# Light has mass?

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Well, well, well! It is a classical physics discussion.

One might be tempted to believe it actually should be an electrodynamics or particle physics discussion.

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What is a light?

long wave--------------------------------------------------------------->short wave

low energy-------------------------------------------------------------->high energy

--------------I.R. , visible light, U.V., X-ray, Gamma-ray--->

one light ==> photon

Why it does not have a mass?

Is it in the no mass action region? First we think about a mass.

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!

Moderator Note

The last post in this thread was split into a new topic, found here.

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Light does not have no moving state, and always move only the speed of light, C. @@What is a mass? F=ma. At a light we can not change an acceleration. So, light does not have a 0 mass, but has no mass.

Edited by alpha2cen
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According to the standard model photons at rest have no mass (of course photons can never be at rest by definition ). Photons moving at the speed of light, however, have energy as in a solar sail and therefore this energy has its mass equivalence. Accordingly to GR EM radiation follows the warp in space surrounding matter, but if not it would bend anyway under the influence of gravity based upon its mass-equivalence as Newton predicted

Edited by pantheory
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Accordingly to GR EM radiation follows the warp in space surrounding matter, but if not it would bend anyway under the influence of gravity based upon its mass-equivalence as Newton predicted

What you say is correct, but the AMOUNT that light bends is consistent with the predictions of Einstein's theory of GR, and not with Newton's theory. Newton's theory predicts an amount of bending of light from stars by our sun that is only half the amount that is experimentally observed.

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What you say is correct, but the AMOUNT that light bends is consistent with the predictions of Einstein's theory of GR, and not with Newton's theory. Newton's theory predicts an amount of bending of light from stars by our sun that is only half the amount that is experimentally observed.

The additional delfection obtained using relativity is due to the spatial curvature around the sun. Without taking the curvature of space into account you're left with only the gravitational acceleration. It can be said that half the delfection is due to gravitational acceleration and half due to spatial curvature. In Einstein's first prediction he didn't take spatial curvature into account and ended up with half the correct value. When he finished his GR theory Einstein recalculated the deflection, now taking spatial curvature into account, and got the correct value.

Edited by pmb
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The additional delfection obtained using relativity is due to the spatial curvature around the sun. Without taking the curvature of space into account you're left with only the gravitational acceleration. It can be said that half the delfection is due to gravitational acceleration and half due to spatial curvature.

Flat spacetime theories as the relativistic field theory of gravity (FTG) give the correct value using a flat space.

For curved spacetime theories as general relativity (GR), it is very misleading to split the acceleration into fictitious 'gravitational' plus 'spatial curvature' components, because quantities in GR are defined over the physical spacetime and this is curved.

The physical reason which GR and FTG give twice light bending than Newtonian gravity is because Newtonian gravity is a scalar theory, whereas GR and FTG are tensor theories (the graviton is spin-2).

In Einstein's first prediction he didn't take spatial curvature into account and ended up with half the correct value. When he finished his GR theory Einstein recalculated the deflection, now taking spatial curvature into account, and got the correct value.

Precisely Einstein's first computation was in his 1911 paper "On the Influence of Gravitation on the Propagation of Light, where Esintein used a scalar theory of gravity developed before GR.

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• 4 weeks later...

A photon in free space travelling at C has momentum;

the mass is calculated from momentum as momentum/velocity=mass

do the math. the nass of a photon is very small (close to zero) but it is more than zero.

Paul

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A photon in free space travelling at C has momentum;

the mass is calculated from momentum as momentum/velocity=mass

do the math. the nass of a photon is very small (close to zero) but it is more than zero.

Paul

Experiment has not been able to show this to be true. All results are consistent with zero, and do not exclude it. This is also not consistent with (very successful) theory, which says that the photon is massless.

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A photon in free space travelling at C has momentum;

the mass is calculated from momentum as momentum/velocity=mass

do the math. the nass of a photon is very small (close to zero) but it is more than zero.

Paul

You are using an old concept of mass mrel. Currently photon is a massless particle m = 0

http://en.wikipedia.org/wiki/Photon#Physical_properties

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If light did have mass, the electromagnetic gauge symmetry would be broken and electromagnetism would no longer be a $1/r^2$ law.

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If light did have mass, the electromagnetic gauge symmetry would be broken and electromagnetism would no longer be a 1/r^2 law.

Most students, including myself at that time, unfortunately do not learn that magnetism actually approximates the inverse cube law concerning the falloff of strength with distance.

http://blazelabs.com...ersecubelaw.pdf

Edited by pantheory
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I was talking about the electromagnetic force between two point charges. I thought that was obvious.

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I was talking about the electromagnetic force between two point charges. I thought that was obvious.

O.K., no problem

Edited by pantheory
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Can we make one photon particle?

$\bigodot ... \bigodot ...... \bigodot$

What is the minimum unit to make or to detect?

the mass of the Sun = total mass of the Sun - photon mass in the Sun

Do we have no problem in this equation?

Edited by alpha2cen
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Can we make one photon particle?

Yes we can 'make' one photon and next study its properties/behavior

What is the minimum unit to make or to detect?

No idea what do you mean by "minimum unit".

the mass of the Sun = total mass of the Sun - photon mass in the Sun

Do we have no problem in this equation?

Photon mass is zero, as said above.

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No idea what do you mean by "minimum unit".

Can we detect one photon or two photons by using the instrument?

Photon mass is zero, as said above.

At the Sun fusion reaction, much gamma ray is trapped near the core area, and high energy photon is also trapped in the convection layer, the retention time is too long.

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A photon has no "at rest" mass - but then it can't exist at rest so its a bit like saying If you got a lead ball that weighs 10 lbs to the speed of light it would have infinite mass - but you can't get it there.

Paul

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A photon has no "at rest" mass - but then it can't exist at rest so its a bit like saying If you got a lead ball that weighs 10 lbs to the speed of light it would have infinite mass - but you can't get it there.

Paul

"Rest mass" is a misnomer [*] and we use the term "invariant mass" or simply "mass" (the immense majority of the literature prefer "mass"). The correct statement is "a photon has no mass".

[*] It is a misnomer also for electrons; e.g., the "rest mass" of an electron moving at 0.1c is the same that when the electron is at rest.

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At any circumstance, does light have only the velocity, C in the Universe? In the water the speed is reduced. At that situation, can we tell light mass is zero?

Light velocity is C

mintrinsic = E/c2

Light velocity is smaller than C

mintrinsic = E/c2 + (- kinetic energy from the small velocity) + (?)

.

.

.

To be a particle

mintrinsic= E/c2 + (-E/c2) + m

Speculation?

Edited by alpha2cen
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At any circumstance, does light have only the velocity, C in the Universe? In the water the speed is reduced. At that situation, can we tell light mass is zero?

Photons always travel at c. Always.

What happens in a transparent medium is that the medium briefly absorbs photons, only to re-emit them shortly thereafter but with some time delay. The end result is to delay the signal. It is the speed of the signal that is less than the speed of light in a transparent medium. Photons still travel at the speed of light. Note that this absorption is not at the atomic (electron) level. That kind of absorption/reemission can only happen at discrete wavelengths. Instead, it's a collective action of the medium. In solids, the physics is best described by phonons. Phonons pop up in all kinds of places in solid state physics. In fluids, it's a bit messier. Fluids are always messier physics. The absorption/reemission is still a collective action even in a fluid.

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Photons always travel at c. Always.

When we travel in the space by space craft at the speed 99/100 C, the light velocity to the moving direction in the space craft is 1/100 C. Of course the absolute light velocity in it is C. Is this right? The light comes from the candle in the space craft.

Edited by alpha2cen
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When we travel in the space by space craft at the speed 99/100 C, the light velocity to the moving direction in the space craft is 1/100 C. Of course the absolute light velocity in it is C. Is this right? The light comes from the candle in the space craft.

No, the light is moving in the direction of the space craft at c. This is what it is measured to be in any inertial frame, regardless of it's relative motion.

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No, the light is moving in the direction of the space craft at c. This is what it is measured to be in any inertial frame, regardless of it's relative motion.

When the light comes out form the space craft, the light speed will be C at the reference frame.

E light, space craft = Elight, reference frame + E kinetic energy difference

hv space craft +( V space craft / C) hv space craft = hv reference frame

V; frequency

At the front of the moving direction wave length of the light will be more shorter than the one in the space craft.

The photon in the star is not different from it in the empty space.

Edited by alpha2cen

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