# Gravity and light

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If gravity is described as a force between two masses, then how exactly can light be effected by it? From my understanding, from what I have been taught, gravity would effect the photons of light which have no mass. Please correct me if I am wrong. Any responses would be much appreciated!

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Light follows what are called geodesics. Light couples to the distortions of spacetime. But yes, gravity effects the spacetime, causing curvature, causing the geometry in which massless radiation follows, called geodesics. I'd write maths but I think an explanation without it suffices.

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Spacetime is curved by energy and momentum. Basically, light follows Newton's first law through curved spacetime. It simply follows the straightest possible path.

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true... even in curvilinear environments, the straightest distance between two points will always be a straight line.

Edited by Aethelwulf
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If gravity is described as a force between two masses, then how exactly can light be effected by it? From my understanding, from what I have been taught, gravity would effect the photons of light which have no mass. Please correct me if I am wrong. Any responses would be much appreciated!

Gravity being a force between two masses is the Newtonian description. While that is approximately correct, that model has been superseded by General Relativity, which is what the other posters have been describing.

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If gravity is described as a force between two masses, then how exactly can light be effected by it? From my understanding, from what I have been taught, gravity would effect the photons of light which have no mass. Please correct me if I am wrong. Any responses would be much appreciated!

When you say "light has no mass" it means that the proper mass of a photon is zero. While the proper mass of a photon is zero it still has what is called passive gravitational mass which means it has the property for which particles are affected by a gravitational field. This is due to its inertial mass which is found by p = mv = mc. Use E = pc or p = E/c and substitue into p = mc and solve for m to get m = E/c2.

In Einstein's first derivation of gravitational deflection he treated it like a wave rather than a particle. Given that the velocity of light depends on the gravitational potential Einstein showed that the wave can be deflected by a gravitating body. I think he got the wrong answer since he didn't take spatial curvature into account.

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Moderator Note

pmb,

Though I understand mass seems to be a buzz topic for you, please be advised that you already have a topic on this and that hijacking other threads with off-topic discussion won't be permitted. Please address the topic in the OP, or move your discussion to somewhere else.

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Though I understand mass seems to be a buzz topic ..

A response is definitely required here:

"buzz topic"? No. I just happen to have some expertise on the topic since its my chosen area of expertise. So when the subject comes up I find it quite easy to respond to them. The responses I post can be found in most SR/GR texts as well as the Feynman Lectures. In fact every single thing I have ever posted can be found in the modern physics literature, without exception. And in all cases I'm quite clear that the proper mass of a photon is zero.

Some examples from the physics literature are in order:

From Relativity: Special, General and Cosmological, Wolfgang Rindler, Oxford Univ., Press, (2001), page 120

According to Einstein, a photon with frequency f has energy hf /c^2, and thus (as he only came to realize several years later) a finite mass and a finite momentum hf/c.

From Introducing Einstein's Relativity, Ray D'Inverno, Oxford Univ. Press, (1992), page 50

Finally, using the energy-mass relationship E = mc^2, we find that the relativistic mass of a photon is non-zero and given by

m = p/c.

Combining these results with Planck's hypothesis, we obtain the following formulae for the energy E, relativistic mass m, and linear momentum p of the photons:

E = hf m = hf/c2 p = hf/c

I choose to use the term "inertial mass" instead of "relativistic mass."

Mind you that these are the texts that many university professors are using to teach relativity.

..., please be advised that you already have a topic on this and that hijacking other threads with off-topic discussion won't be permitted.

A post literally cannot be considered to be hijacking a thread if the response is precisely the response to what the OP asks, which it was.

If one were to read my post carefully they'd see that I responded only to the first post and exactly on the topic the OP asked. A question was asked. I answered it. It's as simple as that. I certaintly can't be blammed for the fact that the subject keeps coming up in several of the physics forums. Especially when people keep responding as do I.

Note that my responses are essentially the same as found in the Physics FAQ, i.e, at

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

Edited by pmb
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Moderator Note

This isn't up for debate in thread. If you have an issue with a mod note, please raise it using the report feature. Any more off-topic replies will be removed.

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Moderator Note

Maybe I should have been more clear.

Do not derail this thread by discussing mod notes. If you have a problem with them, please use the report feature located at the bottom left of each post to bring it to the attention of staff.

Any more responses to mod notes in this thread (by anyone) will be removed.

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Moderator Note

Due to the number of reports about the above mod notes, staff thought it best that we clarify a few things.

Firstly, we are not penalizing pmb for being wrong. This is an ongoing issue where threads are constantly being hyperfocused on the issue of 'what is mass' to the point of them being off topic. Given the number of threads in which this has occurred, it is frustrating for staff and it is just as frustrating for people reading the thread, as it largely obfuscates the main questions asked. Staff are tired of having to clean up these threads, when there is already a perfectly good thread created for the sole purpose of housing such discussion.

While the above mod notes may have seemed overly harsh (it possibly could have done with better wording on my part) and while pmb's comment may seem on topic, prevention is better than cure and we would rather curb the tangential discussion now than clean up the pages of off-topic discussion later.

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• 4 weeks later...

If gravity is described as a force between two masses, then how exactly can light be effected by it? From my understanding, from what I have been taught, gravity would effect the photons of light which have no mass. Please correct me if I am wrong. Any responses would be much appreciated!

Gravity is not described, in modern physics, as a force between two masses. The effect of curved time space is called gravity. Since light does "bend" around large mass objects it is described as following the time / space curvature. Gravity, as a force, cannot exist. Yet some folks still try to use it as a force in defining the universe.

Paul

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• 2 weeks later...

Light follows what are called geodesics. Light couples to the distortions of spacetime. But yes, gravity effects the spacetime, causing curvature, causing the geometry in which massless radiation follows, called geodesics. I'd write maths but I think an explanation without it suffices.

Definitely right Bro

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• 2 weeks later...

Light follows what are called geodesics. Light couples to the distortions of spacetime. But yes, gravity effects the spacetime, causing curvature, causing the geometry in which massless radiation follows, called geodesics. I'd write maths but I think an explanation without it suffices.

It is mass and energy that affects the spacetime not gravity. Gravity is what we call the effect of bent spacetime.

Paul

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