# If photons have zero mass, why do they feel the effects of gravity?

*We know that black holes exert a gravitational force on the objects. How can a massless photon go into black hole and not escape from it? The gravitational force is related to mass isn't it? Is there another force in the black hole or does light has mass?*

This is a great question!

You are right that according to Newton's gravity, the force of gravity on particle that has 0 mass would be zero, and so gravity should not affect light. In fact, according to Newton's gravity Black holes should not exist: no matter how strong gravity is, light would always be able to escape!

However, we know that Newton's gravity is only correct under certain circumstances, when particles travel much slower than the speed of light, and when gravity is weak... This is certainly not the case near a black hole! When we try to understand how black holes work we need to consider the more general law of gravity which is Einstein's General Relativity...

According to General Relativity, gravity is not a force! On the contrary, gravity just affects how distances are measured, and says what shape has the "shortest" path from one place to another... All particles then follow these "shortest path" routes in their motion. Notice that nowhere so far have I mentioned mass, this rule applies for all matter and energy, whether they have mass or not!

It turns out that very close to the black hole, these shortest paths never cross the event horizon... As a result neither light nor anything less can escape from the gravity field of a black hole!

# Still Curious?

**Get More 'Curious?' with Our New PODCAST:**

- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.

**More questions about The Theory of Relativity:** Previous | Next

# How to ask a question:

If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

Main Page | About Us | For Teachers | Astronomy Links | Ask a Question | View a Random Question | Our Podcast

Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't existURL: http://curious.astro.cornell.edu/question.php?number=782

This page has been accessed *18023* times since April 23, 2012.

Last modified: *April 23, 2012 6:41:59 PM*

Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.

**Warning: Your browser is misbehaving! This page
might look ugly.** (Details)