Astrophysicists try to falsify multiverse, find they can’t.

Ben Carson, trying to
make sense of the multiverse.

The idea that we live in a multiverse – an infinite collection of universes from which ours is merely one – is interesting but unscientific. It postulates the existence of entities that are unnecessary to describe what we observe. All those other universes are inaccessible to experiment. Science, therefore, cannot say anything about their existence, neither whether they do exist nor whether they don’t exist.

The EAGLE collaboration now knows this too. They recently published results of a computer simulation that details how the formation of galaxies is affected when one changes the value of the cosmological constant, the constant which quantifies how fast the expansion of the universe accelerates. The idea is that, if you believe in the multiverse, then each simulation shows a different universe. And once you know which universes give rise to galaxies, you can calculate how likely we are to be in a universe that contains galaxies and also has the cosmological constant that we observe.

We already knew before the new EAGLE paper that not all values of the cosmological constant are compatible with our existence. If the cosmological constant is too large, the universe either collapses quickly after formation (if the constant is negative) and galaxies are never formed, or it expands so quickly that structures are torn apart before galaxies can form (if the constant is positive).

New is that by using computer simulations, the EAGLE collaboration is able to quantify and also illustrate just how the structure formation differs with the cosmological constant.

The quick summary of their results is that if you turn up the cosmological constant and keep all other physics the same, then making galaxies becomes difficult once the cosmological constant exceeds about 100 times the measured value. The authors haven’t looked at negative values of the cosmological constant because (so they write) that would be difficult to include in their code.

The below image from their simulation shows an example for the gas density. On the left you see a galaxy prototype in a universe with zero cosmological constant. On the right the cosmological constant is 30 times the measured value. In the right image structures are smaller because the gas halos have difficulties growing in a rapidly expanding universe.

From Figure 7 of Barnes et al, MNRAS 477, 3, 1 3727–3743 (2018).

This, however, is just turning knobs on computer code, so what does this have to do with the multiverse? Nothing really. But it’s fun to see how the authors are trying really hard to make sense of the multiverse business.

A particular headache for multiverse arguments, for example, is that if you want to speak about the probability of an observer finding themselves in a particular part of the multiverse, you have to specify what counts as observer. The EAGLE collaboration explains:

“We might wonder whether any complex life form counts as an observer (an ant?), or whether we need to see evidence of communication (a dolphin?), or active observation of the universe at large (an astronomer?). Our model does not contain anything as detailed as ants, dolphins or astronomers, so we are unable to make such a fine distinction anyway.”

But even after settling the question whether dolphins merit observer-status, a multiverse per se doesn’t allow you to calculate the probability for finding this or that universe. For this you need additional information: a probability distribution or “measure” on the multiverse. And this is where the real problem begins. If the probability of finding yourself in a universe like ours is small you may think that disfavors the multiverse hypothesis. But it doesn’t: It merely disfavors the probability distribution, not the multiverse itself.

The EAGLE collaboration elaborates on the conundrum:

“What would it mean to apply two different measures to this model, to derive two different predictions? How could all the physical facts be the same, and yet the predictions of the model be different in the two cases? What is the measure about, if not the universe? Is it just our own subjective opinion? In that case, you can save yourself all the bother of calculating probabilities by having an opinion about your multiverse model directly.”

Indeed. You can even save yourself the bother of having a multiverse to begin with because it doesn’t explain any observation that a single universe wouldn’t also explain.

The authors eventually find that some probability distributions make our universe more, others less probable. Not that you need a computer cluster for that insight. Still, I guess we should applaud the EAGLE people for trying. In their paper, they conclude: “A specific multiverse model must justify its measure on its own terms, since the freedom to choose a measure is simply the freedom to choose predictions ad hoc.”

But of course a model can never justify itself. The only way to justify a physical model is that it fits observation. And if you make ad hoc choices to fit observations you may as well just chose the cosmological constant to be what we observe and be done with it.

In summary, the paper finds that the multiverse hypothesis isn’t falsifiable. If you paid any attention to the multiverse debate, that’s hardly surprising, but it is interesting to see astrophysicists attempting to squeeze some science out of it.

I think the EAGLE study makes a useful contribution to the literature. Multiverse proponents have so far argued that what they do is science because some versions of the multiverse are testable in our universe, for example by searching for entanglement between universes, or for evidence that our universe has collided with another one in the past.

It is correct that some multiverse types are testable, but to the extent that they have been tested, they have been ruled out. This, of course, has not ruled out the multiverse per se, because there are still infinitely many types of multiverses left. For those, the only thing you can do is make probabilistic arguments. The EAGLE paper now highlights that these can’t be falsified either.

I hope that showcasing the practical problem, as the EAGLE paper does, will help clarify the unscientific basis of the multiverse hypothesis.

Let me be clear that the multiverse is a fringe idea in a small part of the physics community. Compared to the troubled scientific methodologies in some parts of particle physics and cosmology, multiverse madness is a minor pest. No, the major problem with the multiverse is its popularity outside of physics. Physicists from Brian Greene to Leonard Susskind to Andrei Linde have publicly spoken about the multiverse as if it was best scientific practice. And that well-known physicists pass the multiverse off as science isn’t merely annoying, it actively damages the reputation of science. A prominent example for the damage that can result comes from the 2015 Republican Presidential Candidate Ben Carson.

Carson is a retired neurosurgeon who doesn’t know much physics, but what he knows he seems to have learned from multiverse enthusiasts. On September 22, 2015, Carson gave a speech at a Baptist school in Ohio, informing his audience that “science is not always correct,” and then went on to justify his science skepticism by making fun of the multiverse:

“And then they go to the probability theory, and they say “but if there’s enough big bangs over a long enough period of time, one of them will be the perfect big bang and everything will be perfectly organized.””

In an earlier speech he cheerfully added: “I mean, you want to talk about fairy tales? This is amazing.”

Now, Carson has misunderstood much of elementary thermodynamics and cosmology, and I have no idea why he thinks he’s even qualified to give speeches about physics. But really this isn’t the point. I don’t expect neurosurgeons to be experts in the foundations of physics and I hope Carson’s audience doesn’t expect that either. Point is, he shows what happens when scientists mix fact with fiction: Non-experts throw out both together.

In his speech, Carson goes on: “I then say to them, look, I’m not going to criticize you. You have a lot more faith than I have… I give you credit for that. But I’m not going to denigrate you because of your faith and you shouldn’t denigrate me for mine.”

And I’m with him on that. No one should be denigrated for what they believe in. If you want to believe in the existence of infinitely many universes with infinitely many copies of yourself, that’s all fine with me. But please don’t pass it off as science.

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If you want to know more about the conflation between faith and knowledge in theoretical physics, read my book “Lost in Math: How Beauty Leads Physics Astray.”

Ask Dr B: Is the multiverse science? Is the multiverse real?

Kay zum Felde asked:

“Is the multiverse science? How can we test it?”

I added “Is the multiverse real” after Google offered it as autocomplete:

Dear Kay,

This is a timely question, one that has been much on my mind in the last years. Some influential theoretical physicists – like Brian Greene, Lenny Susskind, Sean Carroll, and Max Tegmark – argue that the appearance of multiverses in various contemporary theories signals that we have entered a new era of science. This idea however has been met with fierce opposition by others – like George Ellis, Joe Silk, Paul Steinhardt, and Paul Davies – who criticize the lack of testability.

If the multiverse idea is right, and we live in one of many – maybe infinitely many – different universes, then some of our fundamental questions about nature might never be answered with certainty. We might merely be able to make statements about how likely we are to inhabit a universe with some particular laws of nature. Or maybe we cannot even calculate this probability, but just have to accept that some things are as they are, with no possibility to find deeper answers.

What bugs the multiverse opponents most about this explanation – or rather lack of explanation – is that succumbing to the multiverse paradigm feels like admitting defeat in our quest for understanding nature. They seem to be afraid that merely considering the multiverse an option discourages further inquiries, inquiries that might lead to better answers.

I think the multiverse isn’t remotely as radical an idea as it has been portrayed, and that some aspects of it might turn out to be useful. But before I go on, let me first clarify what we are talking about.

What is the multiverse?

The multiverse is a collection of universes, one of which is ours. The other universes might be very different from the one we find ourselves in. There are various types of multiverses that theoretical physicists believe are logical consequences of their theories. The best known ones are:

• The string theory landscape
  String theory doesn’t uniquely predict which particles, fields, and parameters a universe contains. If one believes that string theory is the final theory, and there is nothing more to say than that, then we have no way to explain why we observe one particular universe. To make the final theory claim consistent with the lack of predictability, one therefore has to accept that any possible universe has the same right to existence as ours. Consequently, we live in a multiverse.
  
  
• Eternal inflation
  In some currently very popular models for the early universe our universe is just a small patch of a larger space. As result of a quantum fluctuation the initially rapid expansion – known as “inflation” – slows down in the region around us and galaxies can be formed. But outside our universe inflation continues, and randomly occurring quantum fluctuations go on to spawn off other universes – eternally. If one believes that this theory is correct and that we understand how the quantum vacuum couples to gravity, then, so the argument, the other universes are equally real as ours.
  
  
• Many worlds interpretation
  In the Copenhagen interpretation of quantum mechanics the act of measurement is ad hoc. It is simply postulated that measurement “collapses” the wave-function from a state with quantum properties (such as being in two places at once) to a distinct state (at only one place). This postulate agrees with all observations, but it is regarded unappealing by many (including myself). One way to avoid this postulate is to instead posit that the wave-function never collapses. Instead it ‘branches’ into different universes, one for each possible measurement outcome – a whole multiverse of measurement outcomes.
  
  
• The Mathematical Universe
  The Mathematical Universe is Max Tegmark’s brain child, in which he takes the final theory claim to its extreme. Any theory that describes only our universe requires the selection of some mathematics among all possible mathematics. But if a theory is a final theory, there is no way to justify any particular selection, because any selection would require another theory to explain it. And so, the only final theory there can be is one in which all mathematics exists somewhere in the multiverse.

This list might raise the impression that the multiverse is a new finding, but that isn’t so. New is only the interpretation. Since every theory requires observational input to fix parameters or pick axioms, every theory leads to a multiverse. Without sufficient observational input any theory becomes ambiguous – it gives rise to a multiverse.

Take Newtonian gravity: Is there a universe for each value of Newton’s constant? Or General Relativity: Do all solutions to the field equations exist? And Loop Quantum Gravity has multiverses with different parameters for an infinite number of solutions like string theory. It’s just that Loop Quantum Gravity never tried to be a theory of everything, so nobody worries about this.

What is new about the multiverse idea is that some physicists are no longer content with having a theory that describes observation. They now have additional requirements for a good theory, like for example that the theory have no ad hoc prescriptions like collapsing wavefunctions; no small, large, or in fact any numbers; or initial conditions that are likely according to some currently accepted probability distribution.

Is the multiverse science?

Science is what describes our observations of nature. But this is the goal and not necessarily the case for each step along the way. And so, taking multiverses seriously, rather than treating them as the mathematical artifact that I think they are, might eventually lead to new insights. The real controversy about the multiverses is how likely it is that new insights will emerge from this approach eventually.

The maybe best example for how multiverses might become scientific is eternal inflation. It has been argued that the different universes might not be entirely disconnected, but can collide, thereby leaving observable signatures in the cosmic microwave background. Another example for testability comes from Mersini-Houghton and Holman who have looked into potentially observable consequences of entanglement between different universes. And in a rather mindbending recent work, Garriga, Vilenkin and Zhang, have argued that the multiverse might give rise to a distribution of small black holes in our universe which also has consequences that could become observable in the future.

As to probability distributions on the string theory landscape, I don’t see any conceptual problem with that. If someone could, based on a few assumptions, come up with a probability measure according to which the universe we observe is the most likely one, that would for me be a valid computation of the standard model parameters. The problem is of course to come up with such a measure.

Similar things could be said about all other multiverses. They don’t presently seem very useful to describe nature. But pursuing the idea might eventually give rise to observable consequences and further insights.

We have known since the dawn of quantum mechanics that it’s wrong to require all mathematical structures of a theory to directly correspond to observables – wave-functions are the best counter example. How willing physicists are to accept non-observable ingredients of a theory as necessary depends on their trust in the theory and on their hope that it might give rise to deeper insights. But there isn’t a priori anything unscientific with a theory that contains elements that are unobservable.

So is the multiverse science? It is an extreme speculation, and opinions widely differ on how promising it is as a route is to deeper understanding. But speculations are a normal part of theory development, and the multiverse is scientific as long as physicists strive to eventually derive observable consequences.

Is the multiverse real?

The multiverse has some brain-bursting consequences. For example that everything that can happen does happen, and it happens an infinite amount of times. There are thus infinitely many copies of you, somewhere out there, doing their own thing, or doing exactly the same as you. What does that mean? I have no clue. But it makes for an interesting dinner conversation through the second bottle of wine.

Is it real? I think it’s a mistake to think of “being real” as a binary variable, a property that an object either has or has not. Reality has many different layers, and how real we perceive something depends on how immediate our inference of the object from sensory input is.

A dog peeing on your leg has a very simple and direct relation to your sensory input that does not require much decoding. You would almost certainly consider it real. On the contrary, evidence for the quark model contained in a large array of data on a screen is a very indirect sensory input that requires a great deal of decoding. How real you consider quarks thus depends on your knowledge of, and trust in, the theory and the data. Or trust in the scientists dealing with the theory and the data as it were. For most physicists the theory underlying the quark model has proved reliable and accurate to such high precision that they consider quarks as real as the peeing dog.

But the longer the chain of inference, and the less trust you have in the theories used for inference, the less real objects become. In this layered reality the multiverse is currently at the outer fringes. It’s as unreal as something can be without being plain fantasy. For some practitioners who greatly trust their theories, the multiverse might appear almost as real as the universe we observe. But for most of us these theories are wild speculations and consequently we have little trust in this inference.

So is the multiverse real? It is “less real” than everything else physicists have deduced from their theories – so far.

More Multiverse Madness

The “multiverse” – the idea that our universe is only one of infinitely many – enjoys some credibility, at least in the weirder corners of theoretical physics. But there are good reasons to be skeptical, and I’m here to tell you all of them.

Before we get started, let us be clear what we are talking about because there isn’t only one but multiple multiverses. The most commonly discussed ones are: (a) The many worlds interpretation of quantum mechanics, (b) eternal inflation, and (c) the string theory landscape.

The many world’s interpretation is, guess what, an interpretation. At least to date, it makes no predictions that differ from other interpretations of quantum mechanics. So it’s up to you whether you believe it. And that’s all I have to say about this.

Eternal inflation is an extrapolation of inflation, which is an extrapolation of the concordance model, which is an extrapolation of the present-day universe back in time. Eternal inflation, like inflation, works by inventing a new field (the “inflaton”) that no one has ever seen because we are told it vanished long ago. Eternal inflation is a story about the quantum fluctuations of the now-vanished field and what these fluctuations did to gravity, which no one really knows, but that’s the game.

There is little evidence for inflation, and zero evidence for eternal inflation. But there is a huge number of models for both because available data don’t constraint the models much. Consequently, theorists theorize the hell out of it. And the more papers they write about it, the more credible the whole thing looks.

And then there’s the string theory landscape, the graveyard of disappointed hopes. It’s what you get if you refuse to accept that string theory does not predict which particles we observe.

String theorists originally hoped that their theory would explain everything. When it became clear that didn’t work, some string theorists declared if they can’t do it then it’s not possible, hence everything that string theory allows must exist – and there’s your multiverse. But you could do the same thing with any other theory if you don’t draw on sufficient observational input to define a concrete model. The landscape, therefore, isn’t so much a prediction of string theory as a consequence of string theorists’ insistence that theirs a theory of everything.

Why then, does anyone take the multiverse seriously? Multiverse proponents usually offer the following four arguments in favor of the idea:

1. It’s falsifiable!

Our Bubble Universe.
Img: NASA/WMAP.

There are certain cases in which some version of the multiverse leads to observable predictions. The most commonly named example is that our universe could have collided with another one in the past, which could have left an imprint in the cosmic microwave background. There is no evidence for this, but of course this doesn’t rule out the multiverse. It just means we are unlikely to live in this particular version of the multiverse.

But (as I explained here) just because a theory makes falsifiable predictions doesn’t mean it’s scientific. A scientific theory should at least have a plausible chance of being correct. If there are infinitely many ways to fudge a theory so that the alleged prediction is no more, that’s not scientific. This malleability is a problem already with inflation, and extrapolating this to eternal inflation only makes things worse. Lumping the string landscape and/or many worlds on top of doesn’t help parsimony either.

So don’t get fooled by this argument, it’s just wrong.

2. Ok, so it’s not falsifiable, but it’s sound logic!

Step two is the claim that the multiverse is a logical consequence of well-established theories. But science isn’t math. And even if you trust the math, no deduction is better than the assumptions you started from and neither string theory nor inflation are well-established. (If you think they are you’ve been reading the wrong blogs.)

I would agree that inflation is a good effective model, but so is approximating the human body as a bag of water, and see how far that gets you making sense of the evening news.

But the problem with the claim that logic suffices to deduce what’s real runs deeper than personal attachment to pretty ideas. The much bigger problem which looms here is that scientists mistake the purpose of science. This can nicely be demonstrated by a phrase in Sean Carroll’s recent paper. In defense of the multiverse he writes “Science is about what is true.” But, no, it’s not. Science is about describing what we observe. Science is about what is useful. Mathematics is about what is true.

Fact is, the multiverse extrapolates known physics by at least 13 orders of magnitude (in energy) beyond what we have tested and then adds unproved assumptions, like strings and inflatons. That’s not science, that’s math fiction.

So don’t buy it. Just because they can calculate something doesn’t mean they describe nature.

3. Ok, then. So it’s neither falsifiable nor sound logic, but it’s still business as usual.

The gist of this argument, also represented in Sean Carroll’s recent paper, is that we can assess the multiverse hypothesis just like any other hypothesis, by using Bayesian inference.

Bayesian inference a way of probability assessment in which you update your information to arrive at what’s the most likely hypothesis. Eg, suppose you want to know how many people on this planet have curly hair. For starters you would estimate it’s probably less than the total world-population. Next, you might assign equal probability to all possible percentages to quantify your lack of knowledge. This is called a “prior.”

You would then probably think of people you know and give a lower probability for very large or very small percentages. After that, you could go and look at photos of people from different countries and count the curly-haired fraction, scale this up by population, and update your estimate. In the end you would get reasonably accurate numbers.

If you replace words with equations, that’s how Bayesian inference works.

You can do pretty much the same for the cosmological constant. Make some guess for the prior, take into account observational constraints, and you will get some estimate for a likely value. Indeed, that’s what Steven Weinberg famously did, and he ended up with a result that wasn’t too badly wrong. Awesome.

But just because you can do Bayesian inference doesn’t mean there must be a planet Earth for each fraction of curly-haired people. You don’t need all these different Earths because in a Bayesian assessment the probability represents your state of knowledge, not the distribution of an actual ensemble. Likewise, you don’t need a multiverse to update the likelihood of parameters when taking into account observations.

So to the extent that it’s science as usual you don’t need the multiverse.

4. So what? We’ll do it anyway.

The fourth, and usually final, line of defense is that if we just assume the multiverse exists, we might learn something, and that could lead to new insights. It’s the good, old Gospel of Serendipity.

In practice this means that multiverse proponents insist on interpreting probabilities for parameters as those of an actual ensemble of universes, ie the multiverse. Then they have the problem of where to get the probability distribution from, a thorny issue since the ensemble is infinitely large. This is known as the “measure problem” of the multiverse.

To solve the problem, they have to construct a probability distribution, which means they must invent a meta-theory for the landscape. Of course that’s just another turtle in the tower and will not help finding a theory of everything. And worse, since there are infinitely many such distributions you better hope they’ll find one that doesn’t need more assumptions than the standard model already has, because if that was so, the multiverse would be shaved off by Occam’s razor.

But let us assume the best possible outcome, that they find a measure for the multiverse according to which the parameters of the standard model are likely, and this measure indeed needs fewer assumptions than just postulating the standard model parameters. That would be pretty cool and I would be duly impressed. But even in this case we don’t need the multiverse! All we need is the equation to calculate what’s presumably a maximum of a probability distribution. Thus, again, Occam’s razor should remove the multiverse.

You could then of course insist that the multiverse is a possible interpretation, so you are allowed to believe in it. And that’s all fine by me. Believe whatever you want, but don’t confuse it with science.

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The multiverse and other wild things that physicists believe in are subject of my upcoming book “Lost in Math” which is now available for preorder.

Multiversing around

multiverse [noun]

From L. multus “much, many” and L. versare “to busy oneself,” lit. “to turn to.” Tech: A hypothetical collection of different variants of -> universes. Colloq: A large collection of no apparent purpose. Expl: “She has a whole multiverse of shoes,” “His essay received a multiverse of comments.”

I am considering to consider to read Brian Greene’s new book The Hidden Reality on the multiverse. On the pro side there’s a likely readable synopsis of an interesting topic. On the con side there’s two of Greene’s books in my shelf that I never finished reading. A writing therapy exercise I thought might be useful.

First, let’s get over with the terminology. Yes, multiverse is a disingenuous nomenclature. If the universe is by definition all that exists, then anything to the multiverse more than the universe does by the same definition not exist. But it’s moot to complain about terminology that has already become common use. What exactly the multiverse is depends on the context, but in either case it’s something that exists in addition to what the presently most widely accepted theories let physicists expect to observe. Some theories seem to imply the existence of “more,” of a multiverse of “more,” and that in other ways than “more of the same.”

Multi versus uni

The central question is what does it mean “to exist?” As a particle physicist I’d say something that can’t be observed doesn’t exist. (Observation doesn’t necessarily mean a direct interaction.) Talking about the “existence” of something that can’t be observed opens the door to fairy tales. Though my invisible friend disapproves, from a scientific point of view I am interested in the multiverse only if it’s observable. And even then my interest is very limited since I find the presently discussed possibilities of observation remote and implausible. But yes, there are versions of the multiverse that may have observable consequences. Eg. we recently discussed a paper on signatures of bubble collisions in eternal inflation, one possible multiverse scenario, and there’s Laura Mersisni’s superhorizon entanglement giving rise to the giant void, and related stories.

For the more entertaining part I’ll now take off my physicist’s hat (okay, it’s an Einstein wig really) and put on my hobby philosopher hat (if you really want to know, it’s actually a tea pot lid).

multiversal [adj]

From noun -> multiverse. Colloq: Of confusing variety. Expl: “By the year 2010, social networking had become multiversal,” “The promises during the election campaign were multiversal.”

A lot of effort has been spent on the search for a “Theory of Everything.” Commonly meant to be a theory unifying General Relativity with the Standard Model of particle physics, it is another misnomer in common use: It is unlikely that a reductionist approach will ever be able to actually explain everything, not in practice and maybe not even in theory. I will however refer here to a TOE in the more general sense as a theory that leaves us with no “Why” questions and reduces all of science to a question of “How” and, knocking on the teapot lid, I’ll refrain from pointing out that we can never know if we’ve found it.

What may such a TOE look like? None of the currently pursued approaches to grand unification or quantum gravity comes even close. Even if string theory or something similar would allow us to compute all the parameters in the Standard Model and in the ΛCDM model, and so on, Nobelprizes would be handed out for certain, but it would just move the Why’s elsewhere, for all these theories have other unexplained assumptions: Why are strings/ loops/ E8/ networks fundamental? Why causality? Why these initial conditions? Why quantization? Why a semi-classical or classical limit? Why matter? Why in fact anything instead of nothing?

For this TOE we cannot use an assumption that constrains the theory to reproduce observation. The only guidance eventually left is mathematical consistency. Most occurrences of the multiverse actually still have additional assumptions, but already the problem is the same: too many possibilities. If you don’t want to settle for a “just because,” a question without answer, an unexplained final cause, you have to swallow that all that can exist, according to current theories, does exist. That doesn’t answer the question, but it removes the need for an answer. That is, in a nutshell, the reason for the recurrence of the multiverse in various branches of theoretical physics: Mathematical consistency just isn’t enough.

(A probabilistic approach for the multiverse with the assumption that our universe is one of the common ones, trying to derive some features of our universe at least as probable, is a reentry into the question-room through the backdoor. It just rephrases the question why our universe is special and what theory allows us to derive the details, to the question why our universe is especially unspecial and what measure allows us to derive the details, and it makes additional assumptions about how to compute probabilities rspt. about the logic used etc. I’m not dismissing the attempts to define a probability measure on the multiverse as useless since sometimes looking at an old problem from a new direction is fruitful. But the attempt in itself isn’t actually progress.)

Mathematical consistency is not a strong requirement. The complex plane and holomorphic functions on it for example are mathematically consistent (unless you insist on some wrong theorem), so is linear algebra on n-dimensional vector spaces. What sort of a universe is that, you might ask. But if mathematical consistency is all that you’re left with, that’s what you get: Everything that’s mathematically consistent “exists” in the same sense as the world around us, a notion of “existence” not in agreement with that put forward by the strange person with the funny wig. This thought then brings us straight to Tegmark’s Mathematical Universe: All of mathematics is real, and all that’s real is mathematics. There is no distinction because there’s no other meaning to “existence.”

My problem with the Mathematical Universe is not that I dislike the idea of being made of math (whatever that might mean). In fact, I quite like the idea (up to a face factor). My problem is that for all I can tell it’s not of use for anything (oops, lid slipped off) and it is based on an assumption I don’t find particularly plausible: That humans in the 21st century have already found the language to describe the fundamental nature of reality.

Formal mathematics is a quite recent achievement in mankind’s evolution. Sure, its precision and usefulness in the description of nature is vastly superior to that of, say, the English language. But 50,000 years ago our ancestors have thought of their precise spoken language as the ultimate tool to describe nature, vastly superior to grunting and waving with paws. So how sure really can we be mathematics is so intimately connected to nature that nature is mathematics?

Versus multi

Now let us turn the argument around. Searching for a TOE we were ultimately left with mathematical consistency as only guidance and it’s not enough of a constraint. It offers too many possibilities and eventually doesn’t explain anything. Unless, that is, mathematical consistency is not the only requirement. (There is of course the requirement to reproduce observation, but that’s too pragmatic for my tea pot.) The only way to avoid a multiverse then seems to be that mathematics is not sufficient to describe the fundamental nature of reality.

So the options are: a) Accept a final cause. b) Accept the multiverse. c) Accept that there’s a way to describe nature better than with mathematics.

If you don’t like a) and b) and therefore have to sympathize with c) you are however left wondering what may describe nature even better than mathematics? Well, you can. Tegmark’s Mathematical Universe irks people because they believe there is a distinction between reality and mathematics, between platonic ideas and the world out there. The common point of view is that the math used in theoretical physics is a description of nature, but humans provide the map between reality and the math. It is possible that this mapping is itself a purely mathematical process. I can imagine there to be an algorithm that searches for mathematical definitions whose properties fit to observed data. Yet presently there is no answer to the question whether there is in fact such an algorithm able to do science like a human. Gödel’s incompleteness theorem is happily waving its tail, waiting for a chance to pee on your leg.

In summary this means if there is neither a final cause nor a multiverse, there likely won’t be any Singularity in 2045 either since no computer algorithm will be able to go beyond math. And vice versa, if a computer algorithm, coded in the language of math, is able to map every aspect of reality to a mathematical structure, then you’re likely stuck with the multiverse, Tegmarkian version, subsuming all other versions. It might then just be that the next revolution in physics comes from neuroscience.

multiverse [verb]

From noun -> multiverse. From L. multus “much, many” and versus, pp. of vertere “to turn.” To make many turns. Colloq: To act or talk incoherently. “She spent the afternoon multiversing around,” “His job interview was a disaster; he totally multiversed it.”

I’ll finish with a quotation from a wise physicist, who wants to remain unnamed but reportedly reads this blog: “The multiverse, the simulation hypothesis, modal realism, or the Singularity –it’s all the same nonsense, really.”

Why the multiverse is religion, not science.

This is the 5th and last part in my series to explain why the multiverse is not a scientific hypothesis. The other parts are: 1. Does the Higgs-boson exist? 2. Do I exist? 3. Does God exist? and 4. The multiverse hypothesis.

I put together these videos because I am frustrated that scientists discard the issue unthinkingly. This is not a polemical argument and it’s not meant as an insult. But believing in the multiverse is logically equivalent to believing in god, therefore it’s religion, not science.

To see why, let me pull together what I laid out in my previous videos. Scientists say that something exists if it is useful to describe observations. By “useful” I mean it is simpler than just collecting data. You can postulate the existence of things that are not useful to describe observations, such as gods, but this is no longer science.

Universes besides our own are logically equivalent to gods. They are unobservable by assumption, hence they can exist only in a religious sense. You can believe in them if you want to, but they are not part of science.

I know that this is not a particularly remarkable argument. But physicists seem to have a hard time following it, especially those who happen to work on the multiverse. Therefore, let me sort out some common misunderstandings.

First. The major misunderstanding is that I am saying the multiverse does not exist. But this is not what I am saying. I am saying science does not tell us anything about universes we cannot observe, therefore claiming they exist is not science.

Second. They will argue the multiverse is simple. Most physicists who are in favor of the multiverse say it’s scientific because it’s simpler to assume that all universes of a certain type exist than it is to assume that only one of them exist.

That’s a questionable claim. But more importantly, it’s beside the point. The simplest assumption is no assumption. And you do not need to make any statement about the existence of the multiverse to explain our observations. Therefore, science says, you should not. As I said, it’s the same with the multiverse as with god. It’s an unnecessary assumption. Not wrong, but superfluous.

You also do not need to postulate the existence of our universe, of course. No scientist ever does that. That would be totally ridiculous.

Third. They’ll claim the existence of the multiverse is a prediction of their theory.

It’s not. That’s just wrong. Just because you can write down a theory for something, doesn’t mean it exists*. We determine that something exists, in the scientific sense, if it is useful to describe observation. That’s exactly what the multiverse is not.

Fourth. But then you are saying that discussing what’s inside a black hole is also not science

That’s equally wrong. Other universes are not science because you cannot observe them. But you can totally observe what’s inside a black hole. You just cannot come back and tell us about it. Besides, no one really thinks that the inside of a black hole will remain inaccessible forever. For these reasons, the situation is entirely different for black holes. If it was correct that the inside of black holes cannot be observed, this would indeed mean that postulating its existence is not scientific.

Fifth. But there are types of multiverses that have observable consequences.

That’s right. Physicists have come up with certain types of multiverses that can be falsified. The problem with these ideas is conceptually entirely different. It’s that there is no reason to think we live in such multiverses to begin with. The requirement that a hypothesis must be falsifiable is certainly necessary to make the hypothesis scientific, but not sufficient. I previously explained this here.

To sum it up. The multiverse is certainly an interesting idea and it attracts a lot of public attention. There is nothing wrong with that in principle. Entertainment has a value and so has thought-stimulating discussion. But do not confuse the multiverse with science, because it is not.



* Revised this sentence after two readers misunderstood the previous version.

Update: The video now has German and Italian subtitles. To see those, click on "CC" in the YouTube toolbar. Choose language under settings/gear icon.

The Multiverse: Science, Religion, or Pseudoscience?

[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]



Why do physicists believe there are universes besides our own? I get a lot of questions about the idea that we live in this “multiverse”. Is it science, religion, pseudoscience, or just wrong? That’s what we’ll talk about today.

The topic of this video is covered in more detail in my new book Existential Physics. 

First things first, what’s a multiverse? You may guess that’s a new form of poetry, and you wouldn’t be entirely wrong. A multiverse is a collection of universes, either infinitely many or a number so large no one’s even bothered giving it a name. It’s an idea that has sprung up in some esoteric corners of theoretical physics and has, not so surprisingly, caught the imagination of science fiction authors, script-writers, and also the public. And it is poetic somehow, isn’t it, all those universes out there.

There isn’t just one multiverse but several different ones, so multiple multiverses, if you wish. The multiverse shouldn’t be confused with the metaverse, which is what universes evolve into when they’ve been fed enough Zuckerberg candy.

How many different multiverses do we have? Well, Brian Greene has written a book in which he lists 9 different ones, but you know how scientists are, the moment the book came out they jumped up to complain about what wasn’t on his list. And I can totally understand that. I mean, everyone knows that a list needs ten items. Nine is just not right. So let me just briefly run through the three types of multiverse that you most often hear about.

1. Many Worlds

The probably best-known and least controversial type of multiverse is the many worlds interpretation of quantum mechanics. If you remember, in quantum mechanics we can make predictions only for probabilities. We can say, for example, a particle goes left or right, each with a 50 percent chance. But then, when we measure the particle, we find it either left or right, and then we know where it is with 100 percent confidence. So, when we have measured the particle, what happened with the other possible outcome?

In the most common interpretation of quantum mechanics, often called the Copenhagen Interpretation, the moment you make a measurement you just update your probabilities because you got new information. The possibilities which you didn’t observe disappear because now you know they didn’t happen. This is called the measurement update, or sometimes the reduction or collapse of the wave-function.

In the many worlds interpretation, in contrast, one postulates that all possible outcomes of an experiment happen, each in a separate universe. It’s just that we live in only one of those universes and never see the other outcomes.

Of course then you have to explain why we don’t spread over all universes like the outcomes of experiments do. Mathematically, this works the same way as the sudden update of the wave-function. This means for what observations are concerned, many worlds is identical to standard quantum mechanics. The difference is what you believe it means.

If you believe in the many worlds interpretation, then every time a quantum object is measured, the universe splits into as many different universes as there were possible outcomes of the measurement. And this doesn’t just happen in laboratories. A measurement in quantum mechanics doesn’t require an apparatus. Anything that’s large enough can cause a “measurement”, that may be Geiger counter, but also banana, or, well you. This means that measurements happens all the time and everywhere. They constantly create new universes, and more are being created as we speak. Which means more bananas! And more yous!

For example, each time the wave-function of a photon spreads into all directions, but then it hits your eye, and the universe splits. In some, the photon arrived in your eye. In others, it hit the wall next to you, in some it went right through your head. And this could be happening to all photons. So, in some universes, an elephant is standing in front of you and you don’t see it. It’s unlikely, but, well, it’s possible, and according to the many worlds interpretation anything that’s possible is also real. I hope you make friends with the invisible elephant. I think that would be nice.

2. Eternal Inflation

We don’t know how our universe began and maybe we will never know. We just talked about this the other week. But according to a presently popular idea called “inflation”, our universe was created from a quantum fluctuation of a field called the “inflaton”. This field supposedly fills an infinitely large space and our universe was created from only a tiny patch of that, the patch where the fluctuation happened.

But the field keeps on fluctuating, so there are infinitely many other universes fluctuating into existence. This universe-creation goes on forever, which is why it’s called eternal inflation. Eternal inflation, by the way lasts forever into the future, but still requires a beginning in the past, so it doesn’t do away with the Big Bang issue.

In Eternal Inflation, the other universes may contain the same matter as ours, but in slightly different arrangements, so there may be copies of you in them. In some versions you became a professional ballet dancer. In some you won a Nobel Prize. In yet another one another you are a professional ballet dancer who won a Nobel prize and dated Elon Musk. And they’re all as real as this one.

Where did this inflaton field go that allegedly created our universe? Well, physicists say it has fallen apart into the particles that we observe now, so it’s gone and that’s why we can’t measure it. Yeah, that is a little sketchy.

3. The String Theory Landscape

String theory is an approach to a unification of gravity with the other forces of nature. Or maybe I should say it was, because it’s rapidly declined in popularity in the past decade. Why? It just didn’t lead anywhere.

String theorists originally hoped that one day it’d be possible to use their theory to calculate the values of the constants of nature, such as the masses of elementary particles and the strength by which they interact and so on. This didn’t work, so they gave up and just postulated that any value is possible. And since they couldn’t explain why we only observe a specific set of values they declared that they all exist.

And so this gives you another version of the multiverse. This collection of universes with all possible values for the constants of nature is called the string theory landscape. It contains universes with different types of matter or that have other laws of nature. For example, in some of them gravity is much weaker than it is in our universe. In some, radioactive decay happens much faster. And some universes expand so quickly that stars can’t form. If you believe in the string theory landscape, this isn’t just theoretically possible, it all actually happens.

You can combine these multiverses in any way you wish. So you can get married to Elon Musk hopping around at half the strength of gravity, with elephants in the room which you coincidentally can’t see. If you believe in the multiverse, then you have to believe this is possible.

There are some other multiverses which I didn’t talk about, like Max Tegmark’s mathematical universe in which all mathematics supposedly exists, or the simulation hypothesis, according to which our universe is a computer simulation. Because if you can simulate our laws of nature, why not simulate some others too? I don’t want to go through all the different multiverses because they all have the same problem.

The issue with all those different multiverses is that they postulate the existence of something you can’t observe, which is those other universes. Not only can you not see them, you can’t interact with them in any way. They are entirely disconnected from ours. There is no possible observation that you could make to infer their presence, not even in principle.

For this reason, postulating that the other universes exist is unnecessary to explain what we do observe, and therefore something that a scientist shouldn’t do. Making an unnecessary assumption is logically equivalent to postulating the existence of an unobservable god, or a flying spaghetti monster, or an omniscient dwarf who lives in your wardrobe. Fine if you do it in private, not so fine if you publish papers about it.

But. This does not mean that other universes do not exist. It merely means that science doesn’t say anything about whether or not they exist. If you postulate that they do not exist, that’s also unnecessary to explain what we observe, and therefore equally unscientific.

So now what, is the multiverse unscientific or pseudoscience or religion? Well, depends on what you do with it.

If you assume that unobservable universes exist and write papers about them, then that’s pseudoscience. Because this is exactly what we mean by pseudoscience: pretends to be science but isn’t. If you accept that science doesn’t say anything about the existence of those other universes one way or another, and you just decide to believe in them, then that’s religion. Either way, multiverses are not science. They’re like Tinker Bell, basically, they exist if you believe in them.

You might find this whole multiverse idea rather silly. And I wouldn’t blame you. But some physicists are quite serious about it. They believe these other universes exist because they show up in their mathematics. You see, they have mathematics, and some of that describes what we observe. And then they claim therefore everything else that their mathematics describes must also exist. They are confusing mathematics with reality.

There are some standard “objections” that physicists always try on me. You have probably heard some of them too, so here’s how you can deal with them.

Objection 1: Black Holes

The first point that multiverse fans always bring up is that we say that the inside of a black hole exists, even though we can’t observe it. But that’s just wrong: You can observe the inside of a black hole, you just can’t come back to tell us what you observed. Besides, we know that black holes evaporate, so they eventually reveal their inside.

Objection 2: Cosmic Horizon

Second objection that I hear is that we can only observe a patch of our own universe because light needs time to travel, and it’s got only so far since the Big Bang. But certainly no one would say that therefore the universe stops existing outside of the part we can observe. No of course not. No one says if you can’t observe it, it doesn’t exist. The point is: if you can’t observe it, science says nothing about whether it exists or not.

Objection 3: Observable Multiverses

The third standard objection is that some physicists have tried to come up with cases in which the presence of other universes would be observable. For example, there has been the idea that another universe could have collided with ours in the past, leaving a specific pattern in the cosmic microwave background. Or our universe could have been entangled with another one. So, the nobel prize winning ballet dancer isn’t married to this Elon Musk but has a quantum connection to an Elon Musk in another universe. Again this would leave a specific pattern in the CMB.

The answer to this objection is that people have looked for these patterns in the CMB and they are just not there. But to be fair, the testable multiverse models are a different problem than the one I named above. The big problem with multiverse ideas is that physicists mistake mathematics for reality. The problem with the testable multiverse ideas is that they think just because a hypothesis is testable it is also scientific. This is not what Popper meant. He said if it isn’t testable it isn’t science. Not “if it’s testable, then it’s science”.

Objection 4: It’s simple

The fourth and final objection is that the multiverse is good because it’s a simple theory. You see, multiverse fans argue that if you don’t make assumptions about what the values of the constants of nature are, but just say “they all exist,” then you have fewer assumptions in your theory. And a simpler theory is better, because Occam’s razor and all.

But look, if that argument was correct, then the best theory would be one with no assumptions at all. There’s just a little problem with that, which is that such a theory doesn’t explain anything. I mean, it literally isn’t a theory, it’s nothing. Just saying that it’s simple doesn’t make a scientific theory a good one. For a theory to be good, it still has to describe what we observe. It’s like just telling my hair to “please stay put” may be simple but doesn’t make it a good hair day.

And that’s exactly what happens in those multiverse theories, they’re too simple to be good for anything. If you don’t specify the values of the constants of nature, then you just can’t make predictions. To be fair, I would agree it’s simpler to not make predictions than making them, but even in physics you can’t publish predictions you didn’t make. At least not yet. Which is why multiverse physicists always end up making assumptions for the values of those constants.

They don’t always do this directly, sometimes they instead postulate probability distributions from which they derive likely values of the constants. But that’s more difficult than just using the constants and certainly not simple.

Same issue with the many world’s interpretations. Those who work on it claim that their theory is simpler than standard quantum mechanics because it just doesn’t use the measurement update. But if you don’t update the wave-function upon measurement, then that just doesn’t describe what we observe. We don’t observe dead-and-alive cats, that was Schrödinger’s whole point.

Therefore, you have to add other assumptions to many worlds, about what a detector is and how the universes split and so on, which for all practical purposes amounts to the same as updating the wave-function. In most cases these prescriptions are actually more complicated than the measurement update. So multiverse theories are either simple but don’t make predictions, or they make predictions but are more complicated than the generally accepted theories.

Let me finish by saying I am not against the multiverse or poetry. I would like to apologize to all the poets watching this. It’s not like I think science is the only thing that matters. You may find the multiverse inspirational, or maybe comforting, or maybe just fun to talk about. And there’s nothing wrong with that – please enjoy your stories and articles and videos about the multiverse. But don’t mistake it for science.

The multiverse is not a paradigm and it’s not shifting anything.

Google “multiverse paradigm” and you get more than a thousand hits. According to Wikipedia a paradigm “describes distinct concepts or thought patterns”. Unfortunately, the multiverse is pretty much the opposite: There’s no distinct concept, but instead a variety of loosely related properties of existing theories that are being construed to have a common theme which, we are then told, is sign of an impending paradigm shift.

I’m starting to take offense in this forward defense. If the spread of multiversal “thought patterns” is sold as a paradigm shift, everybody opposed to the multiverse is discarded as being stuck in yesterday. It’s only the enlightened who are ahead of their time and understand the significance. I really don’t think there’s any paradigm here and certainly nothing is shifting. To see why, it’s helpful to distinguish two different classes of multiverses that are presently being discussed, usually thrown together.

1. The Multiverse of Disappointed Hopes

Science works by constructing models for real world systems. These models can then be used to understand what happens in the real world, and to make predictions. A theory is a map from a model to the real world. The model should not be confused with the theory itself. The theory is what tells you how to identify properties of the model with the real world. The model is the actual stand-in for the real world system.

Einstein’s General Relativity for example is a theory: it’s a prescription for how to deal with space-time and particles moving in it. A model is the space-time of a star or an approximately homogeneous matter distribution. It’s the theory of General Relativity, but the ΛCDM model. Likewise, there’s quantum field theory, and the standard model. Needless to say, not everybody uses this terminology all the time, but that’s how I want to use it.

Models and theories are not only used in physics and don’t necessarily have to be mathematical. Psychologists have models for human behavior that they apply to patients – the ‘real world’. A drawing is a model, in this case the “theory” that connects it to the real world comes for free with your visual cortex. A story is a model, the “theory” is your knowledge of the language that relates letters to real world objects or actions. And so on. The merit of mathematical models is that they have a very strict quality control, which is self-consistency.

And then there are toy models.

Toy models are models that do not have real world counterparts. It’s drawings of creatures that don’t exit or stories of people that have never lived. They’re playgrounds of creativity that can teach us lessons about the theory, which is why studying toy models is a very common and often fruitful exercise. There’s an infinite amount of such toy models. You could say there’s a whole multiverse of them, all these toy models that don’t map to any part of the universe we know. Asking whether what they describe is real is like asking if Harry Potter really exists because a story has been written about him. The difference between fantasy novels and physicist’s toy models is the size of the interested audience, but in spirit they’re the same exercises in creativity.

So, sure there are models that don’t describe the real world, in physics as well as in painting. That’s because mathematical consistency alone does not imply a model describes what we observe, much like using English does not imply you talk about real people. Additional requirements are needed besides consistency to construct a useful model, and these requirements are always agreement with observations, though this isn’t always explicitly phrased this way. When we assume Lorentz-invariance or renormalizability or absence of ghosts, these are physical requirements ultimately based on our experience.

This means a multiverse that you can get rid of by adding the requirement that the model needs to describe observation is neither new, nor surprising, nor something to worry about. It just means that mathematical consistency of whatever theory it is you’re dealing with is not sufficient to make a particular prediction. The string theory landscape is a multiverse of this type. The only reason people talk about this now is that many of them had been hoping string theory would make some requirements that one needs in the standard model unnecessary. Alas, these hopes were disappointed, though the last word might not be spoken yet.

Does it make sense to instead talk about probability distributions over the models you get when you refuse to use existing ties to observations, here specifically the values of certain parameters? No. Because that’s cherry picking the observations you want to neglect.

In the construction of the model there always enter many other observations that are being neglected if one considers such probability distributions, such as the number of (large) dimensions, Lorentz-invariance, or the existence of space-time to begin with – these are not requirements of mathematical consistency, these are physical requirements based on observations. If you wanted to be serious with asking for the probability of particular models, you should sample over all models, in the end over all that is mathematically consistent. You’d be left with Tegmark’s mathematical multiverse. And in that mathematical universe you’d have replaced the question “Which model describes the real world?” with “Where are we in the mathematical universe?” You don’t gain anything.

Once you have seen the power of mathematical models to describe natural systems, it is natural to ask if there is a mathematical model that describes “everything” we see. I believe there is. But people who search for a “theory of everything” today mean more than that. They want in particular a theory that delivers the parameters in the standard model. But even if that would be achieved, we would still have to use other axioms that are ultimately based on observations. So while it is worthwhile to try to find a simpler model that reduces the number of axioms, including values of parameters, we can never avoid using input from observation. If we do, we’ll end up with a multiverse which just tells us that mathematical consistency isn’t sufficient.

So if you have a multiverse that can be eliminated by the requirement that the model is consistent with observation, this isn’t a paradigm shift, it’s just disappointed hopes.

2. The multiverse package deal

But there’s a different type of multiverse, one that you cannot get rid of by requiring match to observation. It’s the case in which a theory applied to a model that describes a real world system necessarily maps into a space that is larger than what we observe. Eternal inflation and the many worlds interpretation of quantum mechanics are of this type. Or, more mundanely, there is nothing in ΛCDM that predicts the universe just ends beyond the distance that we can (presently) observe, so you have a multiverse beyond our observations.

This opens a can of interpretational worms because we can now endlessly discuss whether the not observable images of the map are real or not. Personally, I find this a rather fruitless debate about the meaning of the world ‘real’. To me a model is a tool to describe the real world and if it does that, and if it’s an improvement over other models, I don’t care if there are mathematical elements in the model that don’t correspond to real world observables. Mathematics is full of structures that for all we know don’t correspond to anything we observe anyway. I don’t see a reason why we must be able to observe them all.

But, no, I don’t think you should just shut up and calculate. Because we might be mistaken in thinking that what the theory predicts beyond our observable universe is indeed unobservable. Maybe we just haven’t asked the right questions and there are ways to observe it after all.

So it’s an interesting feature that theories can display, but it’s certainly not a new concept. There’s been a century of discussion about the presence of mathematical objects in quantum mechanics that for all we presently know are fundamentally non-observable. So if that’s a paradigm shift it’s one that has already happened long ago.

3. Wilzcek’s Multiversality

Frank Wilzcek recently had a paper on the arxiv titled “Multiversality”. The first half of the article is a nicely written general introduction, the second half is about axion cosmology and then the paper ends quite abruptly. The most interesting part of the paper are three positive answers to the question

“Are there aspects of observable reality, i.e. the universe, that can be explained by multiversality, but not otherwise?”

It is fruitful to look at the answers to gauge the depth of the existing arguments in favor of the multiverse:

“Yes – one is the apparent indeterminism of quantum mechanics, despite its deterministic equations.”

Wilczek claims here the apparent indeterminism of quantum mechanics can be explained by the many worlds interpretation but not otherwise. That’s an objectionable claim, in particular because the qualifier didn’t include anything about locality.

“Yes – the outrageously small, but non-zero, value of the dark energy density.”

Here he is claiming that there is no other way to explain the measured value of the dark energy density than anthropic reasoning and that anthropic reasoning necessarily implies a multiverse. There are many people who would object on the former and the latter is manifestly wrong. You don’t need a multiverse to do anthropic reasoning, see my post Misconceptions about the anthropic principle.

“Yes – the opaque and scattered values of many standard model parameters that are not subject to the discipline of selection.”

An interesting answer because it is phrased to suggest that the values of the standard model parameters are scattered to begin with. Even if they were however that wouldn’t force us to believe that any possible distribution of values actually exists in a more meaningful sense than Harry Potter exists.

Taken together, these answers tell you aptly just how weak the case for a multiverse really is.

Summary

We should distinguish between multiverses that you can eliminate by adding axioms to the theory that tie the model to the real world, and those that you can’t eliminate this way. The string theory landscape is of the former type, you “just” have to find the right vacuum, and good luck with finding that. Eternal inflation and the many worlds interpretation are of the latter type. In this case you get more than you asked for. One can interpret this type of multiverse as a calculation device which might have its uses. It might also turn out that these multiverses aren’t unobservable after all, so these ideas certainly merit some investigation. In any case however, there’s no paradigm shifting here.

Hawking’s “Final Theory” is not groundbreaking

Yesterday, the media buzzed with the revelation that Stephen Hawking had completed a paper two weeks before his death. This paper supposedly contains some breathtaking insight.

The headlines refer to a paper titled “A Smooth Exit from Eternal Inflation” in collaboration with Thomas Hertog. The paper was originally uploaded to the arXiv in July last year, but it was updated two weeks ago. It is under review with “a leading journal” which I suspect but do not know is Physical Review D. Thomas Hertog gave a talk about this at the conference which I attended last summer. You can watch the video of Hertog’s talk here.

According to The Independent the paper contains “a theory explaining how we might detect parallel universes and a prediction for the end of the world.” Furthermore, we learn, “Hawking also theorised in his final work that scientists could find alternate universes using probes on space ships, allowing humans to form an even better understanding of our own universe, what else is out there and our place in the cosmos.”

In the Sunday Times you can read that the paper “shows how we might find other universes”  and in The Telegraph you find a quote by Carlos Frenk, professor of cosmology at Durham University who said: “The intriguing idea in Hawking’s paper is that [the multiverse] left its imprint on the background radiation permeating our universe and we could measure it with a detector on a spaceship.”

Since the paper doesn’t say anything about detecting parallel universes, I was originally confused whether the headlines were referring to another paper. But no, Thomas Hertog confirmed to me that the paper in question is indeed the paper that is on the arXiv. There is no other paper.

So what does the paper say?

The paper is based on an old idea by Stephen Hawking and Jim Hartle called the “no-boundary” proposal. In the paper, the authors employ a new method to do calculations that were not previously possible. Specifically, they calculate which type of universes a multiverse would contain if this theory was correct. The main conclusion seems to be that our universe is compatible with the idea, and also that this particular multiverse which they deal with is not as large as the usual multiverse one gets from eternal inflation.

It’s not entirely uninteresting if you are into multiverse ideas, because then you need this information to calculate the probability of our universe. But it is also a very theoretical paper that does not say anything about observational consequences.

The only thing that the paper does say is that inflation took place. And inflation predicts that gravitational waves produced in the early universe should leave an imprint in the cosmic microwave background (CMB). This is the CMB polarization signal that BICEP was looking for but didn’t find. There are, however, some satellite missions in the planning that will look for it with better precision.

So how do we detect parallel universes? By detecting the CMB polarization. I do not kid you.

Here’s what Hertog said about this:

“This model predicts that our universe came into existence with a burst of rapid expansion called cosmic inflation. A big bang of this kind amplifies gravitational waves which in turn show up in satellite images of [the pattern of temperature fluctuations in] the cosmic microwave background. Future satellite missions should see this, if the theory is correct.

Observational evidence for the no-boundary model [in the form of gravitational waves from the big bang] would yield strong evidence for a multiverse. This paper provides a step towards a mathematically sound and testable model of the multiverse. That constitutes a significant extension of our notion of physical reality.

Some cosmologists have argued against the multiverse on the basis it can’t be tested. However our model shows that observations in our own universe can provide strong evidence for the existence of other universes. ”

Allow me put this into perspective.

Theoretical physicist have proposed some thousand ideas for what might have happened in the early universe. There are big bangs and big bounces and brane collisions and string cosmologies and loop cosmologies and all kinds of weird fields that might or might not have done this or that. All of this is pure speculation, none of it is supported by evidence. The Hartle-Hawking proposal is one of these speculations.

The vast majority of these ideas contain a phase of inflation and they all predict CMB polarization. In some scenarios the signal is larger than in others. But there isn’t even a specific prediction for the amount of CMB polarization in the Hawking paper. In fact, the paper doesn’t so much as even contain the word “polarization” or “tensor modes.”

The claim that the detection of CMB polarization would mean the multiverse exists makes as much sense as claiming that if I find a coin on the street then Bill Gates must have walked by. And a swarm of invisible angels floated around him playing harp and singing “Ode To Joy.”

In case that was too metaphorical, let me say it once again but plainly. Hawking has not found a new way to measure the existence of other universes.

Stephen Hawking was beloved by everyone I know, both inside and outside the scientific community. He was a great man without doubt, but this paper is utterly unremarkable.

Note added May 2nd 2018: The paper was now published in JHEP.

Is the Anthropic Principle scientific?

Today I want to explain why the anthropic principle is a good, scientific principle. I want to talk about this, because the anthropic principle seems to be surrounded by a lot of misunderstanding, especially for what its relation to the multiverse is concerned.


Let me start with clarifying what we are talking about. I often hear people refer to the anthropic principle to say that a certain property of our universe is how it is because otherwise we would not be here to talk about it. That’s roughly correct, but there are two ways of interpreting this statement, which gives you a strong version of the anthropic principle, and a weak version.

The strong version has it that our existence causes the universe to be how it is. This is not necessarily an unscientific idea, but so-far no one has actually found a way to make it scientifically useful. You could for example imagine that if you managed to define well enough what a “human being” is, then you could show that the universe must contain certain forces with certain properties and thereby explain why the laws of nature are how they are.

However, I sincerely doubt that we will ever have a useful theory based on the strong anthropic principle. The reason is that for such a theory to be scientific, it would need to be a better explanation for our observations than the theories we presently have, which just assume some fundamental forces and particles, and build up everything else from that. I find it hard to see how a theory that starts from something as complicated as a human being could possibly ever be more explanatory than these simple, reductionist theories we currently use in the foundations of physics.

Let us then come to the weak version of the anthropic principle. It says that the universe must have certain properties because otherwise our own existence would not be possible. Please note the difference to the strong version. In the weak version of the anthropic principle, human existence is neither necessary nor unavoidable. It is simply an observed fact that humans exist in this universe. And this observed fact leads to constraints on the laws of nature.

These constraints can be surprisingly insightful. The best-known historical example for the use of the weak anthropic principle is Fred Hoyle’s prediction that a certain isotope of the chemical element carbon must have a resonance because, without that, life as we know it would not be possible. That prediction was correct. As you can see, there is nothing unscientific going on here. An observation gives rise to a hypothesis which makes a prediction that is confirmed by another observation.

Another example that you often find quoted is that you can use the fact of our own existence to tell that the cosmological constant has to be within certain bounds. If the cosmological constant was large and negative, the universe would have collapsed long ago. If the cosmological constant was large and positive, the universe would expand too fast for stars to form. Again, there is nothing mysterious going on here.

You could use a similar argument to deduce that the air in my studio contains oxygen. Because if it didn’t I wouldn’t be talking. Now, that this room contains oxygen is not an insight you can publish in a scientific journal because it’s pretty useless. But as the example with Fred Hoyle’s carbon resonance illustrates, anthropic arguments can be useful.

To be fair, I should add that to the extent that anthropic arguments are being used in physics, they do not usually draw on the existence of human life specifically. They more generally use the existence of certain physical preconditions that are believed to be necessary for life, such as a sufficiently complex chemistry or sufficiently large structures.

So, the anthropic principle is neither unscientific, nor is it in general useless. But then why is the anthropic principle so controversial? It is controversial because it is often brought up by physicists who believe that we live in a multiverse, in which our universe is only one of infinitely many. In each of these universes, the laws of nature can be slightly different. Some may allow for life to exist, some may not.

(If you want to know more about the different versions of the multiverse, please watch my earlier video.)

If you believe in the multiverse, then the anthropic principle can be reformulated to say that the probability we find ourselves in a universe that is not hospitable to life is zero. In the multiverse, the anthropic principle then becomes a statement about the probability distribution over an ensemble of universes. And for multiverse people, that’s an important quantity to calculate. So the anthropic principle smells controversial because of this close connection to the multiverse.

However, the anthropic principle is correct regardless of whether or not you believe in a multiverse. In fact, the anthropic principle is a rather unsurprising and pretty obvious constraint on the properties that the laws of nature must have. The laws of nature must be so that they allow our existence. That’s what the anthropic principle says, no more and no less.

The science of the multiverse.

CMB aniotropies.
Image: NASA/WMAP

A new numerical study of bubble collisions makes a big step towards testing the multiverse hypothesis.

The multiverse might not be the most original or even surprising idea that physicists have proposed recently, but it certainly excelled in capturing the public imagination and in stirring up discussion. On the very top of the discussion list is the question whether the multiverse is a scientific idea at all, or whether it is simply a philosophical retreat for lazy physicists who’ve gotten tired hunting answers that didn’t come knocking on their door.

But there’s no simple answer to that question. Whether the multiverse is a scientific idea depends on what one means with multiverse. As I explained previously, the existence of “multiverses” in many theories is just a consequence of pushing models beyond their phenomenological reach. The only thing these types of multiverses demonstrate is that physicists don’t understand the limits of pure mathematics. In contrast, the best motivated multiverse is bubble universes in eternal inflation. And these can have observational consequences.

Eternal inflation is a variant of inflation, a phase of exponential growth in the early universe. Exactly how this phase proceeds depends on the properties of quantum fields that filled the universe back then. In eternal inflation, it is only parts of the whole space in which the rapid expansion comes to an end and structures like our galaxies form; these parts are referred to as “bubble universes”. In the rest of space, inflation continues and goes on to create new bubbles. This bubble production lasts eternally.

Most of these bubble universes are causally disconnected from ours and we have no chance to ever observe them. However, it is at least theoretically possible that some of these bubbles collide as they expand. This would mean that what is now our observable universe had, when we look back in time, not one seed, but two or more that later came to join. This type of bubble collision can have observable consequences for the formation of structures and leave imprints in the pattern of temperature fluctuations in the cosmic microwave background (CMB).

The accurate mathematical description of these bubble collisions is however difficult because Einstein’s field equations are non-linear. Solutions can normally only analytically be found in cases with many symmetries, and in the case of bubble collisions the geometry prevents one from using such a highly symmetric ansatz. Approximately valid analytical solutions have previously been put forward, but when one wants to make quantitative prediction one needs a sufficiently precise numerical simulation. Such a numerical simulation has to evolve forward in time the metric components, whose fluctuations eventually go to seed the perturbations in the CMB, which we then later measure.

Such a numerical simulation has recently been published in this paper

Simulating the universe(s): from cosmic bubble collisions to cosmological observables with numerical relativity
Carroll L. Wainwright, Matthew C. Johnson, Hiranya V. Peiris, Anthony Aguirre, Luis Lehner, Steven L. Liebling
arXiv:1312.1357 [hep-th]

The authors only study the simplest case, that is the collision of only two bubbles and in addition these bubbles are identical (they have the same vacuum state). This is clearly not the most general case, but even so their simulation allows them to calculate the effects on the CMB better than previously possible.

Roughly speaking, the bubble collision leaves a localized temperature fluctuation - a hot spot or a cold spot - in the CMB that fades off away from the center of the collision. Exactly how it fades is very important for the extraction of such a signal from the data, and yet this could only be estimated before this numerical simulation was completed. Notably, the authors found that the fall-off is faster than was expected from the analytic estimates.

The figure below shows the time-evolution for the scalar field (Φ) that drives the inflation and two functions (a and α) that quantify the behavior of the metric. The horizontal axis is one of the spatial coordinates the vertical axes a measure for the time.

Figure 7 from arXiv:1312.1357 [hep-th]

What you can see in the image is how the configuration starts out being initially split in two halves and then evolves towards a situation where the initial split slowly fades away. That the bubbles both originate at “the same” time can always be achieved by a suitable choice of time-coordinate. The time, or the initial distances between the bubbles, can later be changed to a free parameter by a coordinate transformation.

This numerical study demonstrates nicely that it is possible to connect the underlying model for eternal inflation to observable signatures. It is also valuable in suggesting a useful parameterization for the effects. I find this a very interesting paper which will provide a basis for further studies that are necessary to analyze cosmological data for signals that might show we live in a multiverse.