Our world is focused on time.Everything in the universe has a close relationship with time.But what time is it?Time is still a cosmic phenomenon that science has never been able to catch up with.A theory called Special Relativity was founded by the great 20th century physicist Albert Einstein. Different Relativity theories are very difficult to imagine because they are not about what we experience in daily life, but they have been verified by scientists.Space and time are both facets of the same thing — space-time. For anything that passes through space-time, there is a speed limit of 300,000 kilometers per second (or 186,000 miles per second), and light also travels the speed limit through empty space.Special Relativity also says that when you move through space-time, a surprising thing happens, especially when your speed is close to the speed of light relative to other objects. For you, time goes slower than for the people you left behind. Until you return to those stationary people, you won’t notice this effect.

Remember, when you left Earth in a spaceship traveling at about 99.5 percent of the speed of light (which is much quicker than we can do now), you were 20 years old and celebrated only five birthdays during your space travel.When you get home at the age of 25, all your classmates are 70, married, and loving their grandchildren! Because time has passed for you more gradually, you will only have had five years of life, while your contemporaries will have had a full 50 years experience.

A kind of time travel often happens in gravitational fields of objects.Einstein had another unique theory called General Relativity, which suggests that for objects in gravitational fields (like here on Earth) time passes more slowly than for objects far from these fields.And near black holes there are all sorts of distortions in space and time, where gravity can be very strong.

Gravitational time dilation This effect measures the amount of time that observers at different distances from a gravitational mass have elapsed between two events. In other words, time runs slower wherever gravity is strongest, because gravity curves space-time.