What would you do if you could travel through time?
Would you visit Ancient Egypt, where pharaohs lived?
Would you go watch the very first moon landing?
Or would you choose to explore a distant future?
Albert Einstein showed us this is more than mere science fiction.
For the father of modern physics, the possibility of time traveling
is within our reach.
Learning How to Time Travel
Do we really need to build a time machine to travel through time?
June, 1951.
A commotion takes place in New York
when a man is run over and dies in the famous Times Square.
It soon became clear there was something odd about it.
The victim was a young man wearing clothes that were typical
of the 1870s.
The mystery deepened further when the police found in his pockets
nothing but old items in perfect condition.
A beer token worth five cents,
a letter dated to 1876,
and 70 dollars in banknotes that were no longer used.
But the biggest clue they found
was a business card with the name "Rudolph Fentz."
The officer in charge of the case went through every New York record
looking for that name.
That's how he found out that the answer to this question
deepened the mystery further.
The only person registered under that name in New York
had gone missing in 1876,
in a case never solved.
The young Rudolph Fentz traveled 75 years into the future
and met his fate in a very different period
from the one he was born in.
Did the secret to time travel die with him?
A NETFLIX ORIGINAL SERIES
Hey, didn't the cop look for the time machine?
It should've been somewhere in New York.
Who said time machines are man-made,
with circuits and cables?
Castanhari, I've watched every movie, every TV show.
-I know what I'm talking about. -Time machines weren't invented by man,
but by nature.
Come on. Time machines grow on trees now?
All right. What is time for you?
This.
Actually, I don't know if time is inside the watch, or--
No. The hours and minutes in your watch aren't time.
They're just ways man came up with to count time.
In fact, in Mars,
time would pass differently for you.
Exactly, Castanhari.
One day in Mars lasts 24 hours and 37 minutes.
Here on Earth,
most people assume that a day has 24 hours,
but they're mistaken.
The planet Earth actually takes 23 hours and 56 minutes
to rotate around its own axis.
There's also a difference in time when we think of years.
Mars takes 687 days
to fully orbit the Sun.
This is equivalent to about two Earth years.
But these numbers are simply measures of time,
and it's important to highlight that time depends on no measure
to exist.
Between you and Briggs, let me see...
There's approximately one meter.
But if measures like meters and centimeters hadn't been invented,
this space would still exist.
Time works in a similar way.
And this isn't about the number of days in a Martian year,
but that on Mars, time passes differently than here on Earth.
-There, it goes by faster. -Really?
Yes, and I can prove it to you right here on Earth,
using an extremely precise clock.
Not this one. Luckily,
we have that type of clock right here in Brazil.
USP - UNIVERSITY OF SÃO PAULO SÃO CARLOS CAMPUS
Precisely here, at USP São Carlos, one of the few places
in the Southern Hemisphere where atomic clocks are produced.
Atomic clocks are extremely precise
because they're set according to the behavior of atoms,
like the Cesium-133, which is in this vacuum chamber.
Once exposed to microwaves, these atoms vibrate fast,
over nine billion times per second.
Since atomic clocks register each of these vibrations,
they can break one single second into nine billion parts.
As atoms always respond to the same oscillation,
a little over nine billion oscillations per second,
its accuracy is tremendous.
A regular clock may become one second ahead or behind per day.
But an atomic clock would take over one million years to do so.
That's why they are ideal for proving
that time travel is real.
That's what some scientists demonstrated in a practical way.
Two atomic clocks were synchronized at a beach
and started displaying the exact same time.
Afterwards, one of them was left there
while the other was taken to the peak of Mount Sunapee,
over 800 meters high.
A few days later,
the researchers retrieved the clock atop the mountain
and placed it next to the one that stayed at sea level.
When comparing the times on the two devices,
the scientists realized something astounding.
The clocks were no longer synced.
The one brought down from the mountain was slightly ahead
of the other one that stayed at sea level.
It was the evidence they needed in order to be sure
that time went by faster
for the clock that was left on the mountaintop.
This happened for the same reason time goes by faster in Mars.
Understanding that time changes depending on where you are
is crucial to anyone who wants to travel through time.
But why does time go by faster in Mars?
Because gravity on Mars is lower than on Earth.
But what does gravity have to do with time?
Everything. If you want, I can adapt the experiment I'm setting up
to show it to you.
Gravity has everything to do with time, and with space too.
Both of them are intertwined.
Space has three dimensions: height, width, and depth.
But there's something we're not used to thinking of
as a dimension: time.
According to the Theory of Relativity,
the Universe is composed of a four-dimensional grid
called space-time.
Many of our daily events show how time and space operate together.
Imagine, for example,
a woman who goes to meet with her husband at a mall.
The problem is, she only knows where he'll be,
but not when.
All she can do is stay put,
hoping he'll show up.
On the other hand, if she only knows the time her husband will arrive,
she still won't be able to find him
because she needs his location.
In our reality, time and space go hand in hand.
In order to understand how gravity is related to that,
we need to talk about mass,
which is when science proves to us that size doesn't matter.
Let's drop a marble and a Styrofoam ball in a water tank
and see what happens.
The result isn't all that surprising.
After all,
the object that seemed heavier sank to the bottom.
Right? But wait a second.
There's something wrong here.
Because if we put them on a scale,
we see they have the exact same mass.
So we're the ones who are wrong for thinking about mass.
Having the same mass doesn't mean having the same density,
which is what matters here.
The Styrofoam ball doesn't float because it's lighter,
but because it's less dense than water.
LESS DENSITY
These two objects have the same mass, but their sizes differ
because they're made of materials with different densities.
Between two objects of the same mass,
the smallest will be denser, and that goes for everything else.
This grid is a representation of space-time in two dimensions.
We can see that the large object distorts it very little.
LESS DISTORTION
But look what happens when I put the smaller one.
The denser object distorts space-time
more intensely around it.
And remember, both have the same mass.
Let's pretend this larger ball is the Sun, and the smaller one, Earth.
Space-time being distorted
influences the Earth,
which is attracted by the Sun.
SUN SPACE-TIME DISTORTION
Now let's place an even smaller ball, the Moon,
with the Earth ball.
The Earth is much smaller than the Sun.
But it distorts space-time, too.
Observe how the Moon ball is attracted by our planet.
EARTH SPACE-TIME DISTORTION
We're so used to thinking of this as simply "gravity,"
but it's actually a bend in space-time.
Gravity is just the way we feel the distortion.
But didn't you both say this thing distorts space-time?
When I look at this grid, I only see space.
Remember how I said they're both intertwined?
Time is over this entire grid.
So let's create a visual representation of this.
This ruler represents time.
It is present in the entire universe.
Every mark on the ruler is an instant.
Now observe what happens when an object with a lot of mass
approaches it.
As time is linked to space,
it's distorted by gravity, too.
This is time dilation.
In the area where the grid stretches out,
time slows down.
The stronger the gravity, the more the grid stretches.
In other words, the slower time passes.
We saw that happening in practice with the atomic clocks.
The sea-level clock was closer to the Earth's core
than the one atop the mountain.
That's why it was more influenced by gravity.
Time went by slower for the clock at sea level.
Black holes concentrate
huge amounts of mass within a small space.
Their density is enormous,
so their gravity is immense,
way stronger than those of stars and planets.
Observe what a black hole does to space-time.
The dilation here is huge.
Isn't this related to that recent movie,
the one with the astronauts who approach a black hole...
Yes, Interstellar. One of my favorite movies.
It shows the effects of time dilation perfectly.
INTERSTELLAR
In the movie, two astronauts are on a planet
orbiting a black hole.
Its gravity is so intense
that two seconds there are equivalent to 20 hours
for the scientist that stayed in the ship.
And the astronauts don't notice time dilation happening
because a distortion in space-time doesn't just affect time,
but everything within that area.
So they think time in that place is going by at a normal pace.
But if the scientist observed them,
he'd see the astronauts moving very slowly,
almost standing still.
Imagine seeing a short action,
like dropping an object,
lasting around 20 hours.
And if the astronauts looked at the spacecraft with a telescope,
they would see 20 hours going by in only two seconds.
-Which one of them is right? -I don't know. Neither?
Both. Each sees their own truth,
depending on their location.
They are in spaces differently affected by gravity.
The astronauts are more affected than the scientist.
At the end of the mission, the astronauts return to the spacecraft
and find out that the scientist is much older now.
While they spent three hours on the planet,
twenty-three years went by for the person in the spacecraft.
So they traveled through time!
Thanks for the spoiler, Castanhari.
I don't have the movie in my data bank.
That's one of the ways to travel through time,
using a time machine called gravity.
So there's more than one?
Yes. It's like a coin.
Gravity on one side, speed on the other.
But wouldn't speed need to be another coin?
-I think so. -Would it really?
Dr. Tayane, in order to simplify the explanation,
I suggest we send our dear janitor to Einstein's famous train.
Einstein was a conductor, too?
Adalberto, picture a moving train.
Inside it,
there's a flashlight on the wagon's floor
flashing a beam of light.
The light hits a mirror on the ceiling
and returns to the flashlight.
If you were inside the train,
you'd see the light going up and down as usual,
regardless of the train's speed.
Because from your point of view,
it's as if everything inside the wagon
was not moving.
Now pretend you're waiting by a station,
watching that same train passing by.
As you perceive the train as moving,
you would see the light traversing a slightly different path.
That difference in the light's path
would be even more noticeable
if the train passed by the station
at a speed next to that of light.
In that case, you would see the beam tracing this trajectory.
And it's easy to figure out which path is longer.
This proves that time and speed are intertwined.
Those outside the train see the light traversing a longer path.
If it covered more distance, it also took longer to do so.
The train's speed caused a time distortion.
The greater the speed, the greater the distortion.
That's why it's the other side of our coin.
SPEED
Wait. What are you doing, Betinho?
Look at me, guys. Did it work?
Am I younger now?
You're on the right track,
but you'd need to go way faster to see the difference.
Like Usain Bolt?
Faster.
Actually, in order to feel the effect, you'd have to go as fast
as that train in our example.
That is, as fast as the speed of light.
And there is nothing in the Universe faster than light.
Light moves at around 300,000 kilometers per second.
299,792 KM/SEC
Take a look.
Between Brasilia and Beijing, China,
there are 17,000 kilometers.
In a single second,
light can go back and forth this space
almost nine times.
That kind of distance may seem huge to us,
but it's small to the Universe.
The distances between planets and stars are so big
that scientists created the light-years concept
to measure the distance light covers in a year.
It's almost ten trillion kilometers.
So the light from a star that is ten light-years away
takes ten years to reach us.
You'd need to run at a speed like that
to feel the effects of time travel.
The faster you go, the slower time goes.
Oh, so that's what happened
to that guy who traveled to the future?
Actually, Rudolph Fentz never existed.
He's a character from a story that was so popular,
it became an urban legend.
But people don't realize that the concept behind that story
is scientifically possible.
And look, I'd dare even say
that there are time travelers here on Earth.
They're just very hard to find.
SIRIUS CAMPINAS, SÃO PAULO STATE
This is Sirius, the largest particle accelerator
in Latin America,
built to be one of the best in the world.
At Sirius, electrons are accelerated to a speed near that of light,
and they stay there, running on vacuum, for hours on end.
The goal is to produce a special kind of light,
used by scientists in various experiments all around the world.
In this particle accelerator,
electrons are at 99.999998% of the speed of light.
Therefore, they spin almost 600,000 times per second
around this machine that is 500 meters in circumference.
But traveling at near light speed
makes the electrons perceive space differently.
Imagine if you could travel at that speed.
Distances that may seem huge to you would become tiny.
That's what's happening to the electrons. To them, this enormous accelerator
has a circumference of only nine centimeters.
Similar to this ring.
But it's not just distance that becomes shorter under that speed.
Time also becomes slower.
Moving at near light speed,
these electrons perceive time as going
six thousand times slower than ours.
So, if you could enter this accelerator,
time would pass differently.
It would be like you're traveling through time.
For anyone going at the electrons' speed,
time becomes 6,000 times shorter than it is outside.
In other words, by traveling a single second at this speed,
if you looked at a clock that stayed out here,
you'd see that you traveled one hour and 40 minutes into the future.
Exactly, Castanhari.
It proves we can observe time dilation by accelerating an object.
Now, how about we broaden our scale?
Imagine we are in the future,
and a space explorer is assigned a mission
to travel to another star and then return to Earth.
For that, they use the fastest ship in the galaxy,
capable of reaching half-light speed.
Its destination is Proxima Centauri,
the star closest to the Sun,
located about four light-years away from it.
How long would they take to make this trip?
Well, it depends on the observer.
For people on Earth, the ship would take
approximately 8.5 years to go to the star,
and 8.5 years to return.
Therefore, the trip would take 17 years.
However, we should not forget that the greater the speed,
the slower time goes.
For the explorer,
the same trip would take approximately 14.5 years
because as they are traveling at an immense speed,
time goes by differently for them.
Upon returning to Earth, the explorer notices
that they've traveled a little over two years into the future.
But those two years were not lost.
In fact, they were stretched.
That's how they traveled through time.
That is the speed side of Dr. Tayane's coin.
But which side of the coin do we use to travel into the past?
None. It's hypothetically impossible.
We know that the faster a body is,
the slower time goes.
Theoretically, if the body were to reach light speed,
time would stop for them.
So, if time becomes zero once you reach light speed,
that speed would need to be even greater to make time go backwards.
But the speed of light is the greatest speed there is
and going back to the past is impossible,
according to the Theory of Relativity.
But maybe there's a different way to travel to the past, using wormholes.
Castanhari, again with that?
Scientists believe there might be a tunnel
they call wormhole that connects two points in the Universe.
Under the right conditions, it could work as a time tunnel.
Picture two edges of a wormhole.
One in the present, and the other, in the past.
A person could go from one to the other and go back in time.
But it's a hypothesis. As far as we know, you can't affect the past.
No, but I can explain. This is space.
-If you fold it-- -I think it's pretty clear already.
I'll go back inside
to finish my solar system experiment that I was setting up.
I know it's hard to talk about time travel
because we're so influenced
by the way books, movies, and TV shows depict it,
that it can be hard to see
that this idea is possible, according to science.
However, by leaving those romantic notions of time travel aside
and focusing only on what science shows us,
we see that this concept, as fanciful as it may sound,
is completely possible.
We know it thanks to the genius of scientists like Karl Schwarzschild,
Hendrik Lorentz, Roy Kerr, Albert Einstein,
and so many others who opened the doors
to a new world with their theories.
Many were so advanced that they're only now being proved.
In April 2019, we had a historical scientific breakthrough,
the first real picture of a black hole.
Science predicted with extreme accuracy
how this cosmic phenomenon would look like
a full century before it was captured.
All of these scientists changed
and, most importantly, expanded the way we perceive reality.
Thanks to them, we now not only understand,
but have also proved
that it's really possible to travel through time.