At the 1939 world’s fair in New York,
the exciting new tech was the live television broadcast.
Roosevelt became the first president to address the nation live on TV
But for years leading up to this event,
engineers have been working on one particular technical problem:
How to ensure the audio and video remain perfectly synced during the live broadcast ?
Without this, words and lip movements wouldn’t match up,
which would be annoying and distracting for viewers.
So, how did they do it ?
Well actually, they didn’t,
instead they discovered something pretty incredible:
We are not very good at discerning whether audio and video are in sync
For example, I intentionally delayed the audio of this entire monolog
nearly a 1/10th of a second and did you notice?
I’ll clap to make it more obvious.
The engineers also found that there’s an asymmetry
in our tolerance for this misalignment,
we don’t really notice if a sound lags video by up to 125 milliseconds
but we can tell something is wrong if it’s leading the video by more than 45 milliseconds.
And to understand why, take a look at this:
Here I am bouncing a basketball as I walk away from the camera.
The sight and the sound of the bounces match up perfectly;
but as I walk away, you know the sound will be increasingly delayed
due to the extra time it takes the sound to reach the camera,
but the sounds still appears synced.
This is because your brain is not reporting to you each instance exactly as it happens,
but rather a short interval of time reorganised to make sense,
so in this case your brain automatically aligns the sound with the sight of the bounce.
At least, up to a point.
Once I’m over 30 meters away,
the sound is now delayed by over 100 milliseconds
and your brain no longer integrates the information from your eyes and ears.
Here, let me play the actual sound of the bounce
together with the sound as received by the camera.
This explains why sound can lag video by more than it can lead.
I mean imagine you were at a basketball game,
and because of how far away you’re sitting, the sound is delayed.
Your brain can handle that.
but if the sound precedes the sight of an event
that would look really odd
because that’s something that would never happen in nature.
This is why the broadcast guidelines for acceptable audio and video mismatches
are skewed in favour of audio lagging behind the video,
our brains are good at aligning audio with the vision that preceded it.
We can actually exploit our audio-syncing-capabilities
to produce some strange results
for example, we created this computer program
where when you press the space bar a light appears on the screen.
But not immediately,
there is an 80ms delay between the button push and the light coming on.
In a study participants who familiarised themselves with a similar program
came to believe that the light turned on immediately after they pushed the button,
just as our brains sychronized the sight and sound of the basketball bounce
Press the space key once to begin
This is just the section where you get the idea of what it does, so you push the spacebar
Now watch what happens when you remove the delay.
That last one came up without me even pressing anything!
You didn’t press anything and it just flashed up there?
Some participants were convinced that the light came on before they pushed the button.
They believed that something else caused the light to come on,
even though it was their action that made it happen.
This is remarkable beause causality the idea that
one thing leads to another is fundamental through our understandng of the world
otherwise how would we know who shot first.
Babies as young as eight months can demonstrate an understanding of causality
when watching a caregiver wind a music box if the music stops
the babies touch the caregiver’s hand to get the music to start again.
Causality is something we re hardwired to recognize
but even this core part of our brains can be fooled.
The flash lag effect gives further insight into how this happens
Stare at the red square in the middle of the screen
and remember what you see when the flash happens.
where was the ring when the flash occurred ？
If you’re like most people you probably saw the flash in the top half of the ring.
but what actually happened was the flash appeared in the exact center of the Ring.
So our perception of the flash seems to be delayed.
To explain this result scientists hypothesized that the brain was anticipating motion.
You see the flash behind the ring
because your brain is predictig where the ring will be
not where it actually is when the flash happens to.
To test this hypothesis,
neuroscientist David Eagleman and his team
modified the experiment so that the ring reverses direction right at the moment of the flash.
Now what did you see?
If the brain was predicting the motion of the Ring ,
you should see exactly the same thing as before
with the flash on the top side of the Ring
But most people see it in the bottom half of the ring .
How is this possible considering
all the frames leading up to the flash were exactly the same in both cases?
If our brains were anticipating motion
then we would have had to somehow known in advance
that the ring was going to reverse direction.
and that seems impossible.
So the alternative hypothesis is that where we see the flash occur
depends on what happens after the flash.
There is a delay between the flash occurring and you perceiving it.
The stuff that happens during that delay is actually incorporated into your perception of the event.
This is incredibly cunterintuitive and it calls into question our perceptions of causality .
The underlying cause of all of these illusions is the same.
What we perceive as the present is not just one moment
but a short interval of time which is a round of tenth of a second long .
And during this period your brain can perform manipulations that distort your perception of time.
And rearrange causality.
This interval has been called the specious present
A term coined by E Robert Kelly.
All the way back in 1882,
he said: All the notes of a bar of a song seemed to the listener to be contained in the present
他说 在听众看来 一首歌一小节的所有音符都包含在当下
All the changes of place of a meteor seemed to the beholder to be contained in the present .
The present is really a part of the past a recent past.
delusively given as being a time that intervenes between the past and the future.
Let it be named the specious present.
Why do we experience time in this ilusory way?
We feel like we’re living in each instant
but what we are actually experiencing is a short period of time.
I think it’s not so that our brains can sync up the sight and sound of distant events
so we can watch TV and movies without distractions
but because fundamentally our brains need to hold
multiple moments at once to make e sense of the world
Just as you can’t read a book letter by letter
you can’t make sense of the world and form memories instant by instant.
So we live in the specious present
under the illusion that we experience each moment exactly as it happens
Hey! A lot of the research in this episode was performed
by neuroscientist David Eagleman
and he’s actually written a book entitled The Brain: the Story of You
which includes a lot of details of his research
so if you want to know more about these topics
I highly recommend you check out his book
and in fact you can download it for free
by going to audible.com/veritasium
where this book is written by David Eagleman himself
But if you’re not interested in this one
you can also pick any other book of your choosing for a one-month free trial.
Audible is a great audiobook website
with hundreds of thousands of titles in all areas of literature
including fiction nonfiction and periodicals
Plus Audible is a Longtime supporter of veritasium
so I really want to thank them for helping me to keep doing
what I’m doing and making these videos.
And I want to thank you for watching.
At the 1939 world’s fair in New York,