You don’t have to believe in the multiverse. Here’s why.
Physicists in the last 10 years have been losing the battle against an idea that was once so radical it was scoffed at by the biggest…
Physicists in the last 10 years have been losing the battle against an idea that was once so radical it was scoffed at by the biggest names in physics. I’m talking about the multiverse, the idea that there are countless parallel universes and that the universe is constantly splitting into more.
The notion of parallel universes is far from new, but until the 1950s it existed primarily in the realm of philosophy and religion. But in 1957 an argument over the interpretation of quantum mechanics resulted in a graduate student, Hugh Everett III, publishing his Ph.D. thesis on this astounding idea: that quantum mechanics could be explained more completely if we interpreted it through the lens of parallel universes. His thesis made little impact on the world of physics which was slowly building up its understanding of the fundamental forces of nature and had little patience for philosophical detours. Thesis published, Everett left academics for good.
To understand the nature of this argument, however, we need to go back further in time, back to the 1920s and ’30s. I wrote about one part of the argument extensively in my article on quantum uncertainty. At that time, the argument between the likes of Albert Einstein and Niels Bohr, was over how quantum uncertainty worked, and we found that in any quantum experiment, it appears that God (or Nature) is rolling dice to decide what happens next. For example, if I cause two particles, A and B, to interact and I measure their properties, it appears as if how I measure A or B affects what I measure. Now I want to talk what that means.
Niels Bohr came up with the first explanation. He said that whenever you do some quantum experiment, like shoot particles through a pair of slits in a barrier, for example, and you want to measure some outcome, like which slit the particle went through, the particle exists as a probability field called a wavefunction. When you fire the particle at the barrier, it is like a cloud of probability that propagates through it. As long as you don’t measure it, it continues on as a cloud of probability and can interact with itself, and, by virtue of traveling through both slits at once, appears like a wave, showing an interference pattern like the picture above. But once you try to figure out which slit the particle went through, the wavefunction “collapses” and you get one answer: left or right, blob of light, not pattern. You don’t get a wave. This is sometimes called the wave-particle duality, but it is much bigger than waves or particles. It is about us and what kind of universe(s) we live in.
The wavefunction collapse idea continued on for decades as the only explanation but there was a problem with it. Wavefunction collapse isn’t part of quantum physics. It is something external to it that we observe but doesn’t exist in any mathematical model we have.
That wasn’t the only problem. The other issue with wavefunction collapse was whether it collapsed for everyone everywhere in the universe or just the one person observing the experiment. That is, if you are observing the experiment of which slit the particle went through and I don’t know what you observed, are you somehow in a cloud of probability yourself or has the cloud collapsed for me as well as you. This was what Eugene Wigner, one of Everett’s mentors, was looking into in the 1950s. He wanted to understand if wavefunction collapse was “objective” or “subjective” and did it require a conscious person to cause the collapse or just some kind of physical apparatus?
Everett had a different idea. What if the wavefunction collapse never happened? What if, instead, when we observed the outcome it just looked like the wavefunction collapsed but what really happened was that different versions of us, the observers of the experiment, continued on into different realities? This had the benefit that quantum physics was preserved. Wavefunctions could evolve across multiple universes, decohering (breaking apart) as experiments were conducted and different results observed, but never really being destroyed. This preserved something very important to quantum physics: something called “unitarity” or the preservation of probabilities across the wavefunction. Unitarity could only be preserved if the wavefunction continued on intact after an experiment, and that could only happen if the universes split, so Everett reasoned.
Everett’s idea solved another problem with quantum physics, one that Einstein decried as a serious flaw, nonlocality. This was the idea that when you measure one particle it affects what you measure of another particle, even when that particle is far away. This “spooky action at a distance” was explainable in Everett’s theory as the interaction of different universes. Thus, particles themselves could remain where they were while the colliding universes took care of the spookiness.
So, if it seems so obvious, why don’t we all believe in the multiverse now? Many physicists are coming around to the idea in recent years that the multiverse is the only rational explanation of quantum physics. The likes of David Deutsch, Max Tegmark, Bryce Dewitt, Sean Carroll and others have insisted that the multiverse cannot be considered an “interpretation”. It is simply a fact of living in a quantum universe.
The philosophical problem with having so many parallel universes cannot be overstated. First of all, if there are many, many copies of me all existing in distinct parallel worlds each with a shared past, how do I have a unique self? And if there are many parallel futures and pasts, does the history of the world and the future even matter? If everything possible is going to eventually play out in some universe, do my actions matter? On the other side, can the human species exist without the multiverse providing the answer to the anthropic principle that for a universe to exist finely tuned enough to have us evolve within it all possible universes must exist? Is this just an Atheistic version of the Story of Creation as told in the Book of Genesis with no more hard evidence in its favor?
Maybe, though, the philosophers just need to shut up and pay attention to what physics is telling them. Or is it the other way around?
Consider that in a multiverse viewpoint all possible things occur, even the least probable. There would be multiverses where the Sun randomly explodes. Multiverses where humans don’t exist. Even ones where people just suddenly are all simultaneously teleported into deep space. All this seems utterly absurd.
It is not surprising that many physicists don’t agree on how to interpret the multiverse. There are those, like Stephen Hawking, who regard the alternate universes as unreal, mathematical conveniences much as I considered the manifolds of General Relativity recently. In other words, there is one “real” universe and all those other universes are might-have-beens. There is no wavefunction collapse. God rolls the dice, and one universe is promoted to real over all others.
Quantum Bayesianists (Qbists) regard the whole notion of a “universe” as a fraud. All things are local, they say. My subjective viewpoint of the universe is different from yours and in the end the only one that matters. Quantum outcomes are not based on God rolling the dice. Instead, I roll the dice. I determine what is “real” and what is not and what you may regard as real could easily be different than what I see as real. This is in a literal rather than figurative sense since outcomes of experiments can be different for me than they are for you. After all, you could be in the lab observing which particle passes through the slit while I am observing you as a probability cloud.
The third option is what I call the reduced multiverse interpretation. This is based on the work of Andrew Strominger at the Institute of Advanced Study at Princeton and others in the early 1980s. In this interpretation, only a smaller subset (yet still infinite) of possible outcomes are actually real but those that are real are sufficient to account for all probable outcomes of quantum experiments. This is based, if you are interested, on microcanonical quantum field theory, a fairly dead area of study these days, but once a candidate for resolving quantum gravity. In microcanonical quantum physics, the multiverse has many realities, but they all follow logically from one another, one evolving into the other, so absurdities may be kept out.
The one interpretation that has been at least partly ruled out by experiment is Niel Bohr’s wavefunction collapse, the so called, Copenhagen interpretation. In 2018, researchers at the University of Edinburgh showed a version of the case of one person observing another doing a quantum experiment and that they could have differing views of reality. They did this using the magic of quantum computing. The different “experimenters” in the scenario were represented by qubits, short for quantum bits. The researchers caused them to interact in a way that clearly showed that the “objective” wavefunction collapse interpretation where everyone in the universe has the same interpretation of the outcomes of quantum experiments is flat out wrong. You can observe which particle went through the slit while I see “you” as superimposed realities of you seeing left and you seeing right. Reality is in the eye of the beholder.
Experiments like this and a gradual erosion of the philosophical desire to have an objective reality that grew out of the Enlightenment have led many modern physicists to accept the multiverse. Yet, for all the rush, scientists still don’t understand what happens when we make a measurement of a quantum experiment. Does the universe split? Does it pretend to split but really determines which universe is the real one? Am I just viewing a result that fits into my quantum Bayesian belief? Is there even an experiment I can do that would determine the truth? Even that is debatable as the flaming arrows between Lee Smolin and Leonard Susskind over whether the multiverse was a falsifiable hypothesis proved in 2004.
Given that it isn’t clear what you should believe, I can only tell you that you don’t have to believe in the multiverse. The science to interpret it isn’t there yet. You can believe that the other universes are “unreal” or you can believe that you are a universe unto yourself like the QBists. If you do believe in the multiverse, you might need to choose which interpretation you subscribe to: limited or full of absurdities. I won’t even get into the idea that parallel universes could have different physics or mathematics entirely. The one thing that you can’t believe in, and still be scientific, is that we all share one objective reality. We all live in slightly different universes. That is no longer a metaphor. It is a scientific fact.