If a spaceship from an alternate universe were to appear in our own, we would undoubtedly have numerous inquiries regarding their nature, their origins, and whether they could pose a danger to us. Caution would be essential.

If the existence of parallel universes or a multiverse is valid, there might come a time when we meet beings from these other realms. Here's how to approach such an encounter.

Many aspects of our universe are ingrained in our understanding, yet we seldom ponder the underlying principles of physics, the nature of fundamental constants, or the predominance of matter over antimatter. These attributes are crucial to the characteristics of our universe; a change in any of them would drastically alter our reality.

However, it is plausible that our universe is just one among many within a vast multiverse. This suggests the potential existence of other universes with their own life forms—possibly intelligent and technologically advanced—and different governing laws. Even if their intentions are peaceful, an encounter could lead to unforeseen consequences. Here’s how to leverage physics to ensure your safety.

Should we witness someone suddenly appear, our immediate concern may be whether they are composed of antimatter instead of matter. This scenario doesn't necessarily require different physical laws; it could simply be a result of cosmic processes that favored antimatter over matter in their universe. If the conditions were reversed, we might never realize it.

Nevertheless, there are observable indicators of matter and antimatter that go beyond mere definitions. In our universe, certain mesons composed of strange, charm, or bottom quarks can occasionally convert into their antimatter versions, swapping quarks for antiquarks.

By inquiring about their CP-violation measurements, we could swiftly determine whether they are made of matter or antimatter. For an extra layer of verification, tossing an apple their way would reveal their nature; if it annihilates upon contact, they are indeed antimatter.

If their universe operates under different fundamental constants, their matter may behave distinctly. Altering the mass of particles, even those we consider insignificant, like a top quark, can subtly affect the mass of a proton.

Any changes to these constants would lead to different atomic and molecular properties. This means that atomic transitions could shift, causing variations in emitted or absorbed wavelengths. By observing the reflected sunlight from their vessel spectroscopically, we could discern whether their physical laws align with ours.

What if their universe adhered to entirely different laws of physics? While testing for matter/antimatter and fundamental constants would be vital, these checks may not cover all potential disparities between universes.

Different fundamental forces or particles might exist in their realm. They could be composed of a material that behaves like matter, antimatter, or something completely novel. If they have mastered travel between universes, they may possess a deeper understanding of fundamental physics than we do. A mutual exchange of knowledge could enhance our understanding significantly.

Do the fundamental forces unify at higher energies in their universe? In our universe, the electromagnetic and weak nuclear forces unify at extreme temperatures, and it’s conceivable that the strong or gravitational forces might do so at even higher energies.

In a different universe, however, the unification of forces may not occur in the same manner. Understanding whether their unification symmetries break at the same energies as ours could unveil significant differences in the rules governing particle interactions, a crucial consideration before any potentially hazardous contact.

Are these beings three-dimensional like us, or do they exist in a different dimensional framework? If they can teleport or manipulate their surroundings in ways that seem god-like to us, they likely operate within four or more spatial dimensions. Conversely, if they inhabit two or fewer dimensions, we may appear similar to deities from their perspective.

Our understanding of dimensions is well-defined, but the nature of other universes remains uncertain.

Does the concept of mass hold the same significance in their universe as it does in ours? We can test this through Einstein’s Equivalence Principle, which states that the acceleration of a mass under force aligns with Newton’s law: F = ma.

Conversely, gravitational effects on mass relate directly to the mass itself, as described by Newtonian gravity: F = GMm/r². These equations may differ in other universes.

Could these two mass types—gravitational and inertial—be equivalent in all universes? If not, acceleration under thrust versus gravitational influence might differ fundamentally. While these mass types are nearly equivalent in our universe, we cannot assume this will hold true elsewhere.

If we can only send a message, it might be best to keep it simple and clear. A straightforward declaration like "this universe contains electrons" could convey critical information. Providing details about electric charge, atomic assembly, and CP-violation could help them ascertain whether their laws align with ours.

Is it safe to physically interact with such beings? If they are capable of inter-universe travel, they likely have the knowledge to answer that question. However, we must communicate our understanding to them. If there are risks involved, we need to convey that information before any interaction takes place.

Starts With A Bang is now featured on Forbes, with republishing on Medium thanks to our Patreon supporters. Ethan has authored two books, *Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.*