The double-slit experiment is the central puzzle of quantum mechanics. Because it demonstrates the fundamental limitation of the ability of the observer to predict experimental results. Richard Feynman called it “a phenomenon which is impossible to explain and which has in it the heart of quantum mechanics”. In reality, it contains the only mystery of quantum mechanics.
If you are not familiar with the double-slit experiment then this 9 minute video is the very best and quickest explination I have found.
Double Slit Experiment explained! by Jim Al-Khalili
Jim explaines the double-slit experiment using atoms. But it works just as well with photons, electrons, and even molecules. It has been done using the sixty carbon atom molecule called Bucky Balls. With Bucky Balls, you get an interference pattern as long as you do not try to detect which slit the Bucky Ball travels through but you get a two-slit pattern if you do. Even if you put the detector behind the slits and look at only only one slit, you still cause the wave to collapse before the molecule enters either slit. How does the molecule, or atom, know it it will be detected on the other side of the slit? And how does this cause the slit without the detector to also collapse the wavefunction?
Literally hundreds of different double-slit experiments have been run by physics labs areound the world. They have been run using photons, electrons, whole atoms and large molecules. They all give the same results.
In 1999, there was an experiment run called The Delayed Choice Quantum Eraser. This experiment used a beam splitter to split photons into entangled pairs. Sending one entangled photon to the screen and then the other to one of four detectors. The detectors were placed several nanoseconds beyond the screen and therefore any choice of whether to collapse the wave had to be made before the other entangled photon hit a detector. Two detectors, 1 and 2 had to pass through a half-sivered mirror, with a 50 to 50 chance of going either way. Therefore no which-way information could be detected. But half of the entangled photons would hit detectors 3 or 4 that gave which-way information. Sure enough, the photons that hit detectors 1 or 2, their entangled pair gave an interference pattern. The entangled pairs that hit detectors 3 or 4 gave no interference pattern.
I don’t like that experiment because it only works with photons, entangled photons at that. Otherwise, the experiment works even with huge molecules. It is enough that the detector is placed behind the slit and only one slit is enough. The wave function must collapse before it hits either slit, even the one with no detector.
Ahh, but things get even stranger. Arvin Ash has more mind blowing news in this 10 minute video:
Does Consciousness Create Reality? Double Slit Experiment may show the answer.
As Arvin Ash explains, and the hundreds of other double-slit experiments clearly show is not measurement or observation that collepses the wave functhin, but the mere recording of that information collapses the wavefunction. However there is a minority of physicists that cannot accept that the wavecollapses at all. That theory is known as the “Many Worlds Interpertation” as explained in Ash’s video. The most promnent proponents of that theory are Sean Carrol, Brian Green, Max Tegmark, David Deutsch, and even Nobel Laureate Alan Guth. Guth says it makes a lot of sense to him. However, it makes no sense to me and it makes no sense to a majority of physicists and cosmologists. The Many Worlds Theorists, they call themselves “Everttians” after Hugh Everett who came up with the idea in 1957, are definitely in the minority. However if you really want to have your mind blown by what these guys really believe, you have to watch this 10 minute video:
Quantum Theory reveals Parallel Universes and Quantum Immortality in alternate universes. Understand most physicists simply don’t buy this stuff. Nobel Laureate Roger Penrose calls it “Reductio ad absurdum”.
Back to the double-slit experiment. Here is what we can glean from the many double-slit experiments that have been run in the last several decades.
- Light is not a wave and a particle, it is either a wave or a particle. And the exact same thing can be said about every other particle in the universe. It is always a wave in transit but becomes a particle when its location can be known to the universe.*
- The particle does not have to be observed or measured to make the wave function collapse. Simply recording the which-way information will cause the collapse whether it is ever observed or not. As long as the information is available to the universe, it collapses.
- Here is the most important point. Which way knowledge collapses the wave function. But knowledge is not something that is just recorded somewhere. That is called data. Data can only be collected or displayed. Data cannot physically do anything. Knowledge is something that must be possessed by a… well, by a possessor of knowledge. Who or what possesses that knowledge?
This is the central mystery of quantum mechanics. Don’t feel lonelyif you cannot solve it. I have not solved it. No one has solved this mystery, no one!
*Arvin Ash makes this point very clear in this video: Why don’t quantum effects occur in large objects? double slit experiment with tennis balls. However, there are dozens of Youtube videos on the double-slit experiment as this in the greatest mystery in quantum mechanics.
Please comment your thoughts on this subject. I value your opinion and will reply to any objection you may have.
If you would like an email whenever I have a new post, then let me know at DarwinianOne@gmail.com
Hey Ron, here is a new breakthrough view of the quantum to classical paradox. Watch the entire vid from Wojciech Zurek. He shows how decoherence of the quantum becomes the classical but with amazing implications – Quantum Darwinism. See vid – https://youtu.be/qTE0Xbaz30Q?t=18
Izzy, I just watched the YouTube video you linked to: “Quantum Theory of the Classical ▸ KITP Public Lecture by Wojciech Zurek”.
It was the basic double-slit experiment with some differences.
The double-slit experiment, which has been done at least one hundred times by different physics, does not deal with entanglement. That is except on one very special case where entangled photons are used. That was the Delayed Quantum Eraser Experiment.
Dr. Zurek never uses the term “wave function.” That is a term every other physics, whoever did the experiment, uses. What they call “collapse of the wave function” he calls “decoherence.” That terms are interchangeable, but physicists like Sean Carroll or David Deutsch use the term “decoherence” to mean something totally different.
But he was dead on when he described how the classical environment causes the collapse of the wave function, or what he calls decoherence. That is, once the position of the electron, atom, molecule, or whatever particle you are using in the experiment becomes known to the environment, the wave function collapses. The particle does not have to be observed; its position simply has to be able to be observed. That is, its position just has to be known to the environment.
No human has to be involved in the collapse of the wave function. The wave function collapses if the “which way knowledge” is known to the universe.
The question is, who or what possesses this knowledge?”
I have a serious problem with the experiment right from the start. Jim says that the light will spread out like a ripple after it passes the slits. But that is not how light works. Light creates a shadow and therefore acts more like a current. If flowing water meets an obstacle in the middle of the river it creates a wake and the main current will go round it. Likewise, with sunlight, when it meets an obstacle it doesn’t diffract like a ripple. It creates a shadow (or wake). When light enters newgrange it doesnt light up the whole chamber but only a thin section so this has been known about for thousands of years.
I know you will say but its monochromatic light but then what we are talking about is unnatural light. Also, sending out individual atomic particles does not occur in nature. Atoms are connected to each other like a web, they dont isolate naturally.
So the entire experiment is not based on natural occurrences and therefore the results are unnatural. The real world does not operate like it does in this experiment.
2. “Light is not a wave and a particle, it is either a wave or a particle. And the exact same thing can be said about every other particle in the universe. It is always a wave in transit but becomes a particle when its location can be known to the universe.*”
What exactly is a particle? In the universe single particles don’t fly about the place, they are interconnected like a Web. Are we describing reality here or are we describing what appears to happen in a certain set of conditions (artificial conditions).
A ripple in water is a wave in transit but even when I gaze at it it remains a wave.
So my overall view is one of caution about the two slit experiment.
When energy is inputted to an atom, it may emit EMR anywhere on the spectrum (important), but in some cases visible light. (it can be radio waves). In the case of a sun like star millions of LY s away, visible light is emitted which propagates through space with an outer border in the shape of a sphere ( a big expanding ball shape). A by-product of this process is that this EMR can carry information and can be intercepted by at least two types of receivers. A radio receiver and an eye. Life as we know it could not exist without this process. Sending out this EMR in a form which can be intercepted on demand is so expensive in energy as to be impossible. (Note how a sphere surface area increases with length of its radius ). If it had to travel in a particle form it would be exhausted after a few million miles. Natures conserves energy and sends EMR out in wave form only. If not intercepted, it will continue as waves indefinitely. 99.9999% of all EMR stays as waves. These waves cannot be intercepted by a radio receiver or an eye.
To allow EMR be received and utilized, nature mandates that on observation, the wave collapses into a particle in a radio receiver or an eye. In this way
EMR can travel extreme distances through space as a wave until a tiny proportion of them is observed when it converts into a particle. This happens with an
electric spark in a petrol engine. The electrons have to find out where their contact target is and they can do that, Note we cannot see EMR in wave form when we look up into then sky on a dark night even through the sky is full of it, , we can only see objects using reflected light. What is in space is waves, we have to look at the star to see the collapsed light.
Lets have your thoughts.
Ron, I get your point. It’s just that any kind of interference collapses the wave function. Observing only one slit and putting the detector after the double slit, in my humble understanding of this phenomenon doesn’t change anything. This all has a philosophicsal aspect, as you can see in the case of Schrödinger’s cat: That experiment is so simple, we don’t have to tweak our mind to understand it. But even though it is so simple, the observer collapses the probability wave the moment he opens the box. It’s simple logic. Maybe, just maybe, logic, like numbers, are an intrinsic part of the universe, the same way natural laws are. This would mean you can’t never ever break a law of logic.
You wrote: This all has a philosophicsal aspect, as you can see in the case of Schrödinger’s cat: That experiment is so simple, we don’t have to tweak our mind to understand it. But even though it is so simple, the observer collapses the probability wave the moment he opens the box.
No, that is not the case. Schrodinger gave this thought experiment to show just how silly the Copenheagen Interpertation was. Schrodinger was right! The cat Is not in a state of quantum superposition before the box is opeaned. The cat, either alive or dead was never in a state of quantum superposition. It was not because the data was always available to the world even before the box was opeaned.
That, WTFC, is what the double slit experiment proves. That is, if the data is simply available to the universe, whether read or not, superposition collapses. The cat data was never hidden, it was always available to anyone who looked. They could look just by opeaning the box. But the data was always available, whether they opeaned the box or not.
Ron, since 1935 there came up many interpretations of Schrödinger’s cat. The original intention may have been somewhat polemical, but this thought experiment was that good, that until today it is being discussed in many ways.
Of course the cat never is in a state of quantum superposition. I never said that. I rather referred to what ist called probability wave. You simply don’t know the state of the cat till you confirm ist. Then, immediately, the probability wave collapses. That, in a way, is exactly what happens in the double slit experiment and is rather in line with Objective-collapse theories:
“Objective-collapse theories, also known as models of spontaneous wave function collapse or dynamical reduction models, were formulated as a response to the measurement problem in quantum mechanics, to explain why and how quantum measurements always give definite outcomes, not a superposition of them as predicted by the Schrödinger equation, and more generally how the classical world emerges from quantum theory.”
My personal simplification is this: If you have an empty canvas, it potentially contains every painting possible. When you start painting (or let a machine do the painting), the “potential wave” collapses till you have a single defined work: the precise opposite of such wave.
You wrote: Then, immediately, the probability wave collapses.
Sorry, but that is simply not the case. There was never a probability wave. A probability wave is something real. Commonly just called the wave function. It collapses only when the particle is stopped by an obstacle or is detected by any kind of detector.
I rather referred to what is called probability wave. You simply don’t know the state of the cat till you confirm it.
No, hell no, that is not what a probability wave is. A probability wave is something that actually exists as a wave. It is not something that is probable like the cat is probably alive or probably dead. An electron that passes through the double slit has a probable position but that position cannot be detected until the wave hits the display screen. The highest probable place is right in the center, right between the two slits. But it can be anywhere along the interference pattern, or in a less probable place.
Seems like each and everyone interpretes this experiment in his favor. Essencially, particles behave just like that. If you say real particles you should give your definition of real and unreal. Because once on includes simple logic into the equation and explores the possibility that physical reality is created by mind (a vast an not necessarily individual mind) instead of the mind arising out of something material, things look much easier to grasp.
WTFC I did not mention Real Patricles. Nowhere in any of my post or comments did I use the phrase “real particles”. You just made that shit up.I have no idea what your game is but I ain’t playing it.
Bye now.
My game Ron? Just throwing my two cents in the ring. Don’t worry, you’re not the only one who has a problem with my views. My marxist buddies stick to their dialectic materialism … but if you really want to understand the incomprehensible, you not only need the precision of a scientist, but also the eloquence of a philosopher, the boundlessness of an artist and the openness of a child. Maybe a bit too much for any single person on earth …
Including myself of course.
Ron, the editing function doesn’t work. Could be helpful to avoid or fix some misunderstandings.
Sorry, WTFC, I am still working on this site. I will try to get a plugin that does that but I do not have one yet.
Ron, you’re right, I misspelled: of course I meant microcosm.
I now spent my precious and limited cuban data on that video and can only say that it’s very basic. Nothing new there. I still stick to my post (changing macro into micro).
WTFC, I had no idea you were in Cuba. Anyway that video showed that your statements in your previous video were simply mistaken.
The detector is After the slit but the wave function must collapse before the slit. And the detector looks at only one slit. Yet the collapse happens to both slits.
What it showed WTFC is that the detector did not cause the wave function collapse. And that experiment was not with photons and had nothing to do with entangled particles.
All that was absolutely something new from your understanding of the test expressed in your previous post.
Please add me to your list of subscribers, Ron.
Bill
Everything is a wave, until its position is defined by an observer. This obser doesn’t have to be human, it can be a dedicated machine, something already “observed” and defined, tereby extending this state to the observed object.
As the world we observe can only be the idear of this world (as it always is an interpretation produced by our brain, even if you connect it to the finest instruments), it would be interesting to think about what kind of reality is out there that could explain these phenomenos: the double slit experiment and – for me even stranger – quantum entanglement.
Which way knowledge has to be recorded and available to be observed for the wave function to collapse. A machine that does not record, or otherwise observe the which way information, does not collapse the wave function. A machine that does not record anything, or somehow indicate the which way information, doesn’t do anything. It is the which way information that can become knowledge that collapses the wave function.
How does the wave function collapse. Iwould call it interference. In the macrocosm, you can observe and measure without interfere with the state of the object (at least, the interference is of a much smaller magnitude: when you measure temperature, the thermometer will absorb part of the heat, and this will alter the result. But this is clearly an ignorable magnitude). But in the microcosm, every observation is a significant interference.
Nope, the delayed quantum eraser proved that it is not interference. Anyway, Jim Al-Khalili clearly showed that the detection was taken from only one slot and after the particle had already passed through the slot. The collapse happens before it is detected. And there is no detection whatsoever for the second slot, yet the waveform clearly collapses before it passes through the slot with no detector. Also, the detector is passive. It does not interfere with the particle.
If I remember well, this experiment uses quantum entanglement. This ist the only way I can imagine how one could measure the path before the particle crosses the double slit (if you have a link to the setup – no video please – I would be very grateful). Same with the passive measurement. How could this work in a macrocosmic environment if not vía entanglement?
I now see the problem. You did not watch the 9 minute Jim Al-Khalili video.
https://www.youtube.com/watch?v=A9tKncAdlHQ&t=3s
If you do not have 9 minutes to spare, then I cannot help you.
How could this work in a macrocosmic environment? No one has ever made such a claim. If you are talking about molecules, they are microcosmic, not macrocosmic.
Thanks Ron, a great video. I love how many commenters think they have solved the problem!
(referring to the Jim Al-Khalili vid)