[Colin2B]

then the flaw is the equal sign,  an equivalent sign should be used instead. .....?

No, there is no flaw.
Energy is a measure of the ability of something to do work, it is not a thing in itself. So kinetic energy = 1/2 mv². In the case of E=mc², E can be thought of as the amount of energy tied up in the creation of the mass.
The = sign is used because E is the measure of the energy contained in the mass. A liquid can be measured in litres, but the volume is not the liquid, it is just one of the measurements you can take.

[raf21]

I'm just a poor country boy struggling with my limited intelligence and education,  looking up at the stars and wondering about things. 
I'm going to have to assume that I do not comprehend e=mc^2.
if supposed rock,  whether it is falling or at rest,  were to undergo total mass to energy conversion,  would the energy output not justify the equation? like when two hydrogen atoms fuse to form helium. ..... 1 plus 1 does not quite equal 2, or am I misinformed that there is a small amount of mass that is ' lost ' and the only thing on other side of the equal sign are photons? surely a total conversion would have far more output. ...I appreciate y'all taking the time too help remedy my remedial understanding of what little I grasp.

[PmbPhy]

Raf says:

Anyone ever consider the weight of light?

What do you think?

Yes. Since photons have energy they have an equivalent amount of mass. Actually its by the photons momentum that it has inertial mass. Others here are referring to a photon's proper mass, not their inertial mass, when they claim its zero.


For a good discussion on this by an authority in particle physics and cosmology please watch:
http://www.newenglandphysics.org/common_misconceptions/Alan_Guth_01.mp4

[PmbPhy]

perhaps my rudimentary understanding of physics is not allowing me to see the "light"...... but is not mass and energy the same thing?

No.

[raf21]

no to which question or all? and how does inertial mass differ from proper mass? do they react in any similar fashions?

[raf21]

nevermind,  I misread your response,  I thought you were answering no to a different set of questions.

[raf21]

I'm worried I will create a braching point in my questioning. .... but what the heck, full stream ahead. ..... first,  if some mass is "lost" during a stars fusion process,  does this mean that the universe is losing weight? would this "weight loss" explain why models of the early universe do not work correctly without the addition of more matter then we can account for?

[raf21]

and the tip of the second question. ....... how much volume is in a.....say 4.24 light year radius sphere? how much volume of photons is that? if they have even the most miniscule amount of mass. ........................

[raf21]

even at the surface of said sphere,  and assuming light is massless, it can still impart inertia,  giving everthing at said surface a gentle little push. .....

[Colin2B]

if supposed rock,  whether it is falling or at rest,  were to undergo total mass to energy conversion,  would the energy output not justify the equation? like when two hydrogen atoms fuse to form helium. ..... 1 plus 1 does not quite equal 2, or am I misinformed that there is a small amount of mass that is ' lost ' and the only thing on other side of the equal sign are photons? surely a total conversion would have far more output.

Fortunatly the processes in an atomic bomb are very inefficient. For the fusion bomb dropped on Hiroshima it is estimated that the amount of matter converted to energy was about 700 milligrams, less than one-third the mass of a U.S. dime. If the total mass of the bomb had been converted the result would have been earth shattering - literally.
This energy comes from the binding energy that has to be put in if you want to push the components (protons, neutrons) together to make Uranium, split those components apart and you release energy; however the energy you add also adds to the mass so the uranium atom has more mass than the individual components that make it up. The energy released is mainly heat, shockwave, and some radiation, so it’s really a case of energy changing the form it is held in.

if some mass is "lost" during a stars fusion process,  does this mean that the universe is losing weight? would this "weight loss" explain why models of the early universe do not work correctly without the addition of more matter then we can account for?

The light doesn’t leave the universe, we see some of it, it just gets redistributed.

how much volume is in a.....say 4.24 light year radius sphere? how much volume of photons is that? if they have even the most miniscule amount of mass. ........................

The ‘volume’ of photons will depend on the intensity of the source and how long it keeps emitting. As I said earlier and as @PmbPhy  says, they don’t have mass in the classical sense but have momentum .
It’s worth looking at the link he gave you. On the same site is a very good article on energy which you should read, you’ll find it helpful.

even at the surface of said sphere,  and assuming light is massless, it can still impart inertia,  giving everthing at said surface a gentle little push. .....

Yes. The voyager spacecraft had to have its course adjusted to allow for the push of photons from the sun. Remember though that the sun is close and big, intensity drops off as inverse square law so at the edge of your 4.24 ly sphere there won’t be any noticable push.

Just a point of forum etiquette. If you post a reply and you want to add to it, but no one else has replied in the meantime, can you modify your post rather than post a series of separate posts. Thanks.

[PmbPhy]

Light can power a solar sail so it can impart a force. Isn't that weigh by another name. In this case the energy would be the determining factor rather than mass.

There's two definitions of weight. The one I use here is identical with the gravitational force. With light imparting force on something like  a solar sail its due to the momentum of the photons and since photons have momentum they have mass since p = mv is how inertial mass is defined. Its basically how Newton defined it too.

[raf21]

my apologies for my lack of etiquette, I'm completely lacking in any experience using this forum type of thing,  this was my first go ast something like this.  many apologies for any unintended rudeness.

[geordief]

Light can power a solar sail so it can impart a force. Isn't that weigh by another name. In this case the energy would be the determining factor rather than mass.

There's two definitions of weight. The one I use here is identical with the gravitational force. With light imparting force on something like  a solar sail its due to the momentum of the photons and since photons have momentum they have mass since p = mv is how inertial mass is defined. Its basically how Newton defined it too.

And what is mass defined as ? Is it just a measurable quantity or does it have any different kind of definition?

I am wondering whether  all these "things" such as energy ,momentum ,mass (what have I missed?)  are all self referential  and define each other..

I know the Higgs Field is supposed to give mass to a particular class of particles. Is that the best definition?

Mass is defined as resistance to momentum change isn't it?

[PmbPhy]

As I said earlier and as @PmbPhy  says, they don’t have mass in the classical sense but have momentum .

I didn't say that. Photons do have mass in the classical sense since classically, m i.e. as Newton defined mass, m = p/v which is a classical expression. You'll note that all texts which use relativistic mass define it that way. The classically defined mass is jus another name for relativistic mass.

[raf21]

so...... mass as defined as defined as resistance to momentum change would apply to light striking a surface and applying a minuscule push? then in the instance of my supposed 4,24 light year radius sphere,  (distance to nearest star other then sol ), however hard to measure,  would exist.  multiply that by the number of stars in our galaxy times the number of known galaxies, and this number might add up to a tremendous amount of "light weight" slowly pushing things apart.  I have no idea how photons react when headed in opposite directions,  so I'll not speculate that that might add too it.

[Colin2B]

I didn't say that. Photons do have mass in the classical sense since classically, m i.e. as Newton defined mass, m = p/v which is a classical expression.

You’re right Pete, me misusing the word classical.

...multiply that by the number of stars in our galaxy times the number of known galaxies, and this number might add up to a tremendous amount of "light weight" slowly pushing things apart.

You can answer your own question if you look at the night sky. How bright is it? Can you bear to look at the stars?
Now try looking at the sun in daylight. Actually, don’t because it will damage your eyes which gives you an idea of the amount of power reaching the earth. But, is the sun pushing the solar system apart? No, so what chance for weak starlight?

[jeffreyH]

Light can power a solar sail so it can impart a force. Isn't that weigh by another name. In this case the energy would be the determining factor rather than mass.

There's two definitions of weight. The one I use here is identical with the gravitational force. With light imparting force on something like  a solar sail its due to the momentum of the photons and since photons have momentum they have mass since p = mv is how inertial mass is defined. Its basically how Newton defined it too.

And what is mass defined as ? Is it just a measurable quantity or does it have any different kind of definition?

I am wondering whether  all these "things" such as energy ,momentum ,mass (what have I missed?)  are all self referential  and define each other..

I know the Higgs Field is supposed to give mass to a particular class of particles. Is that the best definition?

Mass is defined as resistance to momentum change isn't it?

Mass can be defined in terms of the Planck values. I worked this through a long time back. The equations are posted somewhere in new theories I think. This can ultimately lead to a mass number. This would be an integer value, and when used with the Planck action h, would lead to quantised mass.

[raf21]

in response to Colin's last statement,  (I still don't have this forum thing down yet,  ain't figured how to copy quotes,  please forgive),  please add time to that equation,  I overlooked that.  a small hammer can drive a big nail given enough time to peck at it.  as far as our sun being brighter then starlight,  perhaps being caught in the suns gravity well can account for us not being pushed away. ....although the sunlight was recently used too flip over some sample vessel Japan sent to an asteroid somewhere,  and this occurred inside our solar system.   btw, all you people rock,  I really appreciate the time y'all have invested discussing these things.  they are probably all very trivial to y'all,  but provide a lot of cool things to think about.  or hot I reckon,  if the subject wanders towards stars. ...

[PmbPhy]

And what is mass defined as ? Is it just a measurable quantity or does it have any different kind of definition?

I already said it. Mass is defined so that momentum us conserved (At least in SR). To be precise we sat that we define m so that the vector quantity mv is conserved. We then call the vector p = mv the momentum of the particle. If you try to go beyond this then I'm sure you can but then its metaphysics, not physics.

Mass is defined as resistance to momentum change isn't it?

Yes.

Mass can be defined in terms of the Planck values. I worked this through a long time back. The equations are posted somewhere in new theories I think. This can ultimately lead to a mass number. This would be an integer value, and when used with the Planck action h, would lead to quantised mass.

Please show me. Expressing one term in terms of another cannot be used as a definition. 
Please watch the train of thought here. Hoping from the classical domain of SR to quantum mechanics you just did can lead to nonsense talk, For example, I can speak of velocity and acceleration as well as a trajectory but when switching to QM all that is meaningless.

[Colin2B]

as far as our sun being brighter then starlight,  perhaps being caught in the suns gravity well can account for us not being pushed away. ....

Good to think about that and it could be true if light intensity and gravitational force varied in different ways, however, they both reduce with distance as the inverse square law so the effects of both diminish together at the same rate.
There are some questions as to whether gravity varies to a different model at extreme distances or at extreme intensities, but in the case we are considering that is unlikely to be a factor, but we should keep an open mind.

PS don’t worry too much about the posting protocols, the moderators and other members will help you through and you’ll soon get the hang of it. The protocols are there to help you, for example if you post a series of ideas or questions as separate posts it’s likely only the last one will be read, so folks can miss what you are asking.