If you know the state of a system, say the position and velocity of a particle, then you can use an equation Newton's second law, to calculate what that particle will do in the future.

In quantum mechanics.

If you know the quantum state of a particle that is its wave function, you can use the Schrodinger equation to calculate what that particle do in the future.

Usually, it spreads out over time, as it is doing here.

Note.

To make this animation, we really solved the Schrodinger equation.

So there's a beautiful symmetry here.

If you know the initial state, you can use an equation to evolve that state smoothly and continuously into the future.

The problem is, in quantum mechanics, we never actually observed the wave function like this.

Instead, when we measure it, we find the particle at a single point in space.

So how are we to reconcile the spread out wave function evolving smoothly under the Schrodinger equation?

With this point like particle detection now, I think it's understandable that when the founders of quantum theory approach this problem, they considered the measurement Maur riel than the way function.

After all, the measurement was something we had actually observed, and it matches our experience of a world of matter particles.

It was harder to say what the wave function was exactly.

Schrodinger formulated his wave equation because scientists, notably de Broglie, suspected that matter has wavelike properties.

But it took 1/3 physicist, Max, born to propose how we should interpret the wave function at each point in space.

The way function has a complex amplitude, essentially just a real number, plus an imaginary number, Max Born suggested.

If you take that amplitude and square it, you get the probability of finding the particle there.

The fact that you have to square the amplitude actually appears as a last minute footnote in borns paper.

But that is how probability was introduced into the core of our picture of reality.

That's a pretty big philosophical leap.

I mean, no longer is the universe deterministic.

This made a lot of scientists, especially Einstein, uncomfortable.

But the born rule, as it is now called, remains at the heart of quantum mechanics because it is spectacularly successful at predicting the outcomes of experiments.

So the way quantum mechanics came to be understood, and the way I learned it is that there are two sets of rules.

When you're not looking, theweek function simply evolves according to the Schrodinger equation.

But when you are looking, when you make a measurement, the wave function collapses suddenly and irreversibly, and the probability of measuring any particular outcome is given by the amplitude of the wave function associated with that outcome squared now.

Schrodinger himself hated this formulation, which is actually why he invented the famous Schrodinger's cat thought experiment.

Put a cat in a box with a radioactive Adam.

Add a radiation detector that triggers the release of poisonous cyanide gas Now, although it was only meant as a thought experiment, Schrodinger helpfully notes, this device must be secured against direct interference by the cat.

Anyway, the whole point of the experiment is to magnify the state of the Adam up to the state of something macroscopic and tangible.

He could have picked anything.

It didn't have to be alive, but Schrodinger selected a cat.

If the Adam decays, the detector detects radiation, releases the poison and the cat dies.

If the Adam doesn't decay, the detector doesn't detect radiation.

Poison is not released, and a cat remains alive.

Since the state of the cat and detector apparatus are directly tied to the state of the Adam, we say they are entangled.

Where things get weird is that, according to quantum mechanics, the state of the Adam does not have to be either decayed or not decayed.

Generally, it's in a superposition of both decayed and not decayed at the same time.

Assuming no measurements have been made, this superposition state of the Adam gets entangled with the detector and then the cat.

So after some time, the wave function of everything inside the boxes in a superposition of the atom has not decayed poison not released Cat Alive state and the Adam has decayed poison released Cat Dead State.

So according to quantum mechanics, the cat really is both alive and dead at the same time.

On Lee, when we open the box and make a measurement, does the wave function collapse and the cat actually becomes either dead or alive?

These days, Schrodinger's cat is often used as a way to show how weird quantum mechanics is.

But that wasn't Schrodinger's point.

He wanted to show that quantum mechanics as formulated was wrong.

So taking up Schrodinger's argument in this video, I want to show that there is a better way to think about Schrodinger's cat.

In fact.

Ah, better way to think about quantum mechanics entirely.

That, I'd argue, is more logical and consistent.

To get there, we have to examine the three essential components of Schrodinger's cat superposition, entanglement and measurement to see if any of them is flawed.

Superposition is the idea that quantum objects can be in two different states at the same time.

This seems like a crazy idea and something we'd never observed.

But we do indirectly with the double slit experiment.

Fire individual electrons through two slits at a screen, and the pattern you see is not just the sum of electrons going separately through one slit and the other slit.

It is an interference pattern.

We are forced to conclude that a single electron somehow goes through one slit and the other slipped simultaneously.

This is superposition, of course.

It's easy to understand superposition with waves.

They are spread out in space, and it's clear how the peak of a wave from one slip cancels with the trough of the wave from another slip to produce the interference pattern.

And luckily we know that when we're not looking, electrons are represented by a wave the wave function.

The double slit experiment, then is concrete evidence that this wave enables individual electrons to pass through both slits at the same time.

So superposition is on solid ground.

The next concept is entanglement.

Consider two electrons fired toward each other with equal and opposite velocities.

We know they will scatter off each other, but we don't know exactly how their trajectories are given by spread out way functions that on Lee give us probabilities.

But as soon as we measure the momentum of one of the electrons, we immediately know the momentum of the other one.

It must be equal and opposite.

Otherwise, conservation of momentum would be violated.

Now.

This may seem obvious, but consider that before the measurement, the momentum of each electron was in a superposition of states, measuring one instantaneously collapsed the wave function of the other.

And this would be true.

Even if those electrons were light years apart.

These electrons are entangled.

What's really going on here is that after interacting, the electrons do not have separate wave functions at all.

They are described by a single wave function, and this is what it means to be entangled.

This explains why measuring one immediately affects the state of the other one because the single wave function has collapsed.

In fact, if we were being rigorous, we'd have to say that there is on Lee one wave function the way function of the entire universe, which includes absolutely everything.

But in the case of isolated, un entangled quantum particles, we can reasonably talk about their individual wave functions and then once they interact with something else, entanglement is the result.

So what we've seen is superposition is really the same thing as describing systems with waves and entanglement means that after particles interact there described by a single wave function.

These are fundamental parts of quantum theory, describing systems with wave functions that evolve according to the Schrodinger equation, which leaves only measurement.

Remember, the measurement postulate was added as a second set of rules to connect the mathematics of quantum mechanics to what we actually observe.

But doesn't it seem weird that there should be one rule for house systems evolve when we're not looking in a different rule for when we are when you boil it down.

Measurement is just the interaction of one quantum system, electrons and photons with another quantum system.

And we know exactly how to deal with that.

We simply evolved their wave functions according to the Schrodinger equation.

So what if we throw out all the rules associated with measurement?

Well, then, in the Schrodinger's cat thought experiment, the radioactive Adam, in a superposition of decayed and not decayed, gets entangled with the detector and in turn, the cat.

Now, remember, we're also made of electrons in atoms which obey the laws of quantum mechanics.

So we are quantum mechanical, too.

So when we open the box, there is no measurement, no wave function collapse.

We simply get entangled with the state of everything inside the box.

So we see the cat alive and we see the cat dead.

Now, how is that possible?

I'm guessing you've never seen both an alive and dead cat before, but the solution is it's because the you that saw the cat alive and the you that saw it dead actually inhabit separate worlds.

By that, I mean, they exist in their own complete realities, and those realities will never interact.

But where did these separate worlds come from?

Well, something I haven't mentioned yet are all the particles of the environment the air molecules, photons, everything that we are not keeping track of.

If a quantum object in a superposition gets entangled with the environment, it is said to undergo environmental D coherence.

This branch is the wave function of the universe, essentially split in the universe into two slightly different copies.

So a more realistic account of Schrodinger's cat goes like this.

The radioactive Adam evolves from 100% not decayed, into a quantum superposition of decayed and not decayed.

The detector becomes entangled with this superposition state of the Adam, but the detector is being bombarded by all these air molecules and photons in the box, which would bounce off differently if it is detected radiation than if it hasn't so.

Almost immediately, the detector becomes entangled with the state of the environment.

It deco.

Here's branching the wave function in two.

At that moment, you are split into two identical copies, one entangled with each outcome of the experiment.

You continue to be identical until you open the box.

But in this case, the cat actually is alive or dead.

You were just finding out by opening the box.

What we are unaware of is that the other outcome also happened just to someone who is not you anymore.

I mean, both observers came from you, but they are no longer you, and they're no longer identical to each other.

This interpretation of quantum mechanics is called many worlds, and it was formulated by Hugh Everett.

And if it's true, the branching of the wave function is happening all the time so frequently, in fact, that the rate may well be infinite, creating infinite, subtly different worlds all the time may sound implausible, to put it mildly, but consider that all those worlds are naturally part of the mathematics of quantum mechanics.

Many worlds just takes them seriously.

To get rid of them requires something like the collapse of the wave function.

And the point is, our experience of reality would be the same in the many worlds picture as it is if the wave function collapses.

But the formalism is so much cleaner and more elegant.

All we have are wave functions that evolve under the Schrodinger equation.

The implication is that the founders of quantum theory may have got it exactly backwards.

The wave function is the complete picture of reality, and our measurement is just a tiny fraction of it, the part we become entangled with when we interact with a quantum object in a superposition, the universe also goes back to being deterministic.

Every outcome happens.

Ah, 100% of the time.

It only doesn't look that way to us because we only experience are tiny sliver of the multi verse.

Now, I imagine that a lot of you have questions and possibly objections to this.

So I went to the expert.

How are you?

Okay, so I wanted to make this video about many worlds, but I was concerned I was gonna screw it up.

So I've come here to meet Caltech Professor Sean Carroll, who has literally written the book on many worlds.

Look something deeply hidden, available wherever books available.

Let's ask.

Probably the common sort of YouTube questions.

The arguments against this?

Yes.

How many?

How many worlds are there now?

The 1st 1 is energy energy oration.

How was energy?

Conserved is completely clear in the math.

Uh, the energy of the whole way function is 100% super duper conserved.

But there's a difference between the energy of the whole way function and the energy that people in each branch Percy so we should think of is not duplicating the whole universe but taking a certain amount of universe and sort of subdividing it, slicing it into two pieces.

The pieces look identical from the inside, except that one has spent up.

The one has been down or something like that, but they're really contributing less than the original to the total energy of everything.

Let's ask the question about how many words there are.

How frequently are they branching like?

We have no idea.

There's a short answer to this on.

I think it's embarrassing to be Don't have any idea.

It's certainly often It's certainly a lot right.

The universe branches whenever a quantum system in superposition becomes entangled with his environment.

So you have atomic nuclei in your body that are radioactive decay.

5000 times a second, there's a radioactive decay in your body.

Every one of those either decays or doesn't do you think of it as a superposition once it decays.

It sort of interacts with what's around.

It becomes entangled in the universe, branches its weight function right.

So branching is happening many, many times a second just because of radioactive decays in your body.

Now, is it happening infinitely often?

We don't know, because we don't know whether the total number of possible branches is infinitely bigger.

Finite.

Uh, it's Jake, Humongous.

By any stretch, there's plenty of room for all these branches to exist, and it might very well be finite, but that the details hinge on things we don't understand about quantum gravity and cosmology and the theory of everything and all that stuff.

So it's a big number, but we don't know how big.

Let's deal with the misconception that many worlds means everything that could possibly happen happens.

Yeah, that's not true.

Many worlds means the way function obeys the shorting recreation.