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Thread: light speed

  1. #1
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    Okay, the reason we can not go FTL is because the faster we go, the more mass we get, the more energy we need.

    So, if understand correctly, photons have mass. That being the case what stops photons from gaining mass, needing more energy etc etc.

    Thanks in advance

    Glen

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    On 2002-11-07 07:48, southern_musca wrote:
    So, if understand correctly, photons have mass.
    They don't.

    Welcome to the board.

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    A gracious constellation, (southern) Musca.

    Welcome in.

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    On 2002-11-07 07:48, southern_musca wrote:
    So, if understand correctly, photons have mass.
    They don't.

    Welcome to the board.
    Isn't there an in-vogue quantum theory that declares photons to have imaginary mass or something? All that quantum physics stuff makes my head spin, and I don't even know that much about it.

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    Well, there is rest mass and mass associated with velocity. Einstein defined the equivalency of mass and energy with his famous E = mc^2. Photons have an energy E = hc/L where L is their wavelength. For photons which have a really short wavelength such that their energy equals twice that of an electrons mass a funny thing happens. That is this. When such a high energy photon hits a target like an electron the photon ceases to exist! It converts into an electron plus an anti-electron also called a positron. Each of these particles have a mass, the sum of their masses being the same as the mass in E = mc^2 where E is the energy of the photon. So the photon seems to have mass after all! Now for General Relativity (GR). Einstein generalized his special relativity that dealt with relative velocities to go to the next derivative and deal with frames of reference accelerating relative to each other. He realized that gravity showed such acceleration. He made a famous thought experiment. Assume you are in an elevator way out in space between galaxies where there is no gravity. Now assume the elevator is accelerating. Shoot a bullet at the wall of the elevator and you will see that the bullet falls by the amount of the acceleration during the time of flight according to d = 1/2 at^2. Now shoot a laser at the same target and you will also see that the light falls by an amount also. Thus Einstein said that the accelerating frame of reference was indistinguishable from gravity. This is the principle of equivalence, sometimes called the equivalence of inertia and gravitational mass, but it's really the equivalence of gravity with an accelerating frame . Then he made his GR become not only a theory of the relativity of accelerating frames, but also a theory of how particles and light work in gravity. So if the light falls in gravity it has an eqivalent mass?? Now the idea that light falls in gravity is generally accepted by just about everyone but me. For entirely different reasons i think that you should be able to distinguish between an accelerating frame and gravity for light. I happen to hold, as a result of my big bang is wrong ideas, that gravity is a push of electromagnetic radiation that has been red shifted to extremely long wavelengths. Thus I think that Einstein was partly right; the bullet's fall in the elevator is indistinguishable from gravity, but the light won't fall in gravity unless that long wavelength electromagnetic radiation pushes on photons just like it does on particles. Now the evidence for falling photons is not totally convincing. It's a very small effect. The aberration of starlight from the sun and so-called gravitational lensing is the primary evidence. But these effects could just as well be due to conventional optics as the light passes through a thicker medium (like the sun's atmosphere or a galaxy) just ordinary optical lensing like light passing through a fishbowl. So, to my mind, whether light has gravitational mass or not is an open question and I truly doubt it has. It certainly has an equivalent mass from special relativity as evidenced by electron-positron pair formation.

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    On 2002-11-07 10:30, John Kierein wrote:
    It certainly has an equivalent mass from special relativity as evidenced by electron-positron pair formation.
    Equivalent mass is different from mass, and anyway, it's finite.

    All the rest of that is against the mainstream, don't you agree?

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    Only my part is against the mainstream. Einstein's stuff was against the mainstream when it was proposed, now it isn't anymore.

    What is rest mass? If you stop a photon and put it at rest, it has mass. Electrons and positrons.

    <font size=-1>[ This Message was edited by: John Kierein on 2002-11-07 10:58 ]</font>

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    On 2002-11-07 10:55, John Kierein wrote:
    Only my part is against the mainstream. Einstein's stuff was against the mainstream when it was proposed, now it isn't anymore.
    Now is now. I thinkk.
    What is rest mass? If you stop a photon and put it at rest, it has mass. Electrons and positrons.
    But it's no photon then.

    <font size=-1>[Fixed quote BBcode]</font>

    <font size=-1>[ This Message was edited by: GrapesOfWrath on 2002-11-07 13:50 ]</font>

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    John, the system of two photons degrading into an electron and a positron will still have zero restmass in the center of momentum frame. This is the peculiar thing about relativistic particle physics: quantities are conserved even when it doesn't seem that they are.

    I appreciate, John, the honesty with which you posted your hypothesis and where you carefully laid out where you differed from the mainstream. Of course, in order for you to be honest, I should ask for your calculations of the refraction indices needed for the effects of lensing to be not due to gravity. Also, could you explain weak lensing to us? Thanks.

    As to the original question, the relativistic mass of the particle depends upon what the "rest mass" of the particle is. It is true that with a non-zero restmass a particle increases its relativistic mass due to the Theory of Special Relativity. However, a particle with a zero rest mass by definition travels at the speed of light and still has a zero rest mass. It's all very convient and works because of the way relativistic mass and rest mass are defined. Right now, we believe that rest mass and gravitational mass are equivalent (Einstein's Equivalence Principle), but that there are some other considerations for gravity too which is why light should bend. It would be hard to argue against this without throwing out gravity as an independent tensor force all together (as what John K. has done). If this is true, that it's not a tensor force, the onus is on John to show why Einstein's GR works so well for, say, the precession of Mercury's orbit. Also, if someone ends up observing any gravitational waves John's idea will be shot out of the water.

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    JS: You don't understand electron positron pair formation! A single gamma ray becomes an electron and a positron. The rest mass divided by c^2 plus the kinetic energy of the two particles equals the energy of the photon . It's e = mc^2 in accordance with special relativity. Go back and read up on physics.
    However, it is also true that Mcdonald of Princeton et al has been able to create pairs with multiple photons. The total mass created equivalent energy plus the kinetic energy of the particles always equals the sum of the energies of the original photons. I agree that mass-energy is certainly conserved. The "stopped" photon converts its energy to mass.

    <font size=-1>[ This Message was edited by: John Kierein on 2002-11-07 13:41 ]</font>

    <font size=-1>[ This Message was edited by: John Kierein on 2002-11-07 14:15 ]</font>

    <font size=-1>[ This Message was edited by: John Kierein on 2002-11-07 14:16 ]</font>

    <font size=-1>[ This Message was edited by: John Kierein on 2002-11-07 14:18 ]</font>

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    On 2002-11-07 10:30, John Kierein wrote:
    Now the evidence for falling photons is not totally convincing. It's a very small effect. The aberration of starlight from the sun and so-called gravitational lensing is the primary evidence. But these effects could just as well be due to conventional optics as the light passes through a thicker medium (like the sun's atmosphere or a galaxy) just ordinary optical lensing like light passing through a fishbowl. So, to my mind, whether light has gravitational mass or not is an open question and I truly doubt it has. It certainly has an equivalent mass from special relativity as evidenced by electron-positron pair formation.
    John, how far out would you expect atmospheric refraction effects to occur?

    Hipparcos results are corrected for gravitational bending out to a large angle. The effect from Jupiter was detected by Hipparcos, but I can't find the reference. Gaia, with much higher accuracy will be able to actually test GR.


    http://astro.esa.int/SA-general/Proj...L/node141.html

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    On 2002-11-07 13:38, John Kierein wrote:
    JS: You don't understand electron positron pair formation! A single gamma ray becomes an electron and a positron.
    This is true only when you are interacting with a different center-of-momentum frame. To get "real" electron positron pairs to exist you need to have two gamma rays ala the decay of an electron and a positron. This is simply so you can conserve momentum, if nothing else. I will give you that the majority of the ways gamma rays with energies above 1 MeV or so interact with matter is through electron-positron pair production.

    However, it is also true that Mcdonald of Princeton et al has been able to create pairs with multiple photons.
    McDonald is the person who whipped my electrodynamics into shape last year. Brilliant guy.

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    It looks like the data is from grazing starlight deflections from the planets and Viking radio data from the sun. Hipparcos is unlikely to work close to the sun (HST can't look towards the hardly sun at all, and it has a very good sunshade). But hipparcos may have measured deflections from other grazing incidence target. It's hard to say what the deflections should be from optics. I have to wave my hands here. But the Viking data has been explained as interaction with the solar plasma and is somewhat controversial. (I think one such writeup was made by Paul Marmet).

    I remain open minded on photon-photon interactions; there has been a long history of discussion about such possibilities with some even ascribing such interactions as causative of the red shift. ( have a hard time with that, myself, since I don't see how there would be a prepnderence of shifts to the red. I think there should be an equal amount to the blue if such interactions were occurring.)
    It's unfortunate that it is not easy to separate a light bending from atmospherics as compared to a possible gravitational effect, because wherever gravity has a high gradient there are also atmosphere gradients caused by the gravity, too.
    One might think this could be observed on the microscopic level near heavy atoms since we have detectors that can actually count photons, but apparently it's not that easy, since we get such things as compton scattering, etc. when light grazes atoms.

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    On 2002-11-07 17:19, John Kierein wrote:
    It looks like the data is from grazing starlight deflections from the planets and Viking radio data from the sun. Hipparcos is unlikely to work close to the sun (HST can't look towards the hardly sun at all, and it has a very good sunshade). But hipparcos may have measured deflections from other grazing incidence target.
    What table are you looking at? The one I see lists a 10mas deflection at 45 degrees from the sun. Hipparcos had ~1-2mas resolution, and they clearly state that correction due to GR effects is required.

    http://adsabs.harvard.edu/cgi-bin/np...dcab987c624008

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    On 2002-11-07 11:25, JS Princeton wrote:
    Right now, we believe that rest mass and gravitational mass are equivalent (Einstein's Equivalence Principle), but that there are some other considerations for gravity too which is why light should bend.
    Now that's what I call aggressive ignorance. Gravity couples to the total energy as (mc^2+K+U), where K/U are kinetic/potential energies of the system.
    That's truly disgusting to have your primitivist views cast here as a gospel, get them back to where they belong already!

  16. #16
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    Thanks for the responses, and thanks also for the warm welcome

    Glen

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    Also, if someone ends up observing any gravitational waves John's idea will be shot out of the water.
    That's already been done by inference. Check out the work that Joseph Taylor and Russell Hulse won their nobel prize for. The found speed of the inspiral of two neutron stars could only be explained by the push back of gravitational waves on the stars as gravitational waves are emmited from the system.

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    My, my, where to start to correct your misconceptions.

    On 2002-11-07 10:30, John Kierein wrote:
    Well, there is rest mass and mass associated with velocity.
    Not quite. There is rest mass and an energy associated with a system.

    Einstein defined the equivalency of mass and energy with his famous E = mc^2. Photons have an energy E = hc/L where L is their wavelength.
    The real equation is E^2 = (mc^2)^2 + (pc)^2
    where E is the total enery, m is the rest mass, and p is the momentum of the system.
    Particles with a rest mass of zero move at the speed of light and the momentum is defined as p = hf/c and f/c is the wavelength and h is Plank's constant.
    For photons which have a really short wavelength such that their energy equals twice that of an electrons mass a funny thing happens. That is this. When such a high energy photon hits a target like an electron the photon ceases to exist!
    ANY photon hitting another particle will cease to exist. Not just high energy photons.

    It converts into an electron plus an anti-electron also called a positron. Each of these particles have a mass, the sum of their masses being the same as the mass in E = mc^2 where E is the energy of the photon.
    It will only convert into a electron-positron pair if the mass of the particles plus the momentum of the particles is less then the energy in the photon.

    So the photon seems to have mass after all!
    Nope, photons have no mass, they have momentum.

    Now for General Relativity (GR). Einstein generalized his special relativity that dealt with relative velocities to go to the next derivative and deal with frames of reference accelerating relative to each other. He realized that gravity showed such acceleration.
    Sorry, gravity does not show an acceleration (you can't accelerate something that is moving at c), it causes an acceleration.

    He made a famous thought experiment. Assume you are in an elevator way out in space between galaxies where there is no gravity.
    Sorry, gravity is everywhere, do you mean the elevator is in free fall?

    Now assume the elevator is accelerating. Shoot a bullet at the wall of the elevator and you will see that the bullet falls by the amount of the acceleration during the time of flight according to d = 1/2 at^2. Now shoot a laser at the same target and you will also see that the light falls by an amount also.
    Sorry, light, bullets, etc. do not fall due to gravity or accelerating frame. They follow a geodesic of spacetime.

    Then he made his GR become not only a theory of the relativity of accelerating frames, but also a theory of how particles and light work in gravity.
    Einstein did not "make" GR into a theory of how particles and light work in gravity, GR describes how particles and light work in the presence of gravity.

    So if the light falls in gravity it has an eqivalent mass??
    Now the idea that light falls in gravity is generally accepted by just about everyone but me.
    Light does not fall in the presence of gravity, it follows a geodesic of spacetime.

    For entirely different reasons i think that you should be able to distinguish between an accelerating frame and gravity for light
    .

    Can you explain the reasons?

    I happen to hold, as a result of my big bang is wrong ideas, that gravity is a push of electromagnetic radiation that has been red shifted to extremely long wavelengths.
    Gravity (as describe by GR) is a warpage (or curvature) of spacetime. If a quantum theory of gravity is every discovered, the mediating boson would be the graviton, not the electron of EM radiation. The spin of the photon is 1 and the graviton would have a spin of 2. Check the work that won Joseph Taylor and Russell Hulse their Nobel prize.
    It showed the only way the inspiral of a pair of neutron stars could be explained was by gravitational waves pushing back on the stars as the waves are emmitted. EM cannot explain the inspiral.

    Now the evidence for falling photons is not totally convincing. It's a very small effect.
    It's a very small effect only near regular stars. Near black holes, the effect becomes so great the the cuvature traps the photon below the event horizon.

    The aberration of starlight from the sun and so-called gravitational lensing is the primary evidence. But these effects could just as well be due to conventional optics as the light passes through a thicker medium (like the sun's atmosphere or a galaxy) just ordinary optical lensing like light passing through a fishbowl.
    The amount of displacement would not be the same, on a consistant basis, for those effects you mentioned. The displacemet matches the GR calculations.

    So, to my mind, whether light has gravitational mass or not is an open question and I truly doubt it has.
    It certainly has an equivalent mass from special relativity as evidenced by electron-positron pair formation.
    Not sure what you mean here. Photons have no rest mass, but they do have an energy which can be converted into a particle pair, provided it has enough energy.

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    Quote
    What table are you looking at? The one I see lists a 10mas deflection at 45 degrees from the sun. Hipparcos had ~1-2mas resolution, and they clearly state that correction due to GR effects is required.
    quote

    Sorry. I stand corrected. Thanks. I now remember reading this a while ago, too. Even 45 degrees from the sun there is a gradient in the atmosphere, but perhaps this is becoming to be more definitive since at that angle there are large time variations in the density due to solar wind variations, etc. which would cause noise in the deflection data. Hipparcos is a remarkable engineering feat! A milliarcsecond is really, really small. I wish they could've looked at lots of quasars with it!

    As for gravity waves, I predict them to travel at c since I think gravity is the shadow cast by a mass in the isotropic long wavelength background. I certainly expect gravity waves to exist. In my view the graviton is the quantization of this shadow like a semiconductor "hole" is the absence of electron in a semiconductor.

    <font size=-1>[ This Message was edited by: John Kierein on 2002-11-08 08:43 ]</font>

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    On 2002-11-07 19:15, AgoraBasta wrote:
    Now that's what I call aggressive ignorance. Gravity couples to the total energy as (mc^2+K+U), where K/U are kinetic/potential energies of the system.
    That's truly disgusting to have your primitivist views cast here as a gospel, get them back to where they belong already!
    I don't know whether I should laugh or cry at such a post. Surely you realize that "mass" is only defined in the rest-mass, newtonian gravitational, and inertial possibilities. One of the reasons GR works is because it arrives at equivalency between those various items in the Newtonian limit.

    However, I suppose you don't need that as a constraint per se on the theory since its reduction is gauranteed by the tensor arithmetic.

    John... you still haven't commented on weak lensing. You just don't think it exists?

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    On 2002-11-07 07:48, southern_musca wrote:
    Okay, the reason we can not go FTL is because the faster we go, the more mass we get, the more energy we need. So, if understand correctly, photons have mass. That being the case what stops photons from gaining mass, needing more energy etc etc.
    Hi there,
    Perhaps you're confusing photons with protons -- as in cosmic rays? Cosmic rays are not photons and travel close to (but not at) the speed of light. Unlike photons, cosmic rays have mass. I remember reading somewhere that due to the effect of Earth's magnetic field, cosmic rays detected near Earth are at kinetic energies of 1 GeV which supposedly equals speeds of around 87% the speed of light. Individual particles with energies of 1020 eV have also been detected. They are made up of protons and alpha particles, and atomic nuclei. Those with energies at 1018 eV are at energies greater that the magnetic field of the entire Milky Way Galaxy and are considered to be an "extragalactic component," capable of inter-galactic travel. I read somewhere else that cosmic rays can cross the Milky Way in "10 minutes" but I don't know if that's true. Sorry, I don't recall the sources I read years ago and I'm not an expert on cosmic rays, so take it with a grain of salt, or perhaps the nuclei of a hydrogen atom. [img]/phpBB/images/smiles/icon_wink.gif[/img]

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    On 2002-11-08 12:09, JS Princeton wrote:
    I don't know whether I should laugh or cry at such a post. Surely you realize that "mass" is only defined in the rest-mass, newtonian gravitational, and inertial possibilities.
    You should learn some physics while you laugh and cry. Surely you realize that your "mass" is not additive and, hence, loses all the sense altogether.

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    "Mass" is defined for the purposes you need it to be defined. Rest mass is used in particle physics. You cannot simply declare the definition to be bad simply because you don't like the context. Indeed, in the domain of particle physics, in the center of momentum frame, rest mass is conserved. Now, in GR formulations the "gravitational mass" might be said to have a bad meaning because the source of gravity is not mass but the stress-energy tensor. However, this doesn't invalidate anything I've said to this point. The point of this thread is to ask the question whether photons have "mass". Indeed they do not accordingly.

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    On 2002-11-08 08:36, John Kierein wrote:

    Sorry. I stand corrected. Thanks. I now remember reading this a while ago, too.
    And I remember reading a much more detailed description of the data reduction process, but can't find it. I recall that they do the relativistic correction out to 90 degrees from the sun.

    Even 45 degrees from the sun there is a gradient in the atmosphere, but perhaps this is becoming to be more definitive since at that angle there are large time variations in the density due to solar wind variations, etc. which would cause noise in the deflection data.
    An angle of 45 degrees would be about tangent with the orbit of Venus, are you suggesting a high enough solar wind density to cause this degree of refraction? If so, I'd like to see your calculation.

    Hipparcos is a remarkable engineering feat! A milliarcsecond is really, really small.
    It has always been one of my favorite missions, it had an huge scientific return and almost no public attention. I have several times complimented my ESA colleagues about it.

    If you read the literature, you will realize that the milliarcsecond level accuracy is only achievable using relativistic corrections. the GAIA mission, which is striving for microarcsecond level accuracy will actually be able to test GR, rather than just utilize it.


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    Quote above from Rick

    (you can't accelerate something that is moving at c)

    Now I haven't studied physics, vectors, etc. in a while but I seem to remember that acceleration is defined as a change in velocity or a change in the direction of travel. So for instance if a photon traveling at c cruises by a massive star or black hole and its direction of travel has changed it would seem logical that it has been accelerated. Gravitational lensing would be a good example of this.

    -AJ

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    On 2002-11-08 14:59, JS Princeton wrote:
    However, this doesn't invalidate anything I've said to this point. The point of this thread is to ask the question whether photons have "mass". Indeed they do not accordingly.
    The thing you've said to which I objected is totally wrong in all and every physics. As to the question of whether photons have mass, they sure do have gravitational mass.

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    On 2002-11-08 15:55, AgoraBasta wrote:
    As to the question of whether photons have mass, they sure do have gravitational mass.
    What does that mean? To you, I mean.

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    "Gravitational" mass is an ill-defined concept because you can transform it away. A high energy photon isn't a high energy photon if I go to a different reference frame. Therefore the gravitational mass of the photon isn't invariant. Therefore we don't use the graviational mass as the definition of mass. We use rest mass. Don't believe me? Read any physics text.

    If you are worried about gravity you solve the problem in GR in which case you are worried about geodesics and not the so-called "gravitational" mass.

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    On 2002-11-07 11:25, JS Princeton wrote:
    John, the system of two photons degrading into an electron and a positron will still have zero restmass in the center of momentum frame.
    I'm afraid this is not correct.

    In the center of momentum frame the rest mass (or just mass) of the two photons equals:

    m<sup>2</sup> = E<sup>2</sup>-p<sup>2</sup>
    (c = 1)
    p = 0 so m = E

    And the rest mass of the electron and positron equals the same.

    m = m<sub>e</sub> + m<sub>p</sub> = E


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    Prior to about 1915 Einstein considered photons to have gravitational mass, but once he finalized general relativity he discarded this idea, and just considered the energy-momentum tensor as the source of the gravitational field.

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