these videos are so dense. if i get distracted for a minute i need to go back 5

this one is seriously mind blowing to me.. the more of these videos you watch, the better they get.. because over time you understand more and more and then you put those pieces together and it's like BLAM! and suddenly your concept of reality becomes so .... unreal

I am positive that Matt just loves to say 'Von Neumann'.

6:55: This guys is so into Quantum Mechanics, even his hair changes when you try to measure it.

“How quantum entanglement creates entropy” Me: I know some of these words.

The answers at the end of this episode just increased exponentially my interest in a full PBS SpaceTime episode about the Quantization of Space. I'd really like to know more about Loop Quantum Gravity's grid of abstract connections between points, and how is possible that Space emerges only on larger scales.

11:12 - Best part: "Our capacity to observe quantum effects like superposition depends on being able to access the entire wave function."

This takes me back to my dissertation - “Defining work in a quantum battery” where I utilised heavily Von Neumann Entropy and the concept of temperature at the quantum level. This was a great episode and made me miss my days in academia, thank you for another phenomenal episode Matt.

You sort of compressed about four episodes into this one.

"Shannon Entropy" sounds like the name of a Bassist in an all girl metal band.

I'm happy to hear your remark about von Neumann and how great he actually was: people often don't realize how much von Neumann was ahead of his time and what kind of genius he was.

I just want to thank you guys for all you do. I love learning about space and I have trouble sleeping. Listening to your videos before bed distracts and keeps my mind busy enough to not over think about life until i can fall asleep. Unsuccessful today since its 6:40am, but I don't know what I would have done without this for the past few years. I have listened to all your videos and been repeating them since they are amazing.

After 3 viewings, I'm still entangled with confusion. More please.

The movie Tenet needs to provide a link to this video in the credits.

How to go from superposition, to entangled quantum states to classical. This was possibly the most brilliant and insightful presentation ever! Thank you!

I love this episode. Everything quantum seems to make more sense now and is less daunting and unfamiliar. The visuals helped so much too. Looks and sounds like the entanglement of all particles weaving the fabric of space. Feels like a familiar warm comforting blanket rather than a feeling like being hopelessly lost driving an unfamiliar car on a super tangly spaghetti junction, in a blizzard or something. I just hope Schrodinger's cat doesn't tear apart the fabric of space time like my cat did my blankets. I mean I'd want revenge if my owner kept putting me in a box for extended periods just to contemplate weather I'm alive still or not.

Whew, definitely going to have to re-watch this one again a few times to actually grok it. Love episodes that really shift my understanding!

Wow. It took me a long to get through digesting it properly but eventually when you linked the quantum with the classical. So many different concepts and puzzle pieces just fell in together in the most beautiful harmonic mannerism. Reality truly is something else

can we get a shout out for whoever designs your thumbnails? they're all so beautiful

Matt O'Dowd and the crew on this show are some of the greatest science communicators of our time.

I didn't realize the same von Neumann whose name pops up in computer science all the time (von Neumann architecture) was also involved in thermodynamics, but there you go!

Why worry about space zombies? As xkcd pointed out, our reliance on reflector telescopes means that space vampires are completely undetectable!

This is the best (short and clear) explanation for Entropy I ever heard. Thank you for the great presentation. Neumann was truly a genius.

I have considered this for ages! Very glad this simple little detail is being presented in depth. I've heard this specifically only in lectures and only in passing, never in any books I've read.

My mind was in the superposition of both understood the episode or didn't, until i broke the uncertainty with a rewatch😂 Really excited for the upcoming episode.

Von Neumann entropy: Sabine Hossenfelder : "doubt" The difference between the pure quantum state with Von Neumann entropy=0 and the known state after measurement is the measurement problem. Hossenfelder says that we don't know enough details about how physics really works to know how there is a transition between the two - we don't have a way to describe that transition within quantum equations. The Copenhagen interpretation is to just say "our experiments work, quantum theory works - so lets just ignore the complicated part that happens during measurement and pretend it isn't a problem." The many worlds theory replaces one handwaving with another handwaving - because it posits a splitting of universes, part of the quantum state become inaccessible, but it has no idea when or how that happens. Just saying "it all happens within the laws of quantum physics and the wave function never pops" doesn't save you from the fact that you have no information about how we get from the unmeasured state to the measured one. If you say it's within the theory, then you have to be able to calculate it within the theory and they can't. So does decoherence solve the measurement problem? Hossenfelder says "no" but she hasn't gotten around to explaining why, at least not in her channel. I haven't finished her book, so I can't say whether she explains it there. But I have to say that the transition you described from entropy=0 to entropy>0 sounds like another one of those "ok, exactly when and how did that happen? Can you calculate it? No." situations.

Its so unbelievably refreshing to see the topics/titles for spacetime vids to not be click baity, ALL CAPS, or a hook! Well done team 👍

If a particle's entanglements only "diffuse" in a sea of entanglements possibly with every other particle in the universe, but never really break, how can we ever produce fresh entangled pairs so isolated that we can use them in quantum computing?

listening to you talk about entropy makes me think about relationships with people and the information we get from each other.

Ooh information theory next. I love information theory! As a software engineer I think that information theory feeds into computer science in a way that is massively influential while barely anyone is even aware of it. You don't need it every day, but knowing about it really helps you understand certain things at a much more fundamental level.

I love this gentleman's knowledge and the way of his video presented without annoying background music. Thanks.

So glad you are here to guide me through these concepts! ❤️

So we just detangle the Quantums and therefore reverse Entropy, problem solved.

First time I've ever heard a physicist say "An Emergent Law". I'm used to hearing them say it's all fundamental, all of it.

Spacetime is the main reason I check my subscription tab. You guys are doing such an incredible job! Every episode is killer!

Wow. Just mind blown by this video. Brilliant script, performance and video editing. Thank you so much!

Thank you, thank you, thank you for explaining these concepts. I am only an engineer, so I don’t always understand, but I can always feel my mind expand.

I have in the past referred to this idea as Quantum Causal Democracy. Basically, as quantum entanglement propagates, each entangled partner "votes" on the weighting of various outcomes of the wave function. The weight of the "votes" that each quantum entangled partner gets is proportional to how causally connected an outcome would be to that particle's own quantum information. The idea behind this is that causality is always preserved, even at the expense of normally invariant factors (on the quantum level) of space and time. The wave function conveys information instantaneously regardless of distance, but it can only propagate at the speed of light (the speed of information). This also provides a resolution for the Quantum Eraser experiment. Variants in which a photon would appear to go through a single slit and not interfere based on their distribution on the screen would be strongly voted against by the particles in detectors C and D because such particles are highly causally connected to information about the position of the particle at particular times. No information travels backwards in time at all, even in theory. Instead, the weighting of the photon's causal connection would depend on its level of entanglement with various other particles and how causally influenced they are by a particular outcome. But I dunno, I don't really understand Quantum Mechanics. I feel like what I described could be formalized into some set of useful equations if I was educated enough, but I'm not. C'est la vie.

Entropy always made my head hurt. It's like, it's all-encompassing, but it's also a measurement of nothing, or of what's potentially there, or what it can do, or how long... It's just... Man, I'm glad there are smart people out there to do this thinking for me.

This format makes concepts so easy to understand. Really great video!

Congratulations on the excellent explanations. PBS does an awesome job in this channel, really outstanding. I will yet be a little bit picky and address a conceptual disagreement about the second law being a statement about the von Neumann entropy of a system. Quantum thermodynamics is mainly concerned with energetic and, further, informational processes; not states. The second law is a statement about a quantum process and the quantity which entails the irreversibility of this process is the entropy production. One should bear in mind that the closed-system evolution in quantum mechanics is unitary and, thus, leaves the entropy of a quantum system untouched. As von Neumann brilliantly notes in his work, irreversibility stems from measurements and, as we understand today, from general quantum channels --- like open quantum systems. By no means I want to say you are wrong in the simplification made. I understand that all those subtleties would make the content very technical and, of course, not as accessible. But, as a young scientist from the quantum thermodynamics community, I thought it'd be nice to add my view on the subject.

Wow, I just was reading The Order of Time by Carlo Rovelli, about Quantum Gravity, and it covers this exact same thing! I almost feel like I am gaining partial understanding! Honestly, this is very cool, and thank you so much

It is so brilliantly explained indeed. Thank you PBS for your much valuable work.

When Matt started talking about the classical coin being influenced by all of its constituent parts and everything they've ever come in contact with, it made me think of almost like subpixel values in Mario. The current behavior is a product of all of the previous interactions interacting to create some sum.

Thank you for this video. As always the explanations are clear even for those who don't study physics.

This video changed my perspective on the nature of the universe. Several times in a row.

37 YEARS OUT OF SCHOOL YOU GIVE FAST REVIEW AND NEW INSIGHTS THANKS

It's been almost 4 years since I've been following this channel, and I love it !!!!!

Information theory is so interesting! Would love to hear more

"Zombies that when you shoot them just turn into thousands of smaller zombies" This is my boomstick.. Shop smart... Shop S-Mart... Also I'm now horrified that this has moved from hilarious Bruce Campbell comedy to my personal zombie apocalypse model. Thanks PBS.

I've been waiting for Quantum Information Theory to be covered for ages. Can't wait for next week's episode! To me QIT is the only satisfying resolution to the question of how quantum things "become macroscopic".

1 video that takes a minimum of 4 times watching it to catch everything and half way kind of understand! Well done

Has he gone 100% Wood Elf? My Granny told me, "You never go 100% Wood Elf!"

I'm just going to sit here and act like I understood everything that was presented to me. Yep.

Hi Matt. I very much like your videos, they do a great job of explaining difficult concepts. I was wondering if you could do an episode on double copy, which appears to be a possible link between QCD and GR/quantum gravity. Thanks!

I had an explanation of quantum entanglement change my perception on it recently. We think of the two “coins” in this case scenario as two separate “coins” that have a “spooky” interaction with each other, but the reality is that because of the interaction between the “coins” their wave probabilities become one single wave, meaning it’s essentially one “coin” that has two representations of its wave function.

Yes I agree when my headphones are in my messy pocket, it gets entangled so I think this is a good proof that entropy is related to quantum entanglement.

*coin lands on side instead of heads or tails* *the big rip starts*

This is really one-of-a-kind... Awesome and inspirational!

I aprecciate how correctly Matt pronounces peoples names, whether scientist's or commenter's. Have I just heard "Michal" with H and silent C? That't the correct polish pronounciation, sir! I'm impressed with your attention to details and being so thoughtful.

Show idea: how do loop quantum gravity, string theory and other theories that claim to successfully unite gravity and quantum mechanics describe what happened at the Big Bang and in black holes?

If entanglement networks are a measure of entropy, when we accelerate particles in accelerators are we temporarily detaching these particles from the planet's entanglement network? And when we measure the results from collisions, are we simply measuring the resultant particles as they enter our entanglement network?

Loving him being honest about the updated information. Also thie fact he took a comment seriously enough to go back and dig to confirm the information.

This perspective on entropy should be added to all undergrad thermodynamics courses. A 15 minute description linking entropy to quantum entanglement has done more to help me understand why the second law is inescapable than all of the engineering courses combined. Well maybe that has more to do with engineers not caring about "why".

I've always thought Shannon's information theory would be fundamental to a ToE.

If the arrow of time arises from increasing entropy, meaning increasing quantum entanglement over time, does that imply that at the beginning of time (assumedly the instant before the big bang) there was no entangled quantum systems?

Answering the questions but knowing what those questions should be! Thank you!

Best description of the concept of decoherance I've ever heard

So, this has me thinking about the relationship between entropy and gravity. If there are more interactions due to higher energy density/strong force kinematic energy, does that essentially slow the ‘clock’ of entropy as the interactions take more ‘time’ to sort out all the entanglements. Could this explain gravity as an emergent property of the speed of causality and the quantum interactions of the wavefunction?

Oh my, this video is soooo insanely good! Matt talking about stuff I barely suspected about quantum world, never definitely knew just had kind of sense of general direction. Thanks so much for this video! Absolute Gem! Explains a lot, I can see pieces here and there finally starting to make more sense!

10:23 "When viewed with its entangled partner, the coin exhibits quantum weirdness like superposition that could be revealed [in bell test] but treated individually each separate entangled quantum coin behaves kind of like a regular classical coin. For example, it doesn't by itself exist in a pure superposition of states; that superposition of states only appears when you include its entangled partner. It's what we call a mixed state." This is the most critical section of the entire thesis that entanglement creates entropy and deserves more drill down. A mixed state is just the mathematical fact that the two particles are anticorrelated and have a definite relationship. (for type II SPDC specifically) The purity of superposition of an individual particle is dependent upon your knowledge that it is, in fact, entangled. You're getting into "consciousness causes collapse" territory here if you are presuming that the superposed nature of the particle depends on our knowledge of its entanglement state. You say "Head or tails, not heads and tails" What is the basis for this inference that entangled particle is equivalent to a fully collapsed wave function? ...you're describing many worlds. You're proselytizing a decoherence worldview and need to be more clear about this. What you've done here is hide faith behind circular reasoning: You believe a mixed state is not actually superposed because you believe in many worlds. You believe in many worlds because it gives a convenient logical escape from problems with decoherence, namely the problem of why one macro state of many is experienced. (why you believe a mixed state and entanglement are the process) It's fine that you like this route of reasoning but please be clear to point out your preference based assumptions so others can more easily pick out these dependency loops. You've tied in the fact that Von Neumann entropy is different for these two situations (mixed versus pure) but you've glazed over the very silly fact that there are "known unknowns" and "unknown unknowns" and you're tying an ontological state to our knowledge about it again, thus returning back to consciousness causes collapse. You're conflating states of knowledge with states of existence without acknowledging it. At 6:18 you say it's Von Neumann entropy tells us how much entanglement is in the system, but around 10:00 you explain that the entanglement hides part of the information thus granting it VN entropy. Obviously you'd have to know the amount of entanglement to determine the VN entropy. (we're in a loop again) Many worlds is your belief because interactions lead to shared wavefunction information that make up consensus networks that define a given macro world. All particles remain forever superposed at their scale and collapsed at the macro scale. There's just propagation of network decoherence through interaction/entanglement. Isolated systems are merely a time-frame based illusion of low interaction during a given period. That all sound right? There's still a problem of a sharp and well defined cut between worlds that should obviously be continuous. At what point is enough entanglement reached to obliterate all superposition? You've just said near the 10:00 mark that the mixed state is equivalent to collapsed so it seems to me that any particle exhibiting the slightest quantum behavior would have to be perfectly unentangled. How'd that happen exactly? I'm not asking about loss of access to a history in another network/world, I mean a loss of entanglement with the current macro network/world. Might you be simultaneously inferring no superposition existing anywhere and also espousing it existing everywhere at the same time but with enough complexification in your explanation that you don't notice the mutual exclusivity of the components of your worldview? I've seen lots of people do this with a lot of different kinds of worldview. Most of these problems with information entropy come from poor definitions of information being fundamentally based upon human symbolic representation and cognition. They come in the form of knowledge level versus fundamental existence. (which is questioned in Copenhagen, but not in many worlds) We can resolve this nonsense by better defining information through first exploring the process of symbolic representation and interaction between non-thinking entities via geometry mediated energy interactions, but I digress. I'd first have to get you to rethink bell tests under a different regime where photons have more degrees of freedom than assumed so you could discard faulty ideas of entanglement. Fat chance of that, right? "Einstein starts from the hypothesis that the laws will look the same to all observers in uniform motion. This permits a very concise and elegant formulation of the theory, as often happens when one big assumption can be made to cover several less big ones... But in my opinion there is also something to be said for taking students along the road made by FitzGerald, Larmor, Lorentz and Poincaré. The longer road sometimes gives more familiarity with the country." - J. Bell "That the guiding wave, in the general case, propagates not in ordinary three-space but in a multidimensional-configuration space is the origin of the notorious ‘nonlocality’ of quantum mechanics. It is a merit of the de Broglie-Bohm version to bring this out so explicitly that it cannot be ignored.” ~ John Bell 'Speakable and Unspeakable in Quantum Mechanics' page 115 “But in 1952 I saw the impossible done. It was in papers by David Bohm. Bohm showed explicitly how parameters could indeed be introduced, into nonrelativistic wave mechanics, with the help of which the indeterministic description could be transformed into a deterministic one. More importantly, in my opinion, the subjectivity of the orthodox version, the necessary reference to the ‘observer,’ could be eliminated… But why then had Bohm not told me of this ‘pilot wave’?... Why did von Neumann not consider it? More extraordinarily, why did people go on producing “impossibility” proofs, after 1952, and as recently as 1978?... Why is the pilot wave picture ignored in textbooks? Should it not be taught, not as the only way, but as an antidote to the prevailing complacency? To show us that vagueness, subjectivity, and indeterminism, are not forced on us by experimental facts, but by deliberate theoretical choice?” - J.S. Bell, On the Impossible Pilot Wave. 1982

Thank you! Keep making these videos.

I find it so surprisingly satisfying when they show that some of the questions and statements people comment are answered by the show's own viewers. People like us. Lets Go! PBS!

the fact that physics studies all scales intrinsic to the universe is what makes physics so beautiful and fundamental, it is what makes physics the god of all knowledge

Thank you. This is an incredible video and an incredible channel.

Great episode on the relationship between entanglement, emergence and entropy, which is arguably one process. I really enjoy how self consistent yet, although disputable, indifferent/invariant nature is, and Matt's and these great minds ability to explain that. I'm more and more being sold on information theory which means it needs to be scrutinized relentlessly. I had a feeling he was getting at a 0 entropy state with quantum entanglement. The states would have to be known, given that you know the partial states of both particles, to have all information and thus no increase in entropy, roughly speaking. Superposition is weird. Entanglement is a little more intuitive. Decoherence is more evasive than a squid covered in vasoline. Wonder if Matt is into constructor theory or geometric unity theory

Cool story: Even the beginner levels of quantum computing use almost all of these concepts and theories. IBM has a pretty neat beginner's guide to quantum theory and even explain some relevant theories AND they give you a cute little playground to mess around in.

So, in summary, O what an entangled web we weave, And hence we suffer entropy.

I watched the video twice, it has something new to me. I was tired of the "multiverse" series, they just seemed sience fiction stories. This is an improvement. Thanks Matt.

That was very well explained, thank you.

Loved this video - had to watch 3x lol... So if we assume more particle interactions over time, are you saying the Shannon entropy will always increase over time for a system while the Von Neumann entropy (when only looking at individual particles rather than the whole system) will always decrease for each particle it measures (as that particle's wavefunction collapses due to interaction)? If so, is the explanation for why we experience a "net" increase in entropy over time at macro scales because at quantum scales, the interactions between particles generate more entropy than the Von Neumann entropy "removes"? Separately, how would the existence of a quantum multiverse impact this - is Von Neumann entropy actually decreasing to zero (for each individual particle as its wavefunction collapses) in this scenario?

See, Gina? I'm not 'letting myself go'... I'm just following a basic law of nature!

The basic pattern of entropy, the pattern of things regressing to the mean while spreading out unless energy is used to prevent this, nothing escapes that, NOTHING escapes that. This pattern can be seen in everything, from ideas and dreams to space and time, from books to electrical generators. This is useful in making predictions about EVERYTHING.

Every time I watch this channel they tell me exactly what I didn't know that I really needed to hear until I heard them tell me kind of thing.

This episode is the hardest so far, at least for me.

You guys should put a number on the episodes so it's easier to find what episode you're referring to when you do the cliff hangers at the end saying what you'll be covering in the next episode

Fascinating discussion! Thank you.

I've always viewed entropy as perpetual motion, like the way the universe expands but on a subatomic level like how bacteria multiplies, not sure if that's another law but looking forward to learning more🌊

PBS Spacetime: Spells entropy with a capital sigma Me: notices the use of rho first than sees sigma.

"Small fast zombies", is that like baby zombies in Minecraft? Those are indeed scary and dangerous!

I like how we can view thermodynamic properties through an informational lens. It is that observation that leads to quantum computing. Will you ever discuss quantum cryptography? It's a really neat application of these ideas.

Blew my mind like always 🙏 I need to rewatch this after some sweet sweet THC

If information has mass, is it possible that “dark matter” could be a form of metadata about the full wave function of all quantum entangled particles in the universe and the fact that the arrow of time moves in one direction could account for the expansion of the universe needing more space for this exponentially growing information?

From what you describe, it seems like particles are getting more and more entengled due to decoherenve. As the entropy of an entangled system is zero (because you have all information with the wave function) then it would mean that the universe would reach an entropy of zero once all particles becomes entangled. I'm lost, please help.

This is very reminiscent of Everett's thesis paper on the multiverse. It's essential to understand you the observer are also in the wave function

That is one of those episodes that I don't get right away, but when I come back, several videos latter I get that "ohh this explains a lot of things!"

Are quantum dice a thing? Is it possible to like make an object that is able to be in more then 2 states? like why does it have to be head or tails? why not 3 different options at the same time?

So are you telling me that entropy is relative to the observer, and every increase in entropy is an entanglement the observer didn't keep track of?

This is one of the best ones!

Ever since I discovered this channel, it's like I've entered a labyrinth of knowledge that I need to fully explore in order to understand each part of it. If only I were like a quantum particle, capable of exploring every path at once 😂