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Thread: photons and neutrinos

  1. #1 photons and neutrinos 
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    As I understand, photons have mass (even though they're not Catholic, sorry), so what is the weight of photons hitting the earth? If a photon behaves like a wave does it still have mass? Photons may be timeless but they must have age. I assume that photons hitting the earth now originated in the sun's core, but how long ago has it taken for them to tunnel out?
    Where is the source of neutrinos? I think I read that they originate in neutron stars but is there another source? Like photons, can they be both particles and waves?


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  3. #2 Re: photons and neutrinos 
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    Quote Originally Posted by greymatt
    As I understand, photons have mass (even though they're not Catholic, sorry), so what is the weight of photons hitting the earth?
    They have no rest mass, but they have momentum, yes. I don't think anyone could put a number to the total momentum of them hitting the Earth, but I am fairly sure it is negligable.

    Quote Originally Posted by greymatt
    If a photon behaves like a wave does it still have mass?
    All photons are always waves, are they not? Seeing as they have a wavelength and are always moving at the same constant speed. And photons do not have a rest-mass.

    Quote Originally Posted by greymatt
    Photons may be timeless but they must have age. I assume that photons hitting the earth now originated in the sun's core, but how long ago has it taken for them to tunnel out?
    It takes 38 minutes for light to reach us from the sun, I think.

    Quote Originally Posted by greymatt
    Where is the source of neutrinos? I think I read that they originate in neutron stars but is there another source? Like photons, can they be both particles and waves?
    Everything can be expressed as either a particle or a wave. I have previously calculated the speed I would need to move at to diffract through a doorway (around 10^-35 m/s)

    Neutrinos are produced in a number of interactions, including beta (-) decay;



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  4. #3 Re: photons and neutrinos 
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    Quote Originally Posted by greymatt
    As I understand, photons have mass (even though they're not Catholic, sorry), so what is the weight of photons hitting the earth? If a photon behaves like a wave does it still have mass? Photons may be timeless but they must have age. I assume that photons hitting the earth now originated in the sun's core, but how long ago has it taken for them to tunnel out?
    Where is the source of neutrinos? I think I read that they originate in neutron stars but is there another source? Like photons, can they be both particles and waves?
    Photons have no rest mass. But they have enerby and since mass and energy are the same thing they also have mass. Photons are in fact an element in the stress-energy tensor of general relativity that determines the curvature of space-time that we call "graavity". You can even, in theory, create a gravitational structure from photons alone. Such structures were studied by John Archibald wheeler who called them geons.

    Neutrinos come from various particle interactions, including nuclear fusion and one source of neutrinos is the nuclear fusion of the sun.

    Neutrinos have been found to have a small rest mass, so they do not travel at quite light speed.

    Photons are particles. But they do not behave like classical "little marbles" but are quantum particles which obey probabilistic laws that are sometimes describable with wave mechanics.

    This Wiki article explains the double slit experiment. Pay particular attention to the pictures showing the classical interference pattern (using electrons in this case) being built up one particle at a time. http://en.wikipedia.org/wiki/Double-slit_experiment
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  5. #4  
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    It takes 38 minutes for light to reach us from the sun, I think.
    8 minutes, drowsy. Not 38.
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  6. #5  
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    *sigh* I need to start looking things up, instead of trying to recite form memory.
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    Yes, I am aware that photons take about 8 minutes to reach earth from the surface of the sun (90 million / 670 million * 60). But surely photons are formed in the sun's centre and they tunnel their way out. So how long on average would it take them to tunnel out? I seem to remember reading that it took the sun a million years to turn itself on.
    By the way, I have just learned that photons are Catholic as they are universal.
    Exactly what is rest mass as I can't imagine a photon being at rest.
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  8. #7  
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    Quote Originally Posted by greymatt
    Yes, I am aware that photons take about 8 minutes to reach earth from the surface of the sun (90 million / 670 million * 60). But surely photons are formed in the sun's centre and they tunnel their way out. So how long on average would it take them to tunnel out? I seem to remember reading that it took the sun a million years to turn itself on.
    Oh, I see what you mean.

    I think the photons we see are all emitted from the surface of the sun, not the centre, explaining why all of the sun we see is equally bright; rather than brighter at the centre.

    Quote Originally Posted by greymatt
    Exactly what is rest mass as I can't imagine a photon being at rest.
    This is exactly the point; when a photon is not moving, it has no energy and so no mass. By contrast, an electron when not moving still has 0.511 MeV of energy, resulting in a mass of . So we say that the electron has rest-mass, and the photon does not.
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    If photons can't escape from black holes, does that mean the inside of the hole is full of light? What about the event horizon of the universe, are photons likely to exist there in a wall of light? Okay, Einstein spent 50 years thinking about light quanta and I've only been thinking about it for 50 minutes.
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    But surely photons are formed in the sun's centre and they tunnel their way out. So how long on average would it take them to tunnel out?
    Photons are formed by electromagnetic interactions between particles in the sun. Since charged particles do not have to be confined to the center, they can be formed at any point on the sun, be it the centre or the surface.

    Because the photons in the centre will more likely be absorbed by nearby particles (I'm not quite sure if fermions like the photon can tunnel out, anyway), we very often receive light from the sun's surface, rather than its centre.

    This is exactly the point; when a photon is not moving, it has no energy and so no mass.
    Good answer, drowsy, but the real reason is that light can never be at rest, and obviously can then never be observed to have any mass at rest. Hence light has no rest mass. Your answer's good too, though.

    By the way, I have just learned that photons are Catholic as they are universal.
    So how does that make them Catholic?

    I seem to remember reading that it took the sun a million years to turn itself on.
    Mainly because it takes quite some time to gather tens of trillions of hydrogen particles to form a star, even one as relatively small as our sun.
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    Quote Originally Posted by greymatt
    If photons can't escape from black holes, does that mean the inside of the hole is full of light? What about the event horizon of the universe, are photons likely to exist there in a wall of light? Okay, Einstein spent 50 years thinking about light quanta and I've only been thinking about it for 50 minutes.
    He spent 50 years on photons? That's news to me...

    You're quite right, by the way. The inside of a black hole is indeed full of trapped light. Hawking radiation would not affect the situation much, either, unless it's an extraordinarily small black hole. Good luck trying to see this light and live, however.

    I doubt you'll find a wall of light at the event horizon, though; a photon would still succumb to the immense gravity and move further into the recesses of the black hole, rather than hover at the event horizon.
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  12. #11 Re: photons and neutrinos 
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    Quote Originally Posted by greymatt
    As I understand, photons have mass (even though they're not Catholic, sorry),

    What on earth does catholic have to do with ANYTHING





    Quote Originally Posted by greymatt
    so what is the weight of photons hitting the earth?

    as far as I know nothing, I think photons are massless, they are forms of energy, which is a separate thing to matter (matter, energy, Anti-matter, Anti-energy and so on and so forth)





    Quote Originally Posted by greymatt
    If a photon behaves like a wave does it still have mass?

    Again, not to my knowledge, Photons behave like Electromagnetic waves, as opposed to sea waves, EM waves I think don't have mass, again simply a transfer of energy, (it's why EM waves can move through space)





    Quote Originally Posted by greymatt
    Photons may be timeless but they must have age.

    Where did you hear Photons are 'timeless' and what does that even mean? seriously it sounds like a cheesy quote from star trek or something, Regardless, of course they have age, everything has age, Technically everything is 14.6B years old but in it's current form a Photon is potentially as old as the star that Created it





    Quote Originally Posted by greymatt
    I assume that photons hitting the earth now originated in the sun's core, but how long ago has it taken for them to tunnel out?

    *sigh* this is the area where Photons and their ilk get REALLY bloody annoyingly complicated and...odd...
    Firstly Photons compose the 'Particle' section of the Wave-Particle duality of Electromagnetic Waves (including visible light)
    Secondly given 'lightspeed' we know that all EM waves travel at roughly 300,000,000 meters per second (3x10^8 mps) Knowing this we can work out Light from the Sun's surface reaches us in aproximately 8 minutes,
    Therefore it's logical that Light and by default Photon's take only a little longer than that to reach us from the centre of the Sun, 10 minutes tops given the Suns crushing pressure and gravity

    Sadly logic falls on it's face when it comes to this kind of thing,
    see even though Light travels at a set speed, (changing negligably in different mediums) the photons that compose it do not travel at a set speed, in fact Photons travel completely randomly, Take the 2 Steps forward and 1 Step back mentality and put the numbers in a random number generator and you have the path of a Photon, they go up down left right forward backward and everywhere in between at a seemingly random pace (there may be some form of thing dictacting their movement but Humans currently don't know about it.
    So this is where the W-P duality comes in, Light Waves only take 8 minutes to reach earth, Light Photons however can take anywhere from 8 seconds to 8 millenia to reach earth
    Due to the complete randomness of Photon movement combined with the immense forces inside the Sun, it's nearly impossible to calculate the actual time taken for a photon to travel from the centre to the surface, estimates range from 18,000 years to 10 million years, However the generally accepted average is 170,000 Years, Either way it takes a Very long time




    Quote Originally Posted by greymatt
    Where is the source of neutrinos? I think I read that they originate in neutron stars but is there another source? Like photons, can they be both particles and waves?
    A Neutrino is a Particle created by specific forms of Radioactive Decay such as those in stars and nuclear reactors, The only one I can explain is in a Supernova, the pressure and heat finally overcomes the forces holding the Electrons and Protons apart, the Supernova crushes both together forming a Neutron and a Neutrino
    As far as I know Neutrino's have Negligible Mass (less than that of an electron) no electric charge and travel close to the speed of light, as a result they pass through normal matter nearly completely undisturbed
    and yes as far as I know they are also Subject to W-P duality
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    Quote Originally Posted by drowsy turtle
    This is exactly the point; when a photon is not moving, it has no energy and so no mass.
    It is a bit more subtle than that. A photon is never "not moving". Since a photon is light, following relativity, it travels at only one speed, c. It never travels faster, and it never travels slower in any inertial reference frame.

    What is called a slower speed of light in a medium is due to the absorption and re-emission of photons, resulting in an apparent slower speed of light, but the photons themselves only travel at c.

    A photon is never at rest. If it is absorbed, it doesn't just "hang around", it ceases to exist. The energy of an absorbed photon becomes energy of whatever absorbed it, commonly an electron. When that something relalxes to a lower energy state and emits an photon, that photon springs into existence and is instantly traveling at c.

    The concept of rest mass is a bit subtle when applied to a photon. It is not as if one could trap a photon that is not moving and somewhow weigh it to determine that it has zero mass. If you "trap it" by absorbing it with some other particle, that particle will have gained mass equal to the mass equivalent of the energy of the photon ().

    "Zero rest mass" is simply a means of encoding the notion that the particle travels at the speed of light and carries energy. Gluons have zero rest mass. Presumably a graviton, if it exists, is also a particle of zero rest mass.
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    Quote Originally Posted by DrRocket
    "Zero rest mass" is simply a means of encoding the notion that the particle travels at the speed of light and carries energy. Gluons have zero rest mass. Presumably a graviton, if it exists, is also a particle of zero rest mass.
    How about W bosons? They, I believe, have rest mass.
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  15. #14 Re: photons and neutrinos 
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    Hi there. May I add something to the discussion?

    Quote Originally Posted by greymatt
    As I understand, photons have mass (even though they're not Catholic, sorry), so what is the weight of photons hitting the earth? If a photon behaves like a wave does it still have mass?
    Photons have no weight. They may have energy that could be transformed into mass to build different particles. The momentum of a photon is given by de Broglie's equation:

    if you substitute

    So, the momentum of a photon depends on its wavelength or energy, not its velocity. This also demonstrates the connection between the wave and particle natures of photons on all other particles. Every particle has both particle AND wave characteristics. The most prominent example are electrons. Why should it be a contradiction? Particles are both.

    Another small calculation: We know the radiative power of the sun reaching the earth, aka the solar constant, is 1367 W/m^2. We can now calculate the force of the solar photons hitting the earth per square metre.

    The force is defined as the time derivative of the momentum:

    We can substitute E with the solar constant leading to:
    ,
    which is the force on one square metre. Not very much. This is similar to the weight of 0.5 milligrammes on the earth's surface.

    Quote Originally Posted by greymatt
    I assume that photons hitting the earth now originated in the sun's core, but how long ago has it taken for them to tunnel out?
    Correct. They originate from the sun's core. Although they only need about 8 minutes to reach the earth from the sun's surface, it takes much longer to pass the comparably small distance from the core to the surface. Sure, some fraction of the photons are fully absorbed and heat the solar plasma. They do not tunnel, but are mostly scattered at the electrons of the hydrogen plasma. With each of those scatterings they lose some energy. This leads to the "cooling" of the photons from the core (15 million K) to the surface (6000 K). The time scale for such a travel is not entirely clear, but based on random walk calculations it can take between several thousand years up to millions of years.
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    Quote Originally Posted by drowsy turtle
    Quote Originally Posted by DrRocket
    "Zero rest mass" is simply a means of encoding the notion that the particle travels at the speed of light and carries energy. Gluons have zero rest mass. Presumably a graviton, if it exists, is also a particle of zero rest mass.
    How about W bosons? They, I believe, have rest mass.
    Yes, they do. So does the other boson that participates in the weak interaction, Z.
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  17. #16 Re: photons and neutrinos 
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    Quote Originally Posted by Dishmaster
    Photons have no weight.
    What does this mean ?

    They have momentum.

    They have energy.

    They participate in the curvature of space-time and therefore can exert the force that we call gravity.

    If you bang two of them together of sufficient energy you can produce something that has rest mass. It itakes some pretty energetic photons to do this.

    If you somehow managed to put one on an ideal balance (very ideal), by having it absorbed by a platform, the platform would register an increase in mass and hence in weight consistent with and . If you perform this calculation you find that because c is very large and h is very small the amount of mass associated with a photon of normal frequencies is extremely small by macroscopic standards, but the principle remains.
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  18. #17 Re: photons and neutrinos 
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by Dishmaster
    Photons have no weight.
    What does this mean ?

    They have momentum.

    They have energy.

    They participate in the curvature of space-time ...
    Agreed.
    Quote Originally Posted by DrRocket
    ... and therefore can exert the force that we call gravity.
    How? During my many years of studying physics, I never came across such an idea. Do you have a reference for that?
    Quote Originally Posted by DrRocket
    If you bang two of them together of sufficient energy you can produce something that has rest mass. It itakes some pretty energetic photons to do this.
    Yes, but this is something else. You turn energy into mass.
    Quote Originally Posted by DrRocket
    If you somehow managed to put one on an ideal balance (very ideal), by having it absorbed by a platform, the platform would register an increase in mass and hence in weight consistent with and . If you perform this calculation you find that because c is very large and h is very small the amount of mass associated with a photon of normal frequencies is extremely small by macroscopic standards, but the principle remains.
    Isn't that the radiative pressure that is produced by the momentum of the photons? Again, I have never heard of this. Are you sure? The radiation energy is turned into heat, isn't it?
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    Presumably the energy-density induced by an EM field would contribute to the stress-energy tensor, and so would contribute to the gravitational field. However, I doubt that the contribution would be very large.

    I say "energy from the EM field" rather than "energy from the photon", because saying the word "photon" implies that we're dealing with a quantum theory of light. In which case, you can't speak of how the photon interacts with the gravitational field unless you have a quantum theory of gravity as well.

    EDIT: wrote something grossly wrong after this, which I have removed.
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    If this is true, a photon with a wavelength of 550 nm (visual light, green) would have a "mass" of 4.0 x 10^-36 kg, which is about 4.4 x 10^-6 electron masses. According to this neutrino mass estimate this is about 50 times more than the lower limit of the neutrino mass eigenstate. Not really a low value in these standards. Could this contribute to the "Dark matter" hypothesis?
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  21. #20 Re: photons and neutrinos 
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    Quote Originally Posted by Dishmaster
    Quote Originally Posted by DrRocket
    ... and therefore can exert the force that we call gravity.
    How? During my many years of studying physics, I never came across such an idea. Do you have a reference for that?

    Electromagnetic energy is included in the stress-energy tensor of general relativity, which is what determines curvature of space-time, which is what we call gravity.

    John Archibald Wheeler studied gravitational structures, that he called geons that could, in principle be formed from electromagnetic energy along, or in fact from gravity alone. Weird stuff, and it takes a LOT of electromagnetic energy to pull this off.

    http://en.wikipedia.org/wiki/Geon_(physics)

    Quote Originally Posted by Dishmaster
    Quote Originally Posted by DrRocket
    If you bang two of them together of sufficient energy you can produce something that has rest mass. It itakes some pretty energetic photons to do this.
    Yes, but this is something else. You turn energy into mass.
    It is just one manifestation of the fact that mass and energy are two aspects of one thing. That is why the mass of elementary particles is often quoted in electron volts.

    The key is to recognize that one does not turn mass into energy or energy into mass, you just change the form in which energy (or mass) is realized.

    One way to think about this is to recogize that there is no conservation of energy and no conservation of mass separately, just conservation of mass/energy. Once we agree that mass and energy are the same thing, then the conservation laws become simple again.


    Quote Originally Posted by Dishmaster
    Quote Originally Posted by DrRocket
    If you somehow managed to put one on an ideal balance (very ideal), by having it absorbed by a platform, the platform would register an increase in mass and hence in weight consistent with and . If you perform this calculation you find that because c is very large and h is very small the amount of mass associated with a photon of normal frequencies is extremely small by macroscopic standards, but the principle remains.
    Isn't that the radiative pressure that is produced by the momentum of the photons? Again, I have never heard of this. Are you sure? The radiation energy is turned into heat, isn't it?
    Yes the radiation energy is turned into heat. But that means that the atoms are in an excited state, or equivalently have more kinetic energy of motion and therefore gain mass in accordance with . If you take a closed bucket of junk and heat it up, it will weigh more after you heat it than before. Not much more.

    The effect is not radiation pressure, since I am talking about what happens from the absorption of some fixed quantity of radiative energy (photons) and not a continuous flux. And I am talking about an actual mass increase.

    One way you might think about it is this. I sit in a laboratory, "at rest", with a closed bucket of some gas. I weight it (measure the mass) and then I heat it up. The molecules in the gas move faster, relative to my laboratory reference. So, following special relativity, in the laboratory reference frame, the mass of the molecules is larger due to the higher relative motion. The net result is that the hot bucket weighs more.
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    If the unit of electromagnetic force is the photon, what is the neutrino the unit of?
    Is it possible that even the value of c is variable on a vast time scale, meaning that nothing is constant in the past, present and future of the universe?

    'Photons have mass? I didn't even know they were catholic'. (Woody Allen)
    'Always look on the light side of life'. (Einstein)
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    Quote Originally Posted by greymatt
    If the unit of electromagnetic force is the photon, what is the neutrino the unit of?
    Is it possible that even the value of c is variable on a vast time scale, meaning that nothing is constant in the past, present and future of the universe?

    'Photons have mass? I didn't even know they were catholic'. (Woody Allen)
    'Always look on the light side of life'. (Einstein)
    The photon is a boson (a carrier of force). The neutrino is not. The neutrino is a lepton.
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by greymatt
    If the unit of electromagnetic force is the photon, what is the neutrino the unit of?
    Is it possible that even the value of c is variable on a vast time scale, meaning that nothing is constant in the past, present and future of the universe?

    'Photons have mass? I didn't even know they were catholic'. (Woody Allen)
    'Always look on the light side of life'. (Einstein)
    The photon is a boson (a carrier of force). The neutrino is not. The neutrino is a lepton.
    I don't think the word "boson" itself implies that the particle is a force carrier. Although I suppose when it comes to elementary particles, that would be the case, wouldn't it?
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    The photon is a boson (a carrier of force). The neutrino is not. The neutrino is a lepton.
    What is the spin for the photon?
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    Quote Originally Posted by salsaonline
    Quote Originally Posted by DrRocket
    Quote Originally Posted by greymatt
    If the unit of electromagnetic force is the photon, what is the neutrino the unit of?
    Is it possible that even the value of c is variable on a vast time scale, meaning that nothing is constant in the past, present and future of the universe?

    'Photons have mass? I didn't even know they were catholic'. (Woody Allen)
    'Always look on the light side of life'. (Einstein)
    The photon is a boson (a carrier of force). The neutrino is not. The neutrino is a lepton.
    I don't think the word "boson" itself implies that the particle is a force carrier. Although I suppose when it comes to elementary particles, that would be the case, wouldn't it?
    The elementary particle classification uses "boson" to describe force carriers -- photon, gluon, Z,W.

    This Wiki article has a nice chart that helps to keep the zoo straight.

    http://en.wikipedia.org/wiki/Elementary_particle
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    Quote Originally Posted by Liongold
    The photon is a boson (a carrier of force). The neutrino is not. The neutrino is a lepton.
    What is the spin for the photon?
    1
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    Thanks to all who have replied.
    However I still have doubt about the nature of photon mass.
    Might the solution be the Higgs Field?
    I mean if a photon does have mass could it get it from the theoretical Higgs?
    Therefore no rest mass.
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    The Higgs mechanism is supposed answer the following question: the carriers of the weak force should come from a gauge-invariant theory in order for the overall theory to be renormalizeable. However, gauge theories appear to require the underlying force carriers to be massless. We know that the weak-force carrying particles have mass. So how does this happen? That's what the Higgs mechanism addresses.

    Now, if we have a gauge theory of a massless particle, like a photon, there's no need to ask questions like, "where does the mass (i.e. rest mass) of the photon come from?" The photon has no rest mass, experimentally and theoretically. So no need to bring in the Higgs mechanism.
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    So if the boson is a force carrier, then can I assume the Higgs boson is also a force carrier? I understand the Higgs field is supposed to pervade the whole universe and give everything mass.
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    Boson is not synonymous with "force carrier". It just so happens that the force carriers we know about happen to be bosons.

    Is it really true that the Higgs field endows ALL matter with mass? I was under the impression that the Higgs boson endowed certain vector bosons with mass. I was not aware that it endowed fermions with mass. I'd be interested in seeing an explicit reference to that effect if that is the case.
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    Upon some further research, it does it indeed appear that the Higgs mechanism is supposed to explain the mass of fermions as well. Would still like to see an official reference on it. (I found someone talking about this on a yahoo answers post, though the person appears to know what they're talking about.)
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    I was under the impression that physicists are moving away from talking about rest mass and relativistic mass and that mass increase due to relative motion is no longer thought of as mass increase per se, but an increase in kinetic energy that manifests as an increase in inertia? How closely related are inertia and gravity?
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    Okay, but we were talking about rest mass, which is still a relevant concept in physics.
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    Incidentally, what do physicists mean when they say a particle is predicted to have mass? It means that in the formula for the Lagrangian, the relevant quantum field has a quadratic term of the form:



    or something like that (if the field is complex, we'd take the norm squared, etc.).
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    Quote Originally Posted by KALSTER
    I was under the impression that physicists are moving away from talking about rest mass and relativistic mass and that mass increase due to relative motion is no longer thought of as mass increase per se, but an increase in kinetic energy that manifests as an increase in inertia? How closely related are inertia and gravity?
    Someone has been reading Wiki too uncritically I think. Not all physicists are of that mind by a long shot. Read a bit more closely and you will find that even those that don't care to use the term relativisitic mass, wind up using the concept under a different name, and it is more closely related to momentum than energy.

    What is inertia ? mass ?

    Then gravity and mass are very closely related. Mass and energy are the same thing. So gravity is also closely related to energy. Mass, energy, prressure all enter into the stress-energy tensor of general relativity and all are part of what determines the curvature of space-time, which is what is called "gravity".

    Does the "relativistic mass" participate in gravity ? Absolutely.

    In fact light can produce space-time curvature, gravity, and it has zero rest mass. If there is a graviton, it will hve zero rest mass, and gravity can produce gravity. Google "geons" and "vacuum solutions to general relativity".
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    Someone has been reading Wiki too uncritically I think.
    I think I heard it from Janus actually, but thanks for the explanation. I will look into those terms, thanks. :wink:
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    The universe is 13.7 billion years old.
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    How do you know the age of the universe for sure? Do they measure earth years at its edge? Is the speed of light a universal constant? Are some stars artificial?
    Do antimatter stars and galaxies distort the estimate of 13.7bn? Does the universe run on a computer program? Is the earth really a quarantine for a computer virus contracted elsewhere in the uni/multiverse? After all DNA is just an information flow. Or is the virus a memetic one which produces the contagion of war and self destruction unwanted in a truly civilised world? Maybe we really are being watched.
    The virus of faith has infected millions into believing that the world is only a few thousand years old, and also the universe. All of this might explain why our planet is positioned at the edge of nowhere. Chilling stuff, but if you think I'm making this up, read the cutting edge books by Marcus Chown.

    'All these 50 years of conscious brooding have brought me no nearer to the answer to the question 'what are light quanta?' Nowadays every Tom, Dick or Harry thinks he knows it, but he is mistaken.' (Einstein)
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    WMAP evidence shows the universe to be 13.7 billion years old plus or minus 2%.
    I read on article on science daily in which one scientist puts the odds of the universe being a simulation at about 1/3. I'll try and find the article.
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    Quote Originally Posted by KALSTER
    Someone has been reading Wiki too uncritically I think.
    I think I heard it from Janus actually, but thanks for the explanation. I will look into those terms, thanks. :wink:
    In this, I think Wiki represents the consensus of the scientific community.

    This has also been a soapbox of my own. I have argued before that this concept of relativistic mass is pretty nonsensical and highly misleading. Sure scientists have used the concept just as they have used the concept of cetrifugal force (the centerfuge has its name from this). But I think it is clear that in an effort to teach the concpets of SR in a clear and consistent manner, this concept of relativistic mass is no better than that of centrifugal force.
    See my physics of spaceflight simulator at http://www.relspace.astahost.com

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    Marcus Chown
    I'll look into his stuff, but, no offence, judging from your description he sounds to be pretty far removed from "ground breaking".

    Edit: After looking at a sample of his works he doesn't look to be a kook, but a pretty much renowned popularizer of science. Where does he discuss the questions you brought up?

    Mitch:

    Thanks. :? I must say that to my limited brain the Wiki/Janus/MitchellMckain version makes more sense. Learning is more interesting than knowing at the moment.
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    Books by Marcus Chown I recommend are 'The Universe Next Door' and 'The Never Ending days of Being Dead'. Some of it is speculation for sure, but he has spoken to a lot of scientists. His book 'Quantum Theory Cannot Hurt You' is a good read, as is 'The Magic Furnace'.

    The Catholic church asks 'Why is there anything at all?' (it is their proof of God), and this is the biggest organisation on the planet.

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    Quote Originally Posted by mitchellmckain
    Quote Originally Posted by KALSTER
    Someone has been reading Wiki too uncritically I think.
    I think I heard it from Janus actually, but thanks for the explanation. I will look into those terms, thanks. :wink:
    In this, I think Wiki represents the consensus of the scientific community.

    This has also been a soapbox of my own. I have argued before that this concept of relativistic mass is pretty nonsensical and highly misleading. Sure scientists have used the concept just as they have used the concept of cetrifugal force (the centerfuge has its name from this). But I think it is clear that in an effort to teach the concpets of SR in a clear and consistent manner, this concept of relativistic mass is no better than that of centrifugal force.
    Better leave Wolfgang Rindler, one of the more notable relativists, out of that consensus. Wiki does not represent a consensus, and sometimes does not even represent accuracy.

    Relativistic mass is nothing more and nothing less than the mass that one would attach to any moving object, moving with respect to the reference frame of the observer whos measure both mass and speed.

    It is a perfectly useful concept, not at all nonsensical and quite conventient for the theory of gravity. It is also quite useful in recognizing the equivalence of mass and energy. is valid if and only if m is relativistic mass.
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    Quote Originally Posted by DrRocket
    Relativistic mass is nothing more and nothing less than the mass that one would attach to any moving object, moving with respect to the reference frame of the observer whos measure both mass and speed.
    I don't. What I attribute to the moving object is kinetic energy which IS relative to the inertial frame and the mass which is NOT. It only seems quite natural to separate these two aspects of the particle.


    Quote Originally Posted by DrRocket
    It is a perfectly useful concept, not at all nonsensical and quite conventient for the theory of gravity.
    ...only for those confused about what is the current theory of gravity. Newtonian gravity depends on mass it is true, but Einstein Field equations depend on the stress energy tensor which is composed of energy density, momentum density, energy flux, momentum flux, shear stress, and pressure. What has relativistic mass to do with it? Nothing.


    Quote Originally Posted by DrRocket
    It is also quite useful in recognizing the equivalence of mass and energy.
    But that is the whole point. Mass and energy are not equivalent. Mass is simply one form of energy and it is a form of energy that is distinct from other forms of energy when this relativistic mass nonsense (which cannot distinguish mass from kinetic energy) is taken out of the picture.


    Quote Originally Posted by DrRocket
    is valid if and only if m is relativistic mass.
    The solar system has nine planets only if pluto is a planet. LOL Even if what you say is true it only amounts to a circular argument. But it is not true. The equation remains valid regardless. If the mass is rest mass then the energy is the rest mass energy and if the mass is "relativistic mass" then it is the rest mass energy plus the kinetic energy.

    The question is which clarification is more fruitful: Using for the mass energy, or using for the mass energy plus the kinetic energy. The latter is more informative since it tells us to look up the formula for as a function of velocity. It separtates the innate property of the object (in the mass) from the dependence on the inertial frame and it is hard to see a down side to that.

    Everywhere else the meaning of m is unambiguous. It is simply the mass which is an innate property of the object i.e. rest mass and not something which is relative to the inertial reference frame from which it is observed. We do use the symbol for rest mass occasionally but this is because the rest mass is important NOT because m is assumed to be relativistic mass. Relativistic mass has no such symbol for clarification, again not because m is assumed to be relativistic mass but simply because the concept of relativistic mass just isn't that important, and so you have to add the clarification in words that relativistic mass is what is meant in your equation. It is an uneccessary ambiguity for which is the perfect solution (eliminating the whole idea of relativistic mass altogether).

    Furthermore it is tied to a confusion of the momentum with mv. But while mv is the momentum of a massive particle when the m is the relativistic mass, the problem is that this does not work for massless particles which have momentum and an energy of motion despite the fact that they have no mass.



    Notice that many if not most of the references which introduce and explain this formula do not use in this equation even though the m in this equation is the rest mass NOT the relativistic mass. Using the relativistic mass instead simplifies this equation (for massive particles) to or but the question is whether this really aids in understanding or is a source of confusion and it is the consensus of those in science eduation is that all you get is a loss of generality and a source of confusion and is thus not helpful at all.
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    The reason I asked about how inertia and gravity is related, is because I am interested in finding out exactly how the properties of mass actually are related. What this concept of relativistic mass is an example of (or at least I think it is), is that the predictions of maths and what actually goes on does not necessarily have to be the same thing. One might choose to think that the object's mass actually increases or that its inertia increases due to momemtum increase, but the math predictions (at least in magnitude and expected observational result) work out exactly the same.

    Is inertia and mass the same thing, is inertia a property of mass? As I understand it, gravity is only determined by rest mass, so the gravity of a moving object does not increase in that reference frame, does it? If indeed this is not so, is this a consequence of there being no such thing as relativistic mass?
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    Quote Originally Posted by mitchellmckain

    ...only for those confused about what is the current theory of gravity. Newtonian gravity depends on mass it is true, but Einstein Field equations depend on the stress energy tensor which is composed of energy density, momentum density, energy flux, momentum flux, shear stress, and pressure. What has relativistic mass to do with it? Nothing.


    Quote Originally Posted by DrRocket
    It is also quite useful in recognizing the equivalence of mass and energy.
    But that is the whole point. Mass and energy are not equivalent. Mass is simply one form of energy and it is a form of energy that is distinct from other forms of energy when this relativistic mass nonsense (which cannot distinguish mass from kinetic energy) is taken out of the picture.
    This basically, er,.... wrong.

    Mass and energy are simply two aspects of one thing. There is a reason that in general relativity mass-energy is one word. And it is relativistic mass that is important in this duality -- you either get to write or you can write and the former is surely the more clear and informative.

    I suggest that you read Gravitation by Misner, Thorne and Wheeler, paying particular attention to chapter 5.

    The stress-energy tensor takes energy density, momentum density, and stress in one reference frame and, when applied as a quadratic form to the vector representing the velocity of an observer gives as output the mass-energy density as measured in the local frame of that observer. It is also precisely this tensor that determines the curvature tensor of space-time to in turn describe what we call gravity.

    Now, this tensor works on 4-vectors, and therefore mass-energy (a scalar) and momentum (a 3-vector) are combined in this formalism into a single 4-vector, but what comes through quite clearly is that it is total mass-energy and not "rest mass" that are important here. And total mass-energy is what is determined by the equation where m is the "relativistic mass". It is this expression for mass (relativistic mass), or equivalently energy, that enters into the stress-energy tensor to determine the curvature of space-time (aka gravity) and one must surely recognize that mass is indeed a significant driver of gravity -- Newton was really pretty close. To misinterpret general relativity so as to exclude mass (aka energy) from one's perspective of gravitational forces is to rather miss the entire point.

    Kinetic enegy has to be handled carefully. It is in fact a bit tricky to define kinetic energy in the context of general relativity. So trying to separate the mass effect is questionable at best. http://math.ucr.edu/home/baez/physic...energy_gr.html http://www.oakland.edu/physics/mog30/node11.html

    Special relativity is a bit simpler on that point.

    Even in special relativity if one want to maintain the usual Newtonian form of conservation of momentum one is forced to use relativistic mass. You can alternately opt to carry around like an albatross, but that is equivalent to simply recognizing that mass itself is dependent on the reference frame. See for instance Introduction to Special Relativity by Wolfgang Rindler. Rindler is most certainly not a member of your "consensus" of teachers. He is however, a world-renowned relativist.
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    You are making no sense at all. Why would you want to conflate mass with the energy momentum 4 vector? They are two different things. The importance of the energy momentum 4 vector certainly has nothing to do with the usefulness of this concept of relativisitc mass.

    We have long standing tradition of particles with mass and particles without mass and you want to rub all this out to say that they are the same thing? It makes no sense. This is why we have the two different words mass and energy, because they mean two different things. And the fact that we have these two different types of particles makes it quite clear that they are not the same thing. Both are forms of energy. Both have energy-momentum 4 vectors. But one type of particle has mass and one type of particle does not.

    Sure the Newtonian gravity is a good approximation but NOTHING I said excluded mass from the theory of gravity. The point was that mass is just one form of energy and it is the energy and momentum densities that are in the field equations NOT relativistic mass, so your claim this concept was essential to the theory of gravity was without foundation.

    Quote Originally Posted by DrRocket
    Even in special relativity if one want to maintain the usual Newtonian form of conservation of momentum one is forced to use relativistic mass. You can alternately opt to carry around like an albatross, but that is equivalent to simply recognizing that mass itself is dependent on the reference frame.
    Only if you want to ignore the massless particles again, which makes it quite clear that p = mv simple isn't a valid definition of momentum anymore, and you demonstrate WHY this insistence on relativistic mass is a holdover from an obsolete understanding of physics.
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    Quote Originally Posted by mitchellmckain
    You are making no sense at all....makes it quite clear that p = mv simple isn't a valid definition of momentum anymore, and you demonstrate WHY this insistence on relativistic mass is a holdover from an obsolete understanding of physics.

    It is really quite simple and not obsolete at all.

    Mass and energy are the same thing.

    That may not make sense to you. But that is not my problem.
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    DrRocket:

    If an object is accelerating away from you and approaching C, does the gravity you experience in its direction change in other than a relation as well? If not, why, since its relativistic mass is increasing? This is not a challange.
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    I can't quite make out what essential argument is being made here. I get the feeling that if it came down to computing an actual trajectory using GR, both DrRocket and mitch would arrive at the same conclusion, but would use different language to describe the mathematics they were using.

    Personally, I like to use the word "mass" only in reference to rest mass in special relativity. In other words, I like to think of rest mass as the norm^2 of the 4-momentum vector in an inertial reference frame. But if DrRocket feels more at home with the relativistic mass point of view, what do I care? These are just words we attach to terms that appear in equations. As long as we both agree that all observers are on equal footing and that the geometry of space-time is locally Lorentzian, we'll arrive at the same answers to physically meaningful questions.
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    Quote Originally Posted by KALSTER
    DrRocket:

    If an object is accelerating away from you approaching C, does the gravity you experience in its direction increase as well? If not, why, since its relativistic mass is increasing? This is not a challange.
    Yikes! Why would it be accelerating? I can think of two reasons. One is that it is in a gravitational field, the other is because it is expelling reaction mass. Sorry I am just wary of allowing things to be magically accelerating. I have no confidence that such will not lead to inconsistencies in physical theory.

    Wouldn't it just be simpler to compare the gravitational pull of an object in its rest frame with the gravitational pull of that same object in another inertial frame. In that case the stress-energy tensor would be different but only by an lorentz boost transformation. Hmmm...

    This seems to be related to a similar question about a high velocity mass particle becoming a black hole. The answer is obviously, no it does not.
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    Yikes! Why would it be accelerating? I can think of two reasons. One is that it is in a gravitational field, the other is because it is expelling reaction mass. Sorry I am just wary of allowing things to be magically accelerating. I have no confidence that such will not lead to inconsistencies in physical theory.
    Fair point.

    Wouldn't it just be simpler to compare the gravitational pull of an object in its rest frame with the gravitational pull of that same object in another inertial frame. In that case the stress-energy tensor would be different but only by an lorentz boost transformation. Hmmm...
    No idea what this means.

    This seems to be related to a similar question about a high velocity mass particle becoming a black hole. The answer is obviously, no it does not.
    Ok.

    Scenario 1:

    An object of mass , linear distance from the observer and relative speed of exerts a gravitational force of N on the observer.

    Scenario 2:

    An object of mass , linear distance from the observer and relative speed of exerts a gravitational force of N on the observer.

    Is ?
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    Since the field equations depend on the energy density, I have no doubt that the motion of the masses in the galaxy adds to its gravitational pull. But Kalister your senario is more difficult to answer. On the one hand you need to qualify it by saying you are comparing the instaneous force when the gravitational pull of the moving mass is at its maximum, which may not be at closest approach. And then since the gravitational effects are limited to the speed of light then you will also have effects that are similar to the aberation of light. I haven't done this problem in GR and I am no expert in GR so how these different effects should be sorted our are not obvious to me.

    BUT, I don't think this is really germain to the issue under discussion because again the gravitational effect is a function of the stress-energy tensor NOT the mass, relativistic or otherwise. Whatever the difference in the gravitational force it is due to the change in the stress-energy tensor not the relativistic mass.
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    Quote Originally Posted by KALSTER
    DrRocket:

    If an object is accelerating away from you and approaching C, does the gravity you experience in its direction change in other than a relation as well? If not, why, since its relativistic mass is increasing? This is not a challange.
    If an object is moving away from you , accelerating or not, then in your reference frame the mass is increasing (or if you like the energy is increasing) and that causes the gravitational effedt to increase. If it is approaching c then the increase is quite large.

    However, the way in which you phrased the question is quite complicated since you are using acceleration and thereby also talking about a mass increase with time. I don't see the details here clearly since there is also an issue of the propagation of graviational waves involved and that gets rather complicated. It becomes even more complicated since you are now talking about a presumably somewhat distant event in a hightly curved space-time -- and now even the definitions of "space" and "time" are not so clear.
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    Quote Originally Posted by mitchellmckain
    BUT, I don't think this is really germain to the issue under discussion because again the gravitational effect is a function of the stress-energy tensor NOT the mass, relativistic or otherwise. Whatever the difference in the gravitational force it is due to the change in the stress-energy tensor not the relativistic mass.
    Yep, it is the stress-energy tensor, also called the momentum tensor, that determines curvature. Said that several times.

    And energy density is a major factor. Energy density is the same thing as mass density since energy and mass are the same thing. And THAT is how mass enters the problem. Moreover, as the success of Newtonian gravity will tell you, mass is usually the dominant term in the stress-energy tensor. Moreover, the other terms are really just mass-energy recognized in somewhat different contexts.

    Yep you can make exotic gravitational fields without ordinary mass. You can even, at least in principle, construct a field out of gravitity itself. Or light. See "geons" as studied by John Archibalc Wheeler. And why can you do this ? It is because light has energy, gravitational field have energy, and energy and mass are the same thing. Potential energy is energy too -- so stress-strain and pressure are also related to energy density.

    It is nothing but mass-energy and turtles all the way down.

    It doesn't matter whether you call it "mass", "energy" or "Oscar". Mass and energy are the same thing. You can fool around with the words if you like, and you can limit the word "mass" to "rest mass" if you like as well. But it all turns out to be one idea going by different names. It all depends on whether you want to tote around and all sorts of aliases for energy.

    Rest mass is not so simple as you might think, at least at the macroscopic level in which gravity is usually a major consideration.

    Consider this, once again. Take a container of gas. Weigh it (on an infinitely accurate scale).. Then heat it up. It will weigh more, precisely because it has higher energy content and therefore greater mass. The molecules are whizzing around and therefore gain mass. This same principle applies to any object.

    Now, ask yourself how you would every measure the "rest mass" of a macroscopic object, one made of real atoms and molecules. The rest mass would have to be measured at (let's ignore quantum effects for a moment) absolute zero. Classical thermodynamics tells you that you can't there from here. Even a body "at rest" is not really at rest when you look at the constituent atoms. [/tex]
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    I meant rest mass of a fundamental particle. And interesting observation--I make no claims about what these concepts mean outside the context of, say, quantum electrodynamics (so no gravity).
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    Nuclear isomers are heavier than the ground state versions. Is this an example of compact energy = mass?
    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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  59. #58  
    Forum Radioactive Isotope mitchellmckain's Avatar
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    Quote Originally Posted by DrRocket
    Rest mass is not so simple as you might think, at least at the macroscopic level in which gravity is usually a major consideration.
    Oh really? LOL I was talking about the mass of those elementary particles that have mass compared to those elementary particles that do not. In what way is their rest mass "not so simple as" I think? LOL

    It is a reasonable supposition that all energy is ultimately some kind of motion since in string theory even the elementary particles themselves are vibrational modes in the geometry of a higher dimensional space time. Is that what you were thinking of? LOL

    But of course you are right, that whenever you use gravitational force in order to measure the mass of something (via a Newtonian formula) then all the effects of the stress-energy tensor are bound to contribute to the mass that you calculate.


    The advantage of is that it seperates out the dependence on the inertial frame from the mass as an innate property of the object, which as I said is an advantage with no down side. You can of course leave the at home when don't need it. LOL





    P.S. Your "infinitely accurate scale" was entertaining.
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  60. #59  
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    Quote Originally Posted by DrRocket
    Now, ask yourself how you would every measure the "rest mass" of a macroscopic object, one made of real atoms and molecules. The rest mass would have to be measured at (let's ignore quantum effects for a moment) absolute zero. Classical thermodynamics tells you that you can't there from here. Even a body "at rest" is not really at rest when you look at the constituent atoms. [/tex]
    I know that in SR, the conserved current that corresponds to Lorentz boosts is the "center of energy", an analogue to center of mass. So possibly this is the best way to assign a meaning to the mass of a system of particles. I don't know to what extent this continues to work in GR, but it's a start toward answering your question.

    mitch: I understand your gripes about thought experiments involving "infinitely accurate scales" and "accelearating objects." At the same time, give me a break.
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  61. #60  
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    Quote Originally Posted by mitchellmckain
    Quote Originally Posted by DrRocket
    Rest mass is not so simple as you might think, at least at the macroscopic level in which gravity is usually a major consideration.
    Oh really? LOL I was talking about the mass of those elementary particles that have mass compared to those elementary particles that do not. In what way is their rest mass "not so simple as" I think? LOL
    Yes really. I doubt seriously that you know what you were talkiing about.

    Rest mass is not so simple as you think (assuming that you do think) for precisely the reasons that I pointed out. It is rather impossible to bring a system of many elementary particles to "rest" simulataneously. Mass, is not an invariant. Rest mass may be an invariant, but it is not something that is realizable for a large system of particles.

    I suggest that you do a bit less LOL and a bit more of that "thinking" stuff. Try it, you might like it.
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