Hi Paul, a wonderful clarification of why observation alters the behaviour of quantum stuff. But I am still unsure how this would work if for example you are doing the double slit experiment in a room with the light on (with photons flying around) then surely “the apple has been bitten” regardless of whether an observer is there. That is, it seems that the observer effect is simply caused by the person turning on the light to look??
That's a great question! The double slit experiment is done using particles that are moving extremely quickly - either at the speed of light (for photons) or close to it (for other particles like electrons). This means that it takes them only nanoseconds to travel from the slits to the screen, so there is not enough time for other particles in the room to affect them. By contrast, when you actually measure which slit the particle went through, you intentionally interact the particle with a measuring apparatus in a way that must alter it. (Specifically, this causes decoherence as discussed in https://quantemplation.substack.com/p/schrodingers-cat-was-never-both-alive.)
We can think of it as being like dealing with old camera film. Running the experiment without deliberately measuring is like having film in a dark room; there is some light that could cause a change, but the total amount of light that hits it over the duration for which it is exposed is small enough that it makes essentially no difference. By contrast, making the measurement is like shining a torch directly onto the film; it will immediately alter the film.
Hi Paul, a wonderful clarification of why observation alters the behaviour of quantum stuff. But I am still unsure how this would work if for example you are doing the double slit experiment in a room with the light on (with photons flying around) then surely “the apple has been bitten” regardless of whether an observer is there. That is, it seems that the observer effect is simply caused by the person turning on the light to look??
That's a great question! The double slit experiment is done using particles that are moving extremely quickly - either at the speed of light (for photons) or close to it (for other particles like electrons). This means that it takes them only nanoseconds to travel from the slits to the screen, so there is not enough time for other particles in the room to affect them. By contrast, when you actually measure which slit the particle went through, you intentionally interact the particle with a measuring apparatus in a way that must alter it. (Specifically, this causes decoherence as discussed in https://quantemplation.substack.com/p/schrodingers-cat-was-never-both-alive.)
We can think of it as being like dealing with old camera film. Running the experiment without deliberately measuring is like having film in a dark room; there is some light that could cause a change, but the total amount of light that hits it over the duration for which it is exposed is small enough that it makes essentially no difference. By contrast, making the measurement is like shining a torch directly onto the film; it will immediately alter the film.