How Do Quantum Particles Behave When NO ONE is Looking?

*The statements in this video have not been evaluated by the Food and Drug Administration. No product mentioned in this video is intended to diagnose, treat, cure or prevent any disease.

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CHAPTERS
0:00 What you've been taught is wrong!
1:35 Particles are not at many places at once
2:11 What is superposition?
3:40 What happens when we measure things?
4:55 How do we know that superposition is real?
8:21 Animations show probability waves
10:19 What do quantum objects look like in 3D

SUMMARY
What is the meaning of superposition and wave/particle duality? What do quantum particles really look like, when we are NOT looking?
All quantum particles exist in a state of superposition prior to any interaction. This does not mean that they exist in multiple states at the same time. So what does superposition really mean?
First know that the math of quantum mechanics does not describe the universe. It describes what we might get if we make a measurement. The math describes the possibility of a quantum object being at any of several positions, at some future time. This does not mean that object is in all those positions now.
Quantum theory describes the potential outcomes of our measurements. Before a measurement is made, there is no outcome. It does not describe the reality of the present. Once you make a measurement, the object is in only a single location. Actual measurements have never found it at a second location or multiple locations at the same time.
The quantum object is in only one place, but that place can be anywhere. Until we measure where it is, the only thing we know is the probability of where it is likely to be.
So what happens when we measure things? Why is the object no longer in superposition when a measurement happens?
The precise mechanism of what happens when a measurement is made, or how the object comes out of a superposed state is not well understood. This is known as the “measurement problem of quantum mechanics.” But we do know measurement is really an interaction. We sometimes also call this interaction an observation, but it is purely a mechanical interaction, and does not rely on anyone having to look at the object. It has nothing to do with consciousness.

An interaction is simply an irreversible exchange of energy with another object. Once an interaction of sufficient magnitude takes place, the superposed object is no longer in superposition, and we observe the particle in only one location.

How do we even know whether a particle is in a superposition, if we only know something about the particle AFTER it is measured? To understand what the particle looks like before we measure it, we look what the particle does BEFORE we measure it.

When we send single electrons through the double slit one at a time, it forms an interference pattern. This can only happen if the electron goes through both slits at the same time and interferes with itself. The proof of superposition is that if we measure which slit the electron goes through, the interference pattern disappears. So this shows that something happens to the electron when a measurement happens. It no longer is in a superposed state and acts like an individual particle. This can be done with any quantum object.

The way to think of this intuitively is not to think of the electron as a point-like particle before it is measured, but as a wave before it is measured. The wave, like any other wave going through two slits will indeed interfere with itself. This is why quantum objects are said to have wave/particle duality. It’s because they exhibit wave-like behavior prior to measurement. But exhibit particle-like behavior, after measurement.

Now, there are several things I want to qualify on this animation. First, quantum objects like an electron are not literally like the wave shown.

Where does this idea of probability wave come from? It comes from the Schrodinger equation. This is basically an energy equation showing how the energy of a quantum system changes over time. The equation has a wave function in it, represented by the Greek letter psi, which is a mathematical expression that represents the quantum state of a particle, or isolated quantum system.

An electron prior to measurement, would be such an isolated quantum system. What’s important to understand is that the wave function, is a variable quantity that describes the wave-like characteristics of a particle.
#quantumphysics
Where an electron shows up after measurement is random, but subject to the calculated probability represented by approximately the square of its wave function.

[Rob_AMX]

Schrödinger Equation: “It may look complicated… and it is” Appreciate that Arvin 😆

[Mahesh_Shenoy]

Loved the video, especially the part where you debunked the misconception that superposition is NOT the same thing as being at multiple locations at the same time.

But, on a similar line, saying ‘electron goes through both slits’ is also a gross over simplification!

Infact if you use any words to explain what the electron is doing, we will end with a misconception.

And so I think the most accurate way to think about it is that the electrons are not going through either of the slits. They are not going through both. They are also not going through neither. This exhausts all the possibilities we can think of and yet, the electron is doing something involving both slits.

And that something is termed a quantum superposition. And that’s that. You can’t articulate what quantum superposition is using any language (except math). You can only ever tell what it is not!

[amihartz]

I think people should read Schrodinger's book _"Nature and the Greek and Science and Humanism."_ Schrodinger had originally argued for using the wave function to replace Heisenberg's original formulation of QM where particles don't have real states between interactions and only have real states in the moment of interaction. Schrodinger hated this, saying "I do not believe the electron hops about like a flea." However, Schrodinger realized that introducing the wave function to fill in the "gaps" between interactions just introduces a new "gap" between the continuously evolving wave and its "collapse" upon measurement, and thus in that book Schrodinger argues in favor of returning to the Heisenberg picture. QM is far more intuitive if you treat it like this and all the "paradoxes" have simple explanations. The only thing that is _real_ is the particle's state during an interaction, there is no _reality_ of it in between interactions. If you accept this one premise then the rest of quantum mechanics follows without introducing a fundamental role for measurement, spooky action at a distance (nonlocality), particles being in two states at once, collapsing wave functions, a multiverse, some sort of special role for the conscious observer, so on and so forth. All of QM follows from just one principle that reality is made up of events and not autonomous objects and that objects "hop about like a flea" between interactions predictable by mathematical laws.

[robertbutsch]

“The math of quantum mechanics does not describe the universe.” All the mysticism surrounding QM disappeared for me when I realized that we have no math, no physics at all, to describes the unobserved quantum world. The wave function and the Schrödinger Equation only describe the classical view of things.

[dhudach]

I'm not a scientists, just someone interested in the strange world of quantum mechanics. Over the past 30+ years I have read many, many books on the topic written by reputable scientists and authors. You have essentially summarized all of them in this video. Outstanding. You've managed to lay out the problem piece by piece in layman terms without sacrificing some technical areas. But I have a question. You talk about the electron wave approaching the double slit and when it hits the screen we know its location. What is the electron when it is fired toward the screen? Is it a particle that turns into the wave? How do we send electrons to the screen and do they start as particles or waves? It's like the story of the little boy walking down the street holding a burning candle. A wise man approaches the boy and wishing to teach him a deep lesson and make him think, he blows out the candle and asks the boy, "Young man, where did the flame go?" The young boy thinks about it then replies, "You tell me where it came from and I'll tell you where it went."

[upzmmwx]

Thanks for the video. I usually watch your videos and really enjoy them, but for this video I have a couple of comments: 1. A measurement is NOT just an interaction. Inside a solid numerous electrons that are in superposition are interacting, but none is measured. Also two hydrogen atoms interacting with Van der Walles force are not being measured, but they still interact. In fact, there is no equation showing what a measurement is (there is no equation describing wavefunction collapse).
2. If you take Schrödinger’s equation seriously, meaning that you take it as really describing reality, then a superposition is the existence of “multiple histories” of the particle. Once it interacts with a macroscopic measurement apparatus, each part of its superposition becomes entangled to it and to its numerous degrees of freedom, generating a practically random phase to its part of the superposition. Once you make many experiments with superposition states that have random phases between them, they no longer interfere with each other (decoherence). In some way all the measurement possible outcomes co-exist but they can no longer interfere with each other. But again, this is if you take the Schrödinger equation seriously and not merely as a calculation tool only.
3. The wavefunction being a complex number has nothing to do with it being non-physical. In fact, one can formulate quantum mechanics without complex numbers, it will just be very cumbersome. Furthermore, even a pendulum amplitude and phase can be described with complex numbers, and it surely is physical.
In summary, all while one ignores the measurement part and treats it like magic, then one gets to failed and inconsistent interpretations.

[ozzymandius]

I think that the much-maligned "shut up and calculate" position is much more insightful and deep than most people imagine.

[binbots]

Particles before measurement exist in a future state. Because causality has a speed limit (c) every point in space where one observes it from will be the closest to the present moment. When one looks out into the universe they see the past which is made of particles (GR). When one tries to measure the position of a particle they are observing smaller distances and getting closer to the present moment (QM). The wave property of particles appears when we start trying to predict the future of that particle. A particle that has not had an interaction exists in a future state. It is a probability wave because the future is probabilistic. Wave function collapse is what we perceive as the present moment and is what divides the past from the future. GR is making measurements in the observed past and therefore, predictable. It can predict the future but only from information collected from the past. QM is attempting to make measurements of the unobserved future and therefore, unpredictable. Only once a particle interacts with the present moment does it become predictable. This is an observational interpretation of the mathematics we currently use based on the limited perspective we have with the experiments we choose to observe the universe with.

[Notdave]

Meanwhile quantum mechanics says, “Yeah, well, you know, that’s just like uh, your opinion man.”

[neIntangible]

Instead of firing through 2 parallel slits, I may suggest experimenting with firing at an "X", or "XX" crossed slits to test for chirality or supplemental effects.

[Pixel-Wisp]

So basically, we don’t know exactly where the particle actually is before we measure it, so we came up with a mathematical formula that expresses this “where it might be” as a probability wave. We can say the same thing about tomorrow’s weather. There is a 30% chance it might rain, or a 70% chance of being a sunny day. Until it actually happens, we don’t know for sure, so we use a probability to describe it. When we do measure the electron, of course then we know where it actually is, so the probability prediction is no longer applicable after that. The mind bender is that, the double slit experiment proves that this probability wave that we made up IS actually happening in reality before we measure the particle.😮

[KaiseruSoze]

QM works within it's own context. It's a description of a limited range of phenomena. It's a placeholder waiting for a finished theory of everything.

We use QM because we're not gods.

[emergentform]

I'm thinking that probably what's going on can best be thought of in terms of quantum field theory. There is no particle but rather a peak in the field in a given area, like how a wave in the water has a highest point. The wave, with a peak as all waves have, isn't isolated but rather spread out over a given amount of space, and constantly moving around at high speed. So when the measurement is made we are simply seeing the highest point of the peak, wherever it is in that instant, and the interaction of taking the measurement has a flattening effect on the rest of the wave so we just see it as a point. So I'm thinking the supposed particle/wave duality is really just a contradiction created by our perception. There is no particle, it's all just waves, and we just label the highest peak of the wave as a particle since it has a definite position once we destroy the rest of the wave by the interaction of measuring. But I'm no particle physicist and I'm basically just talking out of my arse here lol.

[physics_enthusiast_Soorya]

2:01 "Multiple states at the same time"

Yes, a Gross simplification.

Superposition has always disturbed me till now, and I hate how widely the information is spread..


And also, "The math of quantum physics doesn't describe the math of the universe, it describes what we'd get if we made a measurement" ❤

Thanks for the beautiful line, Arwin Ash. I also approve your statement because it is just to simplify our caluclations..

Also, a big thanks for the animation for the 3D probability density of the electron through the double slit! I really wanted it 😄

[timjohnson]

My boxed cat feels better now. Thank you, Arvin!

[b.s.]

1:13 damn it Arvin... I need both to feel the superposition 🔴🔵

[SuperYtc]

The intro with the pills was so funny.

[shoot-n-scoot]

Thanks for clarifying the "superposition" meaning.
Intuitively, superposition defined as existing in multiple places at the same time seemed very wrong.

[listonheinz]

I know this video is made for dumbbells like myself, but my brain still has difficulties understanding what’s going on.

[esra_erimez]

A police officer pulled over Heisenberg, and asked “Do you have any idea how fast you were going?" Heisenberg re[lied, “No but I know precisely where I am.” The cop says “You were doing eighty miles an hour.” “Oh great!” says Heisenberg. “Now I’m lost!”