Is all that exists just whatever exists right
now? Is the past erased and the future a void yet
to be filled? Well the answer lies in between the past and
the future - in the elusive, ever-moving eye-blink that we call the present. Today we’re starting a deep dive into the
nature of time - and down that rabbit hole we’ll encounter the physical origin of time,
the question of determinism, and even what this tells us about consciousness and free
will. Big questions that’ll take us to the edge
of what physics can answer, and perhaps beyond. But today we’re definitely doing physics,
because physics already tell us a lot about the reality of the past, future and present. We think of the world that exists as the world
of the now. The past is gone except in our memories, and
the future is a blank slate ready to be written into the past. But interpreted some ways, physics suggests
that the future and the past exist eternally, while the present is an illusion. To show you why this really might be the case,
we need to actually visit the past. As with many ideas in physics, this one originates
with Isaac Newton. Newton was the first to come up with a set
of laws that allow us to predict the motion of all objects in the universe. Newton’s revelations led to the notion of
the deterministic universe - the idea that, by knowing the current position and velocity
of every particle in the universe, as well as the forces that act between those particles,
you could calculate all future and all past states of the universe. Now quantum mechanics might argue otherwise, depending on which quantum mechanics you prefer, but we’ll come back to in an upcoming episode. Newton’s picture of the universe included
one other assumption that we know for sure is wrong. It included the idea of an absolute, clearly
definable “now”. Newton assumed that all particles, all observers,
all points in space were ruled by a single, constantly ticking clock. This universal clock meant it was possible
to define a notion of “now” that everyone would agree on, and so everyone would also
agree on what was past and what was future. Here’s a nice way to represent Newton’s
picture of space and time. Let’s cut out one dimension of space - space
then becomes a 2-D slice at a given instant in time. Now let’s stack successive instants so that
time becomes the 3rd dimension. Every slice is the same universe at a particular
instant, and it evolves from one slice to the next according to the laws of physics. We only experience a single slice at a time,
and we usually think of the universe in that slice as the one that currently exists. But we can also imagine a perspective from
outside both space AND time in which this entire structure just exists. From that perspective, time doesn’t have
a particular direction, and there’s no preferred “present”. Time is just a dimension like space, and we
only observe a flow of time if we play the slices in sequence like a flip-book. We sometimes use the term “block universe”
or “block time” to refer to this view of all space and time just existing in this
4-dimensional chunk. The flip-book of the block universe has to
be played in the right direction to see emergent phenomena like the sequence of cause and effect, evolving patterns of structure and information, entropy, and even our conscious experience,
which itself emerges in the forward evolution of patterns of information in our brains. Our awareness of the universe rides this forward-moving wave of the present. To us, none of the rest of the block universe
exists, because our existence emerges from the forward evolution of a razor-thin slice. Although by the way - that perception of existing in a single instant in time is definitely an illusion - our awareness - in fact our
conscious existence - is sort of smeared out over a couple of hundred milliseconds. But that’s a topic for another, well, time. Another way I like to think about it is that
4-D spacetime is a vinyl record, and our subjective experience is the music coded in the grooves. Viewed from outside, the whole thing exists,
but the music only emerges if played in the right direction. Nothing “plays” the block universe - it
just is, and temporal phenomena like us are just embedded in it, when you look at it in
the right way. There’s a name for this idea that all of
time, future and past, sort of just exists as this atemporal structure. It’s “eternalism”. The opposite viewpoint would be presentism,
which posits that only the current instant has a meaningful existence - the past is erased,
the future still blank. And the middle viewpoint would be that of
the growing block - that’s the idea that this shock-front of the present creates the
block universe as it proceeds forward, weaving the past out of nothing. Believe it or not, we can actually science
these very philosophical ideas, with just a little help from the smartest guy in the
block universe. The representation of the block universe that
I showed you is very Newtonian. We had a single time axis that everyone agreed
on, and everyone agreed what slice of the block was currently the present, and which
sides were the future or the past. Newton says there’s only one way to slice
the universe. Einstein says otherwise. In his special theory of relativity, Einstein
showed that there’s no absolute way to define whether two events happen simultaneously - the present, past, and future are relative concepts. Let’s pick this apart. How do we even say what “now” means for
other parts of the universe? Nothing, not even information, can travel
faster than light. That means any distant event is already in
the past by the time we see it. We represent the region of the block universe
that we can possibly perceived with a light-cone - signals from things inside that cone have
had time to reach us. The boundary of the cone holds the paths of
light-speed signals. Whatever events live on that cone represents
the universe that we see around us, carried to us by light. Any event above that cone is unseeable in
the present - we’d have to wait until our lightcone moves upwards to encompass it. Which is the same as saying we have to wait
until its light has had time to reach us. Our entire time-slice of our present is never
observable in the present. To map it, we need to wait. From your point of view, you are motionless. After all, motion is relative so we can define
your motion as stillness. That means as time passes you move straight
up - fixed in space, only moving through time. As that happens, signals begin to reach you
from the time-slice you’re trying to map. Successive shells of light from more distant
parts of that time-slice arrive one after the other, expanding your view. OK, so the present seems definable - if only
in the future. But if they’re moving relative to you things
get weird. To you, they seem to make a mistake. They are racing towards one set of incoming
signals and away from the other. At the same time, their entire perception
of the spacetime grid is warped due to their motion. We talked about this warping via the Lorentz
transformation in a lot of detail previously. The upshot is that the moving observer builds
an entirely different map of that time slice - they define the present differently. In fact, it’s possible for another observer
to be in your slice of the present, but for you not to be in theirs. So that’s the effect of Einstein’s special
relativity - space and time tilt into each other, so that different observers will slice
up block time at different angles depending on their velocities. Even your own sense of the present changes
with your cvmotion. Start moving forward and your slice of “now”
will skew. Ahead of you, things once in the future will
become the present, and behind you the past becomes the present and what was once the
present is now in the future. Walk in circles around the room and your entire
now-slice tilts crazily like a ship deck in a storm. Nearby the effect is tiny, but the “present”
at the edge of the observable universe veers back and forth by a couple of centuries every
time you switch direction. Try it, but be careful - it can make you dizzy. Let’s get back to the question: what does
all this say about the reality of the past and the future. Imagine that the future is created as the
wave of the present sweeping out the block universe. But where is that wave? Whose slice? At a given location in space, you can lay
down all possible time-slices representing possible presents. The largest tilts are for things traveling
at the speed of light, and these sweep out the future light-cone. In front of the future light cone is the region
that is in the future for everyone, no matter what their speed. So is that the future that doesn’t exist
until created by the evolving present? Well that can’t be right. Consider the time-slice of our present; we
can imagine other observers on that time slice that we surely must “exist”. After all, they could have travel to those
points from our own past. They have future light cones, simil
to our, offset in space from ours. The space beneath their future light cones defines all possible defines all possible definitions of the past to them. Just like it does for us. So now fill our present time slice with observers and their remains no part of the block universe that couldn't be considered the past according to someone who lives in our present. Let’s take stock: It’s hard to accept
that only the present exists, that only the current “slice” of the block universe
has a meaningful reality - because it’s impossible to define what that slice actually
is. It’s also hard to accept that only the past
exists and the future doesn’t - at least in the sense of a growing block universe,
again, because the “present” of someone else on your slice of present could be literally
any point in your future. So we’re left with two options: 1) the entire
block universe has a meaningful existence; or 2) if you don’t want the future to exist
then you need to deny the existence of the present beyond your own immediate experience, which can include events only from your past light cone. Nothing outside that section of the block
universe can be ascribed a definite reality until you interact with it. That second stance feels extreme - it feels
like the definition of solipsism - that your own subjective experience is the only thing we
can be sure exists. But there’s a way to humble this notion
- to make it more palatable the materialist - to those who like to believe in an actual
universe independent of the self. That means most physicists, including this
one. To rescue materialism without demanding a
perfectly defined personal future we need quantum mechanics. Quantum mechanics tells us that all this stuff
outside our past lightcone - and even unobserved aspects of the world within that cone - exist
in a state of indeterminacy until observed. Depending on how you interpret quantum mechanics, that could be evidence in favor of this solipsistic denial of external reality. But quantum can also save materialism and
determinism if you like- but to do so you need all possible realities to exist simultaneously and persist into the future. We’ve talked about these ideas before - the
Copenhagen and Many Worlds interpretations of quantum mechanics - but soon we’ll dive back in, to see what
these idea imply about time, determinism, and what it would mean to live in a block
multiverse space time. To everyone who helps us out on Patreon - once again, thank you so much for your support. It makes a huge difference. And today's special shoutout goes to Marty
Yudkovitz who's supporting us at the Big Bang level. Marty, we're officially designating you the
coolest dude in the block universe - that means past, future, and present - never mind
that all of those are an illusion. At any rate, may your future lightcone contain
only wonderful things. Last episode we reported on the incredible
discovery of possible biosignatures in the atmosphere of Venus. Remember, it's probably not life - but it
just might not not be. . . So let's see what you had to say. Henri Williams points out that any Venusian
life that we find is very likely from Earth - having transported to Venus on rocks that
were blasted from Earth's surface by impacts. And Afto Kinito points out that it could easily
have happened in the other direction - Earth life being seeded by Venusian meteorites. Well I think you're both on to something. Earth and Venus do exchange a lot of material,
so it's almost certain that early microbes would have traveled between the two, in the
process we call panspermia. The question then becomes - would it have
been easier to kickstart life from scratch on two planets independently, or have a spacefaring bugs propogate after crashlanding? Honestly, the latter seems pretty compelling. Man, it would be so cool to learn that we're
all actually from Venus. Robin Nogueira takes issue with my statement
that it would be good news if the venus life was confirmed, pointing out that it would
mean the great filter is in front of us. Just to explain this excellent point for those
who don't remember every episode we ever did. The mysterious absense of civilizations in
our galaxy is sometimes explained in terms of a great filter - something reliably stops
planets from spawning space-faring civilizations. That something may be ahead of us - like nuclear war, environmental destruction, ... Or it may be behind us, like it might be extremely
difficult for life to form in the first place. If we discover that life forms very easily
- for example by discovering it on Venus, then it makes it more likely that the great
filter is ahead. Which is bad news, not good. So yeah, maybe - but there are still plenty
of past great filters, like evolving complex lifeforms or intelligence or whatever. Or maybe life did only spawn once in our solar
system and then spread. Either way, I'm still rooting for life on
venus. Mark Pendragon reminds us that industrial
processes also produce lots of phosphine, so maybe we just discovered Venusian heavy
industry and not microbial life. Bummer. Maybe if we can make contact we can ask whatever advanced civilization to kindly shut down their factories so we can more carefully analize
the atmosphere for much more interesting bacteria.
Every time I watch his videos I feel like I’m being bullied with science. Like I can follow along for 5 minutes and then bam the whole tone changes. I think I love the abuse cause I’ve been subscribed forever.
Block universe blows my mind. I got all the way through a physics degree without even learning that that was essentially the dogma. I guess if I'd sat a course in GR I probably would have been told.
I really think more people need to know about this fundamental aspect of reality!
I struggle to comprehend this guy's videos. It always feels like he is reading a written piece of work rather than explaining something verbally. Maybe I'm slow, but it feels like each sentence is way too dense to understand quickly enough to keep up as he's speaking. Love the concepts he tackles, but I can never get the most out of his videos because my brain is scrambling to decipher what he's saying.
Just kept thinking of the time knife
Finally! I've never heard of any of this, but I always wondered if my past life still exists. I have happy blocks of time in my past that I cherish and I would love to visit once more. Like an old record, being pressed and waiting to be played again.
I'm so glad that these happy blocks are considered as still in existence.
If black holes turn out to destroy information, then information can and is destroyed in the Universe. If information can be destroyed, then there is no one true past. The past becomes probabilistic like the future.
This was hard to follow. And i'm not even multitasking!
I just kept thinking about that horse in the JLo movie The Cell
Yes and No at the same Time.