Monday, January 11, 2010

LHC alarmism debunked

I just attended a seminar by Michelangelo Mangano titled "Black holes at the LHC: safety and society". While of course the bottom line was that there is no real risk of the LHC destroying the universe it is still worthwhile to discuss it a little bit.

So, a little compendium of why you shouldn't be scared of LHC produced black holes:

  • LHC is almost surely not likely to produce black holes, simple as that, most of the models predicting such scenarios work with additional dimensions and require a higher center of mass energy than would be available at LHC. Anyway, there are indeed some scenarios where black holes can indeed produced at LHC, they are not very plausible but a case can be surely made from them.
  • Black holes are unstable: Black holes decay via Hawking radiation. Particles can be created out of "nowhere" in the vacuum, a rather sloppy argument essentially says that the uncertainty principle allows particles to pop out of nowhere as long as the energy loan required to create them is payed really fast. Now, consider one of this virtual particles being created just outside the black hole and the other one just inside of the black hole. The net effect for an observer outside the black hole is that black hole is radiating, in the process the black hole is loosing mass that is just escaping in this radiation. This radiation is not only electromagnetic, in principle a black hole can radiate anything.
    This makes the black hole to "evaporate" as it looses mass via radiation, it turns out that the lifetime of a black hole is proportional to its mass. For black holes with really small mass, like the ones speculated to be produced at LHC this yields out such a short lifetime that there is not a reason to worry at all.
    We can now be cynical and say that QFT in curved spacetime (the fancy name for the framework used to calculate this kind of things) is not really experimentally established, maybe there is something wrong with it and black holes could be stable, then you should consider that...
  • Black holes produced in particle collisions are charged: Collisions at hadron colliders happen between the charged components of protons and conservation of charge requires that the final product (in this case a black hole) remains charged. Any charged particle, even a microscopic black hole interacts with matter in a way that has being throughly studied and is summarized in the so called Bethe-Bloch equation for the range of energies likely involved in the LHC. The result is that the black hole will just radiate all its energy away in a short distance. But hey, what if we are somehow missing something in the creation of microscopic black holes and it is possible to produce stable and neutral black holes? This looks extremely unlikely but lets go ahead and see what can happen if they are indeed produced.
  • Accretion rates from microscopic black holes are negligible: Lets say we have a slow moving, stable and neutral black hole. Can it eat the Earth and cause all other kinds of havoc? Well, if we model mass infall into a black hole via Bondi accretion (a fancy name for spherical accretion) it turns out that to gain a considerable mass, say 1 ton, a huge amount of time is required. Right now I don't remember now the exact number but it was of the order of a thousand millions of years. If a predator capable of running at most a 1mm per year is chasing you with bad intentions you wouldn't be scared, would you?
Well, despite this arguments there is still people around trying to halt LHC and the issue has even been brought to court, where it has always been dismissed based on technicalities, mostly because the LHC is out of jurisdiction of the court.

So, why I am telling you all this? Well, wait for tomorrow...

Friday, January 01, 2010

Create wallpaper slideshow in Ubuntu 9.10

First of all, happy new year! Nothing like the morning of a new year to be messing around with Linux and Python!

One of the new things in Ubuntu 9.10 is that you can now use a wallpaper slideshow for your desktop, which will cycle your desktop wallpaper at regular intervals from a selected pool of wallpapers. The default Ubuntu 9.10 comes with a space-themed wallpaper slideshow, but there is apparently no option to create your own ones. However, the configuration is stored in plaintext XML files, one in /usr/share/gnome-background-properties and the other in the corresponding directory where the images are stored, in /usr/share/backgrounds.

The following Python script will automatically create these two XML files so that you can set a wallpaper slideshow with the images of your preferences.

Slideshow Builder script
(Right-click and select "Save Link As..", or the equivalent in your browser).

Here's the instructions. You will need root access since the directory where you're doing the operations, /usr/share/, is owned by root. Simply use sudo (or sudo -i if you're lazy).

1) Create a directory in /usr/share/backgrounds with the name of your slideshow. For instance, if the desired name is 'space', the dir should be called '/usr/share/backgrounds/space/'.

2) Copy all the wallpaper images to that folder. The supported formats are jpg, png, gif and bmp.

3) Run the Python script in the images directory. This will automatically create two xml files: one in the (current) images directory, and another one in /usr/share/gnome-background-properties.

4) That's all! You can now select the slideshow in the Gnome wallpaper selection interface.

A sample usage of the script is shown in the following image:

And the slideshow now appears in the desktop Background selection screen:

Update: I've fixed the dead link to the script.