Theoretical physicist Marcelo Gleiser raises the issue that the multiverse hypothesis suffers from the unscientific property of non-falsifiability. Embedded in his article is a solid acknowledgement of the fine-tuning of physical parameters for life to exist in our universe.
Today let’s take a walk on the wild side and assume, for the sake of argument, that our Universe is not the only one that exists. Let’s consider that there are many other universes, possibly infinitely many. The totality of these universes, including our own, is what cosmologists call the Multiverse. It sounds more like a myth than a scientific hypothesis, and this conceptual troublemaker inspires some while it outrages others.
How far can we push the theories of physics?
The controversy started in the 1980s. Two physicists, Andrei Linde at Stanford University and Alex Vilenkin at Tufts University, independently proposed that if the Universe underwent a very fast expansion early on in its existence — we call this an inflationary expansion — then our Universe would not be the only one.
When a sufficiently large region of space is filled with the field of a certain energy, it will expand at a rate related to that energy.
The result for cosmology is a plethora of madly inflating regions of space, each expanding at its own rate. Very quickly, the Universe would consist of myriad inflating regions that grow, unaware of their surroundings. The Universe morphs into a Multiverse. Even within each region, quantum fluctuations may drive a sub-region to inflate. The picture, then, is one of an eternally replicating cosmos, filled with bubbles within bubbles. Ours would be but one of them — a single bubble in a frothing Multiverse.
Is the multiverse testable?
This is wildly inspiring. But is it science? To be scientific, a hypothesis needs to be testable. Can you test the Multiverse? The answer, in a strict sense, is no. Each of these inflating regions — or contracting ones, as there could also be failed universes — is outside our cosmic horizon, the region that delimits how far light has traveled since the beginning of time. As such, we cannot see these cosmoids, nor receive any signals from them. The best that we can hope for is to find a sign that one of our neighboring universes bruised our own space in the past. If this had happened, we would see some specific patterns in the sky — more precisely, in the radiation left over after hydrogen atoms formed some 400,000 years after the Big Bang. So far, no such signal has been found. The chances of finding one are, quite frankly, remote.
We are thus stuck with a plausible scientific idea that seems untestable. Even if we were to find evidence for inflation, that would not necessarily support the inflationary Multiverse. What are we to do?
Different kinds of different in the multiverse
The Multiverse suggests another ingredient — the possibility that physics is different in different universes. Things get pretty nebulous here, because there are two kinds of “different” to describe. The first is different values for the constants of nature (such as the electron charge or the strength of gravity), while the second raises the possibility that there are different laws of nature altogether.
In order to harbor life as we know it, our Universe has to obey a series of very strict requirements. Small deviations are not tolerated in the values of nature’s constants. But the Multiverse brings forth the question of naturalness, or of how common our Universe and its laws are among the myriad universes belonging to the Multiverse. Are we the exception, or do we follow the rule?
The problem is that we have no way to tell. To know whether we are common, we need to know something about the other universes and the kinds of physics they have. But we don’t. Nor do we know how many universes there are, and this makes it very hard to estimate how common we are. To make things worse, if there are infinitely many cosmoids, we cannot say anything at all. Inductive thinking is useless here. Infinity gets us tangled up in knots. When everything is possible, nothing stands out, and nothing is learned.
That is why some physicists worry about the Multiverse to the point of loathing 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.
BigThink
If there is “nothing more important to science than its ability to prove ideas wrong,” is it fair to ask, “What is the means by which the theory of evolution could be proved wrong?”