B Photon Mass: Does Light Have Direction?

[Lucas Nunes Rosa]

TL;DR Summary

Simple photon question...

Could someone ell me if photon has mass? How can light have its direction changed by massive bodies?

 

[Ibix]

A photon has zero mass.

In general relativity all things are affected by gravity because gravity is the curvature of spacetime - it affects the distances and angles between points in spacetime and nothing can ignore that. And all things produce gravity because the source of gravity is the stress-energy tensor, which includes mass but also other things such as energy and momentum - which light has (edit: that light produces gravity is theoretically true, but we've never experimentally tested it because absolutely unbelievable amounts of energy are needed to make something that we would expect to be detectable with current technology).

Newtonian gravity requires things to have mass, but it turns out that Newton's theory of gravity is an approximation of general relativity, and it's an approximation that's valid when mass is the only significant component of the stress-energy tensor and when everything is moving slowly compared to the speed of light. It does a terrible job of describing light interacting with gravity.

 

[DaveE]

Summary:: Simple photon question...

Could someone ell me if photon has mass? How can light have its direction changed by massive bodies?

This question was the basis of the first experimental verification of GR. This is what proved (OK, one thing that proved) that Einstein was right and Newton was wrong.
https://www.britannica.com/science/relativity/Experimental-evidence-for-general-relativity

 

[Ibix]

This is what proved (OK, one thing that proved) that Einstein was right and Newton was wrong.

To be pedantic, it was one piece of evidence that showed that Newton's theory was only an approximation, albeit one which remains valid in all but some fairly extreme circumstances.

 

[DaveE]

To be pedantic, it was one piece of evidence that showed that Newton's theory was only an approximation, albeit one which remains valid in all but some fairly extreme circumstances.

Fair enough. I guess Quantum Gravity will prove Einstein was also wrong, someday.

 

[Ibix]

Fair enough. I guess Quantum Gravity will prove Einstein was also wrong, someday.

Well, to be pedantic again, relativity must be an approximation to quantum gravity which is accurate in all but some very extreme circumstances. Any successor theory must simplify to something indistinguishable from the current one, because it must explain all the experimental data we've got that matches the current theory to our best precision.

 

[vanhees71]

Summary:: Simple photon question...

Could someone ell me if photon has mass? How can light have its direction changed by massive bodies?

As far as we know, photons are massless. In GR the gravitational interaction is due to the energy, momentum, and stress and not only due to mass energy. That's why photons (I'd rather say the electromagnetic field) are subject to the gravitational interaction as anything that has energy, momentum, and stress.