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ALICE lead ion collision, Large Hadron Collider
This computer graphic shows one of the first collisions recorded between two lead ions at the Large Hadron Collider’s top energy. The energy in the center-of-mass system is approximately a quadrillion electron-volts. (Credit: CERN / ALICE Collaboration)

The Large Hadron Collider set another record for particle-smashing energy levels this week – which set off another round of hyped-up rumblings about the end of the world.

Before the LHC’s startup in 2008, the Internet was set abuzz with worries that high-energy collisions could create globe-gobbling black holes or cosmos-wrecking strangelets. Protests were mounted, lawsuits were filed, and physicists at Europe’s CERN particle physics center had to explain in depth why the nightmare scenarios were nothing more than nightmares. Once the collider went into operation, the lawsuits were dismissed and the hand-wringing settled down.

Now the world’s largest collider is operating at near its design limits, and this week, CERN reported that lead-ion collisions in the LHC’s ALICE detector reached energies beyond a quadrillion electron-volts – a level also known as 1 peta-electron-volt, or 1 PeV.

“This energy is that of a bumblebee hitting us on the cheek on a summer day. But the energy is concentrated in a volume that is approximately 10 -27 (a billion-billion-billion) times smaller,” Jens Jørgen Gaardhøje, professor at the Niels Bohr Institute at the University of Copenhagen and head of the Danish research group within the ALICE experiment, said in a news release.

At first blush, a quadrillion electron-volts sounds like a huge ramp-up from 13 trillion to 14 trillion electron-volts, or 13 to 14 TeV, the traditionally quoted figures for the high end of the LHC’s collision energy. That’s what set off the doomsayers. In the weeks leading up to the ALICE collisions, there was a drumbeat of postings claiming that “CERN LIED” and warning that 1-PeV smashups would have catastrophic consequences.

The reason for the different figures has to do with particle theory rather than conspiracy theories. The 14-TeV figure relates to proton-on-proton collisions. The lead-ion collisions being conducted for the ALICE experiment, which aims to recreate the conditions that existed in the universe just an instant after the Big Bang, are much more energetic because the ions are so much bigger.

By the same token, the energy of the collision is shared by far more subatomic particles. Each of the 208 particles in the lead-ion nucleus – 126 neutrons and 82 protons – ends up receiving less than 40 percent of the energy that the LHC imparts to each of the particles in a proton-on-proton collision, CERN physicist John Jowett said.

“Still, the concentration of so much energy into the tiny nuclear volume is enough to establish truly colossal densities and temperatures about a quarter of a million times those at the core of the sun,” Jowett wrote. “Heavy-ion collisions recreate the quark-gluon plasma, the extreme state of matter that is believed to have filled the universe when it was only microseconds old.”

Even though Gaardhøje said collisions at the level of a quadrillion electron-volts have “never been realized before under terrestrial conditions,” such energy levels are common in cosmic-ray collisions beyond Earth. In fact, neutrinos with energies higher than 2 quadrillion electron-volts have been detected by the IceCube experiment in Antarctica. So if you hear that the LHC is going to destroy the world … again … the best advice is, DON’T PANIC.

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