[ghh]

The “equivalence” depends upon “rest mass”. A moving mass has inertial momentum, or a total energy of mc^2 + mv, so the solution is not so much that a photon has no mass, but has no inertial momentum but through E = pc does have an intrinsic momentum but always with the velocity c. There is no definition of momentum which doesn’t contain “m”. And as Edward2007 points out photons still exhibit boson like properties.
Whilst we are digesting that, you might also refresh your reading on the convergence of the photon gas calculations of Maxwell-Boltzman, Bose-Einstein and Fermi-Dirac. And then look up Wikipedia on the subject of J-Band radars and the cavity magnetron, and the resolution of pinhole cameras. This suggests that for microwaves the energy quantum, as defined by Planck's constant may have a wave width equal to the wave length ie a volume, and before you say “but…” there is an interesting calculation. The discovery of the CMBR shows that space is not empty. There is a continuous flux of radiation, defined by the “black body temperature”. Since this is a steady state we can ignore the velocity and the number of quanta in the volume at any instant is constant. If we consider a volume of space, say 1 cubic metre, and calculate how many quanta (of a wavelength diameter) will fit in that space, and apply The equation of state for an ideal gas, which is PV/NkT, where P = pressure, V=volume, k is the Boltzman constant, T is the temperature (Kelvin) and N is the number of molecules (in this case quanta) of the gas. So pressure P = NkT/V. Since kT = E, and since E = hf , (the number of quanta per second) the number of quanta in the volume at any instant is kT/f. This returns a constant value 6.626069x 10-34, recognisably “h”. The Pressure is therefore Nh/V. Similarly the density can be obtained from the number (N) quanta contained in the volume at the mass equivalent of h
So density = N x 7.37249577722913E-51/V. Now the speed of sound in a gas is √pressure/density. If you do that calculation for any wavelength you get the same very interesting number.
Graham

[lightarrow]

If 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.

[G-1 Theory]

Last 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

[G-1 Theory]

If 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 and it’s reflected

light counteracts the on coming light and the sail moves forwards.

Here at the University Texas’s laser lab we use lasers to push solar sails

in a vacuum chamber for testing.

Ed

[maff]

In order to decide if light is heavy you need to work out what kind of energy it's mass would inflict on this world.
It's not at rest so rest energy or potential energy is eliminated. Does it have kenetic or moving energy? It doesn't seem to cause damage while travelling at super high speeds when colliding with something. There's no fission or fusion involved so nuclear is eliminated. Electrical energy isn't a factor because there's no electrons involved. So if it has mass where is there a measurable result of that mass on this Universe.
On the other hand photons get pulled into large gravitational fields such has black holes and it can be slung shot around large stars. So it behaves sometimes as if it had mass. We all treat light as energy but light is the only form of energy that has a moving particle which remains constant throughout the delivery of that energy.
Rest, kenetic, nuclear and electric energy all have a net product involving mass while light seems generally not to do this.
It gives the human eye the ability to see all the other energies at work.  My thinking is that light is actually heavy but not in our relative Universe.
..maff

[lightarrow]

Last 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

Probably you have missed my post of 19.06.2007. Read it well.

[lightarrow]

If 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.

[that mad man]

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.

[paul.fr]

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.

They still sell them, but i too forget what they are called.

[lightarrow]

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.

If your theory is right, why then the toy rotates in the opposite sense to what you would think?

Answer: the toy is not perfect, some air remains inside even if they try to make the void there. So, the black side of the "flags" is heated from the light more than the reflecting (or white) side; this heats the air molecules near the black side more than the air mol. on the other side, and for reaction they are thrown away, pushing ahead the black side.

If there was complete void, the rotation would be in the opposite sense: the momentum that the reflecting side receives from the light is twicw that received from the black one (theorically, for a total absorbing and total reflecting case), so the reflecting side would be pushed more.

In presence of air, the first effect prevails on the second, and the black side is pushed more than the other.

If you think that light has mass, you can try to prove it. Unfortunately, physics will prove to you the opposite, so I don't think you would be easily able to prove your theory.

[G-1 Theory]

If 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

[that mad man]

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.

[lightarrow]

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

Yes, you're right.

[lightarrow]

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 agree with your last sentence.

[marklee]

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?

[lightarrow]

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?

Momentum is not m*v, in general. For example, is not m*v for photons and for light in general. For light momentum p = E/c where E = energy. So, there is no need to have mass to have momentum.

[Kryptid]

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).

[lightarrow]

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).

1. They do attract each other (no need of experiment, Einstein's equation
G_μν = (8πG/c⁴)T_μν says it).

2. This doesn't mean light has mass. The Einstein's equation written up contains the tensor T_μν which depends on mass AND on energy. The equation essentially says that space-time curvature (expressed by the tensor G_μν) is given by mass and energy (expressed in T_μν).

3. The fact two (stationary) laser beams attract each other doesn't mean the same thing happens with photons (you haven't said it but in case someone could think it).

[Ian Scott]

Light and other electromagnetic radiation interests me.

Does light have finite "rest mass" I believe we can only know to an uncertainty. Lets say light travels a little bit less than "c" - no worries here as light velocity is slower in glass than a vacuum.

So could light have a finite rest mass? If light etc traveled just less than "c" this would be plausible would it not? After all "c" is just a number based on magnetism and electrostatic forces. Permeability is a defined number and other constants are made in relation to it.

Why should light not be stoppable?

Some people I read use a resonant helium "soup" to slow light to the passage of a day in a small chamber. It must have energy still. I guess it has information.

Anyway such lasers aside ...

[Ian Scott]

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