The Quantum Bomb-Tester!

[@matthewluecke3704]

I watched Sabine Hossenfelder's video about this same topic, posted late August 2021. KZbin's algorithm suggested this video shortly thereafter. Your video explains it better. Thank you for prescience science. :)

[@vallea7770]

I totally agree. I love Sabine but this woman explained it much better.

[@frankt9156]

Physic professor at MIT says it didn’t take both path. And you can prove it by look at one path.

[@amihart9269]

You should check out in the search bar "[KEYNOTE] Why Interference Phenomena Do Not Capture the Essence of Quantum Theory".

[@bjmcintyre5080]

I agree I understood this better than Sabine’s

[@michellejohnson2280]

Same! Came here to clarify Sabine's video. Thank you, I understand now!

[@JanStrojil]

Like many, I read the recent Saturday Morning Breakfast Cereal comic, went to wikipedia, had no idea what it was talking about and then was delighted to find that you have a video on the topic. Thank you!

[@adrians20]

Lol, there is another comic there that mentions it and that brought me here. Have a nice day!

[@atlantisvelforening]

Could it not be that when there is a live bomb without explosion, the bomb do not absorb the 50% chance of a photon along this path, but only disturbs it, (for instance with its electromagnetic field), so that the destructive interference preventing B from measuring any photons no longer holds?

[@haythamabdel-qader6934]

That was not an easy concept to tackle, roughly 6 minutes, beautifully simplified. I've watched a lecture about this years ago, and even then I thought a video with some editing could make it more palatable, you've definitely done that.

[@upandatom]

+Acrimonious Haze thank you for the kind words. I always thought the elitzur-vaidman bomb tester thought experiment was fascinating, but for some reason no one knows about it? So I tried to put it out there in an easy manner.

[@bhaveshmahale1179]

@@upandatom finally I found someone interested in scientific stuff like this. You explained very well.

[@blackcyklops]

Totally agree. I watched two videos before this, and though it was clear in the beginning, they'd lose me after half way. The animations in this video really helped me to understand what was really going on. Glad I checked this video

[@amihart9269]

There is _potentially_ fallacious reasoning here which makes it look bizarre. Notice how the second beam splitter can take an input from both the top and bottom path. Beam splitters are two-input devices, not one-input devices, and all logic gates in quantum computation are unitary meaning it must also necessarily be a two-output device as well. In the diagram, the light is entering the first beam splitter at the same angle as it hits the second from the top path, meaning the first beam splitter has an input of 1 in the top path and 0 in the bottom path. It then outputs either a 0 or 1 for the two different paths. The confusion arises because the 0 state here presumed not to be tangible. In the 1 state, a photon is observed, in the 0 state, a photon is not observed so it is presumed not to exist. This experiment becomes less weird if you replace it with something where both states are tangible. Take, for example, a bit defined by an electron where 1 refers to a spin up and 0 refers to a spin down, and you repeat this using two electrons which are negatively correlated (see code at bottom). It becomes less bizarre because when you learn the bomb is not a dud despite it not exploding, this was from the bomb measuring a 0, and 0 is the electron spin down, so something physically interacted with the detector which is how you learned the information, and thus there's no reason to conclude it was an "interaction-free measurement." The implication here is that rather than concluding the first photon "took both paths at the same time," you could also conclude that photons in the 0 state are discrete physical objects which would be measured as a 0 but could still carry information, that they are still real photons that propagate through the electromagnetic field but in a way that would show up as no photon there on a measuring device. This isn't an entirely original idea even, the paper "Why interference phenomena do not capture the essence of quantum theory" covers the same topic and shows how a 0-bit photon could actually carry information and how that could be used to derive the same results as the bomb tester experiment without it being an interaction-free measurement. The logic gate you'd use to carry this out would be: |00⟩ → |00⟩ |01⟩ → 1/√2|01⟩ + 1/√2|10⟩ |10⟩ → 1/√2|01⟩ - 1/√2|10⟩ |11⟩ → |11⟩

[@vijayshankar102]

I didn't realize someone could be this happy while explaining physics.

[@behnam263]

Until there is no exam, I am happy to hear and talk about physics.

[@unbeatablehero]

OMG All these years I have struggled to exactly (or most of it as they are totally weird) understand the weirdness of quantum physics. Your analogy is so beautiful that I can understand the weirdness of quantum particles mostly such as how "They are in both places" AND "Superposition" AND "They change themselves" etc so well. You are awesome

[@omsingharjit]

After 5:30 ? Confusing ? Because you didn't tell why A shows non explosive And B shows Explosive 25 % of time ? How can it will interference itself when it goes through only one slit upper one ?

[@Cocandre]

That's why you shouldn't allow theoretical physicists in a lab : they would blow it up 50% of the time

[@upandatom]

+Cocandre haha and then be surprised

[@hansblod3379]

If the bomb is a dud the photon is also absorbed but does not trigger the explosion, right? Why would the dud not absorb the photon?

[@Christopher-ye6cv]

The triggers on the dud bombs have no photon sensor, so any light incident on the bomb will not be absorbed and will instead pass straight through

[@matswessling6600]

@@Christopher-ye6cvthat doesnt make sense. if there is a bomb it will block the way for the photon. Dud or not. The bomb part of the experiment is just weird and make no sense.

[@jaelte]

Thank you for the video. I just saw a reference to this and tried to read through the Wikipedia article, but I wasn't getting it. The video went a long way to helping me understand this concept.

[@LookingGlassUniverse]

Oh my *god*, I love this! Your channel is fantastic, I'm subscribing

[@adityakhanna113]

Try giving it a shout-out! Or a collaboration!

[@upandatom]

ahh thank you so much! so flattered :)

[@LookingGlassUniverse]

Aditya Khanna I did on my twitter cos more people need to know about this!

[@upandatom]

Looking Glass Universe I can't thank you enough I'm so grateful! :))))

[@LookingGlassUniverse]

No no! You're content is great so you deserve it! What's your twitter name btw?

[@gamdsch]

Sorry to be a nitpicker - in a Mach-Zehnder interferometer detector B will get a constructive and A a destructive interference. The interference here stems from the phase difference induced by the beamsplitter (pi/2). It's rather simple actually: If one half of the beam gets transmitted twice and the other one reflected twice, they will be of opposite phase (destructive). If they each get reflected and transmitted once they will be of the same phase (constructive). Good job otherwise!

[@upandatom]

Oh really? I must have misread the article. Thank you for the correction :)

[@jasonpersonal8517]

This is true

[@bearyboy9088]

Yea I was gonna say the same thing

[@fewwiggle]

@ gamdsch Couldn't you manipulate the lengths of the paths so that A would get constructive interference?

[@Salsuero]

@@fewwiggle Manipulating the experiment to get the results you want. LOL Sounds like a great idea to me!

[@keppela1]

This makes sense until the very end. If the photon shows up on B, it gives you no more information than if it shows up on A. It took the upper path so the bomb could have been live or a dud and we wouldn't know.

[@NathanRichHotpot]

Simply having something in the bottom path which interacts with the photon collapses the superposition, which doesn't happen with a dud. If the superposition is collapsed we still don't know if it's going to be detected at A or B. But what we do know is that 100% of B detections were caused by superposition collapse, which is only from a live bomb.

[@dudethethe2548]

What happens if you remove the lower right mirror? No bomb, and no measurement, would this mean no collapse of the superposition? Would this not give the same 25% chance of A or B?

[@mohammedzeidan8417]

thank you so much. you saved so many lives with this video XD overall u simplify things perfectly. good job.

[@Goldenretriever-k8m]

ahh this was so much easier to understand than the video i just watched which was Sabine's video about this! i'm not a physics major or anything so I need it really spelled out for me, and even though Sabine's video was veeeeery similar, i was quite confused about what was going on

[@Temeliak]

Ended up here from there too, Sabine went a bit too fast indeed, and left me very confused, this one was definitely clearer

[@Goldenretriever-k8m]

@@Temeliak yeah, I love Sabine's videos but sometimes I need an extra thorough breakdown in addition to her videos.

[@robbedemey]

How is this different from the double slit experiment? The bomb is clearly making a measurement, so there wouldnt be any interference before the detectors. What am I missing?

[@lepidoptera9337]

You aren't missing anything. This is a modified version of the double slit where you hold a dark card in front of one of the slits to make it into a single slit. The classical pattern changes accordingly.

[@blakefarnsworth]

Intstead of a bomb, think of it instead as a detector, but we don't know if the detector works or not. We're essentially able to learn if the detector is working, even without the detector going off, 25% of the time. That's what takes it one step beyond the basic double slit experiement.

[@tushargohil9569]

@@blakefarnsworth I still don't get it. Please help me understand just the last part. How do we know and confirm that the bomb is live if the photon is detected on B?

[@Ccaste1967]

To completly understand the concept, something needs to be said. In addition to not having any sensor in the dud bomb we also need to set it up in a way that any light incident on the dud bomb will not be absorbed and will instead pass straight through. Otherwise it will just be an obstacle in the way which is blocking one path.

[@HolySoliDeoGloria]

I think you need to move the bomb off the lower path and move it to the lower right, as here: faculty.hampshire.edu/lspector/aaai-99-www/sld015.htm. With the bomb in the middle of the bottom path, it absorbs the photon whether it's a dud or not. If the bomb is a dud, then it becomes a mere obstruction. Even if there is some explanation for how the photon passes through the dud bomb's sensor, the way the bomb is depicted in the existing diagram is just unnecessarily confusing. Otherwise, you did a good job explaining this. Thanks!

[@upandatom]

Mike Toleno thanks! And really? That's how it is drawn on Wikipedia.

[@upandatom]

Mike Toleno I took it to mean that if the bomb is a dud it can't absorb the photon

[@upandatom]

Mike Toleno maybe I was wrong

[@HolySoliDeoGloria]

Wikipedia (as it appears now) has two different pictures. The one at the top of the article has the bomb in place of the lower-right mirror. The multiple pictures on the right side in the main body have the bomb (detector) in the middle of the lower path. Those pictures show an open circle, which sort of explains (though not explicitly) how the photon passes through if the bomb (detector) is a dud. As I alluded to, either method gives the same result, but if you put the bomb in the middle of the path, you have to explain how the photon passes through the dud bomb. I think putting the bomb in place of the mirror is simpler and more elegant.

[@upandatom]

Ok thanks for the update :)

[@garryslocombe]

Great video Jade. Why are so many of the best KZbinrs aussies? I haven't really accepted the concept of wave-particle duality and have always believed that there must be an explanation that isn't contradictory. I thought I had one but this video tells me I have a bit more thinking to do. This model must be testable without blowing up the lab 25% of the time. Also you should be able to use a bomb analogue where you know whether it is live or not. It is a great to test my day dreams against.

[@gavinhackland8440]

All I got from this is that radioactive cats can detect bombs. Curse my comic book education!

[@vallea7770]

This was the best explanation I have watched. Thanks for making this video.

[@Edgard422]

I think this could have more than a 25% chance of success, since when the photon is detected in the sensor "A" we can still test it again. So we have a 25% of 25% added chance of recognizing the live bomb without detonating it. I think it ends with something more than 32%.

[@upandatom]

+ Edgard Perez Palma you're right. These Japanese scientists actually found a way to improve the success rate to 82% with a real experiment. It was a bit different though, less bombs and more fabry-perot resonators. Here's the link if you wanna check it out. books.google.com/books?id=XAzVCgAAQBAJ&pg=PA148&lpg=PA148&dq=Experimental+demonstration+of+two+dimensional+interaction+free+measurement&source=bl&ots=VU_kKWoTpT&sig=Ubi7Xi8kvSoGJlq3DQ_mGOIEyns&hl=en&sa=X&ved=0ahUKEwjh48i27Z_QAhUL9IMKHXh0C2AQ6AEILzAE#v=onepage&q=Experimental%20demonstration%20of%20two%20dimensional%20interaction%20free%20measurement&f=false

[@desmondorsinelli8850]

At 5:59 you say 25% detected on B - live with no explosion. This implies that the setup knows what'll happen if it were to go the lower path. My mind is in a superposition of being mind blown and mind snowed.

[@paulgemperlein626]

I'm trying to understand the actual practical application. Could the military legitimately test bombs this way? Like if they just accepted that it wasted half their bombs in exchange for knowing the other half weren't duds, that seems like it could be worth it. Could this be setup at the end of a consumer goods production line as a quality control measure? Like to check if a flashlight works after assembly?

[@samuelprice538]

I read something in new scientist years ago about this but they used medusa instead of a bomb. They also said by tweaking the experiment you can increase your chances of success and without too much difficulty to the point where the bomb would never be detonated in the lifetime of the universe

[@jemyhuan]

if the photon detected by B can be the lower pass was blocked or the phase was changed on the pass. it has nothing to do with bomb ok.

[@TechItOut]

Physics and Philosophy. OK I am hooked. Subscribed

[@upandatom]

+Tech It Out Thank you! :)

[@davidbernalromero6571]

How can a photon test a dud bomb and that not be an interaction (i.e. be affected by it, i.e. measured)? If a photon "tests" a bomb and it'ss live, the photon is absorbed and the bomb explodes. But if the photon "tests" a bomb that is dud, there has to be come kind of interaction, even if it is not absorbed. If there weren't any kind of interaction with the dud bomb, it may as well be live bomb and nothing would happen.

[@lepidoptera9337]

You got it. A bomb is a classical system. It's either a 100% absorber or empty space. There is nothing quantum mechanics can change about that.

[@desmondorsinelli8850]

@ 2:35 you say 100% of the beam shows up at detector A. Wouldn't it be 50%? The other half went toward detector B, was destructively interfered upon and that caused nothing to be detected at detector B. But just because no light was detected at B, it doesn't mean it went to A instead. Right?

[@MrQuantumInc]

The only problem with this thought experiment is that use the word "Bomb" to mean anything that blocks photons but only when working correctly. I am pretty sure a dud bomb still casts a shadow, and an explosion counts as a light source, so your results could be backward.

[@astrolillo]

that makes 0 sense kid

[@nber82]

I was confused, hopefully this helps. Game changer explanation with live bomb. Couple statements. If it goes both ways, and wasn't measured (exploded), it ALWAYS goes to A. It interferes with itself, therefore makes it there. 100% If it IS measured and exploded, it collapses and can go to A or B, so you'll get a 50/50 split Now, if it IS measured (collapsed), it can go to A, because now it's just a bouncing ball which falls randomly 50% A or B If it ISN'T measured (not collapsed), it can ONLY go to A because it HAS to intefere with itself to get to A. If it ISN'T measured (not collapsed), it goes both ways, it HAS to go to A (100%), but NEVER can go to B. B really only can happen if the beam ONLY goes one way. The beam CANNOT interfere with itself AND make it to B. The only way the beam gets to B is if it DOESNT interfere with itself. Which means it ONLY goes one path. Again, the ways the beam can make it to A. if it interferes with itself OR if it is collapsed, and randomly follows the 50/50 physics rule The way it makes it to B, is if the beam is collapsed and follows the classical 50/50 physics rule. It CANNOT make it B AND interfere with itself. It CANNOT interfere with itself and make it to B, it's really like if A is a square opening and B is a round opening . A square and a circle can go through A, but only a circle can go through B. You and concluding the shape of the object not by what it can go through, but simply by what it cannot. A collapsed beam is a round object in this example. A round object can get to A or B, but IF IT MAKES IT TO B, it can ONLY BE ROUND.

[@zakirhussain-js9ku]

Particles move as particle-wave system. It is the wave associated with particle which splits, interferes & guides the path of particle into making interference pattern. For the 2 split waves to reach upper spliter & constructively interfere both paths must be open. Particle always takes a single path & is never on both paths at the same time. 25% of time the particle shows up at detector B is when particle takes the upper path & live bomb detector blocks the path of the wave travelling on lower path. The wave does not have enough energy to trigger the bomb detector. As only one wave reaches the upper spliter there can be no constructive interference. The particle therefore shows up 25% each at both detectors. There is no such thing as quantum superposition. If the particle is on lower path superposition or no superposition, nothing can stop the bomb from exploding except detector malfunction.

[@thechickenduck8377]

First video that explained it so I understood this. Thank you!

[@donepearce]

You have to be pretty quick to run the cat experiment. The wave function of a couple of kg of cat collapses pretty quickly, so the cat is in fact either alive or dead, and not in a state of quantum superposition.

[@cezariuszreginia7724]

its so confusing so half the time we cant detected it ? how is that? i dont get it

[@lepidoptera9337]

The bomb either absorbs a photon or it doesn't. When a live bomb absorbs a photon, then it explodes. It doesn't matter how many beam splitters you put around the bomb and how you distribute the detectors. The first two facts never change.

[@apurbabiswas7218]

Hey, great video! I came to your channel from a comment you made on Science Asylum's video. I like your content, and I'm looking forward to seeing your channel grow. I have some constructive feedback that you may want to take into consideration. I don't make videos, I'm just an avid watcher of Physics on KZbin. When you referred to a previous video (2:17), I found it slightly distracting from the explanation. I may just nitpicking here, but I'd prefer it if you included that as an annotation instead. On a positive note, I like the animations and the narration. Keep up the great work! Subscription earned. :D

[@upandatom]

+Apurba Biswas thank you for the feedback! I honestly have no idea what I'm doing so any feedback is greatly appreciated. I'll make sure to just include it as a card next time :)

[@apurbabiswas7218]

You're welcome :)

[@michaelzoran]

PARADOX: This lady is so intelligent. By default, she must be classified as a "Nerd" and/or "Geek." Yet, she is so pretty - which is "Cool." How can this be??? It is a paradox. 🙂

[@dudethethe2548]

What happens if you remove the lower right mirror? No bomb, and no measurement, would this mean no collapse of the superposition? Would this not give the same 25% chance of A or B?

[@司馬舜-w8p]

It would be nice if you can talk about the Quantum Zeno Effect and how it boosts the succession rate of knowing a live bomb without triggering it:D

[@lucio8794]

Some protips: 1. Watch the camera when you talk, the viewer unconsciously feels more involved that way. 2. There's some reverberation. You can fix it with some soundproof panels (they aren't very expensive). For everything else, as I said, excellent job!

[@upandatom]

+Lucio Moriconi thank you for the tips! 1. Can you please tell me what you mean when you say watch the camera when I talk? You are the second person to say this but I honestly have no idea what either of you mean :( When I watch my videos it looks to me like I am looking directly at the camera, and I've shown some friends and they agree. Do you mean every so often I'll look somewhere else or like the whole time I am not looking into the camera? 2. I just bought a new microphone, hopefully it helps! If you watch my Santa video you can see it in action. I think the sound still needs work though but I just use my bedroom as my recording studio lol

[@lucio8794]

1. It seems like you look a bit on the right of the camera (by your point of view), but it's just a trifle. 2. I've just watched it and the difference is very noticeable, sorry! ^^"

[@upandatom]

Lucio Moriconi Ohhhhh. I can't see it but I think I know what the problem is. I'm videoing with a smart phone and instead of looking at the camera lens I'm looking at the screen. That must be it. I'm afraid I can't fix that until I buy a proper camera with a lens and no reflection, but I promise I'll try when the day comes! Thanks for the feedback anyway. Damn having no money! haha

[@Jehannum2000]

The Transactional Interpretation of Quantum Mechanics gives a much simpler picture of what's going on here. Importantly, it removes the need for the Knowledge Interpretation - the mysterious and confusing apparent correlation between changes in an observer's knowledge and in-flight changes to the wave function.

[@lavie403]

In my case, the video did not allow me to understand. By what we say in the premise that it's about detecting live bombs without detonating them. And yet, the experiment involves explosions... Wikipedia (Elitzur-Vaidman bomb tester) is more explicit and adds these very important details to me to preserve logic "With this process 25% of live bombs can be identified without being detonated, 50% will be detonated and 25% remain uncertain."

[@deandeann1541]

I wonder if this would work with a real bomb. Can this have real world useful applications? Or is there an unstated asterisk, like with the quantum eraser experiment?

[@Rodrigopingoi]

I don't think it would work with a real bomb because a dud one would cause an interference in that case. Check out this video by arvin ash: kzbin.info/www/bejne/j5PVqX5njbKgjKM

[@robertmolldius8643]

The werd thing is that the photon did not take the path through the bomb but still it tells that there is a live bomb at that location. 😄

[@lepidoptera9337]

Except that it doesn't. In order to tell you that with certainty, you would have to repeat the experiment an infinite number of times, which would explode a live bomb with 100% certainty. That, however, is exactly what a classical experiment would do, so you have gained nothing. :-)

[@lepidoptera9337]

@@squishy-tomato Dude, what about "not 100% certain" did you not understand just now? :-)

[@lepidoptera9337]

@@squishy-tomato I simply gave you the facts. What you didn't notice is that the quantum mechanical method still gives you an exponential advantage over the classical result. That is exactly why we would love to have working quantum computers. Physics is subtle. Don't let other people pull your strings as I just did. :-)

[@lepidoptera9337]

@@squishy-tomato You lack of physics knowledge, kid. ;-)

[@zakmartin]

Seems like a very complicated, hit or miss way to stop a bomb from going off. I keep mine in the fridge.

[@jefferyfoli4639]

We only have 5 physical senses with which to observe the visible spectrum. With our eyes closed, the wave of potentiality (interference pattern) exists.. With our eyes open, we immediately observe the 3 dimensional world in which we find ourselves. This manifests the single particle reality.

[@astro-zodiac]

Every time I watch your video I get to know more about quantum stuff👌👌👌

[@videofrat3115]

You explained it better than Nadine. Anyway the idea of "detecting if the bomb is live without detonating it" is a bit misleading, as you still have a 75% chance of detonating the bomb

[@HebaruSan]

But if the bomb's a dud... won't the photon still be stopped, just without an explosion?

[@ApoorvaSrinivasa7]

Nope. The principle is that the detector in the bomb detects it, causing the explosion. But if the bomb is a dud, meaning, if the detector doesn't work, then the photon will keep moving. Think of a stage mic: if it's on, we notice that a lot of the sound waves are absorbed for amplification; if off, nothing much happens. It's kinda like that, but not exactly. Only if the detector is working does the photon gets detected (absorbed?), else continues.

[@paulgemperlein626]

I don't understand. Would the photon not physically hit the steel-cased bomb in its way and be absorbed? Is the video showing a simplification or are you saying the photon literally passes through the bomb via some quantum magic?

[@mmmmmmmmmmmmm]

I think we're just assuming something about how the bomb works when we say that

[@Christopher-ye6cv]

@@paulgemperlein626 The triggers on the dud bombs have no sensor (photon absorber), so think of the photon passing only through the absorber on the live bomb or the space where the absorber is missing on the dud bomb, not through the bomb casing.

[@s6th795]

This reminds me of the "meltdown" hack that exploits speculative execution on your CPU... but for real life. Scary.

[@dantebg100]

👍👍 nicely explained

[@blueckaym]

Very nice description of the experiment! I'm all for applying all possible approaches to solving problems, even if not perfect. The main trouble with all that subject I have is that the superposition is our description of the sum of possible states, there's absolutely nothing that tells us that the superposition IS a REAL thing, that can affect anything. Certainly a state of the particle that we've described to be in superposition can affect something sometimes ... but our DESCRIPTION can't. To be completely objective I don't have a proof of that either :) ... except for all of our experience in all of the time ever, but that's not really proof, it's just strong observation. One of the explanations on how non-interaction (of the photon with the bomb) can give us information about the bomb is many worlds interpretation (MW). While MW is a theory that can't ever be proven right or wrong (it's unfalsifiable) it's very convenient to some as excuse for their laziness (I mean when being lazy to consider all alternatives). And while I completely agree that when delving with the unknown (and scratching on the surface of the potentially unknowable) it's practically impossible to list all possible alternatives, it imho doesn't mean we should stop looking for them, especially when the presented options are non-sensical and unfalsifiable. But things actually are not that grim. With many of these experiments (at least lately) more and more people start to suspect that Locality doesn't work as we intuitively expect, or many claim it's dead. However so far I haven't seen or heard of a single experiment designed to test locality itself (disclaimer: I'm not a scientists so it might be result of my ignorance. If so please point me to such experiments!). I read in wikipedia about the quantum bomb interpretations: "Jean Bricmont offered an interpretation of the Elitzur-Vaidman bomb test in terms of Bohmian mechanics.[10] It has also been argued that the bomb test can be constructed within the Spekkens toy model, suggesting that it is a less dramatic illustration of non-classicality than other quantum phenomena like the violation of Bell inequalities" It seems very interesting to me to hear more about these interpretations (especially since Bohmian mechanics is the one that repeatedly comes to save the common sense :)).

[@becoolthisisarobbery]

love the videos. hope you keep them up.

[@upandatom]

Thank you! And I will, working on one right now :)

[@new-knowledge8040]

Has the experiment ever been performed with only one photon. In other words, has the experiment ever been dismantled immediately after the first photon reached a detector. Or, has there always been more than one photon being used by the experiment setup, even though only one photon is released at at time ?

[@lepidoptera9337]

It's a statistical experiment. With one photon the bomb either explodes or it doesn't, but you won't learn anything about the experiment itself. Not that it is an interesting experiment, to begin with. It teaches us absolutely nothing about nature.

[@guardian-X]

I'm glad the cat is alive!

[@nigh7swimming]

If this doesn't blow your mind, nothing will.

[@lepidoptera9337]

If this blows your mind, then you need a mind upgrade.

[@billcosbyeatsbabies9947]

Although I like the idea of this, and the concept really is fascinating... This bomb detector is one that not only detects a live bomb, but also sets it off 50% of the time. Thats like having a metal detector that dissolves half of your buried treasure! Or a microwave that will tell you if something is eddible, but destroys 50% of your food! lol

[@upandatom]

Haha I know, I would not want to be the scientists working with this apparatus...

[@mixer8774]

@@upandatom This is easily fixed by making a series of the beamsplitters. And you can make the experiment almost never set off a bomb.

[@callistomoon461]

Great video!

[@barontait4985]

Trouble is all bombs are arguably duds until they explode, only the event of explosion is definitive. What is the distinction between dud and good which can be settled without explosion? A dud can be made to explode and a good bomb made to fail, so only the explosion counts.

[@Langkowski]

Harmless bombs will not absorb the photon, unlike the dangerous bombs

[@TheDudeINdallas]

That’s the classical way of thinking- explosion is the only definitive way to tell. This experiment says you can tell with an almost 100% confidence whether it is a good bomb without interacting with it in any way.

[@zsbali]

I don't understand. At 3:10 you say 100% of the time the photon shows up on detector A. Then at 3:38 you say if the photon takes the lower part there's a 50-50 chance it will end up on either detector A or B. And if the detector takes the upper part there is still a 50-50 chance it will end up detector A or B. So, the statement you said at 3:10 contradicts to the other 2 at 3:38.

[@adityakhanna113]

Um. What if we place a bomb in front of the screen A with constructive interference? This is after the beams interfere right? As nothing goes towards B, and all the light goes towards A. If it's a dud, then no explosion If live, then BAM!? Doesn't that guarantee 100% certainty in detection? What about single photons? Would it now not show any interference? What am I missing?

[@upandatom]

Great question! You're right, doing that WOULD guarantee 100% certainty in detection, but the point is to detect a live bomb WITHOUT detonating it. If you placed the bomb behind screen A, the duds would be fine but the live bombs would always detonate. We wouldn't be able to keep any live ones. However if you place the bomb in the lower path 25% of the time we get to detect a bomb was live without it exploding. Does that answer your question? And with single photons it does not show any interference because for interference to occur there needs to be at least 2 objects. Something can't interfere with nothing. Do you know what I mean?

[@adityakhanna113]

miss physics Oh, I see your point! That important 25%. But shooting electrons/photons one by one does produce an interference pattern, right? Which we might think of it as travelling all paths? Ooh... Something can't interfere with nothing. But something can interfere with itself. Do I get it right?

[@upandatom]

YES! so the amazing thing that happens here it that the photon DOES interfere with itself, because we thought it would just take the 1 path as it is only 1 thing. But it incredibly takes BOTH paths, which is what causes the interference pattern.

[@sailendrakc549]

(Help please) I dont get the part of sending 1 unit of photon to the splitter?? Sending 1 unit of photon, which is according to my knowledge is fundamental and quantum, to the splitter and thinking it will cause superposition of the photon does not makes sense to me. Thanks for reading!

[@NathanRichHotpot]

Quantum objects which confront undetected futures exist in superpositions of those futures.

[@CeceDeAnn]

Love the creativity! ❤️

[@upandatom]

+Earthiopian Eats thank you!

[@nitsanbenhanoch8691]

What if we alter the first beem-splitter so that it sends 99% of the beems upward and 1% of them toward the bomb?

[@awfuldynne]

You have to alter the second beam-splitter to match so the beams' destructive interference causes them to cancel out going to detector B. Without doing the math, I suspect the end result is that you have to run the experiment several more times because the photon hits detector A much more often, resulting in an "inconclusive" run, and you still end up detonating about 2/3 (or more?) of the live bombs.

[@rockbore]

I like this vid. Question: Is this a consequence of Bell's inequality?

[@Moosa27]

Something no-one explains well is what experiments exactly did the scientists do to come up with the weird quantum superposition mechanics? E.g. why the example in video is a THOUGHT experiment? Why not a real experiment? If this is not a practical test, what practical test did they do that could only be explained by superposition?

[@aaroncurtis8545]

The Double Slit/which-path experiments is where it all starts (Not Young's, the 1900s versions). She said she linked to an explanation of that somewhere.

[@LycanKnight2011]

The light globe finally, switched on for me on superposition :). Thank you

[@paulcohen6727]

If this is just a thought experiment, how do we know that it will really turn out that way? and Is there even a light that can emit just one photon at a time? Since a photon is just a given amount of energy, E = hf, then why can't we divide it in half, to make an analogy, the way we can split current in half with a pair of resistors in parallel?

[@mmmmmmmmmmmmm]

Because energy is quantized

[@billgough867]

There's no such thing as a particle of light. It still is a very minute wave that leaves its energy as it travels along.

[@lepidoptera9337]

A wave is the classical expectation value of many quanta, i.e. it's an ensemble property. There are no waves in a single repetition of the experiment.

[@Adam_Wegert]

I hate when people explain this Schrodinger's cat experiment focusing on what is most ,,click-bite'' like content instead of explaining what we have learnt so far (some time had passed since 1930, right?): 1. First of all it was originally proposed by Schrodinger to show that QM MUST be incomplete since the conclusion is clearly absurd. 2. Cat IS a quantum object, everything is but in the case of macroscopic objects these quantum systems are extremely complicated. Most physicist (excluding objective people pursuing objective collapse theories) believe that QM is universal and apllies to all scales-therefore why aren't we observing cat being both dead and alive? 3. Since there is this notion of decoherence-extremely, extremely, extremely effective and rapid process of surpressing any quantum superpositions due to the massive interaction with the environment. The bigger the object is the more difficult is to shield it from this interaction and decoherence destroys quantum effects more rapidly. One shuld stress that this interaction is not only environment measuring cat but ALSO cat's leaking information to the surrounding environment and even internal interactions within the cat itself which we do not control-this also counts as an environment and is responsible for decoherence. Imagine that somebody is not a physicist and hears the usual Schroinder's cat story: what he or she would remember from it? Cool, science tells us that cats can be both alive or dead-but this is absurd, so something went terribly wrong with science. And then people are surprised when conspiracy theories are so popular...And of course writing such critical comment I will be though as a hater...

[@yanava]

The bomb analogy makes it so hard to understand...

[@MikhailKuznetsovMr]

Uhm, wouldn't the dud stop the photon just the same as a live bomb?

[@blakefarnsworth]

Think of the "bomb" more as a possibly-dud detector. It goes through the detector, it's not stopped by it, whether it's a dud or not. But in the case that it isn't a dud, the photon is aware of the working detector on the bottom path, which collapses it, meaning it can no longer interfere with itself. This would mean that half the time, the bottom detector would go off, half the time the photon would go up and then split equally between A and B because there is no interference due to the detector being on... even if it doesn't go on the path of the detector. I'm hoping I made that make more sense and not less!

[@wolfisr]

Great video! Thanks a lot. Regarding the cool names: Avshalom Elitzur is a Hebrew name ( you miss pronounce Elitzur, but you obviously don't know Hebrew, its OK😉), Lev Vaidman is a Russian name. But they both live and work here in Israel. Vaidman taught me math in my BSc about 20 years ago🙂 It was extremely difficult... Thanks again for the videos and the enthusiasm u bring into it!

[@GameReality]

So you have to sacrifice half of the bombs ?

[@lepidoptera9337]

No, you have to sacrifice every bomb to be 100% certain that it's live. Wait a minute... that is exactly what would happen in the classical case!

[@tolgatezcan8433]

What if we make a several bomb tests and use different people to observe. And tell those people to pick a path for the photon. Then compare the results with their initial decision. I mean, maybe the observer is effecting the bomb test result only by thinking a result before we send the photons ?!? Or even when photons are on the way?

[@davidwilliam152]

Brilliant expression! Thank you

[@Hust91]

Did they actually do experiments on this as wikipedia suggests?

[@miskbalder]

Sry, not sure where to ask this but.. Could u explain how the particles are measured in the double slit experiment? I have heard about and read it in many places about the measuring of the particles but it seems so hard to find details about how it is done, and I love ur way of explaining things so it would be very kewl to see what u make of it too :) Or does anyone else know something about it and care to share?

[@schmetterling4477]

There are no particles. There are only quanta. Quanta are small amounts of energy and all measurement is irreversible energy exchange. If you actually want to measure single photons, then you will have to buy a Hamamatsu photomultiplier tube or similar product. It's not nearly as exciting as most people think. All those things do is to make "click" . :-)

[@miskbalder]

@@schmetterling4477 ah thnx for the input, will try to learn more about it, but u are right, at first before u learn something a bit "complex" it is much more exciting than when u understand the simplicity of it :)

[@vishalmishra3046]

Wonderful video !! Can you modify the experiment to bump up the no-explosion successful-detection of live-bomb a tag bit more than 25% ? e.g. by adding additional stages of beam splitters and detectors.

[@astrolillo]

Yes, take into account that this video has some errors, but yes, you are correct, you can bump up the efficiency up to any big number, say 99%, by using a different arrangement and resonance. Check out prof Zwiebach video (MIT) on this topic

[@lepidoptera9337]

@@astrolillo Good luck with that. You either absorb energy from the field (and create a photon that way) or you don't. Whenever you do create a photon at the bomb's absorber- boom. Whenever you don't, then you just don't know, which means you have to repeat the experiment, until... boom.

[@pushkarsaraf9501]

Thanks for not killing the cat 🙃

[@aidanandrewson7223]

This video is the bomb! Sorry about that, I'll see myself out.

[@upandatom]

lol :)

[@elie00had]

This is awesome!

[@andrewlyall9286]

Suppose photons ONLY interfere with themselves and NEVER interfere with each other, would we be able to tell?

[@Langkowski]

Using bomb as a metaphor is a horrible idea, and only makes it more confusing. Instead of bombs, use what is actually used in the experiment; a detector. If he detector is on, it will absorb the photon in 50% of the time, and if the detector is off then nothing will happen. A photon in a superposition will explore all possible paths at once. It's all a question of energy.

[@lepidoptera9337]

"The photon" can't put the bombs in superposition. That's why it's not a quantum experiment any more than the double slit is. If you want a quantum experiment, then you have to have a system that can actually be in superposition instead of a detector that is, by definition, 100% classical. Put a sodium atom there. Now you got a quantum experiment.

[@Langkowski]

@@lepidoptera9337 Well, that's not exactly what I said. I was talking about the photon, not the bomb. What is a sodium atom supposed to do?

[@lepidoptera9337]

@@Langkowski That's my point. The photon doesn't exist until it is absorbed by either the detector or the bomb. The light source puts some energy into the electromagnetic field which is then taken out somewhere but there are no photons anywhere between emission and absorption. Instead there is a quantum field state but it is not "one photon" that is "flying around" somewhere. The description of this experiment with semi-classical photons is a real hodgepodge that will naturally lead to misunderstandings about the actual physics that's involved.

[@billgough867]

I'm perplexed: Surely it must be 33.33% of the time the photon takes the upper path. or the lower path. And 33.33% of the time the photon takes both paths. Either that or the photon takes both paths at once a 100% of the time.

[@lepidoptera9337]

Photons don't have paths. That's just some bullshit Einstein came up in 1905 and which still gets repeated ad nauseam.

[@starsreflectingsky]

There's something I don't understand with this explanation. If there's a live bomb, it will explode 100% of the time because the wave will always hit the live bomb. That's the whole point of the argument. Superposition has the photon going down both paths every single time. So the wave will always touch the live bomb and the bomb will always explode and that will break down superposition and prevent a particle from going down the other path to detector a or b. 100% of the time the photon becomes a particle when the bomb explodes and it becomes a particle on the path that leads to zero detection from detectors a and b. There is no particle available to go down the other path to detector a or b because that means that the wave function would have remained intact after the bomb detected the photon I might be misunderstanding something but I can't quite get past this logistical conclusion.

[@Langkowski]

There is a simpler version of the experiment. You are probably familiar with the classic double slit experiment. A single photon is sent towards a doble slit, and because of the photon's superposition, it can travel through both slits simultaneously. If you place a detector behind the left slit, but not the right slit, and the detector is dormant, nothing is gonna happen. The single photons will interfere with themselves and over time an interference pattern will emerge on the screen on the opposite end. But if the detector behind the left slit is activated, it will register all photons it encounters, and the superposition will collapse, and there will be no interference pattern. Common sense says that because of the superposition, all photons will come out from both the left and the right slit, and therefore 100% of them will interact with the detector behind the left slit, leaving no photons left that can escape from the right one. But for reasons nobody understand, 50% of the photons will still pass through the right slit, even if they do not interact with the detector on the left side. Apparently it is all about probability. If that means the superposition collapses after the photon has passed the double slit, or if they really do have to chose between either the left or right slit, I have no idea.

[@BritishBeachcomber]

I hate that cat, and do did Schrödinger. He only postulated it to demonstrate how non-intuitive quantum theory is. The cat escaped and the rest is history..

[@SBk408]

thank you

[@UteChewb]

Nice. A clear and penetrating explanation of an abstract idea. I mildly disliked the music but that would be my only complaint, the insight gained overrode that. In future vids could you also add some references, both less technical and more technical, because you know some of us just like our heads hurting from this stuff.

[@upandatom]

+UteChewb thanks for the feedback. What did you dislike about the music? Just out of curiosity. I chose it because I thought it sounded mysterious but fun at the same time, which I think matches my channel pretty well. Haha noted. By references do you mean "Hey you can go read more about this here" type of thing? For this video I pretty much got the whole thing from wikipedia haha.

[@UteChewb]

It's not that the music itself is a problem just that when you have something complex presented and being argued then the music can be more of a distraction. You notice in things like Crash Course and Veritasium there's usually no music. It wasn't a big issue though. By references I mean links to text, pdf, videos etc pitched at more introductory and more advanced information, that way you can cater for the curious who may have various levels of knowledge.

[@upandatom]

Hmm I see what you mean. I guess I thought it made the video more upbeat and stimulated more senses. I guess there are pros and cons :) Ahh I see what you mean, will keep in mind for future videos :) I have a horrible habit of not taking down any sources and then forgetting where I found things. Been like that since college, made referencing essays a bitch.

[@UteChewb]

Well, I saw a video the other day, sorry can't remember by whom. It used music but it was a low key repetitive kind. Probably boring by itself but not taxing either. I guess you just have to practice and test out various pieces. Good work though. But, yeah, probably a good idea to track where you get music from, at least for legal reasons, and someone is bound to ask what it is.

[@upandatom]

+UteChewb ok thanks for the feedback. I'll keep my eye out for the perfect piece ;) or ears out...

[@desmondorsinelli8850]

I'm astounded (though I don't get the experiment)(but I'll be listening to this again until I do understand it). In the meantime, I can tell - this is weird.

[@lepidoptera9337]

The correct scientific title for this paper would have been "Photon statistics and classical limit comparison for leaky two-arm vs. perfect three arm interferometers". Not very exciting, right? Would you read a paper like that? I wouldn't. ;-)

[@matswessling6600]

why do they insist on using a bomb? it makes no sense.

[@jones1351]

You kinda lost me with the A/B detectors. It seems like you're saying in one place that the single photon always winds up at A; but then you seem to say it goes 50%A and 50%B. I'm sure that I missed something. I just keep missing what I missed.

[@ApoorvaSrinivasa7]

If superposition happens, i.e., going through both arms at the same time, due to interference the photon always winds up at A. Else, if it goes through either of the arms, then it's 50-50. Helpful?

[@stefanh8190]

Nice video, now I understand it

[@raleighwalter4250]

Just wanted to say your explanation is first rate but I had to watch this twice because you’re also lovely looking 😉

[@172ngan8]

If the cat is alive, then why wasn't there any sound or movement in side of the box, why lol.

[@ronaldderooij1774]

Nice new channel. Physics is really interesting. Unfortunately for me, it talks in mathematics. That is a no go area for my brain (and I have always deplored that). So, I studied political science, which was interesting too, and environmental science (yep, you guessed, it was interesting too). Everything is interesting. Why does the homo universalis die out? By the way, understanding quantum is really easy, if you are prepared to delete every preconception of reality from your mind first. I think it is time that physics should be more about our environment than about finding new tiny particles, btw. But that is understandible from my viewpoint, I guess.

[@upandatom]

+Ronald de Rooji thank you! Yes there is a lot of mathematics in physics. It's what deters most people from it. It's just not intuitive to most, including myself. I know what you mean about everything being interesting! I want to study psychology and linguistics and literature! There's just no time for everything :(

[@ronaldderooij1774]

There is time, only there are different priorities to deal with time. :-)

[@peersvensson9253]

You can make the success probability arbitrarily high by nesting MZIs

[@someperson6343]

my brain exploded

[@neillibertine3044]

I think it need to be rephrase this hypothetical (so it's clear that it has nothing to do with reality as such but attempt to find that if this predetermine something random) example. If intensity of light is more then probability of light at detector is 100% due to interference and which is double of otherwise. Detector B is excluded and probability is 0% and again half of if there is no interference. If intensity is reduced to level of single photon and photon is detected at B which ensure that there is no interference, there are two possibilities; either 25% chance that bomb is live or 25% chance that bomb is dud. But as total probibility of photon at B is itself 50%, so net probability of bomb is dud is 50% and is live is 50%. But does it gave any more information that already had like there is 50% chance of bomb is live or dud. And after this experiment result is same, so this experiment didn't gives any new or more information.