Young's Double Slit Experiment

[bob_kark]

I've read Schrodinger's Kittens by John Gribbin and The Fabric of Reality by David Deutsch. I don't claim to know much by reading these, I just have an interest in physics. Anyway, in these books (and Wiki), they discuss some of the oddities of Young's Double Slit Experiment and one of these oddities is that if you were to place some sort of photon detector on either of the two slits but still allow the slit to stay open, it causes the interference pattern to disappear.

I'm just a little confused about this. I'm wondering if anyone knows how the photon detector actually detects the photons. Does it test to see if the photon is absorbed by an electron? If so, could this actually cause the disappearance of the interference pattern or are these books making a valid point? I'm just not sure if this is sensationalism to sell the book, if this is possibly outdated information now, or if they're on to something.

I also wanted to see if anyone else could recommend some other books on quantum physics, astrophysics, etc... It's very interesting to me, but I'm a bit of a layman and don't really know how much I should trust the information in these books. I've taken physics classes and I read Scientific American, but I never received a degree in a scientific field so I wouldn't really understand something too advanced. Any ideas? Thanks for the help in advance.

 

[Ducky]

Man that's a misleading title to a randy young man like me...

 

[epepke]

I've read Schrodinger's Kittens by John Gribbin and The Fabric of Reality by David Deutsch. I don't claim to know much by reading these, I just have an interest in physics. Anyway, in these books (and Wiki), they discuss some of the oddities of Young's Double Slit Experiment and one of these oddities is that if you were to place some sort of photon detector on either of the two slits but still allow the slit to stay open, it causes the interference pattern to disappear.

I think this is fairly confused.

Young's experiment was done a long time ago. You cannot detect a photon, in the way you mean, without destroying it. So, yeah, it's going to affect the pattern!

More interesting is doing the two-slit experiment with electrons. You can detect an electron without destroying it: just shine a little light. And still, there is no way to do this without affecting the interference pattern. If you can tell definitely which slit the electron is going through, then the interference pattern vanishes. If you can tell, say, half the time, then you get half an interference pattern.

This led to the realization, upon which quantum physics is based, that everything in the universe works like this. You just cannot have a particle, call it a "bogon," that you can shoot at something to detect it without also affecting it.

It's called "quantum" because you can't make a light dim enough to detect an electron without affecting it. If you keep making it dimmer and dimmer, it comes in lumps, i.e. quanta. And for a given frequency of light, each lump or quantum is the same size. So if a lump interacts, it changes things. If a lump doesn't interact, it doesn't.

You can make the lumps of light (photons) with lower energy, but then the get bigger, and then they are too big to distinguish between the two slits.

Obviously, there's a lot more to QM, but them's the basics.

 

[rjh01]

You know that a electron left an electron gun and ended up on a piece of film. In between it had an equal choice between two slits. The electrons do not make a choice. They go though both at the same time. Hence you can shoot them out the gun one at a time and the pattern stays. However put an electron detector on the slits and the pattern goes away.

Strange but true.

 

[NeilC]

Not strange - mindblowingly strange.

Why does it happen?

 

[rjh01]

See here http://en.wikipedia.org/wiki/Uncertainty_principle. This explains the Heisenberg uncertainty principle.

Not sure it explains why it must be true. I think it is just one of those things that just are. It is almost like asking why is the speed of light that speed?

 

[Angus McPresley]

Not strange - mindblowingly strange.

Why does it happen?

"I think it is safe to say that no one understands quantum mechanics. Do not keep saying to yourself, if you can possibly avoid it, 'But how can it be like that?' because you will go 'down the drain' into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that."
--Richard Feynman


I would also recommend Feynman's "QED" as a fantastic book on quantum physics for the layman. The second chapter in particular will have you wondering why you were never taught this in school.

 

[bob_kark]

Man that's a misleading title to a randy young man like me...

Ah, sorry to get you excited so late at night. It was a bit of a racy title for late night posting.

 

[bob_kark]

I think this is fairly confused.

Young's experiment was done a long time ago. You cannot detect a photon, in the way you mean, without destroying it. So, yeah, it's going to affect the pattern!

Thanks for the response epepke! Sorry, I may not have been too clear in my OP. Here's a better discription via Wiki:

A remarkable refinement of the double-slit experiment consists of putting a detector at each of the two slits, to determine which slit the photon passes through on its way to the screen (If the photon or electron passes through only one slit - which it must do, as, by definition, a photon or an electron is a quanta, or "packet" of energy which cannot be subdivided - then logically it can not interfere with itself and produce an interference pattern). When the experiment is arranged in this way, the fringes disappear.

The Copenhagen interpretation posits the existence of probability waves which describe the likelihood of finding the particle at a given location. Until the particle is detected at any location along this probability wave, it effectively exists at every point. Thus, when the particle could be passing through either of the two slits, it will actually pass through both, and so an interference pattern results. But if the particle is detected at one of the two slits, then it can no longer be passing through both - it must exist at one or the other, and so no interference pattern appears.

This is similar to the path integral formulation of quantum mechanics provided by Richard Feynman (although Feynman stresses that this is merely a mathematical description, not an attempt to describe some "real" process that we cannot see), in which a particle such as a photon takes every possible path through space-time to get from point A to point B. In the double-slit experiment, point A might be the emitter, and point B the screen upon which the interference pattern appears, and a particle takes every possible path - through both slits at once - to get from A to B. When a detector is placed at one of the slits, the situation changes, and we now have a different point B at the detector, and a new path between the detector and the screen - upon which the interference pattern no longer appears).

It doesn't appear at least according to this that the photon is destroyed. I was wondering if the interference pattern disappeared simply because of the reaction of the photon to the detector or if it was due to limiting the photon to travelling through one slit at a time rather than both at once.

More interesting is doing the two-slit experiment with electrons. You can detect an electron without destroying it: just shine a little light. And still, there is no way to do this without affecting the interference pattern. If you can tell definitely which slit the electron is going through, then the interference pattern vanishes. If you can tell, say, half the time, then you get half an interference pattern.

This led to the realization, upon which quantum physics is based, that everything in the universe works like this. You just cannot have a particle, call it a "bogon," that you can shoot at something to detect it without also affecting it.

It's called "quantum" because you can't make a light dim enough to detect an electron without affecting it. If you keep making it dimmer and dimmer, it comes in lumps, i.e. quanta. And for a given frequency of light, each lump or quantum is the same size. So if a lump interacts, it changes things. If a lump doesn't interact, it doesn't.

You can make the lumps of light (photons) with lower energy, but then the get bigger, and then they are too big to distinguish between the two slits.

Obviously, there's a lot more to QM, but them's the basics.

This again leads back to my question though. Does the interference pattern disappear because the electrons react to the photons, by absorbing them, or does it disappear because the electron had to travel through only one slit, thus not allowing it to travel through the other slit at the same time?

 

[Carn]

Thanks for the response epepke!



This again leads back to my question though. Does the interference pattern disappear because the electrons react to the photons, by absorbing them, or does it disappear because the electron had to travel through only one slit, thus not allowing it to travel through the other slit at the same time?

The second, the interference pattern vanishes, because the electron "goes" only through one slit. This effect would be still present, if one had a magic way to measure where the electron "goes" through, without affecting it in the slightest way, would theoretically still produce this effect.

Carn

 

[bob_kark]

The second, the interference pattern vanishes, because the electron "goes" only through one slit. This effect would be still present, if one had a magic way to measure where the electron "goes" through, without affecting it in the slightest way, would theoretically still produce this effect.

Carn

Thanks Carn!

 

[epepke]

Sorry; I started a response to this but had to do some work.

It doesn't appear at least according to this that the photon is destroyed.

It's quite simple. The wikipedia article is wrong.

This again leads back to my question though. Does the interference pattern disappear because the electrons react to the photons, by absorbing them, or does it disappear because the electron had to travel through only one slit, thus not allowing it to travel through the other slit at the same time?

Both, and neither.

You have to remember that this doesn't happen in a classical way, such as with a billiard ball. The photon doesn't bounce off the electon and push it. The rules of QM are clear and easy to calculate, but they're so different from what we're used to, it's impossible to understand.

What happens, really, is that you can't really say that the electron goes through both holes. Rather, in the absence of a state change (which would happen if you shined a light on it), you just can't say that it goes through one hole or the other. There's a subtle difference. I highly recommend the QED book, or even better, the video taped or the DVD of QED in NZ that you can get from tuvatrader.com. The quality is poor, but they're fun.

I don't know if this makes you feel any better, but it makes me feel better. The classical view has a problem. Either it goes down forever, or it bottoms out. If it bottoms out, one can expect where it bottoms out to be very different. The universe seems to be arranged such that it does bottom out, and where it bottoms out is QM, which is indeed very different.

 

[kmortis]

I think this is fairly confused.

Young's experiment was done a long time ago. You cannot detect a photon, in the way you mean, without destroying it. So, yeah, it's going to affect the pattern!

More interesting is doing the two-slit experiment with electrons. You can detect an electron without destroying it: just shine a little light. And still, there is no way to do this without affecting the interference pattern. If you can tell definitely which slit the electron is going through, then the interference pattern vanishes. If you can tell, say, half the time, then you get half an interference pattern.

This led to the realization, upon which quantum physics is based, that everything in the universe works like this. You just cannot have a particle, call it a "bogon," that you can shoot at something to detect it without also affecting it.

It's called "quantum" because you can't make a light dim enough to detect an electron without affecting it. If you keep making it dimmer and dimmer, it comes in lumps, i.e. quanta. And for a given frequency of light, each lump or quantum is the same size. So if a lump interacts, it changes things. If a lump doesn't interact, it doesn't.

You can make the lumps of light (photons) with lower energy, but then the get bigger, and then they are too big to distinguish between the two slits.

Obviously, there's a lot more to QM, but them's the basics.

Something hit me while reading this.

The universe is apparently pulse width modulated.

 

[bob_kark]

Sorry; I started a response to this but had to do some work.

It's quite simple. The wikipedia article is wrong.

Both, and neither.

You have to remember that this doesn't happen in a classical way, such as with a billiard ball. The photon doesn't bounce off the electon and push it. The rules of QM are clear and easy to calculate, but they're so different from what we're used to, it's impossible to understand.

What happens, really, is that you can't really say that the electron goes through both holes. Rather, in the absence of a state change (which would happen if you shined a light on it), you just can't say that it goes through one hole or the other. There's a subtle difference. I highly recommend the QED book, or even better, the video taped or the DVD of QED in NZ that you can get from tuvatrader.com. The quality is poor, but they're fun.

I don't know if this makes you feel any better, but it makes me feel better. The classical view has a problem. Either it goes down forever, or it bottoms out. If it bottoms out, one can expect where it bottoms out to be very different. The universe seems to be arranged such that it does bottom out, and where it bottoms out is QM, which is indeed very different.

Thanks for the information! Sounds like I should really read QED.

 

[epepke]

Thanks for the information! Sounds like I should really read QED.

I do recommend the video tapes, though, or the DVDs. They're a delight. I've bought them twice. They got stolen once. I chuckle to wonder what the theif thought he was getting.

 

[sionep]

I've read Schrodinger's Kittens by John Gribbin and The Fabric of Reality by David Deutsch. I don't claim to know much by reading these, I just have an interest in physics. Anyway, in these books (and Wiki), they discuss some of the oddities of Young's Double Slit Experiment and one of these oddities is that if you were to place some sort of photon detector on either of the two slits but still allow the slit to stay open, it causes the interference pattern to disappear.

I'm just a little confused about this. I'm wondering if anyone knows how the photon detector actually detects the photons. Does it test to see if the photon is absorbed by an electron? If so, could this actually cause the disappearance of the interference pattern or are these books making a valid point? I'm just not sure if this is sensationalism to sell the book, if this is possibly outdated information now, or if they're on to something.

I also wanted to see if anyone else could recommend some other books on quantum physics, astrophysics, etc... It's very interesting to me, but I'm a bit of a layman and don't really know how much I should trust the information in these books. I've taken physics classes and I read Scientific American, but I never received a degree in a scientific field so I wouldn't really understand something too advanced. Any ideas? Thanks for the help in advance.

This has been discussed in the thread on :

Theory of Elementary Waves

I hope that Simon Bridge who initiated that thread will give you his argument about this spooky behaviour of quantum mechanics or give you answer that is satisfactory to your question. I have tried to explain in that thread on how philosphically inconsistent of the quantum mechanics interpretations (not quantum facts of experimental observations). The question you are asking is a philosphical one and physics come second. The phenomena requires the photon to go thru hole A and hole B of the slits simulatneosly to account for the observe diffraction pattern. It is not the physics but the philosophy is the issue.

 

[sionep]

This again leads back to my question though. Does the interference pattern disappear because the electrons react to the photons, by absorbing them, or does it disappear because the electron had to travel through only one slit, thus not allowing it to travel through the other slit at the same time?

Bob,

John Gribin's book you are reading mentioned, that it is the conscious mind that creates that odd behaviour. Copenhagen interpretations say that if you even dare to look (conscious about the external world), it would create such odd behaviour. Once you know which hole (A or B) that the particle went through, the particle simply aware that you, the observer, is conscious of what's its property is, then it simply act according to you being looked at it or being conscious about it as a particle and therefore stop being a wave but change to be a particle leading to disappearance of the interference pattern. This has to be (disappearance of interference patterns), because a stream of particles doesn’t interfere, but waves do. You (observer) create reality rather than reality that is independent of our conscious mind. The issue is philosophical rather than physics. I can point you out to philosophical debate on this, which I admit that I am not a philosopher at all, however I am a physicist by training. To refute this argument, it has to be philosophical counter-argument rather than physics, since there HAS never been an experiment that measured to prove the existence of wave and matter (particle-wave duality) simultaneously. Physicists only make assertions that the cat lives and dies simultaneously or an electron is wave and particle simultaneously but none has ever proved this by measurement since the complementary principles forbids any measurement of particle-wave duality simultaneously. Only one can be measured at one time. If you (observer) decide to measure particle, you will expect to see particles and not wave, that is why knowing about which hole that the particle went thru (A or B), you don't see the interference at all which is wave, you only see a gaussian-like particle distributions on screen. If you decide to measure wave, then you (observer) will see wave, when interference appears on screen. Note, the interference comes back in when you lost your knowledge of which hole (A or B) that the particle went thru, that is when the detector is taken off the slit holes. Interference is observed since the wave front reaches the holes (A and B) simultaneously therefore traverses them at the same time to interfere at the screen.

Here is another swipe. Most physicists deny the existence of God and at the same time believe that material objects can be at 2 different places at once. This mean that the particle goes thru holes A and B simultaneously. This is the problem. I don't see the logic that scientist can deny claims of faith healers using non-local universe to validate their practices and at the same time believing that you can be at point A and point B simultaneously. This is absurd and double standard. This absurdity has been recognized by the founders of quantum mechanics (Neil Bohr, Heisenberg, Max Born, etc,...).

I will post you some links about this argument (primacy of conscientiousness over primacy of existence) because it is debated with philosophy rather than physics, which physics take a back seat. By the way, I am atheist and don't believe God.

 

[Donn]

This is all very interesting stuff. I'm no scientist, simply an average joe.

I keep hearing the argument that QM cannot have an affect on the macro world, yet we observe these interferance patterns which are being projected onto screens that our macro eyes are seeing, so it seems that the effects of the quantum level are very evident; the spooky, philosophical, weirdness of the double slits still results in something macro that we can see?


And, given that, then how can we be so sure that this macro world is divorced from QM principles?

Is it something to do with large herds of things, like photons? Perhaps a constant beam of light will drown the weird effect of one photon as it goes through the slits and we will see the proper wave-interferance pattern we expect?

Is the last post about God = woo but simultanaeity = okay not suggesting that a single photon is a "material" thing and therefore all material things should be acting like a single material thing?

Where is the cut-off between down there and up here?

 

[BillyJoe]

Bob,

Here is another swipe. Most physicists deny the existence of God and at the same time believe that material objects can be at 2 different places at once. This mean that the particle goes thru holes A and B simultaneously. This is the problem. I don't see the logic that scientist can deny claims of faith healers using non-local universe to validate their practices and at the same time believing that you can be at point A and point B simultaneously. This is absurd and double standard. This absurdity has been recognized by the founders of quantum mechanics (Neil Bohr, Heisenberg, Max Born, etc,...)

First of all, the idea is that, if an observation is made, they behave like particles and go through one or the other slit forming a scatter pattern on the screen. If no observation is made they act like waves and pass through both slits and interfere with themselves on the way to the screen. Why it happens, nobody knows. Is it a particle or a wave? Probably neither. If we could understand the nature of this thing, to which we ascribe (in our ignorance) wave/particle duality, we could possibly understand why it behaves as it does.

Secondly, there is no fodder here for the whackos. Sure, observation changes what happens, but it doesn't cause it to happen. It just happens that way. And what happens, happens predictably. If you think the whacks have the support of quantum theory, ask them to change the outcome of one of these experiments. If they can cause an interference pattern to emerge when they make an observation (rather than the predicted scatter pattern) I'll start to take them seriously.

regards,
BillyJoe.

 

[BillyJoe]

This is all very interesting stuff. I'm no scientist, simply an average joe.

Ditto.

I keep hearing the argument that QM cannot have an affect on the macro world....

My understanding is that it can, but that it is extremely unlikely. Theorectically, it is even possible to walk through walls, but the chances of it ever happening is so vanishingly small as to be virtually, and for all intents and purposes, impossible!

...yet we observe these interferance patterns which are being projected onto screens that our macro eyes are seeing, so it seems that the effects of the quantum level are very evident; the spooky, philosophical, weirdness of the double slits still results in something macro that we can see?

They are talking about the probability wave, which, for all intents and purposes (see above) exists only at the quantum level. The results of the two slit experiments are entirely predictable (save a demonstration to the contrary by one of those whacks).

Is it something to do with large herds of things, like photons? Perhaps a constant beam of light will drown the weird effect of one photon as it goes through the slits and we will see the proper wave-interferance pattern we expect?

No. It happens just as surely when the intensity of the light is so low that the "particles" travel one by one towards the screen. If you observe (detect) them, you get a scatter pattern. If you don't, you get an interference pattern.

Is the last post about God = woo but simultanaeity = okay not suggesting that a single photon is a "material" thing and therefore all material things should be acting like a single material thing?

Where is the cut-off between down there and up here?

BillyJoe