# Can ever a beam of light be trapped in an orbit of a star?why?

Dec 19, 2017

No. The Universal Law of Gravitation requires mass

#### Explanation:

The Universal Law of Gravitation is ${F}_{G} = G \frac{{m}_{1} {m}_{2}}{d} ^ 2$

A beam of light doesn't have any mass, so it can't be trapped into an orbit as can happen to objects that have mass.

Dec 19, 2017

A beam of light can't be trapped in an orbit of a star.

#### Explanation:

The mathematics of General Relativity are really hard, so I will try and explain in English.

Now we used to think of three dimensional space with time being an independent variable ticking at a constant rate. We now know that this is not correct.

We live in four dimensional spacetime. Time passes at different rates for different observers. An observer in a spaceship sees time passing more slowly on another spaceship which is travelling faster of is in stronger gravity.

Massive bodies such as stars cause spacetime to curve. A body orbiting a star is actually travelling in a straight line, called a geodesic, in spacetime. The close to the star the orbit the faster the body needs to travel to stay in orbit.

Light consists of photons which have zero mass. This means that they travel at the speed of light, have no mass and do not experience the passing of time.

For light to be trapped in orbit, the orbital speed must be the speed of light. It also means that gravity must be strong enough to stop time completely. This condition is only known to occur at the event horizon of a black hole. Also, the event horizon is a singularity which means that it and everything inside if are completely invisible to everything else in the universe.

To sum up nothing except for a black hole is massive enough or dense enough to capture light in orbit. Light which enters the event horizon of a black hole can't be considered to be in orbit as it is forever lost to our universe.