1. Ups, it was a typo in my last comment "it was shown that small pure fission (read fusion) weapons are possible, but their yield is like 5 tons" . it is in wiki:https://en.wikipedia.org/wiki/Pure_fusion_weapon. No plutonium. I think that we can't exclude possibility that such device may be upgraded (for example, using U-238 blanket) and many other tricks. I have some more links in the map and if one is interested he may follow, but I hope that the best results in pure fusion are kept tightly in secret.
2. Ok.
3. Yes, one probably may get more dangerous isotopes using U-238 blanket, if we consider chemistry of isotopes and their ability to accumulate in human body. Strontium, plutonium, Iodine and cesium may result from U-238 fission and all on them could accumulate in human body because of their similarity to calcium or other elements. Internal radiation is also much more dangerous.

But the main idea of salted bomb is not about which type of blanket to use - it is very technical question which require large calculations, and it may be found that some kind of blanket is even more effective in killing all humanity than either cobalt or U-238 (think about C-14, polonium, tritium, combinations etc). The main idea of salted bomb is that it is specially design to produce long term atmospheric contamination and that it is very large and stationary. And it is defence or blackmail weapon, not offensive. By large I mean like 20 000 tons dry weight. And 10-100 gigaton of explosive power. It is much more than all ICBM nukes combined. And most of them do not use U-238 blanket as they try to make them more clean.

By the way one may use ICBM to create something like U-238 salted gigabomb, if he use them to attack existing nuclear power stations. I heard that it was a fear North Korea may attack Japan's nuclear power stations with their nukes. I can't and don't want make exact calculation about it.

1. The main idea about pure fusion weapons is that we don't know how to make it now, but if we know, it would change a lot in the world. At least 10 different approaches have been studied, and it was shown that small pure fission weapons are possible, but their yield is like 5 tons, so they are not practical now. But if they find the way to combine different approaches it could result in higher yields. For example for laser fusion it was suggested that staged approach may work, where laser fusion of smaller pellet start fusion in higher pellet (http://ufn.ru/ru/articles/1998/11/f/ - article is in russian). It is not proved that pure fusion weapons are possible, but still need to be concerned about them until we will prove that they are impossible.
2. Lowell Wood in his article (http://journals.aps.org/pra/abstract/10.1103/PhysRevA.20.316, I have pdf) about fusion detonation in planar atmospheres showed minimal conditions for such reaction. It mostly depends of concentration of deuteriem (it should be 20 times more than in oceans, and it is small margin as deutherium and lithium could naturally concentrate in some places) and of the size of primary bomb, which should be as I remember around 300 meters - it is almost impossible to create such device on earth without use of self-replicating nanobots. And on Earth it will be overkill itself. One may be surprised to learn that humanity already sent nuclear device with weapon grade plutonium inside Jupiter, so it is not as remote possibility in the future (I mean drowning of spacecraft Galileo with nuclear batteries in the Jupiter). While Galileo was not able to initiate nuclear detonation of Jupiter, in near future one may create self-replicating nanobots which will be able to build large enough nuke to start detonation of Jupiter. Such explosion would be strong enough to kill all human settlements in Solar system. I also never met understanding (or wise contrarguments) from anyone on this topic and don't like to discuss it as possibility of it is small and very remote.
3. The main difference between stationary salted bomb (SSB) and ICBM is that SSB is invulnerable to any enemy attack and is able to create much more radioactivity as it is deliberately created for it. It may have yield of 10-100 GT than is many times more than all ICBM combined. Its only useful for nuclear blackmail. The difference is also is in results of its implementation. In case of large scale nuclear war wit ICBM total fatalities are estimated in hundreds millions maximum. Stationary cobalt bomb may result in billion fatalities or total extinction. So it is more dangerous. But we don't know that any was ever build as it is very expensive device, it may cost 10-100 billions dollars.

It looks like you did not look into details provided inside each of the topic. This proves for me that the idea of interactive pdf was wrong. ((

1. "Home made nukes" are not about plutonium but about pure fusion weapons. Several ways to create them has been suggested, all listed in the document.

2. Natural fusion ignition is not about the Earth but about enriched layers of deuterium in Jupiter. The idea is highly speculative, any way.

3. Salted bomb could create much large contamination as it is specially designed to do so. Most fusion bombs are relatively clean now. Salted bomb may create not only cobalt-60 but other isotopes. Russia in 2015 publish (accidentally) a plan to use 100 MT cobalt bomb torpedos to contaminate US ports.

But the main difference is that cobalt doomsday bomb is very large bomb, probably stationary with yields of gigatons, and most of its energy is used to create dangerous isotopes. In this case it could create enough cobalt-60 to contaminate Earth's surface. It was calculated by Szillard in 1950s and while the exact calculations are secret, I tend to believe him. No known stationary doomsday devices has been created until now ( or we don't know about them.)

I tried to get a discussion going on this exact subject in my post this week, but there seemed to be little interest. A major weakness of the standard Bayesian inference method is that it assumes a problem only has two possible solutions. Many problems involve many possible solutions, and many times the number of possible solutions is unknown, and in many cases the correct solution hasn't been thought of yet. In such instances, confirmation through inductive inference may not be the best way of looking at the problem.

I'd say that the possibility of 'home made nukes' is essentially zero unless fissile isotopes become widely available and cheap, and even if all current controls on nuclear materials and technology were lifted, that would still be far from plausible. The US government pays about $1mil to $10mil worth of plutonium-238 for a single nuclear weapon; this is even though the current industrial process for producing plutonium is highly optimized and efficient (the first nuclear test required $1bn worth of fissile material).

'Home made nuke' is in the same category of plausibility as 'home made fighter jet'.

EDIT: There are some other misconceptions in your document as well. Natural fusion ignition (in materials at densities and pressures you could find anywhere on or in the Earth) is physically impossible to trigger via a nuclear explosion; a thermonuclear weapon only works by imploding some quantity of fuel contained inside the device, and this implosion requires very precise and special conditions to carry out.

'Salted' bombs are largely a myth; ordinary bombs made out of Uranium-238 (most of the current arsenal) produce far more dangerous radioactive isotopes than a hypothetical cobalt bomb. Most nuclear weapons are already 'salted.'

Elon Musk cites first principle thinking in physics as a key to identifying neglected market opportunities. Can someone give me an example of how it may work in that application?

Recently moridinamael wrote about diswashers: As a pampered modern person, the worst part of my life is washing dishes. (Or, rinsing dishes and loading the dish washer.) How long before I can buy a robot to automate this for me?

If you reason from first principles then there's nothing stopping a device in which you input a pile of disher and that afterwards sorts them into the cupboard from existing. Especially with the recent advances in machine vision and google opensourcing Tensor flow.

Another nonautomated kitchen task is cutting vegetables. There no good reason why a robot shouldn't cut vegetables as well as humans.

What this means is that trade opportunities in small, obscure, illiquid niches of financial markets are not exploited by the big fish and so could remain "open" for a long time.

Could you give examples of what you mean that existed a few years ago and that are now exploited, so that no further money can be made and you don't lose something be openly sharing the information?

It could be, but I think theoretical physicists actually are very intelligent. Do you disagree?

edit: But let's leave them aside, and talk about me, since I am actually here. I am not in the same league as Ed Witten, not even close. Do you (generic sense) have something sensible to communicate to me about how I go about my business?

Rationalist community needs to learn a little humility. Do you realize the disparity in intellectual firepower between "you guys" and theoretical physicists?

Last week was a gathering of physicists in Oxford to discuss string theory and the philosophy of science.

From the article:

Nowadays, as several philosophers at the workshop said, Popperian falsificationism has been supplanted by Bayesian confirmation theory, or Bayesianism...

Gross concurred, saying that, upon learning about Bayesian confirmation theory from Dawid’s book, he felt “somewhat like the Molière character who said, ‘Oh my God, I’ve been talking prose all my life!’”

That the Bayesian view is news to so many physicists is itself news to me, and it's very unsettling news. You could say that modern theoretical physics has failed to be in-touch with other areas of science, but you could also make the argument that the rationalist community has failed to properly reach out and communicate with scientists.