Whether it's science fiction or science fiction movies, there's no coolest concept in it than traveling through time. Because based on common sense, everything is always developing into the future, and there is only one direction for the passage of time, whether it is fast or slow.
As science fiction enthusiasts, many people dream of being able to travel through time and space, some hoping to travel to the past, and some hoping to travel to the future.
The future is uncertain, and it is theoretically feasible to travel to the future. If we can fly a spacecraft at nearly the speed of light, or orbit a black hole at close range, and slow down the speed of time in our space-time region in these two ways, we can theoretically travel into the future.
And the past has happened, and if we can really travel to the past on the same timeline, then it is possible to change the present. The most famous is the grandmother paradox, if you go back in time and accidentally kill the young grandmother, your father will naturally not be born, then how did you come to be, and travel to the past to cause the accident? Wouldn't that be a contradiction! This act of traveling back in time breaks the laws of cause and effect.
Parallel universes, however, can resolve this contradiction. According to the idea of parallel universes, if you travel back in time and accidentally harm your young grandmother, then this change will create a new branch of time, splitting a parallel universe from that point in time. With the concept of parallel universes, there is no longer a need to worry about changing the past and affecting the future during the journey, because this change will only produce new branches of time, and the old timeline will not be affected.
According to this hypothesis, there will be almost an infinite number of parallel universes not long after the birth of the universe.
When it comes to parallel universes, this concept was born not from the time travel paradox, but from quantum mechanics. This goes back more than 80 years to a cat whose owner was the famous physicist Schrödinger, so the cat was also called "Schrödinger's cat".
As the founder of wave dynamics in quantum mechanics, Schrödinger proposed the idea experiment of "Schrödinger's cat" in 1935. The purpose is to make people feel how weird the Copenhagen interpretation in quantum mechanics really is, and to attack the incompleteness of the Copenhagen interpretation.
The Copenhagen School holds that the quantum world is inherently uncertain, that only probabilities can be used to describe the properties of the system, that measurement behavior affects the system, and that the set of probabilities shrinks to one of the values of all possibilities. Scientists use wave functions to describe the properties of quantum systems, so this situation is also known as wave function collapse.
Schrödinger said that if a cat is kept in a closed container containing a small amount of radium and highly toxic cyanide, radium has a chance of decay and controls an electronic switch. If radium decays, it triggers the mechanism to break the bottle containing cyanide and the cat dies; if the radium does not decay, the cat is still alive.
According to quantum mechanics, radioactive radium is in the superposition of both decaying and non-decaying states. According to this principle, the cat in the closed container should also be in a superposition of dead cats and live cats, which is both dead and alive "Schrödinger cat". Only when we open the container is the state of life and death of the cat is determined.
For traditional physicists, this is inconceivable, and it is a violation of logic to live and die. They believe that the cat's outcome is predestined before the container is opened: either life or death. It's just that we didn't open the container to observe, so we're not sure if the cat is alive or dead.
It is well known that microscopic matter has wave-particle duality, whereas before observation, particles and fluctuations are superimposed. According to the Copenhagen school of quantum mechanics, the Schrödinger cat is in a state of superposition of life and death before the container is opened. Once the observational behavior is involved, this superposition will be disturbed and manifest as one of the eigenstates.
This thought experiment, which cleverly moves quantum phenomena from the microscopic world to the macroscopic world with a cat, is one of the most classic thought experiments in the history of science. In fact, the reason for this bizarre result is that quantum superposition states do not apply to macroscopic large objects.
Since the state of life of the cat is unknown before the container is opened, and there are two states, there should perhaps be two different worlds, in one of which the cat is still alive and in the other the cat is dead. Based on this idea, the concept of parallel universes was derived.
In the 1950s, physicist Hugh Everett proposed a multi-world interpretation in quantum mechanics and argued that there was no wave function collapse at all, and that cats and dead cats actually existed in two parallel universes.
According to this theory, everything that could have happened in the past and in the future has either actually happened or is about to happen, except that they all happened in other universes and have nothing to do with the universe in which we live. If there really are many parallel universes, then there must be another self in some parallel universes, but their lives are different.
Everything has a beginning and an end. Physicists believe that there must be a proto-universe, and other parallel universes are constantly differentiated on this basis. These parallel universes coexist with each other, and there will be no connection between the newly differentiated parallel universe and the original parallel universe, they exist independently of each other.
Thinking about the cat gave birth to perhaps the boldest conjecture in physics— the parallel universe. It is only for its existence that scientists can only explore it from a theoretical level. But the theory has a big question, will the universe really make such a big change for the sake of a cat's life and death?