Hey cool. But what if you bring your USB stick "backup" to some friends, and then add or modify files on the stick? Unison will let you sync those changes back to your desktop when you get home. Does rsync do that, too?

It is, in fact, a bit more than that. rsync doesn't handle deletions, so your backup will keep growing in size even though you're not really making any additions (say, you're renaming a big file - now that file is copied twice). unison does, however. This is essential for this use case, especially if one of the backup devices has somewhat limited space on it.

+1 for unison, it's an awesome little program. I use it on macosx as well, and I believe there's a windows client. I personally prefer the CLI version, but if the GUI version is anywhere near as good, I'll heartily recommend it. What you would do after coming home is to run sync against both external storages. Should work like a charm.

Fortran is certainly higher-level than C. It's got lots of convenience, such as array expressions, automatic deallocation of memory, and other stuff that you won't find in C. Fortran was written to explicitly let you avoid thinking of the underlying hardware. In my opinion, too much so. After all, there's always a "computer" in "computational science".

I have to say I agree on this point. However, in my own experience, I find that spending some time every now and then cleaning up my data serves to keep my data more useful, by restructuring the file system organization and by reminding myself what I have. Having several GB of data lying around that are in principle interesting or useful is all very well, but if it's not convenient to access and use, I just don't see the point. If my data aren't worth looking at and organizing at least to a minimal extent (this usually leads to at least some amount of deletion), I ask myself why I'm keeping the data around at all.

1 barn is tiny compared to the electromagnetic cross section. The penetration depth is enormous when compared to other particles (save for neutrinos). Hence, it _is_ tiny. 1 barn is "not tiny" only in the context of subatomic particles and -interactions. Uranium has a large cross-section, yes, but do you think it is likely that they will build the reactor cladding from U-235?

That's technically correct, yes. However, the probability of a neutron interacting with an atomic nucleus is tiny, so they penetrate deep into any material and cause serious damage to (for instance) a metal by transmuting the nucleus it interacts strongly with. Protons or electrons, on the other hand, immediately start slowing down when they enter a material due to electromagnetic interactions, which are long-range. That's what I was getting to.

Neutrons interact through the strong and weak forces, but not the electromagnetic forces. The weak force is, in fact, weak, and the strong force is strong, but really short-range only. Basically a neutron has to run smack into an atomic nucleus to "feel its presence", whereas a proton will hit a wall of electrons (both protons and electrons are charged) when it enters a material. Thus a neutron can very well interact strongly (in any meaningful sense of the word, both in physics-speak and regular-english-speak) but the probability is low, meaning that they penetrate deep into the material and then (usually) interact by tearing the nucleus apart or by morphing it into a different (probably radioactive) isotope.

I'm 100% sure. Neutrons are uncharged and interact very weakly with matter compared to electrons and protons. Hence, they penetrate deeply into any material. We're talking on the order of magnitude of meters, though. Neutrinos, however, will interact only once per several thousand light years when passing through solids - that's a whole different business.

tl;dr: Neutrons interact weakly enough to penetrate, but strongly enough to be annoying.

Fusion reactors generate enormous amounts of neutrons, which interact only weakly with matter. Making a reactor casing that can withstand the radiation damage and collect the heat for useful purposes (power generation, desalination of water, heating for industrial processes etc.) for long enough is extremely hard. This is expected to be the ultimate limit to how well fusion power can work. I don't have a citable source, but I got this from a talk at CERN by the guy in charge of the ITER project.