Double slit experiment

[paigetheoracle]

I watched a TV program recently that covered Everetts Many Worlds Theory and showed the results of a double slit experiment. Did I miss something? There appeared to be no measuring devices at the slits but we were told that the target pattern showed that the photon had split into two. If there was nothing at the gaps to show two simultaneous blips, what physical proof is there that this is what happened? Sorry but I don't take too much on faith and as a layman it seemed this was speculation but maybe there was more to the experiment than was shown on the program, not being a physicist I wouldn't be aware of this though. I don't dispute the results just the interpretation maybe. For instance could the material used with the slits in it have affected the passage of the photons through creating some kind of field?

 

Also it was stated that the quantam world may work in a different way to the Newtonian one as particles could be affected at a distance, even though separated by space, if force was applied to one of the pair. What if this resonance or harmony is duplicated as in identical twins, where what happens to one is picked up by the other as in supposed telepathy(illness/accident) or when separated and brought up independently, similar episodes occur in their lives. like marrying someone on the same date or dressing in the same style of clothes etc. Could that which is designated now as psychic or bizarre be a reflection of the subatomic worlds strangeness too but not recognized as such or would this idea be more in keeping with the Strange Claims forum?

[zerothehero]

This is the measuement problem of quantum physics. It exists, it is a problem and it doesn't make sense. Quantum physics is the most successful theory ever invented and proven to a million decimal places. It has given us all the technology we have come to take for granted but is basd upon the assumption that we exist in a state of "wave form probabilities" and it is only by an actual observer making a measurement that collapses this wavefom and reduces it to an actual event.

So without even answering this fundamental problem (and they haven;t(the scientists)) they are now coming up with String Theory (beatiful in it's mathematics and needing to have 10 dimemsions to be true) but never in a million years possible to scientifically prove. Is this science?

[modest]

I watched a TV program recently that covered Everetts Many Worlds Theory and showed the results of a double slit experiment. Did I miss something? There appeared to be no measuring devices at the slits but we were told that the target pattern showed that the photon had split into two. If there was nothing at the gaps to show two simultaneous blips, what physical proof is there that this is what happened? Sorry but I don't take too much on faith and as a layman it seemed this was speculation but maybe there was more to the experiment than was shown on the program, not being a physicist I wouldn't be aware of this though. I don't dispute the results just the interpretation maybe. For instance could the material used with the slits in it have affected the passage of the photons through creating some kind of field?

 

The double slit experiment tells us a lot about the nature of the quantum world. A quick explanation would be this:

 

Particles Passing Through 2 Slits: ............ and 1 Slit:

Here we're firing particles (blue) at a barrier (red). You can imagine them as paint balls. The left image has 2 slits in the obstacle and the right image has only one slit. As we might expect, the pattern on the targets (grey) is very predictable. If there is one slit then there is one cluster of impacts. If there are two slits then there are two clusters of impacts. These targets show the nature of particles passing through a single and double slit barrier.

 

Waves Passing Through 2 Slits: ............ and 1 Slit:

Here we have a wave tank with a barrier in the middle. Same setup as before except filled with water. We generate waves on the left side of the tank and monitor the waves hitting the target on the right side. The dark blue line on the target here represents the wave hitting the target. Looking at the single slit on the right we find greatest intensity opposite the slit but mostly spread out. We would expect this because the wave has a single source. Next look at the double slit and notice something very different. There is a pattern of peak heights and minimums across the target. This is due to constructive interference. Basically, it says the wave is amplified where two crests meet (interfere constructively) and the wave's amplitude is decreased when a crest meets a through (interfere destructively). The waves interfere with each other creating an interference pattern. The pattern in dark blue of the left image above shows the nature of waves passing through a double slit.

 

So, what happens if we pass photons or electrons through our setup:

 

Electron Passing Through 2 Slits: ............ and 1 Slit:

The first thing we notice is that our electron (or photon) acts like a wave and a particle. The pattern on the left (2 slit) experiment shows a pattern of constructive interference just like a wave. The pattern on the right (1 slit) shows a single cluster of target-hits much like a particle.

 

But, your question is how do we know the particle passes through both slits before interacting with the target. It has been shown that electrons can be sent through the setup of a double slit one at a time. When this is done repeatedly a pattern of target-hits reveals an interference pattern. The single electron seems to pass through both slits and interferes with itself. Having one thing in two places at the same time is very much at odds with classical reasoning; but, it is shown to be the case. If a detector is placed over one or both slits to determine which way it went then the interference pattern is destroyed. It is the interference pattern created when electrons (or any quantum sized particles) are sent through a double-slit that indicates the probability wave is going through both slits. Quite cool :shrug:

 

-modest

[Rade]

A recent (2007) publication in a very respected peer reviewed journal raises important new questions about double-slit experiment:

 

Paradox in Wave-Particle Duality

S.Afshar et al. in Foundations of Physics.

 

[quant-ph/0702188] Paradox in Wave-Particle Duality

 

The main conclusion of the paper is that a single quantum mechanical entity (= photon, electron...) can behave simultaneously as both a particle and as wave, and they have conducted the experiment to support their claim (this is most likely the reason the editors of the journal decided to publish the experiment--it is a good example of scientific method at work to test predictions of QM theory).

 

IMO, this experiment is very significant because, in 1927, Niels Bohr said that photon is either wave or particle but never both at the same time--this is known as Bohr Principle of Complementary. The authors of the experiment claim to show that the photon can have simultaneous wave and particle properties and thus disagree with Bohr, and conclude that QM is thus "incomplete" as often claimed by Einstein. But of course this Afshar et al. (2207) paper is very controversial--see this Wikipeda link for addition information, pro and con:

 

Afshar experiment - Wikipedia, the free encyclopedia

 

And here is a very nice powerpoint presentation by physicist John Cramer (John G. Cramer - Wikipedia, the free encyclopedia) that shows how the results of an early version of the the Afshar et al. experiment supports his unique interpretation of QM theory:

 

http://www.physics.ohio-state.edu/~perry/p632/articles/Boskone_0402.ppt#1

 

Note:

 

The Cramer presentation is for a 2005 version of the Afshar experiment(S.S. Afshar, “Sharp complementary wave and particle behaviours in the same welcher weg experiment,” Proc. SPIE 5866, 229-244; [quantph//] Shahriar S. Afshar - Sharp complementary wave and particle behaviours in the same welcher weg experiment)

The 2007 Afshar et al. paper is an "improved" version of the 2005 experiment.

[paigetheoracle]

probability wave[/url] is going through both slits. Quite cool :)

 

-modest

 

Thanks for the explanation. From the first reply I thought we had a 'religion' starting here i.e. 'A mystery beyond our comprehension'.

 

One point though, the detectors I suggested wouldn't be interested in where the photon(s) went, just to detect that it/they went through both slits simultaneously as two separate particles or (forgive my ignorance) isn't this what is happening at all and it is just a wave being created on the other side as per your illustration? (The problem is not the experiment or its results but the gap between my understanding of it and someone elses explanation of what is happening/ supposed to be happening).

 

Could this experiment seem nonsensical because its interpretation is? Maybe what we're witnessing is proof of the fabled 'ether' and that the photon is in fact like a stone in a pond disturbing it? I put this forward not as necessarily being right but because I've noticed that the mind has a habit of accepting things as being 'sound' because it can see 'how' it could be true, not that it necessarily 'is': The world could have seemed flat until space travel proved it wasn't, for instance.

[modest]

Thanks for the explanation. From the first reply I thought we had a 'religion' starting here i.e. 'A mystery beyond our comprehension'.

Yeah, it's not hard to understand how people find spirituality in quantum mechanics

 

One point though, the detectors I suggested wouldn't be interested in where the photon(s) went, just to detect that it/they went through both slits simultaneously as two separate particles or (forgive my ignorance) isn't this what is happening at all and it is just a wave being created on the other side as per your illustration? (The problem is not the experiment or its results but the gap between my understanding of it and someone elses explanation of what is happening/ supposed to be happening).

It's a great question and I'll try to answer with my limited knowledge of the subject. If anyone else wants to explain it differently then I would not at all be offended.

 

The photon travels through the slits as a probability wave. We cannot know where it is until we measure its position. If the photon is emitted and allowed to travel through our slits unobserved until it reaches the screen then the probability wave will 'interfere with itself' and create an interference pattern. However, if we measure its position (as with a detector at one or both slits) then the probability wave collapses - there is then 100% probability as to the location of the photon. There is no longer an equal probability between both slits and the interference pattern goes away. So, according to the rules of quantum mechanics it is impossible to know 'which way' the photon goes and get an interference pattern. According to the Copenhagen interpretation of quantum mechanics the photon has a probability of traveling through every possible path through space (in our experiment - 2 slits). Only when it is detected or interacts with the target does it's probability wave collapse. According to the Heisenberg uncertainty principle there is no way of measuring the photon's position without altering its momentum. So, there is no way of knowing which slit it passes through without affecting the photon or collapsing the probability wave.

 

I know it is difficult to accept that the photon doesn't know which way it's going or how it got where it is. But, this is fundamental to quantum mechanics.

 

Could this experiment seem nonsensical because its interpretation is? Maybe what we're witnessing is proof of the fabled 'ether' and that the photon is in fact like a stone in a pond disturbing it? I put this forward not as necessarily being right but because I've noticed that the mind has a habit of accepting things as being 'sound' because it can see 'how' it could be true, not that it necessarily 'is': The world could have seemed flat until space travel proved it wasn't, for instance.

 

No, the double-slit is actually a very good test of quantum mechanics and quantum mechanics describes it very well (though maybe I'm not).

 

-modest

[paigetheoracle]

Heisenberg uncertainty principle there is no way of measuring the photon's position without altering its momentum. So, there is no way of knowing which slit it passes through without affecting the photon or collapsing the probability wave.

 

I know it is difficult to accept that the photon doesn't know which way it's going or how it got where it is. But, this is fundamental to quantum mechanics.

 

 

So it's the 'fabled' Observer Effect in action - or interference patterns of a different kind? By the way, the diagram makes sense of what I saw in the program, now.

 

I could only understand it if it was waves of particles, not waves or particles: I always used to think of photons as lightning (particles) and the waves as thunder (You can hear the effect but never see it).

 

This sort of goes into spooky territory doesn't it - as in quantam physics terms and psychic? (Bishop Berkeley 'Is the quad still there if not observed by God?'/Things that can only go bump in the night if you're not looking/ the half seen thing out of the corner of your eye). Could ghosts be some kind of quantam effect or work through quantam mechanics? (I visit a blogsitethat covers this kind of thing but from the human perspective - which was why I was watching the program that included this experiment: Parallel worlds, Parallel lives (BBC4) on British TV; centred around Hugh Everett III's musician son and his efforts to understand his dad's Multiverse theory.

[freeztar]

I could only understand it if it was waves of particles, not waves or particles: I always used to think of photons as lightning (particles) and the waves as thunder (You can hear the effect but never see it).

 

Rade's post above gives a link to research on the duality of light. I haven't followed the link, so I don't know how informative it is, but googling light duality should get you some answers.

 

To summarize it, for a long time people thought that light was exhibited as either a particle or a wave. Recent research has shown that light can exhibit both properties at once.

[paigetheoracle]

What I seem to be reading into this is that quatum mechanics seems to reach out into other spheres too - namely morality and philosophy. By this I mean things seem to only work in freedom (trust i.e. unobserved/ left alone) and that includes people: Life is voluntary - death, obligatory (praise versus criticism) - in other words things work through a kind of spontaneous magic and interruptions kill all effort dead: Sight hold things in thrall(stops them)as sound releases them into motion. Everything works through the release of energy and this is where science and spirituality meet (Jesus 'Consider the lillies of the valley...'*). In other words Eastern philosophy touches upon this as does Western physics now (How anything works, is how everything works - as a general principle).

 

* In other words, things left to their own devices turn out to be perfect forms of themselves, in mind, body or spirit (energy) but when interferred with they become ill, distorted, stunted (negative interference patterns).

[modest]

Backwards:

 

* In other words, things left to their own devices turn out to be perfect forms of themselves, in mind, body or spirit (energy) but when interferred with they become ill, distorted, stunted (negative interference patterns).

 

The interference pattern is the unobserved experiment.

[paigetheoracle]

Backwards:

 

 

 

The interference pattern is the unobserved experiment.

 

Methinks what I'm saying and what you're saying is different - in other words you are meaning it in a specific context and I'm using intereference as a general term, unrelated to the experiment but not unrelated to real life: no conflict in reality just a little misunderstanding though.

 

The way I meant it was like a balloon - left alone to fill with air, makes a perfect orb or cell - disturbed in its growth pattern, it gets destroyed or distorted.

[paigetheoracle]

Another thing about this, looked at from a laymans perspective, is that it reflects the observation that if you want something so badly, that you chase it - it runs away from you (love, success, money, the paranormal etc) but if you become passive, it runs to you or appears unconsciously as in dreams or intuition (think of all the problems that have 'solved themselves' when you give up all deliberate effort on them). All discoveries are accidental from this perspective (passive, relaxed, unconscious) and even the state of the universe and the growth of life seems this way too but in reality isn't it that this state 'allows' things to happen (expand into reality) like magic, where deliberation acts as the gatekeeper, to let certain things through and keeps other back(intelligent design but not as the bible thumpers mean it - Life as a car factory i.e. systematic exploitation of an idea on a conveyor belt, ,with design variations through time as more is learnt than the basics, and the ideas refined)? As this is a physics forum, this may be getting off track but it is not where I was deliberately aiming it to go, just the answer I got back started my mind off on another unexpected facet. Sorry.

[paigetheoracle]

One point not answered so far is to what 'degree' does the observer effect interfere with the double slit experiment? If we have the results of what happens when no interference occurs, then surely as scientists, you should be able to take this sacred cow of a theory and see if it holds up totally to physical reality. By this I mean devise some subtle way of measuring as I first suggested: Fire a cannon ball at a sheet of paper and it will burn its way through. Kick a football and it may tear its way through but a balloon blown against it, is unlikely to penetrate the material. I thought science revolved around experiment to prove the theory and so far nobody has told me anybody has tested this belief, in the way I suggested or any other. This doesn't mean I don't believe in the observer effect but it may be that in this respect it's about the size of the objects studied and not necessarily because it is a quantum effect (The American Indians understand that to stalk (hunt) anything, you don't charge at it, shouting and screaming, unless you want to frighten it away but must move carefully, slowly, gently and sometimes in disguise/ circumspectly, so as to create the minimum disturbance to it, so you can get as close as possible to it: This I propose is the only way to study anything - shrink your presence, so as to get as close to the thing studied as possible and see its reactions (life) as near as normal as possible (Bulls in china shops, need not apply)

[snoopy]

One point not answered so far is to what 'degree' does the observer effect interfere with the double slit experiment? If we have the results of what happens when no interference occurs, then surely as scientists, you should be able to take this sacred cow of a theory and see if it holds up totally to physical reality. By this I mean devise some subtle way of measuring as I first suggested: Fire a cannon ball at a sheet of paper and it will burn its way through. Kick a football and it may tear its way through but a balloon blown against it, is unlikely to penetrate the material. I thought science revolved around experiment to prove the theory and so far nobody has told me anybody has tested this belief, in the way I suggested or any other. This doesn't mean I don't believe in the observer effect but it may be that in this respect it's about the size of the objects studied and not necessarily because it is a quantum effect (The American Indians understand that to stalk (hunt) anything, you don't charge at it, shouting and screaming, unless you want to frighten it away but must move carefully, slowly, gently and sometimes in disguise/ circumspectly, so as to create the minimum disturbance to it, so you can get as close as possible to it: This I propose is the only way to study anything - shrink your presence, so as to get as close to the thing studied as possible and see its reactions (life) as near as normal as possible (Bulls in china shops, need not apply)

 

 

I think what you are referring to is Heisenbergs uncertainty principle in which you cannot know precisely a things momentum and position.

 

This is because the act of measuring a thing changes its position or momentum very slightly, this does not mean however you cannot know roughly which is what physicists do by means of a probability distibution.

 

Because the double slit experiment does not involve measuring a things momentum or position then the observer does not interfere with it.

 

Peace

:)