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2002-Nov-07, 12:48 PM
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

GrapesOfWrath
2002-Nov-07, 01:34 PM
On 2002-11-07 07:48, southern_musca wrote:
So, if understand correctly, photons have mass.
They don't.

Welcome to the board.

Argos
2002-Nov-07, 01:43 PM
A gracious constellation, (southern) Musca.

Welcome in.

segfault
2002-Nov-07, 02:39 PM
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.

John Kierein
2002-Nov-07, 03:30 PM
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.

GrapesOfWrath
2002-Nov-07, 03:41 PM
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?

John Kierein
2002-Nov-07, 03:55 PM
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>

GrapesOfWrath
2002-Nov-07, 04:06 PM
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.

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

JS Princeton
2002-Nov-07, 04:25 PM
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.

John Kierein
2002-Nov-07, 06:38 PM
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>

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Karl
2002-Nov-07, 09:03 PM
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/Projects/GAIA_files/LATEX2HTML/node141.html

JS Princeton
2002-Nov-07, 10:10 PM
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.

John Kierein
2002-Nov-07, 10:19 PM
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.

Karl
2002-Nov-07, 10:33 PM
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/nph-bib_query?bibcode=1986STIA...8734720W&db_key=AST&high=3dcab987c624008

AgoraBasta
2002-Nov-08, 12:15 AM
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!

2002-Nov-08, 01:36 AM
Thanks for the responses, and thanks also for the warm welcome

Glen

Tensor
2002-Nov-08, 03:49 AM
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.

Tensor
2002-Nov-08, 04:42 AM
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.

John Kierein
2002-Nov-08, 01:36 PM
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>

JS Princeton
2002-Nov-08, 05:09 PM
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?

Chip
2002-Nov-08, 05:57 PM
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. /phpBB/images/smiles/icon_wink.gif

AgoraBasta
2002-Nov-08, 07:46 PM
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.

JS Princeton
2002-Nov-08, 07:59 PM
"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.

Karl
2002-Nov-08, 08:15 PM
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.

AJ
2002-Nov-08, 08:35 PM
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

AgoraBasta
2002-Nov-08, 08:55 PM
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.

GrapesOfWrath
2002-Nov-09, 01:04 AM
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.

JS Princeton
2002-Nov-09, 01:12 AM
"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.

Ring
2002-Nov-09, 03:04 AM
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

Ring
2002-Nov-09, 03:19 AM
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.

Tensor
2002-Nov-09, 04:22 AM
On 2002-11-08 15:35, AJ wrote:
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.

Good so far.



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.

Seemingly logical does not always apply when dealing with GR. To have an acceleration, you must apply a force to a mass (F= ma). Since the rest mass of a photon (or in the case of your question, a graviton) is zero, no acceleration can take place. The graviton simply follows the shortest path (a geodesic in spacetime) , which near a massive star or a black hole is a curved path (the equivalent of a straight line in Euclidean space). Since the graviton is moving at a constant velocity (c) and is staying on the same path (curved though it is) it is not accelerating.


Gravitational lensing would be a good example of this.

Gravitational lensing is an example of the different paths taken by photons. The closer to the massive object, the more curved the path of the photon.

Celestial Mechanic
2002-Nov-09, 05:32 AM
On 2002-11-08 22:19, Ring wrote:
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.
I agree, and that is what makes gravitation so universal. Leptons are immune to the strong force; neutrinos are immune to the electromagnetic force; if right-handed neutrinos and left-handed antineutrinos exist they might even be immune to the weak force; but everything has an energy-momentum or stress tensor. Nothing is immune to gravity.

I prefer to think of mass as a secondary concept, defined in terms of the primary concept of the energy-momentum 4-vector via its square as follows:

m^2 * c^4 = E^2 - p^2 * c^2.

The velocity 4-vector is just pc/E. (Multiply this by c if you want conventional units of m/s.) When the particle is massless, that is, m=0, then the velocity is 1 (or c) always. When the mass is non-zero, the velocity is always less than 1 (or c).

The way I like to explain it is "Mass is what enables you to sit still and wait."

JS Princeton
2002-Nov-09, 12:30 PM
On 2002-11-08 22:04, Ring wrote:

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




Thanks, Ring! Excellent catch. It's not that the system has zero restmass at all! It's that the system has a nonzero restmass and zero momentum: that's why the thing works. Errors on my part all the way around.

In order for this to be true we have to be able to get rid of the momentum of the light. That's why there can be no simple degradation of a photon into an electron and a positron in free space.

AgoraBasta
2002-Nov-09, 12:43 PM
On 2002-11-08 20:12, JS Princeton wrote:
"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.
I don't care of such delusions.
Reality is as follows - having said "photon" one means mass, period. Whenever you have to deal with spatially localized EM radiation, you better consider it as having mass. Simple examples are - a reflective cavity "filled" with photons, a thin dense beam of light, etc. Not only do the photons have gravitational mass, they have inertial mass in those examples.

But, I guess, you are going to defend your misconceptions no matter how wrong and thoroughly disproven they are.

GrapesOfWrath
2002-Nov-09, 01:08 PM
On 2002-11-09 07:43, AgoraBasta wrote:
I don't care of such delusions.
Reality is as follows - having said "photon" one means mass, period.
Oh, is that all you mean. Well, that's different.

AgoraBasta
2002-Nov-09, 01:18 PM
On 2002-11-09 08:08, GrapesOfWrath wrote:
Oh, is that all you mean.
Sure not all, but you knew that /phpBB/images/smiles/icon_wink.gif

GrapesOfWrath
2002-Nov-09, 01:19 PM
It is against the mainstream, though, don't you agree?

AgoraBasta
2002-Nov-09, 01:23 PM
On 2002-11-09 08:19, GrapesOfWrath wrote:
It is against the mainstream, though, don't you agree?This time I'm pro, JS is contra (the mainstream, I mean).
So whom would you join? Or might you have a votum separatum?

GrapesOfWrath
2002-Nov-09, 01:42 PM
On 2002-11-09 08:23, AgoraBasta wrote:
This time I'm pro, JS is contra (the mainstream, I mean).
So whom would you join? Or might you have a votum separatum?

Is that a Russian Latin term?

So, "photon has mass" is mainstream? I dunno, I think young JS is being inculcated with some of the most au current physics possible.

Let's see, here's an article from Physical Review Focus, 8/16/2002 (http://focus.aps.org/story/v10/st9), photons don't have mass. Here's a Physics News Update, 3/4/1998 (http://www.aip.org/enews/physnews/1998/split/pnu361-3.htm), photon mass usually assumed to be zero, with experimental tests limiting any actual mass. Here's the Physics FAQ version (http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html), "overwhelming consensus among physicists today is to say that photons are massless."

But that last article does allow some room to wiggle. What you got?

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2002-11-09 08:45 ]</font>

AgoraBasta
2002-Nov-09, 03:41 PM
On 2002-11-09 08:42, GrapesOfWrath wrote:
Is that a Russian Latin term?
No, simply Latin.

So, "photon has mass" is mainstream? I dunno...
I've already noticed that you "dunno" /phpBB/images/smiles/icon_razz.gif
(Photons have mass, they don't have rest mass) <- that's mainstream.

GrapesOfWrath
2002-Nov-09, 04:09 PM
From the Physics FAQ first paragraph: This question comes up in the context of wondering whether photons are really "massless," since, after all, they have nonzero energy and energy is equivalent to mass according to Einstein's equation E=mc2. The problem is simply that people are using two different definitions of mass. The overwhelming consensus among physicists today is to say that photons are massless. However, it is possible to assign a "relativistic mass" to a photon which depends upon its wavelength. This is based upon an old usage of the word "mass" which, though not strictly wrong, is not used much today.

So, clearly someone agrees with JS, and they even allow how your viewpoint can be accommodated--but I would hesitate to call it mainstream. You're going to have to come up with some contrary citations.

PS: I sorta see what votum separatum would be from the Latin, but you never know...I can't find it in a legal dictionary, but I see it used on the internet, especially for eastern european countries. What's it mean in that sense?

<font size=-1>[Add PS]</font>

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2002-11-09 11:32 ]</font>

AgoraBasta
2002-Nov-09, 08:40 PM
On 2002-11-09 11:09, GrapesOfWrath wrote:
So, clearly someone agrees with JS, and they even allow how your viewpoint can be accommodated--but I would hesitate to call it mainstream. You're going to have to come up with some contrary citations.
*Some* would agree with JS, but he's dead wrong, period.

Now to illustrate this -

Quoth JS:

Right now, we believe that rest mass and gravitational mass are equivalent (Einstein's Equivalence Principle)...
...
The point of this thread is to ask the question whether photons have "mass". Indeed they do not accordingly.

Quoth Steve Carlip (http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=Cu8JJo.Azs%40ucdavis.edu&rnum=9&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26q%3DCarlip%2Bgravitational%2Bmass%2Bphoton):

I agree that the idea of ``relativistic mass'' causes more confusion than enlightenment, and if I were asked the mass of a photon, I would say zero. On the other hand, there is a useful concept called ``invariant mass'' for a system of photons, which is generally not zero. (Take the sum of the four-momenta of the photons and find its invariant square; this will not be zero, in general, unless all of the momenta are parallel.)

From the same author (http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=9kun7s%24fsi%241%40woodrow.ucdavis.edu&rnum=16&prev=/groups%3Fq%3DCarlip%2Bgravitational%2Bcoupling%26h l%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26start%3D10%26sa%3DN):

...the question ... is basically whether electromagnetic energy contributes E/c^2 to gravitational mass, or whether there's some extra coefficient. There are good measurements of the contribution of electromagnetic binding energy in nuclei to ``passive'' gravitational mass. The contribution is E/c^2 to an accuracy of about a few parts in 10^10. For magnetostatic energy the results are weaker (because the energy is smaller), but one gets E/c^2 to a few parts in a million.


Essentially, whenever a photon is added to a system, the system adds a mass of hf/c^2, no matter if the photon interacts there electromagnetically of just wanders somehow.

So you make your judgement.


PS: I sorta see what votum separatum would be from the Latin, but you never know...I can't find it in a legal dictionary, but I see it used on the internet, especially for eastern european countries. What's it mean in that sense?
It's used in its basic literal meaning - "a separate vote", when someone has to state a dissenting opinion.

<font size=-1>[ This Message was edited by: AgoraBasta on 2002-11-09 16:50 ]</font>

JS Princeton
2002-Nov-09, 11:39 PM
Oh dear. Why are you so antagonistic, Agora?

Here (http://www.google.com/search?hl=en&ie=ISO-8859-1&q=Gravitational+mass+inertial+equivalence) are the Google search results for "Gravitational mass inertial equivalence".

The first site: http://www.jca.umbc.edu/~george/html/courses/glossary/mass_inertial_vs_grav.html gives a good layman's rundown of why I'm right and Agora is wrong.

Let's quote from some of the other results on the Google search. From the Cornell Site:


The Kruezer experiment shows that
active gravitational mass = passive gravitational mass for lab-sized bodies of very different chemical composition (= relative fractions of strong, weak, and electromagnetic binding energy, of protons,
neutrons, and electrons, of differnt quark types). I forget the accuracy limit, but I think it was around 1e-4 or so.

A variety of earth-surface equivalence-principle and 5th-force experiments tell us that passive gravitational mass = inertial mass for lab-sized bodies of very different chemical composition (= relative fractions of various stuff, as above, down to accuracy levels of 1e-12 or so.

OK. Note the equivalence of gravitational mass to inertial mass. Lest you think they are weird in Ithaca, here's more from the Eot-Wash site of the University of Washington:


The Newtonian Version

Gravitational mass is the charge to which gravity couples. Inertial mass is a measure of how fast an object accelerates--given
the same force, increasing the inertial mass implies decreasing acceleration. The simplest way to state the equivalence principle
is this: inertial mass and gravitational mass are the same thing. Then, gravitational force is proportional to inertial mass, and the
proportionality is independent of the kind of matter. This implies the Universality of Free Fall(UFF): in a uniform gravitational
field, all objects fall with the same acceleration, e.g. 9.8m/s2 near the surface of the earth.

The Einsteinian Version

All objects fall the same way under the influence of gravity; therefore, locally, one cannot tell the difference between an
accelerated frame and an unaccelerated frame. Consider the famous example of a person in a falling elevator. The person floats
in the middle of an elevator that is falling down a shaft. Locally, that is during any sufficiently small amount of time or over a
sufficiently small space, the person falling in the elevator can make no distinction between being in the falling elevator or being in completely empty space, where there is no gravity.

Ah, so now you see it isn't easy. The ONLY way you can talk about a "gravitational" mass is if you are dealing with Newtonian gravity. It doesn't make sense to talk about it in Einstein's gravity which uses frames and spacetime to describe the relationships of "states" to gravity and the metric of spacetime.

From the Universiy of Idaho Physics Department on the deal with Einstein's weird gravity:


It is to be noted that although all material is composed of protons and neutrons (and electrons), the amount of stored electric and nuclear energy varies from material to material. These experiments show that all types of energy fall in the same way in a gravitational field. Laser ranging measurements to the Moon show that even gravitational energy itself falls at the same rate. If this were not the case, there would be a slight distortion of Moon's orbit about Earth caused by their slightly different
accelerations towards the Sun. This is known as the Nordvedt effect and it is not found [29].

Why should bodies of different material composition all fall at the same rate in a gravitational field? Einstein's proposal was
that the rate of fall really had nothing directly to do with the mass.

In effect, what we are saying is that "mass" is only "gravitational mass" if it is in the Newtonian treatment in which case it is inertial mass and therefore rest mass.

I can understand your frustration, Agora. In the earliest formulations of relativity (esp. on the part of Lorentz) there was a popular idea that one should consider a "relativistic" mass that wasn't frame invariant (mass times gamma). This confusing definition is no longer used, instead the equation E^2=p^2+m^2 is used to define the rest mass as the only "mass" that really matters in physics. The "gravitational" mass is therefore something of a misnomer. The stress-energy tensor is the source term for gravity: it isn't a "mass" at all. In fact, the stress-energy tensor involves terms of pressure and density. This is the confusing nature of the tensor arithmetic of gravity all rolled up into one. It doesn't seem to us that a truck full of canaries that are sitting should be treated differently for gravity than a truck full of canaries that are flying around and hitting themselves against the sides of the truck. However, that's what the stress-energy tensor tells us. To say that the two different trucks don't have the same mass would be foolish and confusing because you'd have to decide with respect to which frame you were measuring the trucks. Indeed, in the rest frame of either truck they have the same mass. That's why we use the frame of rest to define mass, and that's why my statement is right.

JS Princeton
2002-Nov-09, 11:48 PM
On 2002-11-09 15:40, AgoraBasta wrote:
I agree that the idea of ``relativistic mass'' causes more confusion than enlightenment, and if I were asked the mass of a photon, I would say zero. On the other hand, there is a useful concept called ``invariant mass'' for a system of photons, which is generally not zero. (Take the sum of the four-momenta of the photons and find its invariant square; this will not be zero, in general, unless all of the momenta are parallel.)

Excellent quote and illustrates my point beautifully. There is really, in GR, no such thing as gravitational mass. You need to appeal to stress-energy tensors. As for the "invariant" mass, this doesn't apply, for reasons that are left to the reader to state, to a single photon! Interesting, isn't it?



...the question ... is basically whether electromagnetic energy contributes E/c^2 to gravitational mass, or whether there's some extra coefficient. There are good measurements of the contribution of electromagnetic binding energy in nuclei to ``passive'' gravitational mass. The contribution is E/c^2 to an accuracy of about a few parts in 10^10. For magnetostatic energy the results are weaker (because the energy is smaller), but one gets E/c^2 to a few parts in a million.


Everything is right about this statement and supports my statement. He's basically saying you have to take into account the density and the pressure when calculating gravity. As we saw in my previous post, this is exactly right.

Now, if you wish to take gravitational "mass" to the conclusion you want us to take, Agora (that is say that mass is the source term for gravity), you are going to run into some definition problems. That's why gavity really doesn't have anything to do with "mass" except in the Newtonian field limit. And there gravitational mass is equivalent to inertial mass as Eotvos showed.



Essentially, whenever a photon is added to a system, the system adds a mass of hf/c^2, no matter if the photon interacts there electromagnetically of just wanders somehow.


This is only true if you accept the "relativistic" mass as being the true mass. As I illustrated many times before, this is not what is done in modern physics.



So you make your judgement.

Ring
2002-Nov-10, 01:30 AM
Essentially, whenever a photon is added to a system, the system adds a mass of hf/c^2, no matter if the photon interacts there electromagnetically of just wanders somehow.



Itís easier to understand mass if you stop thinking of it as a thing, and instead consider it to be a property of a system. In the equation for the magnitude of the energy momentum four vector

m<sup>2</sup> = E<sup>2</sup> - p<sup>2</sup>

(c = 1)

m represents the system mass. Since the energy of a single photon equals its momentum it has no mass. But if it is moving through a pipe then the system of the pipe/photon has a higher mass than just the mass of the pipe.

So anytime you add electromagnetic energy to system or any kind of energy that cannot be transformed away you have added mass to the system equal to that energy.

In other words mass equals the energy of a system that cannot be transformed away.

And you can define your system to be anything you want. I.e. Jupiter plus a faraway photon.



<font size=-1>[ This Message was edited by: Ring on 2002-11-09 20:37 ]</font>

JS Princeton
2002-Nov-10, 06:39 AM
It's taken a while, Ring, and two different message boards but I think we've come to an agreement on relativity. Whew! /phpBB/images/smiles/icon_smile.gif

AgoraBasta
2002-Nov-10, 10:23 AM
So may we now rest assured that the young JS has learned a thing or two and won't spit out a blasphemy like "Right now, we believe that rest mass and gravitational mass are equivalent (Einstein's Equivalence Principle)..." anymore? /phpBB/images/smiles/icon_smile.gif

GrapesOfWrath
2002-Nov-10, 10:37 AM
On 2002-11-09 15:40, AgoraBasta wrote:
It's used in its basic literal meaning - "a separate vote", when someone has to state a dissenting opinion.
OK, but how would that apply to me? I thought you two guys were doing all the dissenting.

AgoraBasta
2002-Nov-10, 10:50 AM
On 2002-11-10 05:37, GrapesOfWrath wrote:
OK, but how would that apply to me? I thought you two guys were doing all the dissenting.
So should I conclude that either
a. You've got no special opinion or
b. You agree with either of us two?

<font size=-1>[ This Message was edited by: AgoraBasta on 2002-11-10 05:51 ]</font>

GrapesOfWrath
2002-Nov-10, 11:28 AM
I don't consider it a matter of opinion.

JS Princeton
2002-Nov-10, 02:54 PM
On 2002-11-10 05:23, AgoraBasta wrote:
So may we now rest assured that the young JS has learned a thing or two and won't spit out a blasphemy like "Right now, we believe that rest mass and gravitational mass are equivalent (Einstein's Equivalence Principle)..." anymore? /phpBB/images/smiles/icon_smile.gif


No, I think I was painfully clear in my explanation of exactly what that statement I made meant. It is still true.

AgoraBasta
2002-Nov-10, 03:41 PM
On 2002-11-10 09:54, JS Princeton wrote:
No, I think I was painfully clear in my explanation of exactly what that statement I made meant. It is still true.
So you get an unsatisfactory mark all over your (you choose what).
Once you embrace the "gravitational mass" concept, your photons get to have such mass; and you must equate "relativistic mass" of a system to the gravitational mass. Basically, it means that mixing Newtonian and relativistic therminology you get lost in the woods; which, in turn, means you don't understand the physics of relevant matters...

JS Princeton
2002-Nov-10, 04:10 PM
Agora, it has become painfully apparent that you are lashing out for no reason. I have provided you with the relevant definitions and the appropriate domains. You haven't presented any evidence that contradicts a single word of anything I've said. A system has a measurable inertial mass that is, in the Eotvos-sense, equivalent to its gravitational mass. If you don't care to figure out what that means, that's your issue, not mine.

<font size=-1>[ This Message was edited by: JS Princeton on 2002-11-10 11:11 ]</font>

AgoraBasta
2002-Nov-10, 04:27 PM
On 2002-11-10 11:10, JS Princeton wrote:
A system has a measurable inertial mass that is, in the Eotvos-sense, equivalent to its gravitational mass.
Inertial mass is believed to be equal to gravitational mass, that's true. But inertial mass is not the "rest mass" either; or would you like to argue that a spinning ball has the same mass as when not spinning?

JS Princeton
2002-Nov-10, 08:28 PM
In the inertial frame, the objects are at rest. Therefore inertial mass is equivalent to rest mass.

Now, if you are asking about a spinning object and a non-spinning object and the interactions between them, an inertial frame cannot be defined in the same way. The inertial mass will still be equivalent to the rest mass of the system (or, as Ring puts it, to the component of the energy four vector that cannot be transformed away).

AgoraBasta
2002-Nov-10, 09:23 PM
On 2002-11-10 15:28, JS Princeton wrote:
The inertial mass will still be equivalent to the rest mass of the system (or, as Ring puts it, to the component of the energy four vector that cannot be transformed away).
That's certainly true for the inertial mass. Forget the rest mass already, the concept long deserved to have been left behind and for a good reason. Oscillatory motions in the system add to mass, rotations inside the system add to mass, static fields add to mass, bound photons and other field quanta add to mass, heat adds to mass, invariant mass of radiation adds to mass, background fields add to mass, etc, etc. In fact, the inertial mass of a system may be decomposed into masses and energies of the components in many various ways, and those components may be affected by non-gravitational influences from other systems along with gravitational interaction.

<font size=-1>[ This Message was edited by: AgoraBasta on 2002-11-10 18:36 ]</font>

JS Princeton
2002-Nov-10, 11:26 PM
Rest mass is a quantity used in physics because we assume we can measure things from a lab frame (at rest). While you, Agora, seem to argue that nothing can be measured to an arbitrary certainty to be at rest, the definition stands and is used by physicists as an idea assumed correct.

AgoraBasta
2002-Nov-10, 11:37 PM
JS, read this little quote from Carlip (http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=9srvq5%24pi5%242%40woodrow.ucdavis.edu&rnum=7&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26q%3D%2522invariant%2Bmass%2522%2Bcarlip).

JS Princeton
2002-Nov-11, 01:37 AM
This is the same message you quoted from before. Carlip says,


It's true that rest mass, or invariant mass, doesn't increase with velocity...

You are saying that invariant mass isn't either inertial or gravitational mass. Then you have to give me a different definition of mass. Carlip understands that some people will be as insistent as you, and therefore says:


In any case, though, gravitational mass is not, in general, the same as rest mass.

to mean something different than what I was talking about. Carlip is saying that if you take mass as the source term then you have to modify your definition of gravitational mass. For Eotvos experiments, this isn't done because we assume we are sitting in a Newtonian limit and therefore the Equivalence Principle boils down to looking at ma=GMm/r^2. This is the only sense in which we can talk about mass as a source term for gravity. Otherwise, we are restricted to Einstein's Equations which don't appeal to mass as a source term (thus the quotes I provided earlier to that effect).

AgoraBasta
2002-Nov-11, 07:16 PM
JS,
You may juggle definitions and restrictions till you get your errors look almost correct. This can't help you eradicate your basic misuderstanding of physics, like those that led you to stating that an e-e+ pair has no mass.
So you'd better stop discussions and get back to your learning.

<font size=-1>[ This Message was edited by: AgoraBasta on 2002-11-11 16:17 ]</font>

Fruh-Batz
2002-Nov-11, 08:42 PM
Yeeeeee-haw!
Are you talking to yourself Agora?

JS Princeton
2002-Nov-11, 10:13 PM
On 2002-11-11 14:16, AgoraBasta wrote:
You may juggle definitions and restrictions till you get your errors look almost correct. This can't help you eradicate your basic misuderstanding of physics, like those that led you to stating that an e-e+ pair has no mass.

Everything I said up to the point about the system of two photons was correct. I was very sloppy in my response to John K. in that I wasn't thinking about the system's mass but rather the individual photon's mass. This is akin to lazy reference-frame switching. The photons in their respective isolated reference frames still have no mass. That was my intention in my first post, but I ended up writing exactly the wrong thing. I don't think this means my physical understandings are incorrect. It is still correct that a single isolated photon, no matter how energetic, cannot on its own decay into an electron-positron pair. Virtual pair production, however, is important for other systems involving individual photons.

Sean Clayden
2007-Sep-19, 11:12 AM
If you travelled away from earth at the speed of light, from earths perspective you would appear to be stationary ! when travelling back at the speed of light it would appear the same. Albeit your time away from earth would be longer. This is only as an observer. As a traveller, the period of time would seem longer however the time travelling and the time observed would be the same. How can it be different ?

Sean Clayden
2007-Sep-19, 11:17 AM
Why ? if possible ? would travelling at c mass increase ?