photon energy

[Gerry]

E(mass) = m c*2 is the energy with speed of light in a vacuum and “at rest” mass.

 

The energy of a photon is E = hf.

 

A photon has internal and kinetic energies…..what does this value of E express, exactly.

 

For example, if I put a photon in a box and move this box at the speed of light in spacetime (a non inertial frame), what is the energy inside that box? Is it E? Is E expressing “at rest” energy of a mass-less particle?

[swansont]

You can't move a box with mass at c; the transform diverges, so you can't even discuss the hypothesis.

 

A photon's energy is all kinetic energy. There is no internal energy to it.

[Gerry]

the box may have mass, but the contents have no mass, only a photon. And its hypothetical of course. Without internal energy, then per your comment, the described box has no energy inside of it. Consider if you will the box is imaginary, as is often done in evaluating energy systems.

[iNow]

But, as swansont already informed you, since the box has mass it cannot move at the speed of light. If you consider situations that are against the laws of physics, then you can make it do damn well anything you want.

[Gerry]

Choosing a non inertial frame does not violate anything. The math you are assuming may not work in that condition, which does not render the frame improper. Choose a massless box with imaginary boundaries is also not a violation of a physics approach to a problem. I can only glean from comments that this energy equation does not apply in this frame, but that prospect still doesnt render the question irrelevant.

[Mr Skeptic]

For example, if I put a photon in a box and move this box at the speed of light in spacetime (a non inertial frame), what is the energy inside that box? Is it E? Is E expressing “at rest” energy of a mass-less particle?

 

Not sure what the point of the box is. If the box were to put any constraints on the photon, then the box cannot be massless so you can't move it at c. If the box doesn't put constraints on the photon you don't need the box. E would be the total energy of the photon if m is the relativistic mass. A photon has zero rest mass, only relativistic mass.

 

If what you are asking is whether if you follow a photon at high velocity it's energy tends to zero, I'd say so (doppler effect).

[swansont]

Choosing a non inertial frame does not violate anything. The math you are assuming may not work in that condition, which does not render the frame improper.

 

Sure it does. The transformation does not work, i.e. you cannot transform into that frame, ergo the frame is not valid. Any subsequent discussion is moot. We don't have physics equations that describe what happens in that frame.

[Gerry]

ok, this clarifies things a lot. THANKS!

 

Does string theory postulate some insight to the question? Might I see strings and/or a brane in the box? If energy tends to zero, i would assume not, also.

[sananda]

You can't move a box with mass at c; the transform diverges, so you can't even discuss the hypothesis.

 

A photon's energy is all kinetic energy. There is no internal energy to it.

 

isn't it true that the photon is only a quantum measurement that came from the discovery of the photoelectric effect!!!

 

such short-sightedness to throw away the fact that light is a wave only...

 

you could fill a sail with wave energy be it water, air, light etc.

 

what if the electrons produced are a result of some other process such as the transfer of extra energy into an atom... an inversion of the way light is emitted from atoms when electrons jump between shells???

[insane_alien]

photo electric effect is NOT the only evidence for photons you know.

 

i suggest you go and learn some science before you carry on.

[sananda]

cool insane alien...

 

can I learn here?

[swansont]

photon antibunching

[sananda]

photon antibunching

 

where's the evidence of an actual photon though?

 

imagine a tiny dust particle in air, it could be affected by a sound wave.

 

the dust particle would move with any currents in the air also...

 

electrons vs virtual particles?

[swansont]

where's the evidence of an actual photon though?

 

imagine a tiny dust particle in air, it could be affected by a sound wave.

 

the dust particle would move with any currents in the air also...

 

electrons vs virtual particles?

 

The specific way it would be affected is different. EM energy is quantized.

[sananda]

The specific way it would be affected is different. EM energy is quantized.

 

quantized meaning take a segment out of the wave... sher you could quantize a wave on the ocean if you wanted to.

[Sisyphus]

quantized meaning take a segment out of the wave... sher you could quantize a wave on the ocean if you wanted to.

 

No, that is not what it means. Light is composed of discrete photons, which is something you should probably look into.

[sananda]

discrete photons were proposed because of the photo-electric effect... light ticks the wave box far more often then teh particle box.

[insane_alien]

it always both a wave and a particle though. there really isn't a difference in the quantum world.

[ajb]

discrete photons were proposed because of the photo-electric effect... light ticks the wave box far more often then teh particle box.

 

The most compelling evidence for photons is not the photoelectric effect, though as you said this was the first observation that suggested discrete quanta.

 

The problem is that the photoelectric effect requires the interaction of any atoms with the light. This means it is not intermediately clear if the quantisation of the electromagnetic energy is really fundamental or some artefact of the complicated set-up and can have a semi-classical description. (only quantize the atoms and treat the EM fields as backgrounds )

 

The first compelling evidence comes from Kibble et.al [1] who investigated antibunching which requires the electromagnetic field to be quantised.

 

There is also plenty of evidence of the photon from cavity QED and the Jaynes–Cummings model. There are experimental effects here that cannot be explained by semi-classical methods.

 

 

 

[1] Kimble, H. J. and Dagenais, M. and Mandel, L., Photon Antibunching in Resonance Fluorescence, Phys. Rev. Lett. 39,11, 691--695, 1977.

Edited September 1, 2009 by ajb