If someone returns to Earth after flying at sub light speed for 1 day, is his family still there?

<> according to Einstein's special theory of relativity, time passes slower on a sub-light spacecraft than on Earth because the spacecraft moves relative to Earth. As long as the speed of the spacecraft is close enough to the speed of light, time on the spacecraft tends to stand still for the Earth's frame of reference.

Speaking of which, one might say that since the spaceship moves relative to the Earth, conversely, it can also be considered that the Earth moves relative to the spacecraft, after all, there is no absolutely stationary frame of reference in the universe. In this case, time passes more slowly on Earth than on a spaceship. So, which frame of reference slowed down in time?

In fact, this problem can be explained by the worldline in Minkowski's space, which is detailed in the "Twin Paradox", which will not be done here. Or you can also think about it from another angle, because the motion of the spacecraft requires an acceleration process, so the observer on the spacecraft will really feel the inertial force, and the spacecraft is really accelerating, so it is the spacecraft, not the Earth, that slows down time.

Back to the question. For the sake of calculation, if a person flies for 1 day in a sub-light spaceship (the time of the ship's frame of reference) and then returns to Earth, will he still be able to see his family when he set off?

According to the Bell Slow Effect:

where δt is the time of the spacecraft's reference frame, δt is the time of the Earth's reference frame, V is the speed of the spacecraft relative to the Earth, and C is the speed of light.

If the speed of the spacecraft reaches the speed of light, when the spacecraft lands on Earth after 1 day of flight, the time on Earth is already a year later. The people on Earth are already a year old, and the people on the spaceship will only feel that they left Earth yesterday. Of course, the people on the ship will also be able to meet their families at the time of departure.

But if the speed of the spacecraft is accelerated to the speed of light, the person who has gone on a day's journey to space will return to Earth 1,000 years have passed on Earth, and the family he set out from is long gone.

The closer you get to the speed of light, the slower time passes on the ship. Although the people on the sub-light spacecraft think that their lifespan is only a hundred years at most, this time is very long for people on Earth. So, in this way, it is feasible to fly to the future time of the earth.

However, this kind of time-traveling journey is unrealistic, because the spacecraft needs to accelerate to sub-light speed in an extremely short time, which requires extremely high acceleration, and it is impossible for the human body to withstand this extreme overload. 1G acceleration is a better option, because if the spacecraft moves in space at 1G acceleration, then the people on the spacecraft will feel gravity similar to that on Earth.

In addition to high-speed motion, objects with extreme gravitational pull, such as black holes, can also have extreme time dilation effects. As shown in "Interstellar", on a planet that orbits a black hole at close range, the time of 1 hour will correspond to up to 7 years of Earth time.

However, the nearest black hole is also 3,500 light-years away, and there is currently no way for us to fly there.

According to the general theory of relativity, the speed of light is a tipping point, time can stand still when the speed of light is reached, time around you will become slow when you are infinitely close to the speed of light, and flying back to Earth for a day at sub-light speed will make his family old, but he will not.

If, as mentioned in the question, it is 500 years after flying at sub-light speed for a day and returning to Earth, it may have changed dramatically, and his family may have long since left, but his descendants may still exist.

It's gone. Because sub-light travel slows down time, if someone returns to Earth after 1 day of sub-light flight, their family may not be there.

No. Because traveling through the universe at the speed of light is equivalent to traveling through time, and when you return to Earth, it is no longer the original time, it may be hundreds of years later, and everyone in the family may have died.

I think if someone flies into the universe at the speed of light and then comes back to Earth a day later, he probably won't be able to see his family again, because a day at the speed of light is probably hundreds of years.

If you can't see your family, you must go to work or grocery shopping.

If he flies to the universe at the speed of light and returns to the earth in 1 day, he will not be able to see his family, according to the theory of relativity, the earth has passed billions of years, and it has long been a matter of life.

No, because 1 day in the universe is equivalent to 100 years on Earth, and the family is no longer there when you return to Earth.