knzhou

The Physical Review is a gigantic set of journals, and like anything of its size, many of its published results are wrong. At this very moment I'm writing a rebuttal to a PRD paper that arrived at nonsensical conclusions due to some basic algebra mistakes.

Indeed, the author's 2000-page online textbook, heavily promoted on the internet, is a classic trap for unwary students.

It looks alright at first: volume I is light on math, but full of neat examples. But it's full of intuitively plausible but slightly wrong statements which fall apart in more general situations, reflecting the author's lack of technical expertise. This problem steadily gets worse: volume IV is an oversimplified introduction to quantum mechanics which contains almost no math, and serious conceptual errors on almost every page. Volume V covers a bizarre mix of particle physics, consciousness, and sexual reproduction. And volume VI is the author's almost math-free personal theory of everything. Because the change is gradual, a student can get seriously misled without noticing, like the proverbial boiling frog.

On HN, people are always asking how to get started self-learning topics like physics. The tragedy is that this has been a completely solved problem for decades: the standard textbooks are excellent. But people don't hear that message because self-promoters pollute the discourse.

This is a new proposal, fresh on the arXiv today, from a group of U.S. particle physicists. The introduction is very readable and lays out the mission clearly:

We can now confidently claim that the “Standard Model” of particle physics (SM) is established. At the same time, we are more and more strongly persuaded that this SM is incomplete. [...] It is now common to describe the SM as an “effective” theory that should be derived from some more fundamental theory at higher energies. But we have almost no evidence on the properties of that theory.

Our successes have become a liability in reaching this goal. Scientists from other fields now have the impression that particle physics is a finished subject. They question our motivations to go on to explore still higher energies. The scale of an energy frontier collider is also challenging to the young people in our field. They need to see qualitatively new capabilities realized during their active scientific careers. [...] That is where the urgency lies.

[T]he entire C3 program could be sited in the United States. With the cancellation of the Superconducting Super Collider and the end of Tevatron operations the US has largely abandoned construction of domestic accelerators at the energy frontier. C3 offers the opportunity to realize an affordable energy frontier facility in the US. This may be crucial to realize a Higgs factory in the near term, and it will also position the US to lead the drive to the next, higher energy stage of exploration.

The main innovation is that they propose to use non-superconducting cavities, which allow much higher accelerating fields, cooled to increase their quality factor. The resulting shorter length dramatically decreases the cost, to an estimated $4 billion, which is 80% to 90% less than other proposals. Of course, $4 billion is no small amount of money, but for perspective that's about equal to the monthly budget of the National Institutes of Health, a third of the cost of the James Webb Space Telescope, or 2% of the total cost of the space shuttle.

One should keep in mind that most dark matter "alternatives", including this one, actually include dark matter. It says so right on the 2nd page of their paper:

Consider requirement (iii), that is, successful cosmology. In (2) we have a new d.o.f. φ [...] What should the expectation for a cosmological evolution of ϕ be? The MOND law for galaxies is silent regarding this matter. There is, however, another empirical law which concerns cosmology: the existence of sizable amounts of energy density scaling precisely as a^(−3).

In other words, they are saying that to get the cosmology right, they need to add stuff that behaves exactly like dark matter -- that is what they are alluding to with the "sizable amounts of energy". They make their φ field play this role. It's just like TeVeS, the other major relativistic MOND theory, where the scalar "S" field does the same thing.

The popular press likes to frame the debate as "dark matter vs. modified gravity", but it's really "dark matter vs. dark matter plus modified gravity", which is much less dramatic.

What happened to the recent work showing that galactic rotation curves are consistent with ordinary GR? Last I read, cosmologists were choosing to ignore it.

Gravitomagnetism is a well-understood and experimentally measured effect. It is also a very small effect, of the order v^2 / c^2 where v is the speed of the sources. In the galaxy, stars move with v/c ~ 1/1000, which means the gravitomagnetic correction is one in a million. So while N-body simulations do sometimes account for general relativistic corrections like these, they're not nearly large enough to remove the requirement for dark matter.

That is the simple reason the paper has been ignored by everyone in the scientific community and rejected from decent journals. Of course, this hasn't stopped hundreds of fluffy pop articles being written on it, or it getting posted every week on HN. The blind leading the blind.

This is why I left this site. Endless smug engineers explaining condescendingly to physicists why they’re stupid sheep, without knowing the first thing about anything. Intellectual curiosity, my ass. Do you have a reply to my concrete criticism or not?

The same criticism applies for any other crappy paper HN likes. Whenever I check this site, half the time the front page has something even worse. Just assume everything you see here is wrong.

Gravitomagnetism is a well-understood and experimentally measured effect. It is also a very small effect, of the order v^2 / c^2 where v is the speed of the sources. In the galaxy, stars move with v/c ~ 1/1000, which means the gravitomagnetic correction is one in a million. So while N-body simulations do sometimes account for general relativistic corrections like these, they're not nearly large enough to remove the requirement for dark matter.

The main thing the paper should do is explain why they think the correction is a million times larger than the back of the envelope estimate. But they don't. Instead, they try to solve everything analytically, never plugging in numbers or reasoning about what's big or small, leading to a forest of long combinations of special functions. That's a reliable recipe for making a mistake.

That is the simple reason the paper has been ignored by everyone in the scientific community and rejected from decent journals. Of course, this hasn't stopped hundreds of fluffy pop articles being written on it, or it getting posted every week on HN. The blind leading the blind.

I'm glad we agree that development and opportunity are good things.

The average per capita income in China is lower than countries like Costa Rica or Venezuela. It's less than half of what a minimum wage worker gets in the US. Every year, hundreds of thousands of people still die in China due to preventable diseases like tuberculosis and hepatitis. About half the children in the countryside are malnourished, to a degree that would not be conceivable for anybody in the US. This is the actual situation China is facing, and it's still dramatically better than many countries that were similarly poor decades ago.

In response to this, you and HN in general immediately react with fears of imminent Chinese world domination. Once again I ask you: why does the prospect of a few non-white people not starving to death make you so afraid? Why is this so alarming, so abnormal? I encourage you to spend at least a moment thinking about it.

When did I make that argument?

It was not a hard thing to infer. There is a very particular kind of deep ignorance one needs to write a comment like OP's.

Yes, you didn't, but you weren't really looking, were you?

All of these examples are pieces of entertainment being pulled because they offend Chinese sensibilities.

Here's the thing: pieces of entertainment are also pulled if they offend American sensibilities. In fact, American sensibilities have exclusively determined what is acceptable in big-budget entertainment for at least half a century. Crap like the Blitzchung affair just reflects this balance shifting to 99% America, 1% China.

Yes, China has the power to cancel one thing from time to time, but America constantly has the power to cancel everything. If a big-budget production isn't appealing to an American audience, it just doesn't get made, period. For example, when's the last time you saw a Chinese protagonist in a major film? Or even a single Chinese person who wasn't portrayed as a villain, a dope, or a sex toy?

In a world with equitable global development, influence would be proportional to population. America would have a 5% say, China 20%, Africa 20%, India 25%, and so on. So I'm not worried about the balance shifting towards that.

OP described living in a China-centric world, not one of equals.

Yes, but there's no evidence to suggest this will be the case. The average yearly income in China is not even half what a minimum wage worker gets in the US! And yet OP thinks China will promptly take over the world. That's yellow peril at work.

The original commenter professed a fear of what that would look like for outsiders in a China-centric world.

Yes, and for the third time now: my point is that the absolute worst-case scenario for the original commenter is still miles better than what 90% of the world (i.e. everybody non-white) faces today.

The worst-case scenario in OP's book is that China joins the ranks of first world countries, which then go from being 100% white to 90%. He'd still be rich, and he'd still be comfortable, but he'd have to share first world status with others. And for some reason, that's unacceptable.

Worst case, he might miss out on one business deal because it was struck in Mandarin, which is an absolutely tiny reflection of the loss of opportunity faced today by everybody who doesn't know English. And that in turn is tiny compared to the reality that 100 million people died of starvation in the 20th century. Famine, disease, and poverty are global moral catastrophes of unimaginable scale. And OP just thinks a country managing to stop them for its citizens gives him the creeps.

Do you really think agitating for World War III was productive to start with? I pointed out rationally and straightforwardly where such extreme opinions come form.

The "discrimination" they fear is no longer automatically being worshipped the second they step foot in a non-Western country, for being the richest guy in every room. In other words, what they actually fear is dropping from 99th percentile to 90th, and they think that possibility is worth starting World War III over.

That attitude is typical, and precisely why people in China find the West so unconvincing.

Well, I've read almost all the books she lists and I've been a quantum field theory practitioner for years, and I can at least attest the list is good. People actually learn from these books.

I think your comment also directly illustrates what I was complaining about. You really shouldn't source learning recommendations from the highest ranking people, because these people know the least about what it's like to learn something anew. A Nobel prize doesn't automatically make somebody a good teacher.

This is a great point. Out of all the ones I've looked into, I think MIT has by far the most complete public curriculum (because of MIT OCW), but Cambridge and Oxford are not far behind, with excellent lecture notes and problem sets.

I personally know the vast majority of the material in both roadmaps, so I know that 't Hooft's is far harder to learn from. Anybody can check this for themselves. There's plenty of broken links, extremely rough drafts of lecture notes, and wild fluctuations in sophistication. The ordering puts graduate-level stuff before its sophomore-level prerequisites.

My statement would only be controversial if you believed that arbitrary adversity in learning was necessary to be a good physicist -- and for my own sake I hope that isn't the case!

This is cool as always, but in case anybody is seriously contemplating using it: this list is infamous for its complete uselessness for anybody actually trying to learn. It's mostly recommended because of 't Hooft's name, but it doesn't reflect how he actually learned physics himself, nor how anybody ever has, really.

It's been "under construction" (i.e. completely abandoned) for two decades. Half the links are broken, and the ones that aren't tend to be whatever the top Google hit was in the 90s, not what's pedagogically best. If you're serious about learning physics, there are many much better roadmaps, like Susan Fowler's list (https://www.susanjfowler.com/blog/2016/8/13/so-you-want-to-l...).

Sort of the difference between a wall that keeps you in versus a wall that keeps unwelcome people out.

And how do you think the Chinese state describes the Chinese firewall?

Curious about the downvotes. Do people believe that this curtailment of freedom can't happen in America? I know people already rushing to prepare for it.

Or perhaps they believe that it will happen, but that it's a good thing?

Or perhaps, as is increasingly common, they believe both simultaneously: "China's firewall restricts freedom; our copy of it promotes freedom."

Time to get a VPN if you want to communicate across the Great Firewall of America.

Actually, in the context of astrophysics, that exact objection has been employed many times. For example, the most famous argument against heliocentrism was that it would require the stars to be ridiculously far away and ridiculously big to patch away the lack of parallax, which felt unparsimonious. Similarly, people believed that galaxies weren't galaxies, because it seems unparsimonious to expand the universe far beyond the Milky Way just to patch up some weird features of fuzzy nebula. And even in our galaxy, the mass in dust and interstellar gas exceed that in stars.

Literally all progress in fundamental physics is "just" "invented". Each time it must triumph against the objections of the same, thousand-year-old philosophical arguments.

To me, again as an outside observer, it feels so counter-intuitive to _invent_ a new type of matter you can't observe than to just say that your calculation is close but not right and to start over. Is it not a crutch?

Physicist here. If you're doing applied physics or engineering, this certainly would be a crutch. But when we're talking about fundamental physics, talking about new kinds of matter that nobody has seen before is not a crutch -- it's literally the core thing we do. That's what makes it fundamental!

Saw a track in the bubble chamber curving the wrong way? Invent a new kind of matter: antimatter.

Saw short-lived particles in the bubble chamber that shouldn't have made it there? Invent a new kind of matter: mesons that decay into the observed particles.

Problems with getting solar reactions to work out right? Invent a new kind of matter: neutrinos.

Amount of neutrinos detected not quite right? Invent multiple neutrinos and neutrino oscillations.

Saw some weird long-lived particles? Invent a new kind of matter: "strange" mesons and baryons.

Want to explain the pattern of mesons and baryons? Invent a new particle: "quarks", along with the stipulation that they can never be observed, even in principle.

Standard Model seems a little off-balance at this point? Invent a new particle: "charm" quarks to balance out the strange ones, at an energy high enough that nobody has seen them yet.

But the mesons and hadron patterns still aren't consistent with the Pauli exclusion principle! Invent a new force: color charge, carried by "gluons", which are also postulated to be unobservable.

Some particular meson and baryon decays acting weird? Invent a new force: the weak force, carried by "weak bosons", which are too heavy to be observable at the time.

Can't get the weak bosons to have mass? Invent a new interaction, the Higgs interaction, carried by an invented new field, the Higgs field, which gets a vev from an invented new function, the Higgs potential, whose elementary excitations are an invented new particle, the Higgs boson.

Of course, not every weird thing is explained by a new type of matter; many anomalies fade away after careful checking. But the anomalous observations that motivate dark matter persisted for almost a century, they're been only building in strength as we get more data, and all attempts we've made to explain them in terms of "normal" physics have failed. So the case for explaining it in terms of something new is at least as strong, in fact far stronger, than the examples I gave above.

This is one of those completely false things that people only believe is true by repetition. Go back and actually read the full set of WHO statements in mid-January. They have a bunch of statements saying that nations should get prepared, one saying that specific studies haven’t yet found hard evidence for person-to-person transmission (because at that point most of the cases they’d managed to find were tied to the market). The WHO never, ever said that it can’t be transmitted, and they absolutely never said that people should do nothing about COVID-19. They were urging nations to act for months before they actually did.

The modeling from IMHE shows that in March the number of daily new infections were above 250K whereas today are about 120K.

Yeah, but the IHME model is also extremely bad. It consists almost entirely of fitting a Gaussian to the number of cases over time. It’s mathematically impossible in their model for the case count to not go down.

Again, just substitute the words.

But the point of Olympic medals is to assess skill. If the available pool of talented athletes is larger than the number of gold medals, then there is a need for more gold medals.

No, because school prestige is a positional good. This is like saying you can address the shortage of Olympic gold medals by just manufacturing tons of them.