Quantum 101 Episode 4: Superposition Explained | Schrödinger's Cat

If you've heard anything about quantum mechanics, you've probably heard of Schrödinger's cat. It's a famous thought experiment (*not a real experiment*) in which a cat is said to be both alive and dead at the same time.
It's in a quantum superposition. But what does that really mean? This video discusses what quantum superposition really is. And if this bizarre thought experiment relates to the real world at all. The answer is yes… and no.
Join Katie Mack, Perimeter Institute’s Hawking Chair in Cosmology and Science Communication, over 10 short forays into the weird, wonderful world of quantum science. Episodes are published weekly, subscribe to our channel so you don’t miss an update.
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[@Mizraab2912]

That 'Well Yes...and No' is such a brilliant thing to add in the Superposition episode!

[@paulbadertscher]

Katie Mack + Carlo Rovelli = the best chance a lay person will have at understanding this stuff.

[@whizzer2944]

It was written long before Schrödinger that everything that can happen does happen , maybe our consciousness triggers a timeline ,but I have problems with this theory.

[@AwwwThatsMintocs]

​​@@whizzer2944 the Bible also stated that the world was a sphere long before it was officially discovered according to recorded history. I don't think Ecclesiastices 1:9 is referring to quantum superposition specifically though. In context the whole of chapter 1 one is a disillusioned discourse on the perceived futile elements of earthly life.

[@wmstuckey]

I like that Katie mentions the quantum bit of information (qubit). What people fail to mention when talking about quantum superposition using Schrodinger's Cat is the difference between a classical bit of information (like a computer bit being on or off) and a qubit. Both bits produce one of two outcomes when queried (measured), but a classical bit has only one measurement possible while a qubit can be measured in many different ways (infinitely many, actually), each with two possible outcomes. For example, when you pass an electron through an inhomogeneous magnetic field, the electron is either deflected towards the North magnetic pole ("up") or towards the South magnetic pole ("down"). You can orient the N-S magnetic field in any direction you like and the electrons will still give one of those two outcomes, so electron spin is a qubit with two outcomes of spin "up" and spin "down" relative the the N-S magnetic field. Now suppose you pass electrons through a N-S magnetic field oriented vertically and then send those that deflected "up" (literally up in this case) to a N-S magnetic field oriented horizontally. What do you expect to find? Well since the electrons have vertical spin up and spin is a vector (picture an arrow pointing upward here), then you probably expect the electron to pass straight through the horizontal magnetic field, i.e., they won't be deflected left or right at all ("up" or "down" relative to the horizontal N-S field). That's because the electron's spin vector (arrow) points up which means it doesn't point side-to-side (left of right) at all, so your horizontal spin measurement of a vertical spin up electron should seemingly yield a result of zero horizontal spin. But, what you find instead is that 50% of the vertical spin up electrons are deflected left ("up" towards North pole) and 50% are deflected right ("down" towards South pole). True, 50% left plus 50% right *averages* to zero, but that's not what you expect from the measurement of a vector quantity in ordinary classical mechanics. [Aside: Quantum mechanics gives the classically expected results on average over the discrete or quantum measurement outcomes.] This is quantum superposition, a vertical spin up electron is a quantum superposition of 50% horizontal spin left and 50% horizontal spin right and we write that as |V+> = |H+> + |H-> (divided by root 2 for normalization, but I don't need that to make my point). The point here is the horizontal spin measurement of the quantum state |V+> produces each of its two "up"-"down" (left-right) results in 50-50 fashion. This is exactly what you hear people say about Schrodinger's Cat, i.e., you open the box and find the cat is awake with 50% probability or find the cat is asleep with 50% probability. With that information alone, Schrodinger's Cat could be a classical bit or a qubit. If Schrodinger's Cat is a qubit, then there must be a measurement of the cat-box system like the vertical spin measurement of the state |V+> that produces |V+>, i.e., |H+> + |H->, with 100% certainty. We know the measurement "open the box" producing the "Asleep Cat"-"Awake Cat" results in 50-50 fashion is analogous to the horizontal spin measurement of |V+>, so what is the measurement of the cat-box system producing |Asleep Cat> + |Awake Cat> with 100% certainty in analogy with the vertical spin measurement of the state |V+> that produces |V+> with 100% certainty? And what does its outcome mean physically? If you can't articulate that measurement and outcome of the cat-box system, and every possible measurement between that measurement and the "open the box" measurement, then the cat-box system is just a classical bit ... like opening a box to find a ball or no ball. No quantum superposition there 🙂 To read more about the quantum information approach to superposition and entanglement for the "general reader," see "Einstein's Entanglement: Bell Inequalities, Relativity, and the Qubit" due out in June 2024 with Oxford UP.

[@lenkyl]

this series is fantastic! thank you dr mack for being so passionate about cosmology and being such a great communicator of it!

[@Deipnosophist_the_Gastronomer]

When did Schrödinger's cat go from being [alive / dead] to [asleep / awake]?

[@curtisturner471]

No no, I believe this cat is both woke and unwoke. Simple explanation.

[@SDS-ff8ed]

Because people sensitivity has been prioritized over practicality. Blame liberals.

[@irwinjones3960]

At 6:00 when the alarm went off

[@whizzer2944]

The RSPCA objected

[@joshredding418]

When someone observes it… law of observation….

[@MrPedalpaddle]

Excellent! There clearest explanation of Super Position yet.

[@nassermfouaponyoussouf]

Thanks Katie, Great explanation

[@stevenverrall4527]

Great fun educational video that clearly makes its point!

[@xanth592]

once you've "observed" something and it's "picked" a state, does it go back to superposition when you stop observing it ? Where is it getting the energy needed to change states?

[@rusticrick999]

No, decoherence is not reversible.

[@Paul1239193]

Actually the cat is not "both" asleep and awake, it is *neither* asleep nor awake because it is in a *superposition* of the two, until observation. (NB. in this formulation of the problem one also needs the assumption that the cat does not wake up on its own during the experiment.) A good resolution to all this was recently given by Presentist Fragmentalism.

[@cspeacock61]

Peter Harness delves into this idea of superposition and quantum entanglement in his hit series "Constellation." It's a mind-bending show, available on AppleTV. It's worth a watch.

[@0neIntangible]

I'd like to see if other experiments, similar to the DS, but done with different patterns and shapes, such as a double cross, or an ( X ), if you will, or other configurations, ( O )... have they been tried or are we ultimately limited to the interpretation of only the double slit ( II )... ?🤔

[@gdr7016]

At quantum scale, charged particles with certain additional energies exist as waves. In that state, phase of it changes very fast and hence only amplitude peak direction (phase)detectable by interacting with Photon of suitable energy resulting in non quantum state. This appears instantaneous ( simultaneous) when compared to classical interpretation. Charged particle with certain additional energy comparable to a spinning coin in the air. When coin falls on the surface, loses additional energy and takes stable state: head or tail but one state only.

[@chessmusictheory4644]

A star is an object that emits waves. If we conducted a thought experiment on a large scale, let’s say the size of a solar system, and we projected the sun through a slit with another slit as far away as Mars or Earth, you would see waves coming through the other slit. This is because as the sun travels to one slit, waves are also traveling, creating a wake of waves that travel with the sun the whole time, pouring light through both slits. Electrons do emit some level of energy so I am not fully convinced although the idea of superposition is a fascinating one.

[@biofluxus]

amazing! Thank you

[@zakirhussain-js9ku]

Is there any direct evidence of a single particle at two places at same time? I mean has it been detected at two places simultaneously. I think the answer is no. The particle is never at 2 places or states at the same time. The wave associated with particle in motion creates illusion of superposition as path followed by particle is determined by the wave. On detection the particle stops and its associated dissipate.

[@schmetterling4477]

There is not even evidence for particles. The only evidence we have is that people aren't paying attention when we explain what a quantum is in high school to them. ;-)

[@ericadenison1019]

General Physics I: Classical Mechanics General Physics II: Electromagnetism Modern Physics III: Forget what you have learnt and start from square one! Hence I am here life long learning is fun!

[@Garagelifestyle98]

Superposition is just the effect of the mass of a set particle as it nears the speed of light. Therefore explaining using simply scaling why it appears as if it is in a “superposition”

[@Welshburger]

I FINALLY GET SCHRÖDINGER'S CAT

[@hochathanfire0001]

What a nice CAT 🐱.

[@priyakulkarni9583]

Your music in the video destroyed 😅😅😅😅😅 super position

[@arilatvala2288]

It is sad to note that good lecturing is spoiled with mixing overplay of music.

[@iinventedgoogle1287]

I don't know why many says it's mysterious or difficult to understand? I understand it like this,since we aren't inside the box both options are theoretically valid.what is so difficult? Please fir quantum mechanics fans, correct me if I'm wrong or missing something.❤🎉

[@schmetterling4477]

Schroedinger pointed out that quantum mechanics seems to run up against the conventional "systems" definition of physics. That attempt in itself was valid., but since he made a serious technical mistake in his argument his particular line of reasoning doesn't go anywhere. Neither did Wigner's (and for the same reason). The true resolution of these paradoxes comes in the form of relativistic quantum field theory which has a much cleaner ontology.

[@rusticrick999]

You are saying 'cat is alive' and 'cat is dead' are both "theoretically valid". The real question is - are both states actually true, are they both actually happening at the same time? The cat cannot possible be both alive and dead, that would be absurd, as Schrodinger asserted, but only because the cat is macroscopic. At the level of fundamental particles, they can actually be in all possible states at once. If the particle were a cat it would be alive, dead, and everything in between.

[@ExposingReflections]

This is the soft and fuzzy, version, for all those who grew up using bicycle helmets, that ensured those from the shallow end of the genetic pool, didn't learn risk management, firsthand.(results are still being tabulated and interpreted, expect the results in the future) The original thought experiment included an alive or dead cat, and a vial of poison. It is a difficult task to reduce knowledge to the basics to excite the average person, but you do a great job. Thank you again, Katie and all the rest at the P I for your work at trying to understand the universe, and pass it on, right down to the smallest parts. Keep up the excellent work.

[@cspeacock61]

Intrigued by this idea that when we observe, say Schrodinger's cat we are seeing one state; let's say dead. Would some parallel version of us observe the cat as alive? That would mean superposition always exists. It's the observation that shows any one state in any one reality. I could be wrong.....or I could be right....or both.

[@spoonstr5958]

I think your little example wrong because the sound of the alarm clock would reach outside of the box giving added information from within the box wouldn't it? anyways nice video

[@dimitriosfromgreece4227]

BRAVO!!!!!!!!!!!❤️❤️❤️❤️

[@MayVeryWellBeep]

Watching these just convinces me the there is no hard line between physics and philosophy

[@schmetterling4477]

That's exactly what a person who doesn't understand physics will come to conclude. ;-)

[@spaceinyourface]

Yea ,it's definitely getting blurier,,,especially the distinction between theorists & philosophers

[@schmetterling4477]

@@spaceinyourface Only until you try to pass a theory exam in the physics department. Let me guess... you haven't. ;-)

[@spaceinyourface]

@@schmetterling4477 Good guess Your absaloutley right, I have not. I certainly recon I could though ,,& philosophy,,with a little revision. ;-)

[@schmetterling4477]

@@spaceinyourface Yeah, but why would you want to waste your time on that bullshit. It's not like it's useful and they are hiring a lot of philosophers. Farmhands are more in demand. ;-)

[@Inquiring_Together]

Be careful opening the box, kitty’s can get startled easily 😋. To communicate superposition across is no easy task. Its certainly not coffee table talk. Although, for Einstein and Schrödinger it was. And there we have it, once again, a cute and simple thought experiment which is at the same time something mysterious and perplexing. What other aspects of our human endeavors can benefit from an acceptance of superposition? Perhaps it could be key in getting past stalemates, in turn, alleviating conflict.

[@whizzer2944]

Or even in the formation of a girl group ATOMIC KITTEN did they infringe copyright.

[@Inquiring_Together]

@@whizzer2944 ahh, to copyright the fundamental laws of nature, sounds like the premise to a James Bond movie.

[@barbmitch6866]

Shrodinger : theres no way you can know if the cats alive or dead Without opening the box Cat : meOw..

[@JanetEarthOne711]

Very mysterious! 😮#Superposition

[@user-fl4nw3ub7b]

The superposition is an invention of people to do predict the behaviour of an particle. It doesn’t change the reality. The particle is on point xyz in space of not!

[@TsukuyomiBeats]

super position is kind of scary

[@parabellum4622]

The only reason I sleep with a blanket is because it's the closest thing to experiencing a warm hug -Aleph Null.

[@Pottery4Life]

I will know if the cat is awake if I hear the alarm clock. I like the G rated cat. 🙂

[@Langkowski]

If it is just about probability, then parts to the wave should not have evened each other out

[@amanwithnohands]

Unless the box was made of glass

[@ultra254]

I don't understand, If we don't know what state something is in before we measure it, Wouldn't it be more reasonable to say that it is already in one of those states and we just don't know it yet, Instead of just assuming that it is in both of those states at the same time until we know what it is?

[@GeoffNelson]

Precisely my thought. And if the cat really is in both states at once, why couldn’t we just observe it being in both states?

[@whizzer2944]

@@GeoffNelson well to do that we would have to be able to see into the future which we can't do .

[@deeptivaidyula]

I couldn't understand superposition first. How can an element have two properties at the same time?? Then I realized that it is indeed possible. A funny example is: We are both stupid and intelligent at the same time! Same person, two characteristics. We infact have more such traits in the same person. May be quantum stuff is also like that. The way we measure it, determines the outcome!!!

[@dylankrejci9965]

You still have a singular measurement of “intelligence”, in whatever measure you want to use. Of course you have multiple measurements possible, but that would literally be comparing apples to oranges.

[@incoprea2]

The quantum collapse that everybody is talking about is still ego-based. Just because you perceive a small part of something doesn't mean the rest of it doesn't exist. That's like saying when you see a whole forest and approach it in your car when you get close to it it collapses to the view just around the road which isn't true. The observer doesn't have any power or control but rather sees a small portion and considers it the entirety.

[@jebbait1669]

Why does me observing something make it true? Couldn't there be a state that is 100% true and when I observe it, I see its 100% true state? Could photons be particles that interact with each other similar to playing pool? Where your ball hits another ball can change the the direction of the ball you're hitting.

[@zeroonetime]

WE CAN NEVER MEASURE THE IMMEASURABLE 010 d.

[@JordanFolkz]

I’m confused on why it’s awake AND asleep, shouldn’t it be awake OR asleep? It’s not that it’s at both states but it’s at 1 state which is unknown before observation.

[@rocketjet5813]

It exists in both places at the same time. When it is observed it is forced into one position. Not that it has only had one position the entire time, it has two. When we observe it picks one. Therefore before we observe the cat is awake and asleep.

[@craigstiferbig]

Who says we aren't in superposition

[@jimkiely2121]

Content is great, but don't need the music.

[@pavandaripalli117]

What if they used a visible glass box...? The experiment may not follow the rules since all obstacles are not glass.🧐🤔 😅

[@chillbeatz6805]

that would be observing🤣🤣

[@schmetterling4477]

Yes, that was the wrong explanation for QM... even though it's the one that you have all heard in undergrad courses. We really need to up the teaching game a lot here... otherwise these misconceptions about quantum mechanics will never disappear.

[@whizzer2944]

Yes either the future already exists or we really are living in a sort of simulation where events are continuously being created as we go along ,this is a difficult one to comprehend.

[@schmetterling4477]

@@whizzer2944 And there is the boring person with his simulation hypothesis. ;-)

[@TheRealPaulMarshall]

The human is obviously in a superposition of having observed the cat to be asleep and having observed the cat to be awake until the human interacts with an observer.

[@whizzer2944]

????

[@boonedockjourneyman7979]

Stop editing history.

[@Universe476]

But your scientists still don't believe in parallel universe

[@Randomprecious_hehe2]

Is the cat not sleeping and awake? Or is it not sleeping and not awake??

[@biosphere8488]

Anyone who is offended or feelings are hurt over a thought experiment doesn't deserve quasi virtue signaling consideration.

[@kinglymajor1465]

We are being observed.

[@spaceinyourface]

Love Katie,,,,,Sean Carroll said If you accept quantum mechanics,,you have to belive the electron can be in 2 spins at the same time....Which means your allready on the road to the asylum,,,why not jump straight in & accept the observer & the whole universe is in a superposition ,,then many worlds ......Yea,,,the craziness is right there in the beginning with that sole electron .

[@whizzer2944]

Good explanation

[@Babybugex]

Boom goes kitty..

[@kevinwatsonmd1744]

Hmm…..alarm clock????? Really????

[@PURE.EVIL.]

YES BUT... If instead of cat was a dog? You don't have answer now!

[@M112-Dave]

Maybe quantum is the reason people don’t know what bathrooms they are supposed to use 🤔😂😂

[@HeroTeo]

You still can't really get it

[@abrahamf6124]

Not exactly true. So the cat is not both awake and sleeping. It's just simple awake or sleeping. Not a big deal. The fact that we don't know where the particle is doesn't mean it's everywhere. The fact that we can't find the particle also doesn't mean it's everywhere. One day we will find new ways and discover it's all a theory.

[@schmetterling4477]

There are no particles to begin with. Quanta are small amounts of energy. They are not objects. We have been teaching that in high school for almost 50 years, by the way. That people don't remember high school physics is a good indication that the way we teach high school is wrong. The material simply doesn't get remembered.

[@abrahamf6124]

@@schmetterling4477 Two quantum entangled electróns are always refered to as entangled particles. And electróns are very much matter.

[@schmetterling4477]

@@abrahamf6124 The "I saw it on the internet so it must be true" is strong in this one. :-) Electrons are quanta of energy, just like photons. You are simply being led on by relativity here a bit. Relativity allows for two kinds of dispersion relations, one for massless and one for massive systems. If you look at the theory, however, then you will find that the "naked" fields in the standard model, which is based on gauge field theory, are all massless. They acquire effective mass by their interactions and that mass is scale dependent to boot. In other words, the details are complicated (and not at the level that the layman can overcome with handwaving). What you are being told on the internet, however, is simply not so. It oversimplifies the physics to the point where you might as well engage in finger painting. The only way quantum mechanics will ever make sense to you is if you listen to what we were trying to tell you in high school: quanta are small amounts of energy. Energy doesn't behave like "particles" would. We simply couldn't tell you what deep consequences that has because those are filling 1200 page textbooks on quantum field theory.

[@schmetterling4477]

@@abrahamf6124 If you want to know how "matter" emerges from irreversible energy exchanges, then reading Mott's 1929 paper on the wave mechanics of alpha-particle tracks is a very intuitive introduction to the consequences of weak measurement theory in a non-trivial and important physical system. Heisenberg had similar ideas a few yeas earlier about the connection between states of Rydberg atoms and classical orbits, which have been fully worked out more recently both in theory and experiment. What we call "stuff" is a mirage created by multiple consecutive measurements on the same quantum system.

[@Jay-bu6cd]

Superposition is to Quantum Physics as the Trinity is to Christianity.

[@GomezBro]

Shrodinger did not believe in Superposition and neither do I. Its nonsense, the cat is dead or alive whether or not you look inside the box... duh.... SO STUPID....

[@schmetterling4477]

Physics is not religion. Neither you nor Schroedinger were ever asked to believe in anything. Schroedinger should have known better, but he didn't. That you don't know better is kind of OK, you are not a physicist. I still don't know why you are trying to make a fool of yourself on the internet. :-)

[@hollies4806]

👍👍

[@markusmaximus629]

The Double Slit: Its pretty simple. The wave is the field and the detected " particle" is the expression of it's energy, carried by the field. Mystery solved.

[@bbbb98765]

You don't understand the double slit experiment. You completely MISSED OUT the single most important feature of the experiment, which is that you get an interference pattern when there is no wave. Congrats though on being cleverer than everyone that ever lived in the last 100 years

[@ronaldderooij1774]

I field is not the same as the wave. The wave is in the field. The wave is the energy. The point is, the wave (energy) is not localised. But the impact (energy) behind the slits is. What happens at that moment is a mistery.

[@markusmaximus629]

@@bbbb98765 I don't understand it? Ha! You all don't understand it. You believe in creation events, and dark energy and cats in boxes.

[@markusmaximus629]

@@bbbb98765 The reality is you don't understand your own double slit experiment, as simple as it is. So, when you fully understand it, you can say that I don't understand it. Keep poking.

[@schmetterling4477]

@@markusmaximus629 I understand it quite well. That's at least one in three now. ;-)

[@HeroTeo]

The cat can be awake without opening the box. It's a stupid example.

Albert and Erwin were correct, this is all just nonsense. You might just as well reactivate the age-old debate as to how many angels can dance on the head of a pin.

[@markusmaximus629]

Schrödinger's Cat is dead, because you beat it to death.

[@kathyorourke9273]

I cannot hear about schroedinger’s cat one more effing time!

[@mikes11220]

The way too loud "background" music makes it almost impossible to hear the people talking. I'm sick of people with "background" music Overpowering the vocals.

[@GomezBro]

This is a pathetic video, Shrodingers Cat is not asleep. stupid woke video changed the original story.

[@Spencer-to9gu]

sorry, but the explanation comes across as high level word salad to me.

[@Juohmaru79]

How can anyone be offended by a thought experiment? Please don't insert woke mentality into physics...

[@elvistesla3179]

This is the lousiest description of a quantum bumping particle theory fundamental that I ever heard. How do electrons transmit wirelessly in charging if it is a particle?

[@stevenverrall4527]

EM energy is transferred from one ensemble of electrons to another. I am an EEE PhD who knows some of the pioneers of this technology.

[@bbbb98765]

Electrons don't transmit wireless energy. Photons do. And how they do it is well understood. Don't be rude about others when you don't understand. Get an EEE or physics degree. Do the work before you criticise

[@elvistesla3179]

@@bbbb98765 It's spelled criticize not criticise. Do I also need an English degree or do you just need spellcheck? So anyway why don't you explain how electrons travel to with wireless charging?

[@elvistesla3179]

@@stevenverrall4527 So there are ensembles of electrons floating around through the air with wireless induction? Do explain yourself further.

[@bbbb98765]

@@elvistesla3179 It's spelled either way, depending on where you live. You should really travel more. Broadens the mind. Also, congratulations on the greatest self - mocking attempt at a zinger that I've ever seen. Truly a peach 😂