UKC

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.

“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

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."

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. 

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.

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. 

Have you seen this http://podcasts.ox.ac.uk/ghost-imaging-quantum-light

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.

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?

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.

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

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.

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. 

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...

It does if there are no observations yet.

One of many many great Adams quotes

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

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!

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

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

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!

Duh

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

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

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

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.

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

That is very bloody weird.

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.

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. 

Emailed you re:In Search of Schrodingers Cat

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?

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.

Would be a great name for a crack route

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

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?

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

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.

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.

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.  

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.

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.

But that is also true in the macroscopic world.

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. 

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

How's this for a layman's explantion?

youtube.com/watch?v=A9tKncAdlHQ&

All - thanks for the interesting contributions above!

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

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

or a daring item of fashion

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

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

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.

Hmmm. Photoelectric effect?

Probability waves for a single particle go through both slits?

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. 

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

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!

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. 

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.

Agree, entanglement is a different order of quantum weirdness.

Dave