# Interference with photons **URL:** \ **Category:** Factual Questions\ **Created:** [January 29, 2019, 4:39pm UTC](https://boards.straightdope.com/t/interference-with-photons/828624 "2019-01-29T16:39:39Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [January 29, 2019, 4:39pm UTC](https://boards.straightdope.com/t/interference-with-photons/828624/1 "2019-01-29T16:39:39Z")
Suppose I shine hundreds of laser pointers at a point on a wall. I manage to get the interference patterns to overlap such that there is no visible dot on the wall. 1. Is spot heating up? If not, where is the energy of the light going? 2. If I look at this spot in a mirror, will the interference have disappeared, and will I see a spot? ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [January 29, 2019, 4:57pm UTC](https://boards.straightdope.com/t/interference-with-photons/828624/2 "2019-01-29T16:57:15Z")
> [@Cardinal](#): > > Suppose I shine hundreds of laser pointers at a point on a wall. I manage to get the interference patterns to overlap such that there is no visible dot on the wall. > > 1. Is spot heating up? If not, where is the energy of the light going? > > 2. If I look at this spot in a mirror, will the interference have disappeared, and will I see a spot? I remember I had this discussion with one of my optics professors. If you had two megawatt lasers in an interferometer, and you aligned them so perfectly that you exactly canceled out the beam in one arm of the interferometer, then stuck your hand in there, would you get burned? Short answer – No, because you’ve achieved perfect interference, so there’s no electric field there, so you’re not burned. Achieving perfect interference is kind of like achieving perfect Nirvana – you don’t want to count upon it. So, as one professor said “I still wouldn’t put my hand in there.” So where is the energy going? Into the other arm of the interferometer, of course. when you tried to perfectly align your beams, you had to use a beam splitter. Half of each beam gets reflected, half transmitted. If you zero out one arm with perfect destructive interference, then the other arm, gets perfect CONSTRUCTIVE interference, so all the energy goes there. Don’t stick your hand in there. Okay, but what if I don’t use a beam-splitter or anything like that? well, maybe you’re putting two lasers at angles to each other onto a wall. good luck trying to get perfect interference. I believe that they have been able to interfere two beams not coming from the same source or seed, but it’s far from easy. it’s going to be practiucally impossible to achieve your perfect interference. But even if you do, there is going to be some other place for the “rejected” light. It’ll all show up in reflected light, or something. You don’t destroy those photons – the best you can do is re-arrange them. and, as for looking in a mirror at you perfect interference spot – it’ll still look like a perfect interference spot. nothing you do with mirrors away from the beam is going to affect that spot. If you’ve arranged things so that you do hjave perfect interference, the spot won’t heat up. But the energy is going _somewhere_. And if you ARE using megawatt lasers and your path difference between the arms slips by just a little bit from an integral number of wavelengths, or if your wavelength from one laser shifts ever so slightly, well, you won’t have perfect interference anymore, and your wall is going to start heating up. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 29, 2019, 5:22pm UTC](https://boards.straightdope.com/t/interference-with-photons/828624/3 "2019-01-29T17:22:19Z")
As an aside, it’s going to be much easier to do this with radio transmitters than with visible-light lasers. As to where the energy does go, there are two ways to set this up. If the two wave sources are very close together (as in, much less than a wavelength apart), then you can get them to negatively interfere everywhere, but then the energy is going to almost all just be flowing between the two sources, and they’ll consume very little from whatever their power source is. On the other hand, if the two sources are separated by a significant amount, then you’re going to have an interference pattern: There will be some places on the screen where the interference is destructive, but other places on the screen where it’s constructive. At best, you’ll get a pattern of regularly-spaced bright and dark fringes, but in practice, for a real-world setup, I’d expect that the fringes would be so close together as to be indistinguishable, and it’d all look uniformly medium-bright. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [January 29, 2019, 5:27pm UTC](https://boards.straightdope.com/t/interference-with-photons/828624/4 "2019-01-29T17:27:53Z")
> [@CalMeacham](#): > > And if you ARE using megawatt lasers and your path difference between the arms slips by just a little bit from an integral number of wavelengths, or if your wavelength from one laser shifts ever so slightly, well, you won’t have perfect interference anymore, and your wall is going to start heating up. So you better hope that there are no merging black holes in the neighborhood… ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [January 29, 2019, 5:54pm UTC](https://boards.straightdope.com/t/interference-with-photons/828624/5 "2019-01-29T17:54:29Z")
> [@Darren\_Garrison](#): > > So you better hope that there are no merging black holes in the neighborhood… well, heck, it’s a LOT easier than using gravitational waves to knock a couple of beams out of perfect alignment. You could have a ground tremor shake the setup, or have refractive index changes due to humidity or pressure. Or you could have someone fall and knock over a mirror mount, or something. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [January 29, 2019, 7:13pm UTC](https://boards.straightdope.com/t/interference-with-photons/828624/6 "2019-01-29T19:13:08Z")
Light rays are not streams of photons, the only time light behaves as a photon is when it exchanges energy with something. One way to think of this is that photons are just a mathematical concept and that they are not _physical_ particles at all. In fact once one moves past non-relativistic quantum mechanics it gets fuzzy and this can be said of all particles. Under QFT what we call “photons” are just observable from the [Electromagnetic four-potential](https://en.wikipedia.org/wiki/electromagnetic_four-potential) of a field so that we can have _Lorentz covariance_. A photon can be considered simply a _construct_ that was introduced to explain the experimental observations. A photon is just not a simple object that can be “destroyed” or “rearranged”. Consider it like “virtual particles” in Feynman diagrams, although those are merely pictorial representations of the mathematical expressions, they solve a similar problem. The point being is that how we _observe_ the effects of electromagnetic waves ends up looking _particle like_, there are no _particles_ traveling with an EM wave. _Photons_ as particles is a convenient and useful _construct_ to help us describe our world, they are tools to model real world observations. Well virtual particles are _exclusively_ an artifact of perturbation theory. Photons are pseudo-particles under modern theories like QED, and while there is a view that this is true of all particles let me explain one difference, the photon and an anti-photon are the exact same particle. Assuming a “photon” is a real _“thing”_ has cause many errors in the world of physics. Mathematical constructs have a valid role, but one needs to remember when someone uses a constructs that it is still a construct and not a physical thing. Thinking of a single _photon_ as a “wave packet” with with a nearly zero potential everywhere except in a narrow range of space and time, helps avoid these errors. Unfortunatly until you get to QED or another theory that avoids the miscalculations of classical theories the photon is treated as an actual physical particle. These theories and incredibly useful but just make sure to not assume that the convenient historical construct of a photon is a _physical thing_. Here is a paper that shows even astrophysicists forget or miss this point and it leads to errors. [Coherent Spectroscopy of Supernova Remnant 1987A](http://adsbit.harvard.edu//full/2009ASPC..413..233M/0000242.000.html) > [@](#): > > A little more attention of astrophysicists to coherent spectroscopy should surely explain a lot of mysterious observations In the OP, if you have perfect interference the potential of the wave function is too low for the wave packet to be absorbed. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [January 29, 2019, 7:49pm UTC](https://boards.straightdope.com/t/interference-with-photons/828624/7 "2019-01-29T19:49:34Z")
> [@rat\_avatar](#): > > Light rays are not streams of photons, the only time light behaves as a photon is when it exchanges energy with something. While I agree with the sentiment in the first half, the second half has null content (if you generalize energy to four-momentum), since we can only ascribe _any_ type of behavior to light when there is an interaction involved. > [@](#): > > One way to think of this is that photons are just a mathematical concept and that they are not _physical_ particles at all. If you define “particle” as “classical particle”, then sure, but I think such an absolutist definition here will cause confusion, since other things that one would like to call a particle can’t be called a particle anymore. > [@](#): > > A photon can be considered simply a _construct_ that was introduced to explain the experimental observations. Isn’t everything? I don’t see the distinction here. > [@](#): > > A photon is just not a simple object that can be “destroyed” or “rearranged”. Here’s where things move out of definitional concerns. A single photon absolutely can be created and destroyed (although I agree it isn’t a simple object). > [@](#): > > Consider it like “virtual particles” in Feynman diagrams, although those are merely pictorial representations of the mathematical expressions, they solve a similar problem. This is a red herring. All real particles can be virtual, and a photons virtual-ness isn’t special to it.\* > [@](#): > > Photons are pseudo-particles under modern theories like QED Nope. Full particles. What’s a pseudo-particle? > [@](#): > > and while there is a view that this is true of all particles let me explain one difference, the photon and an anti-photon are the exact same particle. That’s also a red herring. Many particles (elementary and composite) are their own antiparticles. That doesn’t make them any less of a particle. It’s just that it’s one particle instead of two related ones. In the case of neutrinos, we don’t yet know if they are their own antiparticle or not, but when we figure that out, it won’t suddenly give or take away the status of “particle”. > [@](#): > > Assuming a “photon” is a real _“thing”_ has cause many errors in the world of physics. Mathematical constructs have a valid role, but one needs to remember when someone uses a constructs that it is still a construct and not a physical thing. It _is_ a real thing, but I agree with the caution that one must understand what that thing is. And a (quantum, not classical) particle is certainly not equivalent to a billiard ball. [sup]\*Photons do have a lot special about them since they are, in quantum field theory, a gauge field. But that’s more of a motivation for introducing the field rather than a statement about what “rights” that field has, including having excitations that we call “particles”.[/sup] ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [January 29, 2019, 9:01pm UTC](https://boards.straightdope.com/t/interference-with-photons/828624/8 "2019-01-29T21:01:51Z")
> [@Pasta](#): > > While I agree with the sentiment in the first half, the second half has null content (if you generalize energy to four-momentum), since we can only ascribe _any_ type of behavior to light when there is an interaction involved. > If you define “particle” as “classical particle”, then sure, but I think such an absolutist definition here will cause confusion, since other things that one would like to call a particle can’t be called a particle anymore. > Isn’t everything? I don’t see the distinction here. > Here’s where things move out of definitional concerns. A single photon absolutely can be created and destroyed (although I agree it isn’t a simple object). > This is a red herring. All real particles can be virtual, and a photons virtual-ness isn’t special to it.\* > Nope. Full particles. What’s a pseudo-particle? > That’s also a red herring. Many particles (elementary and composite) are their own antiparticles. That doesn’t make them any less of a particle. It’s just that it’s one particle instead of two related ones. In the case of neutrinos, we don’t yet know if they are their own antiparticle or not, but when we figure that out, it won’t suddenly give or take away the status of “particle”. > It _is_ a real thing, but I agree with the caution that one must understand what that thing is. And a (quantum, not classical) particle is certainly not equivalent to a billiard ball. > [sup]\*Photons do have a lot special about them since they are, in quantum field theory, a gauge field. But that’s more of a motivation for introducing the field rather than a statement about what “rights” that field has, including having excitations that we call “particles”.[/sup] I am talking about how they are defined under the various theories, which is mathematical and hard to share here. While I think the construct of a photon is useful, here is a link to Lamb’s fairly famous paper called “Anti-photon” [http://www-3.unipv.it/fis/tamq/Anti-photon.pdf](http://www-3.unipv.it/fis/tamq/Anti-photon.pdf) Try an make his same argument with other “particles”, > [@](#): > > Objects like electrons, neutrinos of finite > rest mass, or helium atoms can, under suitable conditions, > be considered to be particles, since their theories then have > viable non-relativistic and non-quantum limits > **[postulates.pdf](https://www.sydney.edu.au/science/chemistry/~mjtj/CHEM3117/Resources/postulates.pdf)** > > 68.20 KB > [@](#): > > Postulate 2. > > To every observable in classical mechanics there corresponds a linear, Hermitian operator in quantum mechanics. > > This postulate comes from the observation that the expectation value of an operator that corresponds to an observable must be real and therefore the operator must be Hermitian. Try to find the identity of the probability density using “H = p[sup]4[/sup] + v(x)” in a typical textbook type proof as example of something with simple enough math to share on here. While I once again don’t think “photon” as a construct should be dropped note Lamb’s conclusion. > [@](#): > > It is high time to give up the use of the word “photon”, and > of a bad concept which will shortly be a century old. > Radiation does not consist of particles, and the classical, > i.e., non-quantum, limit of QTR is described by Maxwell’s > equations for the electromagnetic fields, which do not > involve particles. Talking about radiation in terms of > particles is like using such ubiquitous phrases as “You > know” or “I mean” which are very much to be heard in > some cultures. For a friend of Charlie Brown, it might > serve as a kind of security blanket. The core concept is that photons are _packets of energy_ of an electromagnetic wave and not bits of matter. It is trivial to treat them as radiation and not a particle without huge implications on the underlying theories. The fact that photons don’t follow the Pauli exclusion principle is another distinction one could make and could be a reason to not describe them as “a stream of particles”. The point is just like “virtual particles” in perturbation based models, which works (if not as nicely for the user) if you remove the concept, radiation as a “a stream of particles” is also not required. If you look at the thread about microwaves and waves if one used a modern “wave group” instead of the particle idea the idea why small holes in a conductive surface block radio waves becomes a lot more clear. Feynman called his diagrams a “bookkeeping device” which makes it easy for people to remember virtual particles are just a construct, but as I offered above the mistake of treating “photons” as a _physical_ thing like other particles actually causes problems. Under current best theories, photons are particles by convention and not out of a fundamental need. I personally like the convenient construct of the “photon” but only if I remember it is a tool and not a description of a physical “thing” in our current best theories. ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [January 29, 2019, 9:17pm UTC](https://boards.straightdope.com/t/interference-with-photons/828624/9 "2019-01-29T21:17:42Z")
This is a good paper to go over the math, while also calling out that interpretational choice and not strictly dependent on a strictly corpuscular theory. (at least in the case of their need) > **[Photon wave functions, wave-packet quantization of light, and coherence theory](https://arxiv.org/abs/0708.0831)** > > The monochromatic Dirac and polychromatic Titulaer-Glauber quantized field theories (QFTs) of electromagnetism are derived from a photon-energy wave function in much the same way that one derives QFT for electrons, that is, by quantization of a... > [@](#): > > We note that there are three common interpretations of the photon. One is the notion of a fundamental particle. Another is that of an excitation of a quantum field. A third is simply what is registered by a photodetector. In this paper, we do not adhere to any of these as being more correct, as the mathematics of wave functions has little to say on this interpretational matter. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [January 30, 2019, 9:56am UTC](https://boards.straightdope.com/t/interference-with-photons/828624/10 "2019-01-30T09:56:59Z")
> [@rat\_avatar](#): > > While I think the construct of a photon is useful, here is a link to Lamb’s fairly famous paper called “Anti-photon” > [http://www-3.unipv.it/fis/tamq/Anti-photon.pdf](http://www-3.unipv.it/fis/tamq/Anti-photon.pdf) Not to mince words, but I completely disagree with Lamb’s conclusions in that lecture. It’s a fun little polemic with some nice history mixed in, but the fact that Lamb was on this “anti-photon” mission for over half his very long life and he found zero traction with it points in some small part to its silliness. We agree that quantum mechanics requires care when pushing up against the limits of a semi-classical view. This holds for systems involving photons or any other particles. A first issue with Lamb’s lecture is that he chooses to give only example systems that are well into the tricky regime (resonating cavities, interferometers, two-slit diffraction) while ignoring the fact that systems with the same tricky properties can be constructed for non-photon particles and ignoring really anything outside of the narrow scope of quantum optics. A telling quote is “With more complicated states it is terribly difficult to talk meaningfully about ‘photons’ at all. QTR gives the only proper description.” This can be said for _any_ particle! “With more complicated states it is terribly difficult to talk meaningfully about \ at all. QFT gives the only proper description.” The distinction he is trying to invoke is artificial and stems entirely from the narrow quantum-optic regime he has confined his arguments to. > [@](#): > > Try an make his same argument with other “particles”: ‘Objects like electrons, neutrinos of finite rest mass, or helium atoms can, under suitable conditions, be considered to be particles, since their theories then have viable non-relativistic and non-quantum limits.’ With all due respect to Dr. Lamb, this is nonsense. A particle can’t be a particle unless it has a non-relativistic and non-quantum limit? Says who? Where does that leave a massless neutrino (which is perfectly allowed in the theory and may even exist)? Where does that leave _all_ fermions in an unbroken theory or in a theory with zeroed Yukawa couplings? Would you call the Z particle a particle? It has mass. It decays into stuff. It’s _very_ much the sort of thing the word “particle” is suited for. The photon and the Z, though, are simply two orthogonal superpositions of the same two underlying fields. It would be very odd to say one is a particle and the other isn’t. > [@](#): > > Try to find the identity of the probability density using “H = p[sup]4[/sup] + v(x)” in a typical textbook type proof as example of something with simple enough math to share on here. Sorry, I couldn’t parse this sentence. > [@](#): > > note Lamb’s conclusion: > _It is high time to give up the use of the word “photon”, and > of a bad concept which will shortly be a century old. > Radiation does not consist of particles, and the classical, > i.e., non-quantum, limit of QTR is described by Maxwell’s > equations for the electromagnetic fields, which do not > involve particles. Talking about radiation in terms of > particles is like using such ubiquitous phrases as “You > know” or “I mean” which are very much to be heard in > some cultures. For a friend of Charlie Brown, it might > serve as a kind of security blanket._ His “a century old” jab is undermined by his own text, where he emphasizes that the early uses of that word by Lewis were different. More to the point: the fact that a classical wave-based limit exists is neither here nor there. It happens to be possible for radiation, which relates to some special properties of the photon, but that doesn’t mean it’s the only way to think of things. > [@](#): > > It is trivial to treat them as radiation and not a particle without huge implications on the underlying theories. Completely disagree, and you can pick the reason, either (1) The logical extension of the argument – that all that matters is the happiness of the underlying theory – holds for _all_ particles. Everything is a field and the theory doesn’t needs _any_ particles. So if we say photons aren’t particles because of this, we have to say that there are no particles. That is linguistically unhelpful. Or, (2) it is either harder or, in some cases, very far from trivial to treat photons as radiation in many physical systems if one is allowed to look outside of Lamb’s cherry picked scope (_e.g._, X-rays, shot noise, photoelectric effect, Compton scattering). > [@](#): > > The fact that photons don’t follow the Pauli exclusion principle is another distinction one could make and could be a reason to not describe them as “a stream of particles”. This is an assertion that bosons can’t be particles. Higgs, W, pions, helium-4, [bound electrons](https://en.wikipedia.org/wiki/Cooper_pair) – all of these are bosons and violate the Pauli exclusion principle, either always or under certain conditions. I think you would call all of these bosons “particles” (some elementary particles, some composite). > [@](#): > > The point is just like “virtual particles” in perturbation based models, which works (if not as nicely for the user) if you remove the concept, radiation as a “a stream of particles” is also not required. As above, the concept of a particle is not required for any particle, if you take the argument to its _ad absurdum_ conclusion. > [@](#): > > If you look at the thread about microwaves and waves if one used a modern “wave group” instead of the particle idea the idea why small holes in a conductive surface block radio waves becomes a lot more clear. Absolutely. Thinking in terms of waves is useful in certain applications. This is true for all particles. For radiation, it so happens that very commonplace situations lend themselves to such treatment. This is less so for other particles. But that’s an anthropic distinction, not a fundamental one.