# Light Questions: Gravity & Mass

• Trevormbarker
In summary: E=mc2 only applies to particles in their rest frames. So photons have no rest mass and cannot have E=mc2.
Trevormbarker
I have two main questions about light, one does gravity affect it? Two, does it have mass?
I have read lots of posts and articles and many say that light is directly affected by gravity while others say that gravity "bends" the spacetime arround it so the light appears to be curved but is in reality traveling straight through the curved spacetime. Also for the question about mass I have read that light has no mass but it has momemntum? My understanding of momentum is that is it mass x velocity so I do not understand how it can have no mass but have momentum.

Photons are affected by gravity in the sense that they trie to follow the 'straightest possible' lines or geodesics of the curved space- time so there is a bending of the light that the photons comprise. Since photons have no rest frame they also have no rest mass so Einstein's energy - mass relation reduces to E = pc for a photon so it does indeed have a momentum based on its energy.

so would it be correct to say that photons themselves are not directly affected by the gravity but they are traveling "straight" through the spacetime which is curved by gravity?

Trevormbarker said:
so would it be correct to say that photons themselves are not directly affected by the gravity but they are traveling "straight" through the spacetime which is curved by gravity?

Light travels on geodisks that are created from the presence of matter. In 3d space, it is generally a curved line. In 4d, it is straight.

I believe light is able to curve space time as well, but you need a lot of energy to make a serious "dent."

is that because the light has momentum and therefor non-rest mass? so enough of it could curve the space time like normal matter?

Trevormbarker said:
is that because the light has momentum and therefor non-rest mass? so enough of it could curve the space time like normal matter?

It has a certain energy and in GR the source of the curvature is the stress - energy tensor so even energy density contributes, if negligible, to the curved geometry.

Yes, it does bend under gravitational field, and is because of mass!

But it is not the inertial mass or rest mass. But according to E=mc2, if it has energy, it must have mass.

Trevormbarker said:
so would it be correct to say that photons themselves are not directly affected by the gravity but they are traveling "straight" through the spacetime which is curved by gravity?

Isn't that exactly how gravity affects everything?

Drakkith said:
Isn't that exactly how gravity affects everything?

But that is only confined to the interpretation of general relativity...

ZealScience said:
But it is not the inertial mass or rest mass. But according to E=mc2, if it has energy, it must have mass.
Except that we are talking about photons and that equation does not apply to photons.

WannabeNewton said:
Except that we are talking about photons and that equation does not apply to photons.

Why?

ZealScience said:
Why?

A photon has no rest frame so it has no rest mass. E = m$c^2$ only applies to particles in their rest frames.

ZealScience said:
But that is only confined to the interpretation of general relativity...

Is there another theory that explains it better than GR?

WannabeNewton said:
A photon has no rest frame so it has no rest mass. E = m$c^2$ only applies to particles in their rest frames.

No, for rest frame m is the rest mass or inertial mass. But for photon m is exactly the mass. Because the mass increase is inertial mass plus mass of kinetic energy, so using E=mc2 got to be the mass of photon.

ZealScience said:
No, for rest frame m is the rest mass or inertial mass. But for photon m is exactly the mass. Because the mass increase is inertial mass plus mass of kinetic energy, so using E=mc2 got to be the mass of photon.

For a particle not in its rest frame the mass - energy equation reads $E = ((m_{0}c^{2})^{2} + (pc)^{2})^{1/2}$ where $m_0$ is the rest mass of the particle meaning its mass in its rest frame. A photon has no such frame so the equation reduces to E = pc. E = m$c^2$ does not apply to photons.

WannabeNewton said:
For a particle not in its rest frame the mass - energy equation reads $E = ((m_{0}c^{2})^{2} + (pc)^{2})^{1/2}$ where $m_0$ is the rest mass of the particle meaning its mass in its rest frame. A photon has no such frame so the equation reduces to E = pc. E = m$c^2$ does not apply to photons.

In your equation, m0 for photon is 0, so it is E=pc=mc2

Why it's not correct? I think E=mc2 applies to all energies, otherwise objects emitting photons would not have conservation of energy/mass

ZealScience said:
In your equation, m0 for photon is 0, so it is E=pc=mc2

Why it's not correct? I think E=mc2 applies to all energies, otherwise objects emitting photons would not have conservation of energy/mass

Photons don't have rest mass, which is what the M stands for in the equation. But this really isn't an issue, as the FULL equation, as WannabeNewton posted above can easily be used in its full form for a photon. The mc^2 part just becomes 0. Since pc^2 + 0 equals pc^2, we simply shorten the equation up to E=pc

Drakkith said:
Photons don't have rest mass, which is what the M stands for in the equation. But this really isn't an issue, as the FULL equation, as WannabeNewton posted above can easily be used in its full form for a photon. The mc^2 part just becomes 0. Since pc^2 + 0 equals pc^2, we simply shorten the equation up to E=pc

So what's the problem with using E=mc2

ZealScience said:
So what's the problem with using E=mc2

For photons? The problem is that E=0 for photons if you use that, which isn't true. E = 0 x c^2 since m = 0. That results in E=0.

ZealScience, what you write is fundamentally incorrect, as people have been trying to tell you. You are misusing formulas.

In addition, it's experimentally excluded - if you use this to predict the deflection of a beam of light by a gravitational source, you will be off by a factor of 2.

Trevormbarker said:
I have two main questions about light, one does gravity affect it? Two, does it have mass?
I have read lots of posts and articles and many say that light is directly affected by gravity while others say that gravity "bends" the spacetime arround it so the light appears to be curved but is in reality traveling straight through the curved spacetime. Also for the question about mass I have read that light has no mass but it has momemntum? My understanding of momentum is that is it mass x velocity so I do not understand how it can have no mass but have momentum.
1st: You can have many different models (points of view) to the same phenomenon. The same with light: you can assume space-time was curved and it's not an Euclidean space anymore (so light actually travels in a straight line there). You can also say space is absolutely Euclidean and gravity affects light. These are two different points of view and both are correct in some cases.
2nd: photon has a mass. It only has no mass at rest. It's fully compatible with the fact photon travels only at speed of light.

alright thanks for all the replies I understand know! I was just confused because, mainly for the first question, I had read many conflicting answers.

## 1. How does gravity affect the speed of light?

The speed of light is not affected by gravity. According to Einstein's theory of relativity, the speed of light is a constant and it does not change regardless of the presence or strength of gravity.

## 2. Does light have mass?

No, light does not have mass. It is made up of massless particles called photons. However, it does have energy and momentum, which can be affected by gravity.

## 3. How does mass affect the bending of light?

Mass can cause the bending of light through the phenomenon of gravitational lensing. When light passes near a massive object, such as a galaxy, its path is bent due to the gravitational pull of the object. This can cause the light to appear distorted or even create multiple images of the same object.

## 4. Can gravity affect the color of light?

Gravity itself does not affect the color of light. However, the warping of spacetime caused by massive objects can lead to a phenomenon known as redshift. This means that light from distant objects appears to have a longer wavelength, or a redder color, due to the effects of gravity on its journey through space.

## 5. How do scientists measure the mass of an object using light?

There are several ways to measure the mass of an object using light. One method is to observe the gravitational lensing of light from a distant object by a massive object, such as a galaxy. By measuring the degree of bending and the distance between the objects, scientists can calculate the mass of the object causing the lensing. Another method is to measure the Doppler shift of light emitted from a rotating object, which can provide information about its mass and rotation speed.

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