How long has it been on Earth since we left at the speed of light and returned at the speed of light a year later?
The speed of light is 300,000 kilometers per second, the fastest speed in the universe
If an astronaut leaves Earth at the speed of light and returns at the speed of light a year later, by common sense, two years should have passed on Earth by the time the astronaut returns.
However, if you know the theory of relativity, then you won't think so. If you look at it from the perspective of relativity, then how much time has passed on Earth by then?
The speed of light is 300,000 kilometers per second, the fastest speed in the universe. According to the theory of relativity, objects with mass are forbidden to exceed the speed of light, and that's because it takes infinite energy to accelerate them to the speed of light. So that astronauts and spacecraft can theoretically only move at speeds close to the speed of light.
And time is relative, in different reference systems would yield different times, so the time on the spacecraft is different from the time on Earth. Time has a unidirectional nature and passes forward forever. And the traditional concept is that time passes at the same rate at every location in space-time, although high-precision experiments have proven that this concept is wrong. So, to understand this problem, we must throw away the absolute view of space-time.
According to special relativity, if the reference system is moving faster, then the speed of time passing under that reference system will be slower. When the speed of motion of an object reaches the speed of light, the time has stopped passing. This phenomenon is called the time dilation effect, or the clock slowdown effect. The more the speed of motion of the reference system converges to the speed of light, the more significant this effect becomes. Simply put, a clock in motion moves slower than a clock at rest.
A spaceship traveling at close to the speed of light is moving at a significantly different speed than the two reference systems, the Earth. The speed of time on the spacecraft is very slow, so when the astronauts return to Earth, for the astronauts, it has only experienced two years, while for the people on Earth, but has passed years, or even hundreds of years. This is really "a day in the sky, a year underground"!
If the astronaut had been moving at 99% of the speed of light, after two years, he would have returned to Earth and found that 14 years had passed for the people on Earth. If he had a twin brother, then his brother would be 12 years older than him at this time.
If moving at 99.99% times the speed of light, after two years, the time difference between the two will be 139.4 years, and if this happens, I'm afraid he won't be able to see his twin brother when he returns to Earth.
However, in reality, the spacecraft can not start with 99% of the speed of light movement, because the human body can not withstand such a large acceleration so the astronauts must go through a process of acceleration and deceleration. This means that the time difference caused by the time dilation effect when the astronauts return to Earth after two years is not as large as previously stated.
Using this phenomenon, we can time travel, as long as the speed is fast enough, only a very short time to travel to the distant future, but this is a single trip, there is no regret medicine.
In addition, in relativity, all reference systems are affine and motion is relative, and a spaceship moving away from the Earth can be seen as the Earth moving away from the spaceship. Then, from the perspective of each, both are slowing down their own time. Then whose time has slowed down?
The reason for this contradiction is that we treat them both as inertial reference systems. Since the spacecraft undergoes acceleration and deceleration during the process of leaving and returning to Earth, then the spacecraft is a non-inertial reference system. The non-inertial reference system must then be treated by general relativity.
And in general relativity, gravity also causes time dilation, and the stronger the gravitational field, the slower the passage of time.
The inertial force during the acceleration of the spacecraft is equivalent to the gravitational force, so the effect of time dilation due to gravity must be considered. All things considered, it is the spacecraft that slows down time, not the Earth.
Even if you think this is absurd, it is the case. This theory has been long and rigorously tested, both in high-energy particle gas pedals and on GPS satellites. GPS relies on precise time, and because of time dilation, the time of atomic clocks on satellites must be calibrated at regular intervals, or the results will be off by about 7 meters every 12 hours.
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