## Friday, July 23, 2021

### Email is broken --- the spammers won

I am an old man. I am convinced email a superior medium for person to person information exchange. It is text based, so you don't need special hardware to use it, it can still transport various media formats and it is inter-operational, you are not tied to one company offering a service but thanks to a long list of RFCs starting with number 822 everybody can run their own service.  Via GPG or S/MIME you can even add somewhat decent encryption and authentication (at least for the body of the message) even though that is not optimal since historically it came later.

But the real advantage is on the client side: You have threading, you have the option to have folders to store your emails, you can search through them, you can set up filters so routine stuff does not clog up your inbox. And when things go really pear shaped (as they tend to do every few years), you can still recover your data from a text file on your hard drive.

This is all opposed to the continuous Now of messengers where already yesterday's message has scrolled up never to be seen again. But the young folks tend to prefer those and I cannot see any reason those would be preferable (except maybe that you get notifications on your phone). But up to now, it was still an option to say to our students "if you want to get all relevant information, check your email, it will be there".

But today, I think, this is finally over. The spammers won.

Over the last months of the pandemic, I already had to realise, mailing lists are harder and harder to use. As far as I can tell, nobody offers decent mailing lists on the net (at least without too much cost and with reasonable data protection ambitions), probably because those would immediately be used by spammers. So you have to run your own. For the families of our kid's classes at school and for daycare, to spread information that could no longer be posted on physical notice boards, I tried to set up lists on mailman like we used to do it on one of my hosted servers. Oh, what a pain. There are so many hoops you have to jump through so the messages actually end up in people's inboxes. For some major email providers (I am looking at you hosteurope) there is little chance unless the message's sender is in the recipient's address book for example. And yes, I have set up my server correctly, with reverse DNS etc working.

But now, it's application season again. We have received over 250 applications for the TMP master program and now I have to inform applicants about their results. The boundary conditions is that I want to send an individual mail to everybody  that contains their name and I want to digitally sign it so applicants know it is authentic and not sent by some prankster impersonating me (that had happened in the past, seriously). And I don't want to send those manually.

So I have a perl script (did I mention I am old), that reads the applicants' data from a database, composes the message with the appropriate template, applies a GPG signature and sends it off to the next SMTP server.

In the past, I had already learned that inside the university network, you have to sleep 10 seconds after each message as every computer that sends emails at a higher rate is considered taken over by some spammers and automatically disconnected from the network for 30 minutes.

This year, I am sending from the home office and as it turns out, some of my messages never make it to the recipients. There is of course no error message to me (thanks spammers) but the message seems to be silently dropped. It's not in the inbox and also cannot be found in the spam folder. Just gone.

I will send paper letters as well (those are required for legal reasons anyway) with a similar script and thanks to TeX being text based on the input side. But is this really the answer for 2021? How am I supposed to inform people I have not (yet) met from around the world in a reliable way?

I am lost.

## Friday, June 25, 2021

### On Choice

This is a follow-up to a Twitter discussion with John Baez that did not fit into the 260 character limit. And before starting, I should warn you that I have never studied set theory in any seriousness and everything I am about to write here is only based on hearsay and is probably wrong.

I am currently teaching "Mathematical Statistical Physics" once more, large part of which is to explain the operator algebraic approach to quantum statistical physics, KMS states and all that. Part of this is that I cover states as continuous linear functionals on the observables (positive and normalised) and in the example of B(H), the bounded linear operators on a Hilbert space H, I mention that trace class operators $$\rho\in {\cal S}^1({\cal H})$$ give rise to those via $$\omega(a) = {\hbox tr}(\rho a).$$

And "pretty much all" states arise in this way meaning that the bounded operators are the (topological) dual to trance class operators but, for infinite dimensional H, not the other way around as the bi-dual is larger. There are bounded linear functionals on B(H) that don't come from trace class operators. But (without too much effort), I have never seen one of those extra states being "presented". I have very low standards here for "presented" meaning that I suspect you need to invoke the axiom of choice to produce them (and this is also what John said) and for class this would be sufficient. We invoke choice in other places as well, like (via Hahn-Banach) every C*-algebra having faithful representations (or realising it as a closed Subalgebra of some huge B(H)).

So much for background. I wanted to tell you about my attitude towards choice. When I was a student, I never had any doubt about it. Sure, every vector space has a basis, there are sets that are not Lebesque measurable. A little abstract, but who cares. It was a blog post by Terrence Tao that made me reconsider that (turns out, I cannot find that post anymore, bummer). It goes like this: On one of these islands, there is this prison where it is announced to the prisoners that the following morning, they are all placed in a row and everybody is given a hat that is either black or white. No prisoner can see his own hat but all of those in front of him. They can guess their color (and all other prisoners hear the guess). Those who guess right get free while those who guess wrong get executed. How many can go free?

The answer is: All but one half. The last one says white if he sees an even number of white hats in front of him. Then all the others can deduce their color from this parity plus the answers of the other prisoners behind them. So all but the last prisoner get out and the last has a 50% chance.

But that was too easy. Now, this is a really big prison and there are countably infinitely many prisoners. How many go free? When they are placed in a row, the row extends to infinity only in one direction and they are looking in this infinite direction. The last prisoner sees all the other prisoners.