Basic Double Slit Experiment Question

[MrFoos]

Laymen and hobby student of physics here. So the only things I know (or at least think I know) I watch from documentaries.

 

In the test to see if an electron is a particle or wave, they say that if you place a viewer before the slits that the resulting pattern on the back wall is a particle result. But if you aren't placing a viewer before the slit the resulting pattern on the back wall is wave interference.

 

So how is watching the result on the back wall itself NOT placing a viewer thereby always causing the electron to act as a particle?

 

Thank you in advance!

Patrick

[Delta1212]

There are two slits. A wave will pass through both and interfere with itself. A particle will pass through only one slit.

 

If the experiment is set up so that you can tell which slit each photon passed through, there will be no interference pattern. If there is no way to determine which path each one took, there will be an interference pattern.

[MrFoos]

Thanks. Maybe I don't underdstand your reply but it doesn't sound like it answers my question. How do we KNOW whether or not there is an interference pattern unless we are looking? If are looking... doesn't that result in the electron behaving like a particle? So how do we see the interference pattern without "looking" in some way?

[Delta1212]

Thanks. Maybe I don't underdstand your reply but it doesn't sound like it answers my question. How do we KNOW whether or not there is an interference pattern unless we are looking? If are looking... doesn't that result in the electron behaving like a particle? So how do we see the interference pattern without "looking" in some way?

You don't. And at the point of detection, you are detecting a point-like particle. You have to fire a lot of them at the detector screen to see the aggregate probability of the particle hitting any given position on the screen. When you are filtering the particles so that you know which slit they went through, the aggregate shows no interference. When you don't know which one each particle went through there is an interference pattern.

 

So while you're detecting a particle, it must have wave-like properties because a classical particle would only be able to travel through one slit or the other, whether you knew which it was or not, and would not be able to interfere with itself.

[Strange]

In the test to see if an electron is a particle or wave, they say that if you place a viewer before the slits that the resulting pattern on the back wall is a particle result. But if you aren't placing a viewer before the slit the resulting pattern on the back wall is wave interference.

 

I think this is a very confusing way to think about this experiment. I'm not sure I can give you a less confusing one though...

 

The important point is that the electron always behaves somewhat like a wave and somewhat like a particle (while not being either).

 

 

So how is watching the result on the back wall itself NOT placing a viewer thereby always causing the electron to act as a particle?

 

When you detect the electron, it will always be detected at a single point ( like a particle).

 

If you don't determine which slit it went through then you will, over time, build up an interference pattern as you would expect from classical waves.

 

If you do detect which slit the electrons go through, then you will end up with a distribution the same as you would expect from classical waves passing through a single slit.

 

So it is almost as if, when you don't check, the electrons go through both slits (*). But if you do check, then they only go through the slit you detect them at and not the other one.

 

(*) That is one interpretation but not one I am very fond of. After all, electrons are indivisible.

[ox1111]

I think he may be talking about the double slit test from posterity. In which to very narrow slits are placed in a row at a distance in a box. When light is let in the first slit, only a straight beam passes through the second slit. when viewing the light pattern on the back of the device, the light beam is slightly wider than the slit, indicating light acts as a wave. Before this test, light was only proven to act as a particle and with this historic test light was proven to behave as both.

Sadly this test changed the way science thought about light, where as now we can see many problems with this test and various reasons for the outcome.

Edited August 30, 2013 by ox1111

[between3and26characterslon]

Laymen and hobby student of physics here. So the only things I know (or at least think I know) I watch from documentaries.

 

In the test to see if an electron is a particle or wave, they say that if you place a viewer before the slits that the resulting pattern on the back wall is a particle result. But if you aren't placing a viewer before the slit the resulting pattern on the back wall is wave interference.

 

So how is watching the result on the back wall itself NOT placing a viewer thereby always causing the electron to act as a particle?

 

Thank you in advance!

Patrick

In order to observe the electron it has to ineract with something (a magnetic field for example, or, the back wall of the apparatus) if this interaction occurs BEFORE the electron goes through the slits it will only go through one slit as a particle. If however there is no observation of the electron before it goes through the slits it goes through as a wave.

 

The remarkable thing is that the electron is always in the Earths gravitational field and magnetic field so is always interacting with something so it seems it is conscious observation that gives rise to the difference between diffraction pattern or not.

[Strange]

it seems it is conscious observation that gives rise to the difference between diffraction pattern or not.

Wrong. Any detection of the location of the particle, conscious or otherwise, will change the result.

[sb635]

 

Wrong. Any detection of the location of the particle, conscious or otherwise, will change the result.

Does the matter of the plate (which has the slits cut in it) "observe or detect or interact with" the electron as it passes from one side of the plate to the other side? If so, this would seem to "force" the electron through one of the slits, not both, at that instant of interaction, which is before it impacts on the observing screen. Also, the double slit experiment can be made more interesting if the plate is put on springs. The plate then moves at the instant the electron goes from one side to the other. The electron itself is not observed to tell which slit it went through. The momentum of the plate is observed, which is different if the electron went through slit A compared to slit B. Supposedly, even if the plate's momentum is observed for each single electron sent to pass through the slits, this still does not destroy the diffraction pattern, but the logic behind that conclusion is flawed to me. The logic clearly shows the delta-momentum (in the UP) is actually the measurement error of the momentum determining/observing machine, which has nothing to do with the nature of an electron. Also, conceptually it seems that after the experiment has been performed, and a diffraction pattern has been produced, a person could still figure out the which slit the first electron went through, and then likewise for the second, third, etc. electrons. A gravitational wave detector could be setup a farther distance away than the observing screen. For each electron sent through, the gravitational wave the plate emitted (due to its motion after acquiring some momentum from the electron) could be observed by a gravitational wave/radiation detector after the electron hit the observing screen. The gravitational radiation could be then used after the diffraction pattern is produced to determine, with a very high degree of confidence (by using a very accurate and precise gravitational wave detector), which slit each electron went through.

 

It is also stated that a single experiment cannot determine both the particulate and wave characteristics of an electron. A single electron is sent to the plate containing the two slits, and a single electron (particle) is observed to impact on the observing screen. That seems to say an electron is a particle, at least when it impacted the observing screen. Many electrons are sent through one at a time and many single particle impact locations are produced, building up the diffraction pattern. Isn't that observing both the wave and particle characteristics of an electron in the same experiment?

[sb635]

... Supposedly, even if the plate's momentum is observed for each single electron sent to pass through the slits, this still does not destroy the diffraction pattern...

I was wrong, the text book that describes this plate-on-springs experiment states the diffraction pattern is destroyed. But still the logic why is flawed, to me. Let p₁ be the induced plate momentum if the particle goes through slit 1, and p₂ be the momentum induced if the particle goes through slit 2, which will be different. The text then states "the uncertainty [math]\Delta p[/math]" in the momentum of the plate must be sufficiently small for us to be able to measure the difference between p₁ and p₂. It is clear to me, this "uncertainty" (which is the uncertainty in the UP itself) is just the measurement error in determining the plate's momentum using a (necessarily) flawed momentum observing machine. And of course, the measurement error in determining the plate's momentum should be much smaller that the actual physical momentum of the plate, otherwise, this momentum will be "lost in the noise" of the measuring device. The is clearly implied in the text book by stating the requirement that [math]\Delta p < < \left| {{p_2} - {p_1}} \right|[/math]. I certainly agree with that, but only if I interpret [math]\Delta p[/math] as an observatory machine error. This measurement error, to me, is completely independent of whether or not the electron is a particle or a wave when it "went" through the plate. What if the momentum observations were never taken? Does the diffraction pattern then appear?

 

[Delbert]

 

The important point is that the electron always behaves somewhat like a wave and somewhat like a particle (while not being either).

 

It always behaves the same. Assuming that is that you accept it exists as a 'thing'. The wave pattern is one experiment. Observing activity at the slits and also observing what then happens at the destination is two experiments, and thus two sets of events. However one designs or constructs it, observing at one of the slits will be detecting it. Thus turning the observing point into not only a detector but a new source. Thus there will be one experiment from the source to a slit, and the other from a slit to the final detector. No double slit was involved in either experiment.

 

And if you accept what I understand to be Richard Feynman's explanation, a photon doesn't 'travel' from source to destination, but rather the energy of what we call a photon is a myriad of interactions. Such that if one is able to construct a appropriate experiment one could possibly detect the said photon in all sorts of places. I suppose one could even argue whether or not there is even such a thing as a photon; leaving the situation whereby all we have is an event at the source and then an event at the destination. What happens in between is in the land of imagination - possibly the fairies!.

 

Trouble is, we have mental hang up whereby because we have an event in one place and then a event a moment later at another place, whereby the second seems to follow the first, we make this ridiculous assumption that something has travelled from the first place to the second!

 

The universe is a weird place.

[swansont]

And if you accept what I understand to be Richard Feynman's explanation, a photon doesn't 'travel' from source to destination, but rather the energy of what we call a photon is a myriad of interactions. Such that if one is able to construct a appropriate experiment one could possibly detect the said photon in all sorts of places. I suppose one could even argue whether or not there is even such a thing as a photon; leaving the situation whereby all we have is an event at the source and then an event at the destination. What happens in between is in the land of imagination - possibly the fairies!.

Depends on what you mean by this. The photon can be modeled as traveling all of those paths, but all but one interfere and cancel. If your emitter is isotropic you don't know where the photon will end up. But if you emit one photon, your experiment can only detect one photon at one location.

[Delbert]

Depends on what you mean by this. The photon can be modeled as traveling all of those paths,...

It seems to me that by implying this thing we call a photon can be 'modelled' as you describe, we are doing nothing more than shoehorning what's really going on into a classical understanding. But I think we all know these things don't follow a classical understanding.

 

It seems to me we've no evidence whatsoever that a photon travels from A to B. But rather whenever we observe it 'in flight', the situation changes, such that our observing point becomes a detector. In other words we presumably simply have the thing travelling from source to observing point, and then from the observing point to the destination, as I mentioned previously. We never observed anything 'in flight'. All we've observed is an event at the source and then another at the destination. What happened in between can only be imagined.

 

As you suggest, we presumably can only understand the thing by modelling many - if not a infinite number of - paths. If I throw a ball from A to B, I only have to model one path to come up with a correct calculation. Mind you, I suppose some might say there maybe tiny errors in my calculation - like slight varying gravity in its trajectory occasioning a tiny relativity affect, maybe. But I think even that wouldn't require a multipath calculation.

 

The conclusion seems to be is that the thing isn't real as a we understand a thing to be, therefore doesn't exist as we understand existence. So, trying to say it travels fails as a consequence. All we can say is we have a sequence of events.

[swansont]

It seems to me that by implying this thing we call a photon can be 'modelled' as you describe, we are doing nothing more than shoehorning what's really going on into a classical understanding. But I think we all know these things don't follow a classical understanding.

 

It seems to me we've no evidence whatsoever that a photon travels from A to B. But rather whenever we observe it 'in flight', the situation changes, such that our observing point becomes a detector. In other words we presumably simply have the thing travelling from source to observing point, and then from the observing point to the destination, as I mentioned previously. We never observed anything 'in flight'. All we've observed is an event at the source and then another at the destination. What happened in between can only be imagined.

 

As you suggest, we presumably can only understand the thing by modelling many - if not a infinite number of - paths. If I throw a ball from A to B, I only have to model one path to come up with a correct calculation. Mind you, I suppose some might say there maybe tiny errors in my calculation - like slight varying gravity in its trajectory occasioning a tiny relativity affect, maybe. But I think even that wouldn't require a multipath calculation.

 

The conclusion seems to be is that the thing isn't real as a we understand a thing to be, therefore doesn't exist as we understand existence. So, trying to say it travels fails as a consequence. All we can say is we have a sequence of events.

 

From what I understand of Feynman's method, all paths are considered. So in that, you are correct AFAIK — the double slit experiment is an indication of this. My concern was the implication that you could detect a photon more than once, at different locations.

[Delbert]

 

My concern was the implication that you could detect a photon more than once, at different locations.

That seems to be a human construct. There were two sets of events, as I described previously. I would suggest that's not one photon moving from source to observing point and then to destination.

 

 

From what I understand of Feynman's method, all paths are considered.

Yes that's right. So how can it be one object?

 

And I understand we're not necessarily limited to two slits. We presumably can have any number of slits for the slit experiment and the photon will travel through the lot on the same journey.

 

And my understanding of Feynman's paths is that they are not just paths, but interactions. Such that as soon as what we call a photon leaves the source it reacts and changes into other particle or particles, which then react again, and so on. As per Feynman's diagrams. So how could a photon have ever travelled at all or even existed!

Edited September 9, 2013 by Delbert

[swansont]

You also have to remember that this is not a statement of how nature really is, but how nature behaves.

[Delbert]

You also have to remember that this is not a statement of how nature really is, but how nature behaves.

Yes. Presumably the way we describe these things enables us to do descriptions and calculations. As I intimated above, the very idea of a 'photon' is a human construct. The thing probably doesn't actually exist, but as mentioned, is rather a countless sequence of events. I suppose the tricky bit is how or what causes the distributed infinite morass of events to somehow condense into a single event at the destination, giving us the illusion of a particle travelling from A to B!

[between3and26characterslon]

Wrong. Any detection of the location of the particle, conscious or otherwise, will change the result.

At the risk of entering into the realm of philosophy what is meant by "detection of the location of the particle". Is observation or detection a conscious thing per se or does it simply mean an interaction? If so why does the electron's interaction with gravity and the Earth's magnetic field not affetc the result?

[Delbert]

At the risk of entering into the realm of philosophy what is meant by "detection of the location of the particle". Is observation or detection a conscious thing per se or does it simply mean an interaction? If so why does the electron's interaction with gravity and the Earth's magnetic field not affetc the result?

It seems to me that what we call detection in fact changes the situation from one ('one' being a mass of interactions as above) process to two processes. Nothing to do with a conscious thing as you seem to imply - unless of course we take it further and say consciousness is a physical process. Observing presumably involves a sequence of events leading to a realisation, so perhaps it is and therefore nothing mysterious.

 

I understand it can get even better with this entanglement business. Like, a photon (or the consequence of events as above!) hits a receptor in my eye whilst I'm looking at the stars, and I can apparently cause the source, millions or billions of miles away, to be a source!!! So was it there before I looked? I didn't know I was so influential.

[JoeLight]

I'm not a physicist so forgive me if this is completely wrong. Its just a thought. I’m under the understanding that potential energy and kinetic energy are very different in that potential energy is the potential for an interaction, and kinetic energy is a measurement of the actual interaction. Since we cannot observe something without witnessing the interaction then potential energy shall never be observed making it nothing more than function of probability. When and electron is fired at the 2 slits it has the potential to pass through either one of the slits or hit the barrier and not pass at all because the electron does not have a defined location until its actually observed. When it isn't observed it has the potential to be on an infinite number of paths. This potential for the electron to travel in any direction could be described much like waves of light and this wave function amplifies and cancels itself just like the wave function of light. For the sake of this explanation I am going to call this a potential wave. The kinetic energy that we can observe does not exist until the measurement is made. When we measure the electrons at the screen they have the potential to be at multiple locations and the probability of the location follows the same interference pattern that light waves do. To measure them we have to interact with them which convert their potential energy into kinetic energy and also define the electron as a particle with a definite location. By simply observing the electrons at the barrier you have now defined its location and it can no longer travel through slit1 if it is already in slit 2. This eliminates the ability for the potential wave from the electron to pass through slit 1 and interfere with the potential wave from slit 2 as there is no electron to have a potential wave in slit 1 and there is in slit 2 and you end up with 2 solid lines rather than the interference pattern. If you were to do the same experiment but add 2 more slits in the middle of the dense line on the second screen and allow them to further pass to a 3rd screen and you observe the electrons at the second screen you would have and interference pattern on the second screen and 2 defined lines on the third. I don’t know if this is already a proposed theory. I had never seen the double slit experiment until the other day and the scientist asked the question “why does this happen”. Immediately this is what came to mind. I’m good at visualizing the way things work but lack the Math skills and Vocabulary to do much with it. Any input on this?

[Delbert]

When and electron is fired at the 2 slits it has the potential to pass through either one of the slits or hit the barrier and not pass at all because the electron does not have a defined location until its actually observed. When it isn't observed it has the potential to be on an infinite number of paths.

"When an electron is fired"? If it was a definable object, then it simply couldn't go through two slits, be everywhere at the same time or any other weirdness we care to ascribe. Saying it 'travels' is an assumption. All we can say is there's an event at the source followed by an event at the destination. What happens in between is anybody's guess.

 

When we attempt to detect the thing (I'm calling it a 'thing' for convenience) at one of the slits, we've in effect created a destination, which then becomes a second source - and so on. In other words a completely different situation or experiment as evidence by the different result (no interference pattern). What happens to this thing whilst what we would refer to as actually being 'in flight' is an assumption - and I suggest there's no place in science for assumptions. I further suggest we have no evidence whatsoever that there is a thing (electron, photon or whatever) actually flying from place to place.

 

And what's more, if it were an actual 'thing', then the thought of it passing through two slits at the same time (or more if there are more than two), then that's something from Alice in wonderland and has no place in science.

 

In other words it can't travel through two slits. And if it's deemed to have actually passed through two slits (as evidenced by the two slit experiment), then it cannot be an identifiable object.

 

What is it then? My conclusion is that there's an event at the source followed by an event at the destination. What happens in between is probably some sort of maelstrom of interactions and interchanges. A thing that we might call an electron or photon never travels from A to B. Which begs the question whether or not such things actually exist - but doubtless my imagination is now running into the heights of hyperbola!

[swansont]

Potential energy is energy present because of the configuration of the system. It's usually not a good idea to try and infer physics properties based on dictionary definitions.

 

You get interference because electrons have wave properties.

[robinpike]

There are two slits. A wave will pass through both and interfere with itself. A particle will pass through only one slit.

 

If the experiment is set up so that you can tell which slit each photon passed through, there will be no interference pattern. If there is no way to determine which path each one took, there will be an interference pattern.

 

Treating the electrons as waves seems to be a rather implausible explanation!

 

When the electrons are fired with the apparatus having NO slits present, the electrons never get through the barrier, they never appear on the other side.

 

From this, it can be concluded that the electrons are not able to pass through the barrier.

 

With the proposed explanation when the two slits are present in the barrier, is that the electron is a wave, or the electron 'travels' along many paths and those different paths enables the electron to interfere with its own direction of movement when it exits on the other side of the barrier.

 

If the electron really were a wave, then part of the wave will always be inside the interior of the barrier, where the electron / wave would not be able to proceed, and it would become trapped inside the barrier.

 

Or, if the electron were travelling along many paths, then most of those paths would also take the electron into the interior of the barrier, and the electron would not be able to proceed, it again would become trapped inside the barrier.

 

How can the electron as a 'wave', or as a 'particle' able to travel along 'multiple path', ever manage to make it to the other side of the barrier, regardless of whether the slit is present or not?

[swansont]

 

Treating the electrons as waves seems to be a rather implausible explanation!

 

 

And yet that's precisely how they behave, which has been known for >75 years.

[barfbag]

If we ignore the hokey consciousness aspects of this video, it is a decent cartoon of how the experiment is done.

 

 

 

So the OP can see the results can be 2 bands.

 

The parts that infer consciousness implications Fred Alan Wolf and some claim are in reality caused by the style of measurement imposed.

 

I think it is a good cartoon if you stick to reality or end it a minute too soon.