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  • Joe 1: Hey smart people, Joe here.

  • 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 timeJust 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? “Nowfor you, as you watch this  video, iswhatever 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 asnowon 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 youis that according to the laws of physics,  

  • and even neuroscience, thenowyou  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 wasnow”. 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 slowerGeneral 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 westwardeffectively 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 ofspeeding 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 groundFrom 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 groundthe 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 onnow

  • 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 isspacetime”.

  • (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 asnow”, 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 makesnowparticularly 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 nexteverywhere. AND I WOULD BE AN ALL POWERFUL  

  • GOD… sorry I drifted away formoment. 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 calledeternalism”. 

  • Imagine stepping outside of the universeto view it as a single block of all moments  

  • and all spaces. This is what some  have called theview 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 realjust not to us. This is your absolute elsewhere,  

  • or perhaps we should sayelsewhen. Moments  here are neither past, nor futurenor present.

  • If you're feeling a little disoriented right nowafter realizing thatnowdoesn't exist out in  

  • the universe, well don't worry, it gets weirderNow 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 timethe 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? Wellmaybe our brains guess ahead, where the ball  

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