How the Multiverse Could Break the Scientific Method
Today, let’s take a walk on the wild side and assume, for the sake of argument, that our universe isn’t the only one out there. Consider that there are many other universes, possibly an infinite one. All of these universes, including our own, are what cosmologists call the Multiverse. It sounds more like a myth than a scientific hypothesis, and this conceptual killjoy inspires some while it outrages others.
How far can we push the theories of physics?
The controversy began in the 1980s. Two physicists, Andrei Linde of Stanford University and Alex Vilenkin of Tufts University, independently proposed that if the Universe was expanding very rapidly at the start of its existence – we call it an inflationary expansion – then our Universe wouldn’t be alone.
This inflationary phase of growth likely occurred a trillionth of a trillionth of a trillionth of a second after time began. That is about 10-36 seconds after the “bang” when the clock that describes the expansion of our universe began to tick. You may ask, “How come these scientists feel comfortable talking about such ridiculously small times?” Wasn’t the Universe so ridiculously dense back then?
Well, the truth is that we don’t yet have a theory describing physics under these conditions. What we have are extrapolations based on what we know today. It’s not ideal, but given our lack of experimental data, it’s the only place to start. Without data, we have to push our theories as far as we see fit. Of course, what is reasonable for some theorists will not be for others. And this is where things get interesting.
The assumption here is that we can apply essentially the same physics to energies that are about a thousand trillion times greater than we can probe at Large Hadron Collider, the giant accelerator hosted by the European Organization for Nuclear Research in Switzerland. And even if you can’t quite apply the same physics, you can at least apply the physics with similar actors.
Rough waters, quantum fields
In high energy physics, all characters are fields. Fields here mean disturbances that fill space and may or may not change over time. A coarse image of a field is that of water filling a pond. Water is everywhere in the basin, with certain properties that take on values at every point: temperature, pressure, salinity, for example. Fields have excitations which we call particles. The electronic field has the electron as its excitation. The Higgs field contains the Higgs boson. In this simple image, we could visualize the particles as ripples of water propagating along the surface of the pond. It’s not a perfect picture, but it helps the imagination.
The most popular protagonist behind inflationary expansion is a scalar field – an entity with properties inspired by the Higgs boson, which was discovered at the Large Hadron Collider in July 2012.
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We don’t know if there were scalar fields in cosmic childhood, but it’s reasonable to assume that there were. Without them, we’d be horribly stuck trying to figure out what happened. As mentioned above, when we don’t have data, the best we can do is build reasonable hypotheses that hopefully future experiments will test.
To see how we use a scalar field to model inflation, imagine a ball rolling downhill. As long as the ball is at a height above the bottom of the hill, it rolls down. It has stored energy. At the bottom, you put your energy at zero. We do the same with the scalar field. As long as it is moved from its minimum, it will fill the Universe with its energy. In large enough regions, this energy causes the rapid expansion of space which is the signature of inflation.
Linde and Vilenkin added quantum physics to this picture. In the quantum world, everything is nervous; everything vibrates endlessly. This is the basis of quantum uncertainty, a notion that defies common sense. So as the field descends, it also undergoes these quantum leaps, which can propel it lower or higher. It is as if the waves in the pond are erratically creating ridges and valleys. Troubled waters, these quantum fields.
Here’s the twist: When a large enough region of space is filled with the field of a certain energy, it will expand at a rate related to that energy. Consider the water temperature in the pond. Different regions of space will have the field at different heights, just as different regions of the pond might have water at different temperatures. The result for cosmology is a plethora of wildly inflating regions of space, each expanding at its own pace. Very soon, the Universe would be made up of a myriad of swelling regions that grow, ignoring their environment. The universe is turning into a multiverse. Even within each region, quantum fluctuations can cause a subregion to bloat. The picture, then, is of an eternally reproducing cosmos, filled with bubbles within bubbles. Ours would be just one of them – a single bubble in a seething multiverse.
Is the multiverse testable?
It’s wildly inspiring. But is it scientific? To be scientific, a hypothesis must be verifiable. Can you test the multiverse? The answer, strictly speaking, is no. Each of these regions that swell — or contract, because there could also be failing universes — is outside our cosmic horizon, the region that delimits the distance traveled by light since the dawn of time. As such, we cannot see these cosmoids, nor receive signals from them. The best we can hope for is to find some sign that one of our neighboring universes has bruised our own space in the past. If that had happened, we would see specific patterns in the sky – more specifically, in the radiation left over after the hydrogen atoms formed some 400,000 years after the Big Bang. So far, no such signal has been found. The odds of finding one are, quite frankly, slim.
So we’re stuck with a plausible scientific idea that seems unverifiable. Even if we were to find evidence of inflation, that wouldn’t necessarily support the inflationary multiverse. What do we have to do?
Different types of different in the multiverse
The multiverse suggests another ingredient – the possibility that physics is different in different universes. Things get pretty nebulous here, as there are two “different” types to describe. The first is different values for the constants of nature (such as electronic charge or the force of gravity), while the second raises the possibility that there are different laws of nature.
In order to support life as we know it, our Universe must obey a series of very strict requirements. Small deviations are not tolerated in the values of the constants of nature. But the multiverse raises the question of naturalness, or how common our universe and its laws are among the myriad universes that belong to the multiverse. Are we the exception or do we follow the rule?
The problem is that we have no way of knowing. To know if we are common, we need to know something about other universes and the types of physics they have. But we don’t. We also don’t know how many universes there are, which makes it very difficult to estimate how common we are. To make matters worse, if there are an infinity of cosmoids, nothing can be said at all. Inductive thinking is useless here. The infinite entangles us in knots. When everything is possible, nothing stands out and nothing is learned.
This is why some physicists worry about the Multiverse to the point of hating it. There is nothing more important to science than its ability to prove ideas wrong. If we lose that, we undermine the very structure of the scientific method.