960. How does gravity influence the passage of time? — AW, Karachi, Pakistan
Gravity's effects on time are the result of general relativity. Any concentration of mass/energy curves the space/time around it, which is ultimately why objects passing near that mass/energy are deflected. This curvature of space/time also slows the passage of time for objects that are near the concentration of mass/energy. To see why this slowing of time must occur, imagine people operating a radio transmitter on the surface of a very massive planet. They transmit their radio wave at exactly 100 MHz. You are far from the planet with your radio receiver and you begin trying to find their transmission. You will find it at a lower frequency, perhaps 99 MHz. That's because their radio wave has had to struggle to escape from the planet's gravity and has used up some of its energy in the process. Since energy is proportional to frequency, the radio wave shifts toward lower frequency as it climbs out of the planet's gravitational well to reach your receiver. Since the people on the planet think that their system is operating at 100 million oscillations per second and you think that it is operating at only 99 million oscillations per second, the people on the planet are evidently experiencing time more slowly than you are. Their second actually lasts longer than yours.
To understand how their time passes more slowly that yours, you can think of the radio wave's frequency as the ticking of a clock. The time it takes the clock's ticks to reach your ear isn't important in measuring the passage of time. What you care about is how often those ticks occur. When you "listen" to the ticking of the clock on the big planet, it ticks 99 million times each second. However, to the people on the planet, it ticks 100 million times each second. This apparent inconsistency is explained by the fact that time is passing faster for you than for the people on the planet. Their second lasts longer than yours, which is why they count more ticks during their second than you count during your second.