/ THE Double Slit Experiment

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Phil Venn07 Jul 2019

Does anyone find the double slit experiment interesting? Or perhaps, moving up a level, the delayed choice quantum eraser experiment? 

1
The Wild Scallion 07 Jul 2019
In reply to Phil Venn:

> Does anyone find the double slit experiment interesting? Or perhaps, moving up a level, the delayed choice quantum eraser experiment? 

Yes . It's mind boggling the only explanation I saw comes from Quantum electrodynamics if I remember correctly.  

I think I saw Richard Feynman give some sort of explanation in a lecture once online.  Suffice to say I can't recall it at this time of the morning.

Post edited at 03:41
DancingOnRock 07 Jul 2019
In reply to Phil Venn:

“Anyone who is not shocked by quantum physics has not understood it” - Niels Bohr

”Anyone who says they understand quantum mechanics, does not understand quantum mechanics” Richard Feynman 

“There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. 

There is another theory which states that this has already happened.” Douglas Adams

Offwidth 07 Jul 2019
In reply to Phil Venn:

Still mind blowing. For those who don't know what it is:

https://en.m.wikipedia.org/wiki/Double-slit_experiment

"The largest entities for which the double-slit experiment has been performed were molecules that each comprised 810 atoms (whose total mass was over 10,000 atomic mass units."

Post edited at 12:47
1
Shani 07 Jul 2019
In reply to Phil Venn:

Came across it a decade after learning of it at school and it really blew me away the second time round. It is spooky, amazing, and in some ways terrifying, all at the same time. 

DancingOnRock 07 Jul 2019
In reply to Shani:

Similar experience to me. We did it at school, then again at university but 10 years after that I read about it in a book by John R Gribbin - In Search of Shrodingers Cat. There’s also Shrodingers Kittens. 

They were much better explanations than those I’d had before.

Post edited at 13:07
Shani 07 Jul 2019
In reply to DancingOnRock:

That's the thing; once you 'understand how much we don't understand' and specific examples of weirdness (such as the observation of a photon collapsing the wave-particle duality), it leads you on to thoughts about consciousness and whether there was a universe before consciousness evolved to collapse the duality. It's absolutely mind-boggling.

+1 for Gribben. His book on the Rheimann Hypothesis is also excellent. 

Post edited at 13:20
rka 07 Jul 2019
Offwidth 07 Jul 2019
In reply to Shani:

Photons might be excused being weird things that most people don't even understand but not pretty big molecules of 810 atoms in that two slit experiment.

wintertree 07 Jul 2019
In reply to Offwidth:

> Photons might be excused being weird things that most people don't even understand but not pretty big molecules of 810 atoms in that two slit experiment.

It’s only “weird” because it doesn’t correspond to the macroscopic world we are used to.  Those 810 atoms are minuscule compared to the ~1 mm oil droplets that follow the classic “two slit” experiment at macroscopic scales and without any (non-trivial) quantum stuff involved.

A lot of the apparent weirdness lies in many of the interpretations that try and explain quantum behaviour to people, rather than in the behaviour itself.  It’s all a bit moot whilst the various interpretations remain just that - but one day we will know what’s really going on and that could be judges as genuinely weird or perhaps just as bog standard particle and wave mechanics with a bit of coupling, some randomness and so on.

what the hex 07 Jul 2019
In reply to DancingOnRock:

A colleague is always going on at length and in detail about physics, if I could say "what's the point of trying to understand a universe that changes as soon as you turn your back on it" it could shut him up for a while and on that basis I'm tempted to buy the book discussed. But would I understand it?

Offwidth 07 Jul 2019
In reply to wintertree:

You have missed my point.

The history of Physics to me indicates there is no liklihood at all that we will ever know what exactly is going on. Every stone we uncover reveals more stones that are harder to move.  As it is, it's getting so difficult to arrange experimental testing of theories that some of modern theoretical physics looks more like metaphysics to me.

Post edited at 14:56
2
mbh 07 Jul 2019
In reply to Phil Venn:

Akira Tonomura of Hitachi's videos of the two slit experiment unfolding are always worth a watch.

https://www.youtube.com/watch?v=jvO0P5-SMxk

Very little chance of there being one electron in the instrument at any one time, let alone two, and yet the interference fringes build up, point by individual point.

DancingOnRock 07 Jul 2019
In reply to what the hex:

Ha, unfortunately the Douglas Adams quote is from Restautant at the End of the Universe. It’s a well known humorous fictional work, the follow up to Hitchhikers Guide to the Universe.

The other two books are pretty good for a layman. No maths, all very descriptive. 

1
wintertree 07 Jul 2019
In reply to Offwidth:

> some of modern theoretical physics looks more like metaphysics to me.

Steady on, metaphysics doesn’t stoop to inventing something riddled with free parameters and fine tuning them to match observations before declaring to have detected that something...

2
Offwidth 07 Jul 2019
In reply to wintertree:

It does if there are no observations yet.

ChrisClark1 07 Jul 2019
In reply to DancingOnRock:

> “There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. 

> There is another theory which states that this has already happened.” Douglas Adams

One of many many great Adams quotes

Darren Jackson 07 Jul 2019
In reply to Phil Venn:

I've no idea what you're talking about... Could you perhaps Phil me in with a Venn diagram? 

Phil Venn07 Jul 2019
In reply to The Wild Scallion:

Feynman's passion for modern physics was astounding and inspiring. If you have time check out a youtube clip of Leonard Susskind. Another world class communicator with a personality!

Phil Venn07 Jul 2019
In reply to Shani:

Ha! This is the correct response for a person with genuine intellect! I

Phil Venn07 Jul 2019
In reply to rka:

I had a go but couldn't get the video going on my tablet. Shame. Thanks for the thread anyway.

Phil Venn07 Jul 2019
In reply to mbh:

That was a biprism experiment. It's not something I'm up to speed on. I'd guess it's another version of the double slit experiment, showing the same result. Single electrons are fired at the screen in these experiments. The result suggests that the electrons "know" where previous electrons have landed! Check out the Delayed Choice Quantum Eraser experiment and discover that electrons may also go back in time and change the final result if necessary. You have to agree that this IS interesting!

Phil Venn07 Jul 2019
In reply to Darren Jackson:

Duh

Phil Venn07 Jul 2019
In reply to what the hex:

No. No one understands it. That's why it's interesting.

Phil Venn07 Jul 2019
In reply to wintertree:

It's only weird because it is. Unless you know better, in which case there's a Nobel Prize awaiting you.

wintertree 08 Jul 2019
In reply to Phil Venn:

> Check out the Delayed Choice Quantum Eraser experiment and discover that electrons may also go back in time and change the final result if necessary.

Once again the apparent weirdness - and all suggestions of going back in time - in this experiment comes from some interpretations. 

tlouth7 08 Jul 2019
In reply to what the hex:

>I'm tempted to buy the book discussed [In Search of Schrodinger's Cat]. But would I understand it?

I am sure you would understand (most of) it, being basically a history of quantum mechanics rather than a scientific textbook. I'm very happy to sell you my very lightly used copy, as I am unlikely to read it again (though it is a good read).

I also have a very lightly used copy of Feynman's QED if anyone wants that, it's rather more maths-y but perfectly accessible for anyone who is reasonably numerate. £5 each plus postage.

Also I don't find the double slit experiment weird (so clearly don't understand it). I suspect this is because as with everything learnt in science at school it was presented as established fact rather than an exciting, mysterious discovery.

Post edited at 09:09
Shani 08 Jul 2019
In reply to tlouth7:

> Also I don't find the double slit experiment weird (so clearly don't understand it). I suspect this is because as with everything learnt in science at school it was presented as established fact rather than an exciting, mysterious discovery.

How can ONE photon appear to go through TWO slits at the SAME TIME?

That is very bloody weird.

kestrelspl 08 Jul 2019
In reply to Shani:

The answer is that the concept "ONE photon" isn't really how things are. You might have heard of wave-particle duality, i.e. things we normally think of as particles sometimes behave like waves.

This is exactly what's happening in the double slit experiment. If you were to see a very small water wave encounter that pattern of slits it wouldn't be surprising at all that it made the interference fringes. The reason it's surprising here is that people thought photons were particles.

The really cool thing when you look into it further is that actually all particles are waves. The underlying objects in our most up to date theories are all things called fields. A field is just a quantity that has a value at all points in space, much like the ocean has a height at all points in space. The field can have waves flow through it exactly like the ocean can have waves go over it.

It turns out that what we think are particles are actually just a particular type of vibration in the underlying quantum fields, so really everything is a wave.

Post edited at 10:32
1
DancingOnRock 08 Jul 2019
In reply to tlouth7:

Depends when you went to school. When I went to school it was presented as light behaved with a particle/wave duality and that was that. At age 15 you probably don’t think too much about why this should be wrong. I don’t know how they present it now.

They also presented electrons as particles spinning round a nuclei and proton particles.  Which nobody questions until they’re told differently. 

We are all children I suppose with this. 

If you look at the speed of light as the speed of propagation through the universe, where the universe is made of something, (even the vacuum of space), rather than a limit that nothing can go faster than, then having photons that know what each other are doing becomes only spooky. 

Current Big Bang theory only covers what happened after everything started to slow down and cool after the initial extremely rapid expansion. Stars didn’t start to form until c200m years. 

Post edited at 10:44
what the hex 08 Jul 2019
In reply to tlouth7:

Emailed you re:In Search of Schrodingers Cat

DubyaJamesDubya 08 Jul 2019
In reply to wintertree:

> > Check out the Delayed Choice Quantum Eraser experiment and discover that electrons may also go back in time and change the final result if necessary.

> Once again the apparent weirdness - and all suggestions of going back in time - in this experiment comes from some interpretations. 

And if those interpretations are correct?

wintertree 08 Jul 2019
In reply to DubyaJamesDubya:

> And if those interpretations are correct?

Then I will eat my hat, but first I will send a message back in time telling my younger self to buy an edible hat.

Less flippantly, I think it says a lot that some interpretations are more compatible with causality than others, but what is the point in debating interpretations with no differentiation in terms of testable points?

Post edited at 11:39
tlouth7 08 Jul 2019
In reply to DancingOnRock:

> Depends when you went to school. When I went to school it was presented as light behaved with a particle/wave duality and that was that. At age 15 you probably don’t think too much about why this should be wrong.

This is what I was getting at. If your teacher just says "this is the way it is" then as a kid you just accept it, and don't think about the implications.

Feynman in QED is pretty explicit that according to his formulation there is no need to ever consider light as a wave. The sum-over-paths method predicts the result of the double slit experiment (end diffraction etc) without recourse to waves, though the maths of waves are simpler and give useful results for macroscopic cases.

In reply to Phil Venn:

Would be a great name for a crack route

Shani 08 Jul 2019
In reply to kestrelspl:

> The answer is that the concept "ONE photon" isn't really how things are. You might have heard of wave-particle duality, i.e. things we normally think of as particles sometimes behave like waves.

> This is exactly what's happening in the double slit experiment. If you were to see a very small water wave encounter that pattern of slits it wouldn't be surprising at all that it made the interference fringes. The reason it's surprising here is that people thought photons were particles.

> The really cool thing when you look into it further is that actually all particles are waves. The underlying objects in our most up to date theories are all things called fields. A field is just a quantity that has a value at all points in space, much like the ocean has a height at all points in space. The field can have waves flow through it exactly like the ocean can have waves go over it.

> It turns out that what we think are particles are actually just a particular type of vibration in the underlying quantum fields, so really everything is a wave.

I get all this. But how come when you observe this wave, it reverts to behaving like a particle? That's the weird bit.

jess13 08 Jul 2019
In reply to Phil Venn:

2 doors in a wall a person going through each door at the same time, would they interfere with each other on the other side? Would they behave as waves or particles?

kestrelspl 08 Jul 2019
In reply to Shani:

What we call "behaving like a particle" is just what happens when you don't put the wave through gaps similar to its wavelength or look too closely at it. If you look at what the theory predicts in those cases the fields "behave like particles".

The issue is that we just find it odd when the things we'd only previously seen behaving like particles revert to behaving like waves when you study them under different conditions.

There are of course lots of other weird and funky things about quantum mechanics (entanglement anyone?!) but the double slits experiment makes perfect sense in Quantum Field Theory

Post edited at 14:24
kestrelspl 08 Jul 2019
In reply to jess13:

They'd behave in the way that you think particles behave. This is because the wavelength of a person is very different to the size of the doors. They are however a big collection of waves in fields that just happen to collectively end up acting like a particle.

mbh 08 Jul 2019
In reply to wintertree:

>  but what is the point in debating interpretations with no differentiation in terms of testable points?

Not much, I think. More fruitful would be to seek out testable points, as Bell's inequality in the 60s enabled Aspect and others to test whether local hidden variables could account for what otherwise implied non-locality.

Shani 08 Jul 2019
In reply to kestrelspl:

> The issue is that we just find it odd when the things we'd only previously seen behaving like particles revert to behaving like waves when you study them under different conditions.

The word 'odd' is doing a lot of heavy lifting here! 

I find it weird that the simple act of observing something should change its behaviour.  

wintertree 08 Jul 2019
In reply to Shani:

> I find it weird that the simple act of observing something should change its behaviour.  

If you think about “observing properties” as gaining information, and think about “properties” not “behaviour” it’s not so much strange as inevitable.  Especially if information is real and therefore thermodynamic.  Information can’t be created, only transferred.  So measurement is impossible without thermodynamic interaction.

kestrelspl 08 Jul 2019
In reply to Shani:

Ah I see what you mean, I think I misunderstood what you meant the first time. Yes measurement changing behaviour is odd, but if you just accept that the thing that's deterministic is the probability not the actual events it's not so bad.

Rob Parsons 08 Jul 2019
In reply to Shani:

> I find it weird that the simple act of observing something should change its behaviour.  

But that is also true in the macroscopic world.

Coel Hellier 08 Jul 2019
In reply to Shani:

> I find it weird that the simple act of observing something should change its behaviour.  

That's actually one of the less weird things about quantum mechanics!   

How does one "observe" something? Well, one has to interact with it, such as bouncing a photon off it, or doing something similar.    Thus, in order for us to "observe" something, that thing must change the photon (otherwise we couldn't deduce anything about its existence from the photon).  But, if the thing is interacting with and changing the photon, then it's not so strange that that interaction will also change the thing. 

In classical physics, one could get round this argument by simply reducing the strength of the interaction down to arbitrarily small levels. Quantum mechanics is then just saying that we can't make the interaction insignificant. 

DancingOnRock 08 Jul 2019
In reply to Coel Hellier:

And if you’re interacting at a point in a field it’s probably going to affect the whole field in some way. 

Siward 08 Jul 2019
In reply to The Wild Scallion:

How's this for a layman's explantion?

https://www.youtube.com/watch?v=A9tKncAdlHQ

Shani 08 Jul 2019

All - thanks for the interesting contributions above!

Dave the Rave 08 Jul 2019
In reply to Coel Hellier:

> That's actually one of the less weird things about quantum mechanics!   

> How does one "observe" something? Well, one has to interact with it, such as bouncing a photon off it, or doing something similar.    Thus, in order for us to "observe" something, that thing must change the photon (otherwise we couldn't deduce anything about its existence from the photon).  But, if the thing is interacting with and changing the photon, then it's not so strange that that interaction will also change the thing. 

I once bounced off a Photon whilst pursuing a double slit experiment on a busy campsite. 

graeme jackson 09 Jul 2019
In reply to Phil Venn:

Disappointed. Thought this was going to be a thread about threesomes

2
wercat 09 Jul 2019
In reply to willworkforfoodjnr:

> Would be a great name for a crack route


or a daring item of fashion

wercat 09 Jul 2019
In reply to jess13:

Imagine the doorways are tight and made something like rubber or resistant inflated balls that causes the people to bounce from side to side as they go through, wobble-walking.  If the doors are close enough they may well wobble into each other and wobble apart, drunkenly

Post edited at 19:51
Phil Venn10 Jul 2019
In reply to kestrelspl:

Photons always behave like waves. They only appear as particles when observed (scientifically that is)

Phil Venn10 Jul 2019
In reply to kestrelspl:

The double slit experiment doesn't make sense in quantum mechanics. It's still unexplained natural phenomena. Of course, if you know better there's a Nobel Prize waiting for you.

3
elsewhere 10 Jul 2019
In reply to Phil Venn:

> Photons always behave like waves. They only appear as particles when observed (scientifically that is)

Hmmm. Photoelectric effect?

elsewhere 10 Jul 2019
In reply to Phil Venn:

> The double slit experiment doesn't make sense in quantum mechanics. 

Probability waves for a single particle go through both slits?

Phil Venn10 Jul 2019
In reply to elsewhere:

It appears that way but there has been no test to verify what really happens. But that's as close as we can imagine it.  If you check out a youtube clip on the Delayed Choice Quatum Eraser experiment you can see that it gets mind bogglingly complicated. 

elsewhere 11 Jul 2019
In reply to Phil Venn:

> It appears that way but there has been no test to verify what really happens.

There never is. All we can test is does it agree or not agree with theory.

wercat 11 Jul 2019
In reply to Phil Venn:

much of our apparatus makes things look like particles at the point of perception, least that's how I see it.  Lots of waves made of what we used to call nothing, matter is a vacuum behaving badly, perhaps just waves trapped, chasing their own tail through the Higgs field

Burst them open and let the waves flow!

Post edited at 08:45
Jamie Wakeham 11 Jul 2019
In reply to DancingOnRock:

> Depends when you went to school. When I went to school it was presented as light behaved with a particle/wave duality and that was that. At age 15 you probably don’t think too much about why this should be wrong. I don’t know how they present it now.

Usually at A level I introduce the PE effect first, as a relatively gentle introduction to waves behaving with a particle-like nature.  Then suggest that the opposite is also true, and look at electron diffraction and a very simple model of electron energy levels.  Then the two-slit experiment. 

Most A level syllabi don't actually require you to look at electrons in the two-slit experiment, but they've already covered it with light, so it's an obvious extension to talk about how electrons can also exhibit wave-like behaviour in this context.  And for brighter students we can then go and talk about how to try to understand all this craziness via the Copenhagen interpretation.  I certainly try hard to keep a sense of wonder at the whole idea of QM throughout it. 

kestrelspl 11 Jul 2019
In reply to Phil Venn:

Especially after seeing your other thread I think you may find that you fall into the very category that you were mocking...

Apologies if you do actually have deeper knowledge, but you're not displaying it very well on here.

Tringa 11 Jul 2019
In reply to kestrelspl:

Agree, entanglement is a different order of quantum weirdness.

Dave


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