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The answer I usually get (and I'm paraphrasing here) is that they disappear and are instead absorbed as heat energy.

But I find it hard to believe that the photon simply "disappears." Common sense tells me it must turn into something or other, not just simply poof out of existence; then again, common sense has betrayed me before.

Forgive me if this is obvious; high school physics student here who's just learned about light and is greatly confused by all this.

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    $\begingroup$ Imagine a vibrating rubber band - now use something to stop that vibration. Whatever you stopped it with will have increased in energy (having absorbed it from the rubber band). Nothing "poofed" out of existence. A photon is not strictly a wave, of course, nor is the EM field a rubber band, but for the purpose of analogy I think it works. $\endgroup$ – J... Mar 6 '15 at 9:17
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    $\begingroup$ I find it hard to believe... Please, see this: youtube.com/watch?v=iMDTcMD6pOw $\endgroup$ – aaaaa says reinstate Monica Mar 6 '15 at 12:11
  • $\begingroup$ If you prefer, it wouldn't be too wrong to think of an electric charge as a hole in the universe into which photons may disappear, or out of which photons may appear. That is, the best known theory (Standard Model) for photon interactions could be interpreted in that way without changing the mathematics. HOWEVER, this sort of thing is fluff; it doesn't really have a proper meaning (the only real meaning is the mathematics) and this particular interpretation is not conventional, so it would not be a good idea to mention it to your teacher. (And I probably shouldn't mention it here either...) $\endgroup$ – Harry Johnston Mar 7 '15 at 3:29
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    $\begingroup$ How is "the photon disappears and is absorbed as heat energy" any different from "the photon turns into something"? Specifically, it turned into heat energy. Because it turned into something else, it's not a photon, so the photon did disappear. But +1 for recognizing that common sense is fallible and I'd give you another +1 if I could, since the actual answers are way more interesting than the logic-based argument I just gave. $\endgroup$ – David Richerby Mar 7 '15 at 12:07
  • $\begingroup$ @aandreev I find all this quantum stuff hard to believe. It's so unlike everything I've learned, where none of its rules seem to apply, and I find it extremely hard to believe. I think that's natural. And this same mysteriousness absolutely fascinates me. And I think that's natural as well. $\endgroup$ – user3932000 Mar 10 '15 at 17:22
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Well, the answer you usually get is half right. They do disappear (more on this in a second). I'd hesitate to say they turn into "heat energy," both because we don't use the term "heat" that way in a technical sense and because most of the time we like to talk about atoms absorbing photons. In this case the energy of the photon becomes potential energy of the electron that made the transition, and there's no need to talk about heat.

Now, can the photon disappear? The short answer is yes. When you talk about things "not simply poofing out of existence" what you're really describing is like a conservation law. For instance, we say that energy is neither created nor destroyed. Your intuition that things aren't just "poofed" out of existence is probably due to your everyday experience that objects generally can be broken into parts, but not usually destroyed. This isn't true in the particle physics sense, usually. The energy carried by that photon has to be accounted for, as does its momentum and angular momentum. But "photon number" is not a conserved quantity the way that energy or (for instance) electric charge are. A photon really is just a way of looking at disturbances/excitations in the electric field, and so its "destruction" just represents that energy that was present in the field has been moved into some other mode.

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    $\begingroup$ You can also correlate it to the photo electric effect, in which the energy( the photon) is absorbed by the electrons(?)and their energy increases, if the energy goes beyond the work function, the electron is emitted. $\endgroup$ – RogUE Mar 6 '15 at 15:52
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    $\begingroup$ True. I just wanted to pick an answer that was not kinetic energy and so quite clearly not "heat." $\endgroup$ – zeldredge Mar 6 '15 at 16:23
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When you turn on a lightbulb, you easily create many photons. They can go away just as easily. That's because they are bosons and they have no charge.

Think of waves on a pond. Where do they "come from" when you throw a stone in? Where do they go when they dissipate?

That's actually a very good analogy in some ways because the math that describes transverse waves is the same, but different in a very fundamental way: the waves are quantized.

In quantum field theory, the field (the pond surface) is everywhere, and it may become excited (throwing a stone in). It's the additional step of "all or nothing" that gives you particles, but that's another step added on top of the issue. Where does a lump in a hall carpet go if you manage to stomp it out rather then shift it? The lump is not a "thing" but a "state". (I'm reminded of "where does your lap go when you stand up?". It is funny in ascribing thing-ness in the same way as an object, but it is a description of a state, not an atom of matter.)

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    $\begingroup$ I've heard the "where does your lap go" question before, but only as a joke. Using it as an analogy for physical states like that is brilliant. (P.S., standing waves on a pond surface are quantized.) $\endgroup$ – Solomon Slow Mar 7 '15 at 17:22
  • $\begingroup$ Standing waves don't have laps; sitting waves do. Why can't standing waves on a pond, like on a gutar, have any aplitude at the wavelength required to fit? $\endgroup$ – JDługosz Mar 8 '15 at 0:13
  • $\begingroup$ I'm pretty sure the lumpiness of a carpet (excess area w.r.t. the underlying floor) is a conserved quantity, that only fails to be conserved on the boundaries of the carpet. What symmetry the conservation of excess area comes from, I'm not sure. $\endgroup$ – John Dvorak Mar 8 '15 at 5:46
  • $\begingroup$ Interesting to think about, there is excess in two different dimensions: a perfect canal-soluton can't be pushed off the long edges, even though they are very close always. A real rug has a small amount of elasticity and can slide against the wood floor. That allows the to dispurse, and the opposite explains why humps can spontaneously appear in the middle of a large room rug. $\endgroup$ – JDługosz Mar 9 '15 at 5:55
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How are photons created?

An accelerating charged particle generates photons tangentially as well as a decelarating one. Where do these photons come from? From the energy carried by the electron. In this sense photons are just a packet of energy which is associated with the electromagnetic field. This type of interactions of electrons and ions with fields happens in the photsphere of the sun, for example, generating the light spectrum we observe.

A photon can interact with charged particles and give up part of its energy or even all of it, and then it "disappears".

Photons can also be produced when electrons that are bound in atoms by the electric field of the nucleus , in steady orbitals but in an excited energy level, fall to the lower energy level releasing a photon. A photon of the same energy will be able to kick the electron to the higher energy level, disappearing in the process.

This appearing and disappearing is not an attribute of photons only. In general particles meeting their antiparticles disappear, because all quantum numbers add up to zero. An electron meeting a positron disappears into two photons. Where do the electron and positrons go? The photon is a simpler particle as it has less quantum numbers to conserve, but the phenomenon exists for all particles in special situations, which you will find out if you carry on to study physics.

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  • $\begingroup$ Could it be fair to say that an electron or positron is a photon that got tangled up with a holon and a spinon and became an orbiton for awhile? The fact that electrons can get entangled makes more sense to me if there's a boson whizzing around inside it at the speed if light, even if it's virtual or a gague boson of sorts. Because anything moving at c experiences no time, and something not experiencing time makes quantum weirdness much easier to reconcile. $\endgroup$ – CommaToast Jan 7 '17 at 1:58
  • $\begingroup$ No, physics is not about fairness but of consistency of mathematical models with data. Virtual particles are fictitious , they are a variable mathematical expression, not measurable in any sense. A model construct. Electrons have no inside, they are point particles. $\endgroup$ – anna v Jan 7 '17 at 5:18
  • $\begingroup$ But photons come out of electrons in experiments all the time... can you prove they don't? I know physicists like to imagine there are no photons inside of a little pocket inside of an electron that come in and out as photons are absorbed/emitted, but if there were, what would it change? $\endgroup$ – CommaToast Jan 7 '17 at 5:53
  • $\begingroup$ @CommaToast The standard model is called standard model because it fits with great accuracy a huge number of measurements, and predicts future measurements too, as has been seen at LHC. The electron is a point particle in this theory. There is no inside. If there were an inside as the one you propose, the standard model would not work as well as it does. Such a hypothesis cannot be incorporated in the standard model. $\endgroup$ – anna v Jan 7 '17 at 6:37
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A photon, unlike some other particles, has no number that must be conserved, thus when absorbed all of the energy present goes into exciting the particle which absorbed it, allowing no laws to be broken. This is due to Noether's Theorem. http://en.m.wikipedia.org/wiki/Noether%27s_theorem

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    $\begingroup$ This is a terrible answer, and a terrible way to teach physics. It begs the question of "Why do photon numbers not need to be conserved?", a question to which there is a very physical, fundamental answer. Nature does not behave because the laws of physics say so - the laws of physics are as they are because nature and logic compel them to be so. Physics is not dogmatic - teaching students that it is dogmatic does a disservice to both. $\endgroup$ – J... Mar 6 '15 at 9:44
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    $\begingroup$ This is a great answer, and a great way to teach physics. It is a concise and direct answer to a specific question about a specific aspect of physics. It does not create an artificial "direction" or distinction between "physics" and "the laws of physics": it does not create an artificial abstraction where none is required. Artificial abstractions are an effective way of structuring learned knowledge, but an ineffective way of communicating new concepts. $\endgroup$ – david Mar 6 '15 at 10:45
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    $\begingroup$ @J... You act as if we have to say why things are not conserved. But why should we assume they are? $\endgroup$ – ACuriousMind Mar 6 '15 at 11:46
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    $\begingroup$ @ACuriousMind I didn't suggest that we should assume this, I'm protesting that we should accept the contrary without proof. It begs the question merely because the answer has no substance. To paraphrase the question - "How can photons poof out of existence?" and then answer with "Because physics has a rule that says they can" is not getting us anywhere. If photon numbers must conserve, we should have a reason why. If they must not conserve, we also should have a reason why. "Eat your vegetables because they are good for you" is dogmatic-it doesn't explain why veggies are good for you. $\endgroup$ – J... Mar 6 '15 at 14:08
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    $\begingroup$ @Jimmy360 Trying not to be confusing is a great goal. But "number that must be conserved" is not a high-school level concept and all you've done is paraphrase the question. "How can photons just poof out of existence?" "Because photons have no number that must be conserved." "What does that mean?" "It means that photons can be created and destroyed." "OK, so photons can poof out of existence. But how is that?" "..." $\endgroup$ – David Richerby Mar 7 '15 at 12:16
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I am going to answer the question in the simplest terms I know.
Assume you have only one hydrogen atom. If a photon of the "correct" frequency "hits" the atom, its electron will jump to a higher excited state. What this means, is that the energy of the photon (the photon itself, since it has no mass) is used (goes into) to make the electron move to a higher excited state. In other words, the kinetic energy of the photon is converted to potential energy (the electron has higher potential energy in the higher excited state).

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  • $\begingroup$ I'm sorry, but potential energy is another thing that I have trouble grasping. $\endgroup$ – user3932000 Mar 12 '15 at 23:56
  • $\begingroup$ Potential energy is the "potential to do work." A spring, a rubber band, water on top of a waterfall, these are all examples of potential energy. When the electron moves to a "higher" energy estate, it has the potential to give up some energy, and this is done by emitting another photon. $\endgroup$ – Guill Apr 16 '17 at 6:16
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Once emitted photons are indivisible units. When a photon hits an electron,both moving in the same direction, the photon will be partially absorbed and the electron emits another photon with lower energy. This happens for example at linear particle accelerators. The energy from the photon partially goes over to the electron and the electron moves faster. If a photon hits the electron, both moving against each other, the electron gets decelerated and emits photon(s) with in sum more energy than the incoming photon has.

There is a chance to imagine how the photon goes over to the electron. For this one have to think about how an electric field - and a magnetic field too - can be quantized. For this one need two different quanta, one of them is on the end of the electrons and the antiprotons electric fields and one quanta, which is on the end of the positrons and the protons electric fields. I see clearly the objection of well educated people. Physics claims the infinity of the electric field of electrons and the other charged particles.

The postulation of such two quanta has some charme. From this quanta it's easy to design electric and magnetic field lines. And the most important point is that all the photons (of different energy) are made from this two quanta too.

Photons always are composed from the equal number of both quanta. Negative charged particles have more of the negative quanta and this difference is equal for all electrons and antiprotons. The same difference but with majority for positive quanta positrons and protons have. Emitted and absorbed photons don't change the particles charge. But the quanta from the photons will be stored partially on the charged particles or these particles gave the quanta back in form of photons of lower energy.

This concept allows to show, that accelerated particles have higher mass and their charge get shilded more and more. And this concept - under the presumption that field lines exist and this field lines are made from this two quanta in clusters with continuous changing numbers - allows to show that the attraction of the electron and the proton in the atom has discrete limits.

And now please forget about this two quanta. They are only a Gedankenexperiment because until now the electromagnetic spectrum seems to be continuous and there is no evidence for quantisation. But it's an amazing imagination to see how photons do not disappear when they hit a electron but travel biggyback on charged particles.

I hope not to be sunk.

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    $\begingroup$ PSA: This answer may be blatantly wrong, but it is an attempt to answer the question. Please downvote it instead of flagging it if you disagree with it. $\endgroup$ – ACuriousMind Mar 7 '15 at 18:54
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    $\begingroup$ -1 This is rambling nonsense. $\endgroup$ – Sean Mar 8 '15 at 0:52
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    $\begingroup$ This answer confuses so many concepts. $\endgroup$ – Jimmy360 Mar 9 '15 at 9:44
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    $\begingroup$ @dmckee The interaction electron <-> photon is always accompanied by an secondary emission of photon(s) from the excited electron. Furthermore the electron gain mass then accelerated - regarding to Einsteins Relativitätstheorie. $\endgroup$ – HolgerFiedler Jan 31 '16 at 17:36
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    $\begingroup$ By "mass" I (and every other particle physicist) mean the invariant mass. That's my point, electrons gain energy but their invariant mass doesn't change, which contradicts your construction immediately, because binding more energy to the electron would change its invariant mass. $\endgroup$ – dmckee Jan 31 '16 at 17:54

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