Journey to the center of the universe
Fellow blogger, Ransom Stephens, published an excellent 8-part series last year about the LHC and the Higgs Boson discovery, which you can read here. I highly recommend it. I will relate my own first-hand experience in today's post, and the impressive engineering required to create the experiments. Though I lived near Geneva for nearly four years, this was the first time I ever visited the facility.
Let’s start with a little physics. We all know that matter cannot travel at the speed of light, and mass increases asymptotically as an object approaches light-speed. Freshmen physics taught us that mass increases (and time slows) by the factor of gamma, calculated by the equation below.
Having largely unnoticeable effects in our non-relativistic world, gamma approaches infinity as v, the velocity of a particle, approaches c, the velocity of light. Enormous energy is required to further accelerate these increasingly heavy protons circling in the LHC ever closer to the speed of light.
How fast are the protons accelerated? Here’s a thought experiment. The closest star system to our sun is Alpha Centauri, over four light-years away. Put another way, light is photons, and a photon would require four years to reach this distant neighbor. After the current upgrade to the LHC, an LHC-accelerated proton would arrive just two seconds later. It’s that fast.
Like speed skaters in an ice rink, the proton clumps have their own non-intersection paths in the dual vacuum tubes before they collide in the center of a particle detector. Over 10,000 superconducting magnets are chilled to near absolute zero, colder than deep space, to accelerate and focus the proton clumps. The protons in the LHC have a cumulative energy of about 70 kilos of TNT. When the clumps intersect, approximately 30 proton pairs have head-on collisions, and generate energy and particles that approximate the big bang. And with that, man has a slight glimpse of the beginning of the universe.
27 kilometer tunnel circles under Switzerland and France. Over 10,000 superconducting magnet segments propel protons to nearly the speed of light. (Photo courtesy of CERN)
While the search for the Higgs Boson (the particle associated with the Higgs field that gives some particles mass, popularly known as the “God particle”) has been the highlight of the LHC, the CERN facility is actually host to a number of experiments to give us fundamental insight into why our universe is the way it is. Why is there matter, but little anti-matter? What makes gravity? Why is the universe not only expanding, but, as surprisingly detected by the Hubble telescope, accelerating? What is the nature of dark matter and dark energy propelling it?
A rap video from 2008 explains all the major experiments associated with the LHC. Credit goes to Kate McAlpine, a.k.a. Alpinekat, the world’s most famous physics rapper. While the creators probably won’t win a video music award, the video cleverly describes the LHC and the major questions of physics it is trying to answer. If you only have five minutes to understand the LHC, the Large Hadron Rap is remarkably effective in summing up the project.
Twenty-seven kilometers of tunnel under ground
Designed with mind to send protons around
A circle that crosses through Switzerland and France
Sixty nations contribute to scientific advance
Two beams of protons swing round, through the ring they ride
‘Til in the hearts of the detectors, they’re made to collide
And all that energy packed in such a tiny bit of room
Becomes mass, particles created from the vacuum
And then… Which brings me to the purpose of my trip. Because, and then the particle detectors go to work. We’ve only discussed the accelerator up to now. It’s one thing to recreate the Big Bang, it’s another to watch it. When the protons collide and new particles are created, they must be detected and tracked somehow. Placed around the point of collision is the ATLAS detector, which does exactly that. Half the size of Notre Dame cathedral and the weight of 100 Boeing 747s, the ATLAS detector is an engineering marvel in its own right.
I met with Markus Joos, an engineer at CERN, and his colleagues at the CERN facility. Collectively they have responsibility for the instrumentation for the detectors. CERN is large enough and specialized enough that the detectors have their own instrumentation group. Frequent readers of Test Cafe know of my Prediction #5 She Blinded Me By Science about modular instrument adoption in the sciences and a subsequent article about an astrochemist’s 1000x speed improvement using AXIe. Indeed, National Instrument’s Labview and PXI are used to control the beams of the LHC itself. While commercial products serve much of the scientific community nicely, specialty detectors like ATLAS are custom built from the bottom up. One reason is that the intense radiation from the collisions require radiation-hardened circuitry, a core competency at CERN.
The LHC shuts down every four years for upgrades and maintenance. It is operated around the clock when running. Markus explained to me they were now planning the upgrade for the year 2022, which was the focus of our discussions. While commercial products aren’t suitable for these exotic detectors, commercial architectures often are. Indeed, the current detectors are based on VME. Without disclosing the particulars, we had in-depth discussions of various industrial and instrumentation bus architectures, and the trade-offs between them. There is a global network of physics labs that source instrumentation, so choosing an architecture that can be leveraged is very important. Also, an architecture must be ultra-reliable and supportable for decades. Finally, the architecture must excel at data processing.
This last point was brought home when we toured the ATLAS detector. CERN is in the midst of a planned maintenance and upgrade shutdown, so this was a rare opportunity to enter the belly of the beast. Strolling across street to the control room, I couldn’t help but remember TV comedy Big Bang Theory character Sheldon Cooper’s expectations of going to see the LHC. As we rode the elevator down to the detector, Markus said we were just going down one floor, though that one floor was nearly 100 meters below us! Walking through a long tunnel, we emerged on the ledge of a large cavern. Vertigo gripped me as I stared at an incredible sight- ATLAS.
Markus Joos from CERN, in front of the ATLAS detector. Markus is part of a team responsible for the instrumentation inside the detectors.
It is hard to conceive the scale until one actually sees it. Symmetric rings of detectors and superconducting magnets surround the LHC at the point of collision. Though the weight of the Eiffel Tower, the ATLAS detector was rolled away from the collision point for maintenance. Markus explained that detectors, even those far from the center, are aligned to a small fraction of a millimeter. Resting on a limestone foundation that is slowly rising, the entire assembly is periodically aligned to needle-point precision. The assembly will be rolled back in place for operation. Days before going live again, the huge facility is searched and scrubbed of any metallic object (a screw, a pen, or screwdriver, ) less they become projectiles when the superconducting magnets are ignited.
Each detecting element generates huge amounts of data from the high-speed digitizers. Unfiltered, they cumulatively generate in excess of 50,000 GB (Gigabytes) of data every second - 100,000 CDs! Recall that most of this data is uninteresting, as only 30 of the 100 billion protons collide. The trick is to use local processing algorithms to filter and detect these collisions. But like a photograph that has been exposed with 30 overlapping images, the separate collisions must be isolated. Distributed supercomputing correlates events at each detector to connect the dots. This brings the permanently stored data “merely” to the neighborhood of a CD every second. Huge storage vaults capture the data for further analysis.
Rising to surface-level again, I was able to see the control room for ATLAS. A NASA-like room full of consoles, scientists staff the center around the clock when the LHC is operating. When an experiment is prepared, one console displays a “START” button. Pressing that button launches 10,000 simultaneous computer programs into action. It should be noted that there is an independently designed and built particle detector called CMS located a few kilometers away in France. Equally impressive, CMS and ATLAS together allow experiments to be verified by two independent means.
All of this is just a build-up to its ultimate purpose- the scientific investigations themselves. Last year, CERN made history when it announced that it confirmed the existence of the Higgs Boson, predicted by the Standard Model. The energy upgrade now underway from 8TeV to 14TeV will allow scientists to characterize it more fully. Markus commented that there certainly is a Nobel Prize to be awarded, but to whom? Certainly to Dr. Higgs and others that derived the Standard Model. But what about the 3000 physicists who are part of the ATLAS collaboration? What about the designers, and constructors of this amazing machine, including the LHC? What about those who had the audacity to conceive that such a machine could even be constructed and supported?
When the Nobel Prize is awarded, they should all take pride in this amazing achievement.
The author, Larry Desjardin stands 100 meters underground, next to the ATLAS detector. Much to his surprise, the hard hat is NOT for protection against an errant Higgs Boson.