Two slit experiment : question from my 12 year old

[EddieSteadyGo]

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).

I know what you mean - I was using those words on purpose to refer to what I thought were the inadequacies with the theory explaining the observed behaviour.

[Dr Jekyll]

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?

[Atomic12C]

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

Edited by Atomic12C on Wednesday 23 May 09:14

[EddieSteadyGo]

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.

[EddieSteadyGo]

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...

That is very cool. It does seem to support the idea that everything is a wave just with a different wavelength, until it is observed.

[EddieSteadyGo]

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.

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?

[Atomic12C]

EddieSteadyGo said:

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?

There is still a wave function at the root of this theory, and although the small energy packet of light is guided by this function it still means that probability is still at play. Pilot Wave, if I understand it correctly, simply "goes around" the notion that the photon takes every possible path from source to destination at the same time. Instead the pilot wave is the most probable route the particle would take.
But again on measurement, it is the wave function of the pilot wave (not the particle) that is disturbed when measured - meaning that the pattern does not form.

(I may have mislead with previous reply - apologies - I was referring to the observer magically collapsing a 'particle position' under quantum mechanics previously, whereby Pilot theory still seems to involve a collapsing of wave function - although there is a subtle difference on how the wave function is applied between the two theories.

Some find this a little easier to bear on the mind as a wave does propagate out from source to interact at multiple points simultaneously, whereas a particle "shouldn't".

To date science has models to best fit observations, but doesn't have an exact description of why particle-wave duality exists or the exact mechanism behind it.
Possibly one of those things that we'll never know.

(Bit like string theory - the theory works but no way plausible to actually detect the existence of a 'string' - so although the model crosses in to philosophy, its an accurate model for predicting measurements/observations)

[Gandahar]

It is due to probability waves

The mere fact the mention of probability in the sentence just means we have not so far pinned it down yet due to lack of understanding on the physical level and using the tools provided by mathematics on the theoretical level.

All these puzzling features will eventually be described accurately in time. History proves so.

[EddieSteadyGo]

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.

[Atomic12C]

EddieSteadyGo said:

I'd be grateful if someone could clarify one point for me on this topic please.

I'll try if I can
I'm not pertaining to be a physicist but I've had an interest in science since my school days and went on to do a Aerodynamics Degree at Loughborough University which employed a lot of science content.

There are also some other good knowledgable types on this science section of the forum that will keep us all straight if there are any glaring errors.

EddieSteadyGo said:

What I find most puzzling is that the electron behaviour changes when it is observed.

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.

[EddieSteadyGo]

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 lol

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.

[Dr Jekyll]

Atomic12C said:

EddieSteadyGo said:

What I find most puzzling is that the electron behaviour changes when it is observed.

And its as you say.....its simply just the 'knowing' part that magically collapses the wave function.
It baffles me also.

Which is why I like the idea that it's the observer's range of possibilities that collapse, not the electrons.

[tbourner]

I like to think of it more like the particles don't exist, there is only a probability that they exist, and that probability increases in certain places so the particle is more likely to exist in one slit or another. In electronics when you work out how semiconductor materials work, you find out about different electron shell densities around different elements, so doping a silicon crystal with phosphorous puts more electrons into the substance, and the extra electrons in the outer shells of phosphorous are a bit frisky and they don't need much persuading to jump to the next particle - this is how electricity flows - it's basically adding some energy and changing the wave function, so that the electron is "more likely" to be found existing in the next particle along - this over the entire substance equates to most of the electrons "moving" across the silicon. But it's all just probability, they don't actually exist until they hit the LED and become a photon (let's not go into that one though, eh).

Going back to Schroedinger's cat, it obviously doesn't work because a cat is a lot of particles. The experiments only work with 1 or maybe 10s of particles, beyond that the probabilities multiply together exponentially meaning the decision has already been made before "the observation" takes place. The probability of a cat existing in one location is ridiculously high, so the cat is either alive or dead, not undecided.

That probably doesn't help with the actual question around observation and time-travelling particles.

[Dr Jekyll]

tbourner said:

Going back to Schroedinger's cat, it obviously doesn't work because a cat is a lot of particles. The experiments only work with 1 or maybe 10s of particles, beyond that the probabilities multiply together exponentially meaning the decision has already been made before "the observation" takes place. The probability of a cat existing in one location is ridiculously high, so the cat is either alive or dead, not undecided.

That probably doesn't help with the actual question around observation and time-travelling particles.

But isn't the whole point of Schroedinger's cat that it's survival or otherwise depends on the state of a particular sub atomic particle? So if that's 0.5/0.5 then the chance of the cat surviving is near enough 05.

Or does the cat effectively become the observer?

[tbourner]

Dr Jekyll said:

tbourner said:

Going back to Schroedinger's cat, it obviously doesn't work because a cat is a lot of particles. The experiments only work with 1 or maybe 10s of particles, beyond that the probabilities multiply together exponentially meaning the decision has already been made before "the observation" takes place. The probability of a cat existing in one location is ridiculously high, so the cat is either alive or dead, not undecided.

That probably doesn't help with the actual question around observation and time-travelling particles.

But isn't the whole point of Schroedinger's cat that it's survival or otherwise depends on the state of a particular sub atomic particle? So if that's 0.5/0.5 then the chance of the cat surviving is near enough 05.

Or does the cat effectively become the observer?

No, that's just the trigger that has a 50% chance of activating - it could be a computer controlled random trigger, the thought experiment is that an un-observed cat with the person outside the box having no visibility and the probability of it being dead is 50%, then the cat is both alive and dead - it's an analogy to probability wave functions and quantum superposition.

[Dr Jekyll]

tbourner said:

No, that's just the trigger that has a 50% chance of activating - it could be a computer controlled random trigger, the thought experiment is that an un-observed cat with the person outside the box having no visibility and the probability of it being dead is 50%, then the cat is both alive and dead - it's an analogy to probability wave functions and quantum superposition.

That's not my understanding at all. The whole point is that a sub atomic particle controls the trigger. Therefore you cannot have a situation where the particle is in two states at once, but the cat is either alive or dead. So if you accept that a particle can have a probability wave then you are also accepting that the cat must have one. The whole point of the analogy was to argue against quantum theory by illustrating the issue that arises once you accept that anything however small can be in two states at once.

[Halmyre]

tbourner said:

Dr Jekyll said:

tbourner said:

Going back to Schroedinger's cat, it obviously doesn't work because a cat is a lot of particles. The experiments only work with 1 or maybe 10s of particles, beyond that the probabilities multiply together exponentially meaning the decision has already been made before "the observation" takes place. The probability of a cat existing in one location is ridiculously high, so the cat is either alive or dead, not undecided.

That probably doesn't help with the actual question around observation and time-travelling particles.

But isn't the whole point of Schroedinger's cat that it's survival or otherwise depends on the state of a particular sub atomic particle? So if that's 0.5/0.5 then the chance of the cat surviving is near enough 05.

Or does the cat effectively become the observer?

No, that's just the trigger that has a 50% chance of activating - it could be a computer controlled random trigger, the thought experiment is that an un-observed cat with the person outside the box having no visibility and the probability of it being dead is 50%, then the cat is both alive and dead - it's an analogy to probability wave functions and quantum superposition.

Terry Pratchett postulated a third state of a cat locked in a box - "bloody furious", and woe beside an observer who looked inside...

[tbourner]

Dr Jekyll said:

tbourner said:

No, that's just the trigger that has a 50% chance of activating - it could be a computer controlled random trigger, the thought experiment is that an un-observed cat with the person outside the box having no visibility and the probability of it being dead is 50%, then the cat is both alive and dead - it's an analogy to probability wave functions and quantum superposition.

That's not my understanding at all. The whole point is that a sub atomic particle controls the trigger. Therefore you cannot have a situation where the particle is in two states at once, but the cat is either alive or dead. So if you accept that a particle can have a probability wave then you are also accepting that the cat must have one. The whole point of the analogy was to argue against quantum theory by illustrating the issue that arises once you accept that anything however small can be in two states at once.

Well I'm no expert, and I don't personally think it needs to be a consciousness as mentioned earlier, but an 'agent which would require an output from an event', if that makes sense? So the electron sensors in the splits, which I have no idea how they work, are looking for a result and that initiates the wave function collapse. The trigger in the box that releases the poison is also looking for a decay of the radioactive particle (the Geiger counter) so that also collapses the wave function, and selects an outcome.

[Halmyre]

For what it's worth, Schroedinger postulated the cat in a box experiment to illustrate the absurdity of trying to apply quantum theory to the 'real' world.

[67Dino]

I had a conversation once with an alien who landed his UFO on my lawn. I asked him what was going on here and he said our problem is we can’t see most of what is happening. When we roll a dice, all 6 results occur, but we only perceive one. The results also exist before we roll the dice, but we can’t see that either. I said, “No, I mean what is going on with you landing on my lawn. Now get off it”.