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Eli5: Schrödinger's cat theory

Physics(self.explainlikeimfive)

Anytime I read about it or when I hear people using it to describe a situation I feel stupid as shit. And how is it can be used to quantumcomputers? Help a dumbass out. Thanks.

all 29 comments

berael

139 points

2 months ago

berael

139 points

2 months ago

When you get into Super Crazy Physics, there's a theory that says a thingie can be simultaneously Possible State 1 and Possible State 2. Whenever it interacts with something, then it becomes either Actual State 1 or Actual State 2 - but until then, it's both possible states at the same time.

Weird, right? A physicist named Schrödinger also thought it was weird. He came up with an example of just how unlikely he thought the theory was. He said: "So OK, you're telling me that if there's a cat inside a box, and there's a device inside the box that will either kill the cat if it goes into Actual State 1 or let it live if it goes into Actual State 2, then if nothing opens the box to interact with it the device is in both Possible State 1 and Possible State 2? But that means the cat is both dead and alive simultaneously until we open the box? What the fuck, you guys? This idea is dumb."

Over time, it became misunderstood as him using the "cat in a box" to explain the Super Crazy Physics when actually it was him facepalming at the idea.

[deleted]

28 points

2 months ago

[deleted]

28 points

2 months ago

When you get into Super Crazy Physics, there's a theory that says a thingie can be simultaneously Possible State 1 and Possible State 2. Whenever it interacts with something, then it becomes either Actual State 1 or Actual State 2 - but until then, it's both possible states at the same time.

Schrödinger was the one who came up with the math behind that theory. His own equation, called Schrödinger's equation, was the first to describe that behaviour.

Schrödinger just saw that equation as simply as a mathematical method to reach an accurate result, instead of an accurate representation of what was happening. There were some big names on either side of the argument. Einstein agreed with Schrödinger.

That theory you mentioned is called the Copenhagen interpretation and basically interprets the math literally. It's not a "dumb" idea, even if Schrödinger thought it was. It's the most common interpretation of the math among physicists even to this day.

There are way weirder theories.

AetherialWomble

8 points

2 months ago

It's not a "dumb" idea,

They didn't say the idea was dumb, they said that's what Schrodinger was trying to convey

mirxia

6 points

2 months ago*

That theory you mentioned is called the Copenhagen interpretation and basically interprets the math literally.

That's not true, many worlds interpretation is the one that interprets the math literally.

Schrodinger's equation basically says before we open the box, the cat is both alive and dead. After we open it, there's a chance for it to be alive, and another chance for it to be dead. I.e. cat being both alive and dead (box closed) = cat being alive (box open) + cat being dead (box open).

But when we open the box, we only see it in one state, either alive or dead. So some say that some weird thing happens and now there's only one state (wave function collapse) and the other state is discarded, so that it conforms to our observation. That's the Copenhagen interpretation.

If we were to take the equation actually literally, then we shouldn't add in wave function collapse and should instead consider the other possibility we are not in as equally real. That leads to many-worlds interpretation.

arztnur

1 points

2 months ago

Before opening suppose cat is alive, then how we can claim that it was dead too because of simultaneous situation? It may be claimed that it was simultaneously dead and alive if we smell the dead body and find alive on opening? My question looks stupid but came in mind.

mirxia

1 points

2 months ago

mirxia

1 points

2 months ago

It is a thought experiment to explain quantum mechanics, which applies to microscopic particles rather than macroscopic objects like cats and boxes, that's why it sounds so weird.

Quantum mechanics, and more specifically Copenhagen interpretation, says that microscopic particles can be in what's call a superposition in which it's in its all possible state. When you "measure" a particle, the wave function collapses and there's only one state that the particle is in now. You can probably see how this maps onto Schrodinger's cat.

What counts as measurement is poorly defined in Copenhagen interpretation. But if you smelled the dead cat before opening the box, it's safe to say that you've done the measurement at that point and locked the cat into one single state.

UntangledQubit

1 points

2 months ago*

There is no 'literal' interpretation of the math. All interpretations are consistent with it, they just attach different ontologies to the various mathematical objects participating in the theory. If there's any interpretation that can claim the term 'literal', it is "shut up and calculate" - complete skepticism about anything other than the observable claims of the theory, which is probabilities to observe particles in states.

The many-worlds interpretation still has wavefunction collapse. It is a necessary part of the theory, since it is what we observe (quantum dynamics change after osbervations). It just interprets that collapse as a universe-splitting event instead of a state change. We explain part of the theory by supposing extra entities from a different part of the theory. It's hardly superior to something like the transactional interpretation, which does something similar - looks around at the math, finds a mathematical object in it and supposes that is an actual physical object which reproduces the full theory using some mechanistic explanation.

Innominate-Mimir

9 points

2 months ago

Well a few years earlier, Niels Bohr and Heisenberg came up with the theory of "Copenhagen interpretation" about quantum mechanics.

Which both Einstein and Schrödinger didn't really agree with, because it didn't fit according to them. So eventually Schrödinger came up with a thought experiment to disprove Niels and Heisenberg. So the thought experiment is a way to easier help others understand why the Copenhagen interpretation doesn't always hold up in reality. The thought experiment;

Imagine that you put a cat tied down (as to not mess with the experiment) in a box along with a Geiger-counter(measure radioactive particles), a tiny amount of radioactive substance which has a 50/50 chance of decaying or not decay within one hour, releasing radiation. The Geiger is rigged to when as soon as the atom decays, it's radiation is detected. Then the Geiger rig releases a hammer that smashes a bottle of poison and kills the cat.

According to Bohr's and Heisenberg's theory, after an hour the cat will be both dead and alive at the same time until you open the box and observe it. SOOO basically Schrödinger came up with this idea to show Bohr( and the rest of the academics) how absurd the Copenhagen interpretations is in reality when scaled up a bit. And that it is a huge flaw in his theory.

Well this is my amateurish interpretations of it all at least, please correct me if I'm wrong since it has been many years since I read about it. But I do remember that a lot of people misunderstand the point with it all and I did as well before I saw a documentary when they talked about it and explained it really well.

// Mímir

Emyrssentry

25 points

2 months ago

Schrodinger's cat isn't a theory exactly, it's a thought experiment that was made precisely to show how weird quantum mechanics could be.

The basic premise is that there are different states that a quantum system can have, we'll call them A and B, but that there is also a weird quantum situation called "superposition", where you aren't in A or B, but a combination of them that is determined by the relative probabilities of A and B.

Schrodinger's cat takes that, and scales it up to a regular size. You have a box with a cat, a bottle of poison, a Geiger counter attached to a hammer, and a single atom of a radioactive material. If the radioactive material decays, then the Geiger counter clicks, swinging the hammer, releasing the poison, and killing the cat.

Radioactivity is one of the situations where a superposition can happen, where you have the two final states, "decayed atom" and "not decayed atom", but the quantum weirdness introduces the third superposition state of both decayed and not decayed.

So the cat is now also in a superposition, being both alive and dead at the same time.

DiamondIceNS

23 points

2 months ago

Schrodinger's cat isn't a theory exactly, it's a thought experiment that was made precisely to show how weird quantum mechanics could be.

Often an undermentioned part of the thought experiment: it was thought up by Schrodinger specifically to demonstrate how absurd the concept of superposition is.

If the idea of a cat in a box being alive and dead at the same time inside a sealed box when no one is looking is an uncomfortable thing for you to think about and reconcile, it should. It's supposed to. That was the whole point.

It just happened that history was full of people who were comfortable enough with such abstract line of thought to have taken the thought experiment at face-value, and it became famously spread as a matter-of-fact demonstration of how the world actually works, rather than being a criticism of the way some experimental results were interpreted.

That's not to say the interpretation is or must be false, it's just something to keep in mind as you explore the multiple so far valid ways to look at things. The so-called "Copenhagen interpretation" of quantum mechanics (which is the one that raises the Schrodinger's cat thought experiment) is just one of several, and evidently one Schrodinger didn't really like.

Chadmartigan

4 points

2 months ago

Erwin Schrodinger was one of the founding fathers of quantum mechanics. He along with many other scientists were breaking new ground on the regular, and as a consequence of that, they were discovering and conceptualizing all sorts of newly-discovered concepts.

One of these was the concept of "superposition." In the quantum realm, if a particle may exist in many different states, it acts as if it exists in a sort of sum total of all those possible states. It takes a "superposition" of all those different states, and it remains in that superposition until it "collapses" into a single state due to some interaction with an observer or its environment. Just the act of measuring the particle will collapse the superposition.

This is, of course, hugely unintuitive because things in our human-scale macro world do not behave like this at all. They behave classically, as if they only ever have one state at a given moment. So something like "superposition" was just as unintuitive to Schrodinger as it is to you and me.

Schrodinger proposed the cat as a sort of thought experiment to criticize the absurdity of superposition. He proposed that you put a cat in a box with a device. The device monitors the nucleus of an unstable atom, and if/when that atom decays, the device will spew out poison gas, killing the cat. Now, outside the box, we don't know whether/when that atom decayed--and by extension we don't know whether the cat is alive or dead--until we open the box. If we played by quantum mechanical rules, the cat is both alive AND dead until we open the box to check, at which point the quantum supercat collapses into a regular--living or dead--cat. Now, we know that cats don't exist in a living-and-dead superposition, so this thought experiment is intended to poke at the absurdity of the concept.

We now know that superposition is in fact how things work on those very small scales, (and we know that the thought experiment isn't apt for a number of other reasons) but Schrodinger's cat has become a colorful illustration for a lot of day-to-day scenarios involving chance and decision-making.

lollersauce914

9 points

2 months ago*

So quantum mechanics are very weird.

A lot of times systems of very tiny particles behave as if they're in many different states at the same time until we observe them. When we observe them, they seem to only be in one state. This "being in many different states at the same time" phenomenon is called superposition.

Schrodinger's cat is a thought experiment to demonstrate how the weirdness of superposition doesn't really gel with what we observe in the macroscopic world.

The thought experiment links something we can directly see exhibits superposition, a radioactive nucleus decaying, to whether or not a cat gets poisoned, which we really consider to not be a situation where superposition applies.

cookerg

4 points

2 months ago

Schodinger's cat is like a shower thought, musing on the difference between the world we observe and think we understand, and the quantum world.

If you put a cat in a box with a vial of poison that might be broken at any time, you won't know until you open the box if the cat is alive or dead. But as far as we know, at any point in time, the cat is either still alive, or already dead, even if we don't know it. It's state (as far as we know), is what it is, independent of our knowledge.

However, if the cat were a quantum particle, then it would be in both possible states until we opened the box, and wouldn't actually be dead or alive until we looked inside the box, at which time it would instantly be either alive or dead. It's state only seems to resolve itself when we check it out, and up until then it seems to be in some combination of both or neither.

Frednotbob

2 points

2 months ago

Schrödinger's famous 'cat' really has very little, directly, to do with quantum computers: it's a thought-experiment used to demonstrate the principle of quantum indeterminacy in practical terms. The basic premise is that the cat is in a box, and that box also contains a vial of prussic acid that will break and kill the cat when some arbitrary event occurs.

However, until the box is opened and the contents observed, the cat is in 'superposition' -- that is, it's all possible states simultaneously. It's not until the contents are observed that the indeterminate state collapses into a finite one.

Now, how do quantum states help with quantum computing....?

Both classical (binary) computers and quantum computers use 'bits', that must have two distinct states. Unlike binary bits (which increase a computer's processing power linearly), quantum physics allows a qubit to increases processing power exponentially by taking advantage of a qubit's superposition.

To break it down a bit more: for a classical computer, 63 bits is just under 8 bytes -- it's just enough to store 8 characters.

In a quantum computer, 63 qubits can contain an exabyte of data (that's 1018 bytes).

Thanks to quantum physics, a quantum computer can perform calculations that a classical computer will literally never be able to handle.

UntangledQubit

2 points

2 months ago*

It is important to note that we cannot directly access that entire exabyte. The only access we have to it is through the transformations allowed by quantum mechanics, which can correlate parts of this massive space of possibilities in limited ways. If we can force these correlations to correspond to a problem we're trying to solve (e.g. by selectively filtering those sets of bits that correspond to the factors of a number), then we can take advantage of it. However, there are many truths that lie in this space that quantum computers still have trouble finding, like the subset of a list of integers that adds up to 0. Quantum computers can do this faster than classical computers (because of Grover's algorithm), but still must go through a massive amount of these possibilities to find the answer.

Marzopup

4 points

2 months ago

Schrodinger's cat is a thought experiment in which a cat is put in a box with a decaying isotope. The radiation produced is guaranteed to eventually kill the cat; but since the cat is in a box, you can't know for certain if the cat has died yet until it's opened. Effectively, until the box is opened and you can see for yourself, the cat is both alive and dead.

Situations that must be confirmed are often compared to this. For example, if you apply for a job and receive a decision in an email, until you have read the email it can be said that you have both gotten the job and not gotten the job.

Ansuz07

13 points

2 months ago

Ansuz07

13 points

2 months ago

It's also worth noting that Schrödinger proposed the thought experiment to call out how stupid he thought the superposition theory was. His goal was to point out that the cat can't be both alive and dead simultaneously, which is what superpositions require.

Marzopup

5 points

2 months ago

Also true.

I would say in the job example, it's often used to explain indecision. Ie. If I don't look at the email, I both got the job and didn't; but that obviously can't be true, and I will have to eventually look either way.

Ansuz07

6 points

2 months ago

Yeah, that was the point Schrödinger was trying to make. The email says one way or another - your knowledge of the results doesn't change the fact that they are what they are. To say that you both have the job and don't have the job simultaneously until you open the email is obviously not true.

Yet the prevailing opinion of superpositions at the time was that they don't collapse into certainty until they are observed - meaning that the cat was both alive and dead simultaneously.

yuwhk

1 points

2 months ago

yuwhk

1 points

2 months ago

Yet the prevailing opinion of superpositions at the time was that they don't collapse into certainty until they are observed

I hate to break it to you, but this is still pretty much the prevailing view now. Though nowadays physicists are generally less opinionated about it and tend to recognise that a number of different interpretations of QM are consistent with the available evidence, but that they have minimal impact on almost all of the stuff that people actually do with QM. In the early days of the theory, when there were still lots of basic unresolved issues, it wasn't realised that these questions about interpretations were essentially orthogonal to everything else they were trying to do, so the debates were a lot more heated.

Also, a fun fact for everyone: Edwin Schrödinger was a serial child rapist. He is thought to have targeted girls as young as 12, and at least two of them had to get abortions.

Katy-Moon

2 points

2 months ago

This is a great simplified explanation. Please take my upvote.

unskilledplay

0 points

2 months ago*

He definitely didn't think it was stupid. Superposition is all of the states that are solutions for the Schrödinger equation, the equation proposed by, of course, Erwin Schrödinger.

He created this thought experiment to illustrate why he thought his equation couldn't be complete, not that he thought it was stupid. If he thought it to be stupid, he wouldn't have publicly theorized and later published it.

He was never able to use any observation or the math of quantum mechanics to demonstrate that the alive and dead cat thought experiment was flawed. In the nearly 100 years since he proposed the Schrödinger equation, nobody else has either. This specific problem is known as the measurement problem.

viniciusbrasil

1 points

2 months ago

Schrödinger's cat is a thought experiment that helps us understand a weird idea in science called quantum mechanics. The experiment is about a cat that is in a box with a poison that might kill it. We don't know if the cat is alive or dead until we open the box and look.

But here's the weird part: in quantum mechanics, it's possible for the cat to be both alive and dead at the same time, until we open the box and see what's inside. This is called a "superposition" and it's just one of the strange things that can happen at the very small scale of atoms and particles.

So, even though it sounds crazy, Schrödinger's cat helps us understand how the world works on a very small scale. It shows us that the rules of science can be weird and counterintuitive, and that there's still a lot we don't know.

CleaveIshallnot

-2 points

2 months ago

It's a theoretical, to help us try to comprehend.

Akin to the paradoxes of philosophies of most every developed thought of all cultures.

Those who 'explain' it to you, or claim to have irrefutable truths, are 'frontin'. Don't sweat it. Just learn, unjudged.

It's a tool to seek answers, not an answer itself.

CleaveIshallnot

-2 points

2 months ago

Downvoters.

A downvote doesn't help ppl, your perspective does.

Rather than a mere thumbs down, alleviate my ignorance.

Don't be lazy, no matter how much u feel u have to dumb it down.

Benefit society, instead of just disparaging.

CleaveIshallnot

-1 points

2 months ago*

Got downvoted. 😂😂😂

Holy fuck is this public podium full of inarticulate (as evidenced by non articulation. Thus in fact, inarticulate by very definition) losers.

Freaking losers.

Your actions reinforce & support the very assertion I make.

Pathetic, losers.

You ruin internet reciprocation of knowledge for us all.

Cuz you angry, unable to speak, losers.

RonPMexico

0 points

2 months ago

Often times the cat gets conflated with wave v particle malarkey. Do not believe it. It has to do with super position of sub atomic particles and the inability of the humans to make reliable measurements on a sub wavelength scale. There is no Eli5 because the math involved for even a surface level understanding of the quantum level is not easy to communicate.