# Question about double slit experiment **URL:** \ **Category:** Factual Questions\ **Created:** [May 22, 2022, 7:51pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893 "2022-05-22T19:51:51Z")\ **Posts on this page:** 20\ **Page:** 2
**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:** [May 26, 2022, 1:38pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/21 "2022-05-26T13:38:52Z")
> [@EastUmpqua](#): > > I don’t understand why a _slightly_ massive photon would imply our understanding of physics is wrong. GR didn’t wreck our understanding of Newtonian Physics. There was an interesting article in Scientific American back in May 1976 on what would happen if photons have a _litle_ mass. – > [@](#): > > # The Mass of the Photon > > The particle of electromagnetic radiation is often assumed to be massless, but the laws of physics do not require that assumption. If the photon has a mass, however, it must be exceedingly small > > - By [Alfred Scharff Goldhaber](https://www.scientificamerican.com/author/alfred-scharff-goldhaber/), [Michael Martin Nieto](https://www.scientificamerican.com/author/michael-martin-nieto/) on May 1, 1976 > **[The Mass of the Photon](https://www.scientificamerican.com/article/the-mass-of-the-photon/)** > > The particle of electromagnetic radiation is often assumed to be massless, but the laws of physics do not require that assumption. If the photon has a mass, however, it must be exceedingly small In 1955 Erwin Shrodinger asked about this Bass, Ludvik, and Erwin Schrodinger. “Must the photon mass be zero?.” _Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences_ 232, no. 1188 (1955): 1-6. Here’s a much more recent paper from 2004 Tu, Liang-Cheng, Jun Luo, and George T. Gillies. “The mass of the photon.” _Reports on Progress in Physics_ 68, no. 1 (2004): 77. > **[PhotonMass\_rpp5\_1\_RO2.pdf](https://optica.machorro.net/Lecturas/PhotonMass_rpp5_1_RO2.pdf)** > > 928.12 KB This one’s been cited 337 times to date. If you look it up on Google Scholar you can find lots of other papers talking about the implications on nonzero photon mass. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [May 26, 2022, 2:48pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/22 "2022-05-26T14:48:53Z")
> [@Dr.Strangelove](#): > > A dedicated experiment could potentially reveal a massive photon, but it’s not going to be easy. You’d have to start with an incredibly intense light source, and put it next to a particle detector–probably a neutrino detector. My understanding, and my understanding could well be wrong, so if it is, nevermind, but that _high energy_ photons can decay into positron-electron pairs. They can only do this in the presence of a particle, however, and that, if they had mass of their own they could do so spontaneously. So, not seeing pair production from high energy gamma rays should prove that photons are massless. ---
**Author:** ![EastUmpqua](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eastumpqua/32/6025_2.png) [@EastUmpqua](https://boards.straightdope.com/u/EastUmpqua)\ **Post date:** [May 26, 2022, 10:31pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/23 "2022-05-26T22:31:52Z")
> [@CalMeacham](#): > > There was an interesting article in Scientific American back in May 1976 on what would happen if photons have a _litle_ mass. – > > > [@](#): > > > > # The Mass of the Photon > > > > > The particle of electromagnetic radiation is often assumed to be massless, but the laws of physics do not require that assumption. If the photon has a mass, however, it must be exceedingly small > > > > > > - By [Alfred Scharff Goldhaber](https://www.scientificamerican.com/author/alfred-scharff-goldhaber/), [Michael Martin Nieto](https://www.scientificamerican.com/author/michael-martin-nieto/) on May 1, 1976 > > [The Mass of the Photon | Scientific American](https://www.scientificamerican.com/article/the-mass-of-the-photon/) I couldn’t read these articles without paying for it. > [@CalMeacham](#): > > Here’s a much more recent paper from 2004 > > Tu, Liang-Cheng, Jun Luo, and George T. Gillies. “The mass of the photon.” _Reports on Progress in Physics_ 68, no. 1 (2004): 77. > > [http://www.optica.machorro.net/Lecturas/PhotonMass\_rpp5\_1\_RO2.pdf](http://www.optica.machorro.net/Lecturas/PhotonMass_rpp5_1_RO2.pdf) > > This one’s been cited 337 times to date. If you look it up on Google Scholar you can find lots of other papers talking about the implications on nonzero photon mass. This article is interesting. I will try my best to understand it. Btw I did look on Google Scholar. I guess you made a better search topic than I did. Thanks for the link. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 26, 2022, 10:45pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/24 "2022-05-26T22:45:30Z")
> [@k9bfriender](#): > > So, not seeing pair production from high energy gamma rays should prove that photons are massless. Perhaps, but we see gamma-ray bursts from distant galaxies. It seems like they should have all decayed by the time they get to Earth if it happens at any kind of non-negligible rate. If we had a huge flux of them, they might be detectable anyway–after all, it takes a light-year of lead to stop a neutrino, but we can detect them with 10-meter instruments because there are just so many–but high-energy gamma rays are just not nearly as common as neutrinos. ---
**Author:** ![EastUmpqua](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eastumpqua/32/6025_2.png) [@EastUmpqua](https://boards.straightdope.com/u/EastUmpqua)\ **Post date:** [May 26, 2022, 10:56pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/25 "2022-05-26T22:56:22Z")
@CalMeacham From the article: "As the fundamental particle that mediates electromagnetic radiation, the photon conveys energy and momentum through space-time and propagates in vacuum at the constant velocity c, independent of the frame of reference, as per the second postulate of Einstein’s theory of special relativity. A corollary of this is that a particle with finite mass can never attain the speed of light, c, or in other words, such a particle cannot exist in the frame of rest of a photon. **The fact that light could not be brought to a stand-still made this point of view reasonable and it is theoretically difficult to find any kind of contradictory counter-example".** [Glass fiber that brings light to a stand-still](https://phys.org/news/2015-04-glass-fiber-standstill.html#:~:text=A%20team%20of%20scientists%20at,speed%20of%20180%20km%2Fh.). ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [May 26, 2022, 11:02pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/26 "2022-05-26T23:02:52Z")
> [@EastUmpqua](#): > > propagates in **vacuum** at the constant velocity c (my bold) is an important part of that quote. ---
**Author:** ![EastUmpqua](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eastumpqua/32/6025_2.png) [@EastUmpqua](https://boards.straightdope.com/u/EastUmpqua)\ **Post date:** [May 27, 2022, 12:01am UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/27 "2022-05-27T00:01:50Z")
> [@k9bfriender](#): > > > [@EastUmpqua](#): > > > > propagates in **vacuum** at the constant velocity c > > (my bold) > > is an important part of that quote. > [@EastUmpqua](#): > > The fact that light could not be brought to a stand-still I don’t think this quote from the article was referring to light being brought to a stand-still in a vacuum. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [May 27, 2022, 12:09am UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/28 "2022-05-27T00:09:46Z")
> [@EastUmpqua](#): > > I don’t think this quote from the article was referring to light being brought to a stand-still in a vacuum. Yes, it absolutely is. We have long known that light slows down when not in a vacuum. Playing with the properties of how it propagates through a medium so that the energy of the photon is stored within it for an indefinite period is certainly an interesting line of research for certain applications, but it’s not in any way contradicting relativity. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [May 27, 2022, 3:54am UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/29 "2022-05-27T03:54:21Z")
> [@EastUmpqua](#): > > [Glass fiber that brings light to a stand-still](https://phys.org/news/2015-04-glass-fiber-standstill.html#:~:text=A%20team%20of%20scientists%20at,speed%20of%20180%20km%2Fh.). Calling that a standstill is misleading. Yes, it’s very slow compared to the speed of light in a vacuum, but it’s not stopped, as the word “standstill” implies. You cannot stop light. ---
**Author:** ![EastUmpqua](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eastumpqua/32/6025_2.png) [@EastUmpqua](https://boards.straightdope.com/u/EastUmpqua)\ **Post date:** [May 27, 2022, 3:01pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/30 "2022-05-27T15:01:11Z")
> [@dtilque](#): > > You cannot stop light. According to this article, [Light stopped completely for a minute inside a crystal: The basis of quantum memory.](https://www.extremetech.com/extreme/162289-light-stopped-completely-for-a-minute-inside-a-crystal-the-basis-of-quantum-memory) Maybe the implications of this would be better suited for a different post? ---
**Author:** ![EastUmpqua](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eastumpqua/32/6025_2.png) [@EastUmpqua](https://boards.straightdope.com/u/EastUmpqua)\ **Post date:** [May 27, 2022, 8:44pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/31 "2022-05-27T20:44:29Z")
> [@EastUmpqua](#): > > According to this article, [Light stopped completely for a minute inside a crystal: The basis of quantum memory.](https://www.extremetech.com/extreme/162289-light-stopped-completely-for-a-minute-inside-a-crystal-the-basis-of-quantum-memory) Maybe the implications of this would be better suited for a different post? I read the comment section of this article. I like this one: please be more precise 29 July, 2013 THIS ARTICLE IS MISLEADING. Light was not stopped, it was ‘converted’ twice… Does the writer not understand the issue?..See more ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [May 29, 2022, 1:00am UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/32 "2022-05-29T01:00:16Z")
> [@Pleonast](#): > > And this is essentially what a slit is doing: making a measurement of the phase. Is it true if we watch the slits to see which slit the photon went through the interference pattern disappears? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 29, 2022, 1:19am UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/33 "2022-05-29T01:19:59Z")
Basically yes; cf. Volume III, Chapter 3, Section 2 in Feynman, also Chapter 1. “Seeing” which slit the particle went through entails disturbing it. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [May 29, 2022, 5:48am UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/34 "2022-05-29T05:48:31Z")
Now here’s something I always wondered about these double slit experiments: How do you “observe” which slit a photon goes through? How do you “observe” a photon as it passes by, other than by getting directly in its path so it hits you and you absorb it and thus “see” it? All the variations on this experiment involved a “detector”, and all the simplified illustrations depict it as a kind of observer that stands to the side and watches the photons go by. They never explain how that works. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [May 29, 2022, 7:35am UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/35 "2022-05-29T07:35:30Z")
> [@Senegoid](#): > > other than by getting directly in its path so it hits you and you absorb it and thus “see” it? You have pretty much answered the problem. You can’t detect it without interacting with it in some manner. There may be more subtle things you can do, certainly you can if you are trying the double slit with say electrons, were there are ways of sort of getting a bit of a clue about which slit, but not 100% certainty, and - guess what? The double slit interference pattern fades away as you become more certain. You can’t fool the experiment. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [May 29, 2022, 7:40am UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/36 "2022-05-29T07:40:54Z")
> [@Francis\_Vaughan](#): > > You have pretty much answered the problem. You can’t detect it without interacting with it in some manner. . . . and I’ll still be a hornswoggled complex probability quark if I can even begin to wrap my mind around the Delayed Choice Quantum Eraser experiment! ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 29, 2022, 9:31am UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/37 "2022-05-29T09:31:08Z")
> [@Francis\_Vaughan](#): > > There may be more subtle things you can do, certainly you can if you are trying the double slit with say electrons You can fire a photon at the electron as its passing through a slit, and if it deflects then it must have been taken that path. But then you’ve deflected the electron, which disrupts the pattern. You can lower the frequency of the photon to decrease the momentum, hoping not to jostle the electron too much–but then the wavelength of the photon overlaps both slits, and you don’t really know which one the electron passed through. It’s easy to think though that maybe with just a bit more cleverness, one could still work around the issues. But it’s rather like conservation of energy and perpetual motion machines. In each special case, you can discover exactly why you run into a limit. But the law itself is involatile. And there may be scenarios that are difficult to explain except purely by applying the uncertainty principle. You don’t need to know the details to know that the law holds. It applies to everything, not just photons and such. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [May 29, 2022, 10:37am UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/38 "2022-05-29T10:37:28Z")
To put it succinctly, “You can’t measure something without changing it.” Metrologists understand this very well. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [May 29, 2022, 1:18pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/39 "2022-05-29T13:18:42Z")
> [@Senegoid](#): > > I can even begin to wrap my mind around the Delayed Choice Quantum Eraser experiment! Delayed choice always seems spookier than it really is. The trick to it is that what you do “downstream” doesn’t change what shows up behind the double slit. You will _not_ have an interference pattern. However, you have cloned the photons, so you can get information about the photons that went through the slit by measuring those cloned photons. And, if you measure them in _one_ way, then you can tell which slit their clone went through. If you measure them in _another_ way, you can tell the polarization of the photon and its clone. If you correlate the photons that had a particular polarization, with the photons they were cloned from, you can see that those photons formed an interference pattern. You can’t measure both, and you aren’t affecting the past, you are just choosing what information to retrieve. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 29, 2022, 11:56pm UTC](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893/40 "2022-05-29T23:56:30Z")
> [@Crafter\_Man](#): > > To put it succinctly, “You can’t measure something without changing it.” Metrologists understand this very well It’s absolutely true, but phrasing it that way could lead one to think that it’s just a matter of refining the instruments more. I work in software, and we joke about “Heisenbugs” that only reproduce when you aren’t looking for them (a common example: inserting extra logging information can change the timing of a program, which may prevent a timing-related bug from manifesting). But these are really just practical problems. The vast majority of Heisenbugs could be solved easily with better tools. There’s nothing fundamental in computing that requires invasive instruments. But the quantum world is different. Measurements changing the outcome are truly fundamental. When people talk about the differences between quantum vs. classical mechanics, they tend to bring up the two-slit experiment, or the EPR paradox, etc. But these are just special cases of something more fundamental. Classical mechanics views the state of the universe as being something that can be observed from afar, at least in principle–a god could inspect any particle and get its position, momentum, spin, polarization, and so on. Quantum mechanics replaces this viewpoint with that of an _operator_. There simply is no fundamental state to inspect. Instead, observation consists of applying the operator, which gives you the value of the observation but alters the rest of the universe. There is no observation aside from this–it’s completely fundamental. How it manifests in practice depends on the details–and usually those details do consist of something like “to measure this particle, you have to bounce another particle off of it, which alters the trajectory”, but if somehow you find a means of bypassing this (as with quantum eraser experiments), it does not matter; you’ll still be bound by the same limit. The operator for the observation you care about applies no matter how you go about the experiment. [Previous page](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893.md?page=1) [Next page](https://boards.straightdope.com/t/question-about-double-slit-experiment/964893.md?page=3)