Conservation of Photons?

[Mr Skeptic]

Is there anything to prevent two photons from combining to form a photon with energy equal to the sum of their energies? Is there anything preventing a photon from splitting into two photons who's sum is of equal energy? I know both of these cases can happen if the photon is absorbed and then re-emitted, but what prevents it from happening in a vacuum?

Edited October 20, 2008 by Mr Skeptic
clarification of conservation of energy portion

[YT2095]

as far as I`m aware the problem would come in the "Combining" part as Photons don`t collide (I could be wrong though).

 

and from a Logical standpoint, if such interactions could occur then I would expect a photon with an energy of say 100 units, if Split into 2 photons would have 50 units each or 70/30 etc... and not 2 lots of 100.

similarly for combining.

[Mr Skeptic]

as far as I`m aware the problem would come in the "Combining" part as Photons don`t collide (I could be wrong though).

 

I think I get what you are saying. In order to get two photons with the same position and momentum so that they could combine, that would be very hard to do if they didn't also start off at the same place.

 

As to the second part of your post, yes, I meant following conservation of energy, and edited my post to make that clearer.

[Severian]

Is there anything to prevent two photons from combining to form a photon with energy equal to the sum of their energies? Is there anything preventing a photon from splitting into two photons who's sum is of equal energy? I know both of these cases can happen if the photon is absorbed and then re-emitted, but what prevents it from happening in a vacuum?

 

It doesn't work for real photons, that is ones that obey [math]E=|\vec{p}|[/math]. Try it for yourself!

 

Consider 3 photons with energies [math]E_i[/math] and momenta [math]\vec{p}_i[/math] where [math]i=0,1,2[/math]. Now set [math]E_0=E_1+E_3[/math] and [math]\vec{p}_0=\vec{p}_1+\vec{p}_1[/math] for energy and momentum conservation. Now check the mass-shell condition for particle 0, given particles 1and 2 obey [math]E_{1,2}=|\vec{p}_{1,2}|[/math].

 

It works for the case where one is absorbed and remitted because the intermediate photon can be virtual. Which is of course why it has to re-emit the photon so quickly.

[swansont]

You can do it, though, in a nonlinear crystal, where momentum can be conserved in the bulk material — it's called four-wave mixing. Specifically, this includes frequency doubling (second harmonic generation)

 

http://en.wikipedia.org/wiki/Nonlinear_optics

[Mr Skeptic]

Severian, I've never heard of the mass-shell condition. I know that for what I suggested to work, all photons would have to be traveling in the same direction, or the momentum and energies wouldn't add up. Is that what you were trying to say?

 

swansont, do the photons get absorbed and re-emitted in those crystals, or are they joined together directly?

[swansont]

Severian, I've never heard of the mass-shell condition. I know that for what I suggested to work, all photons would have to be traveling in the same direction, or the momentum and energies wouldn't add up. Is that what you were trying to say?

 

swansont, do the photons get absorbed and re-emitted in those crystals, or are they joined together directly?

 

How could you tell?

 

It includes processes that involve a virtual intermediate state (Raman scattering) that is enhanced when near a real state, so I'm not sure if that counts as absorption — the electron is never actually found in the excited state. This is a quantum effect, and the question is sort of classical.

[Pete]

Severian, I've never heard of the mass-shell condition.

That mass-shelll condition refers to the fact that no matter what energy a particle has its energy and momentum will always satisfy the condition wherein E^2 - p^2 = m^2 where E = inertial energy, p = momentum and m = particles proper mass.

[Mr Skeptic]

Thanks, Pete. I'm familiar with that equation (if c=1 in it), but I've never heard it called a mass-shell condition.

[Tom Vose]

Is there anything to prevent two photons from combining to form a photon with energy equal to the sum of their energies? Is there anything preventing a photon from splitting into two photons who's sum is of equal energy? I know both of these cases can happen if the photon is absorbed and then re-emitted, but what prevents it from happening in a vacuum?

 

The problem with trying to split a fundamental (smallest unit of energy)- such as a photon, is that it cannot be split in any smaller, because it surely must be the smallest amount of energy, energy itself can come in.

 

These packets where first called ''quanta'', and if a photon could be split, you would surely risk a rip in the fabric of space and time itself. I will explain this if you want me too.

 

Two photons however must combine to create other forms of matter. The reason why, is that two particles such as an electron and a positron can come together to create two photons, as a by-product. So yes, photons can come together, but never individually divided.

[Klaynos]

The problem with trying to split a fundamental (smallest unit of energy)- such as a photon, is that it cannot be split in any smaller, because it surely must be the smallest amount of energy, energy itself can come in.

 

These packets where first called ''quanta'', and if a photon could be split, you would surely risk a rip in the fabric of space and time itself. I will explain this if you want me too.

 

Two photons however must combine to create other forms of matter. The reason why, is that two particles such as an electron and a positron can come together to create two photons, as a by-product. So yes, photons can come together, but never individually divided.

 

So, how do we get photons of different energies if they are the "smallest unit of energy"?

[Tom Vose]

So, how do we get photons of different energies if they are the "smallest unit of energy"?

 

Photons can have different energies due to their wavelength.

[Klaynos]

Photons can have different energies due to their wavelength.

 

So I've a 5eV photon, which you state is the "smallest unit of energy".

 

I also have a 1.5eV photon, which you also state is "smallest unit of energy," how can they both be the smallest? One is clearly smaller than the other.

[Tom Vose]

So I've a 5eV photon, which you state is the "smallest unit of energy".

 

I also have a 1.5eV photon, which you also state is "smallest unit of energy," how can they both be the smallest? One is clearly smaller than the other.

 

Are you intentionally being ignorant... i am sorry, but you of all people (according to your position here), should close in on the fact that differential frequencies of photon wavelengths have themselves different energies...

 

 

... do you not know this...?

[Sayonara]

Just to save lots of time, let me assure you that you won't evade his question by asking one back.

[insane_alien]

yes but you are arguing that a photon is the smallest unit of energy. this is clearly not true as photons can have a vast spectrum of energies.

 

for instance if i have an electron(mass energy equivalence of ~500keV) and a 1MeV then clearly the photon has more energy than the electron.

 

this is the point klaynos is trying to make. ...well he did make it, you just never picked up on it.

[Klaynos]

Are you intentionally being ignorant... i am sorry, but you of all people (according to your position here), should close in on the fact that differential frequencies of photon wavelengths have themselves different energies...

 

 

... do you not know this...?

 

I am trying to make you think threw what you have written. Something can't be the smallest value and vary in that value.

[Mr Skeptic]

Yes, and let us not forget that photons can also have more energy than a particle's mass-energy. Photons are quantized in that photons of a particular color have the same amount of energy all in one "packet" unlike the classical wave theory where you might be able to take half a photon's energy.

[Tom Vose]

How ill-advised. I am saying a photon can have a spectrum of energy, if that spectrum is revised by a history of a wave function. This should be known, without question.

[insane_alien]

The problem with trying to split a fundamental (smallest unit of energy)- such as a photon, is that it cannot be split in any smaller...

 

you said a photon was the smallest unit of energy, not that it was variable.

 

as it is variable it is not always the smallest. and as it is variable, it is possible to split a photon into two(or more) photons of lesser energy, the sum of which would be the energy of the origional photon.

[Sayonara]

How ill-advised. I am saying a photon can have a spectrum of energy, if that spectrum is revised by a history of a wave function. This should be known, without question.

What people are challenging is that, given that a photon can have a spectrum of energy values, that a photon should be called "the minimum unit of energy".

 

Do you not see the contradiction?

[Tom Vose]

you said a photon was the smallest unit of energy, not that it was variable.

 

as it is variable it is not always the smallest. and as it is variable, it is possible to split a photon into two(or more) photons of lesser energy, the sum of which would be the energy of the origional photon.

 

 

Are you kidding me?

 

A photon expriences a null-path... ... ... Variable photon energy has to be... if it was not, variable photon paths in variable speeds would not hold.

[insane_alien]

oh ffs, you must be misinterpreting that deliberately.

 

you know damn well that what was meant was 'photons can have different energies'.

[Tom Vose]

What people are challenging is that, given that a photon can have a spectrum of energy values, that a photon should be called "the minimum unit of energy".

 

Do you not see the contradiction?

 

Well that would make sense...

 

 

...I suppose...

 

Except, energy spectrum of an ethereal wave makes as much sense as an actual virtual wave... a wave that does not exist in the natural world...

 

 

... The minimal amount of energy that any natural action takes, will be the smallest amount.

 

oh ffs, you must be misinterpreting that deliberately.

 

you know damn well that what was meant was 'photons can have different energies'.

 

Thank you, that is appreciated.

[swansont]

Are you kidding me?

 

A photon expriences a null-path... ... ... Variable photon energy has to be... if it was not, variable photon paths in variable speeds would not hold.

 

A photon's energy does not depend on the speed. Under what context are you claiming variable photon speed?