[lightarrow]

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]

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 Also to clarify, i told you all this because you inferred to the outdated concept of relativistic mass - outdated in the sense that in a qualitative physics course, lecturers usually inform us that the term relativistic mass is hardly ever used nowadays in an academic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]

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.

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]

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.

[Vern]

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.

[Pmb]

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*.

That's not true. Relativistic mass is the ration m = p/v. If a body is under stress then m does not equal E/c^2.

Here is an example: http://www.geocities.com/physics_world/sr/inertial_energy_vs_mass.htm

[Pmb]

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.

Perhaps I'm not clearon what you mean by "region of space is fixed". What does that mean? Thanks.

Pete

[Pmb]

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!

That is not correct. Relativistic mass is not an outdated concept. It's used in Cosmology a lot. A survey of recent relativity literature was done by Gary Oas which showed that it's widely used in modern textbooks. I've seen in used in the American Journal of Physics too. It's a very meaningful concept and you can get into trouble if you try to use invariant mass as the definition of mass when you venture outside its use in particle physics

[lightarrow]

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*.

That's not true. Relativistic mass is the ration m = p/v. If a body is under stress then m does not equal E/c^2.

Here is an example: http://www.geocities.com/physics_world/sr/inertial_energy_vs_mass.htm

Yes, I had seen that page. Unfortunately I couldn't understand it. [:-'(]

[lightarrow]

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.

Perhaps I'm not clear what you mean by "region of space is fixed". What does that mean? Thanks.
Pete

It mean stationary in the frame of reference considered. IMHO, if you consider that region of space as system, as long as a beam of light (for example) crosses that region, the system acquires (proper) mass, even if the light beam itself hasn't. I know it sounds a bit...esoteric []  but I cannot see how it could be wrong.

[lightarrow]

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!

That is not correct. Relativistic mass is not an outdated concept. It's used in Cosmology a lot. A survey of recent relativity literature was done by Gary Oas which showed that it's widely used in modern textbooks. I've seen in used in the American Journal of Physics too. It's a very meaningful concept and you can get into trouble if you try to use invariant mass as the definition of mass when you venture outside its use in particle physics

For example?

[yor_on]

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.

It sound as if we're born under the same full moon here Vern

Electromagnetic charge?
How much would Earth and the moon need to have to attract each other?
Or the solarsystem as a whole, and why don't we notice it?

[Vern]

Well; the earth and the moon have as much as they have in the form of the accumulations of their matter. It works just fine. In the innards of the matter the electromagnetic change is taking place giving substance to the matter.

I don't see a problem there.

[Pmb]

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!

That is not correct. Relativistic mass is not an outdated concept. It's used in Cosmology a lot. A survey of recent relativity literature was done by Gary Oas which showed that it's widely used in modern textbooks. I've seen in used in the American Journal of Physics too. It's a very meaningful concept and you can get into trouble if you try to use invariant mass as the definition of mass when you venture outside its use in particle physics

For example?

I constantly see people make the following mistakes

(1) The weight of a body does not depend on its speed
(2) The gravitational field of a body does not depend on its speed
(3) The mass density of radiation is zero
(4) Light cannot generate a gravitational field
(5) The ratio p/v is always equal to E/c^2
(6) Relativistic mass is not conserved in nuclear reactions
etc

[lightarrow]

I constantly see people make the following mistakes

(1) The weight of a body does not depend on its speed
(2) The gravitational field of a body does not depend on its speed
(3) The mass density of radiation is zero
(4) Light cannot generate a gravitational field

Maybe it is not necessary to talk about relativistic mass in these cases, if we start from my previous considerations.

[Vern]

(4) Light cannot generate a gravitational field

I suspect that this statement is wrong. How do photons attract each other gravitationally if this is so?

Edit: Maybe i misunderstood. What is the mistake? Are the statements mistakenly correct or mistakenly incorrect?

[Mr. Scientist]

Yup he is wrong.

A photon generates a gravitational curvature as it accelerates through spacetime. In fact, curvature, gravitational waves and acceleration are all part and parcel the same thing.

[lightarrow]

(4) Light cannot generate a gravitational field

I suspect that this statement is wrong. How do photons attract each other gravitationally if this is so?

Edit: Maybe i misunderstood. What is the mistake? Are the statements mistakenly correct or mistakenly incorrect?

PMB wrote that those statements are *incorrect*.

[Mr. Scientist]

If that's the case; the first statement is true. So the author has made one mistake or another.

[Mr. Scientist]

In fact, so are the rest apart from the one mentioned.