Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Lloyd on 04/11/2019 09:16:36
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Hi all. Like so many others, not involved with science but interested in most aspects especially physics.
I have watched a version/explanation of the double slit experiment on the internet and of course it was very interesting. I watched as single photons passed through a single slit producing a single bar pattern. Then a single photon passing through 2 slits producing the interference pattern. To count how many actual photons were going through each particular slit a counter was set up at one of the slits. The pattern then reverted back to a single pattern as no photons were recorded going through the watched slit! Strange but acceptable. My question ... what happens when both slits are observed by a counter?
Hopefully I have that correct and you can understand why I am asking that particular question.
Lloyd
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How are the slits observed? In other words, how can one detect a photon at one slit and still expect it to go through? They're not like cars you can count as they pass by in one lane or the other.
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er ... yes I probably got that all wrong! sorry. I'll do a bit more reading and watching.
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I'll do a bit more reading and watching.
What @Halc is pointing out is that you have to know in detail how an experiment has been done in order to understand the result. In particular, how did they detect the photons without destroying them of affecting them in some way.
Do come back with more questions on this, because I think you will have a lot.
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You have hit at the heart of quantum mechanics, after a century there are still many interpretations.
Sean Carroll narrowed it down to really only three contenders --
1. Many worlds.
2. Hidden variables.
3. Collapse theories.
In the future it will be said: "they had quantum mechanics fall into their lap, they used it but did not understand it".
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Oh boy ... I spent 45 minutes writing a reply and then decided to delete that start again and include a link for the video I had watched. After another 20 minutes of writing and trying to upload my reply that then didn't appear ... so try again!
I am posting a link to the video I watched to better explain myself. (agh ... not allowed to post external links.) edit ( Double Slit Experiment explained! by Jim Al-Khalili)
In the video a flashing/beeping detector is placed above the upper slit of 2 in order to detect which slits atoms were travelling through. Apparently it is 50/50. From what I gather this is to attempt to discover how/why it is atoms revert from an interference pattern to a particle pattern when directly observed. (although from observing the experiment without the detector I would class as observation anyway?)
The question I was trying to ask was ... what if there were 2 detectors observing both slits as opposed to just the top one?
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In the video a flashing/beeping detector is placed above the upper slit of 2 in order to detect which slits atoms were travelling through.
OK, it is atoms now, not photons. Those can theoretically be detected without destroying them. I've read that it can be done with photons as well, but the thing you describe in the OP seems to stop them altogether.
(although from observing the experiment without the detector I would class as observation anyway?)
Yes, they're still measuring the interference pattern, so it's still an observation. Without the slit detector, we lose the information about which slit they went through.
From what I gather this is to attempt to discover how/why it is atoms revert from an interference pattern to a particle pattern when directly observed.
That's the whole crux of QM. QM says you will get this behavior, predicts it with wicked accuracy, but only in probabilistic terms. It isn't really a particle pattern. If it was, it would form a focused spot on the target (a shadow of the slit barrier), not a distribution on a bell curve.
Asking why this happens has been the subject of debate for over a century. There are thus many interpretations as to why this happens, and some of these interpretations are very different from each other, so they give very different answers to why this happens. Problem is, none of them make any unique prediction that allows some interpretations to be falsified. They all behave empirically the same. Hence we cannot assert any particular reason why this happens, but we know it does.
A collapse interpretation (there are many of them) would say that the slit detector collapses the wave function of the atom, reducing the experiment to a single-slit experiment, which yields single-slit results (no interference pattern).
The question I was trying to ask was ... what if there were 2 detectors observing both slits as opposed to just the top one?
Same as one detector. The information is known by the one measurement, so the 2nd one gathers nothing new.
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(although from observing the experiment without the detector I would class as observation anyway?)
In quantum physics ‘observe’ has a very specific meaning. It doesn’t mean that someone stood there looking at the experiment, it means that the presence of a particle or wave was detected.
The question I was trying to ask was ... what if there were 2 detectors observing both slits as opposed to just the top one?
It’s important to recognise that the 2 beams are not just any old beams. In order to get interference between 2 electron beams they have to be coherent; that means all the electrons have the same energy (frequency) and the phase relationship is stable - this latter being arranged by passing the beam through a narrow slit called a collimator. These are the same requirements for a light beam to be considered coherent. If anything disturbs the energy or phase then you will not get an interference pattern.
If you place a detector above a slit to measure the passage of the electron it must interact with the electron in some way and if this changes the energy or phase the beam will no longer be coherent - this is known as decoherence. This is why I said that you need to understand the detail of an experiment to know what is happening.
Each time this experiment is performed in different ways we learn more about the quantum world, unfortunately the popsci press rarely give enough detail, choosing to present the results in the most top level and dramatic terms possible.
I think @Halc has answered your main question, you won’t get a double slit interference pattern if you disturb the electron in any meaningful way at either or both slits.
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I think it's fair to say that I could not get into any meaningful discussion with you guys because of our differing levels of scientific/mathematical understanding on the whole subject. You guys have years of education and study on the subject ... I on the other hand have no scientific education, online animated videos and a inborn inability to understand mathematics. Okay, perhaps not meaningful to you but perhaps yes, meaningful to me because it brings about a greater understanding of the subject I am asking about, in this case the double slit experiment. So that's me kind of saying thanks for your patience ...
So I now understand that a detector at each slit would not alter the results ... the dual nature of the, shall we stick with a single photon, would still remain ... seemingly a particle before the double slit and a wave after the double slit ... and reverting back to a particle when 'observed' after the double slit. Also delaying the observation until the photon has passed through the slit and has nearly reached its destination results in the photon reverting back to it's particle state as it was before entering through the double slit. The point is that the photons travelling through the slits should produce an impact pattern similar to the double slits. But in actual fact they produce an interference pattern. So ... wave - particle duality. Why ? I now need to discover the 'suggested' reasons as to how/why this is possible. Anyone care to suggest / put forward a reason? (oh ... and for hecks sake I hope my understanding above indicates I've understood correctly so far !)
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@Lloyd Quantum mechanics is just weird. That's the way it is. Probability is the key. It wasn't chosen to be that way by physicists. Look into polarised light and filters.
https://www.khanacademy.org/science/physics/light-waves/introduction-to-light-waves/v/polarization-of-light-linear-and-circular (https://www.khanacademy.org/science/physics/light-waves/introduction-to-light-waves/v/polarization-of-light-linear-and-circular)
This is where we eventually get to Bell's inequality. Go read up on it!
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Also worth viewing
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(https://upload.wikimedia.org/wikipedia/commons/thumb/8/8b/Two-Slit_Experiment_Light.svg/2000px-Two-Slit_Experiment_Light.svg.png)The point of the double slit experiment is that one or two slits, the transmission medium behave the same. Its the wave effect, if you put waves through two slits you get destructive and constructive interferance, leading to high and low points along a wave crest. Ie a wave crest now has peaks and troughs along it aswll as a trough between the following and preceeding waves. Lots like when 2 oceans meet. Point is that light is behaving with these wave characteristics.
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Don't need mathematics to think about it Lloyd. Only if you have another interpretation and then want to have it published. Most of the stuff at the QM level is probabilistic, everything is thought to be a result of that and decoherence. Decoherence can be seen as this small probabilistic level reaching some sort of threshold of interactions and scale leaving the world as we experience it normally.
So in a two slit experiment there is a probability of either one or both slits being 'engaged' by one particle. And the way it unfolds will be defined through your setup. The idea of indirect evidence is increasingly popular in those situations as every time you probe a particle you also force it into a set behavior aka a 'wave collapse'. Whether one want to think about it first as waves or as particles is more of a question of what you believe than what is right here. I've read physicists stating the particle view while other state the wave view.
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If you instead of that want to think about it as fields of different energy density interacting it becomes one more interpretation :)
In that case most of what we take for granted disappear. A hole isn't a hole, it's something of a 'energy density' or maybe lack of, looking as a hole to us. and a 'particle' is not a projectile, instead it becomes a emanation of either ' one field's ' interaction with itself or fields interacting. I think that suits my own taste better than the other approaches. As you wrote light has a wave particle duality, and I see that as absolutely correct.
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there is one big hurdle with the field idea, to me then. And that is how to make it fit relativity. What that means is that our universe is 'observer dependent' looked at from relativity. You can 'shrink' this universe through mass f.ex or by speeds. And those effects are no artifacts but physical laws. So if we define a universe as a field interacting with itself, or fields, we still need to incorporate relativity in it. And that is where it becomes really weird.
What it means is that everything might be said to have different 'speeds' versus your observation/platform. Which means that every object you observe will have its own definition of a size and time of/in this universe. Scaling it down we meet particles and passing that 'breaking it up' fields. That's also where it starts to hurt my head.
there is one more dimension we need to add to fields and that is time. Depending on how you look at it it becomes a arrow pointing one way, or a ocean. If you think of it as a ocean then that can contain a lot of possibilities, and also catch the way Einstein defined SpaceTime, consisting of four dimensions. three double ended and one with only one direction. And if you set that together with observer dependencies you get not only one universe but a multitude, one each for each 'observer'.
If you conclude that neither Relativity, nor QM (and fields) are satisfied with what we naively think of as our reality and universe, I would agree :)
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you could also think of it as an 'ocean' consisting of probabilities in where 'time' is directly connected to decoherence giving us outcomes which then becomes our arrow. But 'probabilities' is also a snake biting its own tail in that it comes from us collecting statistics. so defining it as probabilities doesn't lead us any further as I think, but that may be where I'm wrong?
It may depend on how we define time.
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You really need to abandon the particle/wave duality business to make progress in physics. Photons, atoms, indeed everything, behaves as it does because it is what it is, not what you choose to model it as.
Quantum mechanics gives us an excellent predictive model of the behaviour of very small things, but doesn't say what they "are". Continuum mechanics gives us an adequately predictive model of mesoscopic entities that allows us to build houses and fly to the planets. The important question to ask of quantum mechanics is "does it scale up to the observed behaviour of radio waves and billiard balls?", which it does. Likewise the test of relativity is "does it scale down to Newtonian mechanics if v<<c?", which it does.
The problem with poking a particulate photon through two slits is manifold.
1. Only half the energy can go through each slit, so the wavelength of the emerging "photons" will be twice as long - but it isn't!
2. If we rotate the receiver, we will alter the time at which the two bits of photon reach it, so they can't interfere - but they do!
Equally, however, if we presume a wave passing through both slits, all the interference peaks will occur simultaneously, however weak the source (down to one photon at a time) - but they don't!
It gets even more exciting when we pass "solid" objects like electrons, atoms or buckyballs through a diffraction grating. There's no way they can disintegrate and recombine at the receptor (if we move the receptor further away, where does the recombination happen? If we remove it altogether, have we created partial electrons wandering through space?) but they form the predicted pattern!
The answer: believe what you see, and choose the most appropriate model to predict what you might see next time, but don't be surprised if something else happens - it just means your model was incomplete.
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So much to take in its impossible to put a response together! A nice concise up to date book might help me collate all the information into something coherent.
It would seem that Quantum Mechanics is more relevant now than ever in the world we live in. Well it has become more relevant to me anyway. And at 62 years old I thought I had most things sussed. I have no problem in accepting Quantum Mechanics for what it is, even though ... yes ... most of it appears completely bonkers! But there are many questions to be answered and QM seems to be going a fair way to answering them. And hey ... what's the alternative?
I wonder what we would be thinking about Relativity now if we had discovered Quantum Mechanics first? So ... after the reading I've done so far, just what is the biggest obstacle to reconciling Relativity with Quantum Mechanics?
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Relativity is a classical and deterministic theory. Quantum mechanics introduces probability, which is not deterministic.
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I think it's fair to say that I could not get into any meaningful discussion with you guys because of our differing levels of scientific/mathematical understanding on the whole subject.
I hope we can still have a meaningful discussion, and I’ll try not to be too technical. The maths is only a description of what happens, but sometimes it’s hard to put into words which is why the popsci press opt out and give oversimplified explanations.
seemingly a particle before the double slit and a wave after the double slit ... and reverting back to a particle when 'observed' after the double slit.
The wave or particle is only a description of how the light behaves, not really a description of what it is.
Ok, to keep it simple and stick with light. When light is emitted by an atom it is as a very short burst of electromagnetic oscillation - a short wave. So that burst, which we call a photon travels as a wave, but the way we detect it requires it to interact with another atom. That atom could be in a photographic film or in a camera ccd, but that interaction shows up as a dot, just as if a particle had hit the detector. In other words, when the wave hits the detector it can be modelled as a particle. This is why you often hear physicists say the photon travels as a wave but is detected as a particle.
It is fair to say that most modern physicists consider the wave/particle duality thing a bit ‘old ways thinking’, they are more likely to ask how you are detecting or measuring the result of your experiment.
The point is that the photons travelling through the slits should produce an impact pattern similar to the double slits.
‘Should’, but only if they are travelling and behaving as particles. Electrons are interesting because we tend to think of them as particles - little hard bullets - but when they are moving they clearly behave like waves, if fact when they were first discovered they were called cathode rays (which is where we get the name cathode ray tube).
One thing I should mention here is that the prime objective of physics is not to provide an explanation of why something happens, the objective is to observe, model and predict behaviour; that’s what Newton did, he didn’t explain gravitation he just described what it did - with very useful accuracy. His model was of a force acting at a distance, today we model it as a field through Einstein’s field equations and that’s a little more accurate.
Sometimes we can see clearly how a thing works, eg we start a car and the wheels push against the ground providing a force that moves the car forward. Some other thing are less easy to predict eg we flip a coin, we can’t predict exactly which way up it will land, but we do know that in a large number of flips we will get approximately 50:50 and we can do a lot of predicting with the uncertainty of which way it will land. This same uncertainty works at the quantum level and allows us to use probability to make some very accurate predictions - we might not know the exact position of an electron in an atom, but we can make very accurate predictions about where it will spend the majority of its time.
Probability is worth learning about because it really helps us understand a lot about the world around us. So, I’m going to go a bit technical and hope you will follow us by learning more about probability.
When we fire bullets (particles) at a slit they will, as you said, go through one slit or the other and form the 2 slit shadow we expect. This is because in probability we say the events of a bullet passing through one slit or the other are mutually exclusive (it can’t do both) and more importantly neither action can influence or have any effect on the other (we say they are uncorrelated). In probability mutually exclusive, uncorrelated evens follow the sum rule, their probabilities add which is what we see with bullets, but with quantum objects which are coherent, it is the square of the sum which gives us the probability of what we see on the screen. There is growing evidence that this is the way the world really works, but at our big world level we lose the coherence between quantum objects and we start to see the probabilities we are used to. However, when we come to modelling the slit experiment we can look at it and say that the electrons going through both slits is one way of modelling it because it works. So when physicists say it goes through both slits and interferes with itself, they are really using a shorthand for ‘that’s the way we can model it’. At the moment we don’t have enough information to know exactly how this works, but we can predict what happens to a very high degree of accuracy.
@jeffreyH has put up a video of polarisation, this raises another problem which occurs when we view the quantum world. We view polarisation as horizontal/vertical, right/left and electron spin as up/down, just as we would say a coin is heads/tails. But it’s not that simple and the polarisation and spin are best represented as complex numbers (which can also be represented as vectors).
PS Looking back I notice @alancalverd has given a very similar explanation to mine, please read it carefully it’s important. I hope mine has some extra detail you find useful. Remember, 62 is not too old to learn about probability, vectors etc, keeps the grey cells on their toes.
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Time to do some printing I think. Can't beat paper in the hand!
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When we fire bullets (particles) at a slit they will, as you said, go through one slit or the other and form the 2 slit shadow we expect.
No! There is a calculable probability that a bullet will, in effect, "go through both slits". As Eddington said, "If a student of physics should fall through the floor and find himself in the room below, he would not be surprised but mildly elated at having observed an extremely rare phenomenon."
That's what I meant by quantum mechanics scaling up to continuum physics at a mesoscopic level. So far, it's only been demonstrated with large molecules like buckyballs, but the maths is sound.
You can work the model two ways: either map an enormous number of parallel universes onto one timeline to get "now", or wait a very long time in one universe for everything to happen. Problem is that once you get beyond 60 carbon atoms, the numbers become ridiculous.
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@Halc This is worth a read.
https://physicsworld.com/a/does-general-relativity-violate-determinism-inside-charged-black-holes/ (https://physicsworld.com/a/does-general-relativity-violate-determinism-inside-charged-black-holes/)
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@Halc This is worth a read.
physicsworld :does-general-relativity-violate-determinism-inside-charged-black-holes
Interesting link. Yes, it says that Newton envisioned a classic billiard-ball sort of physics, and it was that way all the way down, and relativity didn't really challenge that (except maybe it does, as the article points out).
Quantum mechanics definitely threw a wrench into the works for the determinists, but it as well doesn't disprove it since there are very much deterministic interpretations of QM.
I think we'd need a unified field theory to begin to say whether relativity has any concrete stance on the issue.
From the article:
Newton’s mechanics allow us in principle to calculate the exact state of a physical system at any point in the future, provided that we know its initial state perfectly. So too with general relativity: a precise knowledge of space’s geometry and its rate of change in the present enables us in theory to predict exactly how space-time will evolve.
This is clearly false, and known to Einstein at the time. QM theory says no amount of measuring of a system will allow you to predict it. Einstein was definitely a determinist, as evidenced by his "God doesn't throw dice" quip, but I don't think any of the deterministic interpretations where developed or well known at the time. There was the 'hidden variables' postulate which said there are variables which cannot be known, but if they were, the future would be perfectly predictable.
As such, Einstein’s theory is considered by most physicists to be entirely deterministic.
... as it doesn't contradict Newton's 'classic all the way down' assumption. But the theory also doesn't posit this assumption. It merely declines to challenge it. QM very much challenges it.
Charged black holes, however, challenge this deterministic picture. The “Reissner-Nordström” solution of general relativity describes a black hole created when a star that is electrically charged and spherical collapses in on itself under the force of gravity. Hidden from view inside such a black hole’s event horizon lies a second boundary known as the Cauchy horizon, beyond which space-time is smooth but indeterminate. In other words, the future can no longer be predicted.
It has also yet to be demonstrated that it is meaningful to speak of the physics inside the event horizon. It is mathematically infinitely far into our future, and since the black hole will evaporate in finite time, it is questionable if anything 'gets in' so to speak. Again, a unified theory would help. The subject has proponents on both sides, and this would best be discussed in a separate thread. I'm quite opinionated on it myself, having the luxury of not completely knowing what I'm talking about.
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QM theory says no amount of measuring of a system will allow you to predict it.
Be careful how you state this. The test of quantum mechanics is not merely that it explains obvious quantum phenomena like line spectra, but that it scales to classical mechanics for large assemblies. And it does.
The wave function of a billiard ball is negligible in comparison with its apparent diameter, so for all practical purposes (e.g. potting the black) it is adequately predictable. But its structure, mass and elasticity are all calculable from quantum mechanics.
Indeed the "measuring" test isn't fundamental to QM. The classical model of bouncing a photon off an electron sort of illustrates the problem of measurement influencing the measurand, but masks the inherent indeterminacy of Heisenberg's statement that Δp.Δx ≥ h.
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In English Billiards, scoring is as follows:
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A pot: This is when the red ball is struck by the player's cue ball so that the red ends up entering a pocket. This scores three points. If the player’s cue ball strikes the other cue ball resulting it going down the pocket, then this scores two points...…..
My apologies, there are pockets, but the target ball is red, not black.
Anyway, the fact remains that for a billiard ball p and x are so large compared with h that the indeterminacy of its position is negligible even when p is zero, which makes the game playable and double-slit diffraction (or spontaneous appearance in another pocket) of a billiard ball very difficult to observe. Or, if you like wave functions, the wave function barely extends beyond its classical surface.
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Macroscopic objects follow deterministic paths which is why combining relativity and quantum mechanics is so hard. You need a very small scale and high energies. These conditions are found when particles are close to an event horizon. So it is going to be difficult to observe. It may be possible to recreate these condition in future particle accelerators.
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Yes, I misinterpreted your scenario. We are in fact in perfect agreement. Because the wave function of a large object is negligible outside its classical radius (indeed you can use that as a definition of "mesoscopic") we can predict where a billiard ball will go. Because the wave function of an electron is much bigger than its radius, atoms and molecules do not collapse.
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In English Billiards ...
My apologies, there are pockets.
I guess what I've played isn't English Billiards. Thanks for the education.
Anyway, the fact remains that for a billiard ball p and x are so large compared with h that the indeterminacy of its position is negligible even when p is zero
Totally agree, but you're talking about measuring it, and not about predicting where it will be in the future. Yes, I can measure the position and momentum of a billiard ball to a lot of zeros of accuracy. No argument there.
Kindly interpret my post as talking about predictions (from a wave function) of the future, and not of measurement of its current state.
Because the wave function of a large object is negligible outside its classical radius (indeed you can use that as a definition of "mesoscopic") we can predict where a billiard ball will go.
If this were true, they could publish tonight's billiards scores in today's paper. Only the one interpretation I mentioned actually asserts the game is fixed (completely determined) like that, and even then there is neither a way to 1) take anywhere near the necessary measurements, nor 2) perform the calculation, so no computer, even hypothetical, can actually perform such a calculation.
This is what I was talking about when discussing predictions of macroscopic objects.
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" enough charge, damping wins out over amplification and the oscillations die away quickly. As Cardoso explains, the charge and cosmological constant essentially provide repulsive forces that counteract the pull of gravity and so diminish its amplifying effects. "
" In the context of cosmology the cosmological constant is a homogeneous energy density that causes the expansion of the universe to accelerate. Originally proposed early in the development of general relativity in order to allow a static universe solution it was subsequently abandoned when the universe was found to be expanding. Now the cosmological constant is invoked to explain the observed acceleration of the expansion of the universe. The cosmological constant is the simplest realization of dark energy, which is the more generic name given to the unknown cause of the acceleration of the universe. Its existence is also predicted by quantum physics, where it enters as a form of vacuum energy, although the magnitude predicted by quantum theory does not match that observed in cosmology. "
Maybe?
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If this were true, they could publish tonight's billiards scores in today's paper.
Indeed, if you knew in advance what shot each player was going to make. Problem is, you don't. Billiards is a competitive game of skill, not random chance. At least that's what good players tell me! You always have a choice of shots and, like snooker, you can choose a shot that gives you a score or one that puts your opponent in a position from which he cannot score but is likely to foul. The fact that good players rarely foul suggests that for all practical purposes, the behaviour of billiard balls is predictable.
If we took your assertions at face value, nobody would dare to drive a car because it is impossible to predict where it will go.
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I watched as single photons passed through a single slit producing a single bar pattern.
I believe this only happens in simulation, never really happens in actual experiment. The images below show what you'd get from single slit and double slit experiment.
(https://upload.wikimedia.org/wikipedia/commons/d/dd/Single_slit_and_double_slit3.jpg)
With a normal single slit apperture, you will still get interference pattern due to diffraction effect.
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That's interesting hamandi ... a lot of information I am coming across is based on simulation and not the results of experimentation. Simulation is fine up to a point I suspect. But there comes a point when the simulation has to be replicated by experiment in order for it to carry any credence.
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It has been argued elsewhere in this forum that interference and diffraction are not the same, and the difference between Hamandi's excellent images shows this. It's a bit academic and a lot of the maths is common to both phenomena but logically you need two sources or particles to interfere, and only one edge to diffract.
The quantum two-slit problem is that a single particle seems to interfere with itself, thus showing that classical continuum wave equations don't adequately model very sparse (single photon) systems.