# How does Light have momentum without mass? [duplicate]

This question already has an answer here:

My question is regarding gravity in black holes. It is said that light can’t escape the enormous gravitational force in black holes; however, is it not true that gravity is directly proportional to the object’s MASS and inversely proportional to the distance between the two objects (Newtonian, I think). If so, light has no mass. So how would light be effected by this phenomenon? Please answer with as much detail as possible

## marked as duplicate by Community♦Nov 19 '18 at 19:16

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• Your question in the title is different than the question you've asked here. Which question are you asking? – enumaris Nov 19 '18 at 19:08
• Possible duplicate of If photons have no mass, how can they have momentum? and How is light affected by gravity?. – ahemmetter Nov 19 '18 at 19:14
• Title question (v1) and question in main body seem different. – Qmechanic Nov 19 '18 at 19:32
• Why do you spell mass in caps? Why do you think Newtonian gravity is the final word on the subject? General relativity has been around since 1915. – my2cts Nov 19 '18 at 20:07

## 1 Answer

So that's not necessarily the best way to think about the way that light travels through a gravitational field. A better approach is to consider the following two statements:

• Very massive objects warp / curve the space around them.

• Light travels between two points along the line with the shortest length.

So ok, in space that is flat and not warped the second statement seems to make sense. If I shine a flashlight in a huge empty region of space, I shouldn't expect the light to make a 90 degree turn at Albuquerque (sorry).

However, as per the first statement, massive objects warp space. So now the definition of "straight" that we defined in flat space doesn't suffice anymore. So in reality light will still follow the most straight path, but to us it will appear curved because we're looking from the outside in at a curved space. So it will look like gravity is "bending" light, when in reality gravity is really bending space and light is still traveling in a straight line as it always does.

Hope that helps!