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does it?

Since photons are never at rest, the rest mass of a photon probably isn't what was being asked about.

A photon has no rest mass, which implies that the Minkowskian norm of its energy-momentum vector is zero. However, it does not follow that the components of its energy-momentum vector are all zero, because the Minkowskian norm is not positive-definite. For a photon we have E2 - px2 - py2 - pz2 = 0 (where E = hn), so the energy-momentum vectors of two photons, one moving in the positive x direction and the other moving in the negative x direction, are of the form [E, E, 0, 0] and [E, -E, 0, 0] respectively. The Minkowski norms of each of these vectors individually are zero, but the sum of these two vectors is [2E, 0, 0, 0], which has a Minkowski norm of 2E. This shows that the rest mass of two identical photons moving in opposite directions is m0 = 2E = 2hn, even though the individual photons have no rest mass.

I have read the article quoted by Lightarrow and my head is in a whorl, I am not going in future even to read any questions concerning mass, energy etc.

The principle of equivalence comes from the equation E^2 = m^2c^4 + p^2c^2 In which m is mass, p is defined as momentum, and c is an absolute velocity: It is generally accepted that electromagnetic phenomena have “momentum, but no mass”, the proof being E2 = 02c4 + p2c2therefore √E2 = √( p2c2)and so E = pc (1.0.1)and similarly “rest” mass has no momentum, therefore E2 = m2c4 + 02c2 E2 = m2c4 E = mc2 (1.0.2)

However, the definition of p, momentum, is mass x velocity,

albeit in this case the velocity has the absolute value “c”.So in SI units pc = Kg.(m.sec-1)(m.sec-1), and mc2 = (Kg.m2.sec-2) (1.0.3) These are dimensionally identical, so the solutions (1.0.1) and (1.0.2) do not seem to be legitimate, although they are claimed by experiment to be “proved”. If this is the case, and E = pc and E = mc2 are valid then either (pc)2 + (mc2) = E2 and-a-bit, or if equation (1.0) is to be upheld, the possibility has to be considered that m has to be non-zero, and c has to be variable.

Slightly tangential, but interestingly mass still remains the one SI unit that cannot yet be defined in terms of something else: 'mass' is DEFINED by the kilogram sample of platinum alloy kept in a box in Paris !

No-one really knows, at a fundamental level, what mass 'is.' You may as well ask what a quark is made of , or what a magnetic field is made of, these questions have no answers in current physics. Think of it like this: as a particle (say an electron) speeds up, it slowly gains mass according to GR.

just exactly how is this 'mass' added? - little tiny bits of electron magicking themselves out of nothing and sticking to the surface of the moving electron?!! Very unlikely. 'mass' at the particle level becomes a purely mathematical abstraction - don't let anyone kid you otherwise :-) Therefore for all intents and purposes a photon DOES have mass, purely by virtue of it's motion, i.e. its energy.

Last time I checked mass and energy were equivalent; doesn't that mean that light, which clearly has energy, must have mass?

If light has no mass, how would you explain the working of a solar sail?

Quote from: edward2007 on 20/06/2007 11:36:42If light has no mass, how would you explain the working of a solar sail?Light has momentum p even if it has no mass: p = E/c.The electric and magnetic field of an EM radiation make a force which is orthogonal to the surface, to the material's electric charges. This force pushes the sail.

Quote from: Bored chemist on 10/06/2007 13:28:51Last time I checked mass and energy were equivalent; doesn't that mean that light, which clearly has energy, must have mass?Bored Chemist is RIGHT!Even through we will never be able to mesure it as it allways moves @ C because if we say that it doesn't mass then we are saying than E=MC2 is totally wrong. I donn't think anyone here is going to think that E=MC2 is Totally Wrong !!!!!!Ed

Quote from: lightarrow on 20/06/2007 22:14:40Quote from: edward2007 on 20/06/2007 11:36:42If light has no mass, how would you explain the working of a solar sail?Light has momentum p even if it has no mass: p = E/c.The electric and magnetic field of an EM radiation make a force which is orthogonal to the surface, to the material's electric charges. This force pushes the sail.Sorry, I must dissagree, EM forces has nothing to do with solar sails.It is the fact that the solar sail is highly reflective...

I remember having as a child what looked like an upside down glass light bulb.It had a small set of sails inside that were white on one side and black on the other and they rotated in sunlight or from a strong beam of light. Cant remember what it was called though.I think that a photon is a particle that does have mass and its the gravitational force (wave) that propels it. I don't believe in the theory of any strong or weak forces.

Quote from: G-1 Theory on 19/07/2007 14:10:05Quote from: lightarrow on 20/06/2007 22:14:40Quote from: edward2007 on 20/06/2007 11:36:42If light has no mass, how would you explain the working of a solar sail?Light has momentum p even if it has no mass: p = E/c.The electric and magnetic field of an EM radiation make a force which is orthogonal to the surface, to the material's electric charges. This force pushes the sail.Sorry, I must dissagree, EM forces has nothing to do with solar sails.It is the fact that the solar sail is highly reflective...1.Have you ever asked yourself why and how light can be reflected? Ok, I tell you: because light is an electromagnetic wave and when hits a surface, it puts in motion the electrons of the surface, making them oscillate, so they, in turn, generate an EM radiation because of their accelerated motion, and this is the light which comes out of the surface. Do you see strong or weak nuclear forces, or gravitational forces in this process?2.There would be a push on the sail even if it was completely absorbing.

Dear Lightarrow;You are right that electromagnetic waves are in the light spectrum and you have a concept here that makes very good conscience. And I truly do like how you worded it, for it really makes sense to me.This is a concept that has not crossed my mind, and I like it. But then you ask, Do you see strong or weak nuclear forces, or gravitational forces in this process?Well; Yes I do see “ The weak nuclear force here.” Because a good friend and mentor of mine here at UT has proven that, “The weak nuclear force and the electromagnetic force are one and the same, and he has a Nobel for his work on this. his Name is, Steven Weindberg.Ed

Thanks for that explanation Light Arrow.If I remember the vanes in the glass bulb were in a semi vacuum.Yes I still think light has mass and I also think that gravity is a wave and not produced by mass.Time it does take and as there are many many gaps and assumptions in our current theories of mass, gravity and light that I think a unified field theory is hard to construct without taking a different approach.

i feel that since light has a push on objects such as solar-sail-equipped satellites, it should have mass. and for electronic waves, they should not have an extra push against a mirror. but sunlight does. shouldn't that mean that the light itself has mass?

Here's an idea for an experiment. Build two lasers that, ideally, would be perfectly parallel to one another and would project laser beams that are also perfectly parallel to one another and do not spread out as they travel (or spread out negligably for the purposes of this experiment).Take these parallel lasers in into an area of outer space where outside interferences are negligably small (gravity, dust, gas, etc.). Build a detector and put it some distance away, perhaps several million miles. Now activate the lasers simultaneously. The distance apart that the two laser beams are at the moment they are fired is labeled "x", and the distance apart that the two laser beams are when they arrive at the detector is labeled "y". The detector is designed to measure "y".If the laser beams generate their own gravitational fields, then they should be mutually attracted to one another as they travel through space. If this is the case, then "x" will be greater than "y". If the beams do not gravitationally attract one another, then "x" should equal "y".Therefore, if the detector finds that "x" > "y", then the laser beams generated their own gravitational fields and therefore had mass. If the detector finds that "x" = "y", then the laser beams did not generate their own gravitational fields and therefore did not have mass (according to my understanding, at least).

My guess for what it's worth is that light has a momentum but could have a rest mass > 0 so that its speed "c" is just a bit less. Will anyone here have the brain space to understand this?Or maybe such people choose to obscure simple ideas

Varying c in classical physicsThe photon, the particle of light which mediates the electromagnetic force is believed to be massless. The so-called Proca action describes a theory of a massive photon.[1] Classically, it is possible to have a photon which is extremely light but nonetheless has a tiny mass, like the neutrino. These photons would propagate at less than the speed of light defined by special relativity and have three directions of polarization. However, in quantum field theory, the photon mass is not consistent with gauge invariance or renormalizability and so is usually ignored. However, a quantum theory of the massive photon can be considered in the Wilsonian effective field theory approach to quantum field theory, where, depending on whether the photon mass is generated by a Higgs mechanism or is inserted in an ad hoc way in the Proca Lagrangian, the limits implied by various observations/experiments may be different.[2]

without even reading through the thread..... you bet energy has mass

take 2 base element; how about H and O;isolate themin BEC cold stateweigh them separatelycombine them; allow to sit to room temperaturenow weight them again combinedwill the combined weigh more than the 2 added separately?Energy has mass

But photons do not have 'Rest Mass'. There is a serious distinction there.

And why bother to read through a thread? It might interfere with one's opinions.

You don't need such complicated way to show that giving energy to a body AT REST increases its mass: heat a piece of iron and you increases its mass; spin it and you increases its mass, ecc, ecc.

Not only: while a single photon has NO mass,

Quote Not only: while a single photon has NO mass, ooops...i disagree, that is a math error, not realityto see your 2 examples you can see the contradictions

No contradictions. Mass is NOT additive.