What Is A Photon?

[Tekime]

Hopefully this doesn't sound too absurdly elementary, but it's still very difficult for me to adequately define exactly what a photon is!

 

It's a term that's thrown around all the time, but amazingly everyone that I ask starts stumbling over their words, mumbling about electromagnetic radiation and whatnot, and suffice to say usually throws up their hands and says "It'a a particle of light!". But what exactly is a particle? For that matter what exactly is light?!

 

Anyway, I've made some attempt at coming to a more cohesive, comprehensive definition of a photon, and most certainly have made some ridiculous blunders along the way, but here goes nothing...

 

A photon is a quantum of electromagnetic radiation; an elementary particle that is its own antiparticle. [1]

 

Yay! Defining words with other words we don't really understand. What exactly is a quantum?

 

A quantum is the smallest discrete quantity of some physical property that a system can possess. [2]

 

So basically a quantum is the smallest unit of measurement for a property of a system.

 

An elementary particle is a particle that is less complex than an atom; regarded as constituents of all matter. [3]

 

Okay, so... a photon is the smallest observable unit of some subatomic particle. Exactly what is that subatomic particle?

 

In particle physics the photon is classified as a subatomic particle of the "gauge boson" type.

 

A gauge boson particle is a "carrier" of fundamental forces of nature (gravity, electromagnetic force, weak nuclear force, and strong nuclear force) - in this case, a photon is a carrier of the electromagnetic force.

 

So a photon is the smallest observable unit of a non-observable particle?!

 

I'm obviously deeper in the mire than I was to begin with. Can anybody help untangle this mess?

[Qfwfq]

Not a dumb question at all dude! :) The truth is that nobody knows what a photon really is.

 

But what exactly is a particle? For that matter what exactly is light?!

The word particle is just the diminutive of part. Instead of light, you could say electromagnetic radiation (light is the kind our eyes see). Obviously this means something that radiates away and has to do with the electric and magnetic forces.

 

What exactly is a quantum?

The name which Planck gave to the smallest amount of energy that could be emitted and absorbed for a given frequency, according to his hypothesis which matches up with blackbody radiation. It solved the problem of the so-called ultraviolet catastrophe. Einstein later found it also matches up with data on the photoelectric efffect. He also proposed reasons to believe this quantization might be inherent in the nature of the radiation, not just in its emission and absorption by matter as Planck suggested. They were both aware that suggesting it to be a little corpuscle led to a very mysterious conundrum; now we know quantization is a fundamental fact of reality.

 

Okay, so... a photon is the smallest observable unit of some subatomic particle.

OK, here you must have read things wherein the word particle means the species rather than the "individual one" (which it turns out doesn't even make all that much sense). It's complicated, you can think of it like, whenever the quantity of the radiation in some circumstance is somehow determined, it can be 0, 1 or a larger integer multiple of the "quantum" (at least, keeping things simple).

 

Elementary just means non composite; it turned out that protons, neutrons and other things (called hadrons) are composite. The elementary things are in three categories: quarks, leptons and bosons and the first two are both also called fermions. Whew! The reason for the word gauge has to do with how bosons constitute the interactions between fermions. It's really wierd, don't try to figure it out unless you're prepared to blow your poor mind inside out. :lol:

 

So a photon is the smallest observable unit of a non-observable particle?!

Oooh, not quite, the photon is observable (when it isn't virtual), perhaps you read something saying that all what we can observe is the "quantum" aspect, which lends it a corpuscle-like nature, and not the amplitude, which lends it the wavy-fieldy-like nature but only calculates probabilities. Edited March 10, 2011 by Qfwfq
slight revision

[Little Bang]

Tekime, there are physicists that have explained it in a logical manner. http://scienceforums.com/topic/20998-photon-creation/page__p__289720__hl__%2Bphoton+%2Bcreation__fromsearch__1#entry289720

[CraigD]

Anyway, I've made some attempt at coming to a more cohesive, comprehensive definition of a photon, and most certainly have made some ridiculous blunders along the way, but here goes nothing...

You approach to the question “what is a photon” looks good to me, Tekime. I’ll jump in where I think it goes astray.

 

A photon is a quantum of electromagnetic radiation; an elementary particle that is its own antiparticle. [1]

 

Yay! Defining words with other words we don't really understand. What exactly is a quantum?

OK so far ...

 

A quantum is the smallest discrete quantity of some physical property that a system can possess. [2]

 

So basically a quantum is the smallest unit of measurement for a property of a system.

No. A quantum is not a unit, because units are arbitrary, and can be arbitrarily divided.

 

I think the key to interpreting the phrase “smallest discrete quantity” here, is to avoid complicating it with references to systems, and focus on the essential attribute of a quantum: its indivisibility. Let’s try using the term quantum first in some non-physics contexts:

• A quantum of apples is an apple. You can divide truckloads of apples into cases of apples, cases into bags, bags into handfuls, and so on, but when you get to a single apple, if you divide further, you no longer have any apples, but rather pieces of apples.
  
• A quantum of sheep is a sheep. You can split up a herd until you have a single sheep, but split that sheep, and you no longer have any sheep.
  
• A quantum of electromagnetic radiation is a photon. If you “divide” an ensemble of photons – for example, reduce the intensity of a beam of light with smoked glass filters so the light reads a light meter about 50% what it did before each filter – you reach a point where you get not 50% of the previous meter reading, but zero. This is not a failure of the meter’s sensitivity, but a real physical effect – eventually, the light beam consists of only one photon, and absorbing it with the filter results in no light.
  Unlike the previous examples, there’s no un-light-like residue of the “one division too many” of an ensemble of photons, but the principle’s the same – like apples and sheep, light comes in discrete, indivisible quanta.
  
• (I’ll leave as an exercise for the reader an explanation of the meaning of a “Quantum of Solace”, IMHO one of the best story and movie titles ever. :) )
  

There was a good word this quality of indivisibility: atomicity. However, its root word, “atom”, was ruined use as an synonym for “quantum” by prior use, when the objects to which it was applied, atoms, were discovered to actually be divisible, so these days it’s used mostly in computer programming to describe programming language elements and operations that are considered not to be built out of others.

 

Another essential quality of all of the quanta of quantum physics, including the photon, is that their existence are described in a way that can’t be interpreted as having definite locations, but rather only as having a distribution of probabilities of existing in arbitrarily selected volumes of space – a quality typically called “wave/particle duality”, but which I think is better described as “quantum weirdness”. This quality doesn’t lend itself as well to non-physics analogies as atomicity does, because outside of carnival funhouses, stage magic shows, and similar deceptive tricks, there really are no examples of it with everyday objects.

[joekgamer]

Essentially, a photon is two fields, one magnetic and the other electric, that are perpendicular to each other and the photon's direction of propagation at one instant of time.

[Tekime]

Thanks for the response guys! I'm still digesting all of this, and starting see where my reasoning was breaking down. I have a dozen new questions now but I'll let this unfold in my mind for a bit. :)

 

I guess it's challenging for me to comprehend because even if "photon" is a well-defined concept, the underlying nature of the system who's attributes it represents is still sort of a mystery. Or perhaps a complete mystery, depending on who you ask. :)

 

This might be going astray, but you know those moments when you are considering something so profoundly huge and incomprehensible that you are left without a single word in your vocabulary to describe the overwhelming and beautiful nature of that one idea? Those moments of awe, when you know that somewhere in there is the truth, but all you feel is humbled and hopeless in the face of the grandiosity of nature? I love those moments, yet hate them... they bring me to life and scare me to death.

 

One of those moments, for me, is considering the observable universe. I'm not sure if I can sum this up very well, but these questions about the nature of light often bubble to the surface for me in such moments. When we look up at a clear night sky, we are being bombarded by countless photons in an endless shower, from the deepest reaches of space. What our eyes detect is just a small portion of what is really there. Photons from galaxies 13.2 billion light years away are just now reaching our planet, just as they are reaching into every conceivable inch of the known universe not blocked from light. At the same time, light from every other conceivable inch of space is spreading out through the entirety of the visible universe. At any given moment as one peers up at the sky, I can only imagine trillions and trillions (or googles?) of light waves from every single visible point in the universe existing in every possible viewable position for the entirety of the universe. If I had Hubble at my disposal, I could zoom, zoom, zoom revealing deeper and deeper detail and resolution, limited, as far as I know, only by the hardware involved. And that's just visible light! I guess this is where I start running out of words, partly due to my ignorance, thinking of photons as particles of energy or waves all compacted into a finite physical space. It seems like nature would run out of room eventually!

 

Anyway, enough of that. I did want to add second part to the question. How would you describe a photon to a seven-year-old?

 

(No, I'm not seven! Unfortunately, nowhere near that... ;) Maybe in spirit!)

[lawcat]

Simply put, photon is the fundamental quanta of light. We call it quanta instead of a term like particle, because a particle denotes something material with volume and mass, and quanta is a more appropriate term because light is more about energy, radiation, rather than about material properties. You could say, a photon is a fundamental particle of light. Everyone would understand you better even though it is technically incorrect.

[Qfwfq]

How would you describe a photon to a seven-year-old?

It's a little bit of light.

[Little Bang]

Here is a guy or gal that's copied one of my posts, which I couldn't find, or at least the idea, since it is slightly different. http://universe1.freeforums.org/the-photon-t3.html