I'm confused about how massless particle is influenced by gravitational force. Some source point to the energy conversion to mass and vice versa so light is treated similarly, other source mentioned "space tells light how to move" eliminating the need for a particle to have mass. What is the correct explanation that explains how gravitational lensing works? (Don't tell me duality, the unique properties of light and this question is from a 3 yrs old liar.)


1 Answer 1


Mass curves space.

However light follows the path of least distance between point A and B. The path of least distance on a flat surface is a straight line.

However if you consider a 2 dimensional surface then put a dent pushing it into the 3rd dimension - the shortest trajectory from point A to B across this dent changes from a straight line to something which appears to curve around the radius.

The shortest path in general from point A to B is called the geodesic. Sometimes this is called the straight line in curved space. https://en.wikipedia.org/wiki/Geodesic

So extending the idea to three dimensions rather two you will need to make a dent into the fourth dimension. The end effect will be something that would look like the image below but you rotate the plane surface about the axis of symmetry - consequently light is spread around the radius entire massive object.

Gravitational lensing Gravitational lensing in 2 dimensions. The observed position is ignoring the fact that the light was travelling across curved space! Source: http://davidjarvis.ca/dave/gallery/lg/gravitational-lens-01.jpg

  • $\begingroup$ therefore gravitational lensing phenomenon is not due to Einstein's equation where momentum is at play, so am I correct to say that objects with mass distorts space and thus space will affect all objects regardless of any amount of mass even zero? I tried searching it seems there are 2 sides of a coin. $\endgroup$
    – user6760
    Mar 22, 2015 at 8:35
  • $\begingroup$ Gravitation lensing is due to another one of Einstein's equations. I'm assuming the momentum one you're referring to is $E^2 = (mc^2)^2 + (pc)^2$. What gravitational lensing is essentially saying is that even massless particles can't escape gravity. $\endgroup$ Mar 22, 2015 at 8:42

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