# The Naked Scientists Forum

### Author Topic: Does light have mass?  (Read 77484 times)

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Does light have mass?
« Reply #100 on: 30/09/2009 02:19:50 »
Any *fixed* region of space containing an energy E has a mass E/c2.
Hi lightarrow! How goes it? I found this response from you to be unexpected. Normally in the passt you have used the term "mass" to mean proper mass. Here you use it to mean relativistic mass. Is there a reason for this that I'm not aware of? Thanks.

Pete

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #101 on: 30/09/2009 07:41:53 »
Any *fixed* region of space containing an energy E has a mass E/c2.
Hi lightarrow! How goes it? I found this response from you to be unexpected. Normally in the passt you have used the term "mass" to mean proper mass. Here you use it to mean relativistic mass. Is there a reason for this that I'm not aware of? Thanks.

Pete
No, it's proper = invariant mass even here. If the region of space is fixed, then the total momentum is zero, so from E2 = (cp)2 + (mc2)2 we can infer that m = E/c2.
« Last Edit: 30/09/2009 07:48:19 by lightarrow »

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #102 on: 30/09/2009 18:17:34 »
Any *fixed* region of space containing an energy E has a mass E/c2.
Hi lightarrow! How goes it? I found this response from you to be unexpected. Normally in the passt you have used the term "mass" to mean proper mass. Here you use it to mean relativistic mass. Is there a reason for this that I'm not aware of? Thanks.

Pete
No, it's proper = invariant mass even here. If the region of space is fixed, then the total momentum is zero, so from E2 = (cp)2 + (mc2)2 we can infer that m = E/c2.

You'd make this so much easier if you would just shorthand this to having gamma next to Mc^2.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #103 on: 30/09/2009 20:46:28 »
Any *fixed* region of space containing an energy E has a mass E/c2.
Hi lightarrow! How goes it? I found this response from you to be unexpected. Normally in the passt you have used the term "mass" to mean proper mass. Here you use it to mean relativistic mass. Is there a reason for this that I'm not aware of? Thanks.

Pete
No, it's proper = invariant mass even here. If the region of space is fixed, then the total momentum is zero, so from E2 = (cp)2 + (mc2)2 we can infer that m = E/c2.

You'd make this so much easier if you would just shorthand this to having gamma next to Mc^2.
Sorry?

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #104 on: 01/10/2009 02:36:44 »
You know, relativistic forumla that come in the form E= \gamma Mc^2. No need for the messy definitions concerning mass.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #105 on: 01/10/2009 13:11:21 »
You know, relativistic forumla that come in the form E= \gamma Mc^2. No need for the messy definitions concerning mass.
But I can't understand what exactly you mean. I proved that a system which is not moving in a specific frame of reference and which has energy, also has invariant mass. Relativistic mass is a different concept, that is, is just energy divided by c2, *always*.

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #106 on: 01/10/2009 14:18:11 »
You know, relativistic forumla that come in the form E= \gamma Mc^2. No need for the messy definitions concerning mass.
But I can't understand what exactly you mean. I proved that a system which is not moving in a specific frame of reference and which has energy, also has invariant mass. Relativistic mass is a different concept, that is, is just energy divided by c2, *always*.
No, its not. reltivistic mass invokes M= \gamma m. This means that it has zero mass. A reativistic mass is never simply E/c^2=M.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #107 on: 01/10/2009 14:59:22 »
You know, relativistic forumla that come in the form E= \gamma Mc^2. No need for the messy definitions concerning mass.
But I can't understand what exactly you mean. I proved that a system which is not moving in a specific frame of reference and which has energy, also has invariant mass. Relativistic mass is a different concept, that is, is just energy divided by c2, *always*.
No, its not. reltivistic mass invokes M= \gamma m. This means that it has zero mass. A reativistic mass is never simply E/c^2=M.
Which is the energy E of a non-zero mass particle? Which is his relativistic mass? Is it different from E/c2?
Which is a photon's energy E? Which is his relativistic mass? Is it different from E/c2?

http://crib.corepower.com:8080/~relfaq/mass.html
http://w3.atomki.hu/fizmind/mag/photon_mass.html
http://crib.corepower.com:8080/~relfaq/light_mass.html
« Last Edit: 01/10/2009 15:20:06 by lightarrow »

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #108 on: 01/10/2009 20:49:33 »
You can either listen or not. But I can assure you one last time; relativistic mass is not correct under E=Mc^2, or any algebraic manipulation.

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #109 on: 01/10/2009 20:52:34 »
Look into this: http://en.wikipedia.org/wiki/Mass_in_special_relativity

Relativistic mass is an outdated concept. Read especially : The relativistic mass concept
 Early developments: transverse and longitudinal mass

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #110 on: 02/10/2009 13:27:25 »
Look into this: http://en.wikipedia.org/wiki/Mass_in_special_relativity

Relativistic mass is an outdated concept.
Good news! I thought to be the only one to say this!

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #111 on: 02/10/2009 13:29:00 »
You can either listen or not. But I can assure you one last time; relativistic mass is not correct under E=Mc^2, or any algebraic manipulation.
Ok. So, can you please give me the definition of relativistic mass of a photon?

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #112 on: 03/10/2009 01:43:11 »
You can either listen or not. But I can assure you one last time; relativistic mass is not correct under E=Mc^2, or any algebraic manipulation.
Ok. So, can you please give me the definition of relativistic mass of a photon?

The mass of a photon in relativity is zero but has itself, a non-zero energy:

E=M^2c^4+p^2c^2

Plugging in the appropriate values, one finally assumes that for Mc^2, we actually have:

E= \gamma Mc^2

Which reduces its mass total to zero; this is the mass of the photon.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #113 on: 03/10/2009 09:09:00 »
You can either listen or not. But I can assure you one last time; relativistic mass is not correct under E=Mc^2, or any algebraic manipulation.
Ok. So, can you please give me the definition of relativistic mass of a photon?

The mass of a photon in relativity is zero but has itself, a non-zero energy:

E=M^2c^4+p^2c^2
This is correct.

Quote
Plugging in the appropriate values, one finally assumes that for Mc^2, we actually have:

E= \gamma Mc^2
This is *not* correct for photons. What does infinite multiplied by zero means?

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #114 on: 03/10/2009 17:17:53 »
You can either listen or not. But I can assure you one last time; relativistic mass is not correct under E=Mc^2, or any algebraic manipulation.
Ok. So, can you please give me the definition of relativistic mass of a photon?

The mass of a photon in relativity is zero but has itself, a non-zero energy:

E=M^2c^4+p^2c^2
This is correct.

Quote
Plugging in the appropriate values, one finally assumes that for Mc^2, we actually have:

E= \gamma Mc^2
This is *not* correct for photons. What does infinite multiplied by zero means?

It is correct for photons, because M is the rest mass, and \gamma makes the value of matter to zero. That is why the rest energy of a photon is given by: E=\gamma Mc^2. You can learn this stuff quite independantly and easily on web sites spralled all over the place.

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #115 on: 03/10/2009 17:19:07 »
You can either listen or not. But I can assure you one last time; relativistic mass is not correct under E=Mc^2, or any algebraic manipulation.
Ok. So, can you please give me the definition of relativistic mass of a photon?

The mass of a photon in relativity is zero but has itself, a non-zero energy:

E=M^2c^4+p^2c^2
This is correct.

Quote
Plugging in the appropriate values, one finally assumes that for Mc^2, we actually have:

E= \gamma Mc^2
This is *not* correct for photons. What does infinite multiplied by zero means?

Also, where i have bolded; what infinite value? There is no infinite value in question here. ts a simple case of algebra.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #116 on: 04/10/2009 13:21:29 »
It is correct for photons, because M is the rest mass, and \gamma makes the value of matter to zero. That is why the rest energy of a photon is given by: E=\gamma Mc^2. You can learn this stuff quite independantly and easily on web sites spralled all over the place.

And how much is gamma for a photon?
You shouldn't base your knowledge on internet sites only, you should also go to school, at least...

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #117 on: 04/10/2009 14:10:50 »
It is correct for photons, because M is the rest mass, and \gamma makes the value of matter to zero. That is why the rest energy of a photon is given by: E=\gamma Mc^2. You can learn this stuff quite independantly and easily on web sites spralled all over the place.

And how much is gamma for a photon?
You shouldn't base your knowledge on internet sites only, you should also go to school, at least...

Talking about ''infinities'' did nothing for the conversation. And I dont need more school. I've had a shitload of it so far; i have education and certificates in physics too.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #118 on: 04/10/2009 19:06:22 »
Talking about ''infinities'' did nothing for the conversation. And I dont need more school. I've had a shitload of it so far; i have education and certificates in physics too.
Well, so you should know that m*gamma is meaningless for a photon.
At high school they call it "Indeterminate form":
http://en.wikipedia.org/wiki/Indeterminate_form
« Last Edit: 04/10/2009 19:08:34 by lightarrow »

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #119 on: 05/10/2009 09:20:10 »
Talking about ''infinities'' did nothing for the conversation. And I dont need more school. I've had a shitload of it so far; i have education and certificates in physics too.
Well, so you should know that m*gamma is meaningless for a photon.
At high school they call it "Indeterminate form":
http://en.wikipedia.org/wiki/Indeterminate_form

M alone is meanngless, not to mention incorrect.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #120 on: 05/10/2009 12:15:14 »
You wrote:

<<E= \gamma Mc^2
Which reduces its mass total to zero; this is the mass of the photon>>

If you didn't mean gamma*Mc^2, what did you mean, then???

#### Mr. Scientist

• Neilep Level Member
• Posts: 1451
• Thanked: 2 times
##### Does light have mass?
« Reply #121 on: 05/10/2009 14:23:54 »
Let me rephrase this.

E is not the ''energy'' alone. When relativity formulated the equation E=Mc^2 in this specific form referred to the rest mass of a particle. Which means does not include [ in fact - never involved] the description of photons. The photon has a non-zero energy as it is the packet of pure kinetical energy, but this energy is not of a rest form associated to a particle with a mass M.

Instead one needs to reduce to mass to zero, to describe the rest energy of a photon to also be zero in quantity; E=\gamma Mc^2. These are equations used frequently in relativity for the same purposes posted above. Also to clarify, i told you all this because you inferred to the outdated concept of relativistic mass - outdated in the sense that in a qualitative physics course, lecturers usually inform us that the term relativistic mass is hardly ever used nowadays in an academic sense, to cause less confusion; something which i earlier highlighted which you where inexorably conducting.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### Does light have mass?
« Reply #122 on: 05/10/2009 20:31:13 »
Let me rephrase this.

E is not the ''energy'' alone. When relativity formulated the equation E=Mc^2 in this specific form referred to the rest mass of a particle. Which means does not include [ in fact - never involved] the description of photons. The photon has a non-zero energy as it is the packet of pure kinetical energy, but this energy is not of a rest form associated to a particle with a mass M.
...and it's exactly for this reason that writing it for a photon, as you did, is meaningless.

Quote
Instead one needs to reduce to mass to zero, to describe the rest energy of a photon to also be zero in quantity; E=\gamma Mc^2. These are equations used frequently in relativity for the same purposes posted above.
I still cannot understand what purposes it can have.
Before your intervention in response of my post, I had used the correct equation E2 = (mc2)2 + (cp)2 and I had never talked of relativistic mass; then you come with the equation E = \gamma Mc2 which is not correct, in general, because is valid only for non-zero mass bodies and which uses a different symbol for the mass (and here it is my erroneous believe that you were talking about relativistic mass).

Then arrives your post with this statement:
<<You'd make this so much easier if you would just shorthand this to having gamma next to Mc^2>>
and I ask you again: what does it mean? It seems meaningless to me.

#### yor_on

• Naked Science Forum GOD!
• Posts: 11978
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Does light have mass?
« Reply #123 on: 08/10/2009 00:48:21 »
does it?

Not as I see it?

If light had a mass how much heavier would it make a supernova by any chosen magnitude? Should I assume that the supernova before exploding versus after, if able to assemble all its light, to then 'mass' the same?

Light constantly do 'things' mass can't. They are immaterial not able to define except when impacting (photons) like being seen by your eye (photons/waves).

"The definition of the invariant mass of an object is  m = sqrt{E2/c4 - p2/c2}. By this definition a beam of light, is massless like the photons it is composed of. However, if light is trapped in a box with perfect mirrors so the photons are continually reflected back and forth in the box, then the total momentum is zero in the boxes frame of reference but the energy is not. Therefore the light adds a small contribution to the mass of the box. This could be measured - in principle at least - either by an increase in inertia when the box is slowly accelerated or by an increase in its gravitational pull. You might say that the light in the box has mass but it would be more correct to say that the light contributes to the total mass of the box of light. You should not use this to justify the statement that light has mass in general."  http://crib.corepower.com:8080/~relfaq/light_mass.html

To that I would like to add that this 'system' as discussed above may be defined as having no 'momentum' as the light 'bounces' inside the box, and as we define the box itself to be the total 'system'. But this light 'bouncing' still have both a speed and a distance traveled in time as observed by us. ( if we assume that light do 'travel' that is :). If it does so then there will be intervals between its 'bounces' where that box will get no action/reaction. So to prove the concept I think you would need to have the light somehow 'frozen' floating freely inside that box and then weight it. And if you 'freeze' it you are acting on it, introducing a force, and as I see it invalidating the claim of it having a 'restmass' like a particle.
« Last Edit: 08/10/2009 00:51:01 by yor_on »

#### Vern

• Neilep Level Member
• Posts: 2072
##### Does light have mass?
« Reply #124 on: 08/10/2009 03:32:32 »
yor_on; you are thinking You can also consider photons of light as mass themselves. They then do not have mass. They are mass. I have made that statement a few times lately and it has not been challenged, but I am sure most folks are not comfortable with it. I arrived at that by just looking at the arithmetic. m = hv / c2. Then just choose the units to eliminate the constants and we are left with m = v; or mass = electromagnetic change. Then restate it simply; mass is electromagnetic change.

Electromagnetic change is any change in the electric and magnetic charge amplitude in a localized area that can be considered as a system.

« Last Edit: 08/10/2009 03:36:44 by Vern »

#### The Naked Scientists Forum

##### Does light have mass?
« Reply #124 on: 08/10/2009 03:32:32 »