→ The twin paradox theory is a special relativity thought experiment involving identical twins, one of whom travels into space in a high-speed rocket and returns home to discover that the twin who stayed on Earth has aged more.

→ Because of time dilation, time moves more slowly in the spacecraft than it does on Earth, and the moving twin will discover that the earthbound twin is older when they return.

Why Time Slow Down ?

→ According to Einstein, nothing can be faster than speed of light in vacuum. So if you approach the speed,
And fire a light beam then you should see the beam travel at speed of light.

Someone at rest will see the speed of beam, then, equal to speed of light + your speed which is not possible.

So time for the moving body should slow down so that the situation could be balanced.