Is the multiverse hypothesis a legitimate scientific theory? That is, are there regularities that illuminate and reflect underlying laws of nature by testing these laws and making predictions that can be either verified or refuted by experimentation and observation? Generally, these are the guidelines for something to be scientific, can it be verified and falsified? Before I continue, we need to make a distinction in two fundamental philosophies of science: instrumentalism and realism.
Instrumentalism: Scientific theories are not intended to be literally true and accepting a theory requires us to believe only that its observational consequences are true. Observation statements are literally true and science is only about these statements and the observations that verify them. A few strengths of this philosophy is that it doesn’t conflict with common sense realism; we can believe in straightforward observations. Plus, it’s more modest and non-commital than scientific realism. A few weaknesses are that scientists seem to assume the realist view of the world in their “un-thinking” moments. The instrumentalist should be able to draw a clear cut distinction between what is and what is not observable, which creates limitations on what really is observable (i.e. naked eye, magnifying glass, microscope, electron microscope, cloud chamber, etc.). This also raises the question, at what point is the objecting being observed really being observed, and so real, but then one bit smaller is not observable and thus not really existent?
Scientific Realism: Scientific theories are intended to be literally true, and accepting a theory involves believing that it gives a true description of reality, “as it really is.” A few strengths of this is that it makes the aspect of explanatory power superior to instrumentalism because explanation requires real things that cause the chain of causality. Explanation by means of fictitious entity is not explanation at all. Instrumentalism cannot explain the actual success of science, especially science’s making predictions, which are empirically adequate (i.e. Boyles-Charles Law, pv=k).
I’m going to argue that we should adopt the realist position partly because it is common sense and because it means and ends in explanation provide a robust sense of explanatory power that lacks instrumentalism and the metaphysical baggage it may carry is less deleterious than instrumentalism.
Before proceeding any further I need to define what I mean by multiverse. Multiverse isn’t monolithic. For simplicity’s sake, let’s go with Max Tegmark’s versions of the multiverse (Brian Greene makes a distinction of five levels but aren’t too different from Tegmark’s). A level one multiverse is, for the most part, more space beyond the observable universe. So, if we were to go to the “edge” of this universe, there would be more space. This isn’t controversial and is pretty much a settled issue (same landscape). A level two multiverse comes out of inflationary cosmology, which is typically associated with other bubble universe spawning from a cosmic landscape and slow-roll inflation. A level three multiverse is where it gets pretty weird. This is usually associated with the Many Worlds Interpretation of quantum mechanics and a rule of thumb, what can happen does happen. Consider a film of every state of affairs. A man asks a woman for a drink. In one scenario she says yes and in another she says no. Both happen. A level four multiverse is the cherry, it equates physical existence with mathematical existence–it’s the ultimate ensemble.
Let’s consider the issue of accessibility. Can we access another universe? For the level two multiverse there is room for interaction. There is more space between two Hubble volumes (the observable space in a closed system/bubble universe). If the rate at which the bubbles expand exceeds the rate at which the swelling of space that propels them to separate (keep in my inflationary cosmology), the bubbles will collide. If they’re really close then there will be so little intervening space that their rate of separation will be slower than their rate of expansion. Calculations show that if we had such a collision with another bubble universe then the impact would send shock waves rippling through space, generating modifications to the pattern of hot and cold regions in the microwave background radiation (see Brian Greene’s The Hidden Reality, 166-167). Consider the image below. This image is of the CMB and the key for bubble collision detection was using a specified algorithm for detecting temperature modulations that would occur in such events (See this paper for calculations).
This can also be tested at CERN. The Brane multiverse and level three mutliverse are striking similar. Brian Greene discusses how we can test the validity of Branes:
If we are living on one of these giant membranes, then the following can happen: When you slam particles together — which is what happens at the LHC — some debris from those collisions can be ejected off of our membrane and be ejected into the greater cosmos in which our membrane floats,” he says. “If that happens, that debris will take away some energy. So if we measure the amount of energy just before the protons collide and compare it with the amount of energy just after they collide, if there’s a little less after — and it’s less in just the right way — it would indicate that some had flown off, indicating that this membrane picture is correct.
Is this too far fetched? Is this special pleading? Well, consider the advances in science within the last century. We build incredibly sophisticated equipment and measure the undetectable on many levels, the question is at which point to we consider something undetectable or inaccessible? Space, time, and spacetime are the scaffolding for general and special relativity, yet you’ve probably never grasped spacetime in the same way you grasp a book. There are several historical instances of a theory’s success being used as an after-the-fact justification for its basic architecture even though that architecture remains beyond our ability to access it directly. Consider so many examples in quantum mechanics such as the cloud chamber. For a non quantum example, consider the problem of the cosmic horizon (the horizon problem). At the beginning of our universe everything existed in a tiny dense point. Inflation occurred and it propelled space faster than light. It slowed down and is now speeding back up. Objects, or for the sake of illustration, observers beyond the cosmic horizon can never interact with us and we can never interact with them. Should we then believe that the space beyond that point of horizon doesn’t exist because it is inaccessible to us? My point is that science is no stranger to theories that include elements from basic ingredients to derived consequences that are inaccessible.
The next question is obvious. Let’s backtrack to our philosophy of science, are all of our measurements merely depictions of a fundamental fiction or do they equate to reality? I would argue for reasons I’ve already suggested, and for theological reasons, that this is real. Theologically, philosophical realism is our best access to natural inquiry, from which we base a natural theology and revelation on. The concept of an exponentially large landscape of reality is rich with beauty and elegance. How does this not reflect the creativity and aesthetic beauty of a Creator?