Physicist Sean Carroll has a new paper trying to drum up support for the multiverse, the groundless idea that there is some vast collection of universes outside of our universe. The paper is entitled “Beyond Falsifiability: Normal Science in a Multiverse.”
In his introduction, Carroll states this:
Multiverse models are scientific in an utterly conventional sense; they describe definite physical situations, and are ultimately judged on their ability to provide an explanation for data collected in observations and experiments. But the kind of science they are is perfectly ordinary science. The ways in which we evaluate the multiverse as a scientific hypothesis are precisely the ways in which hypotheses have always been judged.
None of this is true. Specifically:
- Multiverse theories are not actually models. In scientific terminology a model is a simplified conceptual representation of a complicated physical situation. For example, the Bohr model of the atom involved a simplified representation in which the complicated reality in an atom was depicted like a simple solar system, with the nucleus being like the sun and the electrons like the planet. Instead of imagining something simpler than the known reality, as in a model, a multiverse is imagining some reality almost infinitely more complicated than our known observable reality.
- Carroll's insinuation that something is scientific because it describes “definite physical situations” is wrong. If I imagine there are mile-high dragons on the far side of the moon, I have imagined a “definite physical situation,” but my fantasy is not scientific.
- Typical multiverse assertions do not at all assert “definite physical situations.” Instead of precise descriptions, we get thinking along the lines of “maybe there are countless other universes, each with a different set of characteristics.” Such airy fantasies are not cases of postulating a definite physical situation.
- Science can properly be defined as either (1) the body of facts that have been established by observations and experiments; or (2) the activity of scientists gathering observations, performing experiments, and interpreting such results. According to neither of these definitions is the concept of a multiverse science. And it is certainly laughable to call this very bizarre and non-scientific idea of the multiverse “perfectly ordinary science.”
- We can hardly show that the multiverse is science by arguing that “the ways in which we evaluate the multiverse as a scientific hypothesis are precisely the ways in which hypotheses have always been judged.” You could use the same logic to argue that palmistry and magic spells are science, on the grounds that the way such things are properly evaluated is precisely the ways in which hypotheses have always been judged.
- Multiverse theories do not actually explain anything, and it is not correct that they are “judged on their ability to provide an explanation for data collected in observations and experiments.”
Carroll gets into a discussion of whether falsifiability helps us distinguish between science and non-science. Falsifiability means when we can imagine observations that might disprove a theory. Carroll tries to suggest that we should discard falsifiability as an acid test for whether a theory is scientific.
It is true that it doesn't make sense to say that distinguishing between science and non-science is as simple as asking whether a theory is falsifiable. There are actually perfectly reasonable scientific theories that could never be falsified, such as the theory that life exists on some other planet. Falsifying such a theory would require observing all of the planets in the universe within some short time period, which is physically impossible because of the vast number of stars that exist, and the difficulty of traveling between them. But the fact that falsifiability is not suitable as a sole determinant of whether something is scientific does not mean that we should go “beyond falsifiability,” and pay no attention to falsifiability.
A reasonable program for judging whether an idea is scientific would be to have a kind of six-point system in which multiple criteria were considered. It might work like this:
- Your theory gets one point if there are possible observations that could verify the theory.
- Your theory gets one point if there are possible observations that could disprove the theory (meaning the theory is falsifiable).
- Your theory gets one point if there is substantial evidence suggesting the theory is true.
- Your theory gets one point if there is not substantial evidence that the theory is false.
- Your theory gets one point if it makes precise numerical predictions that can be tested.
- Your theory gets one point if it is simple and economical, and consistent with the long-standing scientific principle of Occam's Razor, that entities should not be multiplied beyond necessity.
This type of evaluation algorithm makes sense. It includes the idea of falsifiability, but only as one of multiple things that will be considered. We may note that according to such a reasonable evaluation algorithm, the idea of the multiverse scores only one point (as does the theory that there is somewhere a planet ruled by purple unicorns). The only one of these requirements met by the idea of the multiverse is number 4: there is no substantial evidence that there is not a multiverse. The multiverse fails all of the other evaluation criteria. It certainly fails requirement 1, as there are no observations that we could possibly have that could show there are other universes. Anything we might observe in our telescopes would be something part of our universe, not some other universe.
Conversely, a real scientific theory will score at least 3 points under such an evaluation system.
Most absurdly, Carroll attempts to persuade us that postulating a multiverse would be a case of “inference to the best explanation,” sometimes called abduction. He states the following:
Abduction lets you go the other way around: if you get sick, and there are no other obvious reasons why, you might conclude that you probably ate some spoiled food. This isn’t a logically necessary conclusion, but it’s the best explanation given the context. Science works along analogous lines....The multiverse, therefore, is a case of science as usual: we evaluate it on the basis of how likely it is to be true, given what we know on the basis of what we actually have observed.
This comparison is highly erroneous, and calling a non-scientific multiverse fantasy “science as usual” is preposterous. In the case of drawing a conclusion about a stomach pain, the numerical reasoning occurs like this:
- We consider the most common causes for a stomach pain – what percentage of the time it is caused by food, what percentage of the time it is caused by cancer, what percentage of the time it is caused by a kidney stone, etc.
- If one of these causes is more common than any of the other causes, we might deem that as the best explanation.
But no such reasoning can possibly can go on in regard to a multiverse. For we have no data at at all about anything caused by a multiverse, nor do we have any evidence that a multiverse exists. So there can be no “inference to the best explanation” comparable to what goes on in assuming that a stomach pain was probably caused by food. Inference is based on observations, and there have been no observations about a multiverse – no observation of other universes. It is wrong for Carroll to claim that a multiverse theory is evaluated “on the basis of how likely it is to be true, given what we know on the basis of what we actually have observed.” To the contrary, multiverse theories are speculations involving other universes very different from those we have observed.
There is a perfectly good term that has been used for centuries to describe speculations about that which is not observable. The term is metaphysics. Metaphysics is what is going on when you speculate about the nature of God, angels, life after death, and unobservable universes beyond our own. Multiverse speculations are properly classified as metaphysics. A multiverse speculation isn't science, which is properly defined as facts established by observations and experiments, or the activity of making observations and doing experiments that try to establish facts (or interpreting such observations and experiments). If you try to put a mask of science on the multiverse, such a ruse is best described as a masquerade. A multiverse speculation masquerading as “perfectly ordinary science” is as outrageous an impostor as some escaped convict masquerading as a doctor by putting on a white coat and a stethoscope.
Postscript: We had some more inane reasoning from the multiverse fantasists in this post by cosmologist Ethan Siegel. He reasons as follows:
Why must the Multiverse exist? Quite simply: there must be more Universe than the part that is observable to us.
This reasoning is silly. If there is more of the universe than we can observe, that simply tells us something about the size of our universe, not that there are other universes beyond our own.
On the same day I wrote this post, philosopher of science Massimo Pigliucci put up a post saying, "Indeed, at the moment, at least, the notion of a multiverse should be classed as scientifically-informed metaphysics." That's the same conclusion I reached.