The largest particle accelerator and collider ever built is the Large Hadron Collider at CERN. Why not go much, much bigger?

The Large Hadron Collider (LHC) stands as the most formidable particle accelerator ever constructed on our planet, propelling protons to energies around 7 TeV—approximately 7000 times their rest mass energy, as outlined by E=mc². This immense machine enables protons moving in opposite directions to collide at designated points, where substantial detectors analyze the results to gain insights into fundamental physics. Since the groundbreaking discovery of the Higgs boson in 2012, the LHC has continued to explore the subatomic realm with unparalleled precision.

However, to further advance our understanding of physics, a new, even more powerful particle collider will be necessary. While several concepts are under consideration, many researchers aspire to develop an accelerator that could match or exceed the size of Earth. Gary Camp has contemplated this idea and posed the following question:

> “Since bigger is better (so far), I am toying with the idea of a collider that circles the Earth. Many advantages are there as well as problems. If you can make a sun shade efficient enough, there is little need for cryogenic cooling of magnets since space is very cold without the sun. Earth magnets need to be quite strong due to the tight radius of the collider, but much smaller in Earth’s orbit. Solar power may be enough to power each magnet, or it may be necessary to use the new small modular nuclear generators which might be made more cheaply in quantity and perhaps require less shielding. DOD might want to contribute to the development as they want that portable/remote generator badly.”

There are numerous motivations for constructing a collider either around the Earth or in outer space, but “cost-effectiveness” is not one of them. Here are some considerations we should keep in mind regarding particle accelerators.

Particle accelerators fundamentally begin with stationary particles, which are typically atoms. The process of accelerating these particles to high energies involves several steps:

• Ionizing the initial (usually hydrogen) atoms to break them into electrons and nuclei (often single protons).
• Applying a strong electric field to these charged particles to accelerate them.
• Utilizing electromagnets to collimate the now swift particles into a focused “beam.”
• Finally, employing electric fields to further accelerate these collimated particles to the maximum energies achievable.

It's essential to remember that only two collider designs can efficiently achieve the highest energies: linear colliders, which use light particles like electrons and positrons in a long track with a continuous electric field, or circular colliders that recirculate particles—be they electrons, positrons, protons, or other leptons or hadrons—while bending and periodically accelerating them until they are ready to collide.

Why is this distinction important? For circular colliders, which are the only viable option for accelerating heavy particles to substantial energies, we cannot rely on fixed permanent magnets; instead, we must utilize electromagnets.

This consideration significantly influences particle accelerator design. To bend charged particles along a circular path, a magnetic field is essential. However, when particles enter the circular accelerator, they are not yet at peak speed; they initially travel at lower energies. As they gain energy, the strength of the magnetic field must be increased to maintain their circular trajectory. If the magnetic field is insufficient, the particles risk colliding with the accelerator walls.

At CERN, Fermilab, and other circular accelerators, this design has dominated for years. Smaller accelerators—whether circular or linear—are used to prepare particle bunches that are:

• High in quantity and density.
• Collimated as tightly as possible.
• Injected into the main accelerator ring at the highest initial energies attainable.

These bunches are then accelerated to their maximum energies before colliding with particles moving in the opposite direction within the same ring.

Electromagnets play a crucial role in circular particle accelerators, as bending magnets keep particles on their designated paths, while collimating electromagnets (e.g., quadrupole and octupole electromagnets) prevent repulsion between similarly charged particles, which could otherwise cause them to disperse and collide with the accelerator walls. Electric fields, often utilized during straight segments of the accelerator, provide a “kick” to the particles, nudging them closer to the speed of light.

Understanding these accelerator components is critical for constructing a large accelerator efficiently. Ultimately, the maximum energy achievable by a circular accelerator is determined by three primary factors:

1. The strength of the magnetic field, particularly the maximum strength of the bending magnets that confine the particles to a circular path. Stronger magnets yield higher energies.
2. The physical size of the accelerator and the radius of the circular path are also limiting factors. Doubling the radius increases the maximum energies achievable.
3. Synchrotron radiation, which is energy emitted as charged particles accelerate in a magnetic field. This radiation most significantly affects particles with a high charge-to-mass ratio, like electrons, while having a lesser impact on heavier particles like muons or protons.

To construct the most powerful particle accelerator, maximizing magnetic field strength and accelerator ring radius while minimizing synchrotron radiation is vital.

Currently, the LHC features the most potent bending magnets (approximately 8 T) and the largest radius (around 4.3 kilometers) of any accelerator ever built. Previously, this tunnel housed the Large Electron-Positron Collider (LEP), which, despite being the same size as the LHC, could only accelerate electrons and positrons to about 100 GeV (0.1 TeV) due to synchrotron radiation.

Fortunately, a proton emits just one ten-trillionth (1 in 10¹³) of the synchrotron radiation emitted by an electron. Hence, while electron and positron energies are limited in circular accelerators, protons are not. In fact, increasing the radius of the accelerator can reduce synchrotron radiation significantly, allowing for the theoretical possibility of achieving energies up to the Planck scale (~10¹? GeV) with a sufficiently large circular accelerator.

This discussion is crucial because it outlines the considerations for constructing the largest and most powerful accelerator imaginable. In the 1950s, physicist Enrico Fermi proposed the idea of a particle accelerator encircling the Earth’s equator. This hypothetical machine, with a radius of 6378 kilometers (about 1500 times that of the LHC), could potentially achieve energies around ~20 PeV, where a PeV equals 1000 TeV or 1,000,000 GeV.

However, achieving such energies would require retaining the same strength of bending magnets as those in the LHC. If we opted for conventional (or passively) cooled electromagnets instead of the superconducting ones actively cooled with liquid helium, our magnets would be limited to around 1–1.5 T, significantly weaker than those at the LHC. This reduction in capability would not be compensated by cost savings, as the expense of a larger ring far exceeds the savings from using cheaper magnets.

Instead of focusing on cost-cutting measures, we should consider the ultimate potential of magnets. While permanent magnets can be powerful, electromagnets far surpass them, with LHC’s bending magnets achieving around 8 T and experimental electromagnets reaching field strengths of 25–45 T, with potential for even greater advancements.

The energy of a proton-proton collider is directly proportional to the magnetic field strength. If we could enhance the LHC’s electromagnets to double, triple, or quadruple their current strength, we could similarly increase the entire machine’s energy capacity. The LHC’s superiority over Fermilab’s Tevatron is not merely due to its larger size; it also benefits from stronger bending magnets—almost double the field strength.

It’s crucial to recognize that particle accelerators are not simply machines that require a large ring and strong magnets to produce high-energy particles. They must be meticulously designed to progressively bend, collimate, and accelerate particles from low to high energies. This demands extensive infrastructure, coordinated electronics, liquid helium cooling for electromagnets, and substantial shielding for the accelerator. At elevated energies, potential radiation hazards to living organisms, including humans, must also be addressed.

Achieving these goals is impractical without substantial power, energy, and advanced magnetic technology. With sufficiently robust electromagnets, an Earth-circling particle accelerator could potentially reach collision energies around ~100 PeV (or ~10? GeV). While this ambitious proposal presents significant challenges, it remains a theoretically attainable goal, potentially enabling machines that could achieve energy levels up to 10,000 times that of the current LHC.

However, we must remember that the energy scales we aspire to reach are exceedingly high. The maximum energy of the most powerful cosmic rays is around 10¹¹–10¹² GeV, necessitating an accelerator the size of Earth's orbit. The theoretical energy scale for grand unified theories is estimated around 10¹? or 10¹? GeV, which would require an accelerator 1000 to 10,000 times larger than Earth's orbit—between 1000 and 10,000 A.U. in radius. Achieving these energies could raise concerns about inadvertently triggering catastrophic events, as this energy scale aligns with the theoretical energy levels associated with cosmic inflation, potentially leading to uncontrollable expansion of space.

At even greater scales, one could envision an accelerator stretching up to ~100 light-years, capable of reaching Planck energies—the point where our understanding of physics begins to unravel. Coordinating such a vast infrastructure, where communication is limited to the speed of light, may seem unattainable, yet future advancements in experimental particle physics could render this endeavor feasible. While these grand visions may appear impractical today, they might inspire future generations to explore the frontiers of knowledge we cannot yet comprehend. The entire subatomic universe remains to be investigated.

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Starts With A Bang is authored by Ethan Siegel, Ph.D., who has written works including Beyond The Galaxy, Treknology, and The Littlest Girl Goes Inside An Atom. His forthcoming National Geographic book, Infinite Cosmos, is set to release on October 8th!