Are We Alone? Essays & Opinions
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Marcelo Gleiser

Why We Matter - To the Multiverse and Back

Appleton Professor of Natural Philosophy at Dartmouth College and author of "The Island of Knowledge."

When, in 1543, Copernicus demoted the Earth to a mere planet, he had no idea that he would, albeit somewhat slowly, turn the world upside-down. The good canon of Warmia, Poland, set off a chain of events that would, in the centuries following, lead to the progressive humiliation of our species, once the proud center of creation. So goes the bad karma of science: the more we learn about the universe, the less important we become. Is this really what modern science is telling us?

In some ways, yes. Take, for example, the laws of physics and chemistry. They are the same across the vastness of space and the duration of time, at least from fractions of a second after the Big Bang to today. If the same laws hold across our cosmic horizon—the bubble of information defined by the distance light has travelled since the dawn of time 13.8 billion years ago—we should expect that events that happened on our planet in its 4.6 billion years of existence—the development of unicellular life and its evolution to intelligent multicellular species—are not remarkable. This is the “big numbers hypothesis”: in our galaxy alone there are some 300 billion stars, most with planets and moons, making up for over a trillion worlds, each different, each with its own history. Now add the existence of some 200 billion other galaxies within our cosmic horizon, and the diversity is staggering. How can we hold any claim to uniqueness?

Modern Copernicanism thus states that we are typical observers, what Tufts University cosmologist Alexander Vilenkin and many others often cite as the Principle of Mediocrity: the properties and evolution of the solar system are not unusual in any important way. It gets worse. When it comes to the composition of the cosmos, the star stuff we are made of amounts to a mere five percent of the total. The rest is comprised of dark matter (stuff not made of the usual protons and electrons) and dark energy (stuff of unknown nature that is responsible for accelerating the cosmic expansion). Our cosmic insignificance multiplies.

But let us not stop there. Modern theories of particle physics suggest that our universe may be but a bubble in the so-called multiverse: a conglomerate of countless universes, some growing, others shrinking, some young, others old, most with physical laws different from our own. If the multiverse is real, physics must answer questions it is not prepared to answer, related to selection principles that specify not just how the laws of Nature operate but which laws of Nature should act and where. Physics would need a kind of meta-law, a law that determines other laws, including the ones operating within our cosmic bubble.

And, most intriguingly, why would our universe have laws that allow for life?

Some want the multiverse to go away. It puts physics in a awkward position, of pondering the existence of an entity whose nature is not physical in the same sense that a galaxy is physical: what’s outside our cosmic horizon is out of reach. Others welcome the multiverse as an elegant solution to another conundrum: Why this universe? What determines the constants of Nature that we measure (like the electron’s charge and mass, the gravitational constant, etc.) and the laws that dictate how matter and energy behave in our chunk of space and time?

Arizona State University physicist Paul Davies calls our cosmos the Goldilocks Universe, given that the constants of Nature and its properties had to be “just right” for life to emerge here: tweak the value of the proton mass, and stars fail to burn properly; expand space too quickly, and galaxies never form. This is known as fine-tuning: life is possible only if the values of physical constants lie within a very narrow range. Astronomer Don York, from the University of Chicago, wrote in an email: “as an observer of Nature, things certainly seem to have had a genesis that was top down (designed).” Astronomer Royal Sir Martin Rees—a supporter of the multiverse as a solution to fine-tuning—pointed out that it all hinges on six fundamental numbers. Is our universe an accident or the result of premeditation?

In the 1970s, astrophysicist Brandon Carter proposed the Anthropic Principle (AP), according to which intelligent observers are a consequence of specific physical properties built into the fabric of the cosmos.

The principle has a strong and a weak version (with some variations in between), which have very different interpretations. In the strong version, we are the result of cosmic intent. The weak version states that our universe harbors life because it has the right properties to do so. This may sound silly at first, but physicists use the weak version to place bounds on physical quantities. Researchers apply the weak AP in an a posteriori approach, using the fact that we are here to delimit what values fundamental constants must have. In the context of the multiverse, we can use the AP to place bounds on what universes could possibly harbor life, as Stanford physicist Leonard Susskind explained in his book The Cosmic Landscape. It appropriates the big numbers hypothesis from stellar systems to whole universes: when the number of universes is so vast, even a rare one like ours is not impossible. No designer required.

There is, however, an orthogonal, anti-Copernican way, according to which fine-tuning is a straw man. There are values of fundamental constants that have been measured for the past 400 years; they are the alphabet of physics, and don’t need a fundamental, unified explanation. Even if the weak version of the AP does away with the need for a “tuner,” it does so at the cost of invoking the mysterious multiverse, itself based on still-speculative physics.

What if we stay within our known universe? Is life really ubiquitous, as the big numbers hypothesis would claim? We only have one data point so far, Earth. And what we learn from life here is that isolating nonliving chemicals inside protective membranes to form self-replicating molecular machines is a very complex step; so is the step from there to cells with protected nuclear material. Terrestrial life was stuck in this unicellular stage for some three billion years out of the 3.5 billion years that we know it has been around.

Without a heavy moon, Earth’s rotational axis would wobble and seasons would not exist; without a protective atmosphere and magnetic poles, cosmic radiation would sterilize the surface, and so on.

The bottom line: Earth is not mediocre.

If life is out there (and there is no good reason why it should not be), it will most probably be simple. If there are intelligent creatures in our galaxy, they are likely so far away as to make us effectively alone. According to this view, which I call humancentrism, modern science is telling us the opposite of Copernicanism: as molecular machines capable of self-awareness, we matter because we are rare; our planet matters because it is rare.

Science’s karma is not so bad after all. 

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