Ideas - How mind-bending theories could solve mysteries in physics

Episode Date: November 4, 2025

Physics has been full of astonishing discoveries over the past century. But they open up even bigger mysteries that scientists are working feverishly to explain. What is dark energy? And why is the expansion of the universe accelerating? In public talks at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, two prominent physicists – Sarah Shandera of Penn State University and Stanford University’s Savas Dimopoulos – discuss the breakthroughs of recent decades and what it will take to solve the most nettlesome mysteries that have deepened in their wake.

This program is brought to you in part by Spex Savers. Every day, your eyes go through a lot. Squinting at screens, driving into the bright sun, reading in dim light, even late-night drives. That's why regular eye exams are so important. At Spec Savers, every standard eye exam includes an advanced OCT 3D eye scan, technology that helps independent optometrists detect eye and health conditions at their earliest stages. Take care of your eyes. Book your eye exam at Specsavers today from just $99, including an OCT scan.

Book at Spexavers.cavers.caps are provided by independent optometrists. Prices may vary by location. Visit Spexavers.cavers.cai to learn more. This is a CBC podcast. I'm Nala Ayed, host of Ideas, and I'm inviting you to a birthday party, ours, because Ideas is turning 60. So we're having a celebration at the Isabel Bader Theater in Toronto on the evening of November 11th. Tickets are free, but you must be able to be. register. Just visit cbc.ca.ca slash ideas. Ideas at 60. That's November 11th at the Isabel Bader Theater in Toronto. See you there. Welcome to Ideas. I'm Nala Ayad.

Coffee is a very serious business in the life of a theory. In a scene from the documentary particle fever, a physicist makes himself an espresso. If you succeed, it's great. If you fail, you get to try another one in another minute. In particle physics, you construct a theory 20 years ago, and it may take that long before you know if you are on the right track. Jumping from failure to failure with undiminished enthusiasm is the big secret to success.

It's not a vocation for the impatient or easily discouraged. For thousands of years, and without the benefit of a user's guide to the universe, humans have uncovered some astonishing truths about nature and the cosmos using intuition, math, observation, and bold leaves of the imagination. That's how we know about relativity, the Big Bang, and the standard model of particle physics. But like the universe itself, the gap between what's known and unknown seems to keep expanding. For example, dark matter and dark energy, we know they actually make up most of the universe, but we don't know what they are.

All kinds of new or tweaked theories and powerful instruments are poking and prodding at the enigmas of the universe. Higher energy particle colliders, like the Large Hadron Collider, which found evidence of the Higgs boson, or LIGO, which first detected gravitational waves in 2015. But for all this technological and intellectual horsepower, physics has come up empty in the search for answers to the most stubborn mysteries. So I'm very happy to be here at Perimeter, one of my favorite institutions in theoretical physics in the world. Savas Demopoulos is an award-winning professor of physics,

Stanford University. As befits his Greek heritage, he's also the Archimedes Chair in Theoretical Physics at the Perimeter Institute in Waterloo, Ontario. Today I'll talk about new physics, in particular how we've been doing research during the last century and what I see as going forward in the next few decades. As part of the Perimeter Institute's 25th anniversary, anniversary celebrations this year, Savas Demopoulos recounted a history of milestones in physics and suggested where the next breakthroughs might come from in a public lecture called New Physics in a post-Big Science world. So the objective of physics is to build one theory that explains everything in the universe.

Physics is using two major tools to do that. One is mathematics and the other is experiment. Mathematics is simply the expression of physical laws in mathematical formula. And the purpose of experiment is to figure out what's actually true by observation. The realization that mathematics is relevant to nature goes back to the Pythagorean's. The Pythagorean used to say, Theos Ais Geometry, which means God, geometrises everything. A modern paraphrase would be God uses mathematics to describe nature. This belief went down to Plato, who was a student of the Pythagorean School. So when he built his academy, the Academy of Athens, the first university, he had an inscription outside the door

saying that no one ignorant of geometry should enter. Eugene Vigner in 1960 wrote a very nice essay called the unreasonable effectiveness of mathematics in describing nature. There he said mathematics is a wonderful gift which we neither understand nor deserve. I want to dwell a bit why it's so mysterious. First of all, the connection of mathematics and nature, is a big question mark, why does nature use the language of mathematics?

What is also an amazing coincidence, that's the gift part, is that we, tiny human beings on a tiny rock in a vast universe, can understand mathematics, can understand the language of nature to such a degree that we can think in our labs or in our offices and write down equations and figure out what happens. everywhere in the universe. So it is a complete likely coincidence that mathematics describes nature

and we're just barely smart enough to understand enough mathematics to understand at least some aspects of nature. Now, mathematics is so powerful. It's not just language. It is like having an extremely powerful friend if you give him the right rules and ask a well-defined question, it will give you the answer. The more precise the question, the more precise answer you'll get.

So it's a miraculous tool, it's divine, and yet there is an equally important part, which is the experiment. And the experiment has the last say. This has been said beautifully by Richard Feynman, who, back in 64, said, it doesn't matter how beautiful your theory is. It doesn't matter how smart you are. If it doesn't agree with the experiment, it's wrong.

And of course, physics without experiment doesn't really describe nature. Experiment without mathematics is a chaos. You might as well collect all the data and all the lad books of the last 2,500 years, put them in a big computer and call it physics. Physics is much more than that. The key point that mathematics allows us to use is a few equations from which you can derive everything, and in principle we can derive them to arbitrary precision. And if you can measure them to arbitrary precision, you have an amazing coincidence that the two should agree.

The relationship between science, math, and experiment dates back more than 2,500 years, to the time of Pythagoras, and three centuries later, Archimedes. So Archimedes, when he was 22 years old, he was tasked by the king to determine if his crown was made out of gold, 100% gold. So Archimedes got the idea when he was entering the bathtub, when he noticed that the more he immersed himself,

the more the level of water was raised. And then he made the British, brilliant hypothesis that perhaps the amount of volume displaced when he immerses the crown in the water, he can figure out by just figuring out the volume of the water. So an irregular shaped object like a crown, he could determine precisely the volume of that. He used that and eventually it turned out it had some silver in it, the crown. So the person who built the crown went to jail. And another famous story is when he made this realization, he got so excited.

he jumped out of the hot tub, I come from California, of the tub, and I was running around nude in the streets, saying Eureka, Eureka, which means I found it. Similar, Eratosthenes. Eratosthenes was the first person who figured out by looking at the difference in the size of shadows in two different cities separated by everything. 800 kilometers seen in Alexandria, by figuring out the difference in the length of shadows at the same time, he was able to determine that the earth is round, and he even estimated it turns out quite precisely the circumference of the earth would be about 40 million meters.

And this was done, of course, 2400 years before the flat Earthers of our times. Both of these people were mathematicians. Archimedes, he actually independently invented calculus. Newton was also a theoretician, and he also did experiment, just like Archimedes and Ratostenis. He's considered the greatest theorist ever. And in fact, he was a very dedicated experimentalist.

In fact, he was so dedicated that he wasn't just an experimentalist. He would act as experimental equipment, his own self. So he's famous for doing experiments in optics, sticking a needle under his eye and moving it around to figure out the laws of optics and also the physiology of the eye and things like that. Now, all of these experiments that I showed are small-scale experiments. They were in a room or in a small amount of space. The transition to the big science era started with a Manhattan project in 1945. That's when the public realized that science is important

and a lot of funding started coming into science. As a result, we started building bigger and bigger projects. So what used to be tabletop experiments, this is what's called an accessibility. or sometimes it's called a collider, collides elementary particles. So accelerates started out being pound-sized object, then tabletop, and now the biggest accelerator in the world, which is the Large Hadron Collider in Geneva, Switzerland, is 27 kilometers around.

The Large Hadron Collider completed what is called the standard model of particle physics. In other words, we needed bigger and bigger accelerators, accelerators, and we studied phenomena involving more and more sub-nuclear constituents, elementary particles. So this gave rise to the standard model. You have roughly 20 particles and roughly 20 parameters in the standard model. So in terms of 20 particles and 20 parameters, you explain, as far as we know, almost everything that we observe in the universe.

It then explains things all the way from subatomic distances, all the way to galaxies and the universe as a whole. So the standard model, as far as we know, applies in almost all phenomena, with one exception that I'll come to, which is called the dark matter. It's an incredibly successful theory in many ways. It has been tested with literally hundreds of measurements. and the most impressive success of the standard model is in predicting a quantity which is called the geomagnetic ratio of the electron.

Essentially, it measures how rapidly an electron processes in a magnetic field. First of all, the theoretical calculation is very involved, and you can carry out the theoretical calculation to the point where you can make a prediction about the magnitude of this quantity to one part per trillion. extreme precision. This means 12 decimal places precision. And this is an astonishing thing.

To be able to have a theory that comes from your mind, agree with experiment where you go out and find out what nature is actually doing to 12 decimal places, and it underlines how important the standard model is and how fortunate we are to live in a time where it has been confirmed. The range of validity of the standard model and the precision with which it has been tested is incredible.

So if the standard model explains so much, so brilliantly, why aren't physicists satisfied with it? A few reasons. One is that we know there is some extra stuff out there. In particular, there is something called dark matter. Ordinary matter, what we are made out of, is only 5% of the energy of the universe. There is dark matter, which is five times more than ordinary matter. Dark matter doesn't emit light, doesn't radiate light, so we cannot see it, but we know it's there

because we can measure its gravitational influence on stellar objects. Second reason why we want to go beyond the standard model is 20 parameters and 20 particles is very impressive, considering the range of phenomena that you describe. But it's still too much. If it's really fundamental, you would think that maybe a truly fundamental theory should have one parameter and maybe fewer particles. So it sounds like we can economize more and have a more elegant simpler theory. then there is two outstanding problems, which I'll come back to a couple of times. One is called the cosmological constant problem, which essentially is the question of how come

the universe is so big. Turns out, there is a quantity called the dark energy, which we know is the dominant portion of the energy of the universe. This dark energy, which is essentially the energy density of the universe, In the standard model, its natural value would be so large that the universe would only grow to be about a millimeter. So in the standard model, the natural expectation for the value of dark energies such that the universe wouldn't grow beyond a millimeter. To make the universe much bigger, you have to tweak the theory, fine-tune some parameter of the theory to an incredible precision.

a hundred decimals or so precision, and then you can have a large enough universe. So that's called the cosmological constant problem. It is the quantitatively the most important problem that we face today. And then there is the hierarchy problem. The hierarchy problem is simply the question, why is gravity so weak

compared to the other forces of nature, like electricity? And here I can do an experiment to demonstrate the weakness of gravity. When I lift my arm, the atomic forces on my shoulder are able to overcome the gravitational attraction of the entire planet Earth.

Atomic forces, which are in essence are electrical forces. So the electrical forces of my shoulder overcome the gravity of the entire Earth. That only happens because gravity is intrinsically much, much, much weaker than electricity. This problem is probably the second most important problem in physics. It's easier than the first, so it has had several approaches that are being tested in the lab.

Okay, so now we go back to the drawing board and we want to build a theory beyond the standard model. What principles do we follow to build a theory? We want to follow the same principles that led us to the standard model. And the first principle, the most fundamental, is the principle of minimalism. Minimalism goes back to a priest called William of Occam, who stated essentially the principle of minimality that we shouldn't add more components to our theory than are necessary. So this is called Occam's Razor, and it underlies much of the physics we've been doing over the last centuries,

since the 13th century one Occam pointed this principle. In modern physics, the way we implement Occam's Razor is mostly through something called unification. Unification is the idea that two very different phenomena are explainable by one and the same principle. For example, Newton in the 17th century was able to unify celestial and terrestrial phenomena. Now, before Newton, most people thought that terrestrial and celestial phenomena are on totally different grounds. Terrestrial phenomena is something that perhaps the human brain can comprehend. Celestial phenomena, they need, you know, gods pulling the planets or angels pushing on, etc.

So they thought that it was a different domain that the two were unrelated. It was Newton, who, in his universal theory of gravitation, showed that the way that an apple falls and the way planets move around the sun are described by exactly the same equations. This was the beginning of modern science, because people realized that we, tiny little people, on a small planet, can think, can do experiments with apples and discover truths that are valid everywhere in the universe. Distant galaxies, at any time in the universe, in the very early

universe, upon its creation, we find universal truth that are valid everywhere and at all times just by doing experiments. This was very liberating. It showed that, yes, we can think about celestial phenomena, and he started the explosion of modern science. And Newton's recipe of unification was repeated at least twice. First, Maxwell in the 19th century showed that electricity, magnetism, and light are all related. They're described by one set of beautiful equations, Maxwell's equations. and then in the 70s, people figured out a way to unifies the electromagnetism of Maxwell

with the weak force. Again, weak force is something that causes radioactivity. So there was the unification of electromagnetism and the weak force into the so-called electro-week force, which was completed theoretically in the 60s and fully discovered and confirmed by several colliders, including the LHC. Now that we would like to repeat this success to the other forces of nature, in addition to the electro-weak force,

we have the nuclear force, the so-called strong force that keeps an atomic nucleus together, and then the force of gravity. So we would like to unify all of them together into a grand unified theory, and this proves extremely difficult. There are ideas on how to approach it, but it's very challenging for the reason that I told you before when I was raising my arm and pointing out how much weaker gravity is than electricity.

So for example, if you compare the electrical force and the gravitational force between two elementary particles, let's say an electron and a proton, you find out that the force of gravity is 40 orders of magnitude smaller than the force of electromagneties. This is an astonishing number. So this means electricity is one with 40 zeros times stronger than gravity. So then if two things are so different numerically, how can you stand the chance to unify them into one entity? So that's a huge problem.

There are three types of approaches. and they have nice names like supersymmetry and compositeness. I will talk about only one of them, the third one, and this is called the Theory of Large Extra Dimensions. The basic hypothesis is the following. So this is a two-dimensional piece of paper. Imagine that our universe is constrained to this two-dimensional paper represents our three-dimensional universe.

So think of the surface of this piece of paper representing the whole universe. And imagine that in addition to the three dimensions of the universe, there are additional dimensions that we have not yet seen, we have not yet observed, that are perpendicular, they are represented by the height. So the vertical direction is a new dimension, and the horizontal directions on the... the surface of the paper are the familiar three dimensions. Furthermore, imagine that all the things we are familiar with,

namely electrons, light, atoms, etc., reside strictly in the three-dimensional space, just like we normally imagine, except for gravity. Imagine that gravity that is not constrained to three dimensions, but it spreads inside the extra-dimensional space. If that is the case, then the strength of gravity gets diluted by spreading in the space of extra dimensions, and therefore it becomes smaller. So gravity becomes a weak because it propagates in extra dimensions. And the best analogy of this is the flow of a river, just like the flow of a river weakens

when the river splits into tributaries, when it spreads in extra space. Similarly, because gravity spreads in extra dimensions, it becomes weaker than electricity, even if to start with electricity and gravity had equal strengths, because gravity spreads inside more dimensions. And of course, this idea has a very nice mathematical representation, extradimensional theories and as well as string theory. And this is one of the exciting things that people have focused on. Perhaps another analogy may help. So imagine that you have a billiard table, a pool table.

So think of the surface of the pool table as corresponding to our three-dimensional space. The billiard balls, they can really be anything that lives in three-dimensional, but let's think of them as elementary particles, let's say protons. Now, we know from experience when the billiard balls collide, although the billiard balls move in the two-dimensional surface of the billiard table, they produce sound, which also propagates in the third dimension perpendicular to the billiard table. And that sound corresponds to gravity, gravity that spreads in the extra dimensions. This also suggests a possible signature of such a framework.

When billiard balls collide, they produce sound waves. this means some of the energy of the billiard balls gets dissipated, goes into the sound waves. So if you measure the energy of the billiard balls before collision and after collision, you'll find out that some energy was lost to the sound wave. And that could be a signature that there is an extra dimension. And similarly, you can imagine doing this in colliders, or if you collide two protons,

again, they can produce gravity that spreads inside the extra dimension, and that results in what's called the missing energy signature. So where do we stand now? We stand with one way to explain the weakness of gravity is to introduce extra dimensions. Now, again, imagine that we have extra dimensions, so we can have one universe there. But extra dimensions has a lot of space. If you admit the idea of extra dimensions,

then there is no reason why there should be only one universe there. You can fit another one. You can have two parallel universes. Or you said it. Ah, you're a theorist. Or three or more. So this can all coexist in the space of extra dimensions. What does this mean?

This means that each parallel universe can have different laws of nature. So having extra dimensions, automatically implies the possibility that you can have parallel universes. And in fact, it's unreasonable to think that we are the only thing, if there is extra dimension, or it's only us and it's a very self-centered view, which has been proven wrong over and over again in our universe. Every time we thought we were unique, we proved not to be unique. Each one of these universes can have different laws.

This program is brought to you in part by Specsavers. Every day, your eyes go through. a lot. Squinting at screens, driving into the bright sun, reading in dim light, even late night drives. That's why regular eye exams are so important. At Specsavers, every standard eye exam includes an advanced OCT 3D eye scan, technology that helps independent optometrists detect eye and health conditions at their earliest stages. Take care of your eyes. Book your eye exam at Specsavers today from just $99, including an OCT scan. Book at Spexsavers.ca.caps are provided by independent optometrists. Prices may vary by location. Visit

It's backsavers.ca to learn more. Hi, I'm Sarah Nicole Landry, and I'm the host of the papaya podcast where each week I ask curious questions to people with incredible stories or expertise in their fields. I'm somebody who has found so much inspiration in storytelling and learning from them, and I wanted to bring that to a podcast where each week we walk away learning something that might just change our lives for the better. Check us out every Monday on the papaya podcast. See you there.

This universe may not have a photon. maybe this has nothing, et cetera, et cetera. So they can be very different than ours. Once again, science strains the imagination even more than science fiction. That's Savas Demopolis, speaking at the Perimeter Institute for Theoretical Physics. This is Ideas. I'm Nala Ayyad.

The achievements of four. physics over the past century are truly mind-boggling, cracking open truths about the universe and our very being that could scarcely have been imagined before. But the science seems to have hit a wall where some unresolved mysteries remain, like dark matter, dark energy, the weakness of gravity and the elusive theory of everything. Sarah Chondera is another physicist who thinks new approaches to the big unanswered questions might be needed to get cosmology unstuck. She's a professor and director of the Institute for Gravitation and the Cosmos at Pennsylvania State University. We're going to hear excerpts from her public lecture called Dispatches from the Hidden

Universe, which was also delivered as part of the Perimeter Institute's 25th anniversary. As Professor Chondera tells it, physics is a story. as old as humanity's ability to wonder, with lots of unexpected plot twists ahead. The story that I want to tell you today is the continuation of one of the oldest human scientific stories that we have. It's the story of looking up and looking out and asking, you know, how to make sense of it all. Or looking up and looking out and saying, man, there's got to be so much more out there. And as we've figured out how to make better, more precise eyes on the universe, we've taken this passion of Stargate.

and turned it into a discipline of cosmology. So the story I want to tell you tonight is sort of where we're at in cosmology, what we know, where we think we're going. So what I'd like to convince you by the end of the talk is that we are not only in an era where we have extraordinary amounts of data coming in at an extraordinary rate,

but we're also in an era where the modeling, the sharp mass tools that we use to make sense of the universe could maybe use an upgrade and maybe a little bit different perspective. So it's a really exciting time because it's full of possibility and full of data. to check what we're doing. Of course, the instruments that we use to look beyond,

beyond just the camera you can hold, are also extraordinary. And so if you were to go out in this night sky and hold up, say, a pencil, and look at just where the tip of the pencil is, maybe on some part of the sky that looks dark, and you were to put the very best eyes that we have, technologically, on that patch of the sky,

what you would find is that, oh, yeah, there's more. This is an image released earlier this year by the James Webb Space Telescope. And what it sees in that tiny, tiny patch of the sky is all of this, which includes, of course, entire galaxies. So the amount of stuff that's out there is just mind-boggling. So when we make these observations and the instruments get better, they're really doing two things, right?

They're collecting more light, so we see things that were faint farther away. They're also looking back in time. And because the universe expands, that means that light's stretched, and so the instruments have to be able to look at those stretched wavelengths of light to reconstruct what was happening. Those are some of the very, very, very first things that were ever formed in the universe, stars and galaxies. And it really is, you know, looking back in time to a point when the universe was very young. But there are many, many events in the nearby universe that happen all the time,

that are energetic phenomena that happen in a flash, you know, things are changing, stars are changing as we look out. And the instrument that's designed to make a picture of all of those transient events, those things that happen on short time scales, is this Vera Rubin Observatory and this legacy survey of space in time. time. So Vera Rubin will make those extraordinarily sharp images every single night. The science is the entire sky every night at a level of detail that is 15 terabytes of data, right? That is amazing. And in that data, astronomers will find millions of things that have changed, right, from night to night, and send millions of alerts out to astronomers and say, hey, go look at that, go look at that, go look at that, go look at that, go look at that. And there's a whole industry of

people, you know, trying to fill, decide which of these million events do I want to go look at, which do I want to look at, which I want to look at. So this will be truly, truly phenomenal in terms of understanding the active universe today. That was light. All of that had to do with light. Of course, we don't just see the universe in light, and I say, of course, but that's actually really new, right?

We have several fundamental forces in nature. Light is one of them. Another one is gravity. And one of the most extraordinary things that's happened over the past decade is that we can actually see, or here, maybe, the universe in gravitational waves. So the LIGO interferometers can detect,

how much space bends by the passage, or it stretches, by the passage of gravitational waves that are released when really heavy objects like black holes spiral together and merge into a single black hole. When the first sort of large-scale detectors of this type were built, they could see pretty far. This is 100 million light years. So a distance light can travel in 100 million years.

The detectors could see about this far. That's really far. On the other hand, it's nothing compared to the size of the space that we've explored, right, were seen, visualized. Those were impressive detectors. They weren't impressive enough to find gravitational waves. That had to wait until almost exactly a decade ago, September 14th, 2015.

And that was the first time that we were able to see gravitational waves from the merger of two black holes, so really see the universe in a force other than light. Ten years later, again, just a few weeks ago, the new release of all of the events that LIGO has seen, a black hole is merging, right, or neutron star is merging, something like 218 events were released. And again, that's something that will continue to expand

as our ability to measure these gravitational waves increases. So this is an extraordinary moment in terms of data. And what we've known for some time, I want to summarize in four facts about the universe. The first one seems maybe in today's world, of course we know that, but it actually wasn't that long ago that we didn't know it, that the universe is not in a steady state. The universe is evolving, it's changing.

The second fact is, again, quite obvious and yet still extraordinary, that the universe is nearly smooth, but not quite. Especially in the very early times before gravity had a chance to act, it was nearly smooth, but not quite. That not quite is where all the fun is, right? Had the universe been perfectly, perfectly smooth, none of this amazing stuff that we're seeing with JDBST, with gravitational waves, none of that stuff would have existed.

We would have had a pretty boring universe. The third fact, which again we've known for a couple decades, is that the current expansion rate of the universe is accelerating. So something's going on that's sort of driving space apart. We've labeled the stuff that does that, dark energy. And if you ask how important is dark energy today in the expansion rate of the universe, it's incredibly important. It's like 70% of this pie.

We call it dark because, well, it doesn't emit light. We're not exactly sure what it is. The fourth fact, not only do we have this dark energy, which makes the universe expand at a faster rate, we've got this stuff called dark matter. So if you ask, of all the stuff that acts like matter out there, it's not this dark energy stuff that gives you accelerated expansion, it's also not some radiation, it really acts like matter.

How much of that is stuff that we understand, the stuff that we're made of here in this room? And the answer is not much. Most of it is this other stuff, dark matter. Dark matter, why is it dark? Because it doesn't emit light. So we don't see it. So these are four facts about the universe.

They're facts that we've known for decades, all of them. And if you turn those into scientific questions, you know, what's going on in the universe? Those four facts give us four really enduring puzzles. For decades, we've asked, you know, why is it not a steady state? That why for scientists, you know, how did it get started? What's the initial state? Is that an initial state that makes sense that I feel is natural? And where is it headed?

What is the fate of the universe? The everywhere is smooth question, again, is why? Where did that bumpiness come from, right, that ends up giving me galaxies today? Expansion rate is more the same. You know, why is it there at all? Why is it this much? And the same thing with matter. Well, what is it?

It's something new? Fine, we know it's there, but what is it? And all of these questions are ways in which, of course, we infer things that we can't see directly. That matter emits no light. Dark energy emits no light. We infer stuff that we can't see directly, of course, by what we do see. Physicists have discovered a lot through inference and managed to boil down a very complicated cosmos into a handful of particles and fundamental forces. By all accounts, a big success.

And based on that success, we have a lot of really good ideas about how to go out and answer these questions. And that's what we've been doing, and doing, and doing, and we still have these questions, which is a pretty interesting situation to be in. So what we've been working on for decades, decades, is to say, well, there's probably some particles that explain each of those. We can attack those sort of piecemeal. I mean, ideally, we'd have a framework where, you know, the particle that do one, there's some way that it makes sense and fits into the same framework as the particles they do all of the others.

The fact that this works at all, this standard approach, is actually a really remarkable fact about nature, that it's possible to find really simple building blocks with pretty simple interactions and put them together to make a complex system. This has been the story of the success of physics, right? For example, it turns out we're really lucky that you can just go on a tall place and drop a few things, and the only thing that matters is the giant Earth and those balls that you drop from the tower

to figure out the basics of gravity. And, of course, we love to smash stuff together. That's how we learn a lot. Our smashing has gotten pretty sophisticated. We don't just smash billiard balls together in the introductory physics course, but we actually figured out how to smash fundamental particles together. So this is a picture from the Large Hadron Collider,

where the billiard balls are protons that you put together in big bunches and speed up and send them violently circling around so that they crash into each other. And because you've worked very, very, very hard to make the system of protons simple and to control them very precisely as you send them around this ring,

you think that you can figure out from the output if there's anything there that was unexpected. You understood so well the initial state in how you put them together that if anything unexpected, it happens, that can be a signature of something new. The reason all of this works is that it's both simple and in some ways a reductionist. You can take some

specific components with different identities. So this is a billiard ball and this is a billiard ball, or this is a proton and this is proton. And I know very well what these things are when I have them in my two hands. And then I can smash them together and see what comes out. So physicists have been so successful with this sort of reductionist approach that we try this everywhere. And it's become really easy to make fun of us for it because here we are, right? It's true. This is amazing that we can do physics this way. It's amazing that we can find out what we have. But a lot of nettlesome issues about the universe are still beyond our understanding. One of them that we've been grappling

with for forever, right, is that we can only observe this non-steady state system from right here and right now. And in a universe that's changing as much as this one is, that's a pretty interesting constraint. Another constraint is that we cannot see the whole thing. We can't see the beginning. You know, the light that reaches us is only from so far back. And beyond that, we got nothing, right? We've got to do inference. We don't know what really happened. I like to call this the blinding flash of ignorance. We don't know. And we can't see beyond the edges, right? There's a finite distance out to which we can see, which has to do with how far light can travel and what the expansion rate of the universes. There are lots of parts of the universe that we can't even

actually see. I mean, to be human is to be a partial observer, and most scientists who aren't particle physicists are also partial observers. In physics, we do have formal techniques for dealing with systems like this. We usually call them open systems, meaning, you know, that I can't describe everything right here and wrap it up in a nice, neat ball. There's stuff I can't quite observe, so I have to talk about what happens with the environment. What's great about this is that, you know, I told you in cosmology, we're sort of stuck. We absorb here and now. We can't go out and make the simple systems that we're then going to use to test our theories.

That's a problem. What's nice about this is that understanding quantum systems, where there are correlations, and say I only observe this part of the system and I want to talk about its evolution, that does happen in the lab, absolutely. And that's something that we can go and manipulate and make experiments on and test whether our models are making sense. And in fact, this is a really active area of research, because, of course, quantum systems

incredibly important for lots of reasons, including computing. So figuring out how to isolate systems and what happens when the systems talk to other quantum systems is a big deal in general. But that means there's data, and that means there are experiments. So I want to compare then these sort of two stories that we have. One is the story that I've worked on most of my career

that everybody that's been doing cosmology for the past four decades has been doing, which comes out of the success of particle physics and these really good ideas that we've had by thinking about what are the fundamental particles in the universe. But that story is a story that's very much a closed system story. We usually assume we've got it all in hand, and we know or we can tell the story about what those components are, and then see if it works out,

see if we can get something that describes the universe. On the other hand, we have a different view, which is both open, meaning I have to ask which parts of the hole can I see? That's the first question I have to ask, not let's assume I have these parts and I know what they are. But I also take a different approach to what the complex behavior is. This is actually something that has a key, like a buzzwordy phrase in physics, which is called Moore is different.

And this has been a really important theme of much of physics, you know, over the past several decades, but it hasn't quite percolated into aspects of cosmology and particle physics as much as it might, partly because it wasn't clear what the connections were. It's possible that we're starting to have a connection, and really what we need to ask in the context of cosmology is this complex behavior that we see, when does that tell me something about the structure of the parts? So I should take as fundamental or as important key knowledge the fact that the universe gives me this diversity of structures at late times. That is a feature of the universe I've got. That's something I really need to explain. And I'm particularly

interested in seeing what happens when we address cosmological questions from this point of view and see if that gives us any new insight on those very difficult and long-standing problems. The universe is not in a steady state. It's nearly smooth everywhere, but not quite. The expansion is accelerating, and most of the matter is something new. We've known these things for a very long time. What I wonder is if figuring out the answer is partly comes from rewriting our math framework for this, that really embraces the fact that part of the universe is hidden. And I think that's especially true with these questions here. And so my bet is that when we understand more about the answer to these questions,

we will have created a theoretical framework for cosmology that at its, really, its foundations, finds a very natural home for these open quantum systems. And at the same time, it gives us these ensembles of systems that can have a diversity of behavior in the subsystems, and that's how we extract information about the whole. So that's what I hope is true. What I love about cosmology, though,

is that the data will tell me if I'm wrong, right? It will tell us how to get on the right track. So I look forward to the universe telling me I'm wrong for the rest of my life. Sarah Chondera, a physicist at Pennsylvania State University, speaking at the Perimeter Institute. Savas Demopoulos, whom you heard earlier in the episode, also accepts that being wrong is part of the game in physics.

But he has high hopes for a mind-bending theory, that there are extra dimensions curled up so small that we can't perceive them, but that gravity can work in. As you'll hear in this part of Savas Demopoulos' talk, these extra dimensions would fundamentally alter our understanding of the universe, or should I say, the multiverse. So it seems like the idea of extra dimensions is creating a tension between the principle of minimalism and the principle of planethood.

Minimalists would suggest one universe, plenitude, many universes, and in fact, a famous mathematician Leibnich suggested that the perfect world contains all possibilities. Now, it's not the first time we face this until around 1600. Most people, definitely not all, but most people thought that there was only one solar system. And it was very special, that was the whole universe, it was made just for us, we're at the center, etc. Now we know that there is in fact 10 to the 20, one with 20 zeros, solar systems in the universe. Of course, this is a very natural idea now, but it was somewhat controversial in 1600. And in fact, the person, again, another priest called Giordano Bruno, who wrote a book about it,

claiming that the universe is infinite and it has innumerable worlds in it, That was considered blasphemous because we were no longer special, we were not, you know, God's children, or the only children, blah, blah. Now, having many solar systems has tremendous consequences in two things. First, it changes the way we think about our solar system, because there is nothing special about our solar system. The other thing is it points to something called environmental selection. I want to dwell on this. It has to do with that if you have one solar system, there are some mysteries. For example, let's say ours is the only solar system. There are obvious questions like, how come the Earth's sun distance is just right

so that it's not too hot and it's not too cold, so it's friendly for our existence? Or, much more complicated, how come the chemical composition of our planet, et cetera, et cetera, is just right to allow for our existence. It looks like either someone cared a lot and put us together very carefully, put everything together to make sure that we exist, divine intervention, or there is a second possibility,

environmental selection. That is the point of view. If you have many solar systems, then environmental selection can account for all of these accidents, whether it's the chemical composition of the Earth or the distance between a planet and the star. etc. So it allows you for a new way to reason, which is not quite the usual physics equations, but still very powerful. Now we have a very big problem, the cosmological constant problem,

which as I said, says that the universe shouldn't be more than a millimeter given the value of the dark energy that we expect in the standard model. However, if you have many universes, the solution again becomes environmental selection. Most universes would be too small for us, but if you have enough universes, they have enough different dark energy content, then some of them, like this one, will grow to become as big as ours.

So this is the only known solution to the cosmological constant problem. It relies on having a multiverse, but the moment you accept the idea of extra dimensions, accepting the multiverse is one small step, since the space is already there. So this essentially demonstrates the following important connection. In string theory, you get multiverses because the extra dimensions have to be curled up in a small space. And there is practically an infinite number of ways to curl up extra dimensions in a small space.

And each one of these corresponds to a different universe with different properties. And the existence of this complex extradimensional space with all the holes, etc., implies the existence of extra particles, which have fancy names like axions, dilatons, moduli, etc., that characterize properties of the extradimensional geometry, and these are potential signatures. Seeing some of these things could be signatures for the existence of these extra dimensions.

So looking for such particles could be the analog of Galileo's telescope, which allowed us to look and eventually to discover that there are other worlds out there, there are other solar systems, other galaxies, et cetera. And these particles can be looked for in many ways. These particles can be the dark matter. They may behave like waves or are like particles. But one thing for sure, the properties are such that most of them cannot be seen in colliders. You need to go to small-scale experiments.

Lots of such small-scale experiments have been done, are being done and are being proposed. They are called, as time evolves different things. Small-scale science, table table of experiments, precision frontier. Lately, quantum sensor is the fact. We started out, pointing out that we transitioned. The energy of colliders, over the years there has been an exponential growing energy. lately that exponential has leveled off. It has leveled off because of technological reasons and there is practical reasons, cost. And this leveling off is expected to continue for at least

three decades. So the high energy frontier, going to higher and higher energy, which is the main strategy we've been using to look for new physics, is being saturated. If you compare this what happens in the high precision, for example, frontier? There are several such plots. Optical clocks had the stability of one part in 10 to the 13, and now it has gone to one part in 10 to the 18. What does one part in 10 to the 18 means? It means that this clock in the whole age of the universe loses a second, one second in 10 to the 18 seconds. Okay, so if you have a date, you set a date at the beginning of the universe, you adjust your clocks together, say, we'll meet in 10 billion years, you meet in 10 billion years with an accuracy of one second. It's incredible and it's in fact

by now it has gotten better. This means that you can do exceedingly precise measurements if two quantities are off in the 18th decimal, they'll behave differently. That's how, for example, you can look for oscillating dark matter, two clocks will be a little off. If they're off by one part, 10 to the 18, because they are affected differently by oscillating dark matter, you will see it. So the high precision frontier is anything but saturated. It is still progressing exponentially. And in the next few years, we've been cultivating exciting interfaces between new physics and these fields are experiencing exponential growth. And as I said, these fields involve much smaller labs, money, resources in general.

Essentially, they've been building for several decades while the colliders were going on. They were doing really miraculous work. So new tools fit the open questions, the open questions being, what's the dark matter? there any evidence for the multiverse, these particles, axions, dilat on modula, et cetera, for example. So let us end by contemplating. What if we discover the multiverse? Find evidence by finding several of these particles that the multiverse predicts. We started several centuries ago. We believed

that we were at the center of the universe, the earth. We were very special. We were right at the center and everything else revolved around us. Then we found out that, well, that's not quite true. Actually, the sun is at the center. We are sort of the third planet over. Well, it's not so bad. It's still one solar system. We're still sort of special.

Then we found galaxies. We say, wait a minute. Our galaxy is sort of less important. Then we found out that there is 10 billion galaxies. And ours is not at the center, because there is no center in the galaxy. It's very democratic. Now we are contemplating the possibility that our whole universe may be less important than a speck of dust in a vast multiverse.

No matter how insignificant that speck of dust may be, it holds some very impressive brains. Like the one you just heard from, that of Savas Demopoulos, an award-winning physicist at Stanford University. And before him, Sarah Chondera, a physicist at Pennsylvania State University. Special thanks to the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, where both those public lectures were given. This episode was produced by Chris Wadskow. Our website is cbc.cai slash Ideas, and you can find us on the CBC News app and wherever you get your podcasts.

Technical production, Emily Kiervezio and Sam McNulty. Our web producer is Lisa Ayuso. Senior producer Nicola Luxchich. Greg Kelly is the executive producer of Ideas, and I'm Nala Ayyed. For more CBC podcasts, go to CBC.com slash podcasts.