At a ceremony today, CERN European research center inaugurated its newest accelerator.

Linac 4 will eventually become the first step in CERN’s accelerator chain, delivering proton beams to a wide range of experiments, including those at the Large Hadron Collider.

After an extensive testing period, Linac 4 will be connected to CERN’s accelerator complex during a long technical shutdown in 2019-20. Linac 4 will replace Linac 2, which was put into service in 1978. Linac 4 will feed the CERN accelerator complex with particle beams of higher energy.

“We are delighted to celebrate this remarkable accomplishment,” says CERN Director General Fabiola Gianotti. “Linac 4 is a modern injector and the first key element of our ambitious upgrade program, leading to the High-Luminosity LHC. This high-luminosity phase will considerably increase the potential of the LHC experiments for discovering new physics and measuring the properties of the Higgs particle in more detail.”

“This is an achievement not only for CERN, but also for the partners from many countries who contributed in designing and building this new machine,” says CERN Director for Accelerators and Technology Frédérick Bordry. “We also today celebrate and thank the wide international collaboration that led this project, demonstrating once again what can be accomplished by bringing together the efforts of many nations.”

The linear accelerator is the first essential element of an accelerator chain. In the linear accelerator, the particles are produced and receive the initial acceleration. The density and intensity of the particle beams are also shaped in the linac. Linac 4 is an almost 90-meter-long machine sitting 12 meters below the ground. It took nearly 10 years to build it.

Linac 4 will send negative hydrogen ions, consisting of a hydrogen atom with two electrons, to CERN’s Proton Synchrotron Booster, which further accelerates the negative ions and removes the electrons. Linac 4 will bring the beam up to an energy of 160 million electronvolts, more than 3 times the energy of its predecessor. The increase in energy, together with the use of hydrogen ions, will enable doubling the beam intensity delivered to the LHC, contributing to an increase in the luminosity of the LHC by 2021.

Luminosity is a parameter indicating the number of particles colliding within a defined amount of time. The peak luminosity of the LHC is planned to be increased by a factor of 5 by the year 2025. This will make it possible for the experiments to accumulate about 10 times more data over the period 2025 to 2035 than before.

Editor's note: This article is based on a CERN press release.

What is dark energy? Why aren’t we made of antimatter? How many dimensions are there? 

These are a few of the many unanswered questions that Jorge Cham, creator of the online comic Piled Higher and Deeper, and Daniel Whiteson, an experimental particle physicist at the University of California, Irvine, explain in their new book, We Have No Idea. In the process, they remind readers of one key point: When it comes to our universe, there’s a lot we still don’t know. 

The duo started working together in 2008 after Whiteson reached out to Cham, asking if he’d be willing to help create physics cartoons. “I always thought physics was well connected to the way comics work,” Whiteson says. “Because, what’s a Feynman diagram but a little cartoon of particles hitting each other?” (Feynman diagrams are pictures commonly used in particle physics papers that represent the interactions of subatomic particles.)

Before working on this book, the pair made a handful of popular YouTube videos on topics like dark matter, extra dimensions and the Higgs boson. Many of these subjects are also covered in We Have No Idea.

One of the main motivators of this latest project was to address a “certain apathy toward science,” Cham says. “I think we both came into it having this feeling that the general public either thinks scientists have everything figured out, or they don't really understand what scientists are doing.” 

To get at this issue, the pair focused on topics that even someone without a science background could find compelling. “You don’t need 10 years of physics background to know [that] questions about how the universe started or what it’s made of are interesting,” Whiteson says. “We tried to find questions that were gut-level approachable.”

Another key theme of the book, the authors say, is the line between what science can and cannot tell us. While some of the possible solutions to the universe’s mysteries have testable predictions, others (such as string theory) currently do not. “We wanted questions that were accessible yet answerable,” says Whiteson. “We wanted to show people that there were deep, basic, simple questions that we all had, but that the answers were out there.” 

Many scientists are hard at work trying to fill the gaping holes in our knowledge about the universe. Particle physicists, for example, are exploring a number of these questions, such as those about the nature of antimatter and mass.

Some lines of inquiry have brought different research communities together. Dark matter searches, for example, were primarily the realm of cosmologists, who probe large-scale structures of the universe. However, as the focus shifted to finding out what particle—or particles—dark matter was made of, this area of study started to attract astrophysicists as well. 

Why are people trying to answer these questions? “I think science is an expression of humanity and our curiosity to know the answers to basic questions we ask ourselves: Who are we? Why are we here? How does the world work?” Whiteson says. “On the other hand, questions like these lead to understanding, and understanding leads to being able to have greater power over the environment to solve our problems.

In the very last chapter of the book, the authors explain the idea of a “testable universe,” or the parts of the universe that fall within the bounds of science. In the Stone Ages, when humans had very few tools at their disposal, the testable universe was very small. But it increased as people built telescopes, satellites and particle colliders, and it continues to expand with ongoing advances in science and technology. “That’s the exciting thing,” Cham says. “Our ability to answer these questions is growing.” 

Some mysteries of the universe still live in the realm of philosophy. But tomorrow, next year or a thousand years from now, a scientist may come along and devise an experiment that will be able to find the answers.   

“We’re in a special place in history when most of the world seems explained,” Whiteson says. Thousands of years ago, basic questions, such as why fire burns or where rain comes from, were still largely a mystery. “These days, all those mysteries seem answered, but the truth is, there’s a lot of mysteries left. [If] you want to make a massive imprint on human intellectual history, there’s plenty of room for that.”