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Multiverse may be real, needed

That's why physicists are obliged to think about it

Sabine Hossenfelder wrote another atrocious anti-physics rant, More Multiverse Madness, and the percentage of lies and complete misunderstandings in that rant seems absolutely staggering to me.

You should be skeptical about the multiverse, she says. Fine but you should be even more skeptical of a priori criticisms of the multiverse and efforts to turn it into a taboo. She starts with several widespread "types of multiverses":

(a) The many worlds interpretation of quantum mechanics,
(b) eternal inflation, and
(c) the string theory landscape.
It's unfortunate that very different ideas like that are being conflated in this way but that's the case. (Susskind argues that they should be conflated because "they're the same" but his paper makes no sense to any other physicist.) Well, there's already a stupidity in the three bullets above.

The landscape itself isn't a "multiverse". The landscape is a structure that exists on the configuration space of string theory, basically on the space of the spacetime scalar fields \(\phi_i\). In particular, it's used for the set of candidate vacua i.e. the stationary points (minima) of the potential \(V(\phi_i)\). The multiverse is something that geometrically exists in the "regular spacetime" – it is a type of the (very large) spacetime. So the landscape isn't a multiverse.

At most, the string theory landscape suggests that a specific multiverse could exist (i.e. the "landscape gives rise to a multiverse"), a multiverse in which different parts are Universes built around different vacua in the landscape i.e. obey different effective low-energy laws of physics. But this kind of the multiverse generated by the string theory landscape is really the same thing as the multiverse from eternal inflation.

The eternal inflation is the only known general mechanism to generate this kind of a multiverse and the string theory landscape is just the most well-defined, most persuasive, specific way to realize eternal inflation. So her entry (c) is either wrong as a "type of multiverse" or it's wrong that she separated it from (b). This may look like a subtlety but it's already a proof that she misunderstands something totally elementary about the multiverse, the landscape, and especially their relationship. She's just emitting toxic incoherent junk about very important scientific concepts she has no idea about.




In the following paragraph, we learn the following about MWI:
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.
The many worlds interpretation may be an "interpretation" but there is no viable, legitimate theory in physics that could be called "the many worlds interpretation". Nothing like that works, nothing like that reproduces the rules of quantum mechanics. It's not up to you whether you "believe" it. Physics isn't about beliefs. It's a fact that quantum mechanics requires to treat probabilities as fundamental concepts and there's no consistent way to "fudge them" using many worlds or anything else.




A weird way to use the word "extrapolation":
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 isn't an "extrapolation" of inflation. It is a type of inflation, eternal inflationary models are a subset of inflationary models. Some inflationary models give rise to eternal inflation, others don't. Being a "subgroup" isn't the same as being an "extrapolation". The eternally inflating history might be called an extrapolation of "one inflationary burst" but that is the right extrapolation only within some models, those that do imply eternal inflation.

In the same way, inflation isn't an "extrapolation" of the concordance model. It is a extension of the concordance model. These two ex- words aren't the same. An extrapolation of X is something that follows from X according to straightforward rules. The inflationary epoch in no way "follows" from the big bang theory. After all, the big bang theory implies a power law expansion of the Universe while the inflationary expansion is exponential. An exponential function surely doesn't follow from a power law by an "extrapolation".

Similarly, the big bang theory isn't just an "extrapolation" of the observed expansion of the Universe. It's a specific theory that allows us to make the extrapolation, one based on Einstein's general theory of relativity. The word "extrapolation" indicates that the empirical data and a ruler are everything you need to find the "extrapolated" theory and its predictions but it's complete nonsense in these three examples. You need lots of deep science that had to be guessed by someone (often geniuses like Einstein), picked from the alternatives, and elaborated upon. Ms Hossenfelder has no respect towards these things – she has no respect for science and scientists – but that doesn't mean that these things aren't amazingly deep and valuable.
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.
There is absolutely nothing wrong about a theory just because the theory contains objects that "vanished long ago". A theory says that dinosaurs used to walk on the surface of the Earth. These alive dinosaurs have vanished a long time ago. That doesn't mean that there's something wrong about the scientific claims about the dinosaurs. In the same way, the Universe has pretty much certainly contained tons of things that haven't existed for billions of years. Plasma filling the interstellar space that was opaque, macroscopic pieces of quark-gluon plasma, the phase of the Universe where the electroweak symmetry is unbroken, and hundreds of other things.

A theorist simply has to "invent" concepts that aren't seen today, at least not directly, otherwise the theorist is completely worthless – like Ms Hossenfelder. It's disingenuous for her to demonize physicists just because they do their work.
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.
The answers may be incomplete but it's just plain rubbish that "no one really knows" how quantum fluctuations of a field affect gravity. That's indeed exactly what good physicists in this area know a lot about, it's their work. The answer to this particular question is mostly determined by the equivalence principle. The fact that Ms Hossenfelder doesn't know anything about these things doesn't make her superior, a natural arbiter who may judge physicists. On the contrary, it proves that she is just an arrogant layman and at most a fake scientist who simply doesn't have a clue what she should understand if she were what she claims to be.
There is little evidence for inflation, and zero evidence for eternal inflation.
The evidence for inflation is extremely strong – and just microns away from being a definitive proof (and a micron is even pettier than a Macron). Inflation is by far the most satisfactory explanation of many basic observed properties of the Universe around us – its huge mass and size, its almost complete flatness, density fluctuations and their patterns etc. etc. – and some would justifiably claim that it's the only explanation we have and maybe the only viable one that is possible.

I think it's a sign of a complete collapse of meritocracy in the Academia that someone claiming that "there is little evidence for inflation and zero evidence for eternal inflation" could have been admitted as a theoretical physics graduate student.

Eternal inflation is "more likely than not" to follow from inflation. The probability that an inflationary model gives rise to eternal inflation seems higher than 50% which is a fact that physicists simply have to take into account.
And then there’s the string theory landscape, the graveyard of disappointed hopes.
The string theory landscape is a qualitative answer to an important physics question – one about the size and internal structure of the set of (especially 3+1-dimensional) vacuum-like solutions to the equations of string theory. The answer probably is whatever the evidence and proofs indicate that it is and the scientific method ultimately doesn't revolve around "hopes", any other emotional concepts, or prejudices.

One may have hopes of any way but an honest, competent scientist cannot pretend that "hopes" count as scientific arguments.
String theorists originally hoped that their theory would explain everything.
As Sheldon Cooper would surely clarify as well, physics may only explain things that are worth explaining and that may be explained in principle. Assuming the landscape is relevant, string theory may still explain everything but the explanation of certain facts about the Universe may be that they were more or less randomly chosen out of many options (a discrete set of options), just like the human DNA.
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.
To insist that everything predicted by a theory to exist exists according to the theory is called "integrity". That's another thing that Ms Hossenfelder lacks. String theory is a very specific robust theory that makes predictions and claims about the Universe. Those claims were guessed in the distant past but many of them were properly derived. They are whatever they are, whether or not someone likes them.
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 only problem is that there is no other game in town when it comes to the deep insights about the Universe that string theory clarifies and quantifies. Ms Hossenfelder may do "the same thing" with a non-existent theory but when two are doing the same thing, it's not the same thing because only string theory actually exists. So when she does "the same thing" with non-existent theories, others – such as your humble correspondent – will point out that she is a scammer.
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.
Assuming that the papers that show it are correct, the landscape obviously is a prediction of string theory. The prediction doesn't contradict any empirical data that are known so this prediction cannot be used to punish let alone rule out the theory. String theory is our only candidate for a theory of everything, despite Ms Hossenfelder's efforts to introduce lies into popular articles about this important fact, and this claim is all about the string theory's exclusive ability to make quantitative statements about certain deep enough statements.

Whether or not string theory is a theory of everything cannot be affected by the question whether someone finds some of its so far untested predictions philosophically pleasing. You don't like them pleasing? Too bad, Ms Hossenfelder. Fudge off and move into another multiverse where the laws are more pleasing to you.
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!
2. Ok, so it’s not falsifiable, but it’s sound logic!
3. Ok, then. So it’s neither falsifiable nor sound logic, but it’s still business as usual.
4. So what? We’ll do it anyway.
No real physicist is frequently using the word "falsifiable" as a part of his research. Only fudged up demagogues and fake scientists like Ms Hossenfelder are obsessed with this word and pump it into 75% of their sentences (see the list above).

The multiverse is obviously sufficiently falsifiable to be considered science and any "demand" that it should be "more" falsifiable is just a reflection of someone's prejudices. Only the simple claim (4) is somewhat relevant. Scientists do their research, anyway. They shouldn't be intimidated by deluded aggressive idiots and would-be bullies like Ms Hossenfelder. As Nima Arkani-Hamed urged the younger physicists in a recent lecture, just don't pay attention to someone's efforts to restrict what you're thinking about and what you find interesting.

Scientists study the multiverse simply because it may be real and it may be needed, as I wrote in the title. It's a possible sketch of the future answers to questions about the size and geometry of the world we inhabit at the longest possible length scales and time scales. The world obviously may be this large (the Earth is also larger than the first curious cavemen were imagining) and this possible answer follows from models that explain other things. And this possible large size could explain other things we observe. So it must be admitted as an answer, physicists must consider it as an option, and they may think harder and deeper about it regardless of the whining of irrational, demagogic, and prejudiced fake scientists such as Ms Hossenfelder.

It's not just some annoying obligation. Physicists are curious and many are thinking about the possibilities involving the multiverse because they're fascinated by them. They just can't resist the temptation. Some of the work is very good, some of it is not but it's obviously incompatible with the scientific age to try to ban anything that just touches the concept of the multiverse.

The tasks that need to be solved before the multiverse picture – or a competing one – becomes very clear may be difficult tasks. But their being difficult doesn't mean that the paradigm is wrong or it shouldn't be tried. On the contrary, lots of brilliant physicists – the very opposite ones to Ms Hossenfelder – do physics exactly because it's so difficult and the cracking of these problems seems to require their rare brains.

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