# What does "observe" mean in the context of the double slit experiment? **URL:** \ **Category:** Factual Questions\ **Created:** [December 30, 2022, 8:08pm UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353 "2022-12-30T20:08:36Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Grrr](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/grrr/32/146_2.png) [@Grrr](https://boards.straightdope.com/u/Grrr)\ **Post date:** [December 30, 2022, 8:08pm UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/1 "2022-12-30T20:08:36Z")
Ok so “observing” it changes the outcome. So what has to observe it? Does it have to be a human? What if it’s a housefly? Pretty sure there is something fundamental I’m not understanding here. Thanks ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [December 30, 2022, 10:33pm UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/2 "2022-12-30T22:33:32Z")
Its not the attempt to observe it, its ACTUALLY observing it. The thing would have to be observable to be observed too… My view on it is that the situation that one slit is so constructed to make the thing observable, it must be interfering with distribution of the things energy. If it can observe energy at a slit, it must have captured energy , and that energy is then certainly not at at the other slit. Imagine two slits the size of an atom of phosphorous … because there is an atom of P in each slit. We are shooting electrons with the energy suitable to make the P glow. If the electron passes through without making a P glow, then its behaving as double slit should. If we get a glowing P, its behaving as a single slit (or worse, its blocked by the P absorption. ) The academic talk is “collapsed the wave function” …I don’t think its like a balloon and any change destroys it totally. I mean, a four slit experiment is turned into a three slit experiment by closing off one slit. So the change in the wave function hasn’t totally collapsed it. The real difficulty in observing the event is that in making the event observable at one slit or the other, its got to have turned the slit into a super-slit… something that really loved to suck that energy in… its changed the experiment. How to make the event observable , but not interfere ? ---
**Author:** ![Dallas\_Jones](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dallas_jones/32/3277_2.png) [@Dallas\_Jones](https://boards.straightdope.com/u/Dallas_Jones)\ **Post date:** [December 30, 2022, 10:48pm UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/3 "2022-12-30T22:48:17Z")
In simplistic terms, in order to observe an event requires that you are measuring that event. You cannot know what is going on unless you have measured it. To observe or measure an event requires that you have inputted some energy. Therefore you have influenced the event just by looking at it. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [December 30, 2022, 11:30pm UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/4 "2022-12-30T23:30:39Z")
> [@Dallas\_Jones](#): > > To observe or measure an event requires that you have inputted some energy. This is the more general _observer effect_. An “observation” in QM is when wave function collapse occurs, and that’s rather different, it’s not about energy transfer. > [@Grrr](#): > > Pretty sure there is something fundamental I’m not understanding here. This is the only part of the OP that I feel confident in answering. No, you are not missing some obvious way in which wave function collapse upon observation “makes sense”. If you thought it made sense, you would _not_ have understood it correctly. It is a deep and unresolved problem. > **[Measurement problem](https://en.wikipedia.org/wiki/Measurement_problem)** > > In quantum mechanics, the measurement problem is the problem of how, or whether, wave function collapse occurs. The inability to observe such a collapse directly has given rise to different interpretations of quantum mechanics and poses a key set of questions that each interpretation must answer. > The wave function in quantum mechanics evolves deterministically according to the Schrödinger equation as a linear superposition of different states. However, actual measurements always find the physic... One of the compelling features of the Many Worlds interpretation is that there _is_ no wave function collapse. If you, as an observer, open the box and see a dead cat, another you is observing a living cat. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [December 31, 2022, 4:35am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/5 "2022-12-31T04:35:18Z")
Yeah, no one really knows. Like lots of QM theory, it’s known very well in terms of using it to do stuff, but the question of what is really going on behind the maths is a big mystery that makes many people uncomfortable when they try to think deeply about it. Sean Carroll does a good talk on the Many Worlds theory of Quantum Mechanics. I assume this is one, they’re generally very similar to each other. [![](https://img.youtube.com/vi/p7XIdFbCQyY/maxresdefault.jpg "Sean Carroll: The many worlds of quantum mechanics") ](https://www.youtube.com/watch?v=p7XIdFbCQyY) ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [December 31, 2022, 4:47am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/6 "2022-12-31T04:47:21Z")
One thing to remember is that _all our observations_ precipitate to EM. The final measurement always involves EM. Hene, if the universe seems incorrect, it must be because the light we use to look at it is lying to us. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [December 31, 2022, 6:27am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/7 "2022-12-31T06:27:07Z")
The idea that observation in some way (energy transfer, or whatever) physically influences a photon’s behaviour in the double slit experiment is wrong, and it’s an attempt to use classical reasoning to try to explain a quantum phenomenon. An impressive illustration of the fact that wave function collapse cannot be due to any physical influence is the delayed-choice quantum eraser experiment. The details of the experiment are at the link below, but in simple terms, it works as follows. An optical crystal is placed after the double slits which converts each photon into two photons at half the energy, which are then sent on divergent paths via a polarizing prism. One of those photons in each pair goes straight to a detector D0 which determines whether or not there is an interference pattern (i.e.- whether or not the original photon went through both slits at once, or whether its wave function collapsed and it only went through one). The rest of the experiment operates on the second photon of the pair, and consists of a bunch of mirrors and two pairs of detectors – the details are in the article but here’s the important functionality of the arrangement. One pair of detectors (pair #1) definitively associates the original photon’s path as having come from slit 1 or slit 2. The other pair (pair #2) detects photons but the information is scrambled so it’s not knowable which slits they came through. The remarkable result of this experiment is that when detector pair #1 is used for the observation, the wave function collapses and no interference pattern is observed at D0. But when detector pair #2 is used for the observation, D0 detects an interference pattern – the photon went through both slits at once – even though everything else was exactly the same. The only difference is whether knowledge of its path is available or not. Even more remarkable, the first photon’s path to D0 is shorter than the path to other two pairs of detectors, so it gets there first; whatever observation you choose to make, it seems that the first photon has already retroactively decided how to behave! > **[Delayed-choice quantum eraser | The experiment of Kim et al. (1999)](https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser#The_experiment_of_Kim_et_al._(1999))** > > The experimental setup, described in detail in Kim et al., is illustrated in Fig 2. An argon laser generates individual 351.1 nm photons that pass through a double-slit apparatus (vertical black line in the upper left corner of the diagram). > An individual photon goes through one (or both) of the two slits. In the illustration, the photon paths are color-coded as red or light blue lines to indicate which slit the photon came through (red indicates slit A, light blue indicates slit B). > So far, th... ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [December 31, 2022, 6:58am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/8 "2022-12-31T06:58:10Z")
> [@Riemann](#): > > One of the compelling features of the Many Worlds interpretation is that there _is_ no wave function collapse. If you, as an observer, open the box and see a dead cat, another you is observing a living cat. I feel like this is an attempt to explain away the quantum weirdness of the “Many Worlds”" interpretation. To the extent that you can be completely entangled with a Schroedinger’s Cat state, the _same_ you is _simulaneously_ observing a dead and a living cat. Sounds weird, and yet the double-slit experiment can be successfully performed with large molecules, not merely single particles; and systems consisting of however many particles have been put into a superposition of all-spin up and all-spin-down. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [December 31, 2022, 6:59am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/9 "2022-12-31T06:59:14Z")
> [@wolfpup](#): > > it seems that the first photon has already retroactively decided how to behave! Ascribing “behavior” to a photon is fraught, though. Traveling at 1c it has absolute time dilation, so it “knows” where it is going to arrive because it already has. In a sense, from the frame of reference of a photon, it does not really exist, except as an expression of energy exchange between two things.        Thus, one could easily say that, if a photon has any kind of agency, it is not unreasonable that it makes a decision in a manner that we would perceive as retroactive (so instantaneous that that it looks retroactive to us time-bound critters).        More likely, of course, is that the “behavior” of a photon is governed entirely by external influences, and what we are observing is nothing more than a matter of how our equipment is guiding it. It is merely effecting the instructions of our experiment, in ways that we find confusing. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [December 31, 2022, 6:55pm UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/10 "2022-12-31T18:55:15Z")
> [@eschereal](#): > > Ascribing “behavior” to a photon is fraught, though. Ascribing “behaviour” or “agency” to a photon is just a peculiarity of our language when discussing apparent cause and effect. The shorter path to the detector D0 is important from the perspective that there appears to be a cause-and-effect relationship between observation of a photon and what its behaviour is. In this case, because of the shorter path to D0, the effect occurs earlier than its apparent cause. But putting that aside, the answer to the OP’s question about what “observation” means is that in the context of the double-slit experiment and all its variants, it simply means “the presence of a ‘which-path’ detector”. No actual humans – or houseflies – are required to observe it. In the actual mechanism of the delayed choice quantum eraser experiment conducted by Kim et al., the pair #1 and pair #2 detectors that I mentioned before exist simultaneously in the experiment. The point is, however, that the signal photons impacting D0 whose entangled “idler photon” partners randomly arrive at the pair #1 which-path detectors experience waveform collapse and do not exhibit an interference pattern at D0, while the idler photons arriving at the pair #2 “path unknown” detectors do not have their signal partners experience waveform collapse and they do generate an interference pattern at D0. ---
**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:** [January 1, 2023, 4:16am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/11 "2023-01-01T04:16:45Z")
> [@Riemann](#): > > If you, as an observer, open the box and see a dead cat, another you is observing a living cat. My question is why doesn’t the cat ever seem to count as an observer? Why need a person to open the box? The cat will be observing what is happening in the box so there is never a wave function to collapse (or at least that collapse happens long before a human peers inside). ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [January 1, 2023, 4:45am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/12 "2023-01-01T04:45:38Z")
You are not alone in wondering about this. This is the “Wigner’s Friend” thought experiment. It’s equivalent to what you are saying - surely the cat knows whether it is alive or dead before the box is opened? But it’s usually formulated as the friend inside a lab first making a measurement with a binary outcome, and Wigner outside the lab subsequently opening the lab door to ask what the measurement outcome was. From the perspective of the friend inside the lab, the wavefunction collapses to a definite outcome when the friend makes their measurement. But from Wigner’s perspective, until he opens the lab door to ask about the result there still exists a superposition of a friend who reports one outcome and a friend who reports the other outcome. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [January 1, 2023, 5:18am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/13 "2023-01-01T05:18:42Z")
In this terminology the observer is defined to be a different system interacting with the system in question, not the system itself. There are different equivalent “perspectives”, like the Heisenberg picture versus the Schrödinger picture. ETA sorry, meant to reply to @Whack-a-Mole ---
**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:** [January 1, 2023, 5:43am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/14 "2023-01-01T05:43:04Z")
> [@DPRK](#): > > In this terminology the observer is defined to be a different system interacting with the system in question, not the system itself. I do not see how the cat is part of the system. The system is the radioactive source and the vial of poison. The cat is merely an observer of what happens. It does not affect the operation of the mechanism. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [January 1, 2023, 6:08am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/15 "2023-01-01T06:08:11Z")
That’s fine. I assumed we were talking about the system being everything in the box (cat + mechanism) observed from outside, but if the system is the mechanism then the cat can be the observer, sure. In fact it seems to make more sense to distinguish the system generating a single bit from the rest; describing a cat or even a vial of poison (consisting of who knows how many molecules) as a single quantum state makes for an interesting thought experiment, but not something computationally tractable. ---
**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:** [January 1, 2023, 6:49am UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/16 "2023-01-01T06:49:09Z")
The point of the cat was to suggest the absurdity of the superposition idea. The use of the cat is as an amplifier of a single probabilistic quantum event (decay or not decay) into a macroscopic highly complex essentially irreversible phenomenon. Wigner’s Friend just moves the goalposts slightly by ensuring we have what we hope we agree is a fellow sentient critter in the superposition. This change mostly addresses, or at least tries to address, the “conciousness collapses the wave-function” thesis. If Wigner’s friend is a concious entity - does the wave function collapse or is he in a superposition? No different to the question about the cat, but we might try to dismiss the cat as not a truly concious entity. Conciousness as a part of quantum woo has been something of a favourite idea (Dancing Woo Li Masters etc) in popular culture, with a continuum of conciousness collapsing wave functions through to collapsing wave functions underpinning consciousness. Not exactly much science here, but sold a lot of books. This confuses the entire discussion about “observer” in a very unhelpful manner. ETA: Gosh, it just occurs to me that the Dancing Woo Li Masters might actually be the origin of the word “woo” to describe non-science. It sure fits both the meaning and the time frame. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [January 1, 2023, 10:14pm UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/17 "2023-01-01T22:14:43Z")
> [@Francis\_Vaughan](#): > > it just occurs to me that the Dancing Woo Li Masters might actually be the origin of the word “woo” to describe non-science it appears that wu li (物理) means “physics” – the “wu” part meaning something like “stuff” ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [January 1, 2023, 10:32pm UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/18 "2023-01-01T22:32:21Z")
> [@Francis\_Vaughan](#): > > This confuses the entire discussion about “observer” in a very unhelpful manner. I agree, and I think any notion that wavefunction collapse relates to consciousness is preposterous. But some serious people _have_ taken it seriously, right? It’s not like some of the hardcore woo that completely misrepresents what _any_ real physicist has ever said about QM. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [January 1, 2023, 11:16pm UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/19 "2023-01-01T23:16:22Z")
> [@Francis\_Vaughan](#): > > Gosh, it just occurs to me that the Dancing Woo Li Masters might actually be the origin of the word “woo” to describe non-science. It sure fits both the meaning and the time frame. Almost nothing is known about the origin of the term, but it seems almost certain that it derived from woo-woo, which was applied to New Agers in the 1980s. [Ben Yagoda](https://benyagoda.com/2016/04/02/woo-hoo-for-woo-woo/) has the earliest quote I could find, but it was definitely in used before 1990. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [January 1, 2023, 11:27pm UTC](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353/20 "2023-01-01T23:27:29Z")
> [@Riemann](#): > > I agree, and I think any notion that wavefunction collapse relates to consciousness is preposterous. I agree with your agreement. As I already said in post #10 about the delayed choice quantum eraser experiment, waveform collapse has nothing to do with what any conscious being, housefly, dog or cat is observing. In the double slit experiment, it has everything to do with the detectability of “which-path” information for any and every individual photon, and this is shown to be independent of any possible interference with the signal photon. [Next page](https://boards.straightdope.com/t/what-does-observe-mean-in-the-context-of-the-double-slit-experiment/977353.md?page=2)