The idea of a parallel universe is common in science fiction, such as the “evil Spock” universe of Star Trek, or the “alternate history” universe where the Nazis won World War II. While these ideas are often just plot devices, they do have some basis in theoretical physics. Of course just because something is a theoretical possibility, that doesn’t make it real. In science we rely upon evidence, which raises the question of whether such parallel universes could ever be observed. The answer is … maybe? 

In physics the idea of a parallel universe, or multiverse as it’s more commonly called, has several variations. One interpretation of quantum theory, known as the many-worlds interpretation, proposes that a quantum measurement of a particular event (say, tossing a coin) causes the universe to “split” into universes where a specific outcome (one universe where the outcome is heads, another with tails as the outcome). A simplistic version of this is most commonly used in science fiction, and is what most people think of when using the term parallel universe. Multiverses also arise in some versions of early cosmic inflation, such as eternal inflation, which proposes that parts of the Universe beyond the observable universe are still inflating, resulting in various “pocket” universes within a great multiverse. While in eternal inflation all these ‘verses would have the same configuration of physical constants, some versions of string theory propose a variety of universes each with their own physical constants. Some may have a slow speed of light, others a weaker electric charge. But if all these other universes lie beyond our own observable universe, how could they ever be anything other that philosophical possibilities? What evidence could we have of their existence?

One way to study them would indirectly through the theories we have. If, for example, we’re able to determine the nature of inflation through gravitational waves, we might find that eternal inflation is the model that best fits the evidence. Currently this is the case for dark matter, where we have strong indirect evidence of its existence even though we haven’t observed dark matter directly. Until this year it was also the case for gravitational waves, where we had indirect evidence for decades before their direct observation. With things like dark matter it’s generally thought that we will eventually find a way to detect them directly, but with multiverses that might not be the case. It is quite possible that indirect evidence is all we can hope for. We might have to resign ourselves to never truly knowing whether multiverses exist.

But there are ways that might provide evidence of other universes, such as observing their influence on the cosmic microwave background. A recent survey of the cosmic background hinted at such evidence, but there are reasons to be cautious. When we look at the cosmic microwave background we see small fluctuations in its overall temperature. This is expected, and in fact we can use those fluctuations to measure certain properties of the cosmos. But there are certain regions that seem somewhat out of place. For example, there are regions that appear colder than we’d expect, and there are areas that are statistically anomalous. While these regions are unusual, they aren’t so unusual that they require some kind of exotic physics to explain them. It’s kind of like tossing a coin and having it land on heads five times in a row. It’s possible that you’ve stumbled across a biased coin, but it’s also quite possible that your five coin tosses happened to have a lucky outcome. If you tossed it twenty times with an outcome of heads, then you could strongly suspect the coin.

Hypothetical “bruises” on the CMB due to collisions with other pocket universes.

Still, five heads in a row might lead you to at least speculate about the bias of the coin. In the same way, we can look to explain the oddities of the cosmic background in terms of multiverses. This is exactly what was done in a recent article in the Astrophysical Journal. Within the cosmic background there are anomalies that could be explained by interactions with other universes. Basically, if another pocket universe happened to collide with our universe when it was young, it could produce a “bruise” we could see in the cosmic background. The fluctuations we observe at a specific frequency (143 GHz) show four distinct regions that seem to statistically agree with this kind of bruising.

While this might seem pretty conclusive, even the author of this work is skeptical. One reason is that the anomalies aren’t that statistically strong, on the order of 5 coin tosses rather than 20. Another reason is that while the fluctuations can be made to fit a multiverse model, they can also be explained in less speculative ways, such as voids within our own universe. So the result is interesting, but far from conclusive.

But perhaps what’s most interesting about this work is that we can at least try to study universes beyond the one we can observe. As long as we are cautious in our study, our efforts to see beyond the veil will ensure that even multiverses are more than a mere philosophical dream.

Paper: R. Chary. Spectral Variations of the Sky: Constraints on Alternate Universes. Astrophys.J. 817, no.1, 33 (2016) arXiv:1510.00126 [astro-ph.CO]

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The idea of parallel worlds is widely used in science fiction. Most often this alternate world is either populated by our evil doppelgangers, or the alternate universe is just slightly different from ours, such as having zeppelins in a modern city. Then there is the “alternate history” fiction, where their world is identical except for a key moment in history. Lincoln survived, Harold Godwinson won the battle of Hastings, etc.

Perhaps the earliest example of alternate universes in physics comes from the 1957 paper by Hugh Everett titled “Relative State Formulation of Quantum Mechanics.” Everett’s paper was an attempt to eliminate the need for an “observer” in quantum mechanics.

Although it is a well tested theory, quantum mechanics can be hard to wrap your head around. As the theory was developed, a standard interpretation was adopted known as the Copenhagen interpretation. The basic idea of this interpretation is that a quantum system is described by a wavefunction, which then collapses into a particular state when “observed”. For example, suppose had a quantum “coin”. Before we measure the coin, we only know the result will either be heads or tails. We have no way of knowing what the result will be, only that we have a 50/50 chance of either outcome. In the Copenhagen interpretation the coin is described by a probability wavefunction. Once we measure the coin (and, say, observe that it is “heads”) then the coin is in a definite state of “heads”. The wavefunction is said to have collapsed into a single state.

This seems rather simple, but the “observer” is not clearly defined. In early quantum theory it was simply assumed to be some larger system. But the world is not divided between “quantum” and “classical” things. If our coin is in a quantum state, then we as the observer should also be in a quantum state. We can’t simply declare a classical observer. So the Copenhagen interpretation isn’t really an accurate way to describe quantum mechanics, though it is still the most commonly taught approach.

Everett’s approach was to remove the “classical” observer. Instead, he proposed that observation is an interaction that decouples the outcome states. If we go back to the coin idea, initially the coin is described by a wavefunction where both heads and tails are possible. The observer is also described by a wavefunction in which it can observe either heads or tails as the outcome. When the observer and coin interact, their wavefunctions don’t collapse, but instead the quantum states separate into one quantum state where the coin is “heads” and the observer measures “heads”, and another quantum state where the result is “tails”.

This eliminates the need for a “classical” observer, but it means the system has split into two systems. On the cosmic scale it means the universe has split into one where the result is heads and one where the result is tails. Each measurement separates (decoheres) the quantum state of the universe into two states. Presumably this would happen with every possible outcome. So there is a universe where you asked that cute brunette out to dinner, and one where you didn’t. In the universe where you asked them out, there is a version where they said yes and another where they said no. So the universe splits at every possible outcome, and (in one interpretation) all outcomes are real. All of these parallel universes splitting and splitting at every quantum choice.

Of course in Everett’s formulation, once quantum states split they no longer influence each other. They have a common past, but no future interactions. So there is no way to travel to or communicate with the version of you that is a billionaire and ask for a loan. This also means that Everett’s many-worlds approach is completely untestable. It may have some philosophical appeal, and may even be real, but it is not a testable hypothesis.

Another appearance of parallel universes comes by way of string theory, and its generalization M-theory. In M-theory, there are 11 dimensions, and our 4-dimensional universe is a membrane (or brane) within this higher-dimensional space. Very roughly, you can visualize our universe as a sheet of paper in a large room. But this means there could be other universes in this 11-dimensional “multiverse”. These could be parallel to our universe. In M-theory, they might interact with our universe gravitationally. There are some models that propose this as a solution to dark matter, for example. But of course all of this is very speculative, and at present there is no experimental evidence for string theory or M-theory. And these other universes would not necessarily be parallels of our universe.

So it seems that Everett’s many-worlds hypothesis is the closest physics comes to a parallel universe in the way often presented in science fiction. Whether or not these parallel universes are real, we have no way to reach them.

For us, choices have consequences, and we always have to face ourselves when we look in the mirror.

Tomorrow: Aliens! As we learn more of the universe it seems increasingly likely that there is life on other worlds. But if that’s the case, where are all the intelligent aliens?

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While fantastical stories have been with us as long as we’ve been human, in the early 1800s a new type of story appeared. Often Mary Shelly’s Frankenstein is named as the first example of this genre. Also known as The Modern Prometheus, it gives us the tale of a mad scientist who creates a creature from alchemy and science. By the late 1800s H. G. Wells wrote tales of The Time Machine, and an alien invasion with The War of the Worlds, and Jules Verne gave us adventure stories of an atomic powered submarine in 20,000 Leagues Under the Sea, and the first astronauts in From the Earth to the Moon.

It’s not surprising that the earliest works of science fiction were about time travel, space aliens and starships. We love dreaming of new horizons, and science fiction can create entire tales from the the mere whiff of scientific possibility. This can be both a blessing and a curse. On the one hand science fiction lets us explore the possibilities, and can inspire an interest in science. On the other hand, the trappings of science fiction can make awesome feats of human engineering seem trivial. Who cares if we can land a rover on Mars when science fiction lets us travel to the stars. On the gripping hand, many of the concepts of science fiction are now deeply rooted in our culture. The once futuristic idea of touch screens and verbally asking your shipboard computer for information on a particular star system are now very real. Even some of science fiction’s wildest ideas like teleportation and quantum computers are at the cutting edge of real science.

But some common ideas from science fiction are based only on the most tenuous science. While we all know what time machines, warp drive and wormholes are in the context of science fiction, scientifically these ideas are speculative at best. They also happen to show up all the time in popular science articles. You’ve likely come across the “NASA working on warp drive” articles, or “scientist wants to build time machine”. Popular media often portrays them as merely engineering challenges rather than speculative science. They are also ideas I get asked about all the time.

It’s because I’m so often asked about these topics that I decided to do this particular series. I’ll focus on the ones with a connection to astronomy and astrophysics. So the topics for this week are

• Time Travel
• Warp Drive
• Wormholes
• Ansibles
• Parallel Universes
• Aliens

I’ll focus on what we know both experimentally and theoretically. We’ll go to the very edge of current science, but separate scientific possibility from wild fantasy.

Starting tomorrow: Time travel. Is it really possible to go back in time? If your Mom falls in love with Calvin Klein instead of your Dad, does that mean you’ll cease to exist? Allons-y!

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