Thank you to Brilliant for supporting PBS. Physics progresses by breaking our intuitions,
but we are now at a point where further progress may require us to do away with the most intuitive
and seemingly fundamental concepts of all—space and time themselves. Physics  came into its modern form as a description
of how objects move through space and time. They are the stage on which physics plays
out. But that stage begins to fall apart on the
tiniest scales and the largest energies, and physics falls apart with it. Many believe that the only way to make physics
whole again is to break what may be our most powerful intuition yet. In our minds, space and timeÂ
seem pretty fundamental, but that primacy may not extend beyond our minds. In many of the new theories that are pushing
the edge of physics, spacetime at its elementary level is not what we think it is. We’re going to explore the “realness”
of space and time over a few upcoming episodes. We’ll ask: Do our minds hold a faithful
representation of something real out there, and if not, why do we think about space and
time the way we do? And if space and time aren’t fundamental,
what is? What do space and time emerge from? But today we’re taking the first step by
exploring how the notion of absolute space and time in physics came about in the first
place, and how that notion is beginning to fall apart. We have this sense of space as an extended
emptiness - a volume waiting to be filled with matter - a regular, continuous, mappable
… space, in which everything that exists is embedded. Meanwhile time is the continuous rolling of
future into past through the present, all governed by the same unstoppable clock. But this idea of space and time as having
an existence “out there”, independent of its contents, became cemented in popular
intuition relatively recently, at the same time that they became cemented in physics. However humans have been arguing over the
reality or the fundamentalness of the dimensions for millenia. We can summarise the two main conceptions
of spacetime as either relational— space as a network of positional relationships of
objects —or absolute—a real entity that exists independently of objects, and rather,
contains the objects. The latter seems to have emerged only relatively
recently. Let’s start with the ancients. They certainly thought a lot about space—after
all, they had maps and they invented geometry. But the geometries of Euclid and Pythagorus
and others didn’t need the notion of space as an absolute entity—they were relational. For example, a triangle is defined by the
relative lengths of its sides and its internal angles. You don’t need a coordinate grid to define
a triangle—which is good, because the ancient Greeks didn’t have one. Sure, their maps had longitude and latitude,
but they didn’t have our own mathematical habit of gridding up empty space with x, y,
and z axes. As such, they didn’t tend to think of empty
space as having its own independent existence. The idea of the coordinate grid came much,
much later. Perhaps you’ve heard of the Cartesian coordinate
system. X, y, and z axes, each at 90 degrees to the
others and gridded up so that any point in space can be defined with three numbers - the
value of the closest grid-mark on each of the axes. This idea feels pretty intuitive to many of
us, but it wasn’t commonly used until after 1637, when the FrenchÂ
mathematician and philosopher Rene Descartes made it cool. With the coordinate system, it became possible
to represent abstract numerical concepts in spatial terms—for example, by graphing an
algebraic function. But it also gave us a tool for describing arbitrarily large andÂ
imaginary physical spaces—and this application would soon revolutionise
all of physics. Regarding the actual nature of space, Descartes
was firmly in the camp of philosophers like Plato, who didn’t believe in empty space. Descartes said that space is only real as
far as it defines the extension of objects and matter. But the invention of the first true mathematical
coordinate system opened the door for a very, very different conception of space. And that new conception was almost entirely
due to Isaac Newton. He gave us a set of equations that could,
apparently, completely describe the motion of objects and how those motions change through
the forces of their interactions. Newtonian mechanics is built on Descartes’
coordinates, and assume a universal clock. Those mehcanics proved wildlyÂ
successful— revolutionary, really. So much so that many, including Newton, began
to see the foundational building blocks of the mechanics—the coordinate of space and
time—as in some way physically real. Newton himself insisted that space is absolute;
it exists completely independently of any objects within it. The empty volume implied by the Cartesian
grid is a thing in itself. And according to Newton time is also absolute. From Aristotle to Descartes, “time” was
mostly understood as a counting of events. But In Newton’s view, there’s a single
universal clock that keeps the same time for all observers--time passes “by itself ”, even
in the absence of any change. Newton also believed that there was an absolute
notion of stillness. Like, a master frame of reference whose x,
y, and z axes are unmoving, and if your position was fixed relative to those axes then you
were truly still. This is contrary to the ideas of Galileo a
century prior, who showed us that velocity is relative—the speed you measure for another
traveller depends on your own speed. The laws of physics are theÂ
same in any non-accelerating, or inertial frame, and so all such frames are equal. While Newton accepted theÂ
mathematical consequences of Galilean relativity, heÂ
thought the difficulty we had in defining a preferred inertial frame was
a limitation of the human mind, not of the universe. The success of Newtonian mechanics elevated
the notion of the realness of space and time in everyone’s minds. But there was one prominent naysayer. Newton had a nemesis. Or maybe it was Newton who was the nemesis
to this guy. Ok, he shared a mutually nemetical relationship with the German mathematicianÂ
Gottfried Wilhelm Leibniz. Their most famous rivalry was over the discovery of calculus, which theyÂ
figured out independently—with Leibniz probably getting to it first. Newton however accused him of plagiarism, and being by far the most powerful scientist of his day, secured the credit for himself. But another point of contention between theseÂ
two was on the nature of space and time. Leibniz did not accept Newton’s assertion
that these dimensions were in some sense real and independent of anything in them. Instead, he thought that both space and time
were relational. What does that even mean? Well, it means that objects exist, but they
don’t live in a 3- or any other dimensional space. Rather, what we think of spatial separation
is a quality of the objects themselves—or rather of the connection between them. Exactly why Leibnitz thought this and rejectedÂ
Newton is a whole thing, that we don’t have time to get into right now. Instead, let me try to give you a sense of
what it could mean for space to be encoded in objects or in their relationships, rather
than existing independently to those objects. Let’s start by imagining only one dimension
of space, represented as a line. This is a Newtonian space, where every point
represents an absolute position in a 1-D universe. We can put some particles in the universe. The position of each in space is defined by
- well, its position in space—whatever grid mark it’s next to if we add a coordinate
system. The particles might have intrinsic or internal
properties—say, mass, electric charge, etc., but their position isn’t a quantity that’s
intrinsic to the particle. In Leibniz’s view there is no space, so
we get rid of the line. The particles still exist, but they aren’t
anywhere. They’re sort of just bundles of properties
with no size or location. Space doesn’t exist so maybe we should place
these particles on top of each other, but then again if location is meaningless we might
as well separate them so we can see them. Let’s add a new property to each particle
that we’ll call X. X is what we call a degree of freedom—something
about the particle that can take on different values, and it can change. Other degrees of freedom could be energy and
phase and spin and so on. X behaves in a particular way. For example, it can change freely. If it’s changing, then it keeps changing
at the same rate and in the same direction. Now these particles have no idea about each other's existence, exceptÂ
in a special circumstance. For example, If two particles have values of X that are
close to each other then those X values influence each other, changing theÂ
rate at which the dials turn. Maybe they want to try to be more similar,
or maybe they try to be more different. If we were to represent these X values with
position on a number line - an x-axis - then the behaviour of the particles looks just
like particles moving around in space and attracting or repelling each other only when
they’re close together. We can’t tell the difference between particles
moving in space versus space-like behaviour emerging from a degree of freedom within the
particles. This thought experiment isn’t explicitly what Leibniz described,Â
nor is it how things should really be to explain a universe like
our own. For one thing, we need 3 spatial dimensions,
not one. X, Y, & Z would all have to be close to each
other for particles to interact. Also, Leibnitz thought that position wasÂ
encoded in the relationship between particles, not in the objects themselves. He gave his elementary particles names - monads
- which among other things had rudimentary consciousness, and that space emerged from
their first-person perspectives of each other. But we don’t actually needÂ
those extra qualities--the idea of particles with interacting, internal degrees of freedom illustrates how space can
emerge from the relationships between elements that are themselves not in space. So that’s Leibnitz on space. He disagreed with Newton on time in a similar
way, believing it to be a measure of the change intrinsic to each element, rather than a cosmic
clock that kept the universe in sync. Of course Newton was the undisputed boss of
science back then, and so his preference for absolute space and time won over the physicists,
and ultimately found its way into the popular imagination. But who was really right? Are objects in space and moving through time,
or are space and time somehow in objects and their connections? Are the dimensions absolute or relational? The big next development seemed to support
Newton. Over the 19th century, our understanding of
the phenomena of electricity and magnetism converged, revealing the existence of something
called the electromagnetic field. A field is just some property that can take
on a numerical value at all points in space. For example, temperature is a field defined
in the air around you. It’s emergent from the properties of the
air particles. But the electromagnetic field doesn’t need
particles. For the first time, it seemed that a field
could be a property of space itself. So, surely if space can have properties, then
space must objectively exist. And more intrinsic properties emerged with
the development of quantum mechanics—for example, space was shown to have a sort of
energy even in the absence of particles—so-called vacuum energy. However, if we really want to decide whether
space and time are real—to judge between Leibnitz and Newton—we need the ultimate
arbiter. We need the greatest expert of space and time
that ever lived—and that’s Albert Einstein. We’ve talked about Einstein’s special
and general theories of relativity many times before. Let’s just go over what the theory changed
about our notions of the dimensions. With special relativity, the separation of
3-D space and 1-D time ended. They became 4-D spacetime. Einstein showed that our motion through space
and our motion through time are linked. A clock moving relative to you ticks slower
from your perspective. And then with general relativity we see that
the presence of mass and energy stretch and warp both space and time. This causes straight line trajectories that
we expect on a Cartesian grid to become curved, and the apparent change in an object’s path
in the presence of mass is Einstein’s explanation of gravity. Relativity overturned some of Newton’s notions
about absolute space and time: that they are independent entities, that there’s a universal
clock for time, and that there’s some sort of ultimate, rigid coordinate system for space. But what did these mean for the central question
of this episode: what about the realness of space and time? Actually, spacetime in Einstein’s universe
kind of feels even more substantial than before. It’s like a fabric that can be warped. It can hold energy. It can even propagate waves—gravitational
waves. Einstein showed that empty space has properties,
so it must be real, right? Well, maybe - but Einstein’s view is really
a radical departure from Newton’s—to the extent that Einstein even called himself a
Leibnizian. Newton believed in space as an underlying
stage on which the particles and the fields danced. But Einstein insisted that no such background
existed—and that’s because to him, space and the gravitational field are the same thing. This field is not painted on topÂ
of a coordinate system; rather,  the coordinate system is a quality of the field. Absent this field there is nothing. So all of this landed Einstein somewhere between
Leibniz and Newton. He believed that there is an extended structure
“out there” that can hold objects and on which distances and durations can be defined,
but it’s not absolute and fundamental in the way that Newton thought. According to Einstein, Descartes was right,
and so was Plato: there’s no such thing as empty space. To quote Einstein,Â
"there is no space empty of field" So is Einstein the last word on the matter? Far from it. We know that general relativity breaks down
on very small scales—smaller than around 10^-35 meters, which is the Planck length. There it comes into hopeless conflict with
quantum mechanics, and it becomes impossible to meaningfully define shorter distances. Just as it’s meaningless to define durations
shorter than the Planck time. This conflict between Einstein’s theory
and quantum mechanics is one of the major challenges and inspirations for progressing
to the next level of physics. And essentially all of the possible paths
forward force us to rethink our understanding of the dimensions—whether multiplying their
number as in string theory, or by having them emerge from elements that, themselves, do
not exist within space—such as in loop quantum gravity, which we’ve discussed, or the cellular
automata of Wolframs physics project, or in the entanglements betweenÂ
elements on a holographic horizon, or from Arkani-Hamed’s amplituhedron among others.. If any of these latter are true, then Leibniz
may have been onto something; space exists in the relationships between some sort of
elementary… something, not as an absolute and physically
real fabric. Leibniz also had another controversial idea:
he thought that space was in our minds. This isn’t the same as saying that reality
is in our minds—it’s not even the same as saying that space doesn’t exist. Rather, Leibniz felt that whatever it is that’s
out there that behaves like space only gains the subjective feeling of depth, breadth,
height, and distance when our brains try to organise objects that are separated by an
altogether more abstract property. Kind of like how the subjective experience
of red only exists when brains interpret a frequency of light. It’s incredibly difficult to imagine a universe
without space or time. The dimensions seem hardwired into our brains. Perhaps we need to break this preconception
to move forward in physics. If so, we need to explore how and why our
brains build our very convincingly spatial and temporal inner worlds. And we’ll do that in an episode very soon,
and perhaps get closer to figuring out whether we live in an absolute or a relational spacetime. Thank you to Brilliant for supporting PBS.Â
Brilliant is an online learning platform for  STEM with hands-on, interactive lessons. BrilliantÂ
is for curious learners, both young and old,  professional and inexperienced. Brilliant coursesÂ
teach you how to think (via interactive lessons  and problem-solving activities/exercises.) andÂ
solve problems with interactive lessons in STEM. For example, Artificial neural networksÂ
learn by detecting patterns in huge amounts  of information. Like your own brain,Â
artificial neural nets are flexible,  data-processing machines that make predictionsÂ
and decisions. In fact, the best ones outperform  humans at tasks like chess and cancer diagnoses.
In this course, you'll dissect the internal  machinery of artificial neural nets throughÂ
hands-on experimentation, not hairy mathematics.  You'll develop intuition about the kindsÂ
of problems they are suited to solve,  and by the end you’ll be ready to dive intoÂ
the algorithms, or build one for yourself. To learn more about Brilliant,Â
go to brilliant.org/spacetime Today we’re looking at your comments fromÂ
the last two episodes. There was the one  about how Earth really moves through theÂ
universe, and then the one about how the  nucleus is held together by meson exchange.Â
Starting with the motion of the Earth. Matt Thomas asks, when we put together all ofÂ
our motion through the universe, how fast are  we moving relative to the CMB? And what effectÂ
does that motion have on our experience of time?  The answer is that we’re moving at 368km/sÂ
relative to the CMB. This isn’t unusual—most  things in the universe have some relativeÂ
velocity like this. But you’re right that  there should be a time dilation relative to theÂ
CMB. Let’s assume the frame of reference of the  stuff of the Earth has on average been movingÂ
at that speed over the history of the universe.  Less time has passed in that reference frameÂ
compared to the rest frame of the CMB—the Big  Bang was more recent for our hypothetical movingÂ
frame. I figured it out—the difference is about  10,000 years. Pretty tiny comparedÂ
to the age of the universe. Karl Sheffield asks what is inÂ
front of our path around the Galaxy?  Well, immediately in front: the interstellarÂ
medium. The Sun’s heliosphere—a bubble  containing its outward-flowing solar wind andÂ
magnetic field—is plowing its way through very  low-density gas and dust grains. ThereÂ
are also bigger things that we can’t see  easily—bits of rock or ice like oumuamuaÂ
that were ejected from other star systems.  There will be ejected planets, brown dwarfs,Â
black holes and other stellar remnants. In  terms of stuff we can see—well we’re headingÂ
in the direction of the star Vega, but Vega is  also orbiting the galaxy and so we’re not goingÂ
to collide. That said, we do occasionally get  close enough to a star or stellar remnant to messÂ
with orbits in our system, with the main danger  being an increase in inner-solar system comets.Â
That’s more likely when we’re passing through  the disk and especially in a spiral arm. It’llÂ
be millions of years before that happens again. Moving on to the episode onÂ
the strong nuclear force. Fensox asks whether Hideki Yukawa eventually gotÂ
the recognition he deserves for discovery of the  strong and weak forces. He did. He got theÂ
1949 Nobel Prize for predicting the existence  of the pi meson. And his name is all over theÂ
standard model—the Yukawa interaction governs  the strong force part of the standard modelÂ
Lagrangian as well the Higgs coupling term. Several people asked how it is that theÂ
exchange of virtual particles can cause  particles to be attracted. After all, inÂ
the analogy of particles throwing balls  at each other, it seems that the exchangeÂ
of momentum should only push them apart.  The short answer is that the balls analogyÂ
is a pretty limited one, and even the notion  of virtual particles is something of a metaphor.Â
What’s really happening is that the quantum fields  between and around the particles are disturbed inÂ
a way that can be approximated as the work of many  virtual particles. But those virtual particlesÂ
don’t simply originate at particle one and  travel in a straight line to particle two. TheyÂ
can originate in a wide region governed by the  uncertainty principle, including on the oppositeÂ
side of particle two. They can also have any mass,  including complex masses. All of this enablesÂ
the virtual particles to transfer momentum in a  way that pulls the particles together insteadÂ
of apart. But really, these particles are a  mathematical fiction to describe fieldÂ
behavior. No balls are being thrown. Feelincrispy points out that I could easily  just make something up and %99.9 ofÂ
you would have no idea. I don't know if I agree with that, but otherwise I have no comment. sleekweasel asks how theÂ
island of stability works,  given that if a nucleus grows too big,Â
its mesons can't hold it together.  To remind everyone —the island of stability isÂ
a region of the periodic table of very large  nuclei that is theoretically more stableÂ
than the current heavy end of the table that we've discovered at this point.  Actually, I don’t really know the details of this. But fortunately Gareth Dean jumpedÂ
in to the comment section to answer, so I’m just going to read that. He says: Nuclei aren't just blobs of particles,Â
they have 'nuclear shells'. When these are empty  the few particles in them are far apart andÂ
cannot exchange mesons. When they are full,  lots of particles are packed close and canÂ
bind strongly. 'Islands of stability' are  places where the shells are full, bindingÂ
is strong and the nucleus is more stable. Regarding my use of the labradoodle toÂ
illustrate the amount of force between  adjacent protons in an atomic nucleus. Many ofÂ
you expressed interest in using labradoodles  as some sort of standard unit of measurement.Â
This is a little impractical because we’d need  to use the mean weight of a statistically large number of labradoodles. But I personally volunteer  to run the NIST labradoodle standards facility toÂ
make sure those very good boys and girls get all  their standard treats and pets. Many of you alsoÂ
pointed out that a universe without labradoodles  is not a universe they’d want to live in. AlsoÂ
agreed. Which brings us to Steve. Steve sees  the elimination of the strong nuclear force andÂ
with it the elimination of all chemistry, biology  and life, as a promising way to rid the world ofÂ
labradoodles. Steve, you’ve identified yourself as  labradoodle-foe, and your name has been passedÂ
to a secret elite team at the NIST labradoodle  standards facility. They’ll be watchingÂ
you. In fact all labradoodles will be watching you.
The episode title sounds like it should be followed by "...mannnnnn," except that this is PBS Space Time, and they can back it up.
I can't grasp X,Y,Z being local properties of a particle, but I certainly can imagine time being a property. That would ensure that all of us in the nearby vicinity share the same present. The more past light cone overlap we share, the more gravitational decoherence which forces macroscopic wavefunction collapse to happen for all particles with the same T. Every particle would have a T wavefunction with some uncertainty but gravitational decoherence ensures they synchronize.
Relativity, treated as trajectories in space parameterized by time would still be the same but there would be coupling between T and X,Y,Z somehow.
The presence of high amounts of mass might trigger decoherence to be at slightly smaller values of T compared to further away, which would manifest as time dilation.
Tear this apart and build it back together! Not a self-promotion just an artistic philosophical approach to the question. I am not a scientist so please correct the metaphors if you want. Sorry if this is breaking the rules, I saw y'all are lenient with theories in the comments
Celestial Scientists
What if the universe is a proverbial chemical reaction inside of a beaker? The big bang is the chemicals reacting as they are poured in, each star is a nucleus and the planets are electrons. Life, or the unique identifying characteristic of our planet, requires electricity so we are the embodiment of that electricity. Death is the electrons joining the atoms of the intended element. Black holes are air pockets popping in the liquid. Supernovae are the atoms changing into the final solution/mixture. Dark matter is the "water" of the solution, everything floats in it but it is impossible to observe because it's the state of matter we are in. The universe is chaos but trends towards entropy/nirvana because the end goal is to finish the chemical reaction to have the final product.
Feel free to share it or use it as an artistic prompt, please send me anything if you do I want to put a collection together
This was a really good episode. One of my favorites, if not the favorite episode I've seen.
Which got me thinking about time dilation and how I've taken it for granted. It was one of those things I said, "Yeah, of course."
Now I'm looking closer as to why it seemed so intuitive. The way I see it now, the more, frames, for lacking of corresponding term, an object travels through the more the observer has to process. So the faster an object, the more frames it permeates through. But the observer can only process as many frames they are currently permeating through. There might have been an episode that touched upon this thought. But time dilation is weird, good thinking.
It's funny, I was going through a bout of loneliness some time ago. And to take my mind off things, I started thinking about quantum physics. I was always curious about if light was a particle or a wave. Watching Space Time is a great way of feeding that food for thought.
Thank you all, and I'll be looking forward to the movie. <3