Photons again

[sr.vinay]

According to the law of conservation of mass and energy, they can just be inter-converted.

Thus, considering photons to be completely energy, is it possible to slow them down and then convert them to mass (however small it may be)?

[ajb]

I don't think it is constructive to think of photons as pure energy. Anyway, photons can be absorbed by materials. This increases the energy and thus the mass (as seen in the rest frame of the material).

 

There is no direct conservation of mass law in relativistic theories.

[Amr Morsi]

You are right ajb. Photons are particles that carry energy (, frequency and phase). By the way, although the rest mass of protons is zero, they do have a motion mass, which is h*(mu)/c^2, where mu is the frequency of the photon.

[sr.vinay]

I do agree that they do have mass. But, is it possible to convert most of their energy into mass again?

Though maybe not directly?

[ajb]

I don't really like the notion of a relativistic mass of a photon, but it comes in useful when thinking about the mass of a mirrored box of photons etc...

[swansont]

I do agree that they do have mass. But, is it possible to convert most of their energy into mass again?

Though maybe not directly?

 

They have energy.

 

You can convert it to mass in some reactions, producing a particle and antiparticle.

[Amr Morsi]

Some experiments have been made, after making the suitable circumistances, to collide two photons and convert them to a pair of electron and proton with the same energy. Such operation can be expected by Quantum Electrodynamics.

[ajb]

I do agree that they do have mass. But, is it possible to convert most of their energy into mass again?

Though maybe not directly?

 

You have electron + positron pair production for example. This won't work for a photon in isolation, you will not conserve 4-momentum doing this.

 

You also have two photon interactions that can also produce matter. It is the reverse of electron + positron -> 2 photon.

 

Also interesting is the photon's internal structure due to vacuum polarisation.

[sr.vinay]

Can you suggest some website that explains in detail the structure(if any) and properties of a photon?

[ajb]

Grab a book on QFT.

 

I like Ryder's book "Quantum field theory".

[mannzzu]

how come the zero mass at rest gains mass and energy! if it gains energy something has to lose energy

[Sisyphus]

how come the zero mass at rest gains mass and energy! if it gains energy something has to lose energy

 

Photons don't exist at rest. They only exist moving at C. It is correct that when a photon is created, something else has necessarily lost energy: whatever gave off the light.

[sr.vinay]

The source loses mass that is converted into photons.

So, this can be converted back to mass?