## Tuesday, March 28, 2006

### Sick at home

The past weekend, we had a family reunion at my parents' place and I planned to go back to Bremen late Sunday or Monday morning. However, I had been quite tired the better part of last week (which at that point I had attributed to the late stages of finishing our paper on multipole vectors in the three year WMAP data and the visit by Guiseppe Policastro as we had not always finished discussing by 6 p.m.). Sunday evening, however it became appearant that indeed there were a number of red spots on my face and my Monday morning all over my body (for illustrations see here). The doctor confirmed my internet based diagnosis of rubella (German measels) and told me to stay at home for the time it lasts. Now, I am with my parents, they take care of me and I have fun reading papers and other things.

So, let me tell you about a few things I came across in case you also have some time to waste. Let's start with a poem by Thomas Gsella published in the weekly supplement to the Süddeutsche Zeitung (in German, I attempt no translation):

Berufsbeschreibung
Der Astrophysiker

Gewöhnliches ist nicht sein Ding.
Er aast im Unbekannten.
"Ereignishorizont" und "String",
das "Schwarze Loch", die "Quanten":

Er faselt Super-Quark und hört
dem "Hintergrund" sein "Rauschen".
Auf Partys steht er da und stört,
doch welche gibt's, die lauschen.

So ist ihm schnurz, ob wir's kapiern:
Er faselt guten Mutes.
Er will den Damen imponiern,
und ach, oh weh, er tut es!

Let me just add that from my experience this poem is rather unrealistic.

Next is a geometry problem I learned from my office mate Wolfgang Spitzer: Draw a sqare and mark one random point on each of the four sides. Now, erase the sqare just keeping the marked points. Use compass and ruler to reconstruct the square from the points. For bonus credit find conditions on four random points to lay on the four sides of a square.

Finally, there is a puzzle from "Spektrum der Wissenschaft", the German version of "Scientific American": A biologist starts with one bacterium. Each night, each bacterium splits into two. (Up to now, it sounds like a pretty dull, well known problem, but...). However, he notices that in some nights, exactly one bacterium becomes inactive and does not split on that night or any of the further night but it stays alive. Yesterday, the biologist counted 1638 bacteria, today, there are 3245. How many days ago did he start with the single bacterium? You can sumbit your solution to Spektrum until April 11th and win a carrier bag.

Wolfgang does not offer a prize but his problem if found much harder.

PS: I received a complaint that I had left destructive comments in other blogs. Let me assure you that I try not to be descructive but I don't know how you percieve it. Nevertheless, I always sign comments (as well as usenet posts) with my name and link to this blog. So comments from annoymous people or other Roberts (without link to atdotde) are not by me.

## Thursday, March 09, 2006

Many people have asked about confirmations that emails they sent were actually received and had to learn that there is no reliable way to check this even if some mailers offer "return receipt" features. For one reason they are not part of the RFC's starting with 822 that define the protocol used to transmit emails: They don't work. First, what consitutes 'receipt' of an email? The first host (in my case atdotde.de) receives the SMTP message? Well, from there it's forwarded to an account where I run my mail client (currently at DAMTP). So, the first host, that does local delivery instead of forwarding? Or the first program that opens the message (spamassasin in my case)? Or the perl-script I use to organise my mail folders? Or only a program that displays the message on a screen (mostly 'pine' in my case, but could be 'less' as well when things get rougher)?

The general recommendation is to ask the reader in the body of the message explicitly to reply to the message to confirm reading it. But even reading it does not always mean one understands it. So even better, do a quiz on the contents of the mail. But still, not getting the confirmation does not mean the message was not read.

But today, I learned of a company which offers the solution to this problem: You have to register with them and get an account (free for 10messages/month, \$50 per year for 750messages/month). Then just add .didtheyreadit.com to any email address. This causes the email to be routed through their server where this part of the address is stripped and the message gets an html attachment with a link to an invisible picture (1x1 pixels transparent say) with a unique URL.

The idea is that your email client downloads the picture to display it and they know somebody opened the message in a picture aware browser.

I use pine which is text based and thus does not care about pictures. Thunderbird at least is concerned about my privacy and warns me it didn't download any pictures that were refered to in the message but not contained (note that spammers also use this trick to determine which of the addresses they succed of web pages are actually read and are not directly going into the bit bucket). Still nothing happens. But if I click to download the picture (or use another mail client that is not so careful about my privacy) the sender of the message gets this.

Upshot: If you recieve an email with an attachment that looks like

<br />
<br />
<div><img src="http://xpostmail.com/b8a6a2cd5cd9ff294ecdf69dbb55d469worker.jpg" nosend="1" name="dtri" width="1" height="1"><link href="http://xpostmail.com/b8a6a2cd5cd9ff294ecdf69dbb55d469.css" hreflang="dtri" rel="stylesheet" type="text/css"></div>

the sender spys on you!

## Thursday, March 02, 2006

### Fusing branes

For the first time this semester, yesterday, I had time to attend our school's colloquium. Everybody in academia will know that there is always a temptation to skip colloquia as they typically are not in your own area of expertise (and often that means interest) and if the speaker is not too good there is often not much you get out of it than at best some intellectual entertainment.

Still, my general philosophy is that in the long run it pays to invest 1.5 hours per week into broadening one's perspective and to get an idea what's hot in other areas of science. Physicists that refuse to spend time with anything else than their own little area of research are in my eyes just ignorants. Nevertheless, I usually take something to read with me so I have a chance to do something slightly more useful with my time once the speaker loses me. In recent times, I have also taken sudokus to seminars but I should better not admit that.

Yesterday, however, there was no need to bring anything, even though the talk was on microbiology because it was just excellent. The speaker was Reinhard Jahn and he talked about fusing membranes.

I have an interest in those as well, at least if we are talking about D-branes:

There, 'recombination' is an elementary process in which two branes join into a single one. It is still simple enough to be understood in detail but via dualities it is related (or archetypical) to many topological transitions (like small instantons, M5-branes falling into end-of-the-universe branes in heterotic M-Theory/Ekpyrotic scenarios etc) in string theory.

In biology as well, one can worry about the microscopic workings of this resolution of a singularity. Different from string theory, cell biology has a natural discreteness at small scales and at some point it becomes important that biological membranes are double layers of peptides sticking together by hydrophobicity.

So, I learned that cell membranes are not really elastic and if a cell (or one of its compartments) grows it has to add membrane which comes in small spherical blobs that fuse with the membrane that is about to grow:

Reinhard Jahn explained the inner workings of the process. An important role is played by somewhat longish specific proteins (SNAPs) that have one end sticking into the membrane and an alpha helix sticking out into the surroundings. Now, if two membranes get near each other, the helices of four of these SNAPs can coil up and form a highly stable supercoil. This draws the membranes together so they can fuse as well as releasing binding energy in the coiling up process that is used to overcome an activation energy barrier for the fusing process. Very fascinating and brilliantly presented!