# Do photons have mass?

Why does gravitational lensing occur if photons are massless?

nicksauce
Homework Helper
Because in general relativity, gravity is not a force that acts on masses as it is in Newtonian gravity. In GR, large masses curve spacetime, which causes particles, photons included, to take different paths, namely those that are the shortest "distance" in curved spacetime. One manifestation of this is gravitational lensing.

Because in general relativity, gravity is not a force that acts on masses as it is in Newtonian gravity.
Gravity acts on masses, but also on other things. I.e. it acts on the pressure. Photons do have a pressure.

explanation: mass curves space, and light moving straight through a curved space is observed to be deflected.

JK423
Gold Member
Photons curve space as well! :)
Generally speaking, whatever has energy it curves space

nicksauce
Homework Helper
Photons curve space as well! :)
Generally speaking, whatever has energy it curves space

Indeed, but even if they didn't, gravitational lensing would still exist, which is what the OP was asking about.

Generally speaking, whatever has energy it curves space
If the cosmological constant is nonzero, then even what does not have energy, it curves space :).

ZapperZ
Staff Emeritus
Why does gravitational lensing occur if photons are massless?

Zz.

Last edited:

Zz.

Thanks for the direct.

I have another q. How does gravity warp spacetime? Space is a vacuum, right? Does gravity manipulate a vacuum?

How does gravity warp spacetime? Space is a vacuum, right? Does gravity manipulate a vacuum?

yes, in accordance with the Einstein tensor.....but nobody really knows how..only that Einstein's math describes almost all gravity situations we encounter.....

yes gravity IS the curvature of spacetime in General Relativity.

I prefer to think of spacetime as a physical entity, one we know little about, that is "warped", curved, by energy, mass and pressure. Either that, or Einstein's entire idea will be shown to be incorrect and we'll have an "improved" theory of general relativity.

Gravity as a fundamental force is summarized here:
http://en.wikipedia.org/wiki/Gravity#Gravity_and_quantum_mechanics

Thanks for the direct.

I have another q. How does gravity warp spacetime? Space is a vacuum, right? Does gravity manipulate a vacuum?

This is something that I hope to grow closer to solving. Gravity obviously manipulates a vacuum, for it "warps" space. However, like other people have said no one really knows why that occurs.

The question for me that arises is almost akin to the chicken or the egg, was it mass that creates the gravity or is it the gravity that enables mass to exist? However, if it is the gravity that enables the gravity to exist, then what enables the gravity?

While I have a very limited background in this subject (someone, please correct me wherever you can), it is common knowledge that as objects increase in speed they increase in mass (at least that is what I have come to understand. Again, someone with a greater understanding please correct me). So is it possible that the creation of gravity is partly because particular particles are going extremely fast, therefore increasing their mass, which in turn is increasing their gravity, which THEN in turn attracts more particles to it, etc, etc.

Quite mind boggling...

diazona
Homework Helper
While I have a very limited background in this subject (someone, please correct me wherever you can), it is common knowledge that as objects increase in speed they increase in mass
There are different definitions that people use for "mass," but most modern physicists would say that is false. When an object increases its speed, its energy increases, but not its mass.
So is it possible that the creation of gravity is partly because particular particles are going extremely fast, therefore increasing their mass, which in turn is increasing their gravity, which THEN in turn attracts more particles to it, etc, etc.
Even particles that are not moving produce and respond to gravity.

Even particles that are not moving produce and respond to gravity.

So how could one say that a particle is not moving is responding to gravity? Is not the response to gravity the movement in a curved spacetime?

There are different definitions that people use for "mass," but most modern physicists would say that is false. When an object increases its speed, its energy increases, but not its mass.

That is a great point. I have not exactly considered the fact that photons can increase in energy but do not increase in mass, quite obviously. However, photons are going a finite speed; they are already going so fast that their mass, classically, is considered infinite...

Pythagorean
Gold Member
does a photon have inertia?

I would argue no, it does not, because it classically has no mass (well, infinite mass I guess?). But I believe from experiments we have seen it does, where light does actually exert pressure on objects...

To exect pressure particles dont need to have mass.

Einstein said that an object that has mass cannot travel at the speed of light in a vacuum because therefore it would have infinite amount of relative mass. So a photon cannot have mass. Wrong or right?

Einstein said that an object that has mass cannot travel at the speed of light in a vacuum because therefore it would have infinite amount of relative mass. So a photon cannot have mass. Wrong or right?

It does not have "rest mass"... this has been discussed in the thread already. It still interacts with gravity because mass alone is not the sum total of what contributes to SET.

Right but a new theory developed has said that the photon interacts with gravity because the space around the photon is bent. If the photon does not have rest mass, what allows the photon to have mass as it travels the speed of light. Gravity is a force between two massive objects the photon cannot directly interact with gravity because it does not possess mass.

If the photon does not have rest mass, what allows the photon to have mass as it travels the speed of light.

It's momentum and energy.

Gravity is a force between two massive objects the photon cannot directly interact with gravity because it does not possess mass.

No, it is not.
Mass is not source of gravity since 1917, surprise, surprise.
Gravity is created by stress-energy tensor. rest mass is just one of 16 cells in that tensor. Massless objects, for example, photon gas, can gravitate because of the pressure.

Right but a new theory developed has said that the photon interacts with gravity because the space around the photon is bent. If the photon does not have rest mass, what allows the photon to have mass as it travels the speed of light. Gravity is a force between two massive objects the photon cannot directly interact with gravity because it does not possess mass.

I have to ask, why in the name of god are you making an argument for Newtonian gravity in the quantum physics forums? This seems outlandish, as Dmitry67 pointed out with more elan.

the main thing is that a photon does not have mass due to Einstein saying that if a massive object traveling at the speed of light would have infinite amount of mass.

Ok you guys beat me. I was thinking that gravity only interacts between two massive objects. You guys are right, sorry for the confusion.