# If light is a form of energy, why should it get attracted by the immense gravitational force exerted by a black hole?

Dec 25, 2017

Mass and energy are equivalent and both are affected by gravity.

#### Explanation:

Albert Einstein proved that mass $m$ and energy $E$ are equivalent in his famous equation.

$E = m {c}^{2}$

Where $c$ is the speed of light.

Now, photons of light have zero mass, but they do have both energy and momentum. Having zero mass means that photons travel at the speed of light and follow geodesics in four dimensional spacetime. A geodesic is the four dimensional equivalent of a straight line.

How gravity works is described by Einstein's field equations.

${G}_{\mu \nu} = \frac{8 \pi G}{c} ^ 4 {T}_{\mu \nu}$

That simple and elegant equation is actual ten second order partial differential equations which are very hard to solve. It can be explained quite simply.

The term ${T}_{\mu \nu}$ is the stress-energy tensor. It says that mass, energy and momentum tell spacetime how to curve. We know mass causes gravity, but so does light! It is hard to curve spacetime, so only large masses have a noticeable effect.

The term ${G}_{\mu \nu}$ is the Einstein tensor which describes how spacetime is curved. It also describes the geodesics which tell matter and energy how to move.

So, as matter and energy are equivalent, both cause gravity and are affected by gravity. A photon travels along a geodesic which can be a straight line. Near to a massive body spacetime is curved and hence light follows a curved geodesic.

Dec 27, 2017

#### Explanation:

Gravitational force isn't simply
$\textcolor{g o l d}{{F}_{G} = \frac{G M m}{r} ^ 2}$
That you plug value of mass of light and $\textcolor{red}{{F}_{G} = 0}$

Newton gave the laws of gravitaion , this is what i learnt last year in ${9}^{t h}$ grade , but Newton himself accepted that he didn't knew why there was some force he called gravitational force , however he was able to calculate force of gravity almost correctly .

Here's the entry of Albert Einstein. He thought to make gravitation more reasonable , he gave a unified theory of gravitation , in which he mentioned curvature in space time to be reason for gravitational force .

Space time is bent by every particle whether it has mass or not , yet not sure about theoratical negative mass .

Light (photons) is affected by even extremely weak gravitational force affects photon but the effect is far less to be observed .

The question however was why there's force of gravitation between a massless particle and some other particle or may it be black hole .
The simple answer is that there are a few misconception related tp gravity , the biggest is created by
$\textcolor{\in \mathrm{di} g o}{{F}_{G} = \frac{G M m}{r} ^ 2}$
gravitational force applies to both mass and energy because they bend space time , some bend less while some more .
But why gravitational force exists? May be we don't smart answer here , our universe is built that way .

A new question could be framed from this .
Will gravitational force exist when the universe is at its highest disordered state? highest possible entropy?
I got no answer for you here just more questions!

Perhaps , it isn't that what you don't observe doesn't exist.
For some formulas you can see @Phillip E's answer , that's really good.