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Thread: Does light exert a gravitational force?

  1. #1 Does light exert a gravitational force? 
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    I'm just trying to be clear on this. I know light has no mass, but it does have energy. Energy alone is enough to exert a gravitational force, isn't it? So, while I rather doubt it's substantial enough to have much of an effect on things, light should still exert a small force when it passes by?

    Or is gravity just based on mass, instead of mass + energy?


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  3. #2 Re: Does light exert a gravitational force? 
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    Quote Originally Posted by kojax
    I'm just trying to be clear on this. I know light has no mass, but it does have energy. Energy alone is enough to exert a gravitational force, isn't it? So, while I rather doubt it's substantial enough to have much of an effect on things, light should still exert a small force when it passes by?

    Yes, it does. In fact, it has to in order to maintain conservation of momentum. Photons have momentum. If a photon has its path deflected by a massive body, its momentum is changed. If the massive body did not incur an equal but opposite change in momentum, the total momentum of the system would not be conserved.


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    Moderator note:
    A number of posts have been removed as being off-topic and/or overly speculative.
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    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Bishadi's posts and responses to them have been moved here:
    http://www.thescienceforum.com/Bisha...%3B-20394t.php
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    the reason for the move is because janus is not up to speed on global events and the ideology of what SCIENCE is......


    some like to believe (like religious quacks) and some like evidence........

    janus does not belong in the field of education or moderating a forum!

    Sorry, to the few who are acually doing the read, i can't reach thru the screen to slap the fools, for you.
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    Quote Originally Posted by Bishadi
    the reason for the move is because janus is not up to speed on global events and the ideology of what SCIENCE is......


    some like to believe (like religious quacks) and some like evidence........

    janus does not belong in the field of education or moderating a forum!

    Sorry, to the few who are acually doing the read, i can't reach thru the screen to slap the fools, for you.

    ... 2...
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    I'm going to re-post one of my questions from the moved text, still: does potential energy count as energy for the purposes of increasing an object's gravitational attraction?

    Like if an electron moves to a higher energy level and stays there, for some reason, would the energy it has absorbed increase its gravitational attraction slightly?
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  9. #8  
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    question is why doesnt it have gravity ?
    Seeing the unseen solving the unsolved and linking the hidden is what intelligence is all about.
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  10. #9  
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    Quote Originally Posted by kojax View Post
    I'm just trying to be clear on this. I know light has no mass, but it does have energy. Energy alone is enough to exert a gravitational force, isn't it? So, while I rather doubt it's substantial enough to have much of an effect on things, light should still exert a small force when it passes by?

    Or is gravity just based on mass, instead of mass + energy?
    Tolman, Ehrenfest and Podolsky worked out the solution of the gravitational field of a directed beam of light. If you'd like to perue the derivation then you can see it on my website at
    The Gravitational Field of a Directed Beam of Light

    Once the gravitational fiel is determined the gravitational force cen be found by using the expression for the gravitational force located here - Gravitational Force

    So yes. And you can see from the derivation that the stress in the EM field as well as the momentum density of the field are boith sources, not just the energy.
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    That was a good question, I didn't know the answer yet. I also didn't know Podolsky was dead :P
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    Quote Originally Posted by Kerling View Post
    That was a good question, I didn't know the answer yet. I also didn't know Podolsky was dead :P
    I'm curious. Is there a reason that you raised an old topic? Perhaps being new here you were browsing around looking for something interesting to talk about? Did you want to discuss this?
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    Since we can see light from most parts of the universe does this mean that space is actually filled with photons.
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    Quote Originally Posted by Chrisgorlitz View Post
    Since we can see light from most parts of the universe does this mean that space is actually filled with photons.
    The CMB suffuses all of space. Don't forget: The light we can see is only a tiny fraction of the "light" that's out there. The CMB is everywhere, but the wavelengths are too great to be visible to humans.
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    Quote Originally Posted by Chrisgorlitz View Post
    Since we can see light from most parts of the universe does this mean that space is actually filled with photons.
    Sure. Picture your eye being at any particular location in space. At each point it will pick up and register light so there must be light at every point in space (even given a finite size of the iris etc).
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    So are these photons producing gravity thoughout the universe or are they moving to fast to produce any gravitational effects?
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    Quote Originally Posted by Chrisgorlitz View Post
    So are these photons producing gravity thoughout the universe or are they moving to fast to produce any gravitational effects?
    At this epoch in time I'd say yes but only a very very very insignificant amount. There was an epoch where the universe was radiation dominated at which the lights mass contributed noticibly to the cosmological mass density.
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    Reposted by kojax
    I'm going to re-post one of my questions from the moved text, still: does potential energy count as energy for the purposes of increasing an object's gravitational attraction?
    Like if an electron moves to a higher energy level and stays there, for some reason, would the energy it has absorbed increase its gravitational attraction slightly?


    Answer. Because the electron was compelled to accelerate and has absorbed fundamental energy that has increased its potential energy relative to the nucleus it assists to neutralise, its gravity would increase and also would its gravitational ability. Potential energy is a human concept originating from our ability to imagine possible changes that could result from such an increase in the electrons energy, rate of change of position etceteras.
    As explained in my 160 page paper titled Matter and Associated Mysteries (gravity is much too involved to attempt to explain in posts but available for free download from The General Science Journal) Gravity and Gravitation are two differing phenomenon’s. Gravitation was measured by the Cavendish experiment and is the results of the interference to a particle or a bulk bodies gravity in accordance with the well known laws. On Earth, we refer to gravity as the weakest of all forces and define cohesion as resulting from the Coulomb effect, whilst on the super dense bodies encountered in the Universe, then gravity conveniently becomes the overpowering and dominant force as evidenced by a single isolated rapidly rotating super dense body. The immediate above is presently allowed because GR describes gravity as an illusion and only results because bulk matter is compelled to follow geodesies. There is a large amount of unexplained physic connected with the concept of geodesies with regards to how bulk matter of varied mass is compelled to increase accelerating whilst being compelled to stay within the confines of any particular geodesy.
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    Photon doesn't create gravity because it is by consensus to be massless, however photon can be attracted to gravity & this is called "passive-gravitational-mass" while normal mass is called "active-gravitational-mass". And momentum doesn't imply mass because it is a basic property of wave rather than photon, and photon is wave, also: momentum as defined by "E=pc" so that it is related to energy rather than to mass alone.

    I don't think we have any clear cut mechanical understanding of gravity or mass yet... Some other speculative physic assume mass is derived from other phenomenon which mean mass isn't fundamental (eg: vaccuum friction, higgs field). -and we don't know gravity either and how it relate to mass, that's why particle like Graviton is just a theory and not yet detected.
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    (deleted duplicate)
    Last edited by pmb; July 15th, 2012 at 07:20 AM. Reason: Deleted duplicate
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    Quote Originally Posted by msafwan View Post
    Photon doesn't create gravity because it is by consensus to be massless, however photon can be attracted to gravity & this is called "passive-gravitational-mass" while normal mass is called "active-gravitational-mass".
    While it is true that photons have zero rest mass, as you mention here, they have non-zero active gravitational mass which, by definition, is the mass that acts as the source of gravity. Normal mass is called [I]inertial mass[/b] which is non-zero also. Since light has active gravitational mass it can generate a gravitational field. Please see post #9 where I reference the article On the Gravitational Field Produced by Light, Richard C. Tolman, Paul Ehrenfest and Boris Podolsky, Phys. Rev, 37, March 1931.

    Tolman discusses this in his text on special and general relativity. Section 110 of his text is entitled The gravitational mass of disordered radiation. He makes the very important observation.
    (From section 109)
    Since a disordered distribution of electromagnetic radiation can be regarded as having the mechanical properties of perfect fluid, we have already suggested in sec 85 that we could assign to such radiation the usual expression for the energy-momentum tensor of a perfect fluid […] where the density and pressure of the radiation - as measured by a local observer who finds no net flow of energy – would be connected by the simple relation

    (from section 110)
    Hence, since the pressure p0 of disordered radiation is necessarily equal to one-third its energy density [tex]\rho_{00}[\tex]… we are led to the interesting conclusion that disordered radiation in the interior of the fluid sphere contributes roughly speaking twice as much to the gravitational field of the sphere as the same amount of energy in the form of matter.
    It’s important not to forget the inertia of pressure and stress in matter.

    Quote Originally Posted by msafwan View Post
    And momentum doesn't imply mass ....
    Momentum is defined as the product of inertial mass and velocity. In relativity, which is a classical theory, one treats a photon as a classical particle that always travels at the speed of light, i.e. as a classical luxon, which is a particle which has a velocity, momentum and mass. Since it has momentum it has inertial mass and since it has energy it has inertial mass. And since it has these properties it also has active and passive gravitational mass.

    Quote Originally Posted by msafwan View Post
    …because it is a basic property of wave rather than photon, and photon is wave, also: momentum as defined by "E=pc" so that it is related to energy rather than to mass alone.
    In classical relativity the photon is treated as a classical particle. You’re mixing classical mechanics with quantum mechanics. Since there is no theory of relativistic quantum mechanics one has to use caution when speaking of quantum mechanical particles in a classical physics (relativity is a classical theory). In quantum mechanics the photon is neither a particle nor a wave, but something else. In any case most relativity textbooks assign a mass of m = hf/c2 to photons.
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    Quote Originally Posted by pmb View Post
    ... photon is neither a particle nor a wave, but something else. In any case most relativity textbooks assign a mass of m = hf/c2 to photons.
    People should assign E=pc to photon instead of E=mc^2 to photon (since photon doesn't have mass), where E=hf.

    I think
    E=mc^2 only happens when Gamma-rays hit a nucleus and create electron & positron pair (where hf=E=mc^2 for both particle).
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    Quote Originally Posted by msafwan View Post
    People should assign E=pc to photon instead of E=mc^2 to photon (since photon doesn't have mass), where E=hf.
    p = mv is an identity whereas E = mc2 is an equality.

    Quote Originally Posted by msafwan View Post
    I think
    E=mc^2 only happens when Gamma-rays hit a nucleus and created electron & positron pair (where E=mc^2 for both particle).
    E = mc2 in all concievable cases for a structurless particle or for a free body, nor merely in the case you cite.
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by msafwan View Post
    I think
    E=mc^2 only happens when Gamma-rays hit a nucleus and created electron & positron pair (where E=mc^2 for both particle).
    E = mc2 in all concievable cases for a structurless particle or for a free body, nor merely in the case you cite.
    The full equation was E=mc^2 +pc, and photon is massless so E=pc. But when photon hit nucleus: it dissappear and create positron & electron thus E=mc^2...
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    Quote Originally Posted by msafwan View Post
    The full equation was E=mc^2 +pc, and photon is massless so E=pc. But when photon hit nucleus: it dissappear and create positron & electron thus E=mc^2...
    Actually, no the Full equation, or the Klein Gordon equation is E^2=M^2*C^4 + P^2*C^2. That is an important difference!
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    Quote Originally Posted by pmb View Post
    Quote Originally Posted by Kerling View Post
    That was a good question, I didn't know the answer yet. I also didn't know Podolsky was dead :P
    I'm curious. Is there a reason that you raised an old topic? Perhaps being new here you were browsing around looking for something interesting to talk about? Did you want to discuss this?
    Judging by the amount of replies that follow It is still a good topic. I Use these question to brush up my knowledge, and realising that my books on relativity, well, were literally really dusty :S.
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    [COLOR=#000000]
    Quote Originally Posted by msafwan;337673 [/COLOR
    The full equation was E=mc^2 +pc, and photon is massless so E=pc. But when photon hit nucleus: it dissappear and create positron & electron thus E=mc^2...
    If I'm correct then you're confusing rest mass m0 with inertial mass m = p/v. For a point particle, or an object that can be treated like a point particle (as such it has to be structureless) the two are related by



    When you say the photon is massless what it means is the rest mass of a photon is zero. Simply saying "the photon is massless" is somewhat misleading when one is discussing the various kinds of mass. The expression you wrote, i.e. E=mc^2 +pc is incorrect. I believe that the expression you're thinking of is



    I refer to the E in that expression as inertial energy so as not to confuse it with total energy, W, which contains the potential energy of the particle. E is defined as

    E = K + E0 = Kinetic Energy + Rest Energy

    Wherein it follows that

    W = E + V(x) = K + V + E0 = Kinetic Energy + Potential Energy + Rest Energy

    For a photon E0 so that E = K.

    What I've been explaining is not something I contrived or made up. I've been talking about straight textbook physics. Here are some examples from relativity textbooks

    Relativity: Special, General and Cosmological, Rindler, Oxford Univ., Press, (2001), page 120
    According to Einstein, a photon with frequency has energy h /c2, and thus (as he only came to realize several years later) a finite mass and a finite momentum h/c.
    Introducing Einstein's Relativity, Ray D'Inverno, Oxford Univ. Press, (1992), page 50
    Finally, using the energy-mass relationship E = mc2, we find that the relativistic mass of a photon is non-zero and given by

    m = p/c.

    Combining these results with Planck's hypothesis, we obtain the following formulae for the energy E, relativistic mass m, and linear momentum p of the photons:
    E = hf m = hf/c2 p = hf/c
    and so on. I have an article on this subject called In defense of relativistic mass, T.R. Sandin, Am. J. Phys. 59(11), November 1991. The abstract reads
    The concept of relativistic mass brings a consistency and simplicity to teaching of special relativity to introductory students. For example, E = mc2 then expresses the beautifully simplifying equivalence of mass and energy. Those who claim not to use relativistic mass actually do so--if not by name--when considering systems of particles or photons. Relativistic mass does not depend on the angle between force and velocity-- this supposed dependence results from incorrect use of Newton's second law of motion.
    If you'd like to read it I can e-mail you a copy.
    Last edited by pmb; July 15th, 2012 at 09:49 AM.
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    Quote Originally Posted by Kerling View Post
    Quote Originally Posted by msafwan View Post
    The full equation was E=mc^2 +pc, and photon is massless so E=pc. But when photon hit nucleus: it dissappear and create positron & electron thus E=mc^2...
    Actually, no the Full equation, or the Klein Gordon equation is E^2=M^2*C^4 + P^2*C^2. That is an important difference!
    Why do you refer to it as the Klein-Goddon Equation? The Klein-Gordon Equation is



    where m is the rest mass of the particle. This is an expresion from relativistic quantum mechanics. Its a wave euation.
    Last edited by pmb; July 15th, 2012 at 09:48 AM. Reason: needed to change latex tags
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    Quote Originally Posted by pmb View Post
    [Why do you refer to it as the Klein-Goddon Equation? The Klein-Gordon Equation is



    where m is the rest mass of the particle. This is an expresion from relativistic quantum mechanics. Its a wave euation.
    It has to do with the non locality of Einsteins expression which expresses E, it was Klein and Gordon that circumvented the problem by Using the expression in it's E^2 form. Which you can see in the wave equation that is given by you. In general the wave equation is for general relativity, but the E^2 expression is used in quantum theory often to prove that when the limit is taken to a classical limit the theory still reproduces this Energy equations. As it is much easier then having to derive the actual Klein Gordon Equation from the theory. It is a way to prove things are 'correctly' relative.

    Either way, you pointed out aswell that E=mc^2 + pc isnt'correct.
    And Rindler is in my opinion a good book. So good advice there!
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    Quote Originally Posted by Kerling View Post
    It has to do with the non locality of Einsteins expression which expresses E, it was Klein and Gordon that circumvented the problem by Using the expression in it's E^2 form.
    I don't understand what you're trying to say. Here's where I'm coming from: For readers in general: We start with the following substitutions, substiting observables E and p with their quantum mechanical operators

    E -> and p ->

    We next substitute these expressions into the classical expression for energy, momentum and mass



    which results in



    which is the Klein-Gordon Equation. This is not the same as , which is a classical expression devoid of quantum mechanical information. It's only the combination of this equation and the quantum mechical operators and the wave function which gives the Klein-Gorgon equation. The later is a quatum mechanical expression whereas the former is found used in classical relativity as well as quantum mechanics.

    My point: Its confusing to give two different expressions the same name. I myself have never seen that term used to refer to .
    Last edited by pmb; July 15th, 2012 at 12:10 PM.
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    In my education my professors always used. (in deriving different quantum mechanical notation. that a check needs to be whether or not it (the theory) fulfills the klein gordon equation. You need to check if your theory still applies to this after for instance second quantization etc. It is for instance a check with the Dirac equation. The E^2 equation follows practically when you try and make the Schrödinger equation relativistic. They are 2 different equations, but the represent the same thing. One is merely the result of the other. But my appoligies if I made things to be more confusing this way. I realise that the E^2 is often not called the Klein Gordon equation. But they were the first to utilizes it as a way of solving problems.
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    This thread might be on life support, as it no longer seems to have any useful purpose other than spamming the author's site, and trolling.
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    GWN's posts and replies have been sent here: http://www.thescienceforum.com/new-h...s-gravity.html
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    Quote Originally Posted by MeteorWayne View Post
    This thread might be on life support, as it no longer seems to have any useful purpose other than spamming the author's site, and trolling.
    If I might ask, what author are you referring to and what is this trolling you speak of?
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    I think of the problem of rest mass as being like the last part of this scene from the movie "Layer Cake". It's this crime drama, and Daniel Craig is playing a drug dealer with a college degree who closes deals. So he's walking away from this drug deal with some tough thugs backing him up, and the guy he's dealing with says "You wouldn't be so f***ing flush if you didn't have him behind you.", to which one of the tough thugs replies "Well he f***ing has, ain't he."

    Layer Cake - Funny Scene - YouTube

    Who cares what a photon's mass *would* be *if* it were at rest? A photon never is at rest.
    Some clocks are only right twice a day, but they are still right when they are right.
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    Quote Originally Posted by kojax View Post
    Who cares what a photon's mass *would* be *if* it were at rest? A photon never is at rest.
    I don't know of anybody asking what the mass of a photon would be if it was at rest. This is one of the reasons why some people, such as myself, prefer the term proper mass over the term rest mass.
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    An emitted photon with E=hf,with wavelength of 600nm,has an energy of 3.3E-19J. Since E=hf=ymc^2,where y=1(when v=0) then E=mc^2. Therefore the rest mass of such photon is 3.3E-19=mc^2, m=3.3E-19/c^2=3.7E-36kg. Unless something is wrong,according to mass-energy equivalence,such a photons has 3.7E-36kg, who cares about its rest mass? Maybe i do!
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    Quote Originally Posted by merumario View Post
    An emitted photon with E=hf,with wavelength of 600nm,has an energy of 3.3E-19J. Since E=hf=ymc^2,where y=1(when v=0) then E=mc^2.
    Nope, a photon is never at rest, so the above manipulation is incorrect.
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    Quote Originally Posted by merumario View Post
    E=mc^2
    This is only true for a particle at rest. The full expression is: .

    Here is an explanation: http://www.wlu.ca/physcomp/pc242/Phys242Lec5.pdf
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    yea something was wrong. but check ought to be E^2=p^2c^4+(mc^2)2
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    Just to complicate the question again.... how do we interpret things if the process that lead to the photons' creation resulted in the loss of some rest mass?

    For example if a positron and an electron come into contact? Clearly both particles have rest masses to begin with. Some exotic particles will be created, but the total rest mass of those particles afterward won't be as great as the original total rest mass of the pair, will it?

    So if the photons that were emitted have no rest mass, then where did the rest mass go? Or it sufficient to simply say that rest mass is not a conserved value?
    Some clocks are only right twice a day, but they are still right when they are right.
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    Quote Originally Posted by kojax View Post
    Just to complicate the question again.... how do we interpret things if the process that lead to the photons' creation resulted in the loss of some rest mass?

    For example if a positron and an electron come into contact? Clearly both particles have rest masses to begin with. Some exotic particles will be created, but the total rest mass of those particles afterward won't be as great as the original total rest mass of the pair, will it?

    So if the photons that were emitted have no rest mass, then where did the rest mass go? Or it sufficient to simply say that rest mass is not a conserved value?
    Mass itself is not a conserved quantity. In the case of electron-positron annihilation two photons will be created, each with an energy of 511 keV (i.e. gamma rays), which is the mass of the electron/positron x c2. There have to be two in order to conserve momentum.
    ei incumbit probatio qui dicit, non qui negat
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    by the process of pair production where a photon interact with a neuclus to give two pair particles say electron and positron both have same rest mass of 9.11^-31kg but different charge(e- and e+). the minimum energy at which this photon must move in other to produce such particles=2*9.11^-31*(2.998^8)2=1.62^-13j.(this is true because when these particles are formed they slow down in speed and their kE equals their PE) so when this two annihilate the chagre cancel out causing an uncharged particle and their mass is converted into energy which makes the mass=0. so the enery of this particle=the rest energy of both particle it created).
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    Quote Originally Posted by kojax View Post
    Just to complicate the question again.... how do we interpret things if the process that lead to the photons' creation resulted in the loss of some rest mass?

    For example if a positron and an electron come into contact? Clearly both particles have rest masses to begin with. Some exotic particles will be created, but the total rest mass of those particles afterward won't be as great as the original total rest mass of the pair, will it?

    So if the photons that were emitted have no rest mass, then where did the rest mass go? Or it sufficient to simply say that rest mass is not a conserved value?
    is not conserved. The entity that IS conserved is . This is trivial consequence of the conservation of total energy .
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    I don't know anything about light photons exerting a gravitational force...but I do know that light photons exhibit thrust. For an example: When you turn on a flashlight...the light photons exhibit thrust, even though the person holding the flashlight, cannot feel the photon thrust coming from the flashlight.
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    Quote Originally Posted by Erno86 View Post
    I don't know anything
    Got that right.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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    (You know nothing)-best thing you ever said. Go and know something.
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