Joe 1: Hey smart people, Joe here.
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 1: Just now. We're at now 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!
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,
and when something happens on each timeline doesn’t match. Out in the world, an event occurs.
Then some other stuff happens shortly after it. Our brains take the stuff that happened
just after, and combine that with the event itself to create a single moment in our heads,
a “now” that doesn’t exist in the real world. It’s what Eagleman calls the illusory present.
So in a very real way, we do live in the past. When this moment, when “now” occurs,
it’s already happened. For experiences like the flash-lag illusion, each of us lives in
our own “now” that is 80 to 100 ms in the past of the “now” in the universe outside our skulls.
Why does this happen? Because the brain is surprisingly slow at processing information
that comes in. Different signals arrive at the brain at different speeds and different
times, and they have to be decoded and processed in different regions of the brain
before we can have a conscious experience. You see the world a lot like Saturday Night Live.
It seems like what we’re seeing is now, but really there’s a delay built in to what we see.
Only it’s not in case someone says a bad word or has a wardrobe malfunction, it’s because of
all the information from different senses and places reaching our brain at different times,
and our brain having to decide which bits of that information happened at the same time.
It feels like all of our senses are unified across our body, that what we hear and what
we feel and what we see are all in sync. But this is a feeling created in our brain.
And there’s a really simple way to demonstrate this. Just touch your nose and touch your foot
at the same time. I feel those as simultaneous, but I mean, the signal from my foot took
many milliseconds longer to reach my brain than the signal from my nose, just by sheer distance.
But your brain waits, and takes all these signals arriving from the past, and like a video
editor, it syncs them up and creates a conscious experience where they happen simultaneously. This
also means that tall people like me live farther in the past than short people.
Scientists once tested sprinters to see if the sound from a starter pistol,
or a flash of light would get them off the line faster. Light travels more quickly than sound,
so you would expect a flash works better. But they were actually faster in response to the sound,
because auditory signals are processed faster than visual signals in our brain.
But if that’s the case, why does this (clap) work?
The movement of my hands and the sound appear synced,
even though the hearing part of my brain gets the signal before the seeing part of my brain.
In the early days of TV broadcasting, engineers were worried about how to
keep the sound and picture in sync. See, unlike film, where the soundtrack is physically linked,
and printed on the film strip, TV broadcasts used
separate signals for sound and picture. And it turned out, they didn’t need to be in sync,
at least not perfectly. As long as the sound and picture weren’t more than 80 milliseconds out
of sync, people’s brains made the correction for them. Just like yours have been doing. The sound
and picture have been out of sync for the last several seconds, and I bet you didn’t even notice.
But if I walk away… at a certain distance… your brain decides the sight and sound are out of sync.
That distance? Is about 100 feet, or 30 meters. Because it’s the distance
at which the speed of light and the speed of sound reach you more than 80 ms apart.
Here’s another weirdness: Our eyes make small jumps several times a second, movements called
saccades. If you stand close to a mirror, and look back and forth between your right and left eye,
someone watching you will see your eyes move back and forth,
but you won’t. You should try this, it’s seriously trippy. Our visual system turns off when our eyes
jump from spot to spot, otherwise the world would look like it was constantly blurry. But
we never notice these gaps in time, because our brain uses the past to fill in the now.
Your brain is not a clock that tracks time.
Time is actively constructed by the brain. Whether it’s in our brains, or out in space,
now is not a place, because time is not a map with coordinates that we all agree on. Time is just a
way to measure change, and change can happen at very different speeds, for any two people.
There’s nothing in the laws of physics or even our own brains that says there is one single “now”.
The universe is a stranger place than most of us realize. Thanks for joining me in this moment,