Schrödinger's Cat: Dead, Alive or Both?
About this video
The video explains Schrödinger's Cat, a thought experiment where a cat's life depends on whether a radioactive atom decays. Because quantum particles can be in superposition until observed, the atom, and so perhaps the cat, seems to be in two states at once. Quantum decoherence explains why we never see this in everyday life, and Schrödinger meant the experiment to show how much we still have to learn.
You'll be able to
- describe the Schrödinger's Cat thought experiment.
- explain what superposition means and what happens when we observe it.
- explain why we don't see superposition in everyday objects.
Key words
- Thought experiment
- An imagined experiment used to think through an idea.
- Quantum particles
- The tiniest particles, which don't behave the way we normally expect.
- Superposition
- When quantum particles are in multiple states at once.
- Quantum decoherence
- Why quantum behaviours like superposition don't show up in our everyday world.
Transcript
Inside this box is an adorable cat, a jar of Kitty Killer ® Gas, a hammer, and a radioactive atom. This setup was the basis for a famous thought experiment from the physicist Erwin Schrödinger, known as "Schrödinger's Cat". There's a 50% chance that the atom decays.
If it does, the hammer smashes the glass and releases the poison, killing the cat. Otherwise, the cat lives. Essentially, there's a 50% chance our cat dies from this experiment. The important thing is, we can't be sure what has happened to the cat until we open the box to check.
This experiment is extremely controversial, but because of what happens when the box is closed. You see, science shows us that until we open it and look inside, the atom isn't really decayed OR undecayed: it's both at the same time. Because atoms are so small, they don't act in the ways we'd normally expect.
When the tiniest particles, known as quantum particles, are forced to make a decision, they can choose EVERY option at once. If they were a coin tossed in the air, they would land on both heads AND tails at the same time. You see, quantum particles can be in multiple states at once, known as 'Superposition'.
Scientists don't know why or exactly how this works, but it's been seen in many experiments. It has been demonstrated that if you fired a tiny part of an atom, called an 'electron', through a wall with two gaps, it would actually appear to go through BOTH at once.
Science shows trying to observe superposition causes particles to collapse into a single state. So, the atom in Shrödinger's experiment is really both decayed AND undecayed until we look into the box. This makes things tricky, because our poor cat's life depends on the state of the radioactive atom in the box.
Could it be we have a cat that's both dead AND alive (a sort of zombie cat) until we open the box? What is a zombie cat!?! * crickets chirp ambiently * After all, with all physical things (including us) being made from atoms which CAN be in superposition, it would make sense if the cat could be in superposition, too.
But, we have a problem. Our experience of reality tells us we can't be dead AND alive: it always has to be one or the other, right? Strangely, the behaviours we see in tiny quantum particles simply don't work and don't make sense in our everyday world. Physicists call this 'Quantum Decoherence'.
I mean, just think about the last time you were dead AND alive, or sitting in two different spots in the same room at the same time. It just doesn't happen. The funny thing is: Schrödinger wasn't using his thought experiment as a way to try and explain quantum physics, nor was he intentionally trying to challenge ideas about reality and consciousness.
Rather, Schrodinger wanted to emphasize just how much more we have left to learn about the universe.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.