My most read blogposts of the decade

1. The LHC “nightmare scenario” has come true (Aug 6, 2016)


2. The multiworse is coming (Mar 13, 2018)


3. Outraged about the Google diversity memo? I want you to think about it. (Aug 9, 2017)


4. No, physicists have not created “negative mass” (Apr 21, 2017)


5. A Philosopher Tries to Understand the Black Hole Information Problem (May 11, 2017)


6. The present phase of stagnation in the foundations of physics is not normal (Nov 19, 2018)


7. The crisis in physics is not only about physics (Oct 30, 2019)


8. The Forgotten Solution: Superdeterminism (Jul 28, 2019)


9. String theory pros and cons [video - no singing!] (Oct 22, 2018)


10. Dark matter nightmare: What if we are just using the wrong equations? (Oct 16, 2019)

I wish you all a good start into the New Year!

How did the universe begin?

The year is almost over and a new one about to begin. So today I want to talk about the beginning of everything, the whole universe. What do scientists think how it all started?


We know that the universe expands, and as the universe expands, matter and energy in it dilutes. So when the universe was younger, matter and energy was much denser. Because it was denser, it had a higher temperature. And a higher temperature means that on the average particles collided at higher energies.

Now you can ask, what do we know about particles colliding at high energies? Well, the highest collision energies between particles that we have experimentally tested are those produced at the Large Hadron Collider. These are energies about a Tera-electron Volt or TeV for short, which, if you convert it into a temperature, comes out to be 10¹⁶ Kelvin. In words that’s ten million billion Kelvin which sounds awkward and is the reason no one quotes such temperatures in Kelvin.

So, up to a temperature of about a TeV, we understand the physics of the early universe and we can reliably tell what happened. Before that, we have only speculation.

The simplest way to speculate about the early universe is just to extrapolate the known theories back to even higher temperatures, assuming that the theories do not change. What happens then is that you eventually reach energy densities so high that the quantum fluctuations of space and time become relevant. To calculate what happens then, we would need a theory of quantum gravity, which we do not have. So, in brief, the scientific answer is that we have no idea how the universe began.

But that’s a boring answer and one you cannot publish, so it’s not how the currently most popular theories for the beginning of the universe work. The currently most popular theories assume that the electromagnetic interaction must have been unified with the strong and the weak nuclear force at high energies. They also assume that an additional field exists, which is the so-called inflaton field.

The purpose of the inflaton is to cause the universe to expand very rapidly early on, in a period which is called “inflation”. The inflaton field then has to create all the other matter in the universe and basically disappear because we don’t see it today. In these theories, our universe was born from a quantum fluctuation of the inflaton field and this birth event is called the “Big Bang”.

Actually, if you believe this idea, the quantum fluctuations still go on outside of our universe, so there are constantly other universes being created.

How scientific is this idea? Well, we have zero evidence that the forces were ever unified and have equally good evidence, namely none, that the inflaton field exists. The idea that the early universe underwent a phase of rapid expansion fits to some data, but the evidence is not overwhelming, and in any case, what the cause of this rapid expansion would have been – an inflaton field or something else – the data don’t tell us.

So that the universe began from a quantum fluctuations is one story. Another story has it that the universe was not born once but is born over and over again in what is called a “cyclic” model. In cyclic models, the Big Bang is replaced by an infinite sequence of Big Bounces.

There are several types of cyclic models. One is called the Ekpyrotic Universe. The idea of the Ekpyrotic Universe was originally borrowed from string theory and had it that higher-dimensional membranes collided and our universe was created from that collision.

Another idea of a cyclic universe is due to Roger Penrose and is called Conformal Cyclic Cosmology. Penrose’s idea is basically that when the universe gets very old, it loses all sense of scale, so really there is no point in distinguishing the large from the small anymore, and you can then glue together the end of one universe with the beginning of a new one.

Yet another theory has it that new universes are born inside black holes. You can speculate about this because no one has any idea what goes on inside black holes anyway.

An idea that sounds similar but is actually very different is that the universe started from a black hole in 4 dimensions of space. This is a speculation that was put forward by Niayesh Afshordi some years ago.

 Then there is the possibility that the universe didn’t really “begin” but that before a certain time there was only space without any time. This is called the “no-boundary proposal” and it goes back to Jim Hartle and Stephen Hawking. A very similar disappearance of time was more recently found in calculations based on loop quantum cosmology where the researchers referred to it as “Asymptotic Silence”.

Then we have String Gas Cosmology, in which the early universe lingered in an almost steady state for an infinite amount of time before beginning to expand, and then there is the so-called Unicorn Cosmology, according to which our universe grew out of unicorn shit. Nah, I made this one up.

So, as you see, physicists have many ideas about how the universe began. The trouble is that not a single one of those ideas is backed up by evidence. And they may never be backed up by evidence, because the further back in time you try to look, the fewer data we have. While some of those speculations for the early universe result in predictions, confirming those predictions would not allow us to conclude that the theory must have been correct because there are many different theories that could give rise to the same prediction.

This is a way in which our scientific endeavors are fundamentally limited. Physicists may simply have produced a lot of mathematical stories about how it all began, but these aren’t any better than traditional tales of creation.

Monty Python's Galaxy Song [I've been singing again]

Here is what I did on the weekend. Happy Holidays everybody!



[You can download a mp3 version here.]

What does a theoretical physicist do?

This week, I am on vacation and so I want to answer a question that I get a lot but that doesn’t really fit into the usual program: What does a theoretical physicist do? Do you sit around all day and dream up new particles or fantasize about the beginning of the universe? How does it work?


Research in theoretical physics generally does one of two things: Either we have some data that require explanation for which a theory must be developed. Or we have a theory that requires improvement, and the improved theory leads to a prediction which is then experimentally tested.

I have noticed that some people think theoretical physics is something special to the foundations of physics. But that isn’t so. All subdisciplines of physics have an experimental part and a theoretical part. How much the labor is divided into different groups of people depends strongly on the field. In some parts of astrophysics, for example, data collection, analysis, and theory-development is done by pretty much the same people. That’s also the case in some parts of condensed matter physics. In these areas many experimentalists are also theorists. But if you look at fields like cosmology or high energy particle physics, people tend to specialize either in experiment or in theory development.

Theoretical physics is pretty much a job like any other in that you get an education and then you put your knowledge to work. You find theoretical physicists in public higher education institutions, which is probably what you are most familiar with, but you also find them in the industry or in non-profit research institution like the one I work at. Just what the job entails depends on the employer. Besides the research, a theoretical physicist may have administrational duties, or may teach, mentor students, do public outreach, organize scientific meetings, sit on committees and so on.

When it comes to the research itself, theoretical physics doesn’t work any different from other disciplines of science. The largest part of research, ninetynine percent, is learning what other people have done. This means you read books and papers, go to seminars, attend conferences, listen to lectures and you talk to people until you understand what they have done.

And as you do that, you probably come across some open problems. And from those you pick one for your own research. You would pick a problem that, well, you are interested in, but also something that you think will move the field forward and, importantly, you pick a problem that you think you have a reasonable chance of solving with what you know. Picking a research topic that is both interesting and feasible is not easy and requires quite some familiarity with the literature, which is why younger researchers usually rely on more senior colleagues to pick a topic.

Where theoretical physics is special is in the amount of mathematics that we use in our research. In physics all theories are mathematical. This means both that you must know how to model a natural system with mathematics and you must know how to do calculations within that model. Of course we now do a lot of calculations numerically, on a computer, but you still have to understand the mathematics that goes into this. There is really no way around it. So that’s the heart of the job, you have to find, understand, and use the right mathematics to describe nature.

The thing that a lot people don’t understand is just how constraining mathematics is in theory development. You cannot just dream up a particle, because almost everything that you can think of will not work if you write down the mathematics. It’s either just nonsense or you find quickly that it is in conflict with observation already.

But the job of a theoretical physicist is not done with finishing a calculation. Once you have your results, you have to write them up and publish them and then you will give lectures about it so that other people can understand what you have done and hopefully build on your work.

What’s fascinating about theoretical physics is just how remarkably well mathematics describes nature. I am always surprised if people tell me that they never understood physics because I would say that physics is the only thing you can really understand. It’s the rest of the world that doesn’t make sense to me.

The path we didn’t take

“There are only three people in the world who understand Superdeterminism,” I used to joke, “Me, Gerard ‘t Hooft, and a guy whose name I can’t remember.” In all honesty, I added the third person just in case someone would be offended I hadn’t heard of them.

What the heck is Superdeterminism?, you ask. Superdeterminism is what it takes to solve the measurement problem of quantum mechanics. And not only this. I have become increasingly convinced that our failure to solve the measurement problem is what prevents us from making progress in the foundations of physics overall. Without understanding quantum mechanics, we will not understand quantum field theory, and we will not manage to quantize gravity. And without progress in the foundations of physics, we are left to squeeze incrementally better applications out of the already known theories.

The more I’ve been thinking about this, the more it seems to me that quantum measurement is the mother of all problems. And the more I am talking about what I have been thinking about, the crazier I sound. I’m not even surprised no one wants to hear what I think is the obvious solution: Superdeterminism! No one besides ‘t Hooft, that is. And that no one listens to ‘t Hooft, despite him being a Nobel laureate, doesn’t exactly make me feel optimistic about my prospects of getting someone to listen to me.

The big problem with Superdeterminism is that the few people who know what it is, seem to have never thought about it much, and now they are stuck on the myth that it’s an unscientific “conspiracy theory”. Superdeterminism, so their story goes, is the last resort of the dinosaurs who still believe in hidden variables. According to these arguments, Superdeterminism requires encoding the outcome of every quantum measurement in the initial data of the universe, which is clearly outrageous. Not only that, it deprives humans of free will, which is entirely unacceptable.

If you have followed this blog for some while, you have seen me fending off this crowd that someone once aptly described to me as “Bell’s Apostles”. Bell himself, you see, already disliked Superdeterminism. And the Master cannot err, so it must be me who is erring. Me and ‘t Hooft. And that third person whose name I keep forgetting.

Last time I made my 3-people-joke was in February during a Facebook discussion about the foundations of quantum mechanics. On this occasion, someone offered in response the name “Tim Palmer?” Alas, the only Tim Palmer I’d heard of is a British music producer from whose videos I learned a few things about audio mixing. Seemed like an unlikely match.

But the initial conditions of the universe had a surprise in store for me.

The day of that Facebook comment I was in London for a dinner discussion on Artificial Intelligence. How I came to be invited to this event is a mystery to me. When the email came, I googled the sender, who turned out to be not only the President of the Royal Society of London but also a Nobel Prize winner. Thinking this must be a mistake, I didn’t reply. A few weeks later, I tried to politely decline, pointing out, I paraphrase, that my knowledge about Artificial Intelligence is pretty much exhausted by it being commonly abbreviated AI. In return, however, I was assured no special expertise was expected of me. And so I thought, well, free trip to London, dinner included. Would you have said no?

When I closed my laptop that evening and got on the way to the AI meeting, I was still wondering about the superdeterministic Palmer. Maybe there was a third person after all? The question was still circling in my head when the guy seated next to me introduced himself as... Tim Palmer.

Imagine my befuddlement.

This Tim Palmer, however, talked a lot about clouds, so I filed him under “weather and climate.” Then I updated my priors for British men to be called Tim Palmer. Clearly a more common name than I had anticipated.

But the dinner finished and our group broke up and, as we walked out, the weather-Palmer began talking about free will! You’d think it would have dawned on me then I’d stumbled over the third Superdeterminist. However, I was merely thinking I’d had too much wine. Also, I was now somewhere in London in the middle of the night, alone with a man who wanted to talk to me about free will. I excused myself and left him standing in the street.

But Tim Palmer turned out to not only be a climate physicist with an interest in the foundations of quantum mechanics, he also turned out to be remarkably persistent. He wasn’t remotely deterred by my evident lack of interest. Indeed, I later noticed he had sent me an email already two years earlier. Just that I dumped it unceremoniously in my crackpot folder. Worse, I seem to vaguely recall telling my husband that even the climate people now have ideas for how to revolutionize quantum mechanics, hahaha.

Cough.

Tim, in return, couldn’t possibly have known I was working on Superdeterminism. In February, I had just been awarded a small grant from the Fetzer Franklin Fund to hire a postdoc to work on the topic, but the details weren’t public information.

Indeed, Tim and I didn’t figure out we have a common interest until I interviewed him on a paper he had written about something entirely different, namely how to quantify the uncertainty of climate models.

I’d rather not quote cranks, so I usually spend some time digging up information about people before interviewing them. That’s when I finally realized Tim’s been writing about Superdeterminism when I was still in high school, long before even ‘t Hooft got into the game. Even more interestingly, he wrote his PhD thesis in the 1970s about general relativity before gracefully deciding that working with Stephen Hawking would not be a good investment of his time (a story you can hear here at 1:12:15). Even I was awed by that amount of foresight.

Tim and I then spent some months accusing each other of not really understanding how Superdeterminism works. In the end, we found we agree on more points than not and wrote a paper to explain what Superdeterminism is and why the objections often raised against it are ill-founded. Today, this paper is on the arXiv.

Rethinking Superdeterminism
S. Hossenfelder, T.N. Palmer
arXiv:1912.06462 [quant-ph]

Thanks to support from the Fetzer Franklin Fund, we are also in the midst of organizing a workshop on Superdeterminism and Retrocausality. So this isn’t the end of the story, it’s the beginning.

How Scientists Can Avoid Cognitive Bias

Today I want to talk about a topic that is much, much more important than anything I have previously talked about. And that’s how cognitive biases prevent science from working properly.


Cognitive biases have received some attention in recent years, thanks to books like “Thinking Fast and Slow,” “You Are Not So Smart,” or “Blind Spot.” Unfortunately, this knowledge has not been put into action in scientific research. Scientists do correct for biases in statistical analysis of data and they do correct for biases in their measurement devices, but they still do not correct for biases in the most important apparatus that they use: Their own brain.

Before I tell you what problems this creates, a brief reminder what a cognitive bias is. A cognitive bias is a thinking shortcut which the human brain uses to make faster decisions.

Cognitive biases work much like optical illusions. Take this example of an optical illusion. If your brain works normally, then the square labelled A looks much darker than the square labelled B.

[Example of optical illusion. Image: Wikipedia]

But if you compare the actual color of the pixels, you see that these squares have exactly the same color.

[Example of optical illusion. Image: Wikipedia]

The reason that we intuitively misjudge the color of these squares is that the image suggests it is really showing a three-dimensional scene where part of the floor is covered by a shadow. Your brain factors in the shadow and calculates back to the original color, correctly telling you that the actual color of square B must have been lighter than that of square A.

So, if someone asked you to judge the color in a natural scene, your answer would be correct. But if your task was to evaluate the color of pixels on the screen, you would give a wrong answer – unless you know of your bias and therefore do not rely on your intuition.

Cognitive biases work the same way and can be prevented the same way: by not relying on intuition. Cognitive biases are corrections that your brain applies to input to make your life easier. We all have them, and in every-day life, they are usually beneficial.

The maybe best-known cognitive bias is attentional bias. It means that the more often you hear about something, the more important you think it is. This normally makes a lot of sense. Say, if many people you meet are talking about the flu, chances are the flu’s making the rounds and you are well-advised to pay attention to what they’re saying and get a flu shot.

But attentional bias can draw your attention to false or irrelevant information, for example if the prevalence of a message is artificially amplified by social media, causing you to misjudge its relevance for your own life. A case where this frequently happens is terrorism. Receives a lot of media coverage, has people hugely worried, but if you look at the numbers for most of us terrorism is very unlikely to directly affect our life.

And this attentional bias also affects scientific judgement. If a research topic receives a lot of media coverage, or scientists hear a lot about it from their colleagues, those researchers who do not correct for attentional bias are likely to overrate the scientific relevance of the topic.

There are many other biases that affect scientific research. Take for example loss aversion. This is more commonly known as “throwing good money after bad”. It means that if we have invested time or money into something, we are reluctant to let go of it and continue to invest in it even if it no longer makes sense, because getting out would mean admitting to ourselves that we made a mistake. Loss aversion is one of the reasons scientists continue to work on research agendas that have long stopped being promising.

But the most problematic cognitive bias in science is social reinforcement, also known as group think. This is what happens in almost closed, likeminded, communities, if you have people reassuring each other that they are doing the right thing. They will develop a common narrative that is overly optimistic about their own research, and they will dismiss opinions from people outside their own community. Group think makes it basically impossible for researchers to identify their own mistakes and therefore stands in the way of the self-correction that is so essential for science.

A bias closely linked to social reinforcement is the shared information bias. This bias has the consequence that we are more likely to pay attention to information that is shared by many people we know, rather than to the information held by only few people. You can see right away how this is problematic for science: That’s because how many people know of a certain fact tells you nothing about whether that fact is correct or not. And whether some information is widely shared should not be a factor for evaluating its correctness.

Now, there are lots of studies showing that we all have these cognitive biases and also that intelligence does not make it less likely to have them. It should be obvious, then, that we organize scientific research so that scientists can avoid or at least alleviate their biases. Unfortunately, the way that research is currently organized has exactly the opposite effect: It makes cognitive biases worse.

For example, it is presently very difficult for a scientist to change their research topic, because getting a research grant requires that you document expertise. Likewise, no one will hire you to work on a topic you do not already have experience with.

Superficially this seems like good strategy to invest money into science, because you reward people for bringing expertise. But if you think about the long-term consequences, it is a bad investment strategy. Because now, not only do researchers face a psychological hurdle to leaving behind a topic they have invested time in, they would also cause themselves financial trouble. As a consequence, researchers are basically forced to continue to claim that their research direction is promising and to continue working on topics that lead nowhere.

Another problem with the current organization of research is that it rewards scientists for exaggerating how exciting their research is and for working on popular topics, which makes social reinforcement worse and adds to the shared information bias.

I know this all sounds very negative, but there is good news too: Once you are aware that these cognitive biases exist and you know the problems that they can cause, it is easy to think of ways to work against them.

For example, researchers should be encouraged to change topics rather than basically being forced to continue what they’re already doing. Also, researchers should always list shortcoming of their research topics, in lectures and papers, so that the shortcomings stay on the collective consciousness. Similarly, conferences should always have speakers from competing programs, and scientists should be encouraged to offer criticism on their community and not be avoided for it. These are all little improvements that every scientist can make individually, and once you start thinking about it, it’s not hard to come up with further ideas.

And always keep in mind: Cognitive biases, like seeing optical illusions are a sign of a normally functioning brain. We all have them, it’s nothing to be ashamed about, but it is something that affects our objective evaluation of reality.

The reason this is so, so important to me, is that science drives innovation and if science does not work properly, progress in our societies will slow down. But cognitive bias in science is a problem we can solve, and that we should solve. Now you know how.

Why the laws of nature are not inevitable, never have been, and never will be.

[Still from the 1956 movie The Ten Commandments]

No one has any idea why mathematics works so well to describe nature, but it is arguably an empirical fact that it works. A corollary of this is that you can formulate theories in terms of mathematical axioms and derive consequences from this. This is not how theories in physics have historically been developed, but it’s a good way to think about the relation between our theories and mathematics.

All modern theories of physics are formulated in mathematical terms. To have a physically meaningful theory, however, mathematics alone is not sufficient. One also needs to have an identification of mathematical structures with observable properties of the universe.

The maybe most important lesson physicists have learned over the past centuries is that if a theory has internal inconsistencies, it is wrong. By internal inconsistencies, I mean that the theory’s axioms lead to statements that contradict each other. A typical example is that a quantity defined as a probability turns out to take on values larger than 1. That’s mathematical rubbish; something is wrong.

Of course a theory can also be wrong if it makes predictions that simply disagree with observations, but that is not what I am talking about today. Today, I am writing about the nonsense idea that the laws of nature are somehow “inevitable” just because you can derive consequences from postulated axioms.

It is easy to see that this idea is wrong even if you have never heard the word epistemology. Consequences which you can derive from axioms are exactly as “inevitable” as postulating the axioms, which means the consequences are not inevitable. But that this idea is wrong isn’t the interesting part. The interesting part is that it remains popular among physicists and science writers who seem to believe that physics is somehow magically able to explain itself.

But where do we get the axioms for our theories from? We use the ones that, according to present knowledge, do the best job to describe our observations. Sure, once you have written down some axioms, then anything you can derive from these axioms can be said to be an inevitable consequence. This is just the requirement of internal consistency.

But the axioms themselves can never be proved to be the right ones and hence will never be inevitable themselves. You can say they are “right” only to the extent that they give rise to predictions that agree with observations.

This means not only that we may find tomorrow that a different set of axioms describes our observations better. It means more importantly that any statement about the inevitability of the laws of nature is really a statement about our inability to find a better explanation for our observations.

This confusion between the inevitability of conclusions given certain axioms, and the inevitability of the laws of nature themselves, is not an innocuous one. It is the mistake behind string theorists’ conviction that they must be on the right track just because they have managed to create a mostly consistent mathematical structure. That this structure is consistent is of course necessary for it to be a correct description of nature. But it is not sufficient. Consistency tells you nothing whatsoever about whether the axioms you postulated will do a good job to describe observations.

Similar remarks apply to the Followers of Loop Quantum Gravity who hold background independence to be a self-evident truth, or to everybody who believes that statistical independence is sacred scripture, rather than being what it really is: A mathematical axiom, that may or may not continue to be useful.

Another unfortunate consequence of physicists’ misunderstanding of the role of mathematics in science are multiverse theories.

This comes about as follows. If your theory gives rise to internal contradictions, it means that at least one of your axioms is wrong. But one way to remove internal inconsistencies is to simply discard axioms until the contradiction vanishes.

Dropping axioms is not a scientifically fruitful strategy because you then end up with a theory that is ambiguous and hence unpredictive. But it is a convenient, low-effort solution to get rid of mathematical problems and has therefore become fashionable in physics. And this is in a nutshell where multiverse theories come from: These are theories which lack sufficiently many axioms to describe our universe.

Somehow an increasing number of physicists has managed to convince themselves that multiverse ideas are good scientific theories instead of what they de facto are: Useless.

There are infinitely many sets of axioms that are mathematically consistent but do not describe our universe. The only rationale scientists have to choose one over the other is that the axioms give rise to correct predictions. But there is no way to ever prove that a particular set of axioms is inevitably the correct one. Science has its limits. This is one of them.

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.