The vast universe is full of endless mysteries, so human beings have a dream, that is, to be able to freely travel through interstellar space, and the key to this dream is speed.
The universe is too big, the radius of the solar system alone reaches 1 light-year, and the solar system is just one of hundreds of billions of star systems in the Milky Way, and galaxies like the Milky Way are more than millions in the universe. Therefore, to travel in the universe, the first problem to be solved is speed, and the limit of speed, the universe has long delineated for us, that is, the speed of light. Matter without rest mass can only move at the speed of light from the moment of birth, while matter with rest mass cannot reach the speed of light in any way, because matter with rest mass produces inertial mass equivalent to gravitational mass in the process of motion, and the faster the speed, the greater the inertial mass.
Inertia mass increases, to continue to increase the speed of matter needs to invest more energy, when the speed of matter is infinitely close to the speed of light, the mass is close to infinity, and the energy required to continue to accelerate has reached infinity, and there is no infinite energy in the universe, so it is impossible to push matter with rest mass to reach the speed of light.
The speed of light is the ultimate speed in the universe, which means that no matter how human technology develops in the future, "infinitely approaching the speed of light" is the limit of human beings, and it takes a year to rush out of the solar system at such a speed, it takes more than 4 years to go to proxima centauri, the closest extrastar to Earth, and up to 100,000 years to leave the Milky Way with a radius of 100,000 light-years. Isn't that desperate? More desperate ones are yet to come.
The speed of light is indeed the ultimate speed of the universe, but it is not the ultimate speed of human beings, and the real limit speed of human beings is much slower than the speed of light.
So what defines human ultimate speed? There is a very familiar term here called "microwave background radiation", which is the electromagnetic wave radiation left over from the Big Bang, and in essence, microwave background radiation is the photon that enters the microwave band. These photons originated from the beginning of the birth of the universe, because of the continuous expansion of the cosmic space, so they entered the microwave band, thus forming a microwave radiation, which filled the entire universe. A photon is a substance, an elementary particle that does not have a rest mass, whether it is a macroscopic object or a microscopic matter, and when it collides with a photon, the state of motion does not change at all, but this is only in general.
When an object reaches a certain height, it interacts with the photons of the microwave background radiation.
For example, when a proton hits a photon at a very high speed, a Π meson is generated, and the Π meson has a rest mass. According to the law of conservation of mass and energy, mass does not arise out of thin air, so where does this Π meson with rest mass come from? It is transformed from energy, and it is the proton that is in high motion that provides energy for it. Among the various mesons, the Π meson is the lightest, its mass is 139.569 MeV/c, MeV/c is the mass unit, 1 MeV/c = 1.783X10∧-36kg. Although the Π meson is light, the energy required to generate such a meson cannot be ignored.
According to the mass-energy equation E=mc, energy is equal to mass multiplied by the square of the speed of light, the speed of light is taken at 299792458m/s, the square of the speed of light is a huge number, so even a very small mass corresponds to an extremely large energy.
Therefore, when the high-speed moving proton collides with the photon of the microwave background radiation, a Π meson will be generated, and the generation of this meson will reduce the energy of the moving proton, the energy will decrease, the speed will be reduced, until the speed of the proton is reduced to a certain critical value, in order to no longer interact with the photon of the microwave background radiation, the energy will not continue to decline, and the energy corresponding to this critical value is called the "GZK limit". The GZK limit of protons is 5X10∧19eV.
The speed of everything in the universe is limited by the GZK limit, from cosmic rays to starships.
So in the future, once the spacecraft made by humans exceeds the GZK limit, it will interact with the photons of microwave background radiation, so as to slow down until the speed is reduced below the critical value, and this interaction may cause damage to the spacecraft in addition to limiting the speed of the spacecraft, so don't think about the speed of light, the GZK limit is already the limit speed that humans can achieve. In this way, is there no hope for human beings to carry out interstellar voyages? In fact, regardless of whether the GZK limit exists or not, the hope of human interstellar voyage cannot be pinned on speed, because even if it moves at the speed of light, it is still too slow for the vast universe. Therefore, if human beings want to achieve interstellar voyages, they must hope for space itself, and must find wormholes that can cross space.