How can you see a cube that isn't really there?

(gentle music) Why does this window spin one way and then the other?

How can something shrink before your eyes?

And then grow again?

And why do you see a chair where there is none?

Optical illusions like these are more than just tricks.

They can actually help us unlock the secrets of how we make sense of the world around us, and even how we construct reality itself.

In this video, we're going to explore a whole bunch of really mind-bending optical illusions and dig deep into how they work, to try and answer to one important question: How do you know that what you see is real?

(upbeat music) Hey smart people, Joe here.

Every waking minute of every day, your brain is doing something truly incredible.

It's taking a gazillion tiny electrical impulses from your eyes and creating a fully functional three-dimensional reality.

It's pretty impressive when you put it like that.

The scientific term for this is spatial perception And it's one of the most fundamental ways that you make sense of the world around you.

I mean, think about it, if you want to move around your environment you gonna be able to detect obstacles, to know how they are arranged.

What kinda space is between them.

I mean, I don't care if you want to do something as complex as brain surgery, or as simple as picking that book off the table, Your brain needs a really accurate picture of how big things are and how far away they really are.

And that's much harder than it sounds.

Space has 3 dimensions.

If you look at it sort of mathematically, those 3 dimensions are basically identical.

But when it comes to how your eye sees the world, those 3 dimensions are not created equal.

Because your brain has to make sense of a 3D world using a 2D picture.

You can put objects in order in the up-down dimension by how those images land on your retina bottom to top.

Same with left to right, those images hit the back of your eye in different spots, so it's easy to put them in order in the side to side dimension.

But how do you figure out that other dimension: how far away an object is?

Well, even though all the light coming into your eye hits the back as a flat image, hidden in that 2-dimensional picture are clues.

There's many different kinds of clues.

And we use these bits of information, these clues, totally unconsciously, to build our picture of reality.

These circles are the same size.

But how far away is each circle?

And which one is farther away now?

Looking from here, you can clearly see which one's closer, and that one circle is actually bigger than the other one.

I can just slide this closer or farther away.

The illusion worked, you couldn't figure out size and distance, because you didn't have enough clues.

When you look through here, the only information that your eye has is which circle is bigger?

And well, size matters!

When things are closer, they literally take up more area on the back of your eye than when they're far away.

So with no other clues present, your brain thinks the bigger thing is close, and the little thing is far, even though they're the same distance away.

And when they look the same size, they can look the same distance away, even when they're not.

Of course, if you have more clues, you can get a better picture of reality.

I'm in front of all that stuff back there.

But how do ya know that?

One clue is that it's maybe slightly out of focus but you also can't see all of these things.

I cover up that stuff, so I'm in front and it's in back.

It's the reason this turtle's in front of this one, who's in front of this one, who's in front of this one.

It's turtles all the way back.

If you can't see all of something, then it's farther away than the thing that you can see all of.

At least that's true most of the time.

You need more clues!

Magic.

And another one of those clues explains why the drawings you did in kindergarten look so awful.

The first thing you do is draw the ground, right?

Then maybe you put a tree over here.

And here's a house, and here's a person.

But how do you tell what's closer and what's farther away when they're all standing flat on this line?

You can't!

If you can't see the ground and the horizon, you can't tell how far away things are.

But if you can and when you add in those other clues: how big things are, what's on top of what, then you can start to build a three dimensional reality.

Take a picture like this.

It's full of clues!

These trees closer to the horizon, they're higher up in the picture, so they're farther away.

And these trees down here are overlapping those other trees, so they're closer But the most important clue in this image is linear perspective.

Even though a path like this is made of two parallel lines the same distance apart, they appear to meet at a point on the horizon called the vanishing point.

Sensing these parallel lines, seeing things converge on a vanishing point is what creates the sensation of perspective in your brain, and it's a really powerful clue.

You know, noticing this linear perspective clue in the world, that's one of the big things people in Europe figured out around the beginning of the 15th century, and it's why art went from looking like this to looking like this.

(orchestral music) These are a few of the clues you use every day to build your reality.

And you don't even have to consciously think about them!

You just DO it.

That's incredible!

Give yourself a pat on the back.

And that brings us to one of the all-time great mind-breaking illusions ever constructed, the Ames window.

The window appears to be turning normally.

Until this point, when it stops and spins the other way.

Most people see this window kinda oscillating back and forth infinitely.

Except that it's not.

It's just rotating.

Round and round and round.

Whee!

Very weird though.

This is an incredibly powerful illusion.

And it gets even stranger.

Watch what happens when I put something through the middle here.

At first everything seems normal, until the object appears to rotate backwards?

What in the cerebral cortex is going on here?

For a long time, people thought this Ames window illusion works because we live in physical spaces built out of rectangles and right angles.

What has come to be known as the carpentered world.

Basically we're used to seeing rectangular things that don't actually look like rectangles.

When researchers showed the Ames window illusion to people who lived in rural areas without many rectangles around, well, they expected that people who don't live in carpentered worlds well, they wouldn't see the illusion as strongly.

But when viewed with one eye, as the illusion typically is, both urban and rural subjects saw the illusion basically the same.

So there must be something else going on here.

And I think the explanation is likely much simpler.

This trapezoid is built in such a way that no matter how it rotates, the length of the longer edge is always bigger on the back of your eye than the length of the shorter edge.

And if there's no other clues available in your visual field, your brain does what it always does: it makes the best guess with what it has to work with.

What is bigger is closer.

And lines converging toward a vanishing point are farther away.

Conclusion?

The window always appears to spin away from you.

(air whooshing) These are visual cues that exist regardless of whether the environment is built or natural.

Human brains seem to be wired to construct reality this way, because that is what the world has always shown us.

Can that also explain this?

This is a room that can make things grow and shrink right before your eyes.

I know what you're thinking: This is a dark, evil magic.

But it's not.

It's just a trick room.

It's called an Ames room.

The ceiling, the walls, the floor, the windows everything, they're all skewed so when you view it from one particular spot, your brain ends up building a false reality.

In this case, your brain is getting the clues that we talked about, it's just that those clues are lying.

The really cool thing about these illusions is that even when you know how they work, when you know that you're getting tricked, when you know that what you see defies the very laws of nature, well, that's still what you see.

Having more knowledge doesn't really affect the illusion much.

I think that's kind of the most wonderful thing about the Ames demonstrations.

They're what researchers sometimes describe as cognitively impenetrable.

It doesn't matter what you think about politics or sports or what you had for breakfast.

Your perception is going to work a certain way.

Now I mentioned that this illusion only works from this one viewing angle, this very specific spot.

This kind of distorted view, where you only see the normal thing from a specific vantage point, is what's called an anamorphism.

Artists have been using anamorphic tricks for centuries.

One of the most famous examples is Hans Holbein's "The Ambassadors".

When you view it from a very low angle, a slash of gray becomes a skull.

Or the very flat ceiling of this church in Rome, which was painted in 1690 to look like a dome.

You're probably more likely to see this illusion marking a bike lane or something, though.

But the Ames room is one of the most striking examples of anamorphosis, making a rectangular reality out of a very non-rectangular room.

Okay, now it may seem like you're looking at a chair here.

But it's not a chair.

It's just a jumble of strings pointing this way and that.

But if you look here this one is a chair.

And so is this one This is an illusion called the Ames chair.

It's not as well known as the Ames window or the Ames room.

But it's actually my favorite.

Partly because this was a real pain in the butt to build, so I have to tell myself it was worth all the trouble.

But it's also my favorite because it shows you how sensitive your brain is to certain patterns, and how if you see a really special pattern, you almost can't help but add meaning to it.

And the clue your brain is picking out in this one is connectivity.

From that special point of view, these edges come together to form nice neat corners.

And your brain is sort of doing a probability calculation: If this really were a bunch of separate bits of string, then of all the ways they could be arranged, there's basically no probability that they'd randomly find their way into the arrangement of a chair, right?

So it's like your brain saying, "based on the very limited information I have, the most likely situation here is that all of these lines are connected.

Because if they're not, I'm in this very weird, chance situation."

Of course, it turns out right now you are in a very weird, chance situation where I have specifically arranged the strings to make a chair shape.

But that would basically never happen in the real world.

Because in your normal experience, those connections would never occur unless they have meaning.

Conclusion?

That's a chair.

Now, interestingly this idea of connectivity might explain one of the oldest illusions on Earth.

I'm sure we've all experienced that incredible feeling that you get when you go out, look up at the night sky, and you see shapes emerge from the stars, constellations.

Different cultures have connected the sky dots slightly differently over the past few thousand years, but one that always seems to be seen the same is this.

The rough shape of a human wearing a belt.

We call that one Orion.

Incidentally that reminds me of a joke: Ever look up at the night sky and see Orion's belt?

Big waist of space if you ask me.

(chuckles) It's like a belt around the waist.

(insects chirping) Ah, whatever, that joke only has three stars.

(crowd boos) Anyway, this is probably going to ruin your horoscope, but the shapes we see in the stars they happen for the same reason we see a chair in a bunch of random strings.

Because we happen to be viewing it from a very specific point of view.

If you were somewhere else in space, the stars of Orion (board scratching) would look like this.

They're all different distances from Earth, and well, nothing special to our friends on Alpha Centauri or whatever.

It's only because you happen to be right here that this group of stars means anything at all.

(board scratching) Most of the illusions in this video were invented by a guy named Adelbert Ames.

You don't meet a lot of Adelberts, do you?

Around 1910, Ames was studying to be a lawyer, and decided he wanted to be an artist instead.

It was always a fun conversation to have with mom and dad.

When he started studying how the eye senses the world with the goal of making more realistic-feeling art.

But Adelbert ended up abandoning art, and spending the rest of his life designing a series of elaborate demonstrations to try and unlock the secrets of how we see the world.

Those demonstrations have captivated scientists and the public ever since.

Just before Ames died, the plans for these demonstrations were published in a book that is incredibly hard to find.

But thankfully I was able to find a copy, thanks to something amazing called the library, which is like the internet if it was a building that you had to go to.

And one of the strange things about most of Ames' illusions, is most of them only work when viewed with one eye, from a very specific spot.

Which is why so many of them work so well in video, because the camera is essentially like viewing the world with one eye.

And as powerful as these illusions are, if you were to view most of them with two eyes, or if you were able to move around a bit, they wouldn't work.

So even though they show us a ton about how our visual system can kinda break down, they also kind of show us how awesome our visual system really is when it does get all the information it needs, from both of your eyes!

Unfortunately, you still only get to use one eye for the rest of the video, the camera but there's one last set of visual cues that I have to tell you about, for size and how far away things are, and they only work because you have binocular vision.

Okay, try this: Hold up two pencils or pens, one in front of the other.

When you focus on the rear object, you see a double image of the front object, one on each side.

Close one eye at a time, and you'll see that your right eye sees the left image, and your left eye sees the right image.

Now, focus on the closer one.

And suddenly the back one is doubled.

Now your right eye sees the right image, and your left eye sees the left image.

So what's going on?

When we look at a fixed point, your eyes move individually, so that point hits the center of each eye.

The closer an object is to you, the more extreme the effect but your brain is sort of always unconsciously comparing how different the picture from each of your eyes is.

And it uses that as a clue for how far away things are.

When an object is really close to your eyes you can actually sense the feeling of your eyes crossing inward, to focus on it.

You can even feel how the muscles that focus your eyes lens are flexing and pulling to bring something into focus and get another clue that way.

There's clues like texture.

You can see more detail on closer things than you can far ones.

And clues like parallax which is how objects at different distances move differently relative to you.

You actually have a ton of clues at your disposal to judge what you see.

So we build our picture of reality from these bits and pieces from clues.

Not the full picture.

The software of your brain.

If you wanna think of it that way it looks for certain clues that it thinks are important.

And your reality is always built from that incomplete information.

Most of the time that incomplete information is still good enough to build a an accurate model of reality.

But the Ames illusions show us that sometimes incomplete information isn't enough.

There's this idea that in some ways, especially today if different people are given the same information then we can end up with a very different picture of what is real and true.

But these illusions show us the opposite really that when it comes to seeing at least evolution has made it so that very different people perceive the world in the same way.

And that's kind of nice to hear these days.

Stay curious.