Can Light Orbit A Black Hole?

Can Light Orbit A Black Hole?

Since black holes are the most powerful gravitational spots in the entire Universe, can they distort light so much that it actually goes into orbit? And what would it look like if you could survive and follow light in this trip around a black hole?

I had this great question in from a viewer. Is it possible for light to orbit a black hole?

Consider this thought experiment, first explained by Newton. Imagine you had cannon that could shoot a cannonball far away. The ball would fly downrange and then crash into the dirt. If you shot the cannonball harder it would fly further before slamming into the ground. And if you could shoot the cannonball hard enough and ignore air resistance – it would travel all the way around the Earth. The cannonball would be in orbit. It’s falling towards the Earth, but the curvature of the Earth means that it’s constantly falling just over the horizon.

This works not only with cannonballs, astronauts and satellites, but with light too. This was one of the big discoveries that Einstein made about the nature of gravity. Gravity isn’t an attractive force between masses, it’s actually a distortion of spacetime. When light falls into the gravity well of a massive object, it bends to follow the curvature of spacetime.

Distant galaxies, the Sun, and even our own Earth will cause light to be deflected from its path by their distortion of spacetime. But it’s the incredible gravity of a black hole that can tie spacetime in knots. And yes, there is a region around a black hole where even photons are forced to travel in an orbit. In fact, this region is known as the “photon sphere”.

From far enough away, black holes act like any massive object. If you replaced the Sun with a black hole of the same mass, our Earth would continue to orbit in exactly the same way. But as you get closer and closer to the black hole, the orbiting object needs to go faster and faster as it whips around the massive object. The photon sphere is the final stable orbit you can have around a black hole. And only light, moving at, well, light speed, can actually exist at this altitude.

Artist impression of a black hole. Credit: ESO/L. Calçada
Artist impression of a black hole. Credit: ESO/L. Calçada

Imagine you could exist right at the photon sphere of a black hole. Which you can’t, so don’t try. You could point your flashlight in one direction, and see the light behind you, after it had fully orbited the black hole. You would also be bathed in the radiation of all the photons captured in this region. The visible light might be pretty, but the x-ray and gamma radiation would cook you like an oven.

Below the photon sphere you would see only darkness. Down there is the event horizon, light’s point of no return. And up above you’d see the Universe distorted by the massive gravity of the black hole. You’d see the entire sky in your view, even stars that would be normally obscured by the black hole, as they wrap around its gravity. It would be an awesome and deadly place to be, but it’d sure beat falling down below the event horizon.

If you could get down into the photon sphere, what kind of experiments would you want to do? Tell us in the comments below.

What are Photons

Faraday's Constant


When we think about light we don’t really think about what it is made of. This was actually the subject one of the most important arguments in physics. For the longest time physicists and scientist tried to determine if light was a wave or a particle. There were the physicists of the eighteenth century who strongly believed that light was made of basic units , but certain properties like refraction caused light to be reclassified as a wave. It would take no less than Einstein to resolve the issue. Thanks to him and the work of other renowned physicists we know more about what are photons.

To put it simply photons are the fundamental particle of light. They have a unique property in that they are both a particle and a wave. This is what allows photons unique properties like refraction and diffusion. However light particles are not quite the same as other elementary particles. They have interesting characteristics that are not commonly observed. First, as of right now physicists theorize that photons have no mass. They have some characteristics of particles like angular momentum but their frequency is independent of the influence of mass They also don’t carry a charge.

Photons are basically the most visible portion of the electromagnetic spectrum. This was one of the major breakthroughs Einstein and the father of quantum physics, Planck made about the nature of light. This link is what is behind the photoelectric effect that makes solar power possible.Because light is another form of energy it can be transferred or converted into other types. In the case of the photoelectric effect the energy of light photons is transferred through the photons bumping into the atoms of a giving material. This causes the atom that is hit to lose electrons and thus make electricity.

As mentioned before photons played a key role in the founding of quantum physics. The study of the photons properties opened up a whole new class of fundamental particles called quantum particles. Thanks to photons we know that all quantum particles have both the properties of waves and particles. We also know that energy can be discretely measured on a quantum scale.

Photons also played a big role in Einstein’s theory of relativity. without the photon we would not understand the importance of the speed of light and with it the understanding of the interaction of time and space that it produced. We now know that the speed of light is an absolute that can’t be broken by natural means as it would needs an infinite amount of energy something that is not possible in our universe. So without the photon we would not have the knowledge about our universe that we now possess.

We have written many articles about photons for Universe Today. Here’s an article about how the sun shines, and here’s an article about why stars shine.

If you’d like more info on Photons, check out the Mass of the Photon. And here’s a link to an article about How Gravity Affects Photons.

We’ve also recorded an episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.