Two slit experiment : question from my 12 year old
Discussion
I remember a long time ago at school being shown the famous two slit experiment. My (basic) memory of the experiment was that when electrons pass through the two slits they can be act both as a particle and a wave. At the time I accepted this as a fact and didn't think too much about it until this morning when my 12 year old wanted to talk to me about it.
He was asking why observing the electron passing through the slit would change its behaviour. I said that it wouldn't. So then he showed me the video he had watched;
https://www.youtube.com/watch?v=DfPeprQ7oGc
The premise seems to be that when firing a single electron it will pass through both slits at the same time and so create an interference pattern akin to a wave. The single electron then switches back to behaving as a particle at the point it is observed.
The idea seems to be that an electron must be in two places at the same time, unless you look at it, at which point it changes to behaving as a single particle again. I looked into it a bit further and saw the idea of Schrödinger's Cat as a way of explaining how something becomes determinant at the point it is observed.
I was wondering if the science experts on here had an explanation as to why observing something would change the behaviour of the electron? Could it be something like a photon of light used to make the observation is interfering with the wave type behaviour of the electron and that is what is causing the change in behaviour?
He was asking why observing the electron passing through the slit would change its behaviour. I said that it wouldn't. So then he showed me the video he had watched;
https://www.youtube.com/watch?v=DfPeprQ7oGc
The premise seems to be that when firing a single electron it will pass through both slits at the same time and so create an interference pattern akin to a wave. The single electron then switches back to behaving as a particle at the point it is observed.
The idea seems to be that an electron must be in two places at the same time, unless you look at it, at which point it changes to behaving as a single particle again. I looked into it a bit further and saw the idea of Schrödinger's Cat as a way of explaining how something becomes determinant at the point it is observed.
I was wondering if the science experts on here had an explanation as to why observing something would change the behaviour of the electron? Could it be something like a photon of light used to make the observation is interfering with the wave type behaviour of the electron and that is what is causing the change in behaviour?
Just watched another youtube video which was quite interesting.
The presenter said that the two slit experiment had been performed with the detector activated to see which slit the electron travelled through. The detector's results were being sent to magnetic tape, so they left the detector active but unplugged the tape. When they did this they got the interference pattern i.e. particles acting as waves. It was only when the detector was actively recording the data did the electrons travel as a particle.
So it wasn't an interaction between the electron and the detector that changed the behaviour but the act of consciously being aware of it. Seems quite a peculiar answer
https://www.youtube.com/watch?v=LW6Mq352f0E
Surely there is a more satisfactory answer than that?
Not too worried about the 12 year old at the moment - it has his dad scratching his head lol
Found this video about a "Delayed Choice Quantum Eraser Experiment"
https://www.youtube.com/watch?v=H6HLjpj4Nt4
It is almost unbelievable in that it seems the behaviour of the photon in this case changes based on what is going to happen in the future... very strange.
Found this video about a "Delayed Choice Quantum Eraser Experiment"
https://www.youtube.com/watch?v=H6HLjpj4Nt4
It is almost unbelievable in that it seems the behaviour of the photon in this case changes based on what is going to happen in the future... very strange.
Moonhawk said:
What passes through the slit is the electron's wave function. All particles are described by a wave function which dictates the probability of a particle being detected in a given location in space.
This wave function interferes with itself as it passes through the slits to give the interference pattern on the other side.
When the electron hits the screen on the other side of the slits the wave function collapses and the particle is detected at a location dictated by the probability distribution of the self interfered (ooh-err) wavefunction.
There is a cool video on the wiki page showing how this works.
https://en.wikipedia.org/wiki/Double-slit_experime...
Essentially the brighter the white colour - the higher the probability of detecting the particle at that point if you were to perform a measurement. Repeat this for many particles and you can see how an interference pattern could be built up even though you are only firing single particles.
That I understand (in theory). The bit I don't understand is that when you observe which slit the particle passed through you don't get an interference pattern anymore. You instead get a clumped pattern, showing the electron's are acting as particles, not waves.This wave function interferes with itself as it passes through the slits to give the interference pattern on the other side.
When the electron hits the screen on the other side of the slits the wave function collapses and the particle is detected at a location dictated by the probability distribution of the self interfered (ooh-err) wavefunction.
There is a cool video on the wiki page showing how this works.
https://en.wikipedia.org/wiki/Double-slit_experime...
Essentially the brighter the white colour - the higher the probability of detecting the particle at that point if you were to perform a measurement. Repeat this for many particles and you can see how an interference pattern could be built up even though you are only firing single particles.
So if you measure which slit the electron passed through you change its behaviour. And weirdly this isn't caused by some subtle interaction between the electron and the measuring device. The very fact you know which way it passed means it doesn't pass as a wave. This is what I don't understand...
Moonhawk said:
The electron doesn't change from a particle to a wave then back again as it passes through the slits. It's only ever described by it's wave-function - until you actually detect it.
It is this last part I don't understand. Let's say a wave passes through water. It isn't the same I know as this isn't at a quantum level.However, the wave moves regardless of whether I watch it or not.
In one of the previous video links they used an example of a star which is several billion light years away. The light passes via a dense galaxy and due to gravitational lensing some light could pass one side of the galaxy and some could pass the other.
When you look at the light on earth, you see a interference pattern, showing it travelled as a wave. But if you look to detect which side of the galaxy it passed, you get a clumping effect, which shows the light passed either one side or the other, several billion years ago. So the fact that it is measured today changes how the photon passed several billion years ago. How does that make sense?
mr_fibuli said:
That guy had a whiff of crack-pot about him - I can't find any credible source for his claim that consciousness or data recording affects the result of the double slit experiment - it is the detector itself. He is just trying to sell his book which claims to unify General Relativity, Quantum Mechanics and Metaphysics. He'd have a Nobel prize if it wasn't a load of BS.
It isn't the detector - it is the "knowing" which changes the behaviour of the electron ...so I guess it isn't the actual recording but looking at the results which is the crucial part ... supposedly. deckster said:
The long answer is that if your son can spend the next forty years studying subatomic physics and come up with a way to unify quantum theory and relativity, then we'll be a good way towards explaining everything, he will get several Nobel prizes and as much physics totty as his slide rule can handle.
Moonhawk said:
The wave function is a probability distribution. At each slit for an undetected electron the probability is non zero - and these non zero areas of the wave function can therefore self interfere.
Yes, I see that is the abstract way of understanding it. But how can that way of visualising it explain how it is seemingly de-linked from time?In the video I linked to called the Delayed Choice Quantum Eraser Experiment, they showed the result of an experiment where they split photons onto different paths which allowed them to explore different scenarios in the same experiment.
In some paths they got clumping (i.e. particle activity) based on a future measurement of the photon which hadn't yet happened on that path. So on the face of it you had the photon acting in a way as if it knew it would be measured in the future. There is a similar apparent paradox with the light from the distant star passing via a dense galaxy which I mentioned earlier.
So presumably the concept of time itself must be different when viewed at a quantum level
creampuff said:
I think you mean photon, not electron.
Electrons wandering around by themselves are generally high energy beta radiation... you don't want to be doing a school experiment with that
The original video referenced electrons but I must admit when I was at school I'm pretty sure they referred to photons.Electrons wandering around by themselves are generally high energy beta radiation... you don't want to be doing a school experiment with that
Dr Jekyll said:
I certainly don't like the notion of a 'probability wave' going through the other slot and causing the interference. Whatever goes through the other slot causes actual interference, not probable or possible interference. So calling whatever it is a 'probable' particle seems absurd to me. It may not be another electron, but it's an actual something, because it causes actual interference.
I think I get the abstract concept of a probability wave but I too find it dissatisfying - maybe because it is too abstract to feel like a logical answer.Thesprucegoose said:
Time is the same at all it is how all molecules know that they go forward in the arrow if time that is interesting.
I think that basically everything is part of everything else, connected by things we cannot see.
You could well be right with that idea. I started watching another video about "entangled particles" (although not sure I quite understand what one looks like) and it seems that could be the case.I think that basically everything is part of everything else, connected by things we cannot see.
Moonhawk said:
Even more interesting - these types of electron probability distributions have been directly imaged (kinda)
These are scanning tunneling electric microscope images of the highest and lowest occupied molecular orbitals of pentacene - compared with the results from a theoretical "density functional theory" plot of the electron density calculated using molecular modelling software. There seems to be a very good agreement - certainly in terms of the overall shape and probability distribution.
I like your answers too, even if I don't fully understand them These are scanning tunneling electric microscope images of the highest and lowest occupied molecular orbitals of pentacene - compared with the results from a theoretical "density functional theory" plot of the electron density calculated using molecular modelling software. There seems to be a very good agreement - certainly in terms of the overall shape and probability distribution.
Maybe it is a tough question, but how do you resolve the "time" issue with the probability wave model?
So if you take the theoretical star example I mentioned earlier where the photons from a far away star bend around a dense galaxy. If observed generally you would get an interference pattern but if you looked specifically to see which side of the galaxy they travelled on you would see a clumped distribution, which suggests that observing an electron now affected its path a billion years in the past?
The other issue I was wondering about is the reason to require such a paradox. I mean, we see this problem from our perspective where the wave acts like a wave, unless you look at it, when it acts like a particle. Why would the electron feel it is important to act like a particle when observed - why couldn't it continue to act like a wave?
It would seem to me that the change in behaviour of the electron when observed would likely be incidental to the actual reason for the change in behaviour.
Fascinating topic though - I was genuinely amazed to find that just observing an electron could change its apparent behaviour.
wisbech said:
Thesprucegoose said:
Time is the same at all it is how all molecules know that they go forward in the arrow if time that is interesting.
I think that basically everything is part of everything else, connected by things we cannot see.
Well, yes. Obvious example is gravity connects all mass, but we can’t see it. We can observe it thoughI think that basically everything is part of everything else, connected by things we cannot see.
https://www.youtube.com/watch?v=MTY1Kje0yLg
ash73 said:
In my opinion the Von Neumann Wigner interpretation makes most sense - the waveform collapses in conscious minds so that we can make sense of the universe and interact with it, it never collapses in physical reality.
https://en.m.wikipedia.org/wiki/Von_Neumann–...
This avoids uncomfortable "observation changed reality" paradoxes, we make observations of probability waveforms all around us and our brains instantiate a model of reality in our mind's eye. For that model to be simple enough to process we have to interpret which slit the electron passes through.
Similarly, when we see with our eyes we are interpreting a complex sea of EM waves and creating an image in our heads. There is nothing reaching us other than waves, the visual model is created solely in our heads.
Thanks for posting this link. Interesting way of resolving the conundrum. https://en.m.wikipedia.org/wiki/Von_Neumann–...
This avoids uncomfortable "observation changed reality" paradoxes, we make observations of probability waveforms all around us and our brains instantiate a model of reality in our mind's eye. For that model to be simple enough to process we have to interpret which slit the electron passes through.
Similarly, when we see with our eyes we are interpreting a complex sea of EM waves and creating an image in our heads. There is nothing reaching us other than waves, the visual model is created solely in our heads.
deckster said:
EddieSteadyGo said:
...reason...Why...feel...why...
Subatomic particles don't feel. They don't have reasons.Scientists tend to like 'what' and 'how' questions, which as we can see can be pretty bloody hard to answer at times. They leave they 'why' to philosophers and religionists (who, conversely, tend to jump straight to 'why' without bothering about little things like 'how'. But let's leave that to a lounge thread).
nicklambo said:
I have two thought experiments with this one that has always bothered me ....1: I'm copyrighting this idea if it works but it did occur to me ( in a slightly fuzzy way) that if the outcome is different in the scenario of the electron going through the slit because it is or isn't being measured then could a message somehow be sent backwards through time via observing or not observing the interference pattern. Example: Prior to firing the electron ask yourself if the number one is going to come up in the lottery....If the answer in the future is no then do not measure the result....if the answer is yes then measure it...this would give you the opportunity to ask yes/no questions backwards through time....
That is an interesting question. I think the way you have framed the question you would only know the answer at the point you measured it, and so you wouldn't have passed an answer back through time.You could maybe adapt the experiment to use an entangled pair of photons and send one side of the pair on a longer journey before deciding whether to measure it or not. Of course as the photons are travelling at the speed of light, the extra distance would need to be very big.
ash73 said:
Dr Jekyll said:
Has this effect ever been observed with anything that doesn't travel at the speed of light? Or anything bigger or heavier than a photon?
The double slit experiment also works with electrons, which have mass.More recently it's been tested with molecules containing 100+ atoms.
https://www.sciencealert.com/physicists-run-a-clas...
Atomic12C said:
OP - another way to think about this is adopting the theory of "Pilot wave".....this theory maintains that the emitted light although still in packets of energy (photons) is 'guided'/propagated by a wave function.
Different to the usual quantum mechanics whereby the the observer 'magically' collapses the wave function to give you a particle.
The root issue is that what we have to describe the behaviour of photons is a "best fit" model. The usual quantum mechanics model works very well, but is a mind bender - and actually doesn't really provide a full explanation as to why the photon acts like a wave or a 'particle' depending on how you view it. It basically provides an accurate model to predict results.
Pilot Wave theory in my opinion provides a 'better' way to describe small particle behaviours but then requires a bit of 'bending' on other assumed 'rules' to make it fit.
edits to add:
here's a good video about what Pilot Wave Theory is about : https://www.youtube.com/watch?v=RlXdsyctD50
Super video - thanks for sharing. I can see why pilot waves are attractive. If I understand it correctly it seems there would be some kind of guiding wave. The particles would remain as particles but would be acted upon by this wave. Hence it explains why when you measure a particle it is only ever in one place and so it removes the need for the idea that the wave is somehow collapsing instantly to become a particle when observed.Different to the usual quantum mechanics whereby the the observer 'magically' collapses the wave function to give you a particle.
The root issue is that what we have to describe the behaviour of photons is a "best fit" model. The usual quantum mechanics model works very well, but is a mind bender - and actually doesn't really provide a full explanation as to why the photon acts like a wave or a 'particle' depending on how you view it. It basically provides an accurate model to predict results.
Pilot Wave theory in my opinion provides a 'better' way to describe small particle behaviours but then requires a bit of 'bending' on other assumed 'rules' to make it fit.
edits to add:
here's a good video about what Pilot Wave Theory is about : https://www.youtube.com/watch?v=RlXdsyctD50
But if this theory is on the right lines, surely the pattern would remain the same regardless of whether the particles were being measured, rather than reverting to a clumped pattern when measured and an interference pattern when not measured?
I'd be grateful if someone could clarify one point for me on this topic please.
I don't have a problem with the fact that the electron travels through the two slits as waves. I quite like the pilot theory mentioned earlier as a way of explaining this.
What I find most puzzling is that the electron behaviour changes when it is observed. Now when we are talking about something being observed, something like a photon needs to touch the item being observed in order to "see" it.
And because we are talking about such small particles, I think it is conceivable that just this photon could influence the behaviour of the particle and hence change the observed pattern from being "wave like" to "particle like".
However, on some explanation videos this is being extended to say that it isn't the measurement of the particle but the "knowing" i.e. if you don't record the results from the measuring device for example, then you would get an interference pattern but if you record the results and know the answer you get the clumping associated with particles.
This opens up so many other issues that I just wanted to double check that this is the definitely the consensus view i.e. the change in behaviour can not be explained just by the photons from the measuring device interacting with the particle being measured.
I don't have a problem with the fact that the electron travels through the two slits as waves. I quite like the pilot theory mentioned earlier as a way of explaining this.
What I find most puzzling is that the electron behaviour changes when it is observed. Now when we are talking about something being observed, something like a photon needs to touch the item being observed in order to "see" it.
And because we are talking about such small particles, I think it is conceivable that just this photon could influence the behaviour of the particle and hence change the observed pattern from being "wave like" to "particle like".
However, on some explanation videos this is being extended to say that it isn't the measurement of the particle but the "knowing" i.e. if you don't record the results from the measuring device for example, then you would get an interference pattern but if you record the results and know the answer you get the clumping associated with particles.
This opens up so many other issues that I just wanted to double check that this is the definitely the consensus view i.e. the change in behaviour can not be explained just by the photons from the measuring device interacting with the particle being measured.
Atomic12C said:
This is a quite straightforward answer, but an 'empty' one I'm afraid. Basically this question that you have has puzzled science since the double slit experiment and the development of quantum mechanics.
To this day nobody really knows why beyond a certain scale that wave-particle duality exists and why the pure nature of 'knowing' a particle's state somehow collapses the wave function.
Many experiments have been done and some interesting ones at that - that try to remove the process of interacting with the particle that is to be 'observed'.
And its as you say.....its simply just the 'knowing' part that magically collapses the wave function.
It baffles me also.
If I were to try to pin some sort of possible answer I am thinking that as string theory (or now more commonly M-theory) suggests there are 11 dimensions in play, that in our familiar everyday 4-D world we are missing some connection that is occurring in one of the other small scale dimensions.
I went for a tour of CERN in Switzerland a few years ago and was told that their big mission was to find if super-symmetry exists and the possibility of additional observable dimensions in play when particles collide. This is all to play a part in refining quantum mechanics and linking it to General Relativity to form a Grand Unified Theory of Evevrything.
Hopefully this will occur in my lifetime as I've always wondered WTF is going on.
So it is the knowing that changes the behaviour of the particle. Very frustrating lolTo this day nobody really knows why beyond a certain scale that wave-particle duality exists and why the pure nature of 'knowing' a particle's state somehow collapses the wave function.
Many experiments have been done and some interesting ones at that - that try to remove the process of interacting with the particle that is to be 'observed'.
And its as you say.....its simply just the 'knowing' part that magically collapses the wave function.
It baffles me also.
If I were to try to pin some sort of possible answer I am thinking that as string theory (or now more commonly M-theory) suggests there are 11 dimensions in play, that in our familiar everyday 4-D world we are missing some connection that is occurring in one of the other small scale dimensions.
I went for a tour of CERN in Switzerland a few years ago and was told that their big mission was to find if super-symmetry exists and the possibility of additional observable dimensions in play when particles collide. This is all to play a part in refining quantum mechanics and linking it to General Relativity to form a Grand Unified Theory of Evevrything.
Hopefully this will occur in my lifetime as I've always wondered WTF is going on.
Thinking back to when I studied A-level physics a long time ago, I tended to learn the content as it was written on the page i.e. formula A would be derived in a certain way, it could be used to figure out parameters B and C and may need to use constant D (which would be provided).
I didn't spend much (if any) time thinking about "why" or what it meant. I just concentated on how the information could be applied to solve the exam questions.
So being able to watch these youtube videos and hearing experts discuss this type of phenomenon is fascinating.
And your last sentence in your post summed it up well for me.
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