[Lloyd (OP)]

Hi all. Like so many others, not involved with science but interested in most aspects especially physics.

I have watched a version/explanation of the double slit experiment on the internet and of course it was very interesting. I watched as single photons passed through a single slit producing a single bar pattern. Then a single photon passing through 2 slits producing the interference pattern. To count how many actual photons were going through each particular slit a counter was set up at one of the slits. The pattern then reverted back to a single pattern as no photons were recorded going through the watched slit! Strange but acceptable. My question ... what happens when both slits are observed by a counter?

Hopefully I have that correct and you can understand why I am asking that particular question.

Lloyd

[Halc]

How are the slits observed?  In other words, how can one detect a photon at one slit and still expect it to go through?  They're not like cars you can count as they pass by in one lane or the other.

[Lloyd (OP)]

er ... yes I probably got that all wrong! sorry. I'll do a bit more reading and watching.

[Colin2B]

I'll do a bit more reading and watching.

What @Halc is pointing out is that you have to know in detail how an experiment has been done in order to understand the result. In particular, how did they detect the photons without destroying them of affecting them in some way.
Do come back with more questions on this, because I think you will have a lot.

[RobC]

You have hit at the heart of quantum mechanics, after a century there are still many interpretations.
Sean Carroll narrowed it down to really only three contenders --
1. Many worlds.
2. Hidden variables.
3. Collapse theories.

In the future it will be said: "they had quantum mechanics fall into their lap, they used it but did not understand it".

[Lloyd (OP)]

Oh boy ... I spent 45 minutes writing a reply and then decided to delete that start again and include a link for the video I had watched. After another 20 minutes of writing and trying to upload my reply that then didn't appear ... so try again!

I am posting a link to the video I watched to better explain myself. (agh ... not allowed to post external links.) edit ( Double Slit Experiment explained! by Jim Al-Khalili)


In the video a flashing/beeping detector is placed above the upper slit of 2 in order to detect which slits atoms were travelling through. Apparently it is 50/50. From what I gather this is to attempt to discover how/why it is atoms revert from an interference pattern to a particle pattern when directly observed. (although from observing the experiment without the detector I would class as observation anyway?)

The question I was trying to ask was ... what if there were 2 detectors observing both slits as opposed to just the top one?

[Halc]

In the video a flashing/beeping detector is placed above the upper slit of 2 in order to detect which slits atoms were travelling through.

OK, it is atoms now, not photons.  Those can theoretically be detected without destroying them.  I've read that it can be done with photons as well, but the thing you describe in the OP seems to stop them altogether.

(although from observing the experiment without the detector I would class as observation anyway?)

Yes, they're still measuring the interference pattern, so it's still an observation.  Without the slit detector, we lose the information about which slit they went through.

From what I gather this is to attempt to discover how/why it is atoms revert from an interference pattern to a particle pattern when directly observed.

That's the whole crux of QM. QM says you will get this behavior, predicts it with wicked accuracy, but only in probabilistic terms.  It isn't really a particle pattern.  If it was, it would form a focused spot on the target (a shadow of the slit barrier), not a distribution on a bell curve.

Asking why this happens has been the subject of debate for over a century.  There are thus many interpretations as to why this happens, and some of these interpretations are very different from each other, so they give very different answers to why this happens.  Problem is, none of them make any unique prediction that allows some interpretations to be falsified.  They all behave empirically the same.  Hence we cannot assert any particular reason why this happens, but we know it does.

A collapse interpretation (there are many of them) would say that the slit detector collapses the wave function of the atom, reducing the experiment to a single-slit experiment, which yields single-slit results (no interference pattern).

The question I was trying to ask was ... what if there were 2 detectors observing both slits as opposed to just the top one?

Same as one detector.  The information is known by the one measurement, so the 2nd one gathers nothing new.

[Colin2B]

(although from observing the experiment without the detector I would class as observation anyway?)

In quantum physics ‘observe’ has a very specific meaning. It doesn’t mean that someone stood there looking at the experiment, it means that the presence of a particle or wave was detected.

The question I was trying to ask was ... what if there were 2 detectors observing both slits as opposed to just the top one?

It’s important to recognise that the 2 beams are not just any old beams. In order to get interference between 2 electron beams they have to be coherent; that means all the electrons have the same energy (frequency) and the phase relationship is stable - this latter being arranged by passing the beam through a narrow slit called a collimator. These are the same requirements for a light beam to be considered coherent. If anything disturbs the energy or phase then you will not get an interference pattern.
If you place a detector above a slit to measure the passage of the electron it must interact with the electron in some way and if this changes the energy or phase the beam will no longer be coherent - this is known as decoherence. This is why I said that you need to understand the detail of an experiment to know what is happening.
Each time this experiment is performed in different ways we learn more about the quantum world, unfortunately the popsci press rarely give enough detail, choosing to present the results in the most top level and dramatic terms possible.

I think @Halc has answered your main question, you won’t get a double slit interference pattern if you disturb the electron in any meaningful way at either or both slits.

[Lloyd (OP)]

I think it's fair to say that I could not get into any meaningful discussion with you guys because of our differing levels of scientific/mathematical understanding on the whole subject. You guys have years of education and study on the subject ... I on the other hand have no scientific education, online animated videos and a inborn inability to understand mathematics. Okay, perhaps not meaningful to you but perhaps yes, meaningful to me because it brings about a greater understanding of the subject I am asking about, in this case the double slit experiment. So that's me kind of saying thanks for your patience ...

So I now understand that a detector at each slit would not alter the results ... the dual nature of the, shall we stick with a single photon, would still remain ... seemingly a particle before the double slit and a wave after the double slit ... and reverting back to a particle when 'observed' after the double slit. Also delaying the observation until the photon has passed through the slit and has nearly reached its destination results in the photon reverting back to it's particle state as it was before entering through the double slit. The point is that the photons travelling through the slits should produce an impact pattern similar to the double slits. But in actual fact they produce an interference pattern. So ... wave - particle duality. Why ? I now need to discover the 'suggested' reasons as to how/why this is possible. Anyone care to suggest / put forward a reason?  (oh ... and for hecks sake I hope my understanding above indicates I've understood correctly so far !)

[Halc]

You guys have years of education and study on the subject

Just so you know, I have no physics education beyond what was probably known in the 19th century.  I've never taken a course in relativity or quantum mechanics. Most of what I know comes from reading on the web. I take the word of those who are properly educated in these matters, which is often difficult to find on the web, whose pages are often authored by random writers and not by the physicists themselves.

So I now understand that a detector at each slit would not alter the results ... the dual nature of the, shall we stick with a single photon, would still remain ... seemingly a particle before the double slit and a wave after the double slit ... and reverting back to a particle when 'observed' after the double slit.

A thing like a photon can be described by a wave function.  I've never computed one in my life, but you just need to know that it has one, or is one. That function evolves over time according to the Schrodinger equation (another thing I will never compute).  At any time, the wave function (which is a function of complex numbers, not real numbers) can be squared to yield a description about what properties (like location) might be measured at that time.  So the wave function says the photon is probably around here at time X, and that here is sort of a bell-curve blob of space that we think of as a particle.  It has no position, only a probability of being measured at any given point.  So it's position might be expressed as the center of that distribution: It is most probably 'here'.

So as this wave function evolves through the slits, the probability of where it will be measured becomes not a bell curve but an interference pattern.  It is more likely to be measured in these places (wave peaks) than between them (troughs), but in fact it could appear anywhere.

Any time a measurement is taken, the wave function completely changes.  It is not longer a probability of where it might be measured, but a certainty about where it was measured.  This simplifies the wave function, and is what is called the wave function collapse. This is as close as it gets to the photon having a 'particle' vs 'wave' state. In fact a photon never has any such state, since it is neither particle nor wave at any time.

So if we've measured which slit it went through, it is now a simpler wave function, and not one that yields a probability curve with an interference pattern.

There are multiple interpretations, some of which deny wave function collapse. Some say the photon in fact goes through both slits, others say it actually has an unknown location and thus goes through only one.

[jeffreyH]

@Lloyd Quantum mechanics is just weird. That's the way it is. Probability is the key. It wasn't chosen to be that way by physicists. Look into polarised light and filters.
https://www.khanacademy.org/science/physics/light-waves/introduction-to-light-waves/v/polarization-of-light-linear-and-circular

This is where we eventually get to Bell's inequality. Go read up on it!

[jeffreyH]

Also worth viewing

[Petrochemicals]

The point of the double slit experiment is that one or two slits, the transmission medium behave the same.  Its the wave effect, if you put waves through two slits you get destructive and constructive interferance, leading to high and low points along a wave crest. Ie a wave crest now has peaks and troughs along it aswll as a trough between the following and preceeding waves. Lots like when 2 oceans meet. Point is that light is behaving with these wave characteristics.

[yor_on]

Don't need mathematics to think about it Lloyd. Only if you have another interpretation and then want to have it published. Most of the stuff at the QM level is probabilistic, everything is thought to be a result of that and decoherence. Decoherence can be seen as this small probabilistic level reaching some sort of threshold of interactions and scale leaving the world as we experience it normally.

So in a two slit experiment there is a probability of either one or both slits being 'engaged' by one particle. And the way it unfolds will be defined through your setup. The idea of indirect evidence is increasingly popular in those situations as every time you probe a particle you also force it into a set behavior aka  a 'wave collapse'. Whether one want to think about it first as waves or as particles is more of a question of what you believe than what is right here. I've read physicists stating the particle view while other state the wave view.

[yor_on]

If you instead of that want to think about it as fields of different energy density interacting it becomes one more interpretation :)
In that case most of what we take for granted disappear. A hole isn't a hole, it's something of a 'energy density' or maybe lack of, looking as a hole to us. and a 'particle' is not a projectile, instead it becomes a emanation of either ' one field's ' interaction with itself or fields interacting. I think that suits my own taste better than the other approaches. As you wrote light has a wave particle duality, and I see that as absolutely correct.
==

there is one big hurdle with the field idea, to me then. And that is how to make it fit relativity. What that means is that our universe is 'observer dependent' looked at from relativity. You can 'shrink' this universe through mass f.ex or by speeds. And those effects are no artifacts but physical laws. So if we define a universe as a field interacting with itself, or fields, we still need to incorporate relativity in it. And that is where it becomes really weird.

What it means is that everything might be said to have different 'speeds' versus your observation/platform. Which means that every object you observe will have its own definition of a size and time of/in this universe. Scaling it down we meet particles and passing that 'breaking it up' fields.  That's also where it starts to hurt my head.

there is one more dimension we need to add to fields and that is time. Depending on how you look at it it becomes a arrow pointing one way, or a ocean. If you think of it as a ocean then that can contain a lot of possibilities, and also catch the way Einstein defined SpaceTime, consisting of four dimensions. three double ended and one with only one direction. And if you set that together with observer dependencies you get not only one universe but a multitude, one each for each 'observer'.

If you conclude that neither Relativity, nor QM (and fields) are satisfied with what we naively think of as our reality and universe, I would agree :)

==

you could also think of it as an 'ocean' consisting of probabilities in where 'time' is directly connected to decoherence giving us outcomes which then becomes our arrow. But 'probabilities' is also a snake biting its own tail in that it comes from us collecting statistics. so defining it as probabilities doesn't lead us any further as I think, but that may be where I'm wrong?

It may depend on how we define time.

[Halc]

Lots like when 2 oceans meet.

The pattern is indeed a lot like what you get with water if the water waves on the left are indeed planar and of consistent wavelength.  But water waves are not particles, and in fact no water molecule travels with its wave.

The diagram makes it look like light must be monochromatic and planar, implying that photons going through one slit interferes with different (but synced) photons going through the other.  This is completely false.  Even if one photon at a time is fired (so not planar with other photons in any way), the interference pattern appears.  If the photon went through one slit, it would produce a one-slit pattern (a bell curve distribution).  The only way the interference patter can appear is if each photon behaves as if it goes through both slits. This is what is meant by them saying that the photon is in superposition of going through each of the slits.  Water doesn't behave that way.

[alancalverd]

You really need to abandon the particle/wave duality business to make progress in physics. Photons, atoms, indeed everything, behaves as it does because it is what it is, not what you choose to model it as.

Quantum mechanics gives us an excellent predictive model of the behaviour of very small things, but doesn't say what they "are". Continuum mechanics gives us an adequately predictive model of mesoscopic entities that allows us to build houses and fly to the planets. The important question to ask of quantum mechanics is "does it scale up to the observed behaviour of radio waves and billiard balls?", which it does. Likewise the test of relativity is "does it scale down to Newtonian mechanics if v<<c?", which it does.

The problem with poking a particulate photon through two slits is manifold.

1. Only half the energy can go through each slit, so the wavelength of the emerging "photons" will be twice as long - but it isn't!   

2. If we rotate the receiver, we will alter the time at which the two bits of photon reach it, so they can't interfere - but they do!

Equally, however, if we presume a wave passing through both slits, all the interference peaks will occur simultaneously, however weak the source (down to one photon at a time) - but they don't!

It gets even more exciting when we pass "solid" objects like electrons, atoms or buckyballs through a diffraction grating. There's no way they can disintegrate and recombine at the receptor (if we move the receptor further away, where does the recombination happen?  If we remove it altogether, have we created partial electrons wandering through space?) but they form the predicted pattern!

The answer: believe what you see, and choose the most appropriate model to predict what you might see next time, but don't be surprised if something else happens - it just means your model was incomplete.

[Lloyd (OP)]

So much to take in its impossible to put a response together! A nice concise up to date book might help me collate all the information into something coherent.

It would seem that Quantum Mechanics is more relevant now than ever in the world we live in. Well it has become more relevant to me anyway. And at 62 years old I thought I had most things sussed. I have no problem in accepting Quantum Mechanics for what it is, even though ... yes ... most of it appears completely bonkers! But there are many questions to be answered and QM seems to be going a fair way to answering them. And hey ... what's the alternative?

I wonder what we would be thinking about Relativity now if we had discovered Quantum Mechanics first? So ... after the reading I've done so far, just what is the biggest obstacle to reconciling Relativity with Quantum Mechanics? 

[jeffreyH]

Relativity is a classical and deterministic theory. Quantum mechanics introduces probability, which is not deterministic.

[Halc]

Relativity is a classical and deterministic theory. Quantum mechanics introduces probability, which is not deterministic.

I don't think relativity theory says anything about determinism one way or the other.

QM theory also doesn't say. It just says that the outcome of an experiment cannot be predicted regardless of how much information of the system can be measured ahead of time.
There are interpretations that are hard indeterministic (fundamental randomness), soft deterministic (randomness is phenomenally emergent, but not fundamental), and hard deterministic (there is only one possible future which could be computed if all variables were known).