Update: Did you enjoy our April Fools’ day story? If you want to find out about the antimatter research that is really going on at CERN, check out this recent update: https://home.cern/news/press-release/experiments/alpha-cools-antimatter-using-laser-light-first-time, and read all about the Antiproton Decelerator: https://home.cern/science/accelerators/antiproton-decelerator.

“Do antimatter apples fall up?” is a question that is certain to aggravate physicists working on a new vertical accelerator proposed for CERN. The true question, they say, is whether antimatter apples fall down differently. If a difference were spotted, it would spell the end of “CPT invariance” – a principle that has underpinned every theory of physics since the invention of quantum mechanics.

“The Standard Model of particle physics has been very successful, but it can’t explain the 95% of the universe which is ‘dark’, and neither Einstein nor any physicist since has been able to cook up a working theory of quantum gravity,” says theorist Flora Oilp. “It’s time to challenge its most fundamental principle head-on.”

The way forward, according to Oilp and her colleagues, is to build a vertical accelerator that will put gravity to the test directly. Every previous particle accelerator has been horizontal. A combination of high speeds and frequent course corrections using focusing magnets has always meant that the effect of gravity can be neglected. But by utilising a range of new, revolutionary techniques, including accelerating particles upwards inside a vacuum vessel, and timing how long they take to fall back down to Earth, physicists can study the elusive fourth force directly. Furthermore, by comparing results with protons and antiprotons, they can watch for signs of “CPT violation”. Such behaviour cannot be explained using conventional theories, which rely on this principle to ensure the conservation of probability.

The accelerator would be built in two stages. Stage one proposes a 500 m vertical accelerator, starting from the base of the LHC shafts. An exciting collaboration with NASA may come to fruition by utilising detectors on the International Space Station (ISS) to detect beams of particles fired by the accelerator every time the ISS is overhead. This “reverse cosmic-ray” experiment would allow the measurement of Earth’s gravity on particle trajectories at unprecedented levels. Stage one will seek to match the roughly 1% precision on measurements of the gravitational constant “g”, which is currently being targeted in parallel by experiments with antihydrogen at the Laboratory’s Antimatter Factory. This moderate build will also allow engineers and physicists to understand the intricacies of running a vertical accelerator in preparation for stage two – the space elevator.

First proposed by Russian scientist Konstantin Tsiolkovsky in 1895, a true space elevator would rise from the equator to a height of 35 786 km – the altitude of a geostationary orbit. CERN’s proposed structure is far more modest, rising a mere 2.5 km above the Swiss countryside. If built, however, this advanced particle accelerator would nevertheless be three times taller than the Burj Khalifa in Dubai, which has been the tallest structure in the world since 2009. Each bunch of protons and antiprotons would need to be sent into a so-called “radial elliptic orbit” so that they return to the same point from which they were launched, calling for minute transverse focusing along the way.

Though the technological challenges are formidable, links to industrial and medical applications are promising, and the physics reach of such a machine is compelling. While appearing somewhat outlandish at first sight, the key advantage of the design is that its sensitivity would scale rapidly with its height. With the project eventually being ramped up to 2.5 km – the maximum height thought to be structurally sound, the CPT invariance could be tested with exquisite precision, approaching 0.005%.

“The sky’s the limit!” says CERN’s Pilar Olof, who was recently elected spokesperson of the new Elevator-Inspired Fast-Fermion Endwise Linac collaboration (EIFFEL). “Recent years have seen debates over whether the next accelerator should be linear or circular, but a consensus is now building that it should be vertical. We can’t wait for the world to see the EIFFEL.”