Space and time are the same; space-time. You have probably heard this many times and, to this day, time is a very complex subject for physicists.
Albert Einstein showed us that time and space are linked while the progression of time is relative and not absolute. It is generally agreed that time is the property of the universe and science is based on the laws of physics and its assumptions.
Time on Earth is different from that in space. This is because of “relativistic time dilation.” Relativity tells us that time is the fourth dimension of space-time. As Einstein showed with his famous equation E=MC2, mass and energy are the same. He also showed us that mass and energy help to cause space-time to curve. The true effect of this curvature is gravity.
Velocity and mass cause time to slow. If you are in a strong gravitational field, you will experience a normal passing of time. An observer from outside this field will see time passing more slowly.
The orbital speed of an object causes time to slow. Gravity is reduced when in orbit compared to being on a surface and this can cause time to speed up. While orbiting Earth, these can cancel out an altitude of half the radius of the planet.
This is why astronauts on the ISS age slightly slower than those on Earth. The velocity component of time dilation is far greater than the reduced gravity effect. A clock on a GPS satellite that is in high orbit will run faster than if it was on Earth’s surface. This is because of the reduction in gravity that has a more substantial effect than orbital speed. The clocks on these satellites have to have their clocks slowed down regularly to match the surface time of the Earth.
Time moves slower as gravity increases. This is known as “gravitational time dilation.” This means time passes slower for objects that are closer to the Earth’s center where the gravity is much stronger. However, this does not mean that living in an underground cave to get closer to the center of the Earth will slow down time as the effect on time is not noticeable on such a small scale. Interestingly though, if you spend your entire life in an underground dwelling, you could age a fraction of a second slower than everyone who lives above ground.
Overall, time is variable and changes in the proximity of mass. A person near a black hole would experience a much slower rate of time. With less extreme gravity on Earth, a person here may grow into old age while the person near the black hole would hardly age at all.
It is hard to conceive but this is exactly what the theory of relativity predicts. At the singularity of a black hole, time stops and can cease to exist. Also, at the speed of light, time stops so light traveling across the universe could take no time to travel an infinite distance.