Counterfactual Quantum Computation is Really Weird
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- Äas pÅ™idán 19. 05. 2024
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Events that could have happened but didn't, so called counterfactual events, are but a mind-game in our reality. In quantum reality, however, they are quite real indeed. In this video I explain how this can be used to do a computation without actually running the computer. This is called "counterfactual computation" (or "counterfactual quantum computation" more specifically).
The key papers which I mention are:
Counterfactual Computation
Graeme Mitchison and Richard Jozsa
royalsocietypublishing.org/do...
Counterfactual Quantum Computation through Quantum Interrogation
Hosten et al
www.nature.com/articles/natur...
The two papers I mention as further developments at the end are:
Counterfactual Quantum Cryptography
Tae-Gon Noh
journals.aps.org/prl/abstract...
Counterfactual universal quantum computation
Zhu Cao
journals.aps.org/pra/abstract...
The specific setup that I use is discussed in more detail here:
The Impossibility of the Counterfactual Computation for all Possible Outcomes
Lev Vaidman
arxiv.org/abs/quant-ph/0610174
00:00 Intro
00:38 Quantum Mechanics is Simple
02:00 Lasers, Beam Splitters, and Photons
05:47 Interferometers
07:09 Counterfactual Computation
10:23 What Does It Mean?
13:07 Quantum Mechanics with Sabine on Brilliant
#physics #quantum #science - Věda a technologie
![[Vowel]ë¬¼ê³ ê¸°ëŠ” 물ì—ì„œ 살아야 í•´ðŸŸðŸ¤£Fish have to live in the water #funny](http://i.ytimg.com/vi/3G9MH063Cx4/mqdefault.jpg)
Sabine, even when I don’t completely understand every aspect of your videos, they always leave with this marvelous, almost childlike, sense of wonder and awe at this extraordinary universe in which we live. Thank you so much for this gift!
I wish I could say it better... I can't. I want to add my gratitude as well.
I wish I could say it better.. I can't I want to add my gratitude as well
@@chennaikaraja8391 nice comment
If you don't understand bs it's a sign of sanity.
I wish I could say it better (but) I can’t. I want to add my gratitude as well
“counterfactual is like my husband mowing the lawn.†I love her dry sense of humor.
“counterfactual is like my wife reparing the car.â€
My wife would say that there is a ZERO percent chance of me mowing the lawn…
@@tomgideon3439 oh, I imagine it is very conditional. Possibly determining whether dinner gets cooked or not, or his cohabitation arrangements...
@@tomgideon3439 I know, just joking mate.
It's funny, but feels like another blow to the concept of free will being real. Any thoughts.? Anyone...
1:08 Math is simple, Meaning is difficult
1:25 A Psi Wave Function
2:34 Lasar hits Beam Spliter
3:30 Photon Split in Half
4:48 Collapse of Wave Function
5:51 Interferometer
7:10 More Beam Splitters
8:17 Yes/No , No - photon ends
9:45 Splitting Up
10:30 What does this mean?
11:17 Computing with Not Computing
12:37 It works
Photons can't be split since they are massless particles. Maybe you are thinking of some type of wave.
@@mikemondano3624 4:35
So simple eh?
The more I learn about this weird Quantum Mechanics thing, the more it sounds just like the rules for magic to me, thanks for the video.
Time to start doing counterfactual bitcoin mining to get rich.
I think quantum mechanics is as close to magic as reality gets. hahaha
Yeah, if magic/miracles exist they surely derive from counterfactuals interacting with the meddling with what we experience as instantaneous but isn't. The key to invoking such stuff could be "pretending". Of course, controlling the results would be nigh impossible. Like Bullwinkle's magic. He failed at his trick every time, but pulling, say, a lion out of a hat was real magic (at least in the cartoon).
@@trucid2 hahahahaahhahahahahauaha
Then there was a cat named Schrödinger. He was chased by a dog until he ran through a beam splitter. The result was two cats, Schrö and Dinger. They're still waiting to recombine. Now you have the tail of two kitties as related to me by that dickens of a guy named Charles. Sorry about that. It's time for me to check into the Science Asylum. I'm told the guy who runs it says it's OK to be a little crazy.
Great video, Sabine!
"Maybe the Laser Fairy brought it." Just the first of several perfect little jokes sprinkled through the learning. And *this* is why I love your videos, Sabine! Thanks for all you do, including your wit. You're very appreciated! â¤ï¸â¤ï¸
I'll take that 5K piece puzzle, though! I've got plenty of time on my hands, and lots of things I want to forget, due to some painful things beyond my control. Jigsaw puzzles are helpful, as they keep my mind occupied.
This all seems inexorably tied with the Bell inequality. Or from the superdeterministic point of view, the lack of statistical independence between the computer and the detector.
I feel like there is a deep symmetry at work here.
I think this is the most astute comment so far. Bell pairs are entangled, so act non-locally (a simple semi-classical GR model being ER=EPR). So it's not the computer "James" deciding the question, it's the entire system as a whole, one of the Bell pairs has to go through the computer. The real question then is _can a single photon (which is not a Bell pair) nevertheless behave like a Bell pair ?_ --- i..e, does a photon (or any other quanta) contain in itself some wormhole structure (a minimal ER bridge perhaps)? If so, then entanglement explains it all.
fwiw I think this is the key to QM, the postulate is that the spacetime topology is determining most of this weird particle phenomenonology. The Standard Model gauge groups crop up in the full spacetime algebra (STA) too (multivector algebra or Clifford algebra) and a point-particle has trivial topology, so the SM cannot be "got" from the STA with point-particles (fields). This suggest (to my simple mind) that the elementary particles are not point-like, and also not stringy stuff or p-branes, but must have some internal non-trivial topology (Mark Hadley's 4-geon idea from 1996). Combine that with Maldacena and Susskind's ER=EPR and you can almost see QM is GR (or GR + Planck scale wormholes). Mark Hadley had this idea a decade before Maldacena, just for the record.
I believe so too.
Speaking of bell inequality and super-determinism - (it will be maybe a lengthy digression, but a necessary one)
Say we set up a "typical EPR" setup. Lab "A" and "B" and "C". The "C"enter sends its entangled particles to "A"lice and "B"ob and both are supposed to do their measurements at the same time perceived by Center.
Let's add two spaceships flying in at near light speed from A-B and B-A directions.
While Center says Alice and Bob simultaneously did their measurements, one ship's captain will claim Alice was first, Bob was second, the other will claim the exact opposite.
We get to super-determinism - it all exists and we are just fooled by our perception of reality.
Here - we can only observe the computation's result if it is part of the system, just like the presence of the second slit "creates" the interference pattern in the double slit experiment.
Which seems to make sense in the super-deterministic point of view, just like with the double slit experiment.
@@BartoszChmura Now, I may be missing something, but I don't see how that setup would result in super determinism, as while they would measure at different times, the output would still potentially be random, as both the USS Allice Observer and USS Bob Observer, while they do know what state one is in, and so the other pair, they are indirectly receiving this information.
By that logic, if someone told you what state one is in later on, after seeing one for yourself, that also backs superdeterminism, but it does not seem like the system described violates or particular weights in favor of that, in fact it doesn't seem like it establishes much in the way of new information. But ,maybe I'm misunderstanding or missing something, or maybe just plain wrong or illogical about something in the first place.
@@BartoszChmura It seems like you might be flying over some potential explanations, those being that from some reference frames the waveform collapse is finite in propigation speed, or there being one objective reference frame for the entanglement to rely upon, as if one did really undergo collapse, it might result in the prospect that the collapse is uncertain in time as well as space, just to throw some ideas out there that might explain the apparent non-simultaneous collapse.
Though the waveform collapse would have to be finite from the perspective in one reference frame, but not another.
Read the paper, “The wave-function is real but nonphysical: A view from counterfactual quantum cryptography.†Interesting read that’s backed by a couple of subsequent experiments.
Shenoy & Srikanth's definition of "nonphysical" is nuts. From their paper: _"...nonphysical in counterfactual cryptography when Bob’s communication generates no detection on his apparatus."_ So it is just semantics. I can show you a physical model in GR where a communication from Bob does not generate a detection event at Bob's detector. So is GR "nonphysical"? No! Shenoy & Srikanth are better off sticking to an epistemological interpretation of ψ.
Further: Shenoy & Srikanth admit they are just making a definition: _"Our approach allows us to define not only a concept of “realityâ€, but also another, that of “physicalityâ€. We think that this is crucial, because our work shows that the distinction between physicality and reality lies at the heart of what makes the quantum state nonclassical"_
It is possible I cannot say for sure) Shenoy & Srikanth are not considering the ER=EPR conjecture, which is realist and physical and permits the operations their paper describes.
@@Achrononmaster This is wrong. There have been several experiments that supports the paper. Here they are:
Direct counterfactual communication via quantum Zeno effect
*Abstract*
*Intuition from our everyday lives gives rise to the belief that information exchanged between remote parties is carried by physical particles. Surprisingly, in a recent theoretical study [Salih H, Li ZH, Al-Amri M, Zubairy MS (2013) Phys Rev Lett 110:170502], quantum mechanics was found to allow for communication, even without the actual transmission of physical particles......Using a single-photon source, we experimentally demonstrate the counterfactual communication and successfully transfer a monochrome bitmap from one location to another by using a nested version of the quantum Zeno effect.*
Direct counterfactual quantum communication protocol beyond single photon source
One more:
Trace-free counterfactual communication with a nanophotonic processor
It goes even deeper because we know that what we call subatomic particles aren't particles in the physical sense. QFT tells us they're excitations of quantum fields. Here's papers on that.
*No Evidence for Particles*
and
*There are no particles, there are only fields*
I will leave you with a quote from Werner Heisenberg:
*“[T]he atoms or elementary particles themselves are not real; they form a world of potentialities or possibilities rather than one of things or facts.â€*
*― Werner Heisenberg*
@@Achrononmaster Yes, GR is idealist...
And no, I'm not idealist.
10:39 "if the photon goes through the inner part"
Isn't the whole point of the double slit experiment that the photon isn't picking 1 path, it's taking every path at the same time and only decides where to pop up when the wave meets a detector?
So even if the photon appears in D1, the wave still went through the computer?
Yeah, seems like its the same as double slit experiment for particles; photon, e, n, atoms...
This was exactly what I was thinking too
@reInstalled in the double slit experiment there are two paths (the first slit and the 2nd slit). And we can't say of any photon that "it went through the first path" (or the second), because if each photon was getting through one path, there would be no interference patterns. So we say photons are really waves, going through every path, until they hit the detector. Here there are two paths (either go through the first lens towards the inner interferometter, or bounce off of it). But Sabine is saying "if the photon goes through the inner part". Speaking of photons like they're only picking one path.
@reInstalled grow up and then read again
This channel is a shining beacon in the quantum field of random disinformation! Please continue doing what you’re doing!
now I'm thinking what a scientific feild called "quantum disinformation" would be about
Not a deepak fan i take it
I read "shining bacon" and I agreed that this channel is as appealing as a shiny strip of greasy but crispy bacon.
Then I read again...
Thanks for bearing with me
We cannot have a field equation and ignore it at the same time. One of the questions that we need to ask 'James' (the photon computer) is: How does the computation change the path-length (phase) of the photons that pass through? Will they all have the same delta? And we really do need to ask about the photons that do not pass through ... Are they absorbed? reflected? ... In any case, introducing James to the apparatus changes the wave-function because it introduces new boundary conditions.
This.
Yes: Sabine is wrong, it’s shockingly nuts! She says at the end it is a tabletop experiment on a computer.
The “experiment†is based only on splitting “a photon†and reconnect the same photon(!), multiple times.
It’s religious nonsense.
@@eb4661 You realize physicists do beam splitting a whole lot, right? Like, this is something people actually do in real life, right?
@@UsernameXOXO I have a beam splitter (1/2 and 1/2) and a laser, so I’m very familiar with the basics of a beam. Certainly also interference patterns.
The tabletop experiment is done on a computer program, taking into account those rare (one in a billion splits of a single photon, and another one in a billion to reconnect with itself). Then, if this happens in a series and is measurable, probability statistics prove the claim. The obvious problem here being the premises, and lack of actual experiments.
You can find some of these online where the photons is shown graphically and with your choices of setup.
Been catching your vids for a bit, but this one is da bomb. I have to view this a dozen times to get just a glimpse of what that heck you are explaining, but putting in the humor is just over the top, as the humor is sophisticated but creates a wonderfully bright juxtaposition of/for the overarching communication of quantum phenomenon. Please create more like this.
I absolutely admire the way that you explain things. You never make me sigh.
My jaw literally dropped when you got to the part where the photon that indicates the result of the computation must have taken the path that does not engage the computer. That is stunning. Thank you for this explanation and for the gentle humour you inject into each of the topics you cover
It seems to me that the claim that if the photon doesn't pass through the computer, then no computation happens, is simply incorrect. The wavefunction still passes through the computer, and interacts with the computer's wavefunction, and this interaction performs the computation, not the photon.
I love how you jumped into the topic directly. More CZcamsrs should do this
I love this channel. You're one of my favorite science communicators and you cover the most fascinating concepts. Do you have any lectures on video?
the "how does it know?" part reminded me of high school when we were learning about snell's law. the teacher said it turns out that the path the light takes is not the min distance but the min amount to time to reach the end spot and compared it to being on a beach and seeing someone drowning in the water, we don't run straight to the water (perpendicular to the shore) and then swim to the person but we run a ways on land and then swim the rest of the way because we know from swimming that it's slower. so the question was "how does light "know" that too?" so is there something similar going on? where the rules of reality seem to impart some skill or knowledge to otherwise brainless things?
[oh! and in the first part where you describe counterfactual as something that doesn't happen affects what does happen made me think of the origins of "alternate facts" where things that aren't true definitely have an effect on reality, but i think that's more a phenomenon with society/psychology/misinformation 😞]
@moi2833 exactly; it is a geometrical problem. All non-optimal paths do cancel out.
What you’re referring to is called the Principle of Least Action
@moi2833 i think that's what makes these things confusing is the "meaning" step..like light "knowing" something, or the word observer sounding like someone now has to "know" something about something, it all seems very mystical/magical which probably isn't the right way to view things 😀
@@BM-rm7vr i'll give it a looksee/search! ðŸ‘
Path of least resistance. Energy is a form of pressure which creates effects such as wave, particle or quantum effects.
I feel like this video sparks more questions than it answers. What does it mean to "run" the computer? If the computer works by having a maze for the photon where the layout encodes the problem, then there is nothing to run. The solution already exists as described by the wave equation and we just need to measure it by sending a photon, and it "costs" us the same regardless of which path it takes. If light behaved purely as a wave with no quantum effects, it feels like the outcome would have been, on average, identical and nobody would find it weird. I guess the weirdness comes from the fact that what gets quantized are our measurements of where the particle ends up, rather than the particles themselves? But isn't that the same concept as the famous double slit experiment, just said in a different way?
Running a device to produce a binary outcome (results differ based on parameter set internally when it actually runs/ is triggered by photon) costs energy, and the simple fact that when we detect photons at D1 is still amazing: for that exact moment (that is: the counterfactual moment when it arrived the machine and triggered the computation ) we know the result had it taken the other route ... I find this version more fascinating than the one using the double slit setup.
I love you Sabine. I don't have a degree in maths or sciences but have always had a love and fascination with them. Your channel helps break things down and make them understandable without stripping away too much to the necessary information needed to 'get it' without actually understanding it completely (which I've heard means you know less than you think you do). Keep up the amazing work!
Hi Sabine. Thanks for your enlightening and light hearted videos! I am now reading your Lost in Math book. Very smooth reading!
This is a great follow up to the quantum games video and totally new to me. QM has become so interesting to a non physicist like me, I look forward to hearing more from you.
Thank-you for this. A new Sabine video always brightens my day, and this one - the wonderful humour and material - was exceptional for me.
I get to see her videos at night only and it gets me sleeplessness.
I've been patiently waiting for sabine to make a video on this topic. It's about time
Thanks for another great and funny video! BTW, my copy of your newest book has shipped to me, and I'm excited to read it! Thanks for all you do!
Come for the science; stay for the jokes.
As I told a comedian at a strip club in Vegas: I'm not here for the jokes
Oh the jokes and science are good, sure, but other people do that. What she does is challenge her "The Guardian" and "The New European" reading audience and that really is rare. I would like to a see video where she tests how far she can push them, just for science of course 😉 Climate, green energy, even trans issues now that really is brave.
sigh
@@FreeApophis Obvious Guardian reader
OK, you finally persuaded me!
I just signed up for my free seven days on Brilliant, and just watched your first lesson and it was really good!
And I am a visual learner so the graphics really helped me to better visualize all this,.
And even though I already knew the material it still helped to do that.
For once I see the graphics I can then picture it in my mind. Then the math starts to make sense and is not just a manipulation of terms but a succinct way to describe what I can then see.
Does it end with an exam tho? :D
there's no reason for thinking that
@@empireempire3545 No.
But There are short quizzes along the way.
BTW, I do have a comment on lesson 1, Interference, in regard to the interactive figure labeled:
'The picture shows the length of the wave, measured in multiples of its wavelength. Find the edges of the central bright spot'.
It would have been helpful to have a third handle that varied the slit size so one can see how that affected the beam width.
Also, figure numbers might have be good to add as well for easier reference.
More feedback.
On the lesson that has the animation that follows the text:
'Thanks to superpositions, we now understand why 25\%25% of the laser light makes it through to the other side'.
the animation is hard to follow and needs a step button and a pause button in addition to a play button so one can step through the animation at their own pace.
I am glad Sabine is here. YT is getting full of click bait videos. She brings to light what others refuse to address.
Most of these videos hurt my brain with smoke coming out my ears, this there again some smoke and I also had a few good chuckles. You again were brilliantðŸ‘. Thank You
"Like my husband mowing the lawn"
Oh boy, someone's in trouble.
got mowed, but in a different reality :D
I was going to do some counterfactual quantum computation.--but then things got really busy at work.
Let me guess . . . . you work in a mirror factory 🤔
@@not2busy Well my mirror image self DOES work in one of those factories, yes.
What work does a zebra do?
@@CAThompson It repels flies.
I first encountered her today (Aug. 11) as I caught the tail end of the interview on NPR about her new book, "Existential Physics." I plan on purchasing it, not merely because the subject matter has long been of interest to me but also because in my research on her, I found we have the same birthday. Happy Birthday ... early, Ms. Hossenfelder.
The first sentence in this video is the pinnacle of quantum comedy. Also "You can compute with a computer that doesn't compute". Priceless.
5 hours after being uploaded and this vid has almost 20 k views, and over 3 hundred comments. Now this is really a quantum leap for a science channel in YT, I wish you all the best for this amazing achievement, keep learning anf the teaching for the rest of us!!! Thanks
But oddly, comments on the actual subject are rare. It's like Hawkins Brief History. I'm told it's one of the most not-read books every sold.
I think the video would benefit a lot by inserting, after the sentence about interference that begins at 6:06, a description of how the two recombining beams would interfere if one or both of the beam splitter's probabilities are something other than 1/2 + 1/2. This is needed to understand the outcome of the recombining in the case where food nazi James doesn't like pineapple on pizza.
"food nazi James" lmao
I do not understand how does that matter. say x and (1-x). wave functions will have weights x^(1/2) and (1-x)^(1/2). Interference will depend on the path difference not the amplitude.
@@janami-dharmam : I don't agree with your argument (assuming I understand what you're saying). Interference is the addition of two (or more) wave functions, so how can the sum not depend also on the coefficients (amplitudes) of the terms (eigenstates) of the wave functions? Completely destructive interference occurs when the two terms being added are both opposite sign (180 degrees out of phase) and equal absolute magnitude so their sum is zero.
I understand what you mean, but I couldn't think of any good way to summarize this calculation quickly. Best explanation I could think of would have taken at least an hour if not longer. And seeing that it isn't necessary to understand the main point of the counterfactual protocol, I decided against it. You find all the details in the references that I added below the video.
@@GamingBlake2002 James is quite simply an Italian.
Love the videos, and how effectively and efficiently you explain your subjects.
Thanks for the explanation Dr H.
The second time I watched you depict the counterfactual result, I could conceptualize it clearly, for about 5 seconds.
Nice feeling!
Amazing and mindblowing. I'm inspired and frustrated at the same time.
I believe “Counterfactual†anything may just be another word for Entanglement. Now I’m more of an amateur physicist (math PhD), but it seems to me that when we say a system is no longer entangled, it is only that there are two many entangled particles for us to keep track of, and so no longer a “pure†entanglement that we can parse or compute/validate. But does that mean it’s not there?
I would really like a physicist to explain clearly why in complex systems the answer is “we cannot preserve entanglement†and not “everything is too entangled for us to make sense of anymoreâ€. Perhaps our definition of Entanglement is too limited?
It just seems as though all of these “counterfactualâ€, “spooky†non-local effects could be chalked up in simple terms to the facts that the summation of local quantum particle interactions creates a kind of non-local web of interactions/dependencies, which en masse and at the large limit appear to average out and give us the macro world we see. But as we go further from that limit, entanglement plays a bigger role. It seems like it’s always there in every interaction of every particle, but just like charge spreads on a sphere to cancel out internal field, entanglements are spread out an effects cancel… but where there are asymmetries I expect their effect is still present, although negligible.
In this experiment in a highly isolated environment, I think the “counterfactual†piece is just a macro result of our lack of understanding/information about the entanglement that creates the effect. Entanglement is the non-local, faster-than-light effect that pushes our logic and reality beyond the classical.
I do like to imagine a web, almost like a huge loom across space and time, of entanglement. Averaging limits the effects. I’d love to see Sabine or PBS Space Time do an episode.. “Is Entanglement Everywhere� I would like to know in what sense this view is right or wrong. I don’t see it as magic, it’s an extension of our classical understanding of reality. But still an ever-present, shifting web of immediate interaction across time and space paints an incredible picture of the universe. Is it real????
You seem obsessed with semantics, rather than the concepts themselves. Speaking casually about things going "beyond" classical logic and non-local effects as if that's perfectly comprehensible and without contradiction to human brains.
PBS Space Time already did a video on Quantum Darwinism and has done some vids on decoherence, which sound like what ur talking about
It's only a fact if it's been detected. Otherwise, it's a potential. Outside of this, macro concepts like time and reality don't really apply.
Entanglement requires at least two quantum objects. Here the outcome for a single object is influenced by what could've happened but didn't happen to it. One could call it an entanglement with the same object in the alternative reality instead of an entanglement between two objects in the same reality.
@@yurypetrov5383 No, one could not call it that, at least not with evidence in hand. Alternative realities have not been observed and aren't necessary to explain the phenomena.
this kind of "paradox" only tells me we undertood something wrog or something else is missing in physics...
3:23: unless I have completely forgotten my QM, Schroedinger's equation cannot describe the behaviour of photons.
I have a silly question;
About the output of the computer, if beam hits D1 and we know the answer without computation, in that moment, entropy of the system is the same another with the working computer? I mean, output information is a result of change right? So this change is exactly the same output for the engineers, but system too? If it is, is that mean the "standby" computer effects surroundings exactly the "working" one?
Thanks for the answers in advance.
What does it mean in a simple language?
If you want an answer, you need to learn how to write a question
Today I learned how the laser fairy carries a plastic plant on a long hike, and then collapses...or something like that. (What do I need to leave under my pillow so that the laser fairy will visit me?) And James has conditional Write-Only Memory.
Maybe I should have some coffee before watching these videos.
Fascinating! Thanks, Sabine!!! 😃
Stay safe there with your family! 🖖😊
This was awesome â€¼ï¸ love the easy to understand examples
And James is pretty cool too 😎
Does counterfactual computation have anything to say about whether the Copenhagen or Many Worlds interpretation are true? Could the counterfactual computation be a sort of ‘signal’ from the other world? Asking because this was also an intriguing part of Neal Stephenson’s Anathem. Also, is it just me or was Sabine’s humor especially good this episode?
I didn't laugh at Sabine's humour, but i know it's funny.
Afaik, the different interpretations of quantum mechanics all produce the exact same predictions
The brief answer is no. Counterfactual computation is about different realities *before* measurement. Many Worlds is based on the idea that these realities continue to "exist" in some sense *after* the measurement. (I explain this in my new book btw).
@@SabineHossenfelder Counterfactual computation was discovered after Many Worlds was proposed. Would you guess that Many Worlds advocates or collapse theorists were, on average, more surprised about this innovation?
@@SabineHossenfelder The value here is you've generated information in violation of boltzmann statistics. This should not be dismissed. There are now over a dozen ways in which statistical laws can be violated and I'm starting to believe entropy can be reduced if we employ enough methods coupled with ingenious engineering.
Sabine, your sense of humour is amazing, I had to go back to the beginning because I was laughing so much I missed some of the explanation. Thanks again for making these wonderful concepts so clearly understandable.
I watch all your videos, been a fan for a long time. Thanks for what you do, I loved the book.
This is so much better than when I first learned about it from an hour long talk on youtube and then tried to code it up on IBM's quantum computer to see what happens.
Non-reversible computing is supposed to increase heat (or more precisely entropy) by a particular minimum amount somewhere. Does this happen to the same amount with counterfactual computing in the case when you happen to get the answer in 1 (or in the aggregate?)? And which part of the system heats up? Is it "James" ?
If you take a 32-bit register, load it with a value, then shift it left by, say, 4 bits, where did the 4 bits "go?" In other words, where did the information "go" at the quantum level? Isn't this an irreversible loss of information?
@@KendraAndTheLaw the question it too high level to answer. is the register made of capacitors or resistors with supplied current, does it refresh, how much capacity if you have all the data related to the electrical circuit and impossible to measure data, like difference in temperature in the environment due to the supplied current on the material that contained the information you might deduce what was there - so the information is not really lost, just obfuscated by your lack of knowledge of the system. not unknowable, just unanswerable. Perhaps with a quantum bit register it becomes what you say.
Somebody's been watching Up an Atom.
@@KendraAndTheLaw My question was not about information loss, it is a question about entropy.
And also, to answer your question, the information went lots of places, it radiated out into the universe in the form of electric and magnetic waves, it sent ripples of small voltage fluctuations through your ground line, and also to all of the other conductors in your circuit that are tiny and get "lost" as heat to the resistance of the wires, and I'm sure a thousand other minute effects. Just like with the burning of a piece of paper.
But again that's not what I'm talking about here. For more info:
czcams.com/video/jv2H9fp9dT8/video.html
@@MagruderSpoots lol, youtube did serve that one up to me recently which I guess might be why I thought to ask, but I was thinking about this video as I was asking it :P czcams.com/video/jv2H9fp9dT8/video.html
The problem with these interferometer examples is they are always dependent on EXACTLY how the experiment is set up...and that level of detail is always missing...
@Matthew Morycinski Not really, the issue is simply in the fine detail which is typically omitted as not relevant when in fact its critical.
It’s even worse:
There isn’t a set-up! Sabine states at the end of the all the gobbledegook nonsense that it is a “tabletop experiment done on a computerâ€.
Normal people would reject QM if such claims as this was taken as proof. Not so in the cult, it’s insane.
@@eb4661 QM was 'invented' because reality didnt agree with the expected results of though experiments...
For instance here it is suggested that a result a D1 can only indicate something about the decision computers response, when in fact you will get a result at D1 1/9 of the time regardless of any indication from the decision computer. So a result at D1 says nothing about the rest of the system and the thought experiment proves nothing.
So a thought experiment can end up proving nothing other than your own logic failings...
Set up as a real experiment it gives you facts to deal with not assumptions, and science should deal with facts not assumptions.
Great video showing some of the mind boggling QM.
Could have been a plus to have the different valued beam splitters in individual colors.
How has it taken me so long to find Sabine’s CZcams channel?? I’ve had an interest in science as a young kid & refined that down to physics by the time my GCSEs and A Levels came around. I am 39 now & still sit here fascinated as I read or watch videos on the absurdity that is the post-Einstein world of physics.
Physics aside, Sabine has a great understanding of British humour. Very witty, often clever, generally dry humour, with a dash of dad jokes before mixing up.
Only a recent subscriber, but keep up the good work Sabine. I find when it comes to difficult concepts, no one can break it down to such an easily digestible level like she does. This channel has been my recent go-to nighttime binge watch
Plants (-) Dogs = Plastic Plants / (5000 piece puzzle on shelf)
Laser (+) Cats = {Plastic Plants (+) 5000 Puzzle Pieces (+) Coffee Cup} / Floor
Damn Schrödinger's cat, should have bought a dog and replace your live plants every 3 months.
This is my type of computer, one that does work even if you do not ask it too! its like the quantum cryptic unbreakable "key", you can make the "key" obsolete just by looking at it, this will let the receiver know you have peeked at the secret code " Alice n bob ", therefore making you continuously change the unbreakable code, even if you do not want too?
I had such a computer once. It was known as Mother.
Excellent video! Once again, I'm digging the humor in these videos!
I recall a Scientific American article describing a way to image something without actually hitting it with photons, a way to detect Medusa without actually looking at her.
11:01 you say the photon reaching D1 didn’t go the other path. But it did, it went through both paths and only collapsed at D1 this time. So it actually went through the computer as a wave function. I don’t get why this is so different from usual quantum computing. Please explain me where I am wrong ! 😊
@@TK-ni3qe all particules are waves functions so yes waves are both real and significant :D
Go check quantum field theory
Yes, the wavefunction went both ways, and part of it hit the computer. As a result, computer got entangled with the photon - in parts of the wavefunction where the photon hits the computer, the computation is run and energy is used up, and in parts where it doesn't, no energy is used. Then the resulting wavefunction recombines again and is finally measured by the detectors. The wave function collapses, and let's say it collapses to "photon at D1". But the photon is entangled with the computer now, and collapsing at D1 also means collapsing the computer into "didn't run" state. So you end up detecting a photon at D1 (which is only possible if the answer is yes) without expending any energy at the computer
@@silentobserver3433 okay I see, thanks for the explanations. I should dig more about what kind of computers they use to understand.
My question is: doesn’t this kind of phenomenon happen all the time in usual quantum computing ?
If you try to assign a classical path to the photon, considering that it is a particle that can't be at several places at once, then, to reach D1 the photon must have gone through the lower path, and not the computer. But then Hardy's paradox has shown that this kind of common sense interpretation does not mix well with QM.
There is a way to give more meat to it using weak measurements. That is measurements that give very little information on which path the photon took, and thus don't disturb the setup a lot, but that with enough repetitions allows to recover what can be interpreted to first order as a kind of undisturbed path information (By taking the number of repetitions to infinity, and then taking the limit of the weak measurements strength to zero).
I say a kind because among other weird things, doing so can yield negative probability events : photons can be in several places at the same time, because they can be negatively somewhere else.
In this case, when we get a click in D1, the result is that the photon neither enter or leave the inner interferometer, it is negatively one going through the upper arm of the inner interferometer, and going through the computer arm of the inner interferometer, and going through the lower path.
Using this approach, there is no counterfactual computation, although one can argue that the limits taken are quite artificial.
arxiv.org/pdf/quant-ph/0610174.pdf
core.ac.uk/download/pdf/44160185.pdf
arxiv.org/pdf/quant-ph/0104062.pdf
Thank you for a fun and insightful presentation on a fascinating topic! Question: Would the paths in Feynman's history integrals count as counterfactual reasoning? After all, a photon "knows" where to land at the back of your eye only because of the vast number of paths that it did _not_ take (did not leave any historical trace) as it passes through the lens of your eye.
I'd like a answer to that one too
Interesting you would ask, I have scratched my head over this myself. In the end it depends (as so often) on how you define the term. You could take the point of view that since those paths contribute to the entire amplitude they are somehow "real" hence not "counterfactual". But this only makes sense if you think the wave-function is ontic to begin with, which is questionable given that we can't observe it. On a similar footing, the question whether the counterfactual computation is actually counterfactual has been debated back and forth in the literature, but in the end the term stuck.
@@SabineHossenfelder Let's suppose we live in an universe where it is ontic. How would we proof that? (Or Popperish, how could we fail to disproof that?)
@@georgelionon9050 ontic is not a word in any English dictionary. Please don't do that.
@@thumper8684 Actually it is, look in the dictionary.
I am glad I found this channel..I have lots of questions about stuff..
My dear Sabine. I love the way you stretch my brain. I always look forward to your newest post. Stay strong, free and healthy.... yours 💛
You know that someone ate the cheese, if only holes left over.
Counterfactual mouse-trap ^^
Maybe, with that, you also could proof the anthropic principle.
The quantum mechanical version of it, is called participatory anthropic principle.
Without us (human detectors), the universe never ran.
Physicist seem to have amazing requisition departments: here we have Sabine telling us to just assume we have a laser, while Simon Clark recently told us to just assume a spherical cow. Where do you get all this stuff? Must have something to do with the multiverse… 😉
I have a laser tho
I'm assuming either a hamburger or a chocolate bar.
You'd be surprised how much stuff you can pull out of a frictionless vacuum.
@@lrwerewolf 💩💩💩💩💩
@@lrwerewolf Not to mention how fast it can accelerate.
Ma'am you have a phenomenal intellect and your humor is spot on!!
I just love the sheer absurdity of the setup, photon wave goes through the interferometer maze and with the power of calculated, dumb luck, it crashes head on with... itself, skipping the program path, and going straight to the correct output pathway as if it had calculated it. and concluding the program performs better when not running it
But its true at face value, some discoveries were just made by negligence and good luck
dear sabine, you made me laugh, while shredding me to parts with quantum brainfood. i bow in respect and keeps smiling, sabine, you got our back!
I prefer these types of presentations rather than some you have done recently. This one is worth watching several times and worth keeping in the archives. Pineapple plus pizza equals yes no matter how you split the pizza.
Horrific! Pineapple and pizza should ALWAYS be an empty set
Pineapple and pizza requires the the pineapple to be dried (at least a little) beforehand, as soggy pizza is not allowed.
This is your best video to date! BRAVO!
During your clarity, yes would be lovely to see what you’d have to say through your mind upon social standings and our backspace
So let me get this straight, you could program the computer to perform a task you don't know will halt (as per the halting problem, such as calculate all the digits of PI), and use the counterfactual to find out the answer without even running the program?
🤯
I think its more like detecting the state of a switch
Well, the switch will be on if the program halts (digits of PI has finished being calculated), and off if it doesn't.
Yeah, seems to me you can decide the halting problem this way, on the other hand, you still wouldn't know when it actually halts and what is the result. This is really blowing my mind, because logic states that solving the halting problem is impossible, on the other hand nature seems to have a different opinion.
You have not really solve the halting problem. Think of it this way. You have a program that you do not know if it will halt. So you run the program directly and check 10 minutes later if it halted. Run the same program using this counterfactual setup and the result is the same as running the program directly.
But how do you know, if the program works correctly. In a program you can have an infinite loop, caused by a coding error, that adds nothing to the solution of the problem(digits of pi). To fix the bug, you have to run the program. If you do the proof, that the program works correctly, you have done the work beforehand, too.The quantum setup tells you only, that something, or anything has one of two states.
Would be funny if counterfactual computation could solve the halting problem 😜
I was thinking the same thing, because it kind of sounds as if it could
This is treated in a little more detail in another comment.
I really like the explanation that the wave function is a machine or like a packed value until it is observed and unpacked. Different things can happen to it like a fragile package that you sent to Aunt Martha but we can't know if or how the package was transformed in transit until we unpack the value
Sabine, I love your light devilish smile (in a funny, tongue in cheek way) in the beginning of the video when you say "that is what we talk about today!". Always makes me chuckle.
Thanks for your always really interesting videos for us interested layman or laywoman!
As always, great and interesting video. Thank you!
But I do not get the excitment at 11:00. Of course the photon that hits D1 knows the answer is "yes". If the answer is "no", the photon aiming at D1 is hit by another photon from computer and both destroy each other (destructive interference). Since the "photon destroyer" does not arrive, the answet must be "yes". Where is magic here? Am I missing something?
Actually the destructive interference in case of "no" is more magical. What happens to energy of the photons?
There are only apparently counterfactual events here. Complete collapse of the wave function is subsequent to detection at any of the detectors. That is, once the photon is absorbed in D1, D2 or D3, the photon decides which path it followed based on the set up of the experiment during the beginning and end of the experiment. In this way, at the end of the experiment a kind of retroactivity occurs since the processor computation happened, but was later "removed". From the photon's point of view, all the events from when the photon is emitted until it is absorbed occur simultaneously and immediately since their speed is infinite. It does not matter that the experiment set up is modified by free will during a test, since the result is already known to the photon because it has already occurred at the moment of photon emission. Why we humans interpret reality in a "relativistic" way, I mean, spatio-temporally, must have to do with how the mind works. But at the level of quantum systems, it makes no sense to talk about events or a timeline.
Can you read my comment? I’m curious what you have to say about it. It seems like you are talking about entanglement and faster-than-light effects being the real source of our perceived “counterfactualâ€. Thanks
@@zepkid5678 Exactly, you have understood even though there is no full explanation for it. It seems that quantum reality exists before our deterministic reality or macro world. When I say "before", I mean that it seems that the "classical" world is a brain interpretation of quantum reality just like the theory of relativity from which it is derived. Although we cannot understand that reality that is below, we can intuit, thanks to experiments like this and others, that speed limit laws do not apply to it, because in reality space-time is also an interpretation of it. In this way the wave function collapses at the end and the photon "changes" the path it followed to adapt it to the result of the experiment. This later "change" in the path is what shows that the macro world is an interpretation of a reality that is below. Other experiments like the DCQE also show the same thing. The most important thing is to keep in mind that from our point of view the photon did pass through that path, but that path was later eliminated. I have read your post, I particularly agree with the second paragraph. It would be difficult to follow the argument here.
Much of what you cover is over my head. But, i enjoy hearing about it and learning.
In the universe I live in, I'll be attending Sabine's lecture in a couple of weeks. Looking forward to it 🙂
My experience: IBM offered access to their Quantum Computer. I started simple, 2 + 2. Every iteration, resulted in a different answer. None being 4. Not being a college professor, I quit before I started a trillion computation program. Seeing what the average answer was.
Sounds like the Prolog sort where you generate every permutation, then select the one that is in the desired order.
how many years ago was this? what did you have to write it in?
Sabine is really playing for the laughs in this episode. But I wonder if a universe in which her husband mows the lawn is logically consistent.
It is, but only until the lawn is observed to be unmowed.
They could just get a dog. Presto! No more grass.
@@kensho123456 Wait, were you the computer in the video @ 8:00?
If so, sorry for asking a personal question. 😛
@@kensho123456 Hmm, I wonder if we've misunderstood one another. I was making a silly joke about the computer named James, who didn't want to be asked questions, and disappeared later in the video. I thought you had been making the same joke (about being deleted); but later realized you must mean a deleted comment, when the very next day one of mine was deleted (probably for including an URL, live and learn). Anyway, I edited my joke comment to make more sense, and hope you get at least one chuckle out of it. Have a nice day!
Or agoat
Love this channel. Love Sabine’s sense of humor.
great video as always ðŸ‘
thanks for sharing your expirences with All of us ðŸ‘😃
If you can compute with a computer that doesn't compute, can you think with a brain that doesn't think? This might be how we got the Flat Earth theory.
lmao!
If you start with believing the outcome, then move up the ladder assuming the outcome proves all assumptions of inputs and reactions, you cannot but land on earth being flat.
You just have to start with believing what you see.
It’s the most appalling video Sabine have ever made. Insane.
@@eb4661 I have to admit I didn't completely understand counterfactual logic, but I hung on to the end for the jokes anyway. Still, what I did understand, I imagined was limited to quantum interactions and not to our everyday world experiences. This was probably the underlying reason why I wrote my comment, to highlight the absurdity of extrapolating this concept to the world we can see and feel. What is a brain if not a highly developed computer? Sabine only knows what her husband is doing if the grass is quantum, but if it was it probably wouldn't be green and it probably wouldn't need cutting anyway.
I completely agree with what you say about how people arrive at absurd ideas like the Flat Earth theory, but I don't really think that Sabine really intended to make this connection. She's made so many arguments against this idea, and argued how science is not a belief system, that I can't imagine that this was her intention...
"How to compute with a computer that doesn't compute"? - Burn finite fossil fuels - like no tomorrow.
"Energy, like time, flows from past to future".
Wailing.
If the interference pattern is affected, then the wavefunction encountered (and was affected by) the computer. The wavefunction, being a wave, travels (and is influenced by) all paths.
Wavefunction "collapse" on measurement doesn't erase all of those previous interactions. Those branches of the wavefunction are "regathered" to the "chosen" outcome, but only "after" all of those interactions.
I loved the video sabine, it was a lot of fun. You made a remark towards the beginning in passing, that I think is very profound. That all the mathematics in the world won't help you understand what a dog is, or something to that extent. I think people that struggle with perceiving what quantum interactions actually are would benefit greatly from pondering that. Call it a quantum koan if you will.
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You're right ðŸ‘
I bought Bitcoin and Ethereum August last year and I made 6grand into half a million after taxes. Now I took that half a milli.. basically made me more money than 35yrs in granding within 6 months. That's digital currency
Life is so precious, when you know you own a property or an investment to always keep you on go...I have no regrets starting my journey in cryptocurrency.
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Plastic plants are so sad. Ancient plants became crude oil which gets refined into plastics which we use to make plants.
Can't say I understand how the answer could be "yes" without running the computer. There are two possibilities there: Either the computer computes the same result while its running and while its not running, in which case its equivalent to just a fixed configuration and its not really "computing" anything, or it has different results while its running and while its not running, in which case the path would be selected on a photon-by-photon basis depending on whether or not the computer happened to be running at the time the photon went through the maze. It seems to me that at best, you're answering the question of "is the computer running?"
What would be more interesting is if you switched the state of the computer (either its program, or turning it on/off) after hitting the top-middle beam splitter but before hitting the computer. I suspect that would reduce to something equivalent to the delayed quantum eraser effect, though exactly how the equivalence would be constructed is beyond my abilities.
Obviously I'm missing some insight into how that configuration works if people have managed to actually make this happen in a lab.. but I certainly can't see the knowledge gap based only on this explanation.
Absolutely love your sense of humor. Big fan for years. You have helped me learn much more than I could have without your videos. Thank you Sabine. Looking forward to reading your new book.
First 🥇
2nd, I just happened to be awake 😄
Please stop integrating too many silly jokes into the lecture. The old format worked better for me. Just scrap 50% of all the jokes and everything is ok!
Noooo, they're like pineapple on pizza: extra but great.
It's a matter of taste, like pineaple on pizzas.
time is like a river delta---there is no right timeline anymore than there is a right path for the river to branch out.
every possibility is happening simultaneously.
You said it in a previous video: light “knows†which path uses the less energy to do the work like your husband .
It’s all about field and less energy paths.
The light will arrive in the more energy saving medium, even if it has to split.
Water in pipes function the same way.
The light beam of light just colapses / resolves in the most energy efficient path in the medium among all the probable paths in the field.
It is also posible that for any posible path there is a probability that the beam will resolve through that path, even if the probability is to low.
When you put a barrier, the light will not pas through one point and the light will find others ways to pass, like water or your husband.
Your husband know the loan will be done eventually in the right time to save the most energy to keep it short, that’s at the end of the somer.
I started and am immensely enjoying your book on Existential Physics. For fun, I started with the word cuscuta in Wikipedia. It only took 16 links to get to philosophy. Cool.ðŸ‘ðŸ¼
Always knew you had a lovely and gentle sense of humor ... but I actually laughed out loud at some of your lines this time.
I could listen for ages to you going on about quantum mechanics, so perhaps we should have a big fan meet in a Biergarten and give you a microphone. And never hold back on your humour, I love it!
I kinda/sorta learned something and yet...giggled along the way. It's a win/win for me!
Joining the voices of "Thanks" for all your intelligent and excellent videos. (and the candor)
Hey Sabine, thought I'd let you know your pronunciation is getting better all the time (And it started out pretty good) I noticed you dropped the 'P' in 'Psi'
Thanks. I'm caught between a British friend and a Greek videographer 😅
@@SabineHossenfelder sounds like a delicious sandwich lol I love how little time you have from messing around with thoughts and feelings of close minded fools... You're so smart and sassy yet classy... love the ultraviolet and green plus the rainbow unicorn too. 🦄💯🙌ðŸ˜
That is because the P is silent.
@@curiousuranus810 Like the P in swimmimg?
@@bobh28630 Like the P in Pterodactyl
Very informative. She makes it so easy to understand. I love the graphical representation.
Weird sience made simple.
The only thing missing is that you will have to connect any outcome with the results of LIGO. Because the results of all quantum computers will be interfered by gravity waves.
Another thing I was wondering about:
Is (our) DNA code also the code for a quantum computer? Nothing in this code is finite. All outcome is like a gamble and still it results in species like us.
This new video style is very entertaining!