Joe 2: And here too.

Joe 1: I'll explain the clone later, but we're  excited to test out a new YouTube feature  

that lets us watch the video along with you  in real time! Just let me sync everything up…

Joe 2: What am I looking at? When  does this happen in the video?

Joe 1: Now. You're looking at now. Everything  that happens now, is happening now.

Joe 2: What happened to then?

Joe 1: We passed then.

Joe 2: When?

Joe 1: Just now. We're at now now.

Joe 2: Go back to then.

Joe 1: When?

Joe 2: Now.

Joe 1: I can't.

Joe 2: Why?

Joe 1: We missed it.

Joe 2: When?

Joe 1: Just now.

Joe 2: When will then be now?

Joe 1: Soon

Joe 2: ARGGGHH

Joe 1: When is now?

[OPEN]

When is now? “Now” for you, as you watch this  video, is… whatever time and date it is wherever  

you are at this moment. But that's different  from now for me as I make this video. My now is  

at some point in the past compared to your now. Whose now is the real now? Ok, obviously I'm in  

the past compared to you now, right? I made  this video, and I uploaded it on that date.  

But I can't access your future. For  me, now is right here, in this moment.

Ok, maybe this is all semantics. We're  just arguing over words. I mean, if I was  

right there in the room with you, surely  then we'd be able to agree on the same  

“now”, right? But what if I was on Proxima b?

Proxima b is the nearest potentially habitable  exoplanet, a little over 4 light years away from  

Earth. Let's pretend, for a moment, that  you have an incredibly powerful telescope,  

capable of seeing me, in my little space  cabin, on the surface of that planet. 

What am I doing, right now, on Proxima  b? There's no way that you could know.  

Because of the speed and travel time of  light, you can only ever know what I was doing  

4 years ago. Whatever you think  of as “now” on my distant planet,  

is in your future. For all you know,  

a black hole opened up and swallowed me and my  little planet and I'm not even there anymore.

The point is, and what I'm going to show you,  is that according to the laws of physics,  

and even neuroscience, the “now” you  experience is yours alone, and it depends  

on where you are and what you are or aren't doing. In 1971 scientists loaded atomic clocks on board  

commercial airplanes. These clocks flew twice  around the world, once eastward, once westward.  

Afterwards, the time they recorded was compared  with clocks that had remained stationary,  

and the three sets of clocks no longer agreed. 

They did not malfunction. If you were sitting  next to that atomic clock on the plane, it worked  

perfectly, they had all kept time correctly, but  because they had moved, relative to each other,  

they no longer agreed on when was “now”. And  these differences were precisely consistent  

with Einstein's predictions of  special and general relativity.

Now, general relativity deals with how clocks  tick faster or slower depending on the strength  

of gravitational fields. Closer to Earth's  center, in a higher gravitational field,  

all processes, including clocks, are slower.  General relativity predicts clocks at higher  

altitude tick faster than clocks on Earth's  surface, and that's exactly what they observed.

But it's the predictions of special relativity  that are perhaps more mind-blowing.Movement  

also slows time, at least from the  point of view of someone standing still. 

The clock that flew eastward, effectively  moving faster than the clock on Earth,  

ran more slowly. The clock that flew westward,  effectively moving slower than the clock on Earth,  

ran faster. Or another way to look at that  is from the westward clock's point of view,  

both of the other clocks are moving  away from you, and both run more slowly.

These differences were only on the scale  of nanoseconds, billionths of a second, but  

they were measurable. Today, calculations using  GPS satellites take these effects into account.

And it gets even weirder. Not only can  two clocks disagree about when is now,  

two people might not be able to  agree on the same now either.

Imagine two observers: One in the center of a  speeding train and the other standing next to  

the train as it goes by. As the center of  the train passes this observer, two bolts  

of lightning strike the train car in the front  and rear. The flashes of light from each strike  

reach him at the same time, so he concludes  that the strikes were simultaneous. I mean,  

obviously, right?! We know that the light from  both strikes travelled the same distance to  

his eyes at the same speed: the speed of light. But what does his friend on the train see? From  

his perspective, the train is moving to meet  the front flash, and away from the rear flash.  

The light from the front flash hits him before  the light from the rear flash catches up. And they  

draw a very different conclusion from the person  on the ground: the front flash happened first.

This happens because of one important rule: no  matter if we're on the ground or on the train,  

the speed of light does NOT change. It is  universal everywhere, from any point of view. 

From the perspective of the person on the  train, each pulse of light traveled the  

same distance from each end of the train. So if  the passenger sees one flash before the other,  

they can only conclude that the front  flash happened before the rear flash. 

Our two observers disagree about the order of  events, and whether two events were, in fact,  

simultaneous. Their nows don't match up.

Whose interpretation is correct? Well, what  Einstein showed with special relativity  

is that they're both correct… in their own  reference frames. From different reference  

frames moving relative to one another, there can  never be agreement on the simultaneity of events.  

What we call relativity of simultaneity. Let's look at that train from a different  

perspective. This time, the passenger  at the center of the train takes a photo  

as the train passes his friend on the ground.  From the perspective of someone in the train,  

the light from the flash reaches  each end of the train simultaneously.

But our friend on the ground? From their  perspective, the back of the train is catching  

up with the light from the flash, but on the other  side that light has to catch up with the front of  

the train. From the perspective on the ground,  the light strikes the front of the train last. 

Again, our two observers don't  agree on what is simultaneous,  

and they are both right, in  their own frame of reference.

If we are moving relative to each other, from  your frame of reference there will be a moment  

where two events are simultaneous, and from my  frame of reference there will be a moment where  

A & B are not simultaneous. And if we can't agree  on what's simultaneous, we can't agree on “now”

But hopefully we can agree on  one thing: Tacos are delicious.

Let's say you and I want to meet for tacos. I'd  tell you to meet me at my favorite taco stand,  

at 12 noon. I have to tell you where to be (taco  stand) and when to be (noon). I have to give you  

a set of coordinates, not only in space, but also  in time. The coordinates in space alone are not  

enough, you could show up at a different time than  me. And the time isn't enough either, you could  

show up anywhere. We can only share our delicious  taco moment if we describe both space and time.

Imagine it this way: Since most of us are stuck  to Earth's surface, we can tell where we are  

with just two coordinates. And we can watch as  these coordinates change along a third axis:  

time. All of this together is “spacetime”.

(Now technically, there are three coordinates  of space, and time is a 4th dimension. But  

unfortunately that's a bit hard to illustrate  in our pesky three dimensional universe.)

We think of reality as one  of these moments in time.  

And if you and I exist in the same reality  (which I am pretty sure we do), then we must  

exist in the same moment in time. There must  be one moment we can agree on as “now”, right?

Our everyday experience tells us the  three dimensional universe at this point  

in time is what's real. The past and future  aren't real in the same way as the present.  

Sure, we can remember the past, and we can  predict the future, or we can imagine it,  

but we can't go to either of them. And whatever  we are remembering about the past or predicting  

about the future, we're really just doing  that with our brain, now, in this moment.

This way of thinking about the universe is called  

“presentism”. That the  present is what is most real. 

And this way of thinking might be wrong. One of  the beautiful things about the laws of physics,  

and the equations that describe them is that they  work equally well here or in another galaxy. And  

they also work equally well a million years  from now, or a thousand years in the past.  

There's nothing in the laws of physics  that makes “now” particularly special.

Those laws of physics also lead us to some weird  conclusions: If I knew everything about the  

universe right now, every particle, every bit  of energy, every motion and every bit of data,  

I could predict what will happen next,  everywhere. AND I WOULD BE AN ALL POWERFUL  

GOD… sorry I drifted away for a  moment. With this information,  

I could also reconstruct everything  in the past. Past, present, and future  

moments are all connected through the laws of  physics. This is a view called “eternalism”. 

Imagine stepping outside of the universe,  to view it as a single block of all moments  

and all spaces. This is what some  have called the “view from nowhen”.

This view says that all moments are equally  real, and there's nothing special about the  

present moment except that you are experiencing  it right now. This doesn't fit in with our  

everyday experience of time and space, but it  seems to be a logical consequence of physics. 

So what is real? Physicist  Sean Carroll puts it like this:  

“All moments in time are real, but  some we understand better than others.”

Speaking of understanding, to understand  something, we need to observe it.  

And we observe things using light.

Imagine a flash of light in space. A second  later, that light pulse has created a sphere  

one light-second across, and every second after  that, that sphere will continue to expand.

In two-dimensional space, our expanding  sphere will appear as an enlarging circle,  

tracing the shape of a cone  as it moves forward in time.

This cone represents everything  this light will ever touch,  

expanding into space at  the fastest speed there is.  

That means someone here could never see it. We  can only see it if we are here, inside the cone. 

Likewise, from any point, the mirror image  cone extending down represents all light,  

how old and how far away, that could ever  reach us from the past. From any event,  

light and information advance outward in  every direction as time progresses upward,  

forming an ever enlarging circle throughout all  future moments. Our past light cone is everywhere  

and everywhen we can see in the past universe. Our future light cone is everywhere and  

everywhen we could ever communicate  with or travel to in the future.

We can't interact with, or see, or ever even know  about anything outside our own light cone. There  

are moments in time and space that may be real,  just not to us. This is your absolute elsewhere,  

or perhaps we should say… elsewhen. Moments  here are neither past, nor future… nor present.

If you're feeling a little disoriented right now,  after realizing that “now” doesn't exist out in  

the universe, well don't worry, it gets weirder.  Now might not even exist inside your own brain!

How did I do that?

When we see a ball, or anything, it takes 10  to 50 milliseconds for information from the eye  

to reach the brain, and another 100 or more  milliseconds before we can take actions on  

the basis of that information. During this time,  the ball continues to move. So it seems like the  

brain's information about where the ball is will  always lag behind where the ball actually is. 

So how are we able to catch the ball? Well,  maybe our brains guess ahead, where the ball  

will be? Maybe we use information from  the past to predict the present? Well,  

it's not quite that simple at all. Keep your eyes focused on the X.  

As the ring moves, a white circle flashes.  What most people perceive is that the flash  

lags slightly behind the ring. But if  we freeze at the instant of the flash,  

the white circle is actually completely inside  of the ring. This is the flash-lag illusion.

Why does our brain see this? Well,  thinking back to catching a ball,  

it fits with what we might expect. That our brain  is predicting where it thinks the ring will be.  

That we are “living in the past”, and our brain's  guessing ahead. But this isn't what's happening. 

Psychologist David Eagleman set up a  modified version of this experiment.  

Everything leading up to and  including the flash is the same,  

but what happens after the flash changes.  The ring either continues around, stops,  

or reverses. And here's what people see:

Again, everything leading up to the flash is  the same as the original experiment. If we  

were really using the past motion of the ring  to predict its future motion, we'd expect the  

same result as the original experiment, our  brains putting the ring ahead of the flash.  

But that isn't what we see. What we perceive at the moment  

of the flash depends on what happens in  the future of the flash. This is really  

strange. It means the moment our brain calls  “now” depends on information from the future.

You know, this is starting to sound like we're  time travelers or something. A better way to  

look at it is that there is physical time out  in the world, and time we make up in our brains,