Does Science Really Predict the Existence of Doppelgängers?

There is a concept in popular culture that there are parallel universes populated with our doppelgängers. What’s more, it is even claimed that science predicts, or even has shown, that these alternative universes and our doppelgängers exist. That really confounds me. Prior to researching this newsletter, I knew a little of the science behind alternative universes suggestions, but had never come across any scientific evidence. Could it be that there really are scientific theories or evidence that we need to take seriously? Although alternative histories could be very useful for winning arguments, the idea that there are copies of me does my head in.

What’s the scientific story behind these claims? It turns out that three different fields of physics lead to the concept of multiple universes: cosmology, string theory, and quantum physics. Physicist Briane Greene enumerates no less than 9 different ways that multiple universes could arise from scientific theory. Taken together, that’s quite stunning. Rarely does scientific thought converge this way. Here I’ll outline just two of these theories, the quilted multiverse and the many worlds theory, that include the key implications of most multiverse theories for the existence of doppelgängers.

Let’s start out with some terminology. If we define the universe as everything, then of course there is nothing else and there can be no other universes. And of course this is the traditional understanding of the word universe. Prior to the last few decades, we understood “everything” to include the Big Bang and everything resulting from it, including the space, matter, photons, quarks, and neutrinos that make up our existence. But some scientists now suggest that our Big Bang may not have been the only primordial explosion leading to space and galaxies. They suggest that our universe sprung out of a quantum tunneling event that occurred in a place with higher dimensions than the four we know and love. The tunneling event lead to a vast eruption, the Big Bang, and created all that we can see and everything beyond it that lies within our space-time continuum. If this quantum-tunneling-in-a-high-dimension theory is correct, than it would suggest that other quantum tunneling events likely occurred in these hyper dimensions, creating other big bangs. Scientists call the other big bangs, and their evolution, other universes.

But some astronomers also call really really distance parts of our space-time “other universes” because these distant regions can not, nor will they ever, be able to interact with us. We’ll never be able to see them. They will never influence our gravitational field. Et cetera et cetera. We refer that which we can see, and therefore that which we can interact with, our observable universe. How big is our observable universe? We believe that our universe has a finite age, about 14 billion years. And we also believe that (almost) nothing travels faster than the speed of light. Therefore, in a static universe light could only have traveled 14 billion light years. If our universe is infinite in space, and we believe that it is, then there will be places outside of our observable universe that do not interact with any part of our universe. And given infinite space, there would be an infinite number of causally separate bubbles - in other words - a multiverse. This is the quilted multiverse theory.

As a short diversion into how big our observable universe consider this quick tour. It takes light from the moon about one second to reach us. The closest star, except our sun, is Alpha Centauri and it is about 4 light years away from us. If Alpha Centauri exploded right now *BANG*, we wouldn’t know about it for another 4 years. The center of our galaxy, the Milky Way, is about 25,000 light years away. Until the 1920s we thought the Milky Way made up the entire universe. And indeed, our galaxy is vast. It includes 4 * 10^11 stars (400 billion) and the last few decades have shown that most of these stars have planets. We now know that the universe extends far beyond the Milky Way. One of our nearest neighbors is the stunning Andromeda Galaxy and it is about a hundred times further from us than the center of the Milky Way. The most distant galaxy observed is about 5000 times further away than Andromeda, or about 13.5 billion light years away.

In a static universe, this most distant galaxy would be at the edge of our observable universe. But - we know that our universe is expanding. So light that was emitted long ago, from objects that are now further than 14 billion lights away, may still reach us. That’s because the starting path of the light from those now distant objects was once much much closer to us - within our observable universe. Calculations suggest that objects up to 45 billion light years away are part of our observable universe. It is estimated that our observable universe contains 2 * 10^12 (2 trillion) galaxies. If they each have 4 * 10^11 stars, like the Milky Way, that’s 8 * 10^23 stars in our observable universe. Or 800,000,000,000,000,000,000,000 stars.

And what about beyond the observable universe? Most versions of the theory of cosmic inflation predict that space is infinite. And we believe the theory of inflation is correct because it first explained some observations and subsequently made predictions which turned out to be true. Cosmic inflation is the theory that the universe underwent an almost unfathomable expansion in the very very very early universe - from 10^-36 of a second to 10^-32 of a second after the Big Bang. By ‘unfathomable expansion’ I mean that distances in the universe expanded by a factor of 10^26. For example, the distance across a proton would have expanded to fill space the size of a softball. This expansion pushed apart space faster than the speed of light. Inflation was first proposed to explain the observation that radiation at 3 degrees Kelvin, which we see all around us, is so very uniform. This 3 degree radiation, known as the cosmic microwave background (CMB), is a remnant of the Big Bang and is known to have last interacted with matter, on any meaningful level, a mere 380,000 years after the Big Bang. The CMB therefore comes from very near the edge of our observable universe. Without inflation, astronomers were perplexed as to how light from opposite sides of our observable universe could be the same temperature. These two regions should not have ever been able to influence one another, being seperated by a distance twice the radius of the observable universe. Incidentally, my master’s thesis was an attempt to explain this uniformity through multiple gravitational lensing events as the CMB travelled passed cosmic objects on its journey to us. Our theory didn’t pan out but inflation did. Connectivity prior to inflation can explain uniformity seen in the CMB post inflation as much greater areas of space would be causally connected than mere light speed travel would indicate.

Since its initial proposal, inflation has successfully explained very small fluctuations that have now been seen in the CMB as well as given an explanation (dark energy) of the acceleration of the expansion of our universe, which started around 6 billion years. And why the divergence into inflation? Because most models of inflation predict that the universe is infinite in size.

So now we finally get to the first doppelgänger ‘prediction’. If space is infinite, then it is argued that everything that is possible must happen, and it must happen an infinite number of times. In other words, there are infinite copies of us out there. A couple of weeks of scouring the internet and I can not find anything more meaty than this claim “If it is possible, then in an infinite universe it must have happened, an infinite number of times”. And I just don’t buy that. The mathematics of different kinds of infinity seem to have been conflated here. There are different kinds, and sizes, of infinite sets. For example, the positive integers are 1, 2, 3, and upwards to infinity. This kind of set is referred to as countable. But there are also uncountable sets, such as the real numbers which are the set of continuous numbers. There is infinite variation of real numbers between every pair of numbers. For example between 1 and 2, we can have 1.1, 1.11, 1.111, and so forth. The set of real numbers is bigger than the set of positive integers. It is my belief that number of universes and the variation within universes could be similarly compared. Universes would be numbered with positive integers 1, 2, 3 … Whereas their state, or variation, would be written as real numbers 1.2, 1.21, 1.213, and so on. To my mind, the variation infinity is likely greater than the counting infinity and therefore there is little chance of repetition in the multiverse. Another example of infinity never repeating is in the number Pi. Pi is known to be infinite and yet it never repeats. Just because our universe is infinite doesn’t mean I have doppelgängers.

There is (at least) one sticking point in my logic. There may not be an infinite number of possible states of a given universe. Apparently quantum physics does not allow for an infinite number of possible states for a quantum particle, as classical physics does. This could provide a limit on possible states of a universe and if there are infinite universes each of those possible states would have to repeat. I’m afraid I don’t know the answer, so let me sum up with what greater minds think. Some physicists think doppelgängers arising from infinite space are inevitable, others agree with me that identical worlds seem impossible.

There is another way that science suggests we may have doppelgängers and it doesn’t require infinite probabilities but rather the curious qualities of quantum physics - that’s got a nice ring to it. You may be familiar with the thought experiment of Schrödinger’s cat. A cat is placed in a box in which her life hinges on whether or not a quantum particle decays. Classical physics can not predict the behavior of the quantum particle. Nor can quantum physics. But quantum physics can yield probability estimates of what the particle will do based on an equation, called Shrodinger’s wave equation. If there are two states, A and B, to which the particle could evolve, the maths may give a 50 / 50 chance that it ends up in state A or state B. But what does the 50 / 50 probability mean in reality? That’s the million dollar question. When we open the cat’s box, we observe that the cat is either dead or alive. Some physicists explain this as the variable in Schrödinger’s equation, the wave function, having ‘collapsed’ onto one of the solutions. And perhaps this collapse to one solution only occurred when we observed the system. In other words, the cat was both dead and alive before we looked at it. Other physicists insist that whatever state we see the cat in, the other outcome also happened, but in a different dimension to ours. These scientists believe that every time there is a quantum choice (state A or state B), the universe bifurcates and both states are realized. No, really. There are lots of serious scientists who think this is likely. They would say that both universes are right ‘here’ but in different dimensions of so called Hilbert Space. This is the Many Worlds hypothesis.

Lest you think that is all madness from staring at equations for too long, be mindful that Schrödinger’s wave equation has explained several bizarre observations. Perhaps the most famous of these is the double slit experiment where electrons are shot at a surface with two slits in it and observed to make a pattern on a screen beyond the slits. If you add up the pattern made when slit A is open and B is closed, and the pattern made when only B is open, you do not get the same pattern as when both slits are open simultaneously, as classical mechanics predicts. Somehow, the electrons traveling through slit A know if slit B is open. Quantum mechanics, via Schrödinger’s equation, does explain this, along with many other observations.

While it can be difficult to wrap our heads around these ideas, at least there is resounding answer to my original question “Are doppelgängers predicted by science?” For this quantum case, the resounding answer is no. The idea behind the many worlds view is an interpretation of the wave equation, interpretation being the key word here. There are no observations to support alternative worlds peeling off. Indeed, the maths suggests we could never observe them. I hasten to add that we don’t even know what Schrödinger’s wave function is. We just know that we can use it to calculate the probability of a quantum particle being in a particular state. It seems overly complicated to invoke multiple worlds to explain the bizzareness of the one we know (or at least seem to know) especially using a mathematical function which we don’t understand.

It is quite satisfying to be able to wrap up with the argument that both the many worlds and the quilted multiverse hypotheses are not even considered scientific theory. Some scientists would say that these hypotheses are concepts, rather than theories, because they are not testable. Because we can’t observe these alternate universes, science can’t prove or disprove them. Hypothesis and testable predictions are the bedrock of science and so to call these two multiverse scientific theory is misleading. Some scientists go even further in deriding these concepts as just plain bad for science because they hammer away at the foundation of science to hypothesize and test. And others believe that using scientists’ time to explore such untestable hypotheses is a waste of valuable resources when our society faces so many massive issues. While I myself left off studying astrophysics to study climate change, for just such a reason, I do not agree that we should stop scientific day dreaming altogether. Mathematics is purely theoretical and has been very useful. Philosophy is fun and makes our brains grow. And if you don’t agree, no worries, it’s possible that one of you out there does.