What would happen hypothetically if a particle collider about the diameter of the earth were built?

Building a particle collider the size of the Earth would present a variety of scientific, engineering, and safety challenges, but it also raises fascinating possibilities for our understanding of fundamental physics.

**Scale and Energy**: A collider with a diameter equal to that of the Earth (approximately 12,742 kilometers or about 7,918 miles) could potentially reach unprecedented energy levels. For context, the Large Hadron Collider (LHC) at CERN operates at energies around 13 TeV (tereelectronvolts) per collision. A collider of Earth-size dimensions could feasibly reach energies that are several orders of magnitude higher, possibly pushing into the TeV or even PeV (peta-electronvolt) range. This could provide insights into new physics beyond the Standard Model, including dark matter, supersymmetry, and aspects of quantum gravity.

**Physics Discoveries**: Such a collider could enable direct studies of phenomena that are currently inaccessible due to energy constraints. It might allow the observation of rare processes or new particles that could help address some of the unanswered questions in physics, such as the nature of dark matter or the hierarchy problem concerning the mass of the Higgs boson.

**Engineering Challenges**: Constructing a collider of this magnitude would be an immense engineering feat. It would require vast amounts of resources, advanced technology for cooling and vacuum systems, and innovative designs to manage the circular geometry at such a large scale. The infrastructure necessary to support the collider, including access roads, power supply, maintenance facilities, and safety systems, would also be monumental.

**Safety Concerns**: One of the most significant concerns regarding high-energy colliders is the potential for dangerous outcomes, such as the creation of stable micro black holes or strange matter. Although theoretical studies have largely dismissed these scenarios as implausible and not creating any real risks, a collider of such size would escalate concerns, likely leading to extensive safety assessments.

**Societal Impacts**: The social and economic implications of constructing a collider of this size would be profound. Funding such a project would require unprecedented levels of international collaboration and investment, and there would be ongoing debates about the allocation of resources—especially considering the scientific uncertainty about what exactly might be discovered.

**Scientific Collaboration**: A project of this magnitude would likely necessitate collaboration among nations, as well as a great deal of political and societal support. It could serve as an unprecedented opportunity for global scientific cooperation, engaging thousands of researchers from multiple disciplines.

**Potential Outcomes**: Discoveries from such a collider could redefine our understanding of the universe. The potential to discover new particles or forces could lead to breakthroughs in technology, materials science, and even energy production through advanced knowledge of fundamental physics.

In summary, while a particle collider the size of Earth could revolutionize our understanding of physics, it would also present enormous technical, financial, and ethical challenges along the way.