# The Naked Scientists Forum

### Author Topic: Does light have mass?  (Read 8206 times)

#### jopie64

• First timers
• Posts: 9
##### Does light have mass?
« on: 19/12/2012 22:43:07 »
The answer is no. But consider this:

Suppose in a few days when earth is about to end, earth is approached by an anti-matter earth which starts orbitting our earth. (They actually orbit each other because same mass.) Anti-earth is getting closer and closer to earth. Freaked out by this news people start evacuating earth. Afraid of being radiated away by the energy from the anihilation, some brave people manage to build a giant mirror box around the earths.

Now the question:
The moon is now orbiting the giant mirrorbox with the earths in it. But what happens to the moon when the earths anihilate each other inside the box?

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Does light have mass?
« Reply #1 on: 19/12/2012 22:58:58 »
Quote from: jopie64
Whether its yes or now depends on what one means by "mass." Sometimes the term "mass" refers to inertial mass and at other times it refers to gravitational mass (of which there are two kinds, active g-mass and passive g-mass). Part of the sense you're using it, it means gravitational mass.

Quote
Now the question:
The moon is now orbiting the giant mirrorbox with the earths in it. But what happens to the moon when the earths anihilate each other inside the box?
The mass in the box remains constant. The only thing that would change is the distribution of mass. A box of radiation of total energy E contains mass an amount of mass m according to E = mc^2.

The moon remains in orbit of the box because the box still has mass (both inertial and gravitational mass)

#### jopie64

• First timers
• Posts: 9
##### Re: Does light have mass?
« Reply #2 on: 19/12/2012 23:16:02 »
The moon remains in orbit of the box because the box still has mass (both inertial and gravitational mass)

Ok I'd agree. But what is 'generating' the mass? Individual photons dont have mass, thats what I've been told. So alot of photons would still not have mass right?

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Does light have mass?
« Reply #3 on: 20/12/2012 19:51:48 »
heh, welcome to the world of mathematics :) Photons are energy, matter are convertibles into energy too. Does 'energy' have mass?

#### jopie64

• First timers
• Posts: 9
##### Re: Does light have mass?
« Reply #4 on: 20/12/2012 19:55:42 »
Does 'energy' have mass?

Thats what I dont understand. Photons dont have mass. But they have energy. But energy IS mass (E=mc2). So they have mass.

?????

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Does light have mass?
« Reply #5 on: 21/12/2012 19:55:09 »

Quote from: jopie64
Ok I'd agree. But what is 'generating' the mass?
The photons and the box itself.

Quote from: jopie64
Individual photons dont have mass, thats what I've been told.
As I explained in post #2, whether a photon has mass or not depends on what one means by the term “mass.” In this case, i.e. the gravitational mass of one or more photons is nonzero. When it is said that a photon has zero it is referring to the proper mass of the photon. The proper mass of any particle of momentum p and proper mass m is given by

One of my pages with all the math worked out might help you understand these issues

Quote from: jopie64
So alot of photons would still not have mass right?
Wrong. See above.

If I were you I’d use the more precise term when you’re taking about an issue in which one or more of these definitions are being used at the same time.

#### UltimateTheory

• Sr. Member
• Posts: 107
• Thanked: 1 times
##### Re: Does light have mass?
« Reply #6 on: 22/12/2012 22:09:55 »
The answer is no. But consider this:

Electron, according to Standard Model, has mass. Small, but still.

So the same Positron.

When Electron will hit Positron they will emit photons.

Conclusion must be simple- photons also must have mass.

Otherwise conservation of energy and conservation of mass we would have to consider as incorrect.

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Does light have mass?
« Reply #7 on: 23/12/2012 06:03:25 »
Quote from: UltimateTheory
Electron, according to Standard Model, has mass. Small, but still.
Let's focus in on the main problem with threads like this. People who make assertions like this always refrain from defining the quantity that they're defining. And when they do then the entire thread degenerates to a conversation where they assert that their definition is right and those who disagree with them and/or use another definition are just plain wrong/ignorant/don't understand "modern" definitions or something else derogatory.

This is really a thread whose subject matter has been repeated wy too many times in the past. The subject comes up on a monthly basis. That's why I wrote this article
http://arxiv.org/abs/0709.0687

It addresses all points of debate/discussion in nearly all these debates/discussions. And make no mistake - Both common definitions are widely used in the physics literature and in universities across the entire face of the earth.

All one really needs to do is read all the subject lines in this page that I created
http://home.comcast.net/~peter.m.brown/sr/sr.htm
and you'll be well versed on the topic. Do yourself a favor and give it a whirl
« Last Edit: 23/12/2012 06:05:28 by Pmb »

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Re: Does light have mass?
« Reply #8 on: 23/12/2012 16:16:02 »
Photons are energy,
No. A physical object cannot be one of its properties.
Quote
Does 'energy' have mass?
If it's stationary, yes. If not, you need more informations to answer the question. Notice however that, even here, you shoud better have written "Does on object which has energy also have mass?" Because a property of a body cannot have another property!
« Last Edit: 23/12/2012 16:20:40 by lightarrow »

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Re: Does light have mass?
« Reply #9 on: 23/12/2012 16:19:04 »

Thats what I dont understand. Photons dont have mass. But they have energy. But energy IS mass (E=mc2).
Wrong. Energy is "energy" and mass is "mass".
Quote
So they have mass.
If you mean invariant mass, no, they don't have.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Re: Does light have mass?
« Reply #10 on: 23/12/2012 16:23:34 »
Electron, according to Standard Model, has mass. Small, but still.

So the same Positron.

When Electron will hit Positron they will emit photons.

Conclusion must be simple- photons also must have mass.
But unfortunately the conclusion is wrong...
Quote
Otherwise conservation of energy and conservation of mass we would have to consider as incorrect.
When a pair particle/antiparticle annihilates, it generates a couple of *two* photons. A system of such two photons *has* mass:

E2 = (Mc2)2 + (cP)2

E = energy of the two photons' system = E1 + E2 = 2E1, with two equal photons, where E1 is a single photon's energy (energy is additive).
M = mass of the two photons' system.
P = momentum of the two photons' system = P1 + P2 where P1 and P2 are the momenta of the  photon 1 and 2, respectively.

A single photon's momentum is, in modulus: |P1| = |P2| = E1/c.

So, if the two photons are not travelling in the same direction:

|P| = |P1 + P2| < 2|P1| = 2E1/c

so

P2 = |P|2 < 4E12/c2   →   -P2 > -4E12/c2

(Mc2)2 = E2 - (cP)2 = (2E1)2 - c2P2 > 4E12 - c24E12/c2 = 0

so

(Mc2)2 > 0

that is

M > 0.

« Last Edit: 23/12/2012 16:33:17 by lightarrow »

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Does light have mass?
« Reply #11 on: 24/12/2012 04:05:16 »
Quote from: UltimateTheory
When Electron will hit Positron they will emit photons.
Electrons and positrons don’t emit photons. They annihilate each other and two or more photons are all that will remain after the annihilation.

Neither an electron nor a photon can absorb or emit a photon. Although this is a common misinterpretation of what goes on in Feynman diagrams in QED it never actually happens in real life. Those are what can best be called virtual processes.

"photons also must have mass." - Inertial mass, yes. Proper mass, no. You're nearly always referring to inertial mass is the examples you give (inertial mass of photon is never zero) but when you speak of the mass of a photon being zero you've changed to the definition you’re speaking of to “mass” means “proper mass.” Don't you find that confusing?

Let me give you another example of having to be cautious with what a word means from other areas of physics: Non-Relativistic quantum mechanics. If we were talking about non-relativistic quantum mechanics and I asked you if the momentum of a charged particle in an EM field was a function of its charge, what would you say?

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Does light have mass?
« Reply #12 on: 24/12/2012 04:23:28 »
I'm going to say this one more time and only one more time - Please stop confusing the fact of whether a photon has mass or not.The only thing I see you folks worry about and wasted an entire thread for is symantics. This whole thing has been only about terminology. Nothing about it is physics when all you're talking about is whether a photon has mass or not. No physicist worth his salt worries about this trivial nonsense. If you were to ask one who is worth his salt and has no alterior motives then all phycists would tell you that the proper mass of a photon is zero and that its inertial mass is nonzero (inertial mass is sometimes refered to as relativistic mass). Often the paticle physics people will steal the definition of "inertial mass" and define it for there own purposes. Mostly to win arguements of this trivial nonsense. I sure wish you'd all stop wasting space on this non-sense. All that you will wind up with is people in different fields wavig banners that they've won a debate and are now free to define mass as they want to. Then then the people from the other side of the debate will do the same thing and everbody will still end up with contradiction in terms.

So please stop dragging physics down with such trivial nonsense. It is unworthy of us all and a wase of breath. Can't you all find someting more constructive to do with your time then spend all day worring about semantics? Sheesh. Shame on you all.

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Does light have mass?
« Reply #13 on: 22/01/2013 18:31:29 »
Maybe Lightarrow, but a photon definitely is a energy to me. In fact it is the closest thing I know to a classically, spatially localized, energy in its outcome (when measured). It has no size, no rest mass, it only consist of a momentum/energy. If I assumed a static universe the momentum and the energy (sh)would be undifferentiated as I think.
==

We have the definition of light quanta though, in where we have discrete 'energies' related to the frequency we find light to have. And that makes it hard on a static universe, as the speed of light is a constant one in a vacuum.  The momentum of a light quanta is not related to the intensity of light, or speed, but only to its frequency as I understands it. The intensity is, from a particle perspective, explained as a result of 'more photons over time' (of a same frequency) hitting the device measuring. And assuming a static reality one then has to find a reason to why this is possible. How would a light quanta be able to present different energies in such a universe?
=

I'm ignoring the speed of light for a reason, it's a constant 'c' in relativity, therefore the same no matter what energies we measure a light quanta to have. And using locality as a guiding principle then it is 'c' everywhere :)
« Last Edit: 22/01/2013 19:58:54 by yor_on »

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Re: Does light have mass?
« Reply #14 on: 23/01/2013 17:02:00 »
Maybe Lightarrow, but a photon definitely is a energy to me. In fact it is the closest thing I know to a classically, spatially localized, energy in its outcome (when measured). It has no size, no rest mass, it only consist of a momentum/energy.
It's not true that it only consists of momentum/energy: it also has a spin, for example. Furthermore, photons with the same (average) energy can have different spectrum's broadness: more or less monocromaticity. I'm not talking of average energy of a set of different photons, but of average energy of photons *all equal*. Photons emitted by the source 1, all equal, will have a spectrum 1 of their energies; photons emitted by the source 2, all equal, will have a spectrum 2 of their energies (for example gaussian curves with different broadness).
« Last Edit: 23/01/2013 17:07:01 by lightarrow »

#### AndroidNeox

• Sr. Member
• Posts: 251
• Thanked: 2 times
##### Re: Does light have mass?
« Reply #15 on: 23/01/2013 21:27:11 »
Light has no rest mass, but it has momentum. Momentum is what generates gravity. If you put a kilogram of matter and one of antimatter into an impregnable box (like a Schrödinger cat box) and they annihilate each other, the gravitational mass of the box will not change, even if it's only full of light.

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: Does light have mass?
« Reply #16 on: 25/01/2013 02:34:12 »
Maybe?

Are you defining a single photon/lightquanta as having a spectrum Lightarrow? That should be from a wave perspective if so, right? As you say it has what we call Spin/polarization but thinking of it, all of those definitions come from treating photons as waves, don't they?

To me it's a packet of energy, or excitation in a field maybe? And it definitely has a momentum.

And Android, If momentum is 'gravity' I don't know btw :) But as we assume that certain 'systems' of light quanta can present us with a proper mass, and as we know that proper mass and gravity goes together? But the universe we acknowledge is all made up from those light quanta, impressing themselves on our senses, so in one sense no. I think 'energy' is a better description for 'gravity' if so, but it's not perfect as gravity in itself is nothing at all except a preferred direction having different 'slopes', depending on the mass attracting.

You made me think, and wonder, some more there Lightarrow :)
How many photons does it take to measure a linear polarization?

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Does light have mass?
« Reply #17 on: 25/01/2013 03:59:20 »
Light has no rest mass, but it has momentum. Momentum is what generates gravity.
There are three things which generates gravity

1) mass-energy
2) momentum
3) stress

Light has all of these

Quote
If you put a kilogram of matter and one of antimatter into an impregnable box (like a Schrödinger cat box) and they annihilate each other, the gravitational mass of the box will not change, even if it's only full of light.

Edit: I wrongly said "I disagree. The pressure inside the box would increase drastically and that pressure is a source of gravity."  The mass associated with the stress due to the box containing the photon gas cancels out the mass contribution due to the increase in pressure. Since there is no box containing a star then the pressure inside a star contributes to the mass.
« Last Edit: 25/01/2013 17:10:47 by Pmb »

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Does light have mass?
« Reply #18 on: 25/01/2013 04:02:56 »
You're forgetting the relationship E = mc2 which means that whatever has inertial energy E has inertial mass m and what has inertial mass m has inertial energy E.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Re: Does light have mass?
« Reply #19 on: 25/01/2013 11:30:30 »
Light has no rest mass, but it has momentum. Momentum is what generates gravity. If you put a kilogram of matter and one of antimatter into an impregnable box (like a Schrödinger cat box) and they annihilate each other, the gravitational mass of the box will not change, even if it's only full of light.
Yes, but what this has to do with the a photon's properties? For example, with that, do you believe to have proved that a photon warps spacetime?

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Re: Does light have mass?
« Reply #20 on: 25/01/2013 11:32:13 »
You're forgetting the relationship E = mc2 which means that whatever has inertial energy E has inertial mass m and what has inertial mass m has inertial energy E.
And with this do you think to have proved that a photon (which has an energy E) warps spacetime?

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Does light have mass?
« Reply #21 on: 25/01/2013 17:22:01 »
Quote from: lightarrow
And with this do you think to have proved that a photon (which has an energy E) warps spacetime?
Since I've just spent an inordinate amout of time in another thread where all I did was to post proofs and you to claim they weren't correct. Homey don't play that game no more. :)

This time around please just state whether it will or won't and what it means to warp spacetime and what that has to do with the subject at hand. Then I might respond. Then answer me this. If a pulse of light could generate a gravitational field could a photon?

The reason I ask is because there is no well accepted theory of quantum gravity, hence the reason I never bothered to learn any of it. Since a photon is a quantum particle one needs quantum gravity to properly answer it. Instead let's talk about a pulse of light of energy E and momentum p and thus has a proper mass of zero.

Tell me lightarrow - Does it generate a gravitational field? If yes, then why. If no, then why not?

#### AndroidNeox

• Sr. Member
• Posts: 251
• Thanked: 2 times
##### Re: Does light have mass?
« Reply #22 on: 27/01/2013 17:47:20 »
I'm going to say this one more time and only one more time - Please stop confusing the fact of whether a photon has mass or not.

Perhaps this would be a good topic for a FAQ list? Maybe include half a dozen links to good explanations (your ArXiV posting would should be on the list, I think: http://arxiv.org/pdf/0709.0687v2.pdf)... sometimes one explanation will "click" for one person but not another. For example, I often need to convert equations to physical models to understand how something works.

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Does light have mass?
« Reply #23 on: 27/01/2013 18:00:52 »

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Re: Does light have mass?
« Reply #24 on: 29/01/2013 00:33:50 »
Quote from: lightarrow
And with this do you think to have proved that a photon (which has an energy E) warps spacetime?
Since I've just spent an inordinate amout of time in another thread where all I did was to post proofs and you to claim they weren't correct. Homey don't play that game no more. :)

This time around please just state whether it will or won't and what it means to warp spacetime and what that has to do with the subject at hand.
It has to do with the subject because it was AndroidNeox to write that "Light has no rest mass, but it has momentum. Momentum is what generates gravity", and he was answering me about photons.
Quote
Then I might respond. Then answer me this. If a pulse of light could generate a gravitational field could a photon?
Do you still believe a photon is a "corpuscle localized in spacetime"? It's something very different.
Quote
The reason I ask is because there is no well accepted theory of quantum gravity, hence the reason I never bothered to learn any of it. Since a photon is a quantum particle one needs quantum gravity to properly answer it.
At last you have written it. But not that I hade any doubt you knew it, it's only that you don't seem aware of how people could get confused, because they so easily make the association "photon = corpuscle".
Quote
Instead let's talk about a pulse of light of energy E and momentum p and thus has a proper mass of zero.
Tell me lightarrow - Does it generate a gravitational field? If yes, then why. If no, then why not?
If that energy were continuously present, as in the case of a continuous light beam, I know it generates gravity, but in the case of a pulse, I actually don't know.
I'm tempted to say yes, because when the pulse crosses a specific volume of space, the energy and momentum temporarily present inside that region should do it:

= 8 and in the tensor there are terms which refers to energy, momentum, ecc:
http://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor

But actually I don't know.

« Last Edit: 29/01/2013 00:35:43 by lightarrow »

#### The Naked Scientists Forum

##### Re: Does light have mass?
« Reply #24 on: 29/01/2013 00:33:50 »