‘I hope you’ll create black holes,’ Stephen said with a broad smile.
We emerged from the dumbwaiter that had taken us underground to the five-story cavern that housed the ATLAS experiment at the CERN laboratory, the legendary European Organization for Nuclear Research near Geneva. CERN’s director general, Rolf Heuer, shuffled his feet uneasily. It was 2009 and someone had filed a lawsuit in the United States, concerned that CERN’s newly built Large Hadron Collider, the LHC, would produce black holes or another form of exotic matter that could destroy the Earth.
The LHC is a ring-shaped particle accelerator that was built primarily to create Higgs bosons, the missing link – at the time – in the Standard Model of particle physics. Built in a tunnel under the French-Swiss border, its total circumference is 27 kilometers (nearly 17 miles) and it accelerates protons and antiprotons flowing in counter-rotating beams in its circular vacuum tubes to 99.9999991% of the light speed. At three locations along the ring, beams of accelerated particles can be directed into highly energetic collisions, recreating conditions comparable to those that reigned in the universe a tiny fraction of a second after the scorching big bang, when temperatures exceeded millions of billion degrees. The particle spray trails created in these violent head-on collisions are picked up by millions of sensors stacked like mini-Lego blocks to form giant detectors, including the ATLAS detector and the Compact Muon Solenoid, or CMS.
The lawsuit would soon be dismissed on the grounds that “the speculative fear of future harm does not constitute sufficient factual harm to confer legitimacy.” In November of that year the LHC was successfully ignited – after an explosion in a previous attempt – and the ATLAS and CMS detectors soon found traces of Higgs bosons in the debris of the particle collisions. But, so far, the LHC has not created any black holes.
Why was it not entirely unreasonable for Stephen – and also for Heuer, I think – to hope that it was possible to produce black holes at the LHC? We usually think of black holes as the collapsed remains of massive stars. This is too narrow a view, however, because anything can become a black hole if squeezed into a small enough volume. Even a single proton-antiproton pair accelerated to nearly the speed of light and bumped together in a powerful particle accelerator would form a black hole if the collision concentrated enough energy into a small enough volume. It would surely be a tiny black hole, with a fleeting existence, as it would evaporate instantaneously through the emission of Hawking radiation.
At the same time, had Stephen and Heuer’s hope of producing black holes come true, it would have marked the end of particle physicists’ decades-long quest to explore nature at ever shorter distances by colliding particles with ever-increasing energies. Particle colliders are like microscopes, but severity seems to put a fundamental limit on their resolution, because it triggers the formation of a black hole whenever we boost energy too much by trying to peer into a smaller and smaller volume.
At that point, adding even more energy would produce a bigger black hole instead of further increasing the collider’s magnifying power. Curiously, then, gravity and black holes completely reverse the usual thinking in physics that higher energies probe shorter distances. The end point of building ever larger accelerators appears not to be a tiny building block – the ultimate dream of every reductionist – but an emerging macroscopic curved spacetime. By bringing short distances back to long distances, gravity flouts the deeply ingrained notion that the architecture of physical reality is an ordered system of nested scales that we can detach one by one to arrive at a smaller fundamental constituent. Gravity — and therefore spacetime itself — seems to possess an anti-reductionist element.
So, at what microscopic scale does particle physics without gravity transmute into particle physics with gravity? (Or, in other words, how much would it cost to realize Stephen’s dream of producing black holes?) This is a question that has to do with the unification of all forces, the subject of this chapter. The search for a unified picture encompassing all the fundamental laws of nature was already Einstein’s dream. It depends directly on whether multiverse cosmology really has the potential to offer an alternative perspective on the life-encouraging design of our universe. For only an understanding of how all particles and forces fit harmoniously together can provide further insights into the uniqueness – or lack thereof – of fundamental physical laws, and thus to what degree they can be expected to vary across the multiverse.
Adapted from ON THE ORIGIN OF TIME. Copyright © 2023 by Thomas Hertog.
Published by Bantam, an imprint of Penguin Random House.
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