Question about the double-slit experiment.

[Bararontok]

If a particle has the probability of being in two places at once, then is it possible to use a particle accelerator to accelerate the particle to a high energy and into the two slits and then place two detectors, with one at each slit, to convert the energy of the particle into electricity? Since the particle has become two particles, will this allow the total power output of the two detectors to be higher than the input power from the accelerator? This is assuming that the accelerator is 100% efficient.

 

[dx]

The particle will never appear in two places at once. If it appears at the first detector then you can be sure that the other detector did not go off. If there are no detectors at the slits, then one cannot say that the particle must have gone through slit one or that it went through slit two.

 

[jtbell]

If there are no detectors at the slits, and all you observe is the final arrival of the particle(s) at a detector/screen further "downstream", then there is no general agreement on what "really happens" to the particles in the vicinity of the slits, beyond the probabilities that the wave function gives us. This sort of question is basically what the arguments about interpretations of QM are about.

 

[Bararontok]

So there is only probability if the particles are not being observed by detectors, but once the particles are observed then there is a definite result.

 

[findekano]

What if the detectors merged signals, so no information on which slit the particle went through, but had the energy of both particles/

 

[Tylerbrown13]

There aren't multiple particles that go through the detectors at a given moment in the dual-slit experiment. There is only a single particle that travels through one of the slits, and then another is thrown, and another, etc. There aren't multiple particles being thrown at one time, and the single particle doesn't become multiple particles, it has the probability to be one of many different particles, based on it's location in space, which once observed is known.

This is my naive understanding on the topic, as I'm only 14, but still I can pretty confidently say that you wouldn't be creating energy by capturing the energy of one accelerated particle, as it's still one particle.

 

[findekano]

My post was just a possible way around the problem with the opening post, but chances are the info given by what percent of the slits (0 50% or 100%) the particle went through would cause it to act as a particle.

 

[Unidempicity]

Brilliant idea but, only 50% of a particle goes through each slit, nice try.

I really don't like thinking about the double slit experiment. Has anyone actually PREFORMED the double slit experiment? I have about as much faith in the double slit experiment as I do alien autopsies. In fact I have EXACTLY as much faith in the double slit experiment as I do alien autopsies. But what if you.. but.. um... so... the fudge is BUILT IN to the particle?? But you can unfudge it if you touch it wrong?? I mean first of all you wouldn't say all balls are made of fudge just because your sister threw a ball of fudge at you. You would say that fudge is made of eggs and milk and maybe some other stuff and is delicious and was made by your sister! But what would you say if the ball hit the cat first and then wasn't made of fudge anymore.. Curiouser and curiouser...

 

[Mark M]

See here. The double slit experiment has been performed with photons, electrons, neutrons, and entire atoms.

Let me explain it as simply as possible:

There is only one particle. Except, it no longer has a defined position. So, we treat it as a probability wave, a wavefunction. The wavefunction assigns probability amplitudes to different possible results. Once you make an observation on the wavefunction, probability amplitudes determine the probability of finding the particle in a particular place - you only find it in one place. These probabilities can interfere with each other, which is why interference patterns build up over time in DSEs.

That's all there really is to it. The act of interfering with the wavefunction will give you a particle, so it is pointless to speculate what happens before this. You cannot 'extract energy' from in it.

Brilliant idea but, only 50% of a particle goes through each slit, nice try.

This is totally incorrect. You can only speak of the probabilities of finding the particle in a particular state. It's not like the particle becomes some kind of cloud that passes through both slits.

 

[Whovian]

Brilliant idea but, only 50% of a particle goes through each slit, nice try.

I really don't like thinking about the double slit experiment. Has anyone actually PREFORMED the double slit experiment? I have about as much faith in the double slit experiment as I do alien autopsies. In fact I have EXACTLY as much faith in the double slit experiment as I do alien autopsies. But what if you.. but.. um... so... the fudge is BUILT IN to the particle?? But you can unfudge it if you touch it wrong?? I mean first of all you wouldn't say all balls are made of fudge just because your sister threw a ball of fudge at you. You would say that fudge is made of eggs and milk and maybe some other stuff and is delicious and was made by your sister! But what would you say if the ball hit the cat first and then wasn't made of fudge anymore.. Curiouser and curiouser...

Please don't try to tie in Quantum Mechanics with everyday experiences, you get stuff in direct contradiction with observational evidence. Just because something isn't intuitively explained doesn't mean it's wrong.

 

[BruceW]

What if the detectors merged signals, so no information on which slit the particle went through, but had the energy of both particles/

I don't think we could really call them 'detectors' anymore, but I see what you're getting at. I'll try to explain as best as I can how I see the situation:

When the particle is given a certain amount of energy at the start of the experiment, this means it is put into an energy eigenstate. And as it travels to the detector, the wave gets spread out a bit, but it is still an energy eigenstate. So at the 'detectors', we have a superposition of energy eigenstates. So a measurement of the energy will still equal the energy which was given to the particle at the start.

To explain this in terms of the states 'upper path' α and 'lower path' β, we know that α and β are energy eigenstates, so any linear combination ψ = cα + dβ (where c and d are constants), will mean that ψ is also an energy eigenstate of the same eigenvalue.

Edit: Also, to finish my explanation: When a system is not in an energy eigenstate, the outcome can have several possibilities for a measurement of the energy of the system, but in this particular case, the system is in an energy eigenstate, so a measurement of the energy will always yield the same value.

 

[findekano]

Thanks for clearing that up

 

[Khashishi]

Has anyone actually PREFORMED the double slit experiment?

Millions of students everywhere in elementary physics classes.

 

[nonequilibrium]

Look up "de broglie bohm pilot-wave interpretation of quantum mechanics" for an intuitive/understandable explanation of quantum theory (no funny business there).

 

[Kruzeman]

On topic but a different question. As I understood is that during the slit experiment it can both react as waves or marbles depending if there is an observer or not.
What I'm wondering is, if this is because of the exlusion principal.

By observering would you be changing the energy levels of the "marble" enough, so that its charastics in energy levels would have an increasing difference in levels from its surroundings so it's drag over distance becomes greater then when not observed.

basically like the bow of the ship creating a wave because it's in the water then having it hit the wall. Or just lifting it just above the water and hitting the wall.

and the height of the boat is determined by the difference of energy levels between the boat and its surrounding. If there is little, its in the air. And if the difference is bigger, the ship drops in the sea thus more drag leaving a wake wave. And the wave is representing the other elektrons adjusting to the changes energy levels of the marble since none elektron and be in the same energy level.

 

[krd]

On topic but a different question. As I understood is that during the slit experiment it can both react as waves or marbles depending if there is an observer or not.

No. The way it's stated that light or electrons behaves either like a wave or a particle depending on the observer gets the whole idea wrong.

It depends on what the observer is actually observing. The double slits experiment. I think the most impressive version is the one done with single electrons. What you observe in that experiment is the electrons hit the screen, as if they are particles - but they build up a pattern on the screen, this indicates there has been interference - that the particles had to be waves before they hit the screen. And not only that - since the experiment is done, sending one electron at a time - for the interference pattern to build up as it does, the electron must interfere with itself - so it must pass through both slits. So in the double slits experiment, you, the observer, are actually observing it both being a particle and a wave simultaneously.


Where it gets all screwy. And I don't know the fancy maths - and I have a feeling the fancy maths does not answer the question - so what if the electron has a probability of hitting the screen where it does. The really confusing thing, in the world way above the quantum level, we expect waves when they hit something to evenly distribute the energy they're carrying against whatever they hit. That's not what happens with electrons and photons. When they hit something, all the infinite points collapse down to one single point.

A thought experiment I've thought up myself that's been really bothering me. Say if we do the double slits experiment in deep space. We make the distance between the slits and the screen 10 light years, and we make the screen 20 light years long. Unless I'm completely wrong, the wave function will be light years wide before it hits the screen. Yet it will be able to instantaneously collapse across all those light years to a single microscopic dot, somewhere on the screen.

I've had similar worrying thoughts about light traveling through the cosmos. A photon can travel for billions of years. Its wave must be billions of light years wide, and that wave can collapse when its observed on Earth by an astronomer - or just anyone looking at the stars.

 

[juzzy]

A proper explanation of it all can be found here

 

[krd]

A proper explanation of it all can be found here

That video does not explain the Two slits experiment. He describes the observation - which is the quantum particles simultaneously behaving as waves and particles.

What is strange, and unexplained, is why the wave decides to collapse at a single point, and not distribute its energy evenly over the screen.

The basic Young slits experiment - done with light, does not show photons to be particles, The observer would only be able to determine that light was waves.

When the experiment is done with single electrons, then it becomes apparent that the electron is both simultaneously acting as a wave an particle. Or at least, behaves as a wave - even splitting into two wave fronts, but both wave fronts collapse as a particle - or discrete quantity. That's the real mind boggling bit.

 

[Whovian]

@krd
Welcome to Quantum Mechanics. Or at least the one interpretation of it you're using.

 

[krd]

@krd
Welcome to Quantum Mechanics. Or at least the one interpretation of it you're using.

Believe it or not, I actually studied this - not to a huge depth. I studied applied physics - on my course we didn't get too deeply into the quantum stuff. I haven't done a differential equation in years. I haven't had the skills to investigate the maths heavy end of the theory.

I'm not sure if I personally have an interpretation of it. I'm clear on what is observed in the Youngs slit experiment. But I couldn't give you a mathematical explanation of why the wave collapses to a point. Is there an explanation for that? An explanation that says there is a probability of the wave collapsing where it does, doesn't really explain it. And I wonder about some of the conventional theory - that the elements may be mathematical fictions that agree with experimental results. (what I mean by mathematical fictions is like astronomical calculations that agree with an Earth centred universe - but they're actually contortions - the results are correct but the theory is incorrect)


Is there an answer to my little thought experiment - making the slits apparatus billions of light years wide and long? Making the waves billions of light years wide - and then having them collapse to a point. When the wave reaches the screen, and the photon finds an atom it's going to collapse on, how long does it take the wave to collapse.

 

[Ger]

The particle nature of waves is the way we can observe the wave pattern. Before we do any detection all we know is a probability that -if we put a detector in given spot- we will find a particle there with that much probability.
The only difference we can detect between massless and mass particles is a difference in time needed for building up the diffraction pattern. With electrons it will take time, with massless photons it is instant. For photons time is meaningless.

 

[Ger]

Is there an answer to my little thought experiment - making the slits apparatus billions of light years wide and long? Making the waves billions of light years wide - and then having them collapse to a point. When the wave reaches the screen, and the photon finds an atom it's going to collapse on, how long does it take the wave to collapse.

The universe is full of slits but to find a single coherent light source illuminating both slits will be a bit difficult. Then space is not excatly empty so the wave will interfere with all possible matter. If ever the wave reaches the screen it has been so much diluted (lost already so much energy) that it will 'collapse' into a faint very low re-emiting far infrared photon, going into any direction, lost to your observation.

You also cross some boundaries of different inertial system, complicating the timely detection if there is an interferance patron at all.

 

[krd]

The particle nature of waves is the way we can observe the wave pattern. Before we do any detection all we know is a probability that -if we put a detector in given spot- we will find a particle there with that much probability.

Having maths that can tells us the particle has a higher or lower probability of appearing where it does, and that probability agrees with experiment, doesn't really explain why it does that.

The only difference we can detect between massless and mass particles is a difference in time needed for building up the diffraction pattern. With electrons it will take time, with massless photons it is instant.

Well, the one of the points of doing young slits, with the electrons - where it's done one electron at a time, is to see the pattern built slowly, literally one electron at a time. Since it's one electron at a time, there isn't another electron for the electron to interfere with. When it passes through the slits, it's not going through one or the other - it has to go through both. It absolutely has to - two new wave fronts need to be created - so it can interfere with itself. The idea that the electron is making a random choice between the two slits is a fiction.

I don't believe anyone has tried the two slit experiment with individual photons. There is a the potential for a misinterpretation when doing the experiment with photons. That a large numbers of point photons are taking a random path over a wave, either going through one slit or another, and because of the large numbers, the interference pattern emerges. The observation with the single electron experiment would indicate that electron is not a point until it's wave collapses at the screen. And before it collapses it's a wave that goes through both slits. It would appear, claiming the electron as a point on a path through space, is a mathematical fiction.

For photons time is meaningless.

Not absolutely meaningless. For the wave front at least, it has a speed, c. When the wave collapses - it seems to happen instantaneously. And if my thinking is right. It seems to be able to collapse all the information in the wave, across cosmic distances in an instant.

 

[krd]

The universe is full of slits but to find a single coherent light source illuminating both slits will be a bit difficult. Then space is not excatly empty so the wave will interfere with all possible matter.

No. Okay, let's say for the sake of argument, we use a single atom light emitting diode (I know these aren't possible - or at least I've never heard of anyone making one - I could use the single emission of an electron, which is a lot easier, but for the explanation I'll use light).

Yes space is full of matter. But the further away that stuff is, the lower the probability the wave will collapse when it reaches it.

The light waves from the big bang have a radius of over 13 billion light years. If the waves collapsed at the first matter their fronts encountered, we wouldn't be able to see any of that light. In fact we wouldn't be able to see much of the cosmos. You can see this with the Young's slits experiment with the electrons - move the screen further away from the slit, the pattern will be larger, but you'll need more electrons to fill it. Make the distance huge, and there's much less probability of the wave collapsing.

If ever the wave reaches the screen it has been so much diluted (lost already so much energy) that it will 'collapse' into a faint very low re-emiting far infrared photon, going into any direction, lost to your observation.

No, you're thinking of this wrong. If it's a single photon, or single electron, the wave is not going to be dissipated or diluted. And the photon will not be red shifted to infrared, if the emitter and screen stay stationary relative to each other. When the wave collapses at the screen, it will have the same quantity of energy, as if you were doing it in a lab over a few inches.

The wave cannot collapse in bits and pieces, it must collapse all at once. The photon or the electron, will have the same energy it did when it left the emitter. Even though I do not have a Ph.D in particle physics - I know this is a fundamental idea.

You also cross some boundaries of different inertial system, complicating the timely detection if there is an interferance patron at all.

Yeah. If you make the distance between the slits and the screen a billion light years, you'll be waiting a billion years for your results. It's a thought experiment.

Now, a practical version might be projecting an interference pattern onto the surface of the moon. Probably not that practical.

 

[BruceW]

Is there an answer to my little thought experiment - making the slits apparatus billions of light years wide and long? Making the waves billions of light years wide - and then having them collapse to a point. When the wave reaches the screen, and the photon finds an atom it's going to collapse on, how long does it take the wave to collapse.

Yeah, it is possible in principle. The wavefunction will collapse instantaneously. And this would appear to violate relativity, but it actually does not. Relativity is only violated if information is transmitted at faster-than-light speeds.

 

[Ger]

I have not reached the 10 posts as yet so I am not allowed to put in the link to the wikipedia article of double slit experiments. Photon_dynamics_in_th_double_slit_experiment.

 

[BruceW]

Yeah, it is possible in principle. The wavefunction will collapse instantaneously. And this would appear to violate relativity, but it actually does not. Relativity is only violated if information is transmitted at faster-than-light speeds.

EDIT: practically, the experiment might not be possible. But there might be some changes you could make to the experiment to make it possible practically.

 

[krd]

Yeah, it is possible in principle. The wavefunction will collapse instantaneously. And this would appear to violate relativity, but it actually does not. Relativity is only violated if information is transmitted at faster-than-light speeds.

Okay. I think though, the statement "Relativity is only violated if information is transmitted at faster-than-light speeds." is a cop out. It's also used as the get out of jail card for quantum entanglement - spooky action at a distance. Einstein never said, that if there is inconvenient events happening, as long as they can't transfer usable information (information that could be used in data communications) then they don't violate relativity.

Before I say anything else - take careful note, I'm using the term wave, instead of wavefunction.

The light's wave front traveling through space, does not violate relativity. And if instead of considering a wave, you considered the photon taking a straight beam like path through space - it doesn't violate relativity.

There's a painful trick with these waves. We cannot tell they're actually there, unless we collapse the wave. But by setting them up to create interference patterns, we can know that they were there, before we collapsed them.

Even though we can't interact with the information contained in the waves - we can see the wave distributes information to make the interference patterns possible. Where the interference occurs locally, it doesn't violate relativity. Where the problem really becomes painful apparent, is when the wave collapses - all the information (the interference pattern) is encapsulated in the point collapse of the wave. I don't think relativity has anything to say on this.

What's often said about quantum physics is it's the physics of the really small. This isn't true. A tiny excited atom may release a photon. But that photon can travel through space for billions of years. These waves are the hugest things in the universe. We don't even have to go into outer space to consider how big they are - just think of radio waves from television stations.

A photon travels from the sun. When you see anything illuminated by sunlight, what you're seeing is the collapse of the light wave - and the waves from the sun that collapse here have a circumference of 37.68 x 10^6 million miles. The wave could pick an infinite number of places to collapse - but it chooses your eye. And drops all it's energy into your eye. If the energy is evenly distributed through the wave, when it collapses, the transmission of that energy to the point of collapse, does violate relativity - or needs something else to explain it - probability is not an explanation.

The stars at night - the light from those stars is often thousands of years old. The circumference of those waves expands at faster than the speed of light by a factor of ∏, the entire surface area of the wave front even faster. They are colossal, yet, their waves collapse into your eye, as if they're the tiniest things.

 

[BruceW]

the wavefront of a classical beam of light does (in many cases) move faster than the speed of light. If you have a monochromatic wave in free space, then the wavefront does move at the speed of light, but in many other cases, it will move at some other speed. The important thing is that in all cases, the information is not transmitted faster than the speed of light.

 

[krd]

the wavefront of a classical beam of light does (in many cases) move faster than the speed of light. If you have a monochromatic wave in free space, then the wavefront does move at the speed of light, but in many other cases, it will move at some other speed.

For the sake of consideration. Ignore the other cases, for the moment. And probably the other cases shouldn't be considered at all, being other phenomena.

The important thing is that in all cases, the information is not transmitted faster than the speed of light.

You're right. The interference pattern travels at the speed of light. What travels faster than the speed of light is the information that the wave has collapsed.

Say a photon wave leaves the sun. It spread millions of miles. It reaches Earth and collapses - But for the wave not to collapse when it reaches mars, it must know that it has already collapsed on earth. All the information in the wave must move faster than the speed of light. All the energy in the wave must move faster than the speed of light.

The double slits experiment shows that the electron is passing through both slits - it couldn't produce the interference pattern otherwise. If you put more double slits in series - you could split the wave millions of times, into millions of new wave fronts - they'll all move at the speed of the electron. But, when one of the waves collapses - say hitting a spec of dust in deep space, then all the other wave fronts will know instantaneously the wave has collapsed - and the wave vanishes.


The wave, no matter how big it is, has to collapse all at once in one place. Or it could collapse in two places or more, because it would be unaware it's already collapsed somewhere else - the distance between those places could be billions of light years or more, so the information of the collapse has to travel billions of light years instantaneously.

So, although the wave fronts can't move faster than the speed of light - the information that the wave has collapsed has to.

 

[BruceW]

No, the information definitely doesn't travel faster than the speed of light. When the state collapses, it is true that there is then zero probability of it turning up somewhere else. But this doesn't mean that information has traveled from the point of collapse to all the places where collapse might have happened.

To be clear, when information is transmitted, this means it would be possible to send some kind of message. So it is not possible to send some kind of message faster than the speed of light.

EDIT: maybe your definition of information is different from this, which is why we are getting our wires crossed.

 

[krd]

To be clear, when information is transmitted, this means it would be possible to send some kind of message. So it is not possible to send some kind of message faster than the speed of light.

EDIT: maybe your definition of information is different from this, which is why we are getting our wires crossed.

Yes, I am using a different definition of information. Just in this instance - I do know what you mean.

What I mean by information in this instance, is the information that let's the entire wave know it has collapsed. So it doesn't collapse twice or more, in other places. These waves can be huge - light years across - billions of light years wide. Though when the wave collapses, the collapse can be conveyed across these vast distances.

 

[BruceW]

yeah, I agree. spooky action at-a-distance!

 

[Physics4]

Could someone please explain the actual detector? I've looked around quite a bit and haven't found anyplace yet that explains how a detector placed at one of the slits can both detect a single particle and also allow the same particle to pass through to the target. Logically one would think that in order to detect a single particle, that particle would need to hit the detector, in which case we shouldn't be the least bit surprised to find the interference pattern disappear. After all, the only slit letting anything through would be the one without the detector. Thanks.

 

[BruceW]

I am not sure what they use in actual experiments, but the example I have seen is using a loop of wire around one of the slits. So this will let the particle pass through one of the slits, and you can tell which slit it went through because a current will be induced in the wire loop surrounding the particular slit that the particle went through.

 

[Physics4]

Thanks Bruce. Allow me to elucidate. Home experiments reveal the principles of the effect, but don't actually use single photons or single electrons because the equipment isn't capable of doing that. I've also seen several computer animations of the experiment, but nothing that describes the method and principle that allows the detector to both detect a single particle and allow that same particle ( wave form or whatever ) to pass through the detector undisturbed to the target.

Typically we simply see a simplistic portrayal where an artist inserts an eye or camera into the animation that is supposed to represent a detector. The problem with such interpretations is that for our eyes or a camera to detect a single photon, that photon has to strike the surface of our retina or the camera sensor where it's converted into energy that is sent down a signal path and registered. The photon has been absorbed. Therefore if you were to actually use such a detector in the double slit experiment, the detector will catch the particle and the particle will never never reach the target. Logically this should have the same effect as covering one slit, and consequently the interference pattern should disappear.

The missing information is therefore, how does the detector actually work to detect a single particle? Logically, if it cannot interact with the particle in some way it cannot detect it, and if it does interact with it in some way it must have some interactive effect, and that interactive effect may well be the cause of the change in the pattern seen on the target. Am I making any sense here?

 

[pilotwave]

The double slit experiment can be understood in a completely "classical" way by considering a definite particle passing through one of the slits while an accompanying guiding wave passing through both. A macroscopic pilot-wave system has reproduced this (and there is no trouble with observing the particle):



If one of the slits were closed, the wave would be disrupted and the pattern is not found. Additionally you can easily imagine a measurement technique so heavy-handed (i.e. a floating buoy measuring the surface deflection) that would also destroy the wave-pattern. No mystery here, just a particle AND a wave.

Another recent experiment shows how a complex underlying pilot-wave dynamics can underlay a simple statistical description via deterministic chaos:

 

[Physics4]

Thanks pilotwave, it’s an interesting theory

 

[Nugatory]

The double slit experiment can be understood in a completely "classical" way by considering a definite particle passing through one of the slits while an accompanying guiding wave passing through both.

Good thing you used scare-quotes around the word "classical"

 

[BruceW]

The missing information is therefore, how does the detector actually work to detect a single particle? Logically, if it cannot interact with the particle in some way it cannot detect it, and if it does interact with it in some way it must have some interactive effect, and that interactive effect may well be the cause of the change in the pattern seen on the target. Am I making any sense here?

Yeah, of course we have to interact with the particle to be able to tell which slit it went through. And this is why the interference pattern changes when we choose to place detectors over the slits.

And yes, I have also seen videos explaining quantum mechanics, where someone just watches the particle, and then the particle chooses one slit to go through. Of course this is only telling part of the story, because the person must interact with the particle. You shouldn't take such videos too literally.

 

[Physics4]

I would have chosen “the professor of ignorance” if allowed as a user name, as I’m a member from a completely different forum, asking a question for someone else. However, allow me to ask you this. If only having the option of choosing one video explaining what the term “observer effect” really meant, which video would you suggest?

 

[BruceW]

I would suggest a lecture series. maybe MIT have something online. Or even type QM lecture into youtube. Some of the introductory lectures might have a good explanation of some of the concepts of QM, before actually teaching it.

The term 'observer effect' is a bit misleading itself. The way I see it, is that if we have some particle, then generally it will not be in an eigenstate of some measurable quantity. So when we do measure that quantity, we are forcing the particle into one of the eigenstates of the quantity we are measuring. So we are not merely 'observing', we are forcing it into a different state. It is only when the particle is already in an eigenstate of the quantity we want to measure, that we can take the measurement without affecting the particle.

Out of curiosity, what is the forum you usually go on?

 

[Physics4]

Thank’s, I can see why your recognized as a curious homework helper. You never know.., perhaps the professor of ignorance may show up again