At yesterday’s reception for the start of the academic year, fifty members of Harvard’s Institute for Theory & Computation for which I serve as director, gathered in the backyard of my home.
As I introduced my collaborators to each other for the first time, I noted that Mark Hertzberg collaborates with me on high-energy physics in the early universe whereas Stein Jacobsen does so on low-energy physics in the current universe. The first frontier is more risky but potentially more rewarding. If Mark and I design a particle collider that reaches the Planck energy, which is 10 quintillion (10 followed by 19 zeroes) times the proton rest-energy, it might be able to create a baby universe in which Stein and I could study materials from many more exoplanets.
Particle accelerators which are currently in use, like CERN’s Large Hadron Collider, employ metallic cavities for acceleration which are limited in the electric field they tolerate and only yield an acceleration gradient of up to a tenth of a proton rest-energy per meter.
Using this traditional acceleration method to reach the Planck energy would require an acceleration distance of 10,000 light years, longer than the thickness of the Milky-Way disk. The related engineering project requires a megastructure that can only be constructed through an interstellar collaboration of many civilizations, because the accelerator will have to pass through the territories of many other planetary systems. Such an interstellar partnership could follow the spirit of the international collaboration that led in 1954 to the creation of CERN. It requires our physicists to collaborate with alien physicists from exoplanets in a Galactic science project, grander than ever conceived on Earth.
My PhD thesis forty years ago was dedicated to developing new methods for acceleration of particles to high energies in plasmas. One of these plasma acceleration techniques is currently being tested at CERN as the Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE). This method holds the potential for reaching an acceleration gradient of a few times the proton rest-energy per meter. This could reduce the length of a Planck collider by a factor of 30 to about 300 light years, still requiring an interstellar collaboration. To shrink the accelerator down to the size of the Oort cloud of the Solar system — so that we will not enter the territorial space of other civilizations, would require plasma accelerators that are a hundred times better, with acceleration gradients of 300 times the proton rest-energy per meter. Such a feat might be achieved by our civilization within the next millennium, as long as we will not return to the stone-age by triggering global cooling from a nuclear world war or global warming from excessive emissions of greenhouse gases.
An even bigger challenge is to have enough particles in the opposing collider beams, so that there will be one collision event over the duration of the experiment. This is challenging because the cross-section for collisions at the Planck energy scale is minuscule, of order the Planck length squared based on unitarity and the uncertainty principle of quantum mechanics. The Planck cross-section is 3.3 times 10 to the power of -65 centimeters squared, roughly the surface area of a black hole with the Planck mass of 20 micrograms. Given the much larger cross-section for proton-proton collisions, only one in 10 to the power of 41 collisions of a hadron collider will probe Planck scale physics. As a result, the total energy consumption needed to probe Planck scale physics would be this factor times 20 micrograms, or equivalently a thousand times the rest mass of the Sun. The power requirement would be up to ten billion solar luminosities for an experiment lasting a million years.
All in all, creating a baby universe by a Planck collider requires an interstellar or perhaps an intergalactic collaboration. My first popular book was titled “Extraterrestrial” and the second was titled “Interstellar.” If I were to write the next book on Planck colliders, it would be fitting to title it “Intergalactic.”
As the particle beam of Planck accelerators moves from interstellar to intergalactic distances, it would encounter emptier spaces. The interstellar medium contains roughly one proton per cubic centimeter, about a thousand quintillion times less than the mass density of air. The reason is simple: air is dense. How big is the separation between air molecules relative to their size? The answer is: a factor of ten. This means that packing air molecules into a solid would increase the mass density by a factor of thousand. The mass density of air is indeed a thousand times smaller than that of ice.
Moving farther away, the interstellar medium of the Milky-Way disk is a million times denser than the mean mass density of ordinary matter in the Universe. The average intergalactic density translates to roughly one proton per 3 cubic meters. The average separation between atoms in intergalactic space is about a meter, a billion times larger than the separation in air. Air molecules collide with each other every nanosecond (a billionth of a second). However, at the mean cosmic density, hydrogen atoms collide with each other every billion years. The intergalactic medium is amazingly rarefied.
The emptiness of outer space offers a great opportunity for propagating beams of energetic particles across vast distances. But as of now, the main challenge is to find partner civilizations that may be interested in collaborating with us on this ambitious construction project. To design the Planck collider, I need the expertise of Mark Hertzberg. To find our interstellar partners, I need the expertise of Stein Jacobsen. Scientific accomplishments stem from collaborations, starting here on Earth and ending among the stars.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
