How big is the universe? This is a question to which no one knows the correct answer. What we do know is that within the observable universe alone, there are trillions of galaxies, and the Milky Way is just one of them. The Milky Way, in turn, contains hundreds of billions of stellar systems. Among them, there is a stellar system called the "Solar System," which harbors a blue planet—our Earth. The image above was taken by the Voyager 1 probe from a distance of approximately 6 billion kilometers from Earth. The faint dot of light pointed to by the arrow in the image is actually Earth. It is evident that even within the Solar System, Earth is so tiny, floating like a speck of dust in the vastness of space. Yet, this "speck of dust" is humanity’s only home in the universe. The vastness of the universe inspires awe in humans, and at the same time, it fuels our dream of venturing into the stars and seas. However, realizing this dream is no easy feat. A direct obstacle is that the speed of spacecraft cannot be infinitely fast. Modern physics tells us that no object with rest mass can be accelerated to the speed of light (note: "speed of light" in this article refers to the speed of light in a vacuum). In other words, humanity’s future spacecraft will have a speed limit, at most approaching the speed of light infinitely. Yet, even "infinitely approaching the speed of light" is no easy task, because there exists another speed limit in the universe that is not the speed of light—and this is the real obstacle for humanity. The GZK Limit As early as 1966, scientists discovered that besides the speed of light, there is another speed limit in the universe, known as the "GZK cutoff." Let’s explore the details. According to the "Big Bang theory," our universe originated from a "Big Bang" of a "singularity" with extremely high temperature and density. Due to the limitation of the speed of light, photons from the early universe are still propagating through space. However, due to the expansion of the universe, they have now shifted to the microwave band. From our perspective, this microwave radiation is uniform in all directions, hence the name "cosmic microwave background radiation" (CMB). Photons from the CMB fill the entire universe, and all matter moving in space will inevitably encounter them. Under normal circumstances, CMB photons do not affect the motion of matter. However, scientists found that when the speed of matter exceeds a critical value, it interacts with CMB photons. The energy corresponding to this critical value is called the "GZK limit." For example, the GZK limit for a proton is 5 x 10^19 eV (note: eV stands for electron volt). If a proton moving in space has energy exceeding this value, it will interact with CMB photons and produce pions. This causes the proton to lose energy (since pions have rest mass), and its speed thus decreases. As the proton continues to interact with CMB photons, it loses a significant amount of energy and continuously generates pions until its energy drops below 5 x 10^19 eV. This speed limitation is the "GZK cutoff." Just as sub-light speed is constrained by the GZK limit involving cosmic microwave background radiation and cosmic rays, their energy during flight in the universe leads to photon interactions, i.e., collisions. After such collisions, the speed of particles drops to the original threshold. As they gradually approach the speed of light again, collisions occur once more, causing another slowdown. In other words, ignoring other factors, as an object approaches the speed of light, particle collisions will slow it down, preventing it from ever reaching the speed of light. If a human spacecraft attempted to travel at the speed of light in the universe, it would only be bombarded by particles and eventually disintegrate. It is generally believed that both microscopic particles and macroscopic objects in the universe are subject to the GZK cutoff. This means that if future human spacecraft exceed the GZK limit in terms of energy corresponding to their speed, they will also interact with CMB photons. Prolonged exposure could even result in severe damage to the spacecraft. In cosmic space, such a GZK limit is more like a real obstacle to humanity’s interstellar travel. Additionally, Earth’s gravity limits humanity’s speed potential. While it keeps everything anchored to the planet, it also acts as a "threshold" for entering and exiting Earth’s environment. So, are these limitations obstacles or protections? Obstacle or Something Else? The ever-expanding universe, cosmic particles, radiation, and other limits beyond the speed of light—are they meant to "trap" humanity, restricting the freedom and development of Earth’s civilization? Or are they protecting humanity, preventing us from leaving the "safety zone" prematurely? After all, according to the above, humans cannot currently surpass the speed of light. Moreover, based on our current exploration of interstellar space, no other habitable planets have been found, at least within the Solar System. It can be said that Earth is humanity’s only home for now. If humans were to escape Earth, it would be like activating a self-destruct program, for few life forms could survive normally in other environments outside Earth. Thus, the various substances and conditions of the universe remain a mystery to humanity, with no immediate solutions. Will humanity unlock the mysteries of the universe once we break through the GZK limit one day? Only the future can answer this question. Summary Therefore, in future interstellar travel, humanity’s real obstacle may not be the speed of light, because under the GZK cutoff, we may not even be able to "infinitely approach the speed of light." How should future humans tackle this dilemma? There are two potential solutions: one is to use a powerful "energy shield" to protect the spacecraft. The other is to bypass speed limitations through technologies that manipulate spacetime. For example, the hypothetical "warp drive" could keep the spacecraft in a "warp bubble" formed by distorted spacetime, contracting the space ahead and expanding the space behind. This would allow the spacecraft to move continuously while "remaining completely stationary relative to its surrounding spacetime." Source: Charming Science Jun, Senluo Vientiane Observation