[jeffreyH (OP)]

Since the big bang has the number of photons remained constant? If not then what kind of interactions changes this number?

[Janus]

The number of photons in the universe is constantly changing.  Stars produce them through fusion. radioactive isotopes emit them as gamma rays.  Materials can absorb them and then re-emit the energy as a larger number of lower energy photons...

[jeffreyH (OP)]

Related to the above answer is the following page from CERN.
http://meroli.web.cern.ch/Lecture_photon_interaction.html

[Kryptid]

Magnetic photon splitting is another mechanism that can change the number of photons in a system: http://iopscience.iop.org/article/10.1086/304152/fulltext/35776.text.html

[alancalverd]

I just switched on a light, thus destroying the balance of the universe.

[jeffreyH (OP)]

As soon as I read that I switched off a light. Ha ha!

[Bill S]

http://meroli.web.cern.ch/Lecture_photon_interaction.html

As consequence of such kind of interactions a photon that interacts with the target is completely removed from the incident beam, in other words a beam of photons that cross a medium is not degraded in energy but only attenuated in intensity.

Have I interpreted this correctly?

A beam of photons is travelling through (e.g.) a vacuum.  It hits a transparent/translucent target.  Any photon that interacts with the target is removed from the beam, so the continuing beam is not influenced by any energy exchange involving the removed photon.  Therefore, the photons in the continuing beam retain their original energy, but there are less of them.

[chiralSPO]

Just to muddy the waters a bit... if there are an infinite number of photons in the universe, then this would have to be unchanged by such limited transactions as absorption and emission.

[alancalverd]

Any photon that interacts with the target is removed from the beam, so the continuing beam is not influenced by any energy exchange involving the removed photon.

Yes and no. Depends on the nature of the interaction. At low photon energies the photon generally disappears into heat, a chemical change, or the movement of charge in an electrical circuit, but at energies above the visible spectrum you can get all sorts of secondary emission including photonuclear reactions.

We use all kinds of filters to remove photons of specific energies from a beam to produce a more monoenergetic  (monochromatic) beam of lower intensity.

[evan_au]

Therefore, the photons in the continuing beam retain their original energy, but there are less of them.

If the beam strikes an object that is above the black body temperature, the object will heat up, and will radiate the increased temperature as low-energy photons.

Anything in deep space that is hotter than the CMBR (2.7K) will, overall, radiate many low energy photons.

[yor_on]

You're thinking of the idea in where there only are transformations, but no 'loss' in a universe Jeffrey? If they are 'excitations in a field' then the number should vary with what interactions presents itself under a arrow of time I think.

[jeffreyH (OP)]

You're thinking of the idea in where there only are transformations, but no 'loss' in a universe Jeffrey? If they are 'excitations in a field' then the number should vary with what interactions presents itself under a arrow of time I think.

The question was intended to provoke answers of interest to readers. However, you are correct in interpreting my line of reasoning. It all stems from symmetry and conservation .

[yor_on]

Yes, those concepts are in some ways exceptionally mindnumbing. Symmetries and conservation laws. Remember when JP first explained to me how he looked at a 'photon recoil' as a 'demand' from conservation laws. It's not that far from being in a cave.

https://en.wikipedia.org/wiki/Allegory_of_the_Cave

[Bill S]

Just to muddy the waters a bit... if there are an infinite number of photons in the universe, then this would have to be unchanged by such limited transactions as absorption and emission.

Large quantity of mud!   If infinity is not a number, how can there be an infinite number of anything? 

Possibly you added the smiley because you guessed I couldn’t resist the bait. 

[jeffreyH (OP)]

Does the circumference of a circle with infinite radius equal a straight line? If so no part of the circumference would tell you that it forms a circle. Similarly no one photon would tell you it is part of an infinite set.

[Bill S]

Does the circumference of a circle with infinite radius equal a straight line? If so no part of the circumference would tell you that it forms a circle. Similarly no one photon would tell you it is part of an infinite set.

You should have been a politician   This doesn't answer the question.

[Bill S]

Depends on the nature of the interaction. At low photon energies the photon generally disappears into heat, a chemical change, or the movement of charge in an electrical circuit, but at energies above the visible spectrum you can get all sorts of secondary emission including photonuclear reactions.

What is the likelihood that emissions arising from these interactions would influence the original beam?
Would there be any effect other than "attenuated intensity"?

We use all kinds of filters to remove photons of specific energies from a beam to produce a more monoenergetic  (monochromatic) beam of lower intensity.

In these cases; would a filter equate to a “target”, in the original quote?

[Bill S]

Yes, those concepts are in some ways exceptionally mindnumbing. Symmetries and conservation laws. Remember when JP first explained to me how he looked at a 'photon recoil' as a 'demand' from conservation laws. 

Do you have a link to that explanation?

[yor_on]

Hmm maybe? It's here on TNS, and I miss that guy.
=

Can't find it Bill, but as far I remember it was 'turning my head'

Reality isn't what I expected then.

[Bill S]

Hmm maybe? It's here on TNS, and I miss that guy.

Don't we all?

Is this the thread it came from?  I’ve not had time to look through it yet, but could be, perhaps.

https://www.thenakedscientists.com/forum/index.php?topic=33720.msg326035#msg326035