The ultimate thread about photons (questioning the basics)

[afterburner]

OK...lately i've been thinking a lot about photons, and questioning their very basic and fundemental properties. I am looking to finally grasp the concept of photons. Can someone with knowledge in this area help me out please?
Im asking these questions, so there must be some other people that are asking the same questions...HELP US OUT PLEASE Oh and dont mind if im asking stupid questions, as i dont have much knowledge in this area

Here it goes

1. Photons are real particles right?
As in INDIVIDUAL photons DO exist
Is it a physical particle? I know its supposed to be energy, but in that case - what IS energy?

2. Photons have momentum, but occupy no volume and have no mass
Could it be possible that they DO have mass...just very very little? undetectable? Othervise how are they individual particles at all? once again what is energy?

3. Photon spin?
I know nothing about this, so please explain

4. Photons do not decay
So basically all photons created during and after the big bang (assuming it happened) still exist today (im not sure about this one, becasue i know photons can get absorbed, but then dont they get re-released again, just at a different wavelength?)

5. Moving a photon trough something extremely thick/cold?
I asked this one before, but what about an extremely high energy photon..how do those types of photons travel throuh matter?

6. Do photons created at the center of the Earth ever see the "light of day" meaning ever reach the surface and beyond?
Kind of related to question 5

7. Distinguishing photons?
It is impossible to distinguish between individual photons right, except their wavelength, but photons of the exact same wavelength are indistinguishable? is this correct?

8. Photons deviating when travelling great distances?
When a photon gets created, in some galaxy on the other side of the universe, and we observe it...does or did that photon ever change its cource?
That is to say, does a photon ALWAYS travel in a straight line? (this is assuming there are no gravitational forces acting on the photon) I ask becasue they travel in waves...read below

9. Photons travelling in waves
Photons travel in a wave-like fashion. Is this correct? im not referring to the wave function. What i mean is do photons actually have a peak and a trough. Do these photons go "up and down" aswell as travel at the speed of light in a particular direction?

10. 3d waves?
If #7 is correct, then do these photons "bounce around" within their wavelength?
What i mean is this - You can picture a hypthetical hollow tube in which these photons "bounce around" at their wavelength in all directions, while travelling. Would this be a more accurate description?
And back to #8, would this "bouncing around" affect the path of the photon (assuming the above is remotely correct)

11. The energy of a photon depends on its wavelength
So once again, what exactly is energy, if it is determined simply by the wavelength of a photon?

12. Whats the distance between photons?
According to antonisebs calculation, there are 1.2x10^13 photons per cubic meter of daylight at any one time, not per soecond.
Im assuming that number is smaller, but not significantly smalled out in a remote region of space. Would that be a correct asumption?
Also, that calculation is for photons emitted from from the sun, what about all of the other photons released from other stars, galaxies, this planet, they also must be there. As in we should add all of the other potential photons that are technically there, not just from our sun (basically he number should be greater?)..i dont know if that made sence...

13. Photons as a ether?
With that many photons per square cubic meter, do the math for square millimeter for a better picture - THATS A LOT OF PHOTONS!! at any one time in a millimeter cubed of space
Is it safe to say, that everything in the universe is in an "ocean" of photons? As in we are just floating in this stuff. (as well as gravitons among other things, but ill leave those for another thread )

14. Photon to photon interaction
With that many photons existing in such a small space, there ought to be some kind of interaction between photons, especially that these photons come from ALL directions..literally
I think it was Ken G that said that photons dont actually collide, but "pair up", and then split up ...this appears as a collision though, but its really not? please explain... if they are not a physical thing, what interacts with what? why would they NOT come in contact with eachother?
So they DONT actually collide, right? and they cannot occupy the same space..is that also correct or no?

15. Photon to matter interaction
When we say a photon hits a physicl object like a keyboard...or a mirror, does it come in contact with whatever it hits? I dont understand this becasue a photon is not a physical thing, so how does it "bounce off" of things? When we type and hit the buttons on our keyboard, the molecules from our fingers dont actually come in physical contact with the molecules making up the keyboard right? but thats matter and electrons are responsible for that.....what about photons though?

16. E=mc^2
So if E=mc^2, and photons ARE energy, does that mean, that if we cram enough photons into a small space, that they will become matter? But i though photons have no mass or volume? How could this be?

17. So why am i asking all of this? I had an interesting view of the universe...
I recently pictured the universe in a very srange way..hard to describe, but here it goes. Dont mind this part, becasue its something that should be in the ATM forum perhaps
Color as we know it does not really exist...its just something that our human, earthly eyes see or "pick up". I guess its easier to start thinking of the room you are in with ABSOLUTELY NO PHOTONS...you see what i mean...everything is colorless, the same, indistinguishable, black...Now this is the tough part...Pretend you cant see the objects in your room, but you KNOW where they are...Now keep that same colorlessness when we add photons(pretend you know where they are aswell)...track these photons mentally (obviously not ALL of them, just a few) What do you get? Think of the example where a tree falls in a forest and noone is tehre to see it..just a room where photons(energy) are bouncing around, getting absorbed, being released again..this is the world we live in? You can extend that though of colorlessness to our solar system, galaxy, cluster, and the universe...all in a "ocean" of these photons...but wait a second....E=mc^2, so energy is bouncing off enery?...for the third time..what is energy? i certainly dont know, which is why i made this post in the first place...

Cheers

[George]

I can only help on a general level.

Originally Posted by afterburner

1. Photons are real particles right?
As in INDIVIDUAL photons DO exist
Is it a physical particle? I know its supposed to be energy, but in that case - what IS energy?

I like to think of matter as condensed energy. However, a photon does not have mass (matter), it has energy (and momentum). If you allow energy into your idea of "physical", then it certainly is physical. It hurts to look at the sun, right? [if you do look, tell me what color you see. ]

4. Photons do not decay.
So basically all photons created during and after the big bang (assuming it happened) still exist today (im not sure about this one, becasue i know photons can get absorbed, but then dont they get re-released again, just at a different wavelength?)

I think many gamma rays might be offended with this line of thinking. Sometimes they get "released" at the same wavelength (elastic scattering).

7. Distinguishing photons?
It is impossible to distinguish between individual photons right, except their wavelength, but photons of the exact same wavelength are indistinguishable? is this correct?

Their propogation has aliginment. It is a transverse wave with electric and magnetic field orientations which are definable.

8. Photons deviating when travelling great distances?
When a photon gets created, in some galaxy on the other side of the universe, and we observe it...does or did that photon ever change its cource?
That is to say, does a photon ALWAYS travel in a straight line? (this is assuming there are no gravitational forces acting on the photon) I ask becasue they travel in waves...read below

From its point of view, I think, yes. However, space is warped here and there, so it will appear to us as bending (and stretching).

9. Photons travelling in waves
Photons travel in a wave-like fashion. Is this correct? im not referring to the wave function. What i mean is do photons actually have a peak and a trough. Do these photons go "up and down" aswell as travel at the speed of light in a particular direction?

(see above)

11. The energy of a photon depends on its wavelength
So once again, what exactly is energy, if it is determined simply by the wavelength of a photon?

I am unclear what you are asking. Its speed also is important to its energy. Wavelength is not a provider of energy, but it does help measure how "hot" it is.

12. Whats the distance between photons?
According to antonisebs calculation, there are 1.2x10^13 photons per cubic meter of daylight at any one time, not per soecond.
Im assuming that number is smaller, but not significantly smalled out in a remote region of space. Would that be a correct asumption?
Also, that calculation is for photons emitted from from the sun, what about all of the other photons released from other stars, galaxies, this planet, they also must be there. As in we should add all of the other potential photons that are technically there, not just from our sun (basically he number should be greater?)..i dont know if that made sence...

The number of photons in a given spot varies inversely as the square of the distance. If you move twice as far away from the sun, you will recive 1/4 th the number of photons. This law applies to all the other light sources out there. There are a fininte no. of sources, all aging, and space is expanding, so the number is limited and does vary with location and time.

13. Photons as a ether?
With that many photons per square cubic meter, do the math for square millimeter for a better picture - THATS A LOT OF PHOTONS!! at any one time in a millimeter cubed of space
Is it safe to say, that everything in the universe is in an "ocean" of photons? As in we are just floating in this stuff. (as well as gravitons among other things, but ill leave those for another thread )

Don't forget the humble neutrinos.
(see above for more)

Color as we know it does not really exist...its just something that our human, earthly eyes see or "pick up".

Hmmm, I don't remember reading this anywhere in any heliochromology book. But, ok, it is just what our eye and brain dream up. [Dr. Lamb coined the phrase "retinex" to incorporate the idea that both eye and brain work together to produce images, I think.]

I guess its easier to start thinking of the room you are in with ABSOLUTELY NO PHOTONS...you see what i mean...everything is colorless, the same, indistinguishable, black...Now this is the tough part...Pretend you cant see the objects in your room, but you KNOW where they are...Now keep that same colorlessness when we add photons(pretend you know where they are aswell)...track these photons mentally (obviously not ALL of them, just a few) What do you get? Think of the example where a tree falls in a forest and noone is tehre to see it..just a room where photons(energy) are bouncing around, getting absorbed, being released again..this is the world we live in? You can extend that though of colorlessness to our solar system, galaxy, cluster, and the universe...all in a "ocean" of these photons...

Shoot, I'll just be tickled to know the color of the sun, then I'll consider such a gedanken.

[kzb]

<<3. Photon spin?
I know nothing about this, so please explain>>

As I understand it, photons HAVE to carry spin. Their nature is something that carries quantums of "spin" (whatever that actually means at the atomic level) between charged particles (eg electrons in atoms).

[Fr. Wayne]

Presently there are a number of studies which are years in development which are studying the photon produced by the collision of gluons and quarks which measure the dynamic "packet" of electro-magnetic momentum contained within a specialized measureable photon. See link below for other links too:
http://www-d0.fnal.gov/Run2Physics/W...Q05A/Q05A.html

[Titana]

Originally Posted by afterburner

8. Photons deviating when travelling great distances?
When a photon gets created, in some galaxy on the other side of the universe, and we observe it...does or did that photon ever change its cource?
That is to say, does a photon ALWAYS travel in a straight line? (this is assuming there are no gravitational forces acting on the photon) I ask becasue they travel in waves...read below

Another strange thing happens to our mote of light. As noted earlier it tries to travel in a straight line, but can't quite succeed. Stars affect its path, clusters and even galaxies pull it off its course. Stranger still is that even after these affects are taken into account, it still curves slightly! The mass of the entire universe affects its trajectory. Space itself is curved and our particle follows this curve. In the two million lightyears it travels in its life this curve is a subtle influence, but is real! Astronomers would very much like to know the exact shape of this curve: positive as on a sphere or negative as on a saddle-shaped surface. The photon knows, but isn't telling.

The Life of a Photon


Titana

[Ken G]

I'll give you my take on your questions, as someone who works often with the
photon concept:

Originally Posted by afterburner

1. Photons are real particles right?
As in INDIVIDUAL photons DO exist
Is it a physical particle? I know its supposed to be energy, but in that case - what IS energy?

To the extent that any particle "exists", yes, photons do also. I would not say they "are" energy, I would say that "have" energy. What they "are" is photons, that's it. Energy is just a property that has proven useful to define in the manner that it is defined, and in different situations what we mean by energy is actually a bit different. It is a common misunderstanding that energy is a "thing" of some kind, rather than an attribute.

Originally Posted by afterburner

2. Photons have momentum, but occupy no volume and have no mass
Could it be possible that they DO have mass...just very very little? undetectable? Othervise how are they individual particles at all? once again what is energy?

They could be anything, of course, but our understanding of them that works very well is that they have no mass, in the sense of rest mass. They do not need to have mass to be a particle, as mass, like energy, is simply an attribute, not a thing in and of itself.

Originally Posted by afterburner

3. Photon spin?
I know nothing about this, so please explain

Spin is a very weird and profound attribute of a particle that controls a lot of things. It controls the angular momentum of the particle, from the point of view of angular momentum conservation, and it also controls whether or not you can have more than one identical particle in the exact same state. You can just think of it as internal degrees of freedom that says two otherwise identical particles can be in different states even if they are moving in exactly the same way. Photons have spin 1, which means you can put as many identical photons into the same state as you want, and it also connects to the fact that the classical description of how photons move can have two distinct polarization states (like polarized sunglasses only letting in light that is vertically polarized, not horizontally).

Originally Posted by afterburner

4. Photons do not decay
So basically all photons created during and after the big bang (assuming it happened) still exist today (im not sure about this one, becasue i know photons can get absorbed, but then dont they get re-released again, just at a different wavelength?)

What is meant by "decay" is spontaneous decay, in other words, photons don't just suddently pop into something else for no apparent reason. However, they can be destroyed, not just converted into a different wavelength. Photons from very early in the Big Bang were destroyed, and replaced by lower energy photons, which were clearly different photons, and which have since been redshifted to even lower energy by the expanding universe.

Originally Posted by afterburner

5. Moving a photon trough something extremely thick/cold?
I asked this one before, but what about an extremely high energy photon..how do those types of photons travel throuh matter?

Since photons carry energy, to stop or destroy a photon you need to have some process that can accomodate that energy. The higher the energy the photon, the fewer the processes that foot the bill, and so the less likely the photon will interact. Even if you are talking about scattering the photon rather than destroying it, there is still a dependence on the energy of the photon. An analogy might be firing bullets through a wall-- low energy bullets might have to hit a window to get through, high energy will just plow through.

Originally Posted by afterburner

6. Do photons created at the center of the Earth ever see the "light of day" meaning ever reach the surface and beyond?

No, not very likely. There is always some probability of almost anything happening, but when the probability is so low that it's pointless to even contemplate, we say it doesn't happen. Infrared photons emitted by the Earth must all come from very close to the surface, or they would have been re-absorbed and turned, at least temporarily, back into heat.

Originally Posted by afterburner

7. Distinguishing photons?
It is impossible to distinguish between individual photons right, except their wavelength, but photons of the exact same wavelength are indistinguishable? is this correct?

Yes, photons with the same attributes are indistinguishable, but it's not just wavelength, spin (polarization) is also an attribute.

Originally Posted by afterburner

8. Photons deviating when travelling great distances?
When a photon gets created, in some galaxy on the other side of the universe, and we observe it...does or did that photon ever change its cource? That is to say, does a photon ALWAYS travel in a straight line? (this is assuming there are no gravitational forces acting on the photon) I ask becasue they travel in waves...read below

A photon is a quantum mechanical particle, and as such, it has no definable trajectory. However, in many applications you can treat it as though it did have a trajectory and get the right answer. If you are doing that, then the photon trajectory can be altered by scattering. However, in astronomy we typically observe the photons that have not been scattered on the way. In cases where the wave property of photons is important, such as for radio waves, the concept of trajectory breaks down and the waves are affected by the medium through which they pass even if there is no obvious "scattering".

Originally Posted by afterburner

9. Photons travelling in waves
Photons travel in a wave-like fashion. Is this correct? im not referring to the wave function. What i mean is do photons actually have a peak and a trough. Do these photons go "up and down" aswell as travel at the speed of light in a particular direction?

You are referring to the wave function, that is the only sense to which photons travel like waves. Classically, we can measure an electromagnetic field which is also a wave, but this is due to many photons and is not the particle picture.

Originally Posted by afterburner

10. 3d waves?
If #7 is correct, then do these photons "bounce around" within their wavelength?
What i mean is this - You can picture a hypthetical hollow tube in which these photons "bounce around" at their wavelength in all directions, while travelling. Would this be a more accurate description?
And back to #8, would this "bouncing around" affect the path of the photon (assuming the above is remotely correct)

Not sure what you're getting at here, but it doesn't sound like a picture you'd want to adopt.

Originally Posted by afterburner

11. The energy of a photon depends on its wavelength
So once again, what exactly is energy, if it is determined simply by the wavelength of a photon?

It's an attribute, with various properties that involve conservation of energy and other things. It is not a thing in and of itself. The sci fi idea that there is such a thing as "pure energy" is bogus, you may as well say "pure telephone number".

Originally Posted by afterburner

12. Whats the distance between photons?
According to antonisebs calculation, there are 1.2x10^13 photons per cubic meter of daylight at any one time, not per soecond.
Im assuming that number is smaller, but not significantly smalled out in a remote region of space. Would that be a correct asumption?
Also, that calculation is for photons emitted from from the sun, what about all of the other photons released from other stars, galaxies, this planet, they also must be there. As in we should add all of the other potential photons that are technically there, not just from our sun (basically he number should be greater?)..i dont know if that made sence...

Yes, but distant stars don't contribute much to the photon density from the Sun in the daytime.

Originally Posted by afterburner

13. Photons as a ether?
With that many photons per square cubic meter, do the math for square millimeter for a better picture - THATS A LOT OF PHOTONS!! at any one time in a millimeter cubed of space
Is it safe to say, that everything in the universe is in an "ocean" of photons? As in we are just floating in this stuff. (as well as gravitons among other things, but ill leave those for another thread )

Don't forget neutrinos-- there are a lot of those too.

Originally Posted by afterburner

14. Photon to photon interaction
With that many photons existing in such a small space, there ought to be some kind of interaction between photons, especially that these photons come from ALL directions..literally
I think it was Ken G that said that photons dont actually collide, but "pair up", and then split up ...this appears as a collision though, but its really not? please explain... if they are not a physical thing, what interacts with what? why would they NOT come in contact with eachother?
So they DONT actually collide, right? and they cannot occupy the same space..is that also correct or no?

I don't recall saying that, but low energy photons such as we experience every day do not interact with each other, and they can occupy the same space (a property of their integer spin). Very high energy photons will scatter off each other because they can make particles and antiparticles which may then annihilate. I don't know if very high intensities of low-energy photons can find ways to interact or not, but it's very unlikely.

Originally Posted by afterburner

15. Photon to matter interaction
When we say a photon hits a physicl object like a keyboard...or a mirror, does it come in contact with whatever it hits? I dont understand this becasue a photon is not a physical thing, so how does it "bounce off" of things? When we type and hit the buttons on our keyboard, the molecules from our fingers dont actually come in physical contact with the molecules making up the keyboard right? but thats matter and electrons are responsible for that.....what about photons though?

Yes, photons don't "contact" things. Their wave function is affected by the thing, such that you can get a reflection, like from a mirror. This has to do with the rules of waves, and how they interfere constructively or destructively, just like sound waves bouncing off a wall.

Originally Posted by afterburner

16. E=mc^2
So if E=mc^2, and photons ARE energy, does that mean, that if we cram enough photons into a small space, that they will become matter? But i though photons have no mass or volume? How could this be?

There is no principle of conservation of volume, and energy can be converted into mass, so yes, photons can be converted into matter. The key is not to have a lot of them, it's to have high enough energy in each photon, to be able to convert it into a particle of matter (generally electron/positron pairs). In that case, one photon goes into one electron, they don't aggregate (it's very unlikely).

Originally Posted by afterburner

17. for the third time..what is energy? i certainly dont know, which is why i made this post in the first place...

That is a pretty good question even for trained physicists. All I can say is, energy is as energy does, don't try to think of it as a thing in and of itself. It is a property that obeys various rules, it is a useful construct. It may not be anything "real", in the sense that aliens who do physics might not use that concept at all. It's pretty darn useful though, so it's hard to imagine physics without it.

[Fr. Wayne]

Thank you very much, Ken G. Solid photon understanding with "no spin." (giggle.)

[afterburner]

Originally Posted by Ken G

They could be anything, of course, but our understanding of them that works very well is that they have no mass, in the sense of rest mass. They do not need to have mass to be a particle, as mass, like energy, is simply an attribute, not a thing in and of itself.
.

i just quoted that one paragraph from Ken G's post, but the reply is going to be to the whole thread and everyone that posted

Ok, so if both mass and energy are attributes, and high enough enery can produce mass, then mass and energy are the same thing, just mass is A LOT more energ than just energy? Just like we call a pond and an ocean..they are both water, one is just A LOT more water...is this correct?

If the above is correct then....With enough energy could we theoretically make things like iron, hydrogen or other elements?...Because if quarks are the basic building blocks of matter, and high energy photons produce electron/positron pairs, I dont see how matter as WE know it can be created...Also if matter and energy are the same thing it seems incredible to me that matter is so stable, whats holding all of that energy together?

How would a photon have more or less energy? what does that depend on?

If photons have two distinct polarization states, then they are physical particles right? Just to clear up, what i mean by physical is this...Its physical if we "zoom in" enough then there would be "something" there...I know there is nothing that we can use to "see" photons individually, but in a hypothetical situation, if we stopped all motion(including photons) and IF we had MUCH smaller particles to bombard the photon with, we would "see" this photon...similar how we use photons to see everything else (hypothetically speaking, of course)...THIS is what i mean by physical particles, or that they "exist", and that its not just a concept to help us understand what it REALLY is. I hope i made sence describing what i meant by physical.

Otherwise, its very confusing to me that photons are "nothing" in 3d space, but they have these attributes like energy, spin, and so on. How could this be? Also if photons "have" energy, then matter "has" energy, and if energy is "nothing" then matter is techically "nothing" aswell right? Would this mean that matter also has no volume, just "fields", which have volume, and we perceive as matter?

If these photons from the big bang were destroyed, then where did the energy go? even if they were replaced, how is that conservation of energy if they got destroyed?

I understand the wave function (well sort of), but that still isnt what i asked.
What i meant is the actual path of a photon. In textbooks the photon has a wavelength(in nanometers), with a peak and a trough...is this the case in reality, do photons fly through space in squigly lines or its just a straight line? Maybe i got somehting wrong again?

Whew..i think thats it for now

[George]

Originally Posted by afterburner

Ok, so if both mass and energy are attributes, and high enough enery can produce mass, then mass and energy are the same thing, just mass is A LOT more energ than just energy?

Nope. Mass and energy are different. Think of $100 in gold being equivalent to $100 in dollar bills; same but different, right?

How would a photon have more or less energy? what does that depend on?

I can only tell you how to know when it has more -measure its wavelength. The shorter the wavelength, or higher the frequency, the greater the energy. Some people will wiggle much faster if they have more energy, too. (e.g. dancing)

If photons have two distinct polarization states, then they are physical particles right? Just to clear up, what i mean by physical is this...Its physical if we "zoom in" enough then there would be "something" there...I know there is nothing that we can use to "see" photons individually, but in a hypothetical situation, if we stopped all motion(including photons)...

Strangely enough, you can never stop or slow a photon regardless of what you think you are doing with your speed. No matter what you, or a fast instrument, will ever observe is lights speed will always be the speed of light. This was the breakthrough Einstein made with special relativity in 1905. He beat others to it. Also, elapsed time for a photon is always zero. Those photons from quasars taking billions of years to reach us, from our point of view, took zero seconds from the point of view of a photon (if you could somehow ask it).

Otherwise, its very confusing to me that photons are "nothing" in 3d space, but they have these attributes like energy, spin, and so on. How could this be? Also if photons "have" energy, then matter "has" energy...

I might be corrected, but photons are a form of energy, and not obtain energy unto itself. The idea of energy is a concept that extends to numerous forms, including light. Metals come in different forms, too. Energy, however, is not normally expressed as mass, unless you are a particle physicists.

Would this mean that matter also has no volume, just "fields", which have volume, and we perceive as matter?

I suspect you are simply redefining things. Suppose it turns out that the smallest thing to matter are some peculiar units of pure energy, it still would not change how we perceive and understand the behavior already established for matter.

If these photons from the big bang were destroyed, then where did the energy go? even if they were replaced, how is that conservation of energy if they got destroyed?

They didn't vanish into another universe, hopefully, they reacted with matter and raised the energy level of the matter accordingly.

What i meant is the actual path of a photon. In textbooks the photon has a wavelength(in nanometers), with a peak and a trough...is this the case in reality, do photons fly through space in squigly lines or its just a straight line? Maybe i got somehting wrong again?

It exhibits behavior identical to a wave as you probably would expect a wave to behave. It also behaves as a particle, too. Some call them waveicles.

[Grey]

Originally Posted by afterburner

Ok, so if both mass and energy are attributes, and high enough enery can produce mass, then mass and energy are the same thing, just mass is A LOT more energ than just energy? Just like we call a pond and an ocean..they are both water, one is just A LOT more water...is this correct?

I wouldn't describe it that way. Perhaps a better analogy would be water and ice, where the two are different forms of the same thing. That's still just an analogy, though. I absolutely agree with Ken G that energy and mass are properties, not things in and of themselves.

Originally Posted by afterburner

If the above is correct then....With enough energy could we theoretically make things like iron, hydrogen or other elements?...Because if quarks are the basic building blocks of matter, and high energy photons produce electron/positron pairs, I dont see how matter as WE know it can be created...Also if matter and energy are the same thing it seems incredible to me that matter is so stable, whats holding all of that energy together?

Sure. High energy photons can produce any particle/antiparticle pairs. It's just that to produce, say, a proton and antiproton, you need a lot more energy than for an electron - positron pair, so we don't see that kind of reaction as often.

Originally Posted by afterburner

How would a photon have more or less energy? what does that depend on?

It depends on the amount of energy that was available in the interaction that produced the photon in the first place.

Originally Posted by afterburner

THIS is what i mean by physical particles, or that they "exist", and that its not just a concept to help us understand what it REALLY is. I hope i made sence describing what i meant by physical.

Given that description of physical particles, I don't think that anything actually qualifies. At the smallest scales, particles simply do not behave like miniscule billiard balls, they are quantum objects that behave in stranger ways.

Originally Posted by afterburner

Otherwise, its very confusing to me that photons are "nothing" in 3d space, but they have these attributes like energy, spin, and so on. How could this be? Also if photons "have" energy, then matter "has" energy, and if energy is "nothing" then matter is techically "nothing" aswell right? Would this mean that matter also has no volume, just "fields", which have volume, and we perceive as matter?

That's entirely possible, though it's by no means the only way it could be.

Originally Posted by afterburner

If these photons from the big bang were destroyed, then where did the energy go? even if they were replaced, how is that conservation of energy if they got destroyed?

The energy went into whatever interaction was involved in the absorption of the photon.

Originally Posted by afterburner

I understand the wave function (well sort of), but that still isnt what i asked.
What i meant is the actual path of a photon. In textbooks the photon has a wavelength(in nanometers), with a peak and a trough...is this the case in reality, do photons fly through space in squigly lines or its just a straight line? Maybe i got somehting wrong again?

I hope you won't think I'm being flip, but as long as you're thinking about the "actual path" of a photon, then you don't really understand the wave function. As Ken G pointed out, a photon doesn't really have a classical path. However, if we're in one of the situations where we can use such a classical path to describe a photon, then it's definitely not a "wiggly line". The oscillations shown in illustrations are oscillations of the electric and magnetic fields, not the physical position of the photon.

[ngeo]

If energy - and even mass - are ‘best understood’ as properties or attributes, not ‘things’ in themselves, and yet the ‘thing’ (in this case, a photon, but also a body of matter) is a form of energy, it seems to me the understanding will be limited. Is it perhaps that at this point science is more a matter of what works, regardless of the degree of understanding?

[Jeff Root]

Originally Posted by Grey

The oscillations shown in illustrations are oscillations of the
electric and magnetic fields

Do the amplitudes of the waves represent electric and magnetic
field strengths, or extension in space of the fields? Or what?

Does a photon have extension in space in *any* direction?

-- Jeff, in Minneapolis

[Grey]

Originally Posted by Jeff Root

Do the amplitudes of the waves represent electric and magnetic field strengths, or extension in space of the fields? Or what?

Field strengths.

Originally Posted by Jeff Root

Does a photon have extension in space in *any* direction?

Only in the same sense that other elementary particles have extension in space. That is, the position of a photon is "spread out" like any other particle. As to whether there's something physical occupying volume for any of these particles, I think that question remains unanswered.

[Ken G]

Originally Posted by ngeo

If energy - and even mass - are ‘best understood’ as properties or attributes, not ‘things’ in themselves, and yet the ‘thing’ (in this case, a photon, but also a body of matter) is a form of energy, it seems to me the understanding will be limited. Is it perhaps that at this point science is more a matter of what works, regardless of the degree of understanding?

I think what you are saying is that science attempts to explain the universe by using concepts that work. Whether or not the universe is "really" related to these concepts is not a matter of science, more like metaphysics or philosophy. Science is always what works, from mundane concepts like a ball and a trajectory, to profound ideas like a wave function. Nature must have some other way of "knowing" what to do that is not containable in the human mind, because everything in science is actually an idealization of nature.

[Jeff Root]

Originally Posted by Grey

Field strengths.

Only in the same sense that other elementary particles have
extension in space. That is, the position of a photon is
"spread out" like any other particle.

If photons were pointlike, or raylike, with no extension
perpendicular to the direction of motion, I would imagine
them to be able to pass through holes of arbitrarily small
size, nomatter what the wavelength. But I know they can't.

To put it crudely: Are photons unable to squeeze through
small holes because they extend perpendicular to the direction
of motion, and so "bump into" the sides of the holes, or for
some other reason(s)?

-- Jeff, in Minneapolis

[Grey]

Originally Posted by Jeff Root

To put it crudely: Are photons unable to squeeze through
small holes because they extend perpendicular to the direction
of motion, and so "bump into" the sides of the holes, or for
some other reason(s)?

I would say that photons (and for that matter, all other particles) are unable to squeeze through small holes (or do squeeze through, but in surprising ways) because when the size of the hole becomes comparable to or smaller than the wavelength, they behave in a wavelike manner when passing through it, and not like small solid objects.

[Ken G]

Yeah, what happens when a photon tries to squeeze through a small hole is a property of its wave function, not the photon itself. You can think of the wave function like an instruction manual that the photon consults whenever it moves. It provides the rules that a photon must obey, and the rules say that a small hole will spread out the possible places the photon can end up. This in turn is due to the importance of constructive and destructive interference of the processes that contribute to the wave function, which get altered by the hole.

[ngeo]

Not wanting to interrupt a good discussion but responding to Ken G above, I would agree that science always finds concepts (idealizations) that work, and perhaps that science attempts to explain the universe through these concepts. But I think when utility trumps ‘explanability’ there is a problem.

Slightly off topic by now, in the case of ‘particle/field is form of energy‘ versus ‘energy is property of particle/ field‘, the essential feature of the particle, field, energy, universe, etc. is motion. Everything moves! To me it would make understanding - and explanation - simpler if the word ‘energy’ were redefined as a ’universal fundamental’, namely the ability to move - and not simply to move some ‘thing’, but to move as in to create new events, paths, and systems (and - initial condition - spacetime itself). Then energy would be the ‘prime mover’, and fields and particles would be its creations. While that might or might not lead to a unified theory, it would lead to a unified perspective understandable by all. (Good wave/particle explanation above, though.)

[Jeff Root]

Along the same lines of shoving light through a small hole-- Is it
possible for light to move from side-to-side at all in a mono-mode
optical fiber, or is it packed in there so tight that it can't wiggle
at all, forced to slide straight down the center?

-- Jeff, in Minneapolis

[Grey]

Originally Posted by Jeff Root

Along the same lines of shoving light through a small hole-- Is it
possible for light to move from side-to-side at all in a mono-mode
optical fiber, or is it packed in there so tight that it can't wiggle
at all, forced to slide straight down the center?

I think that even in a single mode fiber, the light bounces off the sides (which is why it can travel around a curve). But when dealing with these issues and designing such cables, they use classical electrodynamics. That is, the light is behaving very much like a wave in this case, and not very much like individual particles.

[afterburner]

How far will a star have to be from an observer unitll the observer starts noticing big gaps of space where he cannot see the star. (even through a telescope)

just to make it more clear...if we put a telescope on one side of the universe, and a star on the other side, will the telescope pick up the photons from that star, or would they have scattered so much that we wouldnt be able to see the star at all?

im wondering becasue if light has a wave function, then technically the photons should be seen from an infinite distance (since the wave goes in all directions, forever, or am i wrong?)...but im thinking the star just wont be able to produce enough photons. hence the question...at what distance will this happen (not being able to see the star with a powerful telescope)?

[Grey]

Originally Posted by afterburner

How far will a star have to be from an observer unitll the observer starts noticing big gaps of space where he cannot see the star. (even through a telescope)

just to make it more clear...if we put a telescope on one side of the universe, and a star on the other side, will the telescope pick up the photons from that star, or would they have scattered so much that we wouldnt be able to see the star at all?

im wondering becasue if light has a wave function, then technically the photons should be seen from an infinite distance (since the wave goes in all directions, forever, or am i wrong?)...but im thinking the star just wont be able to produce enough photons. hence the question...at what distance will this happen (not being able to see the star with a powerful telescope)?

This works the same whether you view light as particles or waves. The intensity drops with the square of distance for the start, so to be able to see something as you move it farther away, you need a bigger and bigger telescope. If you want to think of the light as an electromagnetic wave, then you can think of the telescope as a device to concentrate the effects of a varying electromagnetic field so that you have a large enough amplitude at a specific spot to be able to measure it, when it would otherwise be too small to measure. If you want to think of light as photons, then you can think of the telescope as a device to collect incoming photons over a larger area and redirecting them all to a single point. Either way, in principle you can see a star at any distance, as long as you have a big enough telescope.

[George]

Photon scattering is not clear to me. [I trust this is not a tangent to the thread.]

Just what is going on when a photon encounters a particle much smaller than itself. In elastic scattering (Rayleigh), the wave length is unaltered but its direction, and polarization, I think, are changed. Is it...
1) an electromagnetic reaction to the electron shell(s)?
2) a diffraction issue?
3) a particle bounce?
4) an absorption and re-emission?

I am guessing an inelastic scatter (e.g. Mie) is no. 4, right?

[Ken G]

Originally Posted by George

Photon scattering is not clear to me. [I trust this is not a tangent to the thread.]

Just what is going on when a photon encounters a particle much smaller than itself. In elastic scattering (Rayleigh), the wave length is unaltered but its direction, and polarization, I think, are changed. Is it...
1) an electromagnetic reaction to the electron shell(s)?
2) a diffraction issue?
3) a particle bounce?
4) an absorption and re-emission?

I am guessing an inelastic scatter (e.g. Mie) is no. 4, right?

Be careful of the other meaning of Rayleigh scattering (the blue sky business, wavelength much longer than the particle size). For elastic scattering, the answers are:
1) yes
2) yes
3) yes
4) yes
In other words, it's not an either-or, each of these answers will work in the appropriate context. (1) means you are treating the problem classically, works fine whenever photon quantum effects aren't crucial. (2) means you are using the quantum mechanical wave function, or it could also be done classically, but it mostly means you are taking account of the coherence effects over the finite size of the target (typically only important for Mie scattering, i.e., not the Born approximation). (3) means you are contenting yourself with a rather crude particle picture, yet if you know how (1) or (2) plays out, you can then substitute picture (3) in cases where interference patterns are not of interest. (4) means you are treating both the photon and the electron quantum mechanically, and so this is the most correct way, although you have to be clear that you are using the generic meaning for "absorption" (that is, some people only use the word absorption to mean when the photon is thermalized and destroyed). By the way, Mie scattering doesn't mean it's inelastic, it means the way a wave interferes with itself is important, i.e., the size of the scatterer is not much smaller than the wavelength.

[George]

Originally Posted by Ken G

...the answers are:
1) yes
2) yes
3) yes
4) yes

Odd, I am more accustomed to more no's.

(4) means you are treating both the photon and the electron quantum mechanically, and so this is the most correct way, although you have to be clear that you are using the generic meaning for "absorption" (that is, some people only use the word absorption to mean when the photon is thermalized and destroyed).

I assume the photon transfers its energy to an electron, raising it to the next, or higher, energy shell. Then, the reverse happens allowing the resurrected photon, if you will, to take off in any direction, I assume. Is this a fair picture? If so, wouldn't this greatly slow down the propogation rate of light through a medium. Is this random walking, or just random thinking?

By the way, Mie scattering doesn't mean it's inelastic, it means the way a wave interferes with itself is important, i.e., the size of the scatterer is not much smaller than the wavelength.

I don't understand the true use of the term. I simply read it somewhere and it stuck. Further help would be appreciated.

[BTW, I want to observe blues, not give them to others. ]

[Ken G]

Originally Posted by George

I assume the photon transfers its energy to an electron, raising it to the next, or higher, energy shell. Then, the reverse happens allowing the resurrected photon, if you will, to take off in any direction, I assume. Is this a fair picture? If so, wouldn't this greatly slow down the propogation rate of light through a medium. Is this random walking, or just random thinking?

Yup, on all counts. Except for the random thinking.

Originally Posted by George

I don't understand the true use of the term. I simply read it somewhere and it stuck. Further help would be appreciated.

[BTW, I want to observe blues, not give them to others. ]

Well, when it looks like a white haze, it's Mie, but when it looks blue, it's Rayleigh (because Rayleigh favors blue, since the scatterers are so small they oscillate faster for higher frequency light, but Mie scattering has scatterers on the same size as the wavelength, so short wavelength light is less favored since the scatterer does not respond entirely coherently. Coherent enough?)

[George]

Originally Posted by Ken G

Yup, on all counts. Except for the random thinking.

Thanks. I'm not all opposed to some of my random thinking, yet it is probably the same as when one is proud of their humility.

Well, when it looks like a white haze, it's Mie, but when it looks blue, it's Rayleigh (because Rayleigh favors blue, since the scatterers are so small they oscillate faster for higher frequency light, but Mie scattering has scatterers on the same size as the wavelength, so short wavelength light is less favored since the scatterer does not respond entirely coherently. Coherent enough?)

[There should be some sort of pun precursor to minimize punishment. ] I was referring, however, to the terms elastic and inelastic. Do they have a deeper meaning other than delta wavelength (between the "in" and "out" of a scatterer), with delta=0 for elastic? I had assumed inelastic would require absorption but with less energy upon release.

[Ken G]

I think elastic just means no change in frequency in the scatterer frame. But terminology tends to evolve to fit a particular application.

[George]

Originally Posted by Ken G

I think elastic just means no change in frequency in the scatterer frame. But terminology tends to evolve to fit a particular application.

Thanks. That makes sense. I get the feeling scattering is no easy subject. I only found, and ordered, one book on it of possible worth at Amazon. Its about 750 pages written by.... S. Chandrasekhar. It should be a quick read. [I doubt I'll get beyond page 10 or so. ]

[afterburner]

So what exactly does the electromagnetic frequency represent? actual movement of a photon in Hz? Example: x-rays 3x10^17 - 3x10^19 Hz

and how does that compare to the neutrons oscillation frequency of 2x10^26 Hz?

does matter "move" much faster than photons? i still dont get it...

please help