Gravitons

[Freethinker]

Originally posted by: wazzuuup
How can photons be massless when at rest,

But I don't think Tormod answered your question. The problem is it is a very difficult question to answer. Partially because of that old complication "semantics". Language is not well equipt to discuss particle physics. So we use words that are close or at best representitive.

The first problem is that there is no such thing as a photon "at rest". It might be said that a photon stops existing if it has no momentum. But even that would not be completely literally correct.

When we talk about photons, the best we can describe is a "rest mass". When an observer sees the particle still and with zero speed. In other words, the particle is at rest as far as this observer is concerned. This is Special Relativity.

It also deals with the wave/particle duality. It's momentum is a particle function and thus would be when mass is a related issue.

So perhaps the question should have been

CAN a photon be at rest as a particle?

But that answer would be no less complicated! :-)

[Tormod]

Originally posted by: Freethinker

E/(C^2)= m

or:

m=E/(C^2)

NOT

m=(C^2)/E

You shouldn't post math this late at night! :-)

Eeeeh...... I never said maths was my best subject in school...my apologies. The point still stands, though.

Mass could reach towards infinity but never be zero.

[Tormod]

Okay, did some research (talked to one of my colleagues who is an astrophysicist).

The photon has no rest mass because it
is never at rest. A photon always travels at c which is the speed of light.

The energy of the photon is (simply put) calculated by multiplying the Planck length with the photon's frequency. the frequency of a photon is related to the energy level it's originating atom had to get rid of due to electrons changing states in "orbit".

One other thing: Massless particles travel at the speed of light because they can. Nothing is slowing them down. Particles with mass CANNOT travel at the speed of light. This is *not* because (as some believe) their mass would become infinite, but because it would require an infinite amount of energy to accelerate the particle to light speed.

I wrote "Mass could reach towards infinity but never be zero" above, which then obviously was wrong.

Hope this gets some of the cake off my face.

[Tormod]

...and this, of course, is excactly what Bo stated before I even started my venture into mathematical explanations.

Oh, I'll just shut up now.

[Freethinker]

Originally posted by: Tormod
Okay, did some research (talked to one of my colleagues who is an astrophysicist).

The photon has no rest mass because it
is never at rest. A photon always travels at c which is the speed of light.

OK, now I am confused. I sure don't want to argue with an astrophysicist. But c is only c in a vacuum. The "speed of light"/ thus the speed of a photon (I would assume) is not a constant except in a vacuum. It slows thru mass.

In fact you have a Hypog News link which discusses slowing the speed of light dramatically

Researchers use semiconductors to set speed limit on light
http://www.hypography.com/article.cfm?id=34291

It says "31,000 times slower than the 186,000 miles (or 300 million meters) per second that light normally clocks while traveling through a vacuum."

Hope this gets some of the cake off my face.

We aren't going to let oyu off that easy! :-)

[Tormod]

I think it wouldn't matter what value c has - the energy is still the Planck length times the frequency of the photon.

Ku is careful to point out that the experiments do not violate the laws of physics, which state that the speed of light is a physical constant that cannot be changed. What can be, and has been, changed is the speed with which a light wave's amplitude, also known as the "envelope" of a wave, is transmitted.

So they are not changing the frequency of the photon at all, just the amplitude.

[lindagarrette]

So even though the photon has energy, it cannot have mass, because it is always travelling at the speed of light which is a non possibility for anything with mass. But he equation doesn't work out that way. But a photon can only be have as a particle when it is not in motion (which is impossible) That pretty much clenches the theory that a photon is not matter. Right?

[Tormod]

I think the quantum-mechanical wave/partical duality should not be taken too literally. It is a duality because light can be described BOTH as moving particles AND as waves. So if you measure light as a set of streaming particles, then that is what you get. If you look for waves, you will see waves. This insight formed the basis of quantum mechanics! Einstein explained that matter can be described in terms of energy. Photons have energy - but not charge, nor mass.

So it is a semantic question. What is matter?

Here is a set of definitions which more or less contradict each other. Take a pick:

http://www.webster-dictionary.org/definition/matter

[BlameTheEx]

Linda

Semantics aside, a photon may not be matter but it does have mass (just not rest mass), and momentum.

It has mass because it has energy, and with that mass naturally follows momentum.

The proof of momentum is that you can make a light sail move by bouncing photons off it.

Given that we can test the momentum we can prove the mass. A beam of light passing close by the sun is bent a little. That was tested to confirm one of Einstein's theories. Now as light has momentum, and this beam's direction has been changed, momentum has to be conserved. Basically as the beam has been dragged towards the sun, the sun must also have been dragged towards the beam, although not by any measurable amount. Still, the mass of light is matched by an equal gravitational force. It can be attracted by a gravitational field, and it also creates a gravitational field, small though it might be.

If you could bottle a sunbeam, say inside a hollow sphere with a perfect mirror coating, it would add mass to the sphere, as if its energy was stored in matter.

[lindagarrette]

I'm still confused. Matter is not mass and mass is not matter., but a phoiton has mass because it is matter in motion -- energy.

Here is a web page I found that probably explains some of the things we are trying to figure out. Don't fuss at me for copying the entire abstract. The author says it explains the particle wave duality mystery. Not to me. Would anyone like to take a stab at it?

The Photon - Electron Interactions in the Electron Positron Lattice (EPOLA) Space Vs. Feyman's QED with his "Nobody Understands" and "Nature is Absurd"

by Menahem Simhony, Hebrew University Retired Associate Professor


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In the epola model [1] electromagnetic (EM) radiation represents elastic epola waves, in which each vibrating epola particle transfers the wave energy quantum - the photon, to the next epola particle. A free particle on the path of this energy transfer may be "knocked" by an epola particle carrying the wave energy quantum. As a result, the photon energy may be tranferred to the free particle. Hence the particle properties of the EM wave are those of the epola particles carrying the waves. The wave properties of a free nuclear particle are those of EM epola waves of de Broglie wavelengths that accompany each motion of these particles in the epola. This explains the "particle - wave duality" mystery. Another mystery is the assumed but physically impossible direct transfer of kinetic energy from a several eV photon to a free electron of a 511,000 eV mass-energy equivalent, as, e.g., in Einstein's 1905 interpertation of the photoelectric effect. The solution is that photons do not exist in nature, and the acting particle is the epola electron or positron that momentarily carries the photon energy and is thus able to transfer it to any nuclear particle of comparable mass (not much above an alpha particle). [1] M. Simhony, Invitation to the Natural Physics of Matter, Space, and Radiation, World Scientific, 1994, ISBN 981-02-1649-1. Website: www.word1.co.il/physics .