BẢN CHÉP ÂM THANH 🔗
WEBVTT
00:00:00.000 --> 00:00:01.760
The famous cat for dummies.
00:00:02.319 --> 00:00:09.039
Quantum on the treadmill it There may be no more famous cat in the history of science than Schrdinger's cat.
00:00:09.199 --> 00:00:11.599
You've probably heard some version of the story.
00:00:11.759 --> 00:00:18.160
A cat is placed in a sealed box with a strange contraption involving a radioactive atom, a detector, and poison.
00:00:18.320 --> 00:00:20.160
Until someone opens the box and looks.
00:00:20.320 --> 00:00:25.199
Quantum mechanics supposedly says that the cat is both alive and dead at the same time.
00:00:25.359 --> 00:00:30.160
And at that point, most normal people say, okay, quantum physics is ridiculous, but sure.
00:00:30.800 --> 00:00:31.839
But here's the important part.
00:00:32.079 --> 00:00:34.479
Schrdinger thought it sounded ridiculous too.
00:00:34.640 --> 00:00:38.240
The cat was not invented to celebrate the weirdness of quantum mechanics.
00:00:38.399 --> 00:00:40.159
It was invented to complain about it.
00:00:40.399 --> 00:00:50.799
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.
00:00:50.960 --> 00:00:55.759
And it starts with something we talked about in the last episode, Schrdinger's wave equation.
00:00:55.920 --> 00:01:02.799
Schrdinger had developed a mathematical description in which something like an electron is represented by a wave function.
00:01:02.960 --> 00:01:12.079
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.
00:01:12.159 --> 00:01:14.400
And the problem was what happened when you actually looked.
00:01:14.560 --> 00:01:17.359
Quantum mechanics seemed to contain two very different rules.
00:01:17.519 --> 00:01:22.079
The first rule said the wave function evolves smoothly according to Schrdinger's equation.
00:01:22.239 --> 00:01:29.840
The second rule said that when you perform a measurement, suddenly you get one definite answer, smooth evolution, then suddenly pop one result.
00:01:30.000 --> 00:01:31.280
That bothered Schrodinger.
00:01:31.439 --> 00:01:37.280
Imagine watching a movie where everything follows the laws of physics until somebody pulls out a camera.
00:01:37.519 --> 00:01:42.640
The instant the camera appears, the movie starts obeying a completely different set of laws.
00:01:42.959 --> 00:01:48.480
You might reasonably ask why, that is essentially the measurement problem, and this is where the cat comes in.
00:01:48.640 --> 00:01:53.680
In Schrodinger's thought experiment, imagine a radioactive atom inside a sealed box.
00:01:53.920 --> 00:01:56.400
Radioactive decay is a quantum event.
00:01:56.560 --> 00:01:59.680
We cannot predict exactly when a particular atom will decay.
00:01:59.920 --> 00:02:02.000
Quantum mechanics gives us probabilities.
00:02:02.159 --> 00:02:04.000
Now connect that atom to a detector.
00:02:04.159 --> 00:02:08.479
If the atom decays, the detector triggers a mechanism that breaks a vial of poison.
00:02:08.560 --> 00:02:10.639
And if the poison is released, the cat dies.
00:02:10.800 --> 00:02:12.319
So now we have created a chain.
00:02:12.479 --> 00:02:14.800
A tiny quantum event controls a detector.
00:02:15.039 --> 00:02:16.719
The detector controls the poison.
00:02:16.879 --> 00:02:20.719
The poison determines the fate of a very large, very ordinary cat.
00:02:20.960 --> 00:02:30.159
Think about an image of microscopic radioactive decay connected through a detector and poison flask to the macroscopic fate of the cat.
00:02:30.319 --> 00:02:36.719
The point is to magnify something happening at the quantum scale until it becomes impossible to ignore at the everyday scale.
00:02:36.800 --> 00:02:38.719
And here's where things become uncomfortable.
00:02:38.960 --> 00:02:44.800
Before the atom is measured, quantum mechanics allows us to describe it as a superposition of possibilities.
00:02:45.039 --> 00:02:51.520
Very loosely, you can think of it as having both possibilities represented in the quantum state, decayed and not decayed.
00:02:51.680 --> 00:02:56.800
But if everything inside the box is connected, then the detector becomes connected to the atom.
00:02:56.879 --> 00:03:01.199
And the poison becomes connected to the detector, and the cat becomes connected to the poison.
00:03:01.520 --> 00:03:14.159
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.
00:03:14.319 --> 00:03:15.039
That is the joke.
00:03:15.199 --> 00:03:17.039
Or more accurately, that is the complaint.
00:03:17.199 --> 00:03:25.199
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?
00:03:25.439 --> 00:03:27.360
Schrödinger was making a complaint.
00:03:27.599 --> 00:03:31.919
The cat is a critique of measurement, not a mascot celebrating quantum weirdness.
00:03:32.080 --> 00:03:37.439
That distinction matters because over time popular culture flipped the meaning of Schrodinger's cat.
00:03:37.599 --> 00:03:42.000
Today we often use it to mean quantum mechanics is amazing, the cat is alive and dead.
00:03:42.159 --> 00:03:51.599
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.
00:03:51.759 --> 00:03:54.400
And that leads us to an important word, superposition.
00:03:54.560 --> 00:04:03.439
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.
00:04:03.759 -->
Load full transcript
00:04:10.240
Imagine I put a coin under a cup, you do not know whether it is heads or tails, but the coin itself already knows.
00:04:10.400 --> 00:04:13.360
There is a definite answer, you simply do not have the information.
00:04:13.520 --> 00:04:15.759
Quantum superposition is stranger than that.
00:04:15.919 --> 00:04:20.079
These are multiple possibilities before measurement, not merely hidden information.
00:04:20.319 --> 00:04:26.639
The quantum state really contains the different possible outcomes in a way that can interfere with one another.
00:04:26.879 --> 00:04:29.680
The classic example is the double slit experiment.
00:04:29.839 --> 00:04:38.000
Send quantum particles toward two openings without determining which opening each particle passes through, and over time you get an interference pattern.
00:04:38.160 --> 00:04:40.160
That interference pattern is the giveaway.
00:04:40.319 --> 00:04:43.360
The possibilities behave like waves and interfere with each other.
00:04:43.519 --> 00:04:49.279
But if you arrange the experiment so you can determine which slit the particle passed through, the interference disappears.
00:04:49.439 --> 00:04:54.240
Something about measurement changes what we observe, and that brings us back to the cat.
00:04:54.399 --> 00:04:57.199
The real question is not is the cat alive or dead?
00:04:57.360 --> 00:05:02.000
The real question is when does the quantum world turn into the definite world we experience?
00:05:02.240 --> 00:05:05.439
Is it when the radioactive atom interacts with the detector?
00:05:05.600 --> 00:05:13.839
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.
00:05:14.000 --> 00:05:15.279
That is the measurement problem.
00:05:15.439 --> 00:05:19.680
And physicists still disagree about what the mathematics is telling us about reality.
00:05:19.920 --> 00:05:24.000
One answer is usually associated with the Copenhagen interpretation.
00:05:24.160 --> 00:05:29.279
Very roughly, before measurement, the system is described by a superposition of possibilities.
00:05:29.439 --> 00:05:33.120
When a measurement is taken, we obtain a single definite observed result.
00:05:33.279 --> 00:05:35.199
Open the box, alive, or dead.
00:05:35.360 --> 00:05:35.920
One fact.
00:05:36.160 --> 00:05:40.480
This does not necessarily mean human consciousness magically creates reality.
00:05:40.639 --> 00:05:45.920
Measurement can be understood in terms of physical interaction and the attainment of a definite outcome.
00:05:46.079 --> 00:05:48.160
But Copenhagen leaves a question hanging.
00:05:48.319 --> 00:05:49.920
What exactly counts as a measurement?
00:05:50.079 --> 00:05:51.839
This is where another idea enters the story.
00:05:52.000 --> 00:06:00.319
Decoherence, a perfectly isolated quantum system, can maintain delicate superpositions, but real objects constantly interact with their surroundings.
00:06:00.480 --> 00:06:07.199
Air molecules hit them, photons bounce off them, heat moves through them, the environment effectively becomes entangled with the system.
00:06:07.360 --> 00:06:14.160
Those interactions rapidly destroy the interference effects that make quantum superpositions visible.
00:06:14.399 --> 00:06:21.199
This helps explain why we do not see chairs, baseballs, treadmills, or cats wandering around in obvious superpositions.
00:06:21.360 --> 00:06:25.040
But decoherence does not completely end the philosophical argument.
00:06:25.199 --> 00:06:32.240
Decoherence can explain the loss of interference, but it does not by itself explain why we experience one specific outcome.
00:06:32.399 --> 00:06:34.560
And that opened the door to more interpretations.
00:06:34.879 --> 00:06:37.279
One of the strangest is the many worlds interpretation.
00:06:37.600 --> 00:06:41.279
Many worlds says maybe nothing collapses, maybe both outcomes continue.
00:06:41.360 --> 00:06:45.600
And there is a branch in which the cat is alive, and another branch in which the cat is dead.
00:06:45.759 --> 00:06:49.519
When you open the box, you become correlated with one of those branches.
00:06:49.759 --> 00:06:52.879
You do not create the outcome, you discover which branch you are in.
00:06:53.120 --> 00:06:53.519
Crazy?
00:06:53.680 --> 00:06:54.000
Maybe.
00:06:54.160 --> 00:06:55.199
But notice what happened.
00:06:55.439 --> 00:06:58.959
We started with an absurd thought experiment designed to expose a problem.
00:06:59.199 --> 00:07:05.600
And solving the problem led physicists toward explanations that were arguably even stranger than the cat.
00:07:05.759 --> 00:07:07.279
Then things got stranger again.
00:07:07.439 --> 00:07:13.519
In 1961, physicist Eugene Wigner proposed an extension now called Wigner's Friend.
00:07:13.759 --> 00:07:20.800
Imagine that instead of a cat inside the box, there is a scientist inside a sealed laboratory performing a quantum measurement.
00:07:21.040 --> 00:07:22.879
The scientist sees a definite result.
00:07:23.040 --> 00:07:27.040
From inside the laboratory, there is no ambiguity, but Wigner is standing outside.
00:07:27.120 --> 00:07:36.240
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.
00:07:36.399 --> 00:07:39.920
So now one observer says, I have a definite fact.
00:07:40.079 --> 00:07:46.000
And another observer says, I can still describe your entire laboratory as a quantum superposition.
00:07:46.319 --> 00:07:50.480
The PDF asked the uncomfortable question directly on page 22.
00:07:50.639 --> 00:07:52.879
When does one outcome become real for everyone?
00:07:53.120 --> 00:07:54.800
Now we are no longer talking about cats.
00:07:55.040 --> 00:07:57.199
We are talking about the nature of reality.
00:07:57.279 --> 00:08:00.079
And this is one of the fascinating things
about quantum mechanics.
00:08:00.319 --> 00:08:03.519
Almost everyone agrees that the mathematics works spectacularly well.
00:08:03.680 --> 00:08:12.959
Quantum mechanics gave us technologies including semiconductors, transistors, lasers, LEDs, MRI systems, superconductors, and much of modern electronics.
00:08:13.199 --> 00:08:16.399
The argument is not primarily about whether the equations work.
00:08:16.560 --> 00:08:18.560
The argument is about what the equations mean.
00:08:18.720 --> 00:08:20.560
Math is agreed, meaning is not.
00:08:20.800 --> 00:08:25.600
Physicists broadly agree on the mathematics of superposition and entanglement.
00:08:25.759 --> 00:08:29.920
They do not all agree on what kind of reality that mathematics describes.
00:08:30.240 --> 00:08:31.759
Does measurement reveal reality?
00:08:31.920 --> 00:08:33.600
Does it select one possibility?
00:08:33.840 --> 00:08:34.960
Does the universe branch?
00:08:35.200 --> 00:08:37.519
Does interaction with the environment explain what we see?
00:08:37.919 --> 00:08:42.159
Is there some deeper theory underneath quantum mechanics that we have not discovered yet?
00:08:42.320 --> 00:08:44.080
Those questions remain alive.
00:08:44.320 --> 00:08:47.200
And there is one more reason Schrdinger's cat matters today.
00:08:47.360 --> 00:08:53.440
Quantum computers deliberately work with the same strange quantum properties that made the cat thought experiment possible.
00:08:53.679 --> 00:08:55.519
A classical bit is either zero or one.
00:08:55.759 --> 00:08:59.759
A qubit can exist in a quantum superposition involving zero and one.
00:09:00.159 --> 00:09:05.600
Multiple qubits can become entangled so that they must be described as parts of one larger quantum state.
00:09:05.759 --> 00:09:12.399
Quantum algorithms manipulate those possibilities before eventually performing a measurement and producing ordinary classical results.
00:09:12.639 --> 00:09:19.519
In other words, what once looked like an embarrassing philosophical problem has become something engineers are learning to control.
00:09:19.759 --> 00:09:22.720
We are building machines that depend on superposition.
00:09:22.879 --> 00:09:24.559
We deliberately create entanglement.
00:09:24.720 --> 00:09:29.360
We preserve quantum states long enough to perform computations, and then at the end we measure them.
00:09:29.519 --> 00:09:32.000
So Schrdinger's cat was never really about a cat.
00:09:32.080 --> 00:09:42.159
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.
00:09:42.480 --> 00:09:45.039
Nearly a century later, the equations still work.
00:09:45.279 --> 00:09:47.200
The technologies built from them work.
00:09:47.360 --> 00:09:49.919
Quantum computers are pushing those ideas further.
00:09:50.080 --> 00:09:54.799
But the philosophical question Schrödinger was pointing at has never completely disappeared.
00:09:54.960 --> 00:09:56.639
What does observation actually do?
00:09:56.879 --> 00:09:59.840
Does it reveal reality, a select reality, a split reality?
00:10:00.080 --> 00:10:03.120
Or is our entire way of asking the question somehow wrong?
00:10:03.360 --> 00:10:10.799
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.
00:10:10.960 --> 00:10:15.519
Not exactly Schrdinger invented the cat because he thought that conclusion was absurd.
00:10:15.759 --> 00:10:18.559
The cat was not the answer, the cat was the question.