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- The Connection Between Gravity and Light: Exploring Einstein's Theory
The Connection Between Gravity and Light: Exploring Einstein's Theory
Discover how gravity influences light according to Einstein's general theory, as well as the historical predictions of black holes by early theorists. Learn about the bending of light paths near massive objects and the misconceptions surrounding the effects of gravity on light.
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Video Transcript
We know that gravity exerts its pull on lights, and we have an explanation for why.
Well, actually, we have multiple explanations that all predict the same thing,
and at first glance, these explanations seem to describe completely different causes.
So what is the true connection between light and gravity, or is truth in fact entirely relative?
Gravity bends the path of light. That fact is guaranteed by Einstein's general theory of
But for some reason, we scientists have been convinced of the fact with no good evidence
for hundreds of years.
In 1783, the English clergyman John Mitchell proposed that a particle of like gripped by
the gravitational field of a sufficiently massive star would slow down, stop and fall back,
and so was the first to predict the existence of black holes.
Mitchell wrote to his good friend Henry Cavendish on the subject, and Cavendish followed
similar reasoning to predict that a particle of light would be deflected in its path as it
passed near a massive object. Mitchell and Cavendish used the same utterly wrong assumptions
to do their calculations. They assumed that light could be slowed down and that light
experiences a force of gravity in the same way that a massive object does. They assumed that
Isaac Newton's theory of gravity was the full picture and that light behaves like any other
particle in response to Newtonian gravity. But the weird thing is that despite these incorrect
assumptions, the effects that these guys predicted have proved very real. We now know that both
gravity and light are much weirder than Newton thought. We explored some of this in recent episodes
when we saw that what we experience as a force of gravity is mostly due to the way mass warps
the flow of time. But the photon doesn't experience the flow of time, it doesn't even have any mass.
mass, and yet general relativity demands that gravity does affect light in ways eerily
close to the predictions of Mitchell and Cavendish. The really hard part is understanding the
why of it. What is really happening when light interacts with gravity? Let's see if we
can't figure it out.
In general relativity, the best place to start is always the equivalence principle. This
is Einstein's great insight that there's no experiment that can distinguish between the
backwards pull due to being in an accelerating reference frame and the downward pull of gravity
of the same strength, or of the sense of weightlessness in freefall in a gravitational field
versus the weightlessness felt in the absence of gravity. Imagine the joiner rocket ship.
Accelerating rapidly, I'd know to escape giant alien spiders or something. A spider breaks through
the front hull and you fire your laser at it. By the time the beam reaches its target,
the ship is moving a little faster than when it fired.
The spider still observes the laser travelling at the speed of light because the speed of light
is invariant to all observers, but something else does change.
See, light is a wave.
The distance between the peaks of that wave is its wave length.
After the front peak of your laser pulse reaches the spider, the ship continues to accelerate,
which means the second peak has a little further to travel than the first, and the third
peak further still.
The distance between the peaks gets drawn out, wavelength increases, which means frequency
and energy drop.
Your laser power rating goes from kill to tickle.
Good news for the giant alien spider?
Not so much for you.
The equivalence principle tells us that we must experience all the same physics if at
rest in a gravitational field, say in a fake rocket ship in a Hollywood set.
That for some reason is also being attacked by giant alien spiders.
light emerging from a gravitational field is stretched out. It experiences
gravitational red shift. And we get exactly the same prediction if we use the
