[INTRO MUSIC] Today's episode is
about space, time, and the nature of reality. My name is Gabe, and this time,
it really is "Space Time." [THEME MUSIC] If you pay attention,
this episode is going to blow your mind, so
we're going to take it slow. What is spacetime, exactly? Before I can answer that, I
need you to do something for me. Give up your intuitions about
how time and space work. At first, your brain
might resist and hold onto those intuitions
for dear life. Don't worry. That's normal. This is challenging for
everyone, even Einstein. Ready? OK. Spacetime refers to whichever
external reality underlies our collective experiences
of the space between things and the time between events. Why can't space and
time just be a reality? Why add spacetime
as an extra concept? Here's why. Suppose two observers are moving
relative to each other, and particles count as observers. Fact-- those observers don't
agree about how much time passes between events. Fact-- they don't fully
agree on how much space there is between things
at any given moment. Fact-- they don't even agree
on the chronological order of all events. And yet, each observer
measures things properly and is entirely consistent,
which means neither of them is wrong. Now that sounds
absurd, but it's true. Plenty of other
resources, some of which we link to in the description,
discuss these discrepancies and the experimental
evidence for them. For today, we'll just
take them at face value and focus on what they imply
about the nature of reality. Because if you think about
it, some of the implications are staggering. Take this disagreement
about sequence of events, for instance. It is severe. If two observers can't agree
on the sequence of events, it means that at
present, someone's past is in someone else's future. Now for nearby
events, the effect is microscopic, but so what? Any disagreement means that
there is no universal division of events into past,
present, and future, which opens major philosophical
cans of worms for things like free will and our belief
that we can change the future. So is everyone's experience
of the universe entirely subjective? Or phrased another way, if
time and space as we usually conceive of them aren't
part of objective reality, then what is? Causality. Let me explain. A good starting point
for objective reality is universal agreement. And as luck would
have it, all observers do agree about this thing. It's called the spacetime
interval, or spacetime separation between two events. Even though two observers
in relative motion will measure different distances
and different elapsed times between the same two
events, they always agree about the spacetime
interval between those events. Now if everyone agrees
about spacetime intervals, they must signify something. But what? We'll notice that since
it involves subtraction, a spacetime interval can be
positive, zero, or negative. When it's positive, nothing
can get from one event to the other, and
there are always observers who disagree about
which one happens first. When it's zero or
negative, signals or things can get from one
event to the other and everyone agrees
on their sequence. So it appears that the spacetime
interval between events A and B tells you whether A can
influence B. In other words, even though we can't agree about
past, present, future, time, or distance, we all appear
to agree about causality. Now that may seem
counterintuitive. Normally we think that time
is responsible for causality. But actually it's
the other way around. To the extent that we agree
about temporal anything, it's only because of causality. Causality is what's real. So what does causality
have to do with spacetime? As it turns out, everything. See, shortly after
relativity first came out, a former math professor
of Einstein's named Hermann Minkowski noticed
that the spacetime interval resembles a weird version of
a distance formula in what's called a non-Euclidean space. So he proposed the
following radical idea-- maybe reality is not
a three dimensional space that evolves in time. Instead, it's a four dimensional
non-Euclidean mathematical space that's just there. No evolution. No time. That 4D mathematical
space is spacetime. Its points correspond to events. All events, everywhere, ever. And in this view,
only things that correspond to geometric
relations in that 4D space are objectively real, like for
instance, causal relations. They correspond to
spacetime intervals, which are geometric relations,
a non-Euclidean version of the distances between points. In contrast, our experiences and
measurements of time and space don't correspond to
anything, per se. They're more like the XY
grid we use in math class, useful for talking about
the board, but arbitrary and inherently meaningless. The board, its points
and geometric facts, are simply there whether we
put axis on that board or not. So are you objectively real? Well, kind of. If you are the sequence
of all the events of which you were
present, then you are a geometric object in
spacetime, a line segment joining the points
representing the events of your birth and your death. Do you move along
that line segment? No, no. You are the line segment. There's no motion
through spacetime. It's not this kind of space. It's tense-less. And your future isn't
merely predetermined. It already exists. There's some zen in trying
to express what spacetime is without misleading you,
but I think the following gets the flavor right. Imagine we're are all reading a
flip book made of graph paper. We agree on the
events of the story, but we don't agree
where they happen on the page, on how
many pages there are between events, or
even on the order of some of those events. And yet, we're all
reading the same book, only there's no graph on the
paper, there are no pages, and there is no book. All of that is
just an imposition our brains make in order to
perceive whatever "it" is. So why do we perceive reality
in such a vividly spatial and temporal way? Good question. No one really knows. So have I told you all there
is to know about spacetime? No, far from it. All of this has just been a
loose introduction to what's called a flat spacetime. Once general relativity
entered the mix, we'll find that there are
many possible spacetimes with different geometries,
making it hard to ascertain which one this is. But we got to crawl
before we walk. We will get to that fun
stuff eventually though, so subscribe. And as always, the comments
are for your questions. I'll do my best to answer them
at the next causally-connected point of spacetime. Last week, we asked
whether NASA could start a zombie apocalypse. You guys, as usual,
had a lot to say. Daniel Jenkins commented that
a space-based zombie outbreak assumes that a more virulent
organism would actually spread better. First of all, that
assumption is unnecessary. It's enough for the bug
to just be more harmful and harder fight off with your
space-depressed immune system. But second, as Nicholas
Garrison pointed out, germs do spread more
easily space capsules for a variety of
reasons, including the fact that the gunk
in sneezes and coughs just hangs there
instead of falling. Pretty nasty. Joe G.P. and Dikasad2
both asked what it is about space exactly
that enhances virulence? Is it the low gravity
or the radiation? What is it? Well, the authors
of salmonella study theorised that the signal
to bacteria in microgravity might be lower shear forces
on their cell membranes from the surfaces and
fluids that surround it. But based on the
journal articles I read, that's only one of
several suggestions and the jury is still out. We just don't know. Joey Broda and
McKnowledge1000 both asked why human gene expression
isn't altered in orbit if bacterial gene expression is. That's a great question, but
I'm not a microbiologist. Maybe human cells do change. I don't know. It's a great question,
but I have no idea. DHGameStudios said
that it's a shame we put a zombie tag on this video. What do you mean? Is it a shame that
"Sesame Street" teaches reading and arithmetic
with a vampire and a canary with acromegaly? Have some fun, man. Lighten up. Zombies are the best. Finally, At-Bristol
Science Centre really wants to see the
Curiosity rover battling martian zombies in a movie. Yeah, you do. You know who else does? Bjs301's kids and their
friends, because I'm pretty sure they understand
that even though zombies aren't real, they're
still super awesome. Finally, quick announcement. The PBS Digital
Studios Network has been nominated for a Webby award
in the science and education category. We'd appreciate your support and
your vote at webbyawards.com. You can check out the
link in the description. [THEME MUSIC PLAYING]
I wish he'd given at least an analogy of two observers observing the same event and finding different time and orders of the events.
This is really well done. It's pretty cool to see PBS making a program exclusively for the Youtube format.
It would be nice if it were presented in more of an ELI5 format with some examples and not just a bunch of scientific terms with overly brief explanations.
I understood nothing tbh
Wow, that took an abrupt shift toward dignity.
https://youtu.be/bZHUAKOmFxs?t=28s
Interesting concept and video. It lost me for a moment but I see what he's getting at. And by that concept of flat spacetime, does that mean the birth and death of the universe already exists? And it all already happened in the non-euclidean dimension? And what exactly is going on the non-euclidean dimension? I want to know more!
This is very confusing.
This guy has done the impossible: The youtube comments aren't awful!