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The famous cat for dummies.
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Quantum on the treadmill it There may be no more famous cat in the history of science than Schrdinger's cat.
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You've probably heard some version of the story.
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A cat is placed in a sealed box with a strange contraption involving a radioactive atom, a detector, and poison.
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Until someone opens the box and looks.
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Quantum mechanics supposedly says that the cat is both alive and dead at the same time.
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And at that point, most normal people say, okay, quantum physics is ridiculous, but sure.
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But here's the important part.
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Schrdinger thought it sounded ridiculous too.
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The cat was not invented to celebrate the weirdness of quantum mechanics.
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It was invented to complain about it.
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So today let's do Schrdinger's cat for dummies, because if you understand why Schrdinger put that poor imaginary cat in the box, you understand one of the deepest problems in quantum mechanics.
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And it starts with something we talked about in the last episode, Schrdinger's wave equation.
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Schrdinger had developed a mathematical description in which something like an electron is represented by a wave function.
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Instead of saying the electron is right here, the mathematics describes a collection of possible outcomes and their probabilities, and that mathematics worked remarkably well.
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And the problem was what happened when you actually looked.
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Quantum mechanics seemed to contain two very different rules.
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The first rule said the wave function evolves smoothly according to Schrdinger's equation.
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The second rule said that when you perform a measurement, suddenly you get one definite answer, smooth evolution, then suddenly pop one result.
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That bothered Schrodinger.
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Imagine watching a movie where everything follows the laws of physics until somebody pulls out a camera.
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The instant the camera appears, the movie starts obeying a completely different set of laws.
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You might reasonably ask why, that is essentially the measurement problem, and this is where the cat comes in.
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In Schrodinger's thought experiment, imagine a radioactive atom inside a sealed box.
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Radioactive decay is a quantum event.
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We cannot predict exactly when a particular atom will decay.
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Quantum mechanics gives us probabilities.
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Now connect that atom to a detector.
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If the atom decays, the detector triggers a mechanism that breaks a vial of poison.
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And if the poison is released, the cat dies.
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So now we have created a chain.
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A tiny quantum event controls a detector.
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The detector controls the poison.
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The poison determines the fate of a very large, very ordinary cat.
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Think about an image of microscopic radioactive decay connected through a detector and poison flask to the macroscopic fate of the cat.
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The point is to magnify something happening at the quantum scale until it becomes impossible to ignore at the everyday scale.
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And here's where things become uncomfortable.
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Before the atom is measured, quantum mechanics allows us to describe it as a superposition of possibilities.
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Very loosely, you can think of it as having both possibilities represented in the quantum state, decayed and not decayed.
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But if everything inside the box is connected, then the detector becomes connected to the atom.
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And the poison becomes connected to the detector, and the cat becomes connected to the poison.
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So if we blindly apply the quantum mathematics all the way up the chain, we seem to get something like atom decayed, cat dead, and atom not decayed, cat alive, existing together in one combined quantum state.
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That is the joke.
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Or more accurately, that is the complaint.
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Schrdinger was basically saying, Do you really expect me to believe there is literally a cat that is both alive and dead until somebody opens the box?
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Schrödinger was making a complaint.
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The cat is a critique of measurement, not a mascot celebrating quantum weirdness.
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That distinction matters because over time popular culture flipped the meaning of Schrodinger's cat.
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Today we often use it to mean quantum mechanics is amazing, the cat is alive and dead.
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Schrdinger was closer to saying, guys, if your interpretation of the mathematics leads to an alive and dead cat, maybe we have not finished explaining what the mathematics means.
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And that leads us to an important word, superposition.
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Superposition is one of those quantum words that gets abused, and people sometimes describe it as if a quantum particle is simply hiding its real answer from us.
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Load full transcript
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Imagine I put a coin under a cup, you do not know whether it is heads or tails, but the coin itself already knows.
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There is a definite answer, you simply do not have the information.
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Quantum superposition is stranger than that.
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These are multiple possibilities before measurement, not merely hidden information.
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The quantum state really contains the different possible outcomes in a way that can interfere with one another.
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The classic example is the double slit experiment.
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Send quantum particles toward two openings without determining which opening each particle passes through, and over time you get an interference pattern.
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That interference pattern is the giveaway.
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The possibilities behave like waves and interfere with each other.
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But if you arrange the experiment so you can determine which slit the particle passed through, the interference disappears.
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Something about measurement changes what we observe, and that brings us back to the cat.
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The real question is not is the cat alive or dead?
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The real question is when does the quantum world turn into the definite world we experience?
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Is it when the radioactive atom interacts with the detector?
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When the detector breaks the poison vial, when the cat becomes involved, when the environment interacts with everything inside the box, or when somebody opens the box.
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That is the measurement problem.
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And physicists still disagree about what the mathematics is telling us about reality.
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One answer is usually associated with the Copenhagen interpretation.
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Very roughly, before measurement, the system is described by a superposition of possibilities.
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When a measurement is taken, we obtain a single definite observed result.
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Open the box, alive, or dead.
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One fact.
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This does not necessarily mean human consciousness magically creates reality.
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Measurement can be understood in terms of physical interaction and the attainment of a definite outcome.
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But Copenhagen leaves a question hanging.
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What exactly counts as a measurement?
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This is where another idea enters the story.
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Decoherence, a perfectly isolated quantum system, can maintain delicate superpositions, but real objects constantly interact with their surroundings.
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Air molecules hit them, photons bounce off them, heat moves through them, the environment effectively becomes entangled with the system.
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Those interactions rapidly destroy the interference effects that make quantum superpositions visible.
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This helps explain why we do not see chairs, baseballs, treadmills, or cats wandering around in obvious superpositions.
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But decoherence does not completely end the philosophical argument.
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Decoherence can explain the loss of interference, but it does not by itself explain why we experience one specific outcome.
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And that opened the door to more interpretations.
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One of the strangest is the many worlds interpretation.
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Many worlds says maybe nothing collapses, maybe both outcomes continue.
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And there is a branch in which the cat is alive, and another branch in which the cat is dead.
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When you open the box, you become correlated with one of those branches.
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You do not create the outcome, you discover which branch you are in.
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Crazy?
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Maybe.
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But notice what happened.
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We started with an absurd thought experiment designed to expose a problem.
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And solving the problem led physicists toward explanations that were arguably even stranger than the cat.
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Then things got stranger again.
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In 1961, physicist Eugene Wigner proposed an extension now called Wigner's Friend.
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Imagine that instead of a cat inside the box, there is a scientist inside a sealed laboratory performing a quantum measurement.
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The scientist sees a definite result.
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From inside the laboratory, there is no ambiguity, but Wigner is standing outside.
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If Wigner treats the entire laboratory quantum mechanically, then before he interacts with it, he can describe the scientist, equipment, and result together as a superposition.
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So now one observer says, I have a definite fact.
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And another observer says, I can still describe your entire laboratory as a quantum superposition.
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The PDF asked the uncomfortable question directly on page 22.
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When does one outcome become real for everyone?
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Now we are no longer talking about cats.
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We are talking about the nature of reality.
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And this is one of the fascinating things
about quantum mechanics.
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Almost everyone agrees that the mathematics works spectacularly well.
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Quantum mechanics gave us technologies including semiconductors, transistors, lasers, LEDs, MRI systems, superconductors, and much of modern electronics.
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The argument is not primarily about whether the equations work.
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The argument is about what the equations mean.
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Math is agreed, meaning is not.
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Physicists broadly agree on the mathematics of superposition and entanglement.
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They do not all agree on what kind of reality that mathematics describes.
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Does measurement reveal reality?
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Does it select one possibility?
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Does the universe branch?
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Does interaction with the environment explain what we see?
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Is there some deeper theory underneath quantum mechanics that we have not discovered yet?
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Those questions remain alive.
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And there is one more reason Schrdinger's cat matters today.
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Quantum computers deliberately work with the same strange quantum properties that made the cat thought experiment possible.
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A classical bit is either zero or one.
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A qubit can exist in a quantum superposition involving zero and one.
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Multiple qubits can become entangled so that they must be described as parts of one larger quantum state.
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Quantum algorithms manipulate those possibilities before eventually performing a measurement and producing ordinary classical results.
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In other words, what once looked like an embarrassing philosophical problem has become something engineers are learning to control.
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We are building machines that depend on superposition.
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We deliberately create entanglement.
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We preserve quantum states long enough to perform computations, and then at the end we measure them.
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So Schrdinger's cat was never really about a cat.
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It was Schrodinger holding up a giant warning sign saying, there is something very strange about the transition between quantum possibilities and the single reality we experience.
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Nearly a century later, the equations still work.
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The technologies built from them work.
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Quantum computers are pushing those ideas further.
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But the philosophical question Schrödinger was pointing at has never completely disappeared.
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What does observation actually do?
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Does it reveal reality, a select reality, a split reality?
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Or is our entire way of asking the question somehow wrong?
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So the next time someone tells you that Schrdinger's cat means the cat is both alive and dead, you can give them the slightly more interesting answer.
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Not exactly Schrdinger invented the cat because he thought that conclusion was absurd.
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The cat was not the answer, the cat was the question.