[Eternal Student]

Hi again.

It might be time to move on to your second question:

2. What exactly are Photonic Molecules?

   Sorry, I have very little idea.      There's some reference on Wikipedia:   https://en.wikipedia.org/wiki/Photonic_molecule

   I'm guessing you're interested in the science-fiction potential for these things.  I'm sorry I can't help but maybe someone else will know something about photonic molecules.

Best Wishes.

[Halc]

You know, it's funny. I LITERALLY just had an online chat with a cosmologist yesterday evening and one of the things he said to me was that matter, as we currently know it, may not actually exist at all. Fields exist, and particles such as Protons, Neutrons and Electrons are merely excitations of those fields

Classic things often tend not to exist in the quantum realm, so 'matter' is just a classical thing that defies close analysis, just as a rock (or the moon) might have a classic location in space even when nobody is looking, but that location defies close analysis. Fundamental things don't have a size and thus no density, but that doesn't mean that a rock doesn't have a density.
Yes, I agree that on close inspection, matter as we classically conceive it doesn't exist.

[Colin2B]

You know, it's funny. I LITERALLY just had an online chat with a cosmologist yesterday evening and one of the things he said to me was that matter, as we currently know it, may not actually exist at all. Fields exist, and particles such as Protons, Neutrons and Electrons are merely excitations of those fields

I agree with @Halc on this, but I think you need to sit down and think what you mean by the word exists.
We generally accept that the table your computer is sitting on exists.
A few hundred years ago no one would have known that it was made of molecules, or of the existence of atoms, electrons, etc. Knowing about those does not change the fact that the table exists, maybe not in the form everyone thought, but still the same at the macro level. I am surprised at your cosmologist, describing Protons, Neutrons and Electrons as merely excitations of those fields really doesn’t answer or change anything, it’s just a lower level of detail. We don’t describe a table as merely molecules.
As Ian Hacking said of electrons "if you can spray them, then they are real."

At any rate though, I do understand most of what you're saying. Mostly that we have very little idea what light actually is.

Actually we we know a great deal about what light is, how it works etc. It is another of those “merely” excitations of a field, and that means we can understand it in ways we couldn’t before.

Hi ES. I would like to offer an alternative view on this:

The next important thing is that you have probably read or been taught that    momentum = mass x velocity.    This is probably why you are concerned about the photon having 0 mass   but still having a non-zero momentum.
    There are at least two ways we can address this issue.   The first is to say that many physicists were also troubled about this.  It's a very good question to ask and something that does seem quite puzzling.
     Physicist's were sufficiently determined to maintain this simple concept of momentum that they developed a quantity called "relativistic mass".   They accepted that the invariant mass of a photon wasn't anything you could ever really measure, it certainly wasn't going to be measured as the mass of the particle when it was at rest in some inertial frame.  So they determined that the invariant mass wasn't something that should be used in that formula     momentum = mv.

I may be misreading what you say, but it implies that relativistic mass was ‘developed’ in response to the ‘rest mass’ of the photon being zero, and hence to maintain the concept of momentum for the photon.
The concept of mass varying with relative velocity predates special relativity and the concept of the photon, coming from the work of Lorentz and others. Lorentz was trying to work within a stationary aether theory and postulated that the measuring apparatus designed to detect movement relative to the aether was length contracted (Fitzgerald contraction) and so could never detect the movement. This led to his famous transforms.
There was also parallel work on the concept of electrostatic mass, that a charged body is harder to accelerate than an uncharged one, and this electrostatic mass increases with velocity. Lorentz was working on an electron theory and applying this electrostatic mass via his transforms he developed the concept of relativistic mass (both longitudinal and transverse). Interestingly he also changed the original Newton’s law that “force = rate of change of momentum” into the form we know today, F=ma.
What is really interesting is that Poincaré took the Lorentz transforms and gave them the form we use today. He also showed they were the result of principle of least action, showed that what we call the spacetime interval is invariant, suggested c might be an unsurpassable limit, suggested a clock synchronisation method using light, and suggested gravitational waves might exist. He apparently decided that developing the work would be too much effort for no useful result. So near!
Einstein originally took on the term relativistic mass in his early papers when he showed his famous E²=(m₀c²)²+(pc)² , but spoke against its use later.
Maintaining the concept of momentum for the photon was never an issue, as long as you believe that momentum is conserved (which thou shalt). If the photon has momentum as it leaves the atom, then the atom should recoil, which it does, and that can be measured. Similarly momentum is transferred at the receiving end.

[Eternal Student]

Hi  and especially @Colin2B ,

   What you've said about relativistic mass seems reasonable.  It's also more history than I was aware of and I haven't checked all the details but I'm quite happy to accept what you said.

The original statement probably should be re-phrased:

Physicist's were sufficiently determined to maintain this simple concept of momentum that they developed   had a good reason to cling on to a quantity called "relativistic mass".

    For certain, Relativistic mass was developed as a concept for several reasons and not specifically because there was an issue with photons having momentum.   
    The spirit of what was originally stated does (hopefully) remain intact:   Relativistic mass does help to maintain some relativistic analogues of equations and concepts from Newtonian mechanics.  In particular, a massless particle having momentum does seem strange starting from Newtonian definitions of momentum and using "Relativistic mass" does help to maintain a facade of Newtonian-like concepts.
    (I've also edited the earlier post to include a  **Footnote** to prevent the paragraph being taken as a literal account of any historical development of the term "relativistic mass"). 

Best Wishes.

[Eternal Student]

Hi again.

Let's take another question from the OP:

3. Photons are capable of transferring light (electromagnetic energy) as radiation, but why is light the only form of energy that has the ability to move as a particle? Why don't other forms of kinetic energy like heat and sound have their own particles to move as?

   Well, there is some interest in things called  Phonons   (note the spelling).

In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. Often referred to as a quasiparticle, it is an excited state in the quantum mechanical quantization of the modes of vibrations for elastic structures of interacting particles. Phonons can be thought of as quantized sound waves, similar to photons as quantized light waves.

- Taken from Wikipedia:  https://en.wikipedia.org/wiki/Phonon

    Personally I still think there is too much emphasis on light being a form of energy.   This was discussed in previous posts.   For school level Physics, yes,  light is listed as a form of energy.   At University level, it isn't necessarily useful to imagine that energy is any kind of substance and so light isn't really made of energy, it just carries some energy or has some energy value associated with it.

Best Wishes.

[Petrochemicals]

Photons could have mass, but my guess is to do with special relativity and the fact that the propagation of light is still mysterious, wave particle duality and quantum mechanics. It does seem strange though.

[Eternal Student]

Hi again,

Let's look at the next question from the OP:

4. When the Sun radiates its energy as a storm of particles, many of them travel throughout space, all the way to the planets around it such as Earth, Mars, Venus and the Moon. Once the energy reaches those planets, they radiate an equal amount of energy away back into space in the form of infrared photons and useless radiation. What about the particles that don't reach anything though, and bolt of into the unobservable universe? There's no air or stone to steal the particles energy away, so do they hold onto that energy forever, or do they eventually loose it through a radiation-like process?

     Photons seem like very stable things, they don't decay into other particles as far as I know, no matter how many billions of years you wait.   The main evidence for this would be the CMB (Cosmic background radiation) - this has been travelling for about 14 billion years  (the entire life of the universe) and we can still observe it today.
     So, if you restrict your attention to the light (and other wavelengths of e-m radiation) which the sun radiates away then this would seem capable of travelling through the universe for eternity if it doesn't come into contact with anything that it would interact with.
      That doesn't necessarily mean that the energy carried by these photons doesn't ever change.   Our best models of the universe show that space is expanding and light redshifts (increases it's wavelength) as it travels through the universe.  This would mean that the photons lose energy as they continue travelling through the universe - but we have to be a bit careful to specify exactly which frame of reference we are using to measure the wavelength.  Let's just be sensible and say we would naturally tend to measure the energy of the photons from a frame of reference that is centred around the sun  (the place where they were originally emitted).   Staying at the sun and always taking measurements from there, we would observe the photons leave with some energy value and that energy value would slowly fall as the years pass by.    (There's a lot of assumptions here but if you're interested in more information there's an entire thread and a recent podcast by the NakedScientists:    https://www.thenakedscientists.com/forum/index.php?topic=83011.0).

     You did say "the sun radiates its energy as a storm of particles" and you're quite correct that it isn't all photons.  The sun also emits other energetic particles - loads of neutrinos but also some larger particles like alpha particles.   First of all the chances of these actually interacting with some other matter eventually are probably larger than you might think.  Space is mostly empty but it is also very big.  There is some interstellar gas spread all over the place and there are plenty of planets, stars and even black holes all with gravity wells that a particle can fall into.   However, let's assume a massive particle can find a trajectory where it avoids contact with anything for billions of years.
     Many massive particles, like Neutrons, are thought to be unstable (reference:  https://en.wikipedia.org/wiki/Free_neutron_decay).  Neutrons are more stable when bound inside a nucleus but given billions of years it seems likely that even these will decay into protons and electrons.  Now it's possible that these smaller things continue to decay but we really don't know and it's quite possible that they don't.  Proton decay is extremely hypothetical and seems to break a conservation of baryon number, it's never been observed to the best of my knowledge  (reference: https://en.wikipedia.org/wiki/Proton_decay).  The larger Leptons like Muons certainly decay but the smaller ones like electrons may be stable.
     None-the-less, after quadzi-trillions of years (I'm not really sure of the order of this timescale, we're talking about seriously big time, many orders of magnitude bigger than the universe has been around for so far) all massive particles may decay or revert back into photons through one mechanism or another.
   Some mechanisms may inlcude quantum tunnelling that establishes a small black hole within a massive particle and we believe that ultimately all black holes will "evaporate" by releasing Hawking radiation.   See this video for a non-specialist and friendly discussion of what might happen (if you're interested):

Info:   "How will the Universe end" - PBS Spacetime, available on Youtube.  This Video is nearly 18 minutes long.  Most relevant sections are 5:58 to 7:00 about Proton decay  and also 9:20 to 11:40 about Tunneling to form micro black holes.

    So, ultimately all particles emitted by the sun should break down into just photons and then these follow the same general loss of energy as described above for light that was emitted and travelled through space.
   Over-all then, there are a lot of assumptions and many things could change as the universe evolves but if it did behave as most models expect then all energetic particles emitted sun would eventually lose energy (as seen from a frame of reference that stayed with the sun all the time).  This isn't really like re-radiating energy as some longer wavelength or exactly the sort of thing you suggested in the OP but it reaches the same end result one way or another -->  whatever energetic particles are emitted by the sun they end up as ultra-long radio waves with ever increasing wavelength and decreasing energy as time progresses.

Best Wishes.

Late Editing:   Added info to the video;  directly mentioned that some leptons my be like the proton and may not decay by ordinary means.

[Eternal Student]

Hi again,

So, I think this just leaves the last question in the OP:

5. How feasible on a scale of 1 to 10 (1 being not at all feasible and 10 being very feasible) is the idea of a Photon blaster/cannon? Think of something like Star Trek's particle cannons, but with Photons instead of electrons/protons. Could we realistically weaponize Photons like this? Just for the record, I know I'm more or less describing a laser, but I was thinking more along the lines of Star War's blasters, or Iron Man's Repulsors. Would something like THAT be possible?   

    I don't really know and a detailed discussion is probably more suited to the Just Chat section.  I would have thought those TV shows and stories show blasters and all sorts of other energy weapons as transferring some momentum to their target just because that is what we would expect from a weapon.   It's also much easier to film and more dramatic when someone is pushed backward rather than just having a small hole put in them.  Such a hole would tend to be really neat and tidy and automatically cauterized by the heat of the laser anyway.  Probably not a very effective weapon to be honest with you and certainly not dramatic enough for TV.
    However, there is energy in all of these beams fired in a TV show and all sorts of possibilities for this to cause a change in kinetic energy of the target,  i.e. to  impart some momentum to them.  It's quite possible that a Star Wars blaster bolt isn't just a simple shot of photons but instead it's a shot of some other energetic stuff with the outer layers of it giving off photons as it travels.
     On a scale of 1-10, how likely is it we could weaponise photons to produce some significant momentum transfer to their target?  I'd say  2 out of 10.

Best Wishes.

[Halc]

5. How feasible on a scale of 1 to 10 (1 being not at all feasible and 10 being very feasible) is the idea of a Photon blaster/cannon? Think of something like Star Trek's particle cannons, but with Photons instead of electrons/protons. Could we realistically weaponize Photons like this?

Weaponized photons is just a laser, and yes, they've already got prototypes of stuff like that. The intent is to heat/vaporize the target, at least enough to render it (typically something in orbit) non-functional. There isn't significant momentum transfer there, but plenty of energy. Very hard to hit an accelerating target at any distance with any ballistic weapon like that, so it makes for a poor choice of weapon between space ships. Sorry star wars/trek.
Ever read Haldeman's novel The Forever War? It is about the only semi realistic sci-fi depiction of space battles, except for the jumps that take them distant places faster than light. Part of the strategy is to never let the enemy know where your home planet is.

Such a hole would tend to be really neat and tidy and automatically cauterized by the heat of the laser anyway.

I'd think it would boil any body fluids, and the steam explosion would blow the immediate area to bits.
Why do the storm troopers sport armor? One shot from the lightest weapon seems to fell them, so it's good against nothing but maybe shrapnel, and even then I don't recall one surviving a near artillery hit.

it's a shot of some other energetic stuff with the outer layers of it giving off photons as it travels.

Definitely so since it doesn't move at light speed and you can't see light from the side, only if it is aimed straight at your eyes.
Rule of thumb: For a cinema light weapon that puts out 'bullets' (as opposed to some kind of continuous phaser beam), it takes about 0.1 seconds to cross the screen regardless of scale. Just enough frames to show which way it is going. That means faster than light if the camera is panned back far enough. You'd think the shots that miss would just keep going in space forever, but no, that would violate the 0.1 second rule.