# Mass of a photon

## Main Question or Discussion Point

Photon's rest mass is zero.

What's its 'moving' mass?

Thanks.

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phinds
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Nugatory
Mentor
P
What's its 'moving' mass?
It has energy, so you can calculate a "moving mass" for it from $E=mc^2$.

However, it's nearly always more helpful to think in terms of energy and momentum instead. The link that phinds posted is a good start.

I have understood something from the link..

But, here are a few questions.

1. Photon's rest mass is zero. It's energy is also zero.
It means it is non existent. So, why to define such a term in science " rest mass of a photon "?

2. When photon is emitted by an electron in an atom, does it mean that the photon did not exist prior to its emission in any possible way? It seems to be creating something out of nothing (in a way).

Or, was the photon present in the electron in some rudimentary form?

The real question is what is mass. You can think mass is invariant energy under transformation. Photon mass is zero it only has "moving" energy instead of "rest energy" plus "moving energy" like mass particles

I have understood something from the link..

But, here are a few questions.

1. Photon's rest mass is zero. It's energy is also zero.
It means it is non existent. So, why to define such a term in science " rest mass of a photon "?

2. When photon is emitted by an electron in an atom, does it mean that the photon did not exist prior to its emission in any possible way? It seems to be creating something out of nothing (in a way).

Or, was the photon present in the electron in some rudimentary form?
As I was replying mass is energy so you can get rid of the concept of mass and think just Chuck of energy "flying off".

Drakkith
Staff Emeritus
1. Photon's rest mass is zero. It's energy is also zero.
Photons have non-zero energy.

2. When photon is emitted by an electron in an atom, does it mean that the photon did not exist prior to its emission in any possible way? It seems to be creating something out of nothing (in a way).
It did indeed not exist prior to emission. It was created from the interaction between the atom and the EM field, and its energy comes from the atom. The atom loses energy equal to the amount gained by the photon.

As I was replying mass is energy so you can get rid of the concept of mass and think just Chuck of energy "flying off".
Not quite. There's a good reason the term mass is still around. Mass and energy are related, but they are not the same thing.

ZapperZ
Staff Emeritus
Dale
Mentor
It's energy is also zero.
No, it's energy is not zero. I am not sure what would lead you to believe that, but it is incorrect.

The relationship between energy, momentum, and mass is:
$m^2 c^2=E^2/c^2-p^2$

does it mean that the photon did not exist prior to its emission
Yes, that is correct. The photon is created by the atom relaxing.

No, it's energy is not zero. I am not sure what would lead you to believe that, but it is incorrect.
How can something that has no mass at all, have energy!!!

I wonder how 'rest energy' of a photon is not zero. Aren't mass and energy interchangeable.
So, if photon at rest has energy, can't it be said to have mass also?

@Deepak K Kapur : Do you ever spend time reading the FAQs at all?

Zz.
I do.

BTW, what are you for, writing FAQs??

ZapperZ
Staff Emeritus
I do.

BTW, what are you for, writing FAQs??
I have more than 31,000 posts. Do you think that all I write are FAQs?

I asked because a lot of your questions appear to have already been addressed by many FAQs and Insight articles in the forum.

Zz.

It did indeed not exist prior to emission. It was created from the interaction between the atom and the EM field, and its energy comes from the atom. The atom loses energy equal to the amount gained by the photon.
I can understand when hydrogen and oxygen atoms combine to form water (electron transfer etc.)

What is the mechanism of interaction of the atom and the EM field that leads to the release of this photon.
(other than the mathematical mechanism)

Something like...
The electrical and the magnetic elements of the incoming photon separate inside the electron and then combine again depending upon the energy of the level that the electron returns to......

ZapperZ
Staff Emeritus
I can understand when hydrogen and oxygen atoms combine to form water (electron transfer etc.)

What is the mechanism of interaction of the atom and the EM field that leads to the release of this photon.
(other than the mathematical mechanism)

Something like...
The electrical and the magnetic elements of the incoming photon separate inside the electron and then combine again depending upon the energy of the level that the electron returns to......
That would be faulty, because I don't need only electrons to generate light. I can take a bunch of protons, shake it up and down, and voila! I've generated EM radiation! So your idea that somehow there's something inside an electron (which in itself is dubious since electron has no internal structure that we know of) cannot explain why I can generate light without using electrons.

Please remember that atomic decay is only ONE way to generate light. Light coming from your old incandescent light bulbs, and light coming from the many synchrotron light sources around the world, are NOT from such atomic transition.

Zz.

How can something that has no mass at all, have energy!!!
It can, and as we can see, it has.

Aren't mass and energy interchangeable.
No they are not interchangeable.

So, if photon at rest has energy
Photons are never at rest. You have misconceptions about very basic issues of the topic you are trying to talk about.

Nugatory
Mentor
So, if photon at rest has energy, can't it be said to have mass also?
There is no such thing as a photon at rest (because they always move at the speed of light) and therefore it is meaningless to speak of the energy of a photon at rest.

It really isn't going to get any simpler than the relationship that Dale has already posted, valid for all particles: $E^2=(m_0c^2)^2+(pc)^2$. A photon has non-zero $E$ and $p$, zero $m_0$.

Photons have non-zero energy.

It did indeed not exist prior to emission. It was created from the interaction between the atom and the EM field, and its energy comes from the atom. The atom loses energy equal to the amount gained by the photon.

Not quite. There's a good reason the term mass is still around. Mass and energy are related, but they are not the same thing.
What is the good reason then?

Dale
Mentor
How can something that has no mass at all, have energy
As I said above. The relationship between energy, momentum, and mass is: $m^2 c^2=E^2/c^2-p^2$

So you can have energy and no mass as long as $E=pc$

So, if photon at rest has energy, can't it be said to have mass also?
IF a photon could be at rest and have energy then it would have mass, BUT in reality a photon is never at rest and does not have mass.

PLEASE, examine carefully the equation that I have posted twice now before you respond again.

Last edited:
Drakkith
Staff Emeritus
What is the good reason then?
Hmm... I thought I had a link that explained it, but I can't seem to find it. I'll keep looking.

Hmm... I thought I had a link that explained it, but I can't seem to find it. I'll keep looking.
Yea, please let me know that would be very helpful thanks a lot!

Drakkith
Staff Emeritus
What is the mechanism of interaction of the atom and the EM field that leads to the release of this photon.
(other than the mathematical mechanism)
At the risk of simplifying this to the point of being nearly wrong, you can think of it as a disturbance in the EM field generated by the sudden change in the atom. Since both protons and electrons are charged particles, any sudden change in the configuration of the atom causes a disturbance in the EM field. This disturbance propagates away from the atom as an EM wave, which is composed of photons.

Of course it is not just changes in atoms that creates photons. Acceleration of any charged particle will generate EM waves and photons.

Is this has something to do with Higgs-Boson Field?

Nugatory
Mentor
Is this has something to do with Higgs-Boson Field?
Nothing whatsoever, not in the slightest.

PLEASE, examine carefully the equation that I have posted twice now before you respond again.
I examined this equation even before posing this question.

I think (may be wrongly) that math comes later than concepts. I am after the concept here....

Suppose (just for the sake of argument) that I want to stop a photon. ( I become small and I can see an individual photon etc.). I think I surely would have to apply force just as now I apply force to stop a ball.

1. Isn't this mass?

2. When I stop a ball it doesn't stop existing ( i.e. lose its mass). Why would a photon become massless if I bring it to rest?

I am fully cognizant of the equations, its just that I want to grasp the concept without equations.

From the replies it seems that photons are already moving at the speed of light when they are produced...

Why? What is this going on... a magic show!