where are all the photons before you turn on the light?

Factual Questions
1

Ok, so my grasp of quantum mechanics is not a sophisticated one. But as I learned it, matter is made up of atoms, which are made up of protons, neutrons and electrons. Your standard lightbulb is an enclosed glass envelope containing some sort of filament and a gas. When current hits the filament, the bulb starts emitting photons like crazy. Where were the photons before the current hit the filament? Tied up in the gas somehow and now suddenly freed by the current?

Is matter made up of protons, neutrons, electrons AND photons? Are photons invisible and all around us, and only visible when excited in some fashion?

PS It’s my first post, so please be kind to me. :wink:

2

A photon is a discrete quantum of energy, in the form of electromagnetic radiation. It has no mass. To emit light, an electron is excited (energy applied), which sends it to a higher energy state, and then drops back down some time later (a very short time). That sends out the EM wave, which we see as visible light. The input energy comes from the electricity moving through the wire of course.

3

They come from the energy you are pumping into it via electricity.

The energy is transferred from electrical to photonic (sorry for the Star-Trek-ese), emitted on a wide range of frequencies. Much of it is light, but some is in the infrared range, some in ultra-violet.

4

A few misunderstandings:

1)In incandescent bulbs, there is no gas. It’s a vacuum (hence the cool pop when you drop them off of overpasses into traffic). Flourescent bulbs, on the other hand, have no filament.

  1. Photons are not matter. They don’t lie around in wait until something excites them. Before you turn on the light they (in essence) don’t exist.

Using the incandescent bulb as an example, here’s what happens:

You start the current, and energy starts flowing through the filament. The excess energy excites the electrons in the filament, forcing them into a higher energy quantum step. (Important for this discussion - when electrons are excited, they “step up”, that is, they stay in one state until they are excited enough to jump to the next step - there aren’t any intermediate states for electrons). To anthropmorphize for a sec, the electrons want to get back to their resting step, and to do that they must release the extra energy. This they do in the form of photons (both visible and infrared, which is why bulbs get hot).

So, the photon is really just a measure of energy. The photon itself can ACT as a particle (and as a wave), but it is not a bit of matter being released by the lightbulb.

V.

5

It was about at my level - not too technical or abstract. In fact, Feynmann was a little animated bear who danced to rhyming songs about physics. Just kidding.

Really, I saw this film with Feynmann, and, IIRC, he said once upon a time he asked the same question: If photons come from excited electrons, are the photons in the electrons? The answer was, no. Think of it like your voice - your voice comes out of you, but it’s not really in you to begin with. You just have the energy to make it.

One advantage of the “wave” side of the wave-particle duality is that it’s much easier to conceive of a wave being produced by an object, without that object previously containing the wave. Like, a kid falling into a pool makes waves, right? The “particle” side is admittedly harder to understand in this context. That’s the reason you kind of need both parts of the duality at all times, to truly understand light.

6

In incandescent bulbs the light comes not from electrons jumping quantum energy levels, but just from black body radiation from a very hot filament. This is approximately the same reason why the sun is the color it is - it is just really hot. When any matter is hot, the atoms in it are vibrating - the faster the vibration the hotter. Vibrating particles emit radiation - basically, any charged particle that is accelerating will emit radiation, so all the atoms are spewing radiation in an amount dependent on the heat of the object. Also, this type of radiation is spread over all frequencies from infrared to visible. This is a separate mechanism from the ‘quantum state jumping’ type.

Flourescent bulbs on the other hand use a plasma, a heated soup of ionized gas that does involve electrons making quantum jumps. Because the photons emitted are in discrete frequencies and most of those are in just the frequency range of visible light, the bulb uses less energy and stays cooler for the same amount of perceived light. However, the spectrum of light emitted is discrete - that is you have specific wavelengths of light and not a continuous spectrum. This makes flourescent lights appear harsher to the eye, and incandescent bulbs emit a ‘softer’ light.

This is the approximate reason why incandescent bulbs can have dimmer switches (just cool the filament by running less electricity and you get less light) but in general flourescents cannot (its either on or off - the quantum jumping thing again.)

7

Thanks for the correction, Douglips

8

Some lamps, especially early ones, were evacuated, but most modern lamps contain a mixture of inert gasses. Typically the mixture is 90% argon and 10% nitrogen. They are present to retard evaporation of the hot tungsten filament. Krypton is superior to argon, but rarely used because of its higher cost. See The Great Internet Lightbulb Book

The reason halogen lamps last so long is that the halogen gas combines with the tungsten vapor and the tungsten redeposits on the filament. In a traditional incandescent lamp, the tungsten deposits on the inside surface of the glass, which is why old bulbs have large gray areas.

I believe the inert gases are at less than atmospheric pressure, but I haven’t been able to find a reference.

9

Wait a second, I’m confused. the way it was explained to me in school, a blackbody is a theoretical perfect absorber, versus radiator. Could you cite for me so that I can unlearn?

Thanky.

10

A perfect absorber is also a perfect radiator (emitter). You can’t have an object that emits more radiation than it absorbs, because that object will continue to cool down till it reaches absolute zero. Same goes for an object that absorbs more than it emits - it will reach infinite temperature.

11

From Feynman’s memoir What DoYouCare What Other People Think? :

Heck, if Feynman had difficulty explaining it, I’m sure not gonna try.
(The preceding statement is facetious : we’ve had a lot of time since this incident to understand this concept, and the explanations here seem pretty good.)

panama jack

“there are all kinds of interesting questions that come from a knowledge of science, which only adds to the excitement and mystery and awe of a flower. It only adds. I don’t understand how it subtracts.” – Richard Feynman, in response to an artist friend who claimed scientists take things apart and make them dull.

12

My dad met up with Feynmann back in the 70’s. They discussed Nuclear Physics matters. I someday hope to persuade my dad to get on here.

13

Thanks for clearing that up. I had kind of remembered it backwards, as if Feynmann had asked his father, but that didn’t make sense unless Feynmann’s father had some sort of physics-teaching background.

Don’t worry, I didn’t think Feynmann asked this question in the middle of his professional career. I remember another part of the film, which I now have a BIG jones to see again, when Feynmann described a question he asked his father when he was knee-high to a grasshopper. Again, my memory is fuzzy, but it was about inertia - why does a wheel in a wagon roll backwards when you roll the wagon forwards? A pretty inciteful question from a child. The answer was, it doesn’t roll backwards. It just wants to stay in the same place, and since it isn’t attached to the wagon, it can (almost) do so. And a little physics mind was launched.

14

Black body radiation is caused by electrons jumping energy levels; kinetic energy pushes them to a higher energy level; they then emit a photon to move to a lower level. It was originally the puzzle of black body radiation that impelled Plank to come up with the beginnings of quantum mechanics to make the model of statistical mechanics apply to the problem of radiation.

The photons themselves come from the vacuum. Since time and energy are complementary, Heisenbert’s equation (delta-t * delta-e = h) applies. That means that for short periods of time the uncertainty of energy becomes large enough that virtual photon/antiphoton pairs can spontaneously arise. Normally these virtual particles merely annihilate one another and no one is any the wiser (well, not really; the mere presence of virtual particles have a measurable effects).

However, near an atom with an electron in an energetic “excited” state things get more complicated. Sooner or later a virtual photon/antiphoton pair will appear near the electron with the photons’ wavelength corresponding to the energy the electron has gained by movinging to its excited state. It then becomes possible for the electron to transfer its energy into the photon/antiphoton pair, essentially absorbing the antiphoton, moving itself to its lower energy state and leaving the photon free to fly about as a “real” particle.

Realize that all of this is not really taking place on the “particle” level; what’s really happening is that the wave function of the Universe’s electromagnetic field is interacting with the wave function of the electron (which is not even a particle in the ordinary sense of the word while bound to an atom). For instance, the “emmission” of the photon at time t does not definitively “happen” (or “not happen”) until the photon is actually “observed” (or “not observed”) at time t+n.

You can read more about virtual particles at the Usenet Physics FAQ.

15

Wow! Thanks to all who contributed.

I still don’t pretend to understand quantum mechanics, but I definitely feel more “illuminated” on this subject. Perhaps a few stray photons have managed to penetrate the murky veil of my ignorance…

Again, thanks to all!

16

Don’t ya just love this message board?
Yeah, me too!

17

Feynman writes, from beyond the grave:

You are of course correct, Feynman, but I am also, modulo some miscommunication on our part. I mistakenly took the words “higher energy quantum step” in SuaSponte’s post to mean the energy levels of an electron in an atom, which is not the main source of radiation in blackbody radiation. Atoms or molecules that are vibrating like weights on a spring due to thermal energy also have ‘quantum states’, but they are not so well defined or quantized as the atomic energy levels.

There are two types of quantum energy levels that are being confused (by me) here.

[ul][li]Electron energy levels in an atom/molecule. The amount of energy an electron can have above its ‘ground state’ in an atom is quantized. The ‘ground state’ is the lowest energy state an electron can have in an atom.[/li]
The size of the quantization depends on many factors, chief among them the number of protons in the atom, how many other electrons are present in the atom, and how highly energized the electron already is. To excite the electron from the ground state to the next available state takes quite a bit of energy - to go from that state to the next state takes less, and so on. The energy it takes to get the electron all the way off the atom is the ionization energy, and if you added up all the steps of energy to go from the ground state to the Nth state, as N approaches infinity the total energy approaches the ionization energy. Does that make sense?

In short, in this type of quantum state change, only photons of specific wavelengths may be emitted because there are specific energy levels of the atom to be observed.

[li]Electron/other particle energy levels in general. A photon can carry no less than hv energy, where v is the frequency, so any particle that reduces its energy must emit a photon in the amount of energy appropriate for the reduction in energy. However, there is no restriction on what energy level the particle can go into, because a photon of any energy from near zero up to the entire energy the particle has can be emitted.[/li]
This is the bit of quantum theory that saved us from the ultraviolet catastrophy. This part of quantum theory does not involve electrons jumping levels in atoms, but instead involves electrons acting like pendula that must slow down by emitting a photon. The pendulum can move at any speed between zero and the original speed before photon emission.
[/ul]

So, when an electron goes from one quantum level in an atom to a lower state, it has a limited number of choices of which state to jump into, and must emit a specific photon. But, when an atom goes from one quantum state to another by radiating a photon, it can emit any frequency it chooses up to an upper limit of the total amount of energy it has.

The confusion comes from the ‘jumping levels’ terminology. I was using it in the sense of the levels of an electron in an atom, the first sense of quantization. Feynman, I believe you are using it to describe the second. I don’t really think of atoms that are emitting quanta of blackbody radiation to be ‘jumping quantum energy levels’, because they can go to any energy state less than what they currently have. They are changing from one quantum state to another, but I didn’t consider “energy level” to refer to that because there is no definite energy difference between one and the next. Mea culpa.

In summary, the energy levels you are talking about are just any energy an atom or molecule vibrating in a blackbody can have, and the energy levels I’m talking about are the different ‘orbits’ (chemistry term) of electrons in an atom. Two very different things, but both are quantum states.

Thank you for keeping me honest.

A good quick explanation of the evolution of blackbody radiation theory can be found at http://theory.uwinnipeg.ca/physics/quant/node2.html .

18

You are, of course, entirely correct as well. :slight_smile:

19

Minor quibble: The energy states in a molecule are still quantized, but the levels are extremely close together, and since all energy levels have a slight nonzero width due to the Uncertainty Principle, the levels can blur together.
Besides, molecular blackbodies are boring. You want a real blackbody, try the Universe, or a black hole. Unfortunately, nobody yet can really say what’s going on at the quantum level in either of those.

20

Dr. Hawking seems to have some ideas as to what’s happening at the surface of a black hole. Inside, however, is a different story; AFAIK no one has come up with much more than an educated WAG.