Two slit experiment : question from my 12 year old

[EddieSteadyGo]

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?

[geeks]

Far from my area of expertise, would be nice to have PH confirm if I am correct, would this be Heisenberg's uncertainty principle?

[EddieSteadyGo]

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?

[c6r]

well, there is a more satisfactory explanation in some ways

https://en.wikipedia.org/wiki/Double-slit_experime...

in short, the 'satisfactory' answer is that the question doesn't really make any sense in a quantum world. not sure a 12 year old will buy that, but it's worth a try

[EddieSteadyGo]

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.

[Moonhawk]

EddieSteadyGo said:

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.

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 probability distribution described by the wave function interferes with itself as it passes through the slits to give a probability distribution showing an 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.

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.

Edited by Moonhawk on Tuesday 22 May 17:14

[EddieSteadyGo]

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.

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]

EddieSteadyGo said:

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.

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...

I believe it's because by detecting it as it goes through the slit - you collapse the wave function.

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.

By detecting the electron with 100% certainty at one of the slits, the probability at the other slit becomes zero and thus it the wave function can't self interfere.

Edited by Moonhawk on Tuesday 22 May 17:49

[EddieSteadyGo]

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?

[Krikkit]

It's not an easy thing to explain. QM is a harsh mistress...

This Wiki on Wheeler's delayed choices does make a stab at it though: https://en.wikipedia.org/wiki/Wheeler%27s_delayed_...

It's also worth some of the quantum entanglement Wiki, although it descends pretty quickly beyond graduate-level QM once you get into the detail. https://en.wikipedia.org/wiki/Quantum_entanglement

Edited by Krikkit on Tuesday 22 May 17:33

[deckster]

The short answer is that quantum mechanics has some very, very weird st going on.

The glib answer is that anybody who claims they understand quantum mechanics, doesn't.

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.

[mr_fibuli]

EddieSteadyGo said:

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?

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.

[EddieSteadyGo]

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.

[EddieSteadyGo]

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.

[EddieSteadyGo]

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

[Sheetmaself]

Show him the experiment being explained by Jim Al-Khalili. This is the best version i have found which isnt too dry. There is a sub 10 minute version which is purely the double slit experiment, but it is part of a bigger lecture which i believe is a 50 minute lecture all covering similar issues.

[mr_fibuli]

EddieSteadyGo said:

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.

Yes, this is the part that this nut job has completely made up to sell his mystical book on quantum consciousness.

The fact that he claims to have unified Relativity and Quantum physics, where Stephen Hawking sadly failed, tells you that he is living with the fairies.

[Dr Jekyll]

EddieSteadyGo said:

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.

One interpretation I quite liked was that because the electron could go through either slit there are two realities, one for each slit. . As long as the only difference between the realities is the position of that electron you can be in both realities at once and nothing even seems odd until you start studying what happening at the slits. Once you detect the electron at one slit you aren't changing the electron, you are fixing yourself in one reality, while another detector and observer is fixed in another reality.

OK mind blowing, but one reality is mind blowing come to that, and it does get past the tricky issue of 'consciousness' influencing the electron.

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.

[Moonhawk]

Dr Jekyll said:

Whatever goes through the other slot causes actual interference, not probable or possible interference.

Yes - what goes through the slits is the wave function. It is this wave function that actually interferes.

Since the wave function describes a probability distribution of the position of the electron - what you are getting is an interference pattern in this probability distribution.

When you then take a measurement - the wave function collapses and the electron is detected somewhere dictated by this probability distribution. In essence the shape of the probability distribution is changed by the slits.

Edited by Moonhawk on Tuesday 22 May 20:36

[Dr Jekyll]

Moonhawk said:

Dr Jekyll said:

Whatever goes through the other slot causes actual interference, not probable or possible interference.

Yes - what goes through the slits is the wave function. It is this wave function that actually interferes.

Since the wave function describes a probability distribution of the position of the electron - what you are getting is an interference pattern in this probability distribution

But what is a probability distribution other than 'places it might be'? That's what I don't understand.