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Nonfiction

The Physics of a Populated Universe

In the universe we know, physical laws eventually reach the point of tautology: They are a certain way, simply because they are that way. Gravity is calculated through this coefficient.  Matter is scattered about in that abundance. The universe expanded by x. We assume that that is the only way it could be, but that’s not necessarily true. There could be any number of physical laws in place instead of the ones we know, some of them possibly even more probable than the ones we know. But if other laws existed, we as humans may not. Because of that, we must consider why the laws of physics are the way they are, and what other strange or lifeless universes there might be, or might have been.

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The Coincidence of Life

No one can say exactly why life exists, but we can say that life is allowed to exist thanks to a few simple coincidences. In the earliest instants of the universe, the epoch of inflation grew our universe just large enough that it wouldn’t collapse into black holes later on, but not so large that stars could never form. Also, in the instants of beginning, the matter and anti-matter that formed had just enough of an imbalance that when they collided and annihilated each other, there was still some matter left over to form you, me, and the universe we know. All across physics we see these fine tunings reflected in the amount of dark energy that pushes apart the galaxies, and in fine structural constants such as the one that defines how neon “open” signs glow red.

The things that facilitate life and appear to be the way they are “just because,” is an ever-growing list. One of the holy grails of science is a single underlying theory that explains away these coincidences: The amount of dark energy, the length of the inflation epoch, the proportion of matter to anti-matter—all these things and more we hope will fall out of some magic, perfect equation that elegantly sums up everything.

But we can’t find that theory.

Einstein spent his life searching for it. Hawking has looked. Every great cosmologist has asked, “Hmmm, what if?” and they’ve come back with nothing certain.

So for now, scientists find themselves in a universe with much that seems to be just so—just because—and many of us hate not knowing why this is.

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The Reasons Behind It All

We could, of course, not really understand the universe. It could be there is some yet undiscovered underlying rule, some beautiful set of equations, that will dictate that all of the impossible coincidences must exist.

Another possibility is that there is a force outside our universe—outside our space and time—that is dictating the constants of our cosmos. Perhaps there is a God, a watchmaker, a greater power tweaking our forces to make life possible. (By definition, this possibility is beyond the testing of science.)

A third possibility is, to some scientists, the ugliest and, to others, the most elegant. This theory says that our universe is just one of many and that our improbable reality is able to occur because every possible combination of constants exists somewhere in the multiverse of universes. This theory may or may not be testable. And this possibility falls out of many different ideas.

There is also the probability argument: In theory, if enough monkeys pound on typewriters for long enough, they’ll end up with Hamlet, and so it makes sense to also say that if enough universes are allowed to exist, eventually life will emerge. But saying that something makes sense just isn’t enough.

Luckily, many scientists find more compelling arguments to suggest multiple universes.

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Quantum Multiverses

According to quantum mechanics, the outcome of a quantum event doesn’t exist until it’s observed. This is the premise behind the Schrödinger’s Cat thought experiment.

The experiment goes something like this: You lock a cat in a sealed container. In the container with the cat is a Geiger counter and a small bit of radioactive material.

At any given moment, each atom in the radioactive material has a certain probability of decaying. If, for instance, it’s a bit of 210 Polonium, the half-life is 138 days. This means that statistically, if you have a bunch of atoms of Polonium, half of them will decay within 138 days. The thing is, statistics doesn’t require that half of the atoms must decay. It could be that more decay or less decay; it’s just a probability. Now if my bunch of Polonium happens to add up to 138 atoms, statistics say that on any given day, I have a 50/50 probability of one atom decaying.

So, imagine I have a cat in a box with 138 atoms of Polonium, and I have a Geiger counter to detect if any atoms actually decay. Just to make things interesting, I attach a vial of poison to the Geiger counter, such that if some atom decays, the Geiger counter will trigger and burst the poison, killing the cat.

According to quantum mechanics, the atoms each exist at every moment in both a decayed and a not-decayed state. Only at that philosophically painful moment when the atoms are observed does the wave function collapse, and the atoms become absolutely decayed or absolutely not decayed. This means, until they are observed those atoms hover in the decayed / not-decayed state and the cat hovers in both a state of life and death.

In reality, the cat is a perfectly good observer of its own death. But still, the atom could be decayed and not-decayed until the Geiger counter or something else comes along to interact with its wave function and observe the outcome.

Radioactive decay isn’t the only weirdly probabilistic thing we observe. For instance, if I have the world’s most pathetic laser and it gives off just one photon at a time, and I point my pathetic laser at a series of slits, the photons will go through and scatter out onto a screen on the far side of the slit. If I keep watching where the photons have landed for a period of time, they will build up a pattern that just happens to be identical to the interference pattern that you get if a bunch of photons from a very bright source all going through the slits at once. Thus, for several odd quantum mechanics reasons, photons have some weird probabilistic way going through slits as waves and interacting in probabilistic ways. While no one can predict where any one photon will land, using quantum mechanics we can predict the pattern that lots of photons will build up.

But why should any one photon do one thing, when in the exact same situation another photon does something totally different? According to what is called the Oxford Interpretation or the Many Worlds Interpretation, each photon actually takes every single different option, but each option occurs in a different, parallel, branching universe. Every time a choice is made, the universe branches. In this way, every possibility that could happen, does happen, just not necessarily in the universe we know as the one we live in. Everything that could happen, does happen, somewhere, in some universe.

In this way, every possible value for every possible factor in our universe is played out in some parallel universe.

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The Test of the Untestable

The question is, how do you test this quantum-based multi-universe theory?

Unfortunately, the only way that has been defined really only tests the theory for the poor person running the test. Imagine the person who places a radioactive decay-triggered gun at their head and steps into a box and waits to see if the world ends. With each moment the gun doesn’t fire, the world splits into a world in which the scientist dies, and a world in which she lives. If she continues to live beyond what statistics say is reasonable, than probability weakly claims that there should be other universes where the scientist has died. It’s a weak argument. It’s an immoral experiment, but it’s the one test we have the technology to do, even if no one will—I hope—ever do it.

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The Many Bubbling Multiverses

But quantum mechanics isn’t the only way to get at multiverses. According to Andrei Linde (of Stanford University) and many others, it is possible that the field that drove the early period of inflation didn’t act the same way in all places. What if in some places expansion continued, with fluctuations in the inflation leading to bubble universes expanding one from another, extending on forever?

This “What if?” is layered on top of detailed theories that match our observed universe, and elegantly explain how inflation could have occurred. The multiverse falls naturally out of theories that drive the period of inflation with a (scalar) field that reacts to its environment. The early universe was filled with bubbling quantum fluctuations that acted like waves. As the universe expanded, the waves froze. The largest fluctuations froze first, and as the universe expanded, stretching the small waves with its growth, they eventually reached sizes where they also froze.

In these spikes of chaotic inflation, new bubble universes could form, each growing out of a bit of the universe before it, each branch growing bubbles with its own physical characteristics. These universes can sprout out of one another nearly forever.

But, while this theory conforms to observations and explains what we experience, we currently have no way of knowing if it is true. Again, we have left the realm of testable science.

It seems that with today’s technology and physics knowledge we must, at a certain level, label the first moments of the universe with the warning, as ancient mapmakers did, “Here be dragons.” We don’t know what sets our universe in place, and we don’t know why we are in a universe so precisely tuned to allow life to exist. We have ideas, but ideas aren’t answers.

Scientists look at untestable theories and must conclude that they really can’t be proven to be any more valid than works of fiction like Stephen King’s parallel universe series, The Dark Tower, in which there is one true universe and infinite child universes, with alternate, not real pasts and futures. In that series, one of the characters, at the moment of his death, says, “Go, then. There are other worlds than these.” In these words he propels the main character, the gunslinger, to other truths, other possibilities, and other places in time and space. We don’t know if there are other multi-verses, parallel to this one, but we do know that our reality is not uniquely dictated by physics as we know it today. Our life in our universe is frighteningly improbable, and yet nonetheless real.

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Pamela L. Gay

Pamela GayDr. Pamela L. Gay is an astronomer, writer, and podcaster focused on using new media technologies to engage people in science and technology. You can learn more about astronomy each week through AstronomyCast.com. Want to do science? Help Pamela and other scientists through IceHunters.org.