Black holes and (peer reviewed) revelations

In lieu of the fact that the LHC is about to conduct its first round-the-ring test of the beams, I went to the CERN website to see if I could keep an eye on accelerator conditions in real time [1]. On the front page of the site, I was pleased to see a link to a newly peer-reviewed and published article expanding on and extending the safety review of the LHC originally conducted in 2003. This paper, freely available to the public and detailing very specifically why the LHC is not a threat to this planet, can be downloaded from the publisher’s website [2].

The paper reviews the three “doomsday scenarios” offered up as reasons for stopping the LHC: mini black holes, vacuum bubbles, monopoles, and strangelets. I’ve offered some thoughts on the black holes issue in this blog, hearkening back to my grad school days [3]. My friend and colleague Dan Hooper appeared on NPR’s “Science Friday” program recently to discuss the excitement behind the LHC, and to address this issue as well [4].

It boils down to a simple fact of nature. The cosmos is the greatest particle accelerator ever conceived. Setting aside the fact that at the beginning of our universe, it was in an incredible state of high energy and density, even today we find particles accelerated to unbelievable energy bombarding the earth. Cosmic rays, heavy particles accelerated by distant and still-unknown mechanisms which constantly strike the earth’s atmosphere, are the testing ground for these doomsday scenarios.

The LHC, as the paper notes, will collide either protons or heavy nuclei at a center-of-mass energy of 7 TeV per beam – this is 7 times higher than the energy of a single Fermilab Tevatron beam, to date the highest energy collider on earth. In order for a single cosmic ray event to mimic a single LHC collision, it must have an energy of 100,000 TeV. That’s because a cosmic ray is striking a molecule in our atmosphere that is, for all intents and purposes, at rest. To get the same punch, conservation of energy and momentum requires the energy of the cosmic ray to be substantially higher than a proton in the LHC beam, because the LHC beams are both moving at one another while the cosmic ray is moving toward a stationary molecule.

So, how often is the earth bombarded by a cosmic ray with at least 100,000 TeV of energy? Well, per second and per cm^2 on the earth, the cosmic ray rate at or above this energy is tiny – just 0.00000000000005 per second, per square centimeter. But, there are 3600 seconds per hour and 24 hours a day, 365 days a year, and the earth has existed for about 4.5 billion years. The earth has a surface area of about 5,000,000,000,000,000,000 cm^2. Given that, how many cosmic rays at or above this energy have struck the earth?

The answer is: a BIG number. In the course of earth’s history, across every square centimeter of its surface, the earth has experienced about 4 x 10^22 such collisions – that’s 4, with 22 zeros after it. And how many such collisions of a 7 TeV proton on another 7 TeV proton are expected over the life of the LHC?

If all goes according to plan, the answer is 10^17 – that’s 1, with 17 zeros after it. That’s also 400,000 times FEWER such collisions than have occurred from cosmic rays over the history of the earth.

If cosmic rays, the result of the cosmos’ own glorious accelerators, have not wiped out the earth (or any other body, such as a star) with mini-black holes or other strange phenomena, there is nothing special about the LHC that makes it more likely to do it. Moreover, the way I see it, this even implies that if cosmic rays DO make mini black holes, they indeed operate according to the laws of thermodynamics and rapidly evaporate.

I find this whole exercise fascinating. I hope that one day I can use this as an example in class of how to take some big numbers and draw some big conclusions about the fate of mankind.

Anyway, that said, I quipped at lunch the other day: is it more glorious to go out with a BANG, as the earth is annihilated by black holes, or to go out with a whimper, as we slowly roast our own species in a noxious bath of our own greenhouse effluence?

Well . . . this IS the way the world ends.


Categorized as Science

By steve

I am a husband, son, and a Professor of Physics at Southern Methodist University. Physics may be my favorite thing to do, but I like to do a little bit of everything: writing, running, biking, hiking, drumming, gardening, carpentry, computer programming, painting, drawing, eating and sleeping. I earned a Ph.D. in Physics in 2004 from the University of Wisconsin-Madison, I teach courses in physics and the scientific method at SMU, and I love to spend time with my family. All things written in here are my own, unless otherwise attributed; don't you go blaming my employer or my family for me.