According to the Many Worlds Interpretation of quantum physics, we live in an infinite web of alternate timelines. It's a serious claim that carries some rather serious scientific, philosophical, and existential baggage. And here are the nine weirdest possible implications.
Illustration by Tara Jacoby.
A few years ago we did an explainer on the Many Worlds Interpretation (MWI), but it's worth doing a quick review. For those of you familiar with the concept, feel free to jump ahead.
According to a hypothesis devised by quantum physicist Hugh Everett, we live in a universe — or more accurately a multiverse — where timelines are constantly branching off and creating distinct and coherent worlds, each experienced by a different version of you.
Quantum physicists have used the MWI to reconcile an uncomfortable shortcoming of the Copenhagen Interpretation, namely the assertion that unobserved phenomenon can exist in dual states. So instead of saying that Schrödinger's cat is both alive and dead, Many Worlders would say the cat has simply "branched" off into two different worlds: one in which it is alive and one in which it is dead.
Some 60 years after its introduction, the MWI remains a highly controversial subject. In a 2013 poll of quantum physicists, only a fifth said they subscribe to the MWI (as compared to the 42% who fall into the Copenhagen camp). That said, the list of thinkers who describe themselves as Many Worlders is an impressive one, and includes such eminent thinkers as quantum physicist David Deutsch, theoretical computer scientist Scott Aaronson, and physicist Sean Carroll.
Regardless of where one stands on the theory, it's certainly interesting to think about the implications. Here are nine that are particularly weird.
It's often taken for granted by cosmologists that the world we observe is the only one, hence the "uni" in universe.
Ruminations of multiverses were once considered scientific heresy, but it's looking more and more likely that it's true. Indeed, the suggestion that there exists a multiplicity of universes has been posited by a number of credible scientists and metaphysicians, whether they be classical cosmologists, anthropic reasoners, or those who dabble in the quantum arts.
The primary assertion of the MWI is that all of existence is composed of a quantum superposition of an uncountably large — or even infinite — number of universes. If this interpretation of existence is true, then there must be an absolutely astounding number of alternate worlds.
The MWI also upsets our notion of individuality. We all experience our lives as a coherent and discrete journey through what appears to be space and time. In reality, however, the self is an exponentially expanding set of instances that are branching off from moment to moment. As a result, we should not think of ourselves as individuals, but rather as multiplicities.
The reason for this illusion is that multiple experiences cannot be observed, so we're left with the impression that we're just a single person. Now, this is not to suggest that our experiences of reality are not somehow real or genuine. They are. We just have to acknowledge — via the MWI — that our lives are not exactly as they appear to be.
Which leads us nicely to the next point: If the MWI is true, then there must be a near-infinite (or infinite!) number of versions of you, each of them experiencing the world as individuals separate from and oblivious to each other. Consequently, the sheer volume of alternate life-paths has to be staggeringly large. Since birth, you — or what you think is you — have been branching off into different worlds with each passing superposition. The complete set of "you" is like a massive root system that's growing exponentially, with each root representing a new timeline.
Because the MWI implies constant variability based upon probabilities, each new instance of you should be distinct, observing a world in which an alternative outcome has transpired. Subsequently, there are versions of you who are still with former romantic partners. Many of your alternate selves are happier and more successful than you are, and vice-versa. There should also be versions of you who have already died, or who have experienced the death of a loved one who's still alive in your current world. There may even be "evil" versions of you, a la Star Trek. The possibilities are practically endless, so long as the basics of physics haven't been violated.
Given that all possible decisions will be made by different versions of you, the MWI makes it difficult to reconcile the issue of free will. If all options of a choice are selected in alternate worlds, then why go through all the trouble of weighing all the evidence before choosing? The collective fate of our totality, it would seem, has already been determined.
But as MWI expert Michael Clive Price points out, while all decisions are realized, some are realized more often than others. In other words, each branch of a decision has its own "weight" that's enforcing the usual laws of quantum statistics.
Also, the MWI would imply a certain indeterminism to existence, albeit in an unintuitive way. Whenever we ask ourselves, "Could I have chosen a different course of action?," the MWI would strongly imply that the answer is most definitely yes. What's more, not only could you have chosen a different course of action, an alternate version of you actually did! As for why you chose differently, or why you fared a certain way on a test or sporting event, it all boils down to how the quantum events affected objects at the classical scale — including the cogitations of your brain.
The MWI necessarily leads to some very bizarre possibilities. Again, all branching off points are possible so long as they're probable and don't violate the laws of physics. It's important to note, however, that given the space of all possible worlds, it's vastly more likely that you'll find yourself in the most probable and seemingly rational of worlds because they appear with the highest degrees of frequency (and by several orders of magnitude).
But there will be some worlds in which highly improbable things must happen. For example, if a person were to flip a coin 1,000 times, there has to be a world in which that person flips heads 1,000 times in a row.
Also, there should be a world out there where someone always bets correctly when gambling on sports.
More radically, a person with no musical training whatsoever could sit in front of a piano and instantly start to play Rachmaninoff's 3rd Piano Concerto from start to finish, including all the requisite dynamics; in such a scenario, each resolving superposition results in a brainstate that produces the correct movements. The odds of this happening, however, are beyond astronomical, and would involve an excruciatingly small subset of all possible universes.
But it's here where many skeptics draw the line, arguing that these scenarios are so ludicrous they almost seem to prove that the MWI can't possibly be true.
This is what's referred to as the Quantum Suicide thought experiment. Imagine a scenario in which a person plays Russian Roulette with bullets placed in half of the chambers. In this superposition, each spin of the chamber should reset the odds of that person killing themselves to 50/50. But the MWI tells us there has to be a world in which the person never shoots themselves, even after, say, 50 spins of the barrel. Though the odds of that happening are one in a quadrillion, the MWI tells us it must happen somewhere.
Interestingly, physicist Max Tegmark says this particular experiment could actually serve as proof that the MWI is true, though it would necessarily require the deaths of countless instances just for that one lucky experimenter to reach the finish line.
Another take on quantum immortality is the assertion that a version of us must always be around to observe the universe. Paul Halpern, author of Einstein's Dice and Schrödinger's Cat, put it this way:
What about human survival? We are each a collection of particles, governed on the deepest level by quantum rules. If each time a quantum transition took place, our bodies and consciousness split, there would be copies that experienced each possible result, including those that might determine our life or death. Suppose in one case a particular set of quantum transitions resulted in faulty cell division and ultimately a fatal form of cancer. For each of the transitions, there would always be an alternative that did not lead to cancer. Therefore, there would always be branches with survivors. Add in the assumption that our conscious awareness would flow only to the living copies, and we could survive any number of potentially hazardous events related to quantum transitions.
That said, quantum events must obey conservation laws, so there will likely be situations in which there's no escape from nature's rules.
Back in 1995, quantum physicist Rainer Plaga proposed an experimental test of the MWI in which he described a procedure for the "interworld" exchange of information and energy, i.e. "weak coupling."
By using standard quantum optical equipment, a single ion could be isolated from its environment in an ion trap. A quantum mechanical measurement would then be made with two separate outcomes performed on another system, resulting in the creation of two parallel worlds. Depending on the outcome, an ion should be excited in only one of these parallel worlds before the ion decoheres through its interaction with the environment. Plaga argues that we should be able to detect this excitation in the other parallel world, which would subsequently provide evidence for the MWI — and a potential way to send low bandwidth information to a parallel reality.
It's difficult to know how detailed this information could be, or if it would mean anything to the receiver. It's fascinating to speculate, however, about the potential implications of inter-world communication.
Very simply: The presence of alternate worlds means there isn't a single timeline to screw up.
If a person were to go back in time, they would merely set off an entirely new web of timelines. Subsequently, the MWI suggests paradoxes — like going back in time to kill your grandfather — are nothing to worry about.
The funny thing about the complete set of infinitely variable worlds is that everything has already happened.
Not only that, everything that has already been done will happen again an infinite number of times. So like Bill Murray in Groundhog Day, the current day will be experienced by yourself over and over and over and over...