Since photons are massless how can gravity bend it?

Question

It's my perception that gravity can only act on objects with mass correct? Then how does a massless substance such as light and other EW's bend in the presence of strong gravitational forces such as black holes.

Answer 1

Light THINKS it's going in a perfectly straight line, but spacetime is warped around gravitational bodies. So, that's how light paths end up getting bent, even though photons are massless. In relativisitc parlance, the path of light is a geodestic, or shortest metric distance, which can be curved in warped spacetime.

Answer 2

First, gravity CAN ACT on MASS-ENERGY (not just on rest-mass), that is on the whole "mass-energy ball of wax."

In the theory of relativistic stars, the contributions of gravity on, and the gravitational attraction of, the mass-equivalent of ALL forms of energy, are important parts of both their structure, AND contributors to their possible instabilities. Such energies include the kinetic energy otherwise contributing to their internal pressure, rotational energy, and even the self-energy of the gravitational field itself.

In a similar sense, then, you can think of gravity as acting on the mass-equivalent of the photon's energy. And in fact, one way of deriving the gravitational redshift of photons climbing out of a gravitational potential well is to consider them losing the VERY SAME AMOUNT of kinetic energy that their mass-equivalents would lose on climbing up the same distance. The results from doing this agree with other, more theoretically based and seemingly quite independent derivations.

So talking of gravity acting on the mass-equivalents of photons isn't just some bit of philosophical, airy-fairy, pseudo-scientific wool being pulled over your eyes. It is in fact a deep consequence built into the connection between light, mass and gravity.

And secondly, of course, there's the other rather trite and pat answer that according to the Theory of General Relativity, both material particles and light follow geodesics in a curved space-time. From this point of view, one no longer talks of gravity "bending a particle's path," or "bending light's path." Rather, one only talks of the geodesics in a sense pre-existing in the curved geometry, and material particles or photons both following them at their various rates.

Exactly how both even particles as well as photons "know" how and why they should do that is something generally glossed over. One is asked to merely accept that as a basic hypothesis in the theory, or alternatively one hears some mumbo-jumbo about it "emerging from the self-consistency of the field equations." I myself have some rather novel ideas about this which involve a quantum-mechanical view of the world, but I'm not prepared to divulge them in this arena !

Live long and prosper.

P.S. By the way, be very wary of anyone who tells you that "What gravity does is to bend the space through which objects move," or "it is the fabric of space that is curved by gravity..."

The fact is that ALL of Newtonian gravitational effects (and I really mean ALL) arise SOLELY because time runs differently in different places. The "bending of space," PER SE, plays ABSOLUTELY NO ROLE WHATSOEVER IN NEWTONIAN GRAVITY. Newtonian gravity arises in that sense from the "bending of time," but NOT from the "bending of space." Even many professionals often lose sight of this (though they should know better!), so it's not so surprising that you've seen these two loose statements in other answers to your question.

Answer 3

Well, technically, it is the rest mass of the photon that is zero, but photons move at the speed of light.
Remember the equation E=mc^2? Since a photon does carry energy, you can intuitively assing it a mass equivalent to that of its energy, as they are both the same.
Anything with a mass cannot move at the speed of light, because then its mass becomes infinite. But what happens when something with zero mass has its mass multiplied by infinity? It can be something that is non-zero then.
As crazy as it may seem.
But another way of looking at it is that light still travels in a straight line, it is the fabric of space that is curved by gravity...

Answer 4

What gravity does is to bend the space through which objects move. That is, the geodesics appear to be bent with respect to those which would exist in a space without an attractive object. Light follows geodesics, so it appears to bend in a gravitational field.