Adhersh from India, wants to know:
What will be the behavior of light when it passes between two highly massive bodies?
Let’s elaborate a bit on your question, to make it into that most powerful of theoretical physics tools: a thought experiment!
Imagine we’re out in deep space—in spacesuits, of course—far from any gravitational influence, and illuminated only by the distant starlight. We’re far away from each other, but much, much farther from anything else.
At a certain exact time that we’ve agreed upon beforehand, I shine a laser pointer toward you, aiming precisely at a light-sensitive panel (like a solar panel, which converts light into an electrical signal) installed on your suit. When the laser light strikes this solar panel, a computer in the suit notes that it’s received a signal and the time that it received it—finding that it was a precise seven seconds after the time we agreed I would send the signal.
Since the light signal took seven seconds to get from me to you, we can deduce that you and I are some 2.1 billion meters—or seven “light-seconds”—apart.
|Light-seconds is a measure of distance, like light-years: since the speed of light is|
constant, it's often convenient to talk about astronomical distances in terms of how
long it would take a ray of light to make the journey.
How can this be? There are two ways to interpret it: the most intuitively obvious is that a change in gravitational potential—going from "high" to "low" to "high" again, the way it would as it approaches and then leaves the vicinity of those massive bodies—slows down light. Light moves slower than c all the time—whenever it's traveling through anything other than the vacuum of space. However, one of the fundamental assumptions of modern physics is that the speed of light in a vacuum is constant, regardless of what it's around. To get around this when trying to work out the math of how prominent this effect should be, it's instead assumed that time itself is slowed down by this change.
But time, as we've discussed before, is just a construct—a way of ordering events. To those who can stretch their imaginations into the fourth dimension and embrace "timeless physics", this scenario can be seen another way—as a warping of space.