into a definite state - all particles take on definite properties, and one actual reality is chosen from many possible ones - at least as far as the Heisenberg uncertainty principle allows

.

The Copenhagen interpretation gives us a non-deterministic universe. The wavefunction evolves in a precise way perfectly defined by the equations of quantum mechanics. But at the moment of measurement, a single reality is randomly selected from the distribution of possible realities. The dice are thrown when the wavefunction collapses. That means it’s fundamentally impossible to predict the state of a system after observation.

Another popular interpretation of quantum mechanics is the Many Worlds interpretation, which simply states that the wavefunction never collapses. Instead, all the possible states of the quantum system continue to exist. Rather than choosing one reality out of many when we observe a quantum system, we sort of just become part of one of those realities and lose track of the others.

On the other hand, the Many Worlds interpretation gives us a deterministic universe. Or, rather, a deterministic multiverse. The wavefunction never collapses - it evolves deterministically forever. The illusion of randomness is that we find ourselves in the one branch of that wavefunction corresponding to the reality we perceive. But we aren’t randomly placed in that branch - we just ARE part of that branch.

So how do these interpretations connect with what relativity tells us about past, future and present?

Let’s start with Copenhagen. It’s tempting to equate everything outside our past lightcone with the unobserved wavefunction. You can imagine that lightcone sort of ploughing through undefined universal wavefunction, collapsing it as it goes. Or at least the parts of it for which signals actually reach our awareness.

That leaves your future lightcone undefined - a non-deterministic void that with room for your non-determined choices.

But now let’s say we believe that other observers in the universe can also collapse the same universal wavefunction with their observations. No problem so far. You can imagine this fleet of observers collapsing the universe all the way up to what you perceive as the present. Your future is intact.

But wait - we saw last episode that your impression of the present is relative. A fast moving observer defines the present very differently to you. In fact any part of spacetime NOT in your future lightcone is potentially “the past” for another observer in your present. And THAT observer can imagine a third observer in their different present for whom your future lightcone is in their past lightcone - so it should already have been collapsed.

The upshot is that, with no way to define an absolute definition of “the past”, and if we believe in other observers, there’s no way to keep the wavefunction of your future from being collapsed before you get there.

Now there are subtleties in this wavefunction collapse idea. Copenhagen doesn’t specify what causes the collapse - it could be observers, as in the von Neumann-Wigner interpretation. That’s the hardest to gel with a non-deterministic universe UNLESS you’re the only observer in the universe. More physicists prefer the idea of a non-conscious interaction causing the collapse. In that case, uncollapsed wave functions tend to exist only in microscopic pockets or very special circumstances - but that doesn’t help uncollapse your future. No, the only totally coherent way for a “non-deterministic” wavefunction collapse interpretation like Copenhagen to give you an uncollapsed future is if you’re the only being doing the wavefunction-collapsing.

OK, let’s move on to Many Worlds. This interpretation sits a bit better with the relativity of past, present and future. Your lightcone sweeps through the global wavefunction, but it doesn’t collapse that wavefunction - rather it selects from it. Sometimes the quantum multiverse of Many Worlds is depicted as this branching tree, in which realities multiply as time progresses.

In the block universe picture, it’s tempting to depict this as the entire block multiplying with every quantum event. But that’s not quite right - a division in the block universe can only propagate as quickly as the result of that quantum event can become known. That can happen at the speed of light - so we can also think of the splitting as proceeding in advancing lightcones.

Really what’s happening here is that parts of the universe become entangled with each other - correlated at a quantum level. So we have spreading webs of entanglement. Each such web defines a set of properties of the universe correlated with some prior quantum decision, with an independent web existing for all different possible decisions.

Here we’re bringing in the concept of decoherence and quantum darwinism, which we’ve covered before. Decoherence is an explanation for how the “worlds” can become separate from each other when you shift from simple systems like an atom to complex systems like a cat.

According to the decoherence explanation, the quantum multiverse isn’t a cleanly divided set of alternate realities. Rather it’s a tangled network of alternate sub-realities. Nonetheless, as YOUR lightcone moves forward in time, you encounter these entanglement networks and you have to choose between them. Your reality is the particular set of entanglement networks that you interact with. And there are other realities with different entanglement networks that contain different versions of you.

I should add that the lightcone picture isn’t ideal here, because entanglement can travel faster than light. However light-speed signaling is the surest way to transfer quantum correlations, so this is still a useful picture.

So what does all of this say about determinism and the reality of your past and future? Interesting things, actually. The evolution of the wavefunction is deterministic. That means all future branching of the wavefunction of your present - the entanglement network you currently belong to - is predefined. What isn’t defined is your own experience of that future branching. You’ll be the thread of conscious experience that travels one of those branches. You’ll also travel the others, but each version of you will only FEEL like you one of them.

This interpretation - many worlds + decoherence - perhaps gives us the most concrete notion of an objectively existing present. It’s that part of the global wavefunction that you’re connected to via entanglement and that shares your timestamp of “now”. But the notion of “now” remains relative - there are still other observers in your slice of reality for whom your now is their past or future.

Copenhagen and Many Worlds aren’t the only interpretations of quantum mechanics in town. We should make an honorable mention to de Broglie-Bohm pilot wave theory. This is an entirely deterministic interpretation that doesn’t have multiple realities - just a wavefunction that guides particles in a perfectly determined way - defined by Bohmian mechanics. Of course we talked about all of this before. The universe of pilot wave theory is really a block universe, but unfortunately no one has convincingly figured out how to make Bohmian mechanics work properly with special relativity. So it’s a very simple block universe, and its not a good representation of ours.

There are other interpretations that deserve mention, but we’re out of time. So - does the future already exist according to quantum theory?  The answer depends on your favorite flavor of quantum interpretation. Perhaps we ride the dice of Copenhagen into an unknown future, or surf the splitting quantum multiverse into all futures, of which you will become just one. Of course we still haven’t got to why time needs to progress in one direction at all, or how time even arises in the first place. But we’ll have plenty more time for time another time on spacetime.