Rotating Question Curious About Astronomy? Ask an Astronomer

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!

April 2012, Manolis Papastergis (more by Manolis Papastergis) (Like this Answer)

Still Curious?

Get More 'Curious?' with Our New PODCAST:

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.

Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist

URL: http://curious.astro.cornell.edu/question.php?number=782
This page has been accessed 18073 times since April 23, 2012.
Last modified: April 23, 2012 6:41:59 PM

Legal questions? See our copyright, disclaimer and privacy policy.
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)