Thursday, January 25, 2018

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.


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.

72 comments:

Matthew Rapaport said...

Wow... a debate (sort of) between Ethan S. and Dr. H! I love it! Thank you for another illuminating and clarifying essay Dr. H. Short, sweet, and packs a punch. Of course I ordered your book as soon as you announced it and very much look forward to reading it. I am not a big multiverse fan, but my reasons are not scientific so I would not dare to mention them here. Thank you again. Great essay!

Sabine Hossenfelder said...

Matthew,

I didn't see Ethan's post until after I published this. Also, my post on NPR earlier this week was not in any way related to Sean Carroll's paper. Indeed, I wrote the piece last year already, just that they didn't publish it until this week. So, it's a strange coincidence, but though we've been talking at the same time, we haven't actually been talking with each other!

Michael John Sarnowski said...

Hi Sabine Hossenfelder,

I would like to make a falsifiable proposal about the multiverse. I propose that a specific model of a multiverse will make an imprint on our universe. Specifically I propose, that at 5.27_plus or minus about .03 billion light years from earth, we will see a change in the observed expansion rate of the universe. It is at this point that the external gravity of an adjacent universe will be equal and opposite to the internal gravity of our universe. This would be due to the acceleration of gravitation increasing linearly, from the center of a uniformly dense sphere, to the edge of the sphere, and the external acceleration of gravity decreasing proportionally by the square of the distance from the center of that universe. This number turns out to be one over the square of the golden ratio, or about 0.382, fractionally, from the center of the Hubble Sphere universe.

Greg Sivco said...

Well then maybe you should talk to each other.

Bert Morrien said...

Dr Hossenfelder,
AphaGo Zero played against itself and became an expert only by changing its synaptic weights. Presumably they settled on constant values. I think a perfect example of evolution. The early universe was also evolving by 'playing' with itself, no multiverse required, only self-reference. Maybe the natural constants were not always constants. Could that be a plausible scenario?

Uncle Al said...

... 1) An interactive multiverse creates excess thermodynamic degrees of freedom.
... 2) Euclid demands all triangles' three internal angles sum to exactly 180°. The Northern Hemisphere is a 540° triangle.
... 3) Accepted theory? OPERA superluminal muon neutrinos were “explained.” Bayesian inference? Mammals laying eggs.
... 4) Global Warming as Pascal's Wager is $trillion/year criminality. The Ozone Hole ignored four generations of eco-refrigerants.

http://thewinnower.s3.amazonaws.com/papers/95/v1/sources/image010.png

Make the 2-hydrogen a 2-cyano group (dipole moment). Literature synthesize non-superposable mirror images in 3:1 ratio. Vacuum supersonic expand to ~1 K rotational temperature, beam into a chirped-pulse FT µwave spectrometer, observe rotational spectra. If respective lines are not exactly identical and exactly superposed, even 3:1 lopsided, baryogenesis is sourced and physical theory is falsified. Dr. Maria Schnell should look.

Bruce Rout said...

Excellent critique.

The Rabbi said...

Hi Sabine, what audience will your forthcoming book be aimed at? Will it be accessible to people like me who do not have a maths/physics background? If not, could you recommend any books on the pros and cons of the multiverse aimed at "ordinary" people. Cheers, Ian.

driod33 said...

"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."
The Copenhagen interpretation for me isn't a interpretation, it is a denial,and appears to deny working quantum computers through the "wavefunction collapse". Which does not happen imo.

Mozibur said...

I think multiverses, wormholes, hyperspace and parallel universes are great for science fiction but not so much for serious science...

Matthew Rapaport said...

Well serendipity then! Mysterious are the ways of the Force! :)

Berndt Mueller said...

Hi S.

I read your blog from time to time and usually agree with what you write so lucidly. But I beg to disagree with your posts on the multiverse. Not because I think it's science - it's not (yet) because no one has figured out a way to observe another universe.

But at this moment in time, the multiverse is the simplest logical explanation for why the constants and laws of nature permit the existence of intelligent life (accepting for the moment the wild concept of human "intelligence" - there's a lot of evidence against that...). It's pretty obvious that most other combination of fundamental constants would not permit intelligent life, or even life, to develop. This raises the question, why are their values in the favorable range? There are a few possible answers: (1) There's no good reason, they just happen to be. (2) There's a god who created the world just so that humans could exist. (3) They must have the values we observe because of some mathematical theorem we haven't found yet. (4) There are many, possibly infinitely many, universes with different laws and constants, and we live in a favorable one by default.

Humankind faced the same problem with understanding why the Earth is just the right kind of planet to support human life. We now know that's because there are 10^20 or so planets out there of all sizes, etc., in our universe alone, and we live on one that has the favorable properties. There are probably a few more planets out there that support intelligent life, although we still have no scientific evidence for that. The multiverse, conceptually at least, is just an analogous explanation of a similar quandary. That certain theories of inflation naturally predict multiverses is a nice by-product of the inflationary hypothesis, but the reasoning does not rely on this circumstance. The multiverse hypothesis is the simplest logical solution of the problem of "specialness" of our universe.

And, by the way, assuming the existence of a god, or gods, or angels is a much bigger leap than assuming the existence of other universes. We know for a fact that one universe exists, so why could there not be others? But we have no factual knowledge of the existence of even a single god or angel. To extrapolate from 1 to N makes more sense than to extrapolate from 0 to 1.

Sabine Hossenfelder said...

The Rabbi:

I sincerely hope it will be possible to understand my book with not more than high-school-level physics. My mom read it and says she understood it, and she doesn't have a degree in physics, so maybe that gives you an idea.

I do assume, however, that the reader brings a certain interest in the topic already and is likely to have read some more in-depth expositions elsewhere. I do not, for example, spend a lot of time going through exactly everything we know about the early universe, dark matter, and the standard model and so on. I explain about them what you need to follow the book, but not any more than that. (I have some recommendations for further reading in the endnotes though.)


Best,

B.

Sabine Hossenfelder said...

Berndt,

The idea that we can explain everything from nothing is logically absurd. If the only assumption you allow is mathematical consistency, you will end up with Tegmark's mathematical universe. In this mathematical universe you will then no longer ask "why these laws and parameters" but "why are we here and not elsehwere". For practical purposes, though, that's the same question.

This means that if you want to describe nature, you necessarily ALWAYS have to draw upon observational input to select assumptions. In the course of history, you might be able to replace some of them with simpler, more encompassing, ones, but we will never get rid of all of them. Hence, there will always remain a question "Why this" or "Why here", and I think it's not the business of scientists to discuss these things.

Best,

B.

Roy Lofquist said...

"The map is not the territory" is a phrase coined by Alfred Korzybski. Ignoring its implications has been the ruin of many a poor (Strike)girl(/Strike) theoretician.

Consider polynomial regression analysis, curve fitting. The basic assumption is that for any (n) data points there exists a polynomial of order f(n-1) that will have all the data points as solutions. This often leads to the fatal conceit that this function is "the" solution. Actually, there is an infinite number of polynomials of order f(n) that will encompass all of the (n) points.

Much of theoretical physics is based on extending formulations that fit known data and "interpolating" or inferring possibilities that are not supported by observation. Cosmology and Quantum Theory have become the hardest of hard science fiction. Or, as Rupert Sheldrake is fond of saying: "Give me just one miracle and I can explain everything".

Pascal said...

>"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."

What the many worlds interpretation does is simply to point out that the rules of quantum mechanics as we know them logically imply the existence of multiple universes.
If you do not like that, you have to change the rules of quantum mechanics (for instance, come up with an "objective collapse" mechanism).

Sabine Hossenfelder said...

Pascal,

Ah, the argument from supposed logic. Didn't you read what I wrote? It's just wrong. The existence of an ensemble doesn't follow from the possibility of making probabilistic estimates.

akidbelle said...

Hi Sabine,

nice post. Thanks. In my opinion, you cut thru hypothesis or interpretations that have become opinions and, maybe, are imposed as preconceptions. Hypothesis is science, not opinions - preconceptions is the opposite.

Roy, Korzybski also pointed out a method (General Semantics) to distinguish "layers of maps", that is layers of abstraction - and eliminate one's filters and projections. Mechanically, one puts a mental map to superpose with reality - then eliminate part of the information, and add some (including interpretation) - and names it "knowledge" considered as certainty... that works until one hits a wall. Problem in physics is that math may be non-finite and the wall is the absence of practical results. I understand this is where we are.

Sabine, I remind we discussed projections a long time ago. Now how about filtering:
"I have no problem with a coupling in the range of 1?"
But such beast does not exist. What does that mean?

No aggression meant, we're all human :)

Best,
J.

marten said...

As far as I understand it there is no stringent multiverse.

pete best said...

Hi Sabine

The multiverse appeals to a lot of people (atheists mostly as I understand it) due to anthropics. The more Universes you have, the more likely you get a fine tuned Universe that we can exists in (or life anyway). Now over at Not Even Wrong this came up recently regarding the types of multiverse you can have, Single Inflation field multiverse means each Universe has the same physics (not sure about the initial conditions though) and hence if the physics is all the same in each and every one of them then life is inevitable? and then the string theory landscape multiverse which is needed for each Universe to have different physics and we just happen to live in one of them where is physics is just right.

So maybe science is fighting atheistic takes from scientists themselves about the need for a multiverse to negate religious meaning to our Universe.

Larry said...

There appear to be leaps in logic by some posters, 1. if there are other "universes" we cannot possibly know about them. 2. Multiverses cannot interact with each other as they are separate and distinct.

If a magnetic field, or gravity from one universe interacts with another universe then by definition--since we know about that "other universe" it is part of our universe.

Sabine Hossenfelder said...

Larry,

In most of the currently considered scenarios, they don't interact.

Domenico Barillari, PhD, C-Nucleonics company said...

Hello Sabine

Whenever a distinguished figure such as Sean Carroll has to turn to Bayesian argumentation - as distinct from the professional statistician or engineer's "tight" application of Rev. Bayes insight - I know that their position, whether philosophical or "purely" scientific, is really starting to skid off the road. The tendency to resort to this, I take mainly influenced by Weinberg's early inspirational calculation, is a sign to me of boredom, frustration or in wort cases "grant money worry". I will also say that it is unfair to people who criticize the critics here for being too cowardly or pedantic, or downright too stupid to wrap their minds around fresh ideas. Most of us having been doing the latter, with great excitement, all of our mature lives. Please can we get Dr. Carroll and Co. to get on with the hard part of eventually sussing out the math and physics content that would lead to the accelerator, telescope or conceptual clarification they promise. The alternative is a kind of tiresome pleading for an idea, that as you indicated, becomes repeated often enough as a spectacular notion that people come to believe in its merits just because it has bounced around the physics community for so long.

signed, a weary Domenico

Mighty Drunken said...

I love the ideas like Tegmark's mathematical universe. However I see these ideas as philosophy and not science, fun "what if" musings. I'm sticking to a single Universe until their is very strong physical evidence otherwise. Which is unlikely considering the usual definition of The Universe :)

I have just finished "The Singular Universe and the Reality of Time" book by Lee Smolin and the Roberto Mangabeira Unger. Overall I was a bit disappointed by the book but I think their aim is a good one. To try to wrestle the metaphysical (or as they call it Natural Philosophy) musings of cosmology away from the multiverse to more testable ideas.

JimV said...

Berndt Mueller said.. "It's pretty obvious that most other combination of fundamental constants would not permit intelligent life, or even life, to develop."

I agreed with most of the comment from which that is an excerpt, but don't find that excerpt obvious at all. I would define "life" as any kind of self-organizing, self-reproducing structure which can evolve to greater and greater complexity. Our form of life appears to be very rare and only possible in a minuscule part of this universe. Perhaps in another universe with different physics, life could form from sub-atomic particles or electrical fields, and in that universe "life" could exist almost anywhere.

I do like the multiverse as a concept, to provide a counter-example to those whose arrogant religions tells them that this universe was created so that they could exist, but that doesn't make it scientific (so far). It merely shows that the alternate religious concepts aren't scientific either.

Seth Thatcher said...

Sabine, fabulous! I've been a big fan of Ethan's for a few years now but this multiverse issue is hurting that relationship. His post yesterday basically acting as if the multiverse is fact is mind-blowing and not in a good way. And it's not the first time he has done so. Your recent pieces on this issue have given voice to my admittedly layman's viewpoint. Thank you for that! And even if in the longrun Ethan ends up being correct which as a "scientist" I must allow for, your logical arguments against it will be valuable to him an others who espouse such ideas so that they are forced to develop more rigor.

Phenomenologist said...

I remember reading an article where the author said "this cannot be because it would lead to divide by zero", and I remember thinking that mathematics attempts to model the universe, not constrain it. And it appeared to me that someone who spends the greater part of their life immersed in mathematics is liable to confuse the model with the modeled.
So it is with the multiverse, among other theories.
Take a concept such as "infinity". That (mathematical) concept has no analogue in the empirical data; it is solely a mathematical concept, and there is no evidence for it in a "real" universe. Similarly "eternity".
True science starts with observed data and constructs theories to explain it, but no theory is scientific if it relies on unobserved data to explain observed data.

David Bailey said...

Hi Sabine,

I was interested that you mentioned the Many Worlds interpretation of QM. Although this is accepted as a possible interpretation of QM, it would involve duplicating the entire universe for every wave function collapse. To me, that means that while the idea is mathematically sound, it isn't physically reasonable.

This makes me wonder if the other possible multiverses are of a similar nature - mathematically possible, but physically absurd.

Gabriel Blythe said...

'Multiverse theories', if they are testable by experiment, are scientific, right? Remember Feynman's opinion on what is the final arbiter of truth/reality in science? I have to side with Professor Carroll on this one.

Lawrence Crowell said...

The multiverse is an aspect of quantum gravity, which is itself horridly out of reach of experiment. An Planck scale accelerator would be thousands of light years in radius. Also a proton on proton collision that gives rise to a quantum black hole would decay into about a mole of particles. This would make the track finding algorithms for backing out the physics from the data impossible. So the putative physics of quantum gravity rests on uncertain empirical grounds. This is the case whether one is a looper or a stringer.

The multiverse has it that a fundamental quantum of the quantum gravitational field is not just black holes or gravitational waves (gravitons) but cosmologies. In fact every solution type in the Petrov-Pirani scheme should correspond to a quantum of the quantum gravity field. This means what ever possible empirical support for theories will come from deep space observations and astronomy. The detection of gravitational radiation is a case of this with classical gravity. I think black hole coalescence might hold signatures of quantum gravity as well. The multiverse is then another plausible case.

The argument for the multiverse, a term I actually loathe with a passion, is that RG flows exist on a complex topography or landscape so the outcomes of coupling constants entire gauge field structures depend not on some unique outcome, but rather on a vast number of outcomes. It is a bit like stellar systems of planets; these are not as Kepler at first thought some unique mathematical structure but rather due to initial conditions. It also in a curious way makes cosmology more quantum mechanically sensible, for it there is only one cosmology then how can we assign quantum amplitudes to it?

Your essay here is of course a bit of a call to sanity. In particular we can't complacently say we "know all this," when in fact the support for inflation is so far weak (though not nonexistent) and evidence for the multiverse is absent. We have a lot of ground left to cover.

IP. Freely said...

"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."

Sabine, you've just tracked the progress of so much post-modern science it's astonishing. The number of hypothesis that seem to be in the "OK, it's not falsifiable and we can't observe it, but it makes sense and it's the best we've got" has become overwhelming. From AGW to Zero Point Energy, the beat goes on.

Educated in the 60's, I look at what science has become and it truly scares me. I honestly wonder how this will end up playing out, but thankfully I expect to be dead before it does.

Sabine Hossenfelder said...

Gabriel,

You may be a chicken that picked on a phone that someone left in the barn. That's a falsifiable hypothesis but it's not science. I explained this in more detail here. Sean in his paper also acknowledges that falsification is neither necessary nor sufficient to make a theory scientific. Best,

B.

Matt Grayson said...

Sabine,

Actually, the "chicken pecking" hypothesis would explain a lot of the multiverse debate. My Bayesian estimate is >98% chicken. The alternative is that intelligent people who spent years studying the awesome explanatory power of the works of Galileo, Newton, Maxwell, and Einstein learned nothing.

Not that all the chickens are on one side of the debate. That atheists would need to resort to the multiverse when there are vastly simpler refutations of "God did it" (Who did God, btw?) is thus better explained.

OK, disclaimer: I'm well aware that the "scientists are as susceptible to self-aggrandizement as anyone else and will take whatever position gets them noticed" explanation beats out the chicken - but only slightly.

Best,

Matt

JimV said...

I read Dr. Siegel's article as not directly contradicting this post. He seemed to me to be saying that the only type of "multiverse" that is supported by current data is one universe in which inflation is continually occurring (somewhere), and that, aside from inflation, all the physics is the same. This is not what most people consider to be a "multiverse" but is the only one which he thinks is, or could be, real (based on the CMB and other data). As he points out, this does not provide any explanation for the origin of our physics and is "useless" in that regard. It does however suggest that total heat-death of the universe may never occur, which I find a bit comforting.

The disagreement, if any, seems to me that Dr. Siegel feels there is good evidence for inflation, and Dr. Hossenfelder may not.

Nonlin.org said...

As long as you take down this sacred cow, why not expand into other cosmological myths? Aren't dark matter and dark energy just plug-ins for our ignorance? Isn't cosmic inflation just a dead end conclusion of the "wind-up toy" view of the universe? Are there any doubts about the "ice skater" model of galaxy and planetary system formation? What if running physics backwards is wrong or done with the wrong assumptions? What else is in doubt out there?

There's definitely a market for nice, comfortable stories that most cosmologists address, but skeptics also want to know what's in doubt and how much so. Write that book and it will sell.

Solomon said...

I have a very naïve perhaps incoherent yet sincere question about the self-consistency of the multiverse idea:

My understanding is that, in string theory, particles are excitations of a string. That is, string theory is seen as more fundamental than particle field theories.

In the multiverse idea, however, it seems that eternal inflation is somehow more fundamental than string theory since one must presuppose eternal inflation. That is, eternal inflation is what is required to generate the bubble universes in which some particular string theory solution may "live".

Yet inflation is based on a (scalar) particle field theory.
This seems to me to be a kind of chicken/egg paradox, is it not? (though I of course entirely grant that I am simply not understanding the basic ideas!}

In any event, let me ask: Is the so-called inflaton a (scalar) particle that is the excitation of a string, i.e. is it embedded in string theory, or is it rather somehow beyond string theory?
And if the inflaton is embedded in string theory, how is it that it seems to have so distinctive a role to play (ie generating the multiverse) compared to all other particles likewise embedded in string theory?

(A similar but simpler question might be:
To the extent that most physicists seem to believe in inflation, why isn't the inflaton listed, at least tentatively, as one of the particles of the standard model of particle physics?)

Sabine Hossenfelder said...

Solomon,

There are various ways to get inflation going in string theory, this paper is a pretty good summary. You don't need additional stuff. The inflaton isn't part of the standard model because there's no evidence for its existence.

Sabine Hossenfelder said...

Everyone:

If you a new here, please take a moment to read my comment rules. I am not an ask-the expert forum. I do not approve and not answer random questions on topics that are not related to the blogpost.

George Herold said...

droid33 (and others) I'm mostly agnostic when it comes to various interpretations of QM. But if you haven't heard of it look at pilot wave theory. (Many worlds is of no use to me as a model of how the world works.)

Mozibur said...

I'm glad to hear that some-one is calling out for inflation for what it is - a hypothesis.

It isn't just mathematics which is about truth: Science is about *truthfully* describing what we observe and we are not just truthfully reporting on the facts but also thinking up theories which we truthfully state can come up with these observations in a more economical and conceptual fashion. Otherwise it would be just fake and flaky science.

morgawr said...

Many decades ago, when I was a physics student, I read Richard Tolmans "Special Relativity", published in 1917, now available on Project Gutenberg. Here is the first part of paragraph 163:

"With regard to the present status of the ether in scientific theory, it must be definitely stated that this concept has certainly lost both its fundamentality and the greater part of its usefulness, and this has been brought about by a gradual process which has only found its culmination in the work of Einstein. Since the earliest days of the luminiferous ether, the attempts of science to increase the substantiality of this medium have met with little success. Thus we have had solid elastic ethers of most extreme tenuity, and ethers with a density of a thousand tons per cubic millimeter; we have had quasi-material tubes of force and lines of force; we have had vibratory gyrostatic ethers and perfect gases of zero atomic weight; but after every debauch of model-making, science has recognized anew that a correct mathematical description of the actual phenomena of light propagation is superior to any of these sublimated material media. Already for Lorentz the ether had been reduced to the bare function of providing a stationary system of reference for the measurement of positions and velocities, and now even this function has been taken from it by the work of Einstein, which has shown that any unaccelerated system of reference is just as good as any other."

Lima Alfa said...

One nice thing, if Multiverse is real, is that there are planty of Sabine Hossenfelder.....LOL

Patat Je said...

"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."

This is not true. It is much more than an interpretation. The standard Copenhagen-interpretation says that the wave function collapses. The many worlds interpretation says such a collapse doesn't happen. The difference between these two theories could be measured in principle. There is a possibility that after a measurement, the universes which decohered from each other start to interfere again with each other.

And you also have the quantum suicide experiment. If the many world interpretation is true, you will never die in that experiment, you are only aware of the branch in which you will live. In the Copenhagen interpretation you will truly die, and your consciousness stops.

If there is only one universe, why do quantum physicists have to make such overcomplicated calculations to calculate the energy levels of atoms? For instance in classical quantum mechanics, a system with 4 particles which exert forces on each other, has to be described across 4 times 3 spatial dimensions, which represents the probability distribution of the different universes. Wouldn't 3 spatial dimensions have been enough?

And in relativistic quantum mechanics, why all the fuzz with these superpositions of all these created and annihilated particles with all their different possible trajectories across space and time? Isn't this way too complicated if there is only one universe?

I think if there is only one universe the physicists could heavily simplify their quantum mechanical calculations. I mean, if they want to calculate energy levels of a Helium atom, these levels are fixed. There isn't some probability they have another value. Why not skip the overcomplicated path integrals?

Vincent van der Goes said...

As an aside, the term Multiverse has already been popularized enough to have been absorbed by comics. A Marvel supervillain has to threaten to take over the entire multiverse nowadays if he wants to be taken seriously.

Len Arends said...

You will have strong evidence when you reach your 150th birthday. If every branch occurs, then there is always a branch where you are conscious for one moment longer.
Multiple-paths is testable ... individually ... with patience.

neo said...

do other QG theories from LQG to As to CDT to NCG predict a multiverse?

does the multiverse also include other non-string QG theories like LQG AS CDT NCG etc?

does the multiverse also include universes that have 1, 2 or 3 dimensions, or multiple time dimensions etc?

Jonathan Tooker said...

>"There is little evidence for inflation, and zero evidence for eternal inflation."

Isn't all inflation eternal inflation by reason of fluctuations-within-fluctuations causality, and therefore all evidence of inflation (weak and sparse at is) evidence for eternal inflation?


>"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."

I am surprised to here this from you as it seems to me, from my perspective as a non-expert string theorist, that the 10D stringlike boundary condition could hardly have "more mathematical motivation." Which starting assumptions do criticize in the foundations of the requirement for 10D stringlike dynamics?

>"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. "

What about the scientific discipline called mathematical physics which seeks to describe observables through the cataloging of what is true?

naivetheorist said...

Bee:

"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. "

This would have been a good place to use that Hawking quote i sent you earlier.

"I don't demand that a theory correspond to reality because I don't know what it is. All I'm concerned with is that the theory should predict the results of measurements."

best regards,


naive theorist.

Sabine Hossenfelder said...

Jonathan,

Inflation is an effective theory in which you use a scalar field to approximate a phase of de Sitter expansion. Where, when, and how that phase began is pretty much irrelevant because the data don't say anything about it. Whether it's eternal or not depends on the potential, about not much is known, and not all potentials lead to eternal inflation (it just so happens that most of the studied ones do - I leave it to you to interpret this). Maybe more importantly, as I said elsewhere, you can dump inflation entirely and blame initial conditions that must have come from a quantum gravitational phase - about which we know nothing.
Best,

B.

Sabine Hossenfelder said...

Neo,

All theories lead to a multiverse if you drop sufficiently many ties to observation. Look, you could for example complain that LQG doesn't uniquely predict the Hamiltonian, so there's a universe for each Hamiltonian. CDT doesn't predict the particles of the standard model, so there must be a multiverse. NCG, ASG, QFT, GFT? Whatever axioms you use, just drop any one of them and conclude there must be a universe for any possible choice. This isn't a "logical conclusion" as they want you to believe, it's mistaking the purpose of scientific theories. Best,

B.

Enrico said...

c) String theory landscape

Proponents of this theory forgot how to compute the probability (P) of an event. P is the number of desired outcomes divided by the number of all possible outcomes. In the multiverse, the desired outcome is our observed universe. The possible outcomes are infinite or some big number like 10^500. So P is zero. The probability of finding a universe like our own in the multiverse is zero. Surprised? Back to basics of probability theory.

Probability is a proxy measure of our knowledge or ignorance of an event. If you know I have a coin in one hand but don’t know which hand, you assign P = ½ = 0.5 to each hand. But if you know the coin is in my left hand, then the probabilities are: P (left) = 1, P (right) = 0. Suppose you don’t know where it is and I have ten hands? P = 1/10 = 0.1. If I have infinite number of hands? P = 1/(infinity) = 0. But if you know it’s in my left hand? P (left) = 1. Hence, a posteriori it doesn’t matter how many hands I have, P (left) is always 1. In the multiverse, a priori P = 0, a posteriori P = 1 regardless of the existence of other universes. How does postulating an infinite number of universes make our universe more likely or inevitable? It doesn’t.

The multiverse is a modern version of Pascal’s wager. If the probability is zero, multiply it by infinity to increase the probability. Supposedly dividing infinity by infinity yields P = 1. But if P = 1, then you have absolute certainty and hence no need to assign probabilities. Circular reasoning, reductio ad absurdum. If the language of probability is confusing, let me put it in plain English. If you have zero knowledge of something, anything you say about it must be nonsense.

Michael Jon Lukas said...

Thank you for finally calling Bull Shit on this topic.

JimV said...

Enrico said ... "In the multiverse, the desired outcome is our observed universe. The possible outcomes are infinite or some big number like 10^500. So P is zero. The probability of finding a universe like our own in the multiverse is zero."

By this logic no single-point event described by a continuous probability distribution (e.g, Gaussian, uniform, whatever) has other than a zero probability of occurring. Yet such events do occur.

Typically, in such cases one uses a probability density function, and talks about the probability of a small range of events occurring, not a single point. Therefore one can talk about the probability of finding a range if universes very similar (but not exactly identical - no duplicate Dr. Hossenfelders, for example, as unfortunate as that would be) to our own, with no logical or mathematical problem.

I have enough respect for thinkers like Hawking, Carroll, Guth, and so on not to dismiss their ideas as nonsense. They may be disproven, or never either accepted or disproven, but that does not make them nonsense. Without trials and errors there is no chance of progress, in my opinion.

Phillip Helbig said...

Many might have run into the multiverse in the writings of Max Tegmark. Just to clear up some confusion: what he calls the Level I Multiverse is what most people call the Universe (his "universe" is for most the "observable universe", i.e. that which is within the particle horizon). No-one doubts the existence of the Level I Multiverse, though one could argue that it is not really a multiverse like the others (II being other universes in the eternal-inflation picture, III being the many worlds in the corresponding interpretation of quantum mechanics, and IV being Max's Mathematical Universe). One could disagree with Max's terminology, but at least it is consistent.

He might have chosen it due to analogy: by definition, nothing beyond the particle horizon could have had any effect on us, yet no-one doubts the existence of stuff beyond the particle horizon. One can make similar (and perhaps weaker) arguments for the other types of multiverses.

Tegmark and Aguirre (in some other universes; in this one, Aguirre and Tegmark) also have a scheme in which Levels II and III are aspects of the same underlying idea (Phys.Rev. D84 (2011) 105002), though perhaps they wrote the paper only because the title contains a pun. :-) (As far as I know, this Aguirre is not related to the one in the Werner Herzog film starring Klaus Kinski.)



Charlie said...

Hi Sabine,

One of the points I think I've heard Sean Carroll make is that MW makes quantum mechanics consistent with the idea of a deterministic universe (or multiverse, as it were). The converse is that without it the universe isn't. Your thoughts? Is this a distinction without a difference since we would not have access to those other universes anyway? (I'm a biologist, so please go easy on me.)

neo said...

thanks Sabine,

btw can you comment on Urs Schreiber's critique of LQG program

"the mistake of LQG is in discarding the phase space of gravity and replacing it by a space of "generalized connections" which has little resemblance to the original problem"

Urs says LQG is not a candidate QG since LQG researchers work on "generalized connections"

Is Urs correct and any reason for "generalized connections" and discarding phase space?

Matt Grayson said...

"C’est magnifique, mais ce n’est pas la science: c'est de la folie!" - Pierre Bosquet

Enrico said...

"By this logic no single-point event described by a continuous probability distribution (e.g, Gaussian, uniform, whatever) has other than a zero probability of occurring. Yet such events do occur."

If a rectangle has infinite length, what should be its width to have a finite area?
Non-zero x infinity = infinity

If events do occur, then it's not a rectangle (non-uniform distribution) hence the length is finite as other points have zero values. But they don't know that a priori that's why they keep talking about infinity even if it doesn't make sense. Or they could just say it's finite but we don't know the shape. But they don't say that because that would be admitting they don't know the probability a priori, which leaves them with P = either 0 or unknown. But their answer is certainty P = 1, which cannot be derived a priori

Unlike you, unfortunately nonsense has no respect for big names

Александр said...

The theory of everything is the theory of consciousness. We ourselves are part of the universe and as part of the universe we interact with the rest of the universe. We really only have interaction and all our theories are the product of our consciousness. Our tools are the material continuation of our consciousness. We live in a "computer simulation", which we created ourselves. A multi-dimensional universe is a multi-consciousness. Can we change our consciousness so much to see another universe? It is to see to receive other signals not hallucinations. We are doing this. We expand consciousness and expand the visible universe. The problem in recording the correlation of consciousness and the universe

Louis Tagliaferro said...

Sabine said,

"Believe whatever you want, but don’t confuse it with science.

I read the whole article but that is all I needed to see, I agree.

marten said...

It is wise to get to the bottom of something like this. :-)

robert Wall said...

It’s interesting that you use “we.” We have no factual knowledge of even a single god...”

Of course those operating under the presumption of methodological naturalism (the presumed “we” of the scientific community) have not found evidence of the supernatural while limiting causal candidacy to natural causes. But to say “we” collectively to imply that mankind has no “factual knowledge” of the only God is really more than your presumed epistemology can deliver or comment on, right? Clearly, “factual knowledge” can be had completely outside “verifiability.” To then claim “no knowledge” really only applies to those who have already sel-regulated the knowledge to their preferred knowledge pool alone and that is naturalistic. One could say that for knowledge to be “collective” it must be verifiable? But even that seems to fail under its own weight. (Self referentially incoherent). Is the claim that “only verifiable knowledge is collective,” verifiable? No.
Factual knowledge of “the” only God, (or the only “deity” that is juxtaposed an eternal universe) is held by many. It is held directly in virtue of direct experience of God and Christianity too lacks “ defacto defeaters.” We hold the “factual knowledge” of Gods existence the same way an innocent man on trial for murder holds the true knowledge of his innocence, by direct experience. When he lands in prison via the failure of the best application of verifiability and the scientific method his knowledge doesn’t change to “oh, I must be guilty.” Of course not, that’s silly. Nonetheless, he has true knowledge regardless of the lack of “communal knowledge.” In fact, in light of his true knowledge one might say that we ought to have believed his testimony about himself to avert a great injustice!

Neither can false experiences of God or experiences of false gods be used to falsify true experiences of God. Especially in the light of clear defacto problems with the “competing experiences.” Perhaps, the testimony of those who experience God directly ought to be weighted a lot heavier?

So, to extrapolate a “zero” knowledge of god as compared to the “one” universe is not really accurate unless one has rigged the game to get the desired results. Besides, the existence of the external world (the universe) is also on par with the nature of Gods existence as being “properly basic.” So, it’s hardly a sensible jump to postulate “infinite universes” when one hasn’t honestly grappled with the existent knowledge of God amongst so many people.

marten said...

There is a difference between reasoning and intuition. The anthropic princiole for instance follows from intuition; there being no difference between the anthropic principle and the dinosauric principle follows from reasoning.

Sabine Hossenfelder said...

Marten,

You are wrong. The anthropic principle is not intuition-based. If there is anything that you can conclude based on reasoning it's that you live in a universe that allows you to reason. Descartes would be proud of your insight.

marten said...
This comment has been removed by the author.
marten said...

Sabine,

I don't think Descartes would be proud of my viewpoint. His idea for instance that human beings consist of a mechanical part and a soul seems rather intuitive to me.

tytung said...

I have a naive question about the statement that multiverses “exist”. When we say several things exist together, we usually mean they exist simultaneously, i.e. exist at the same time. But what does it mean to say these universes exist at the same “time”? Do we implicitly assume a notion of time/simultaneity at the meta-universe level?

Sabine Hossenfelder said...

tytung,

Well, you shouldn't say they exist because there's no way of knowing they exist.

Patat Je said...

Sabine Hossenfelder, if what you just said is true, does there exist anything beyond the outer edge (event horizon) of the observable universe or within the event horizon of a black hole? We can't say anything exists out there because there is no way of knowing.

Sabine Hossenfelder said...

Patat,

There's somewhat of a conflation of terminology here in that "the observable universe" isn't what is meant by a "universe" in the multiverse, for our "universe" might extend beyond what's observable.

Having said this, if your theory has elements that are not necessary to describe observations, science has nothing to say about their "existence." This means both it does not say they exist and does not say they do not exist. It just simply has nothing to say about it. Same situation indeed as with god and other deities. They're not science.

JimV said...

"It just simply has nothing to say about it. Same situation indeed as with god and other deities. They're not science."

To expand upon that a little, they are not science because there is no reliable, verifiable evidence for them, and they offer no explanatory or predictive value. However, since their powers by definition are incomprehensible and unbounded, it cannot be proved that they are not hiding in some other universe between random miracles. So in fact they are yet another form of multiverse hypothesis.

Pat Ferrel said...

Very, very nice. Occam would approve. The usual suspects are skewered, but I've always wanted to see inflation on that list. "Uhh, the universe got this way because of a magic faster than light expansion of space-time. Something testable? Well just look at the CMB..." Huh?