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.