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Think you understand Quantum Physics? Try This.
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- Опубліковано 12 лип 2024
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I was recently reminded of a quantum puzzle that tripped me up and I wanted to share it with everyone to see if you could figure it out. Let’s have a look.
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#science #sciencenews #quantumphysics #physics
To quote Enrico Fermi: "Having listened to your lecture I am still confused. But on a higher level."
lemme guess, the lecture's name rhymes with "Zinger"?
🤣
😂😂😂
Can momentum be conserved along the zig-zag axis?
@@GEMSofGOD_com In a granular space model the randomness at Planck lengths is the result of unknown fields interacting at those scales and possibly making a sharing of both momenta as a particle gets deflected by almost negligible amounts at every granule. These deflections make for the variable paths a photon can take and Feynman's work shows it roughly equivalent to a wave function quantifying probabilities.
My first question was "How do they know it's interaction free?" Thanks for explaining that.
They dont know, it means they measured absence and that changed the wave-function somehow.
To say that, you'd need to define what an interaction actually is. And that's something the Copenhagen interpretation of quantum mechanics struggles with
Whenever I hear that the measurement is interaction-free, I automatically assume it is just a thought experiment that cannot be replicated in reality.
This is to a large extent a linguistic problem. "Interaction-free measurement" is a term that does not mean that there is no interaction. It only means that we detect an event without explicit manifestations of such an interaction. If there are no eventw with such manifestations whatsoever, for example, I'm reading a book at home while some fellows in Boston mingle with entanglement of nitrogen vacancies in diamond, nothing happens. So, the answer to your question "how do they know it's interaction-free" is straightforward: they could see the outcome of interaction (photon scattering) but they didn't detect it, hence the measurement was interaction-free.
Can momentum be conserved along the zig-zag axis?
I'm an engineer but I got it immediately because it is like the measurement uncertainity Is reduced in a Kalman filter. In the measurement the wave function doesn't collapse completely, but It Is chopped in two, and this IS interaction.
yes… don’t mess with the superposition 😀
And this interaction changes its Fourier transform, hence the characteristic function.
I'm not sure. But if the assumption was that the partical is actually behaving in a classical manner then collapsing half of the wave function wouldn't of itself be interacting with the particle. The problem with the experiment isn't the partial collapse of the wave function it's actually that the uncertainty principle applies to the measuring apparatus.
I'm thinking that location prediction probability was at it's lowest each time measured.
I suggest that just taking more measurements will find that particle.
@@laars0001: Oh, no. It is long gone by now. All the interactions scared the particle away. 😂
I didn't 'understand' it as thoroughly as you explained it, but "interaction-free measurement" had already set off alarm bells in my intuit understanding of QM.
Then you have a good intuition
This. My thought was, "if the measurement is truly interaction-free, then you're not actually making a measurement". Same intuition, but I think I was just coming at it from the other side.
I guess it's easy to lose sight of the fact that the detector has its own quantum limitations. I wouldn't have thought of that.
Indeed.
The uncertainty grows with the energy required to make the measurement. It can happen at macroscopic scales too.
Classic problem: We forget that we are NOT independent observers of reality. We are a part of realty, so is literally everything we do or could ever do. Easy to overlook that when coming up with experiments like this.
edit: Is YT comments broken? This reply appears as a Highlighted reply when having the other replies closed, once I open them this reply disappears completely from the list.
Quantum is a transitive property of experiments; if the target is quantum, the entire apparatus must be probabilistic. You can say, "probably interaction free", but not "interaction free".
@@bartroberts1514 It's probable there will be a probability..
No cats were harmed in the making of this video.
The only one that´s harmed in quantum physics still sits in a box since hundred years, waiting for someone to set it free. I hope it will be Sabine.
Cats are not real.
Probably 🙂
Meow! (Thank you!)
Schrödinger's Cat is dog whistle for "idk what I'm talking about but I liked watching The Big Bang Theory"
lol you jumping the frames while talking about possibility of macroscopic objects making quantum jumps 😆
🤯
There are two things I understand about quantum physics:
1. I know practically nothing about quantum physics.
2. I understand it even less.
The problem with QM is that they are trying to rectify it with Einstein’s relativity nonsense. With Newton's gravitational attraction nonsense.
E=mc. E and m are equivalent so E=c where c is bounded Acceleration so we get Everything is an emergent property of Acceleration.
To understand the universe, you need to use tools like Newton's Law of Motion, F=ma. Kepler's laws of motion, Acceleration increases as the radius decreases in an elliptical orbit. Lenz law, refraction.
When I see anyone using mass as the actionable force, I just write them off as flat earthers. Gravity/mass attraction is the tool developed to explain why objects fall to the ground on a stationary plane rather than float away.
It's stupid to still be using tools developed for a stationary plane universe.
Hawkings- flat earther.
Einstein - flat esrther.
Newton- flat earther
Galileo - mass does not attract mass. MOTION is what causes the daily tides. Not mass.
The fact that the entire scientific community is still using flat earth tools is mind boggling.
Plato approves this msg.
@@stewiesaidthatc is not acceleration, it's velocity, and CONSTANT. please watch some introductory videos to quantum mechanics, because as always, when you discover famous physicists to be wrong, it's probably you that is wrong instead.
@@lih3391 oh please. Get a real education. (c) is ABSOLUTE MOTION. Comprende.
Energy can be neither created nor destroyed, only transformed. What transforms it? Mass? What is it transformed into? Rainbows and lollipops?
I don't know they've been teaching you kids in public schools but it sure isn't physics.
E=mc (c) is the absolute amount of acceleration for the mass. Comprende.
Go look up the lifespan of the various animals. (C)/lifespan is determined by the animals mass and its heart rate.
The lifespan of humming is 5 years or c, it's mass is accelerated by E, the amount of nectar/sugar it consumes. The more energy it consumes, the greater it's heart rates, the quicker it reaches c - absolute acceleration or in this case, death.
An embryo. (C) is the date it hatches.
M is the size of egg. The larger the egg, the more energy that is required to reach c. Reduce E by a degree and takes the egg a day longer to reach c.
The laws of physics are equally applicable in ALL FRAMES OF REFERENCE. I suggest you go back to school and learn what a frame of reference is.
The Schrödinger equation was created from the classical energy equation E = p^2/2m + V(x) by replacing dynamic variables with operators. To solve the Schrodinger equation you use the classical potential energy. Well, it works, but it just doesn't seem right, in my opinion, that a fundamental atomic theory should depend so much on classical mechanics.
I noticed some macroscopic zig-zagging going on, and it involved a pink-to-purple/gray gradient shirt with buttons.
That's the Sabine effect.
I don't get it
@@Kveldred Sabine's image jumped from side to side near the end
@@naturelover137 oh. ooooh.
something must be wrong with me, 'cause I was like 99% sure that OP meant her tits. yk, like "those babies are swingin' & jigglin' all video!"
...the fact that Sabine ♥️'d the comment did give me pause, a bit
@@naturelover137 oh. ooooh.
maybe something is wrong with me
'cause I was like 99% sure OP actually meant her ta-tas
seeing that Sabine had ♥️'d the comment _did_ give me pause, a bit
What a coincidence! I was having a conversation with a friend about the observation or fail to observe a particle, when you post this. My day now is ruined I cannot stop thinking about it 😆
Thank you for the video!
I discussed this with my husband for like a week, so I sympathize with the problem 😅
@@SabineHossenfelder Lol, hopefully you're still married👍
@@SabineHossenfelder... but not him? 😅
Macroscopic realism, a concept introduced by Leggett and Garg, is a classical worldview that posits that macroscopic objects can be in definite states and can be measured without affecting their state or subsequent dynamics. This is in contrast to quantum mechanics, which allows for superpositions and entanglement. The Leggett-Garg inequality is a mathematical test that can be used to determine if a system exhibits macroscopic realism. Experimental violations of this inequality have been demonstrated using various systems, including superconducting quantum interference devices and massive particles. These experiments have pushed the boundaries of macroscopic realism, showing that even large objects can exhibit quantum behavior.
the problem arises when the superposition is for example between "absorbed photon" and "non-absorbed photon". this clearly is impossible, unless the photon absorption is just a transformation (like strings). However, to be honest, this is becoming embarassing. I mean the fact that we cannot really understand what we are measuring, although the measurement clearly spits out a number.
easy for you to say
@@drgetwrekt869 the photon absorption is just a transformation of a quantum field in QED. The topic of macroscopic realism is less about "what are we measuring" and more about correlation between measurements at different times.
Sounds to me they are trying to offer an alternative to the 'observer effect'
Classical information is in my opinion, not real at any scale. All information if quantum and classical measurements are only the illusory result of interactions between two or more quantum systems, like the crossing of two lines to make a point.
6:26 I love the subtle eyebrow movement of the picture!
Since I have nothing intelligent to add when it comes to quantum anything, I'll help the algorithm by stating that I love that shirt. How do you keep it looking so fresh? Do you have a closet full of identical ones? Or are they kept in a quantum box where every time you open it a quantum path causes the Universe to spilt, leaving a copy of the shirt in a quantum state.
I told you.
This shirt exists in all of the invinite universes.
@@Thomas-gk42lol
it is a green shirt, she just adds the effect in the editing.
@@utkua 😆 I want The Starry Night for the next video.
I really only wear it in the studio, that makes it much easier!
This seems to imply that you can measure a wave function by measuring:
1) Where it is, or,
2) Where it isn't.
Yes, indeed. And it's not just position, you can do the same thing eg with energy states. If you know that, say, an atom is in a superposition of two energy states, and you measure that it is not in one, you know it's in the other.
queue The Missile.
@@SabineHossenfelder so that's sort of the intrinsic logic of the universe?
@@SabineHossenfelder And they wonder why when people can not grasp a thing that is years beyond their understanding people would use the word Magic or use superstition as an explanation. Its not here, so its over there but if we look at it, was it ever there at all but now over here ? Quantum theory its what science looks like if it had pronoun issues.
Great vid as usual Sabine even struggled through with my driving glasses on as for the life of me can not find my reading glasses, just to convey the slightly above average dedication to your episodes. 😁
@@SabineHossenfelderBasically existence seems like appearing and disappearing. Appearing is particle nature, disappearing is velocity and motion.
There are some videos that I have running in the background while I do something else. And then there is this video.
Thank you!
When I try to understand Quantum Physics, my brain makes the same 'glug-glug' sound produced when water is emptied out of a large jug.
Yes, but it makes room for the vodka. Win, win
@@dougsheldon5560 An excellent point!
You just need to make your head spin! That way you’ll have a vortex that drains your brain in seconds…
I understand quantum physics about as much as I understand women
@@adamsheaffer Wild thing, you make my Head Spin
You make everything groovy, wild thing
Wild thing, I think I love you
But I wanna know for sure
Come on and hold me tight
Sabine, thanks for another great video. I really liked this "quiz" style of video and would love to see more in the future!
One can convert the operator from position space, where it is a Heaviside function H(r1)d*(r1-r2) to momentum space where it acquires a term 1/(k1-k2), i.e. the momentum can be changed (I'd rather not try normalizing that thing, it's what you get from cutting off arbitrarily sharp and making the box infinite).
I think another problem with the idea of a "zigzagging" particle is mixing the classical description of a pointlike particle with position and momentum and p=m dx/dt with the quantum world, where we have a wave function that describes the particle and we no longer have some pointlike thing that goes from left to right or right to left implying a positive or negative momentum.
d*=delta function.
Sounds plausible, though my QM is a bit rusty.
My first idea was that in 2d phase space you truncate only one side of the pdf, excluding e.g. the particle being on the left. Technically the particle could have started further right (initial location not perfectly known) and is simply is moving left between measurements, even not considering changes to the momentum distribution. For multiple left/right measurement sets it is getting clear that also the momentum distribution has to be perturbed as well. You provided the math basis for this step, sounds like home work you could get at university.
This video replaces my favourite previous Sabbine video. They keep getting better. My favourite before this one was the video of Sabine learning how to ride a tricycle which also had training wheels on them but she still fell. Great content as always 😁
I like the way you share how you arrived at your favorites from the different periods in your life. There is a season for everything.
I don’t understand quantum physics, I just like listening to Sabine. 🤩 She’s rad!
And sexy in that pink blouse.
True, and Not understanding quantum physics is the only way forward.
I don't think anyone properly understands it, not even the scientists. It's interesting to try & learn about it though.
I don't understand quantum physics either, but I also don't understand you if you just want to hear Sabine talking.
@@marvhollingworth663 Unfortunately too many educated people appear to believe they understand a lot. In my opinion they most often just accept what they have been told. In my own experience even when have gone through it myself and believing it has some times shown up to be wrong. This has not been in physics but in electronics which basically is physics as well.
It just seems impossible to put a quantum particle in an interaction free environment. Not only do we not have full track of the photons, we also don’t have track of whatever else the particle might interact with. An easy thought would be photons emitted by the box it is in, or just the box itself, it could be bouncing between all its walls constantly, and the beam only ensured more likelihood of it interacting with that side of the room.
While that is true, one can measure how often photons scatter on residual molecules in a vacuum and account for that. That is, even if they interact, you would know how likely that is and adjust your expectations accordingly. So I would say you are correct, that "interaction-free" isn't something we can ever establish except theoretically, but reality gives us some taste of what the theory says.
no photon would have been "bouncing" of anything, the photon gets absorbed, in case of mirrors the photon would be losing its energy like almost on constant in interaction with a mirror, you would need perfect mirrors for "bouncing" the light there and back
such mirrors are not real
@@SabineHossenfelder- this notion of “interaction free” measurement has never sat well with me. Even theoretically. I mean, of course you can postulate whatever you want, it’s just that your theory will diverge from reality. We, and our detectors, exist as part of reality and are made of the same quantum stuff as everything else… and operate by the same quantum rules. We don’t have some magical non-quantum measurement mechanism. If we are trying to measure things that are roughly at the same of what we are using to measure, our observations are inevitably going to interact. How can they not if they’re going to accomplish anything? This is very much analogous to computer programming where, if your are limited to using software without the computer system then all your attempts to measure what the computer is doing are going to subtly impact the operate of what you are trying to measure. If the thing you are measuring is big and resource intensive, then the impact of measurement is likely lost in the noise… but if you are trying to (for example) measure the performance of a single line of code, then the impact of your measurement is large and it can be difficult to get it right. Now, if you have an external piece of equipment like an oscilloscope, that is outside the computer, you might be able to measure the computer’s operation without impacting it. In physics though, I don’t think we have anything “outside” of quantum mechanics.
Isn’t it one of the foundational claims of QM that there is no such thing as measurement without interaction? In other words, if the measurement doesn’t interact with the quantum system, no information can be gained from the measurement.
Probably a naive question given I'm far from a physicist (Software Engineer, so lowest of the low when it comes to intellectual requirements), but is it correct to say that everything we are is an expression of the quantum world? Is the 'material world" we experience an abstraction built on top of quantum phenomenon?
Or am I totally misunderstanding and should go back to my 1s and 0s?
That's why I love this channel, every video is a "Thinker", and a quiz too probably. Funny I was thinking those photons were going very straight!
10 ⭐ stars!
NEAL
Look forward to seeing a Riemann Hypothesis video by you. I believe it will be fantastic. Your insights on such a profound and challenging topic would be incredibly valuable. I've always admired your ability to explain complex mathematical concepts clearly and engagingly. A video on the Riemann Hypothesis would be an excellent addition to your content, and I'm sure many others would benefit from and enjoy it as well. Please do it!
The particle knows where it is at all times. It knows this because it knows where it isn't.
The particle it is not sure just where it is. However, it is sure where it isn't, within reason, and it knows where it was.
The particle almost never knows where it is, since it only decides where it might be when you try to observe it; then it entangles with the observer, and neither the particle nor observer know where they are after that...
What does it mean for a particle to know something?
You mean the wave-function knows wher it is at all times, because it extends to infinity and has a probability at every location.
We may also have interactions with the boundaries of the box. ?
This is a nice thought. My first inclination was interactions with virtual photons, but I’m still learning about that odd concept.
5:05 I love the Heisenberg animation! A real "What now?" 😀
something scientists don't seem to get is that probability does not correspond with anything real. It's just an idea. Specifically, is just a fancy way of saying "I'm not completely sure". And the reason why you can't measure both position and velocity at the same time is simply because those two ideas are mutually exclusive. Position is your exact location at a certain time, but momentum measures change in position over time. You all act like it's some magical thing preventing the measurement. Also, you could always measure stuff passively, an eye or an ear just absorbs light or sound that would have been there even if you weren't, so the thing being measured would not be affected.
@@JohnPretty1 oops
You are thinking in classical terms, where indeed probability is just a precise way of saying what we don't know. Take a course in QM and you will find that there is something real with probability in QM, and that the uncertainty principle comes not from probability, but from the wave properties of the probability. It's unrelated to what you are describing.
Once upon a time in the quirky land of Quantumville, a physicist named Schrödinger decided to throw a party. He invited his best pals: Einstein, Heisenberg, and a host of particles. Schrödinger’s cat was the guest of honor, though no one was quite sure if it would actually show up.
As the party kicked off, Einstein tried to explain his theory of relativity, but every time he moved, the speed of the light bulbs seemed to change. "E=mc²," he declared, but the lights just twinkled back, seemingly unimpressed.
Heisenberg, ever the uncertain one, kept wandering around, mumbling, "I'm not sure where I am or how fast I'm going, but I'm definitely somewhere." This left the other guests in fits of laughter, particularly the electrons who were trying to figure out whether to act like waves or particles that evening.
Suddenly, the doorbell rang, and everyone turned to see Schrödinger’s cat entering the room. "Am I here, or am I not?" it purred mysteriously, causing a ripple of quantum confusion. The particles, feeling a bit entangled, decided to stay superposed and enjoy the party both ways.
Maxwell's demon showed up late, complaining about having to sort all the guests into hot and cold categories, which was an impossible task at such a lively gathering. Not to be outdone, Newton tried to get the apple bobbing started, but gravity seemed optional in Quantumville, and the apples floated away.
Feynman grabbed a drink and started doodling on napkins, explaining his diagrams to anyone who’d listen. "It's simple, really," he said, "everything is connected through paths of least action. Even this beer." He promptly knocked it over, causing a small but impactful chain reaction.
As the night went on, the jokes became more arcane. "Why did the tachyon cross the road?" someone asked. "Because it was already there!" The laughter could be heard in every possible timeline.
As the party wound down, Schrödinger's cat, both there and not there, made its exit. "See you in the next quantum state," it meowed. The guests, now thoroughly amused and slightly more entangled, dispersed with new stories to tell.
In Quantumville, parties never really end; they just exist in a state of perpetual possibility. And so, the physicists and particles of Quantumville continued to mingle, entangle, and collapse into laughter, proving that even the most complex theories can bring joy, one wave function at a time.
Literally literature
that looks ai generated
@@darthhunter69still funny though lol.
@@insideman-v1w
It has virtual funniness.
And its mass/funniness is so high that it went over like a lead balloon.
My previous sentence was not necessarily funny.
@@-danR 🤣I’m so high I can’t work out your response.
Wow, this is mindblowing. My understanding of measurement in quantum physics reached a new level. TY
Professor: Do you think you understand quantum physics?
Underappreciated student: Probably.
Seems it's only a thought experiment... I was wondering if there are times when you shine a light on the whole box and still find a lack of a particle in the box at all...
Yes, if you wait long enough there's a chance for it to have tunnelled through the wall of whatever it's contained in.
@@SabineHossenfelder Or the particle has decayed.
@@SabineHossenfelder, the possibility of finding no particle is true only if you made the wall potentials thin enough to allow tunneling during the experiment. So yes, tunneling is possible. However, if it happens, it only indicates a poor experimental design.
@myspeechles that was kind of my point: Simply saying “it leaks” is an oversimplification of the parameters needed to make a correct and meaningful version of this experiment. It fails to recognize that all forms of reflection are quantum interactions to a finite depth of the medium that is doing the reflecting. If you don't acknowledge that complexity, you don't know whether you will get appreciable tunneling. If you are, say, designing an Esaki or tunnel diode, you have to pay attention to this stuff, or it won't work.
This one irks me a bit because it’s a common casual statement that makes the process sound more like magic than mathematics. It is not magic because if you don't have the wave function extending into the medium, you don't get quantum tunneling. But if you say it in terms of waves penetrating the barrier, it doesn't sound quite as magical because you are explicitly acknowledging that the barrier is thin enough to be leaky despite how tall it is. That's not as good for “Oh wow!” entertainment physics, so the leaky-to-the-wave part usually gets skipped.
It is also important because it takes a finite time for the quantum wave to propagate through the barrier or for a photon to interact with electrons at a mirror surface when it reflects.
Thus, if you hear a physicist talking about particles “instantaneously” tunneling through a barrier, all it means is they don't understand how tunneling works. The way it works is that the wave function propagates at a finite speed through the material for a short distance. Once that happens, there's a finite chance of finding the particle in that extended section of wave function. That's just how probability wave functions work.
@myspeechles, thank you. That is a nicely cautious and well-thought-out reply.
Normally, I would reply with specific references, but today is the first day in about a month that I could use my computer for more than a few minutes in my office. (Leg injury with surgery, but fortunately, nothing broken.)
The best I can do from my phone is this: Repulsion of a particle is more than a surface effect. If you have a 6-in piece of steel, every layer of that steel wants to bounce the ball back.
The main difference in the quantum case is that waves are not as well-localized as balls. If you have 6 inches of steel, and the steel repels the electron, the electron will not tunnel through the steel.
Wonderful explanation!
Happy you like it!
Thx Sabine for the link to Dicke's paper. Always good to read primary sources
Makes sense
I thought about it for a bit and what came to mind was "there must be interaction in there somewhere"... the particle couldn't behave like that any other way
By assumption, the state is all that can be known before measurement. So, any measurement that changes the state in a knowable way must affect the particle.
That paper by Dicke illustrates this well. Though you might not find the particle in the interval (-a,a), the act of flattening the wave function on (-a,a) will affect the time evolution of the wave function's components outside that interval. So, the measurement did affect the dynamics of the particle, therefore some kind of interaction did occur.
As Susskind puts it, in quantum mechanics, information isn't where you think it is.
If interaction is the same as changing the wave function, it happens instantly as Einstein pointed. So it seems like a weird interaction, a spooky action at a distance.
@@samgragas8467 Einstein couldn't accept missing the boat on the real physics revolution of his time, the invention of quantum mechanics. Aesthetic slander like "spooky action at a distance" are leftovers from Einstein being bitter.
@@iyziejane He missed nothing, intant collapse makes no sense so he thought QM is not the whole picture.
@@samgragas8467 Quantum mechanics does not require an instant collapse and actually says very little about how the collapse happens. We do know, and for more than half-century already, that collapse is consistent with the usual quantum dynamics. Formally this is expressed by Stinespring's dilation theorem.
@@mishaerementchouk So you measure an entagled particle and you get -1. Then the other one is 100% +1 instantly, no matter how far.
How does it make sense?
Watch out! Sabine starts quantum jumping @6:05.
If she turns off the camera, will her wavefunction spread out?
@@jurajvariny6034 yeah but only until she turns it back on again and then it will collapse.
I did figure out that one possible option was that the measurement wasn't interaction free, but not how any measurement interaction would work exactly. Thank you for the analysis and thoughts, @Sabine.
Wow, that was an excellent and entirely logical explanation. If a measurement produces seemingly incorrect or illogical results, it is worthwhile to examine the measurement itself, as there may be an underlying issue or logical error.
I thoroughly enjoyed this video because it presented a Sherlock Holmes-like narrative within the realm of quantum physics, without requiring extensive scientific knowledge. Instead, it emphasized the importance of logical reasoning and critical thinking.
Thanks Sabine, you nailed it again!
It's amazing how even reputable scientists often mix up microscopic frames of reference and microcosmic frames of reference...
...and this is a big deal?
@@jayr526 I wouldn't say it's a big deal, but it's always nice to remember that scientists screw up sometimes.
@jayr526 Actually it is. The Microscopic scale is everything too small to be visible to the naked eye and big enough to be made visible using direct flow or reflection microscopes. While the microcosmic realm is everything smaller than the smallest singular unit of matter. Therefore a proton. Quarks are not singular as they can not exist without other quarks and a group of quarks automatically becomes a proton. Those can only be seen through measurement, like in an electron microscope. Not though direct flow or reflection. And even then often just approximated. In practice under the size of individual cells light distortion and parallax make a sharp image impossible using direct flow or reflection microscopes but at least you can visualize shapes. Calling everything too small to be visible to the naked eye microscopic ignores the separation of scales. A little bit two dimensional explanation but precise enough for a YT comment. In practical application like high tech and defense this separation of scales is especially important because the greatest performance is generated by microcosmic phenomena, not microscopic phenomena. This is why cracking microcosmic secrets is more important than macroscopic exploration.
it's an easy mistake to make because there is no boundary between the two...
@@grumpytime9375 there is through seperation of scales. But a little known boundary
So... is interaction-free measurement impossible in general?
6:06 A perfect illustration of macro-scopic objects making quantum jumps 🤣
This is such an important concept in quantum mechanics. I’ve heard people say that the quantum world conspires to mess with us. It’s not - they just fail to apply this known phenomenon. Sabine does a great job explaining it. She is an excellent science communicator.
How about the particle bouncing of the box walls. You did not mentioned that scenario. Then the particle and the box momentum is conserved Together. Maybe the walls had detectors?.
I had an explanation about this in the first version of the script, but then I removed it because I thought it'd be too much of a distraction, so thanks for bringing this up...
The thing is you can make the box so large that it becomes very unlikely the particle could have gone so far as to hit the boundary before you made the measurements. So basically for what the puzzle is concerned you can just imagine the box is infinitely large.
Now, strictly speaking the possibility is never zero that the particle hits the wall, but the probability becomes so small that it doesn't make a difference. (I actually checked this numerically, but it's kind of obvious, as a Gaussian wave-packet decreases in amplitude faster than exponentially.)
>SabineHossenfelder : If you assume the box is vast, that presumably increases the amount of time needed to measure whether the particle is in half the box. Isn't this a problem? Or can we also assume the photon source and photon detection array are vast? (Larger than the LHC? 😊)
@@SabineHossenfelder, as I noted elsewhere, this quasi-classical case of always looking opposite of where the ball is was my first thought on reading your description.
However, it’s not fully compatible with your description of a wave function initiated at rest and filling most of the box - that is, unless you accept that each wave function is real and you annihilated half of it on your first test, creating a new, quasi-classical wave function with definite momentum away from the obliterated section.
This case _could_ show up as a solution to some purely mathematical superposition solutions, but if it does, it would be a cyclic solution.
I haven’t seen any clear explanation of what your idea is, but as soon as you start alternating scans, caution is appropriate. Not finding part of the packet _changes_ the part not found. This cannot be done without altering the meaning of the wave function - more specifically, the non-find adds momentum by moving the center of mass of the wave function from the center to one side.
I take a completely different approach to this problem. If you don't find the particle in a measurement on one side of the box, it simply means you would have found it on the other side if you had measured there. It does not imply that it was on the other side all along. This distinction is crucial.
Until you measure it, the particle is in a state of superposition, described by the "cloud" you mentioned. Assigning a specific position or momentum to the particle is meaningless until a measurement is made.
Once the first measurement is made on the right you gain the information that the particle's momentum was to the left. its position and momentum are still not *precisely* defined, but you do have some information. - it must be on the left, and it must be moving left (assuming nothing interacted with the particle).
Measuring the left side and finding that the particle isn't there means that information was wrong.
The only way you can resolve this is with the interaction resulting from the measurement.
Yes that’s exactly my assumption. The superposition still holds across the whole could even though it wasn’t detected on one side of the cloud. It’s interesting to compare this with the double slit where you measure one slit and don’t see the particle there. In this case the superposition does collapse.
@@alansmithee419 Thank you for the explanation. I agree with this, especially that each measurement actually gives you more information about the system.
I interpret it to mean that the probabilities of finding the particle at a location in the box change. This also changes the probabilities for the possible measurable impulses. I suspect that this also leads to the zigzag path described as a result of further measurements. You can call this interaction resulting from the measurement.
However, I do not interpret these as the actual path of the particle but rather as directions for the expected values of its momentum at the times of the measurements.
@@TomOSevens
You're correct that there is no zigzagging of a specific particle. As Sabine says, the probability distribution *is* the particle, it's not hidden in some specific place within the distribution until you find it. It is everywhere it could be at once until you measure it.
However, I think what is important is that this distribution represents all possible paths the particle could have taken if it were a classical particle (this breaks down when you think about quantum tunneling, but that's not relevant to this problem). The idea is that every single path through the probability distribution that could possibly result in your measurement outcomes are zig-zags. Even though this isn't what the particle actually does, the fact that it is universally true for all individual paths the particle could have taken is significant (of course the particle takes all paths at once).
Sabine waves all this away and says "the particle must have zigzagged" for the sake of making a more digestible video I think XD.
The jump cuts are a beautifully integrated hint.
In the way it’s setup here you don’t even need to think about the uncertainty in the photon. The electron’s momentum itself is uncertain, measuring the position by looking at which side it is in only makes the momentum uncertainty greater. So it’s perfectly possible for the electron to zigzag like that. And btw for a general state (not a momentum eigenstate) momentum is only conserved on average.
I'm in a state of superposition so I can both solve it and not solve it at the same time!
A nice explanation. You know that if after hearing it, it seems obvious 👍
Yes, ha, that's what I thought as well. Once you know it, it's kind of obvious!
I'm glad, that the term "interactionless measurement" gave me a severe heartburn. Shows my insincts are in good condition. But I'd not at the first shot would have thought about Heisenberg as the culprit. And we did not even have to superpose the observer with the experiment. I wonder how an explanation of the impossibility of the paper would have sounded like under that angle of view. Damn, remind me to visit Copenhagen at some point in time...
This is a really nice puzzle, and would make a good homework problem.
The particle doesn't have to zigzag, It could spiral (in a wave-like manner ) and maintain angular momentum.
That seemed like a really obvious option to me as well… but I’m not a physicist. If it behaves like a wave, then that makes much more sense than a sawtooth pattern would, under most circumstances. Would be wonderful if someone could elaborate.
@@vkjs2 I think it's a simple as an electron cloud being akin to a miniature electromagnetic field.
OK, but, hang on. If it's not possible to do an interaction-free measurement, then that means the attempts at such measurements can't be distinguished from an actual measurement, right? But then this means that you can never be certain of the results of any measurement, doesn't it? So doesn't that imply you can never actually do a measurement?
I think *in this case* it's not possible to do an interaction-free measurement, but it's possible in other cases. Eg, imagine the example of the bomb-detector. In this case, you first split the particle into a superposition and then the two parts of the wave-function are very far away from each other. If you now make a measurement on one half, that can't affect the other half because that's elsewhere.
In this case you need another interaction, which is that with the beam splitter. But this isn't part of the measurement. So the measurement is indeed interaction-free, I would say, while the entire experiment isn't.
There are "degrees of certainty". If a measurement finds an electron in a place with 99.9999% probability, it is pretty much there for most experiments.
When an interaction is at least unlikely, you account for it by repeating the experiment a number of times, and discern those results least likely to involve an unintended interaction. Probability of unintended interaction should be theoretically calculable, in a well-designed experiment.
@@SabineHossenfelder"[...] the two parts of the wave-function are very far away from each other."
But isn't this just a technicality? In the end the wave function should still extend to each side and a measurement on one side changes the wave-function on the other, no?
@@SabineHossenfelderI'm probably missing something that's ridiculously basic here, but why wouldn't the particles be entangled with one another?
4:36 Spot On! :-) It's a classical concept explained within classical reality with no actual basis in quantum 'reality'. It is true that We all are fuzzy and quantum even to our smallest cells and bacteria but we just can't see with the eyes selection has given us.
The composition of matter is a complex concept that can be challenging to grasp. At the macroscopic level, objects are composed of smaller components. This concept extends to the quantum realm, where even larger quantum particles are comprised of even smaller entities. Consequently, all matter is fundamentally composed of quantum particles. This knowledge may induce cognitive discomfort. However, it is noteworthy that while the brain perceives pain, it lacks pain receptors. ❤
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I feel like you either need to be a genius to comprehend some aspects of quantum mechanics, or on *a lot* of drugs.
The BOX !!! the particle interact with the box and can change momentum / just increase the frequency of the measurements and you well detect it eventually
These videos deserve millions of views.
💯
Q : Why do we want to know the exact position of a particle . ? " The universe is a place where the center is know where and the circumference is every where . " 😊
Short version: Wave functions are real, carry momentum, and do not always rescale to atomic or particle scales. They are always finite in size, so “wave packet” is a better description.
If you scan half of a large particle wave function with photons and see no particle there, you collapse that half of the wave packet and transfer its momentum to the photons, bending them away from the remaining side of the wave packet. Notably, since no particles are involved and space behaves like a lens, this empty-space bending of a light path is as close to anti-gravity as you can get in experimental physics.
Simultaneously, the remaining half of the wave function acquires momentum in the opposite direction. Even though it has not fully collapsed, the wave packet has acquired momentum away from the light beam, giving net-zero linear momentum in the system.
The only cases in which you can alternate the photon scan of the left and right sides are (a) your measurement uncertainty is so high that you had effectively no impact on the wave function, or (b) after your first wave packet reduction, you created a quasi-classical bouncing particle situation, and then timed your alternating scans to match the empty half. Since each wall impact of the bouncing way packet counts as a specular reflection detection of the wave packet location, the wave packet stays compact. This means you can do alternating scans of the empty side indefinitely with a very low risk of finding the particle.
6:36 so your comment on the Journal was "interaction-free" since you interacted with the other "half" of the interested people (i.e., the simply curious ones here in UA-cam, rather than properly defined Scientists)?
LOVELY
Thinking. Great questions. I absolutely enjoy this type of discussion
I did not quite figure it out, but when you talked about interaction-free measurement, it sounded to me like perpetual motion machines… but then when you explained how it « worked », it made sense so I brushed it off. My physicist’s instincts were right but I was fooled by your explanation.
Quantum particles have quantum properties. What a great insight...
Wonderful that physics has these deep, fundamental mysteries. What is measurement, how does it work, etc. How fun!
Thanks for the stimulating video, Sabine! Here's the thoughts of a (soon to be) fellow theoretical physicist.
In the Copenhagen interpretation, there is no need to discuss the interaction-free nature of the measurement: any measurement is a non-unitary process that collapses the wave function, so there is no reason to expect the particle to retain any of its properties afterwards. Consider as an example that we first measure the momentum of a particle in a box, then its position, and then its momentum again. The math is very clear, no further assumption about the measurement process is needed, nor would it make any difference.
What you are trying to do with your answer, I believe, is to go beyond the inconsistency of the collapse postulate and try to understand what the other features of quantum mechanics imply in lack of a better theory. It seems to me that you are excluding a priori both the MWI and the spontaneous collapse models, which in this regard functionally behave just as Copenhagen, leaving only non-local hidden variable theories on the plate. What do you think, am I missing something?
Have a nice day!
My be I will be able to help you understand this puzzle. To understand what is going on, we need to understand the physical processes which is going in this isolated physical system. In my article published "being the iron curtain" I have explained the phenomenon of "Uncertainty" - In a isolated physical system there is no a "Reference Point" Without reference point the particles are there, but there is no physical element to determine their position. The "Act of Observation" or even a single photon drastically changing the situation because we providing physical point of reference. Probably will be of interest of yours to inform you for the existence of (TOE) - the book - "Theory of Everything in Physics and The Universe" Regards
In the double slit experiment, you get particles (rather than waves) if you detect which route it did NOT travel down. So, no interaction but the collapse still happens. It is the knowledge that collapses the wave function, not the measurement itself. (If you measure but do not look at the results, it does not collapse).
Exactly. Now, I have made a measurement of my coffee mug by using the light reflected off its internal surfaces. I have measured a high probability of needing to refill it.
Well, probably exactly.🥴
It is also possible that the interaction that changed the momentum of the particle was not the measurement but some unknown (or unrecognized) force or particle.
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I had a different answer that's also (perhaps more) correct: you put a particle in the middle of the box, so you knew it's precise location, but not it's momentum- the particle could have sideways momentum, bouncing off the sides of the box, so at any instant you look at either side there's a 50% chance it will be on the other. The interaction is with the walls of the box, not with the detector. This solution adheres to requirement C- an interaction free measurement.
The moment you brought up interaction-free observation, I was skeptical because I had never heard of such a thing before. But if I think of it in terms of probability clouds and relational calculus, it actually makes sense to me.
The particle knows where it is by knowing where it isn't.
My way of thinking about it still makes sense. There is a wave (the so called particle). The probability isn't about where the particle is, it's about how likely your photons are to interact with it. But the "particle" is everywhere, since it's no particle at all, but rather a wave.
So you send photons at it. If a bunch of conditions are met at some point (this is my personal belief, having no evidencto back it up), a process will occur that will change the direction of the photon, and you think you have detected the particle.
This process is a non-linear wave interaction, and will soak up energy from the environment, and in that way "remove" the particle from other positions. The process ends at some energy threshold, which just happens to be the energy of the particle itself (its mass). That's why you can measure left and right and not find the particle on either side. Just because that condition wasn't met, whatever it is.
6:08 subtle jump from one side to another is a clever editing.
@ 5:08 Heisenberg's eyebrow movement freaked me out :))))
@6:24 That's Pogi! If you know how fast he's going you don't know where he is! 🤣
Sabine, first, a thought problem:
Launch a neutral, low-momentum particle or molecule toward a circular momentum detector split into two semicircular parts. The momentum detector operates not by absorbing the molecule but by reflecting it. For a real experiment, you would likely want to use low-angle impacts to ensure specular (mirror-like) particle reflection, but for this thought experiment, we can assume simple vertical reflection.
Note that all forms of specular reflection, including everyday reflection of light in a mirror, are unavoidably quantum phenomena since any particle vastly smaller than the flat surface must somehow “see” the larger surface to know the proper angle of reflection. In the Schrödinger wave interpretation, the probability wave function gathers this information.
Note also that momentum is vector-conserved, which sharply contrasts with scalar-conserved energy. That means you can create any number of opposing action-reaction momentum pairs from a potentially tiny amount of energy. All you need is for the total set of vectors to cancel to zero.
This idea of reflection without atomic absorption sounds strange, but it is how NASA light sails use the sun for propulsion without absorbing photons in the black-surface sense. That kind of absorption would result in a very different effect of heating. Also, specular reflection is not just a photon effect since low-momentum neutrons, for example, reflect specularly.
What happens in specular reflection is the creation of a new momentum pair that did not exist before. Counterintuitively, this doubles the momentum the momentum detector gains compared to simply absorbing the particle. The detector receives not just the original momentum of the particle but a second, same-direction unit of momentum from the newly created momentum pair. Specular reflection conserves overall momentum by reversing the momentum of the particle’s probability wave, sending it back in the direction from which it came and giving a total momentum identical to that of the original particle.
Above and to the side of each semicircular detector is a low-intensity laser pulse generator with optics that spread the directed beam across the volume just above each detector. Each can generate pulses synchronized with particle launches, with each pulse having barely enough coverage, energy, and frequency to ensure the particle did not cross that region of space on its way to the reflector.
Why “barely enough” instead of simply blasting the region above the detector with photons? That’s because this thought experiment has one more twist: A detector opposite each laser pulse generator that is sensitive enough to detect whether the laser pulse acquired any lateral momentum while traversing the empty region of space above the detector. If you think about it, for laser pulses, that is equivalent to this question: Does the laser pulse bend ever so slightly as it traverses the empty region of space just above the detector? The best way to detect such an optical distortion would be interferometry, which can be exquisitely sensitive to such minute path distortions.
Each semicircular momentum detector also has this capability. If the momentum received is anything but straight-on vertical momentum, each can detect that the particle has similarly veered off course due to a lens-like bending of its path.
But this is ridiculous! Why would you look for such empty-vacuum lensing effects when quantum theory clearly says no such effects exist? Because this is not a theory. This thought experiment intends to suggest lab experiments that might validate (or invalidate) whether standard quantum theory covers all cases. More on that below.
It’s time to run some experiments.
First, launch particles with both lasers off. The prediction is this: Each semicircular detector receives one unit of momentum as long as the particle reflects specularly. There are no surprises here, as this is standard quantum mechanics. Both semicircular detectors report no deviations in the path of the particle. Note, however, that both also equally report the reflection of the particle. That’s a neat trick if only particles impart momentum.
Second, launch a series of particles accompanied by the low-level laser flashes on one semicircle detector side, say the left side. The question is this: What do the various lateral motion sensors for both particles and laser pulses see in this case?
Copenhagen does not handle this question easily because it explicitly assumes probability waves cannot carry momentum, only particles. Most quantum interpretations, even ones that try to be different from Copenhagen, continue to assume this duality of information-only waves and property-only particles.
But why not just follow what we know from experiments happens in specular reflection, and assume that momentum conservation holds?
That analysis gives this set of predictions:
For every detection where the laser pulse sees only empty, particle-free space over the left detector, the right detector receives twice as much specular reflection momentum as before. Why? Because the laser pulse has reshaped the total particle wave and now falls only on the right detector. Moreover, since the light pulse breaks the symmetry of the wave, the lateral momentum detectors of the right side suddenly kick in and report a slight bending of the particle path to the right.
What about the left laser receivers that detected only empty space? Conservation of momentum requires that if the particle path bends toward the right, the light paths in the detector must bend toward the left as if the empty space had become a weak lens.
If all of this proves true experimentally - and I’m quite confident it will - what are the implications?
The most important implication is that the infinite duality of the Copenhagen translation - the attempt to translate quantum theory into a pair of “perfect” waves and “perfect” particles - is incorrect. Until they are detected, probability waves contain momentum, and that momentum is completely and experimentally real.
When the left laser “proved” no particles went that way, it also interacted with that region of the very-much-real wave function and captured its load of momentum. It was never “empty” space, and the absence of a particle there does not disconnect its presence from experimental reality. On the contrary, that empty space with a wave function behaves like a one-use-only lens that bends the light interacting with it.
Sabine, if you return to your original Backreaction blogs from years ago, you’ll see that what I’m suggesting here is not new. I think I called the transfers of momentum in empty space “sonons,” meaning space phonons since they are cases where space itself behaves like a momentum transfer mechanism. I now lean away from that terminology since it’s more akin to probing the deeper nature of local, single-inertial-frame space and time. Experiments like these poke at what we mean by “space.”
I love videos like these! Super interesting
About 30 years back, when I began my life as a teen, I took it as a challenge to understand qm. Lost a few sleepless nights to unterstand the double slit experiment. After reading Feynman's explanation of probability cloud, I thought I understood it. Then again lost my mind because I realised that it was not actually a very solid "physical" explanation. That has been the story for me to understand most of the quantum phenomena since then. So, as I grew older, I stopped trying to get a "physical" understanding of qm myself and decided to go with whatever physicists like you say about it. So, basically it's a chaotic journey at the moment.
Honestly this whole thing about “you know where it is, so you can’t know where it’s going”… I wonder how on earth they ever figured out THAT.
is a fun way to think about this
let's imagine we are in a foggy field, and you are the particle. Here’s how the concepts translate to this scenario:
Fog as Probability Distribution
Probability Distribution:
Imagine the fog represents your uncertainty about where you are. In the thickest part of the fog, you are most likely to be found. As you move, the fog spreads out, representing the increasing uncertainty of your position over time.
Uncertainty Principle
Uncertainty in Position and Movement:
The more you know about where you are in the field, the less you know about how fast or in which direction you are moving. If you focus on your position, your sense of movement becomes more uncertain, and vice versa.
Interaction-Free Measurement
Using Light (Interaction-Free Measurement):
Suppose a friend is on the edge of the field, shining a flashlight. If the light beam passes through the fog without scattering, your friend knows you’re not on that side of the field. If the light scatters, they know you're there.
The Puzzle in the Field
You Moving in the Field:
You start in the middle of the field. Your friend shines the flashlight to the right side and doesn’t see any scattering (you’re not there). They shine it to the left side and again don’t see any scattering (you’re not there either).
Despite not finding you on either side, the fog keeps spreading. According to quantum mechanics, you could still be somewhere in the field. This spreading of fog represents the probability cloud.
Interaction and Momentum
Momentum Conservation:
Imagine you can’t change direction without an interaction. If you start moving left and your foggy path suggests you moved right without interacting with anything, it would seem like you violated the rule of momentum conservation.
Realization
Interaction-Free Measurement Reality:
To truly know the flashlight’s beam path (to ensure it didn’t interact with you), your friend needs to know its exact direction. This requires precise knowledge of the light’s momentum. However, knowing this means losing track of the light’s exact position, implying there must have been some interaction to know you weren’t there.
Conclusion
In the Field:
From your point of view in the foggy field, the flashlight beams checking the left and right sides interact with you indirectly. This interaction affects your position and momentum, meaning the so-called interaction-free measurement isn’t truly free of interaction.
Macroscopic Realism
Implications for Larger Objects:
If this scenario were scaled up to larger objects (like you instead of a small particle), demonstrating quantum behavior would be much harder because it would require proving interaction-free measurements on a much larger scale, which is challenging due to the interactions involved.
That was quiet intriguing explanation of "interaction free" measurements... Always wondered whats up eith those lol
"violation of macroscopic realism" I have that experience every day.
Firstly, intuition immediately provided the correct answer. To completely isolate one effect from others is not an easy task.
Secondly, it’s amusing to see a multiple choice format in physics and mathematics.
You got me. I made the mistake of believing that one can have interaction free measurement. The uncertainty principle strikes again!
I remember taking quantum at the University of California in the early 80’s. I was doing very well in the class. One day, about 1/3 of the way through the first quarter, the professor asked the class, “How many of you feel like you are getting a good intuitive feel for quantum mechanics?” About half the class raised their hands. I was shocked. I thought I was doing well, but all I was doing was solving problems in accordance with the mathematics. I had no intuitive feel for the stuff, at all. Then the professor said. “Everyone with their hand raised needs to study harder. It is impossible to have an intuitive understanding of quantum!”
your first really good video in a good while.
(it was physics, no personal opinion, and not letting out on known facts like in other videos, where you contrafict strongly with A-level school books (eg. Klett Verlag) ,wikipedia, quick google search.)
Sabine : Did you figure it out...
Me : I was supposed to figure out something???
As time passes, I am more and more convinced that all the weird stuff about quantum mechanics are rooted in not really understanding what we do when measuring stuff. I expect one day someone saying "of course the wave collapses into a point. The sensor is a point, what else did you expect?" or "obviously, if you setup the experiment so it splits randomly into two paths you will measure a random output according to each path, duh!"
When your parents called you Sabinchen in your toddler years, they (probably) had no idea that someday you would develop into such a powerful critical mind and challenging scientist around the Globe plus encouraging common folks, like me, to think just a little bit harder. Keep up the good (critical) work and may the Force be with you for a long, long time.
I'm glad she had a normal toddlerhood including terms of endearment.
Of course I figured it out by simply violating temporal constraints, and watching your answer before it happened.
I do not understand anything, but I feel smart consuming this kind of content
and you are...
6:15 The problem with macro/micro is actually a methodological oversight.
1. The probabilistic interpretation of phenomena by quantum theory implies the symmetry of the results: 50/50.
2. However, such an interpretation by quantum theory is a priori extrapolated to asymmetric phenomena.
3. For example, in the case of Schrodinger's cat, the radiation source is considered as a probabilistic (50/50) symmetrical element of the experiment, and is combined with an asymmetric participant: a cat for which being dead or alive has asymmetric states.
4.For clarity, you can “slightly” modify the experiment: put a dead cat in a "black box" with a radiation source. It seems that now even the most faithful follower of quantum theory will not claim that the cat is both dead and alive at the same time.
5.It is better to put illuminated photo paper in the drawer, rather than a cat: it is unlikely that the paper will react to the actions of the 50/50 source in such a way as to become usable.
P.S. Jokes with time asymmetry are unacceptable even for such a “blindly lucky theory”.
My first thought was that it wasn't interaction free, but given it's quantum mechanics, I didn't think the answer could be that simple. 😄
You give me a headache every day and my brain loves it. Now I'm going out into the garden to dig up tangled roots and think...
Knowledge brings joy to the soul and bitterness to the view of the human tragedy
Thank you for this New insight. Of course measuring space where the article is not there does impact the direction-position of the article.
Trying to understand quantum mechanics is literally like nailing jello to the wall! I have spent close to 50 years of studying this topic on a sporadic basis and I have even more questions now with this bizarre Russian Doll of a science concept.