The Joy of Why - Will We Ever Prove String Theory?
Episode Date: May 29, 2025For decades, string theory has been hailed as the leading candidate for the theory of everything in our universe. Yet despite its mathematical elegance, the theory still lacks empirical evide...nce.One of its most intriguing, yet vexing, implications is that if all matter and forces are composed of vibrations of tiny strands of energy, then this allows for a vast landscape of possible universes with different physical properties, varieties of particles and complex spacetimes. How, then, can we possibly pinpoint our own universe within a field of almost infinite possibilities?Since 2005, Cumrun Vafa at MIT has been working to weed out this crowded landscape by identifying which hypothetical universes lie in a ‘swampland’ with properties inconsistent with the world we observe. In this episode of The Joy of Why, Vafa talks to co-host Janna Levin about the current state of string theory, why there are no more than 11 dimensions, how his swampland concept got an unexpected lift from the BICEP array, and how close we may be to testable predictions.
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I'm Jana Levin.
And I'm Steve Strogatz.
And this is The Joy of Why, a podcast from Quantum Magazine,
exploring some of the biggest unanswered questions in math and science today. Hey Steve. Hi Dana. Different day, different studio. Yes I'm
going for the Orson Welles look here. This is really dramatic. I actually like it. It's looking good.
I want to talk to you today about something that's gotten quite controversial
and so I really want to ask your honest take on string theory.
How do you really feel about string theory?
I don't really have an informed opinion.
That has never stopped anybody.
What I'm told is it's helping a lot in pure math, that the techniques from string
theory are being imported into fields like algebraic geometry.
As for what it's doing for physics, I get the feeling it's the best game in town, but it's hard to tell yet whether
it's going to really describe the physics of our universe.
Yeah, I think it's been a fascinating legacy. So the excitement initially was that string
theory might do for quantum gravity. What all of these famous
theories like the Electroweak theory and the theories about the matter forces did for matter,
which is to say it unified them all into one sort of elegant equation. And so the hope
was that there would be one string theory, right? And instead, what happened is there ended up being this vast landscape of possible string theories,
like 10 to the 500 possible vacu hours, starting points.
That became at first a tremendous disappointment, and then people sort of ran with it and started
talking about the multiverse.
Then that led to a very strong, I think, reaction against string theory.
But that hasn't rocked your world, that sort of reaction against it.
My personal world?
Yeah.
I don't know what to make of it.
I mean, I do hear people criticizing it very much as being untestable in principle.
A lot of people invoke Karl Popper and his whatever era that was,
1940s or 50s era philosophy
of science.
There's a part of me that wants to believe that this could work because it seems so compelling
to be able to get rid of the problems of renormalization that have afflicted a lot of attempts to quantize
gravity.
And that was the initial selling point, wasn't it, that string theory could handle that in
a way that other theories had trouble with?
I would say that the big success was that gravitons, which
are the little quanta of gravity waves,
the way that photons are the quanta of electromagnetic waves,
were natural, a natural ringing of a string in the same way
that the other particles were harmonics on a string.
And so that was really the enthusiasm.
Whether it was not fully quantized is really a different
story.
Well, you know, I brought some of these concerns up to our guest who is a very influential
string theorist, Kumran Vafa.
He's a professor at Harvard.
And it was interesting because he didn't dismiss the concerns.
And one of the ideas he had was to invoke a concept he's called the Swampland.
So it looked like there were all of these dream theories and it was totally proliferating
and we couldn't find our universe in this muck.
And so he had a program instead to kind of call theories that he could prove failed.
First of all, I love the term, Swampland, very suggestive.
But what are some of the admissibility criteria?
What sends you to the swamp?
Yeah, it's interesting.
It's such a complicated program, but it could be that it won't lead to the kind of universe
we observe today.
Or the matter fields don't drop out to match what we expect from the standard model of
particle physics, which is so successful.
General relativity, it can't look like a different theory of gravity.
So we have these key anchors.
We want any theory that goes beyond these to be able to replicate the successes in the right regime.
And so I think it's just kind of a way of whacking through the brush
and clearing away what you know is weed, right, from what you're hoping to
find. I guess which would, in this analogy, be like, I don't know, the roses.
This sounds great.
So that's sort of my synopsis of the scenario, but maybe we should hear from Kumran himself.
Here's theoretical physicist Kumran Vafa.
Kumran, welcome to The Joy of Why. We're excited to have you
on the show.
Thanks. It's a great pleasure to be on this show.
I can see some equations on the blackboard behind you. I want to open for our audience
just to ask, how does a person get into theoretical physics? What's drawn you to that subject?
My interest in theoretical physics started when I was a kid. I vividly remember my awe and amazement the fact that moon does not fall down as something needing some explanation.
And it's bothered me that most people were not bothered by this fact that there's this
thing up there and it might hit them on their head or something but it was kind of yeah it's cool it's beautiful what's the issue here why don't you
ask this question why is it not falling down i mean isn't that the obvious question so these kind of
things were always hitting me in some form or another but it took a while for me to zoom into
theoretical physics as a profession it really is one of the huge insights of Newton
that the moon doesn't fall to the earth.
Why that's the case and connecting that
to the kinds of physics we understand on the earth.
I don't know if people appreciate
what an enormous leap it was to imagine
that the moon was dictated by the same forces
that plucked an apple from a tree.
Exactly. And when I later learned Newton's understanding of how this works, and the fact
that why does the moon fall ends up being, no, actually the moon is falling. And how
does that compatible with the fact that the moon is not hitting the ground? That to me
was quite an awe moment also. That's the way a physicist understands how things work.
And then of course Einstein, hundreds of years later, talks a lot about falling.
He thinks about free fall in an even deeper way than Newton to some extent.
Yes, in some sense the fact that being in an elevator and falling, you wouldn't know
if you're in outer space or falling and the fact that there's no experiment that can detect
the difference was quite an insight.
He has said that this is one of the happiest scientific thoughts he had.
I love that quote, happiest thought of my life.
And the thought is really about gravity being a form of weightlessness.
I feel heavy on my chair right now.
I feel heavy when I'm walking around on my feet.
But really Einstein began to think of gravity as, oh no, it's actually removing all that stuff
and experiencing this free-falling weightlessness. How does that relate to his theory? The fact that
the objects move the way they do regardless of their mass, so to speak, and that point that
the thing does not care about the trajectory you take in space-time. The fact that geometry of space-time
dictates this to all of the objects around it. That was a beautiful geometrization of gravitational force. So you're a student of theoretical
physics at some stage learning Einstein's geometric theory of gravity
and freefall. What draws you now the extra step to something as abstract and
complex as string theory? The interest in string theory is related to the interest
in trying to understand the fundamental laws of nature,
how things work at the deepest level.
What is everything made of?
How are the forces interacting with each other?
What are the forces?
What are the fundamental things?
And in particular, it had been tried for many decades,
but unsuccessfully to understand Einstein's theory
at the microscopic level.
And that somehow didn't work in the context
of mixing it with quantum mechanics,
which is the law that governs the microscopic physics.
So when you try to bring in quantum descriptions, which is needed
for small scale descriptions, with Einstein's theory, which is great for large distances,
the two didn't quite fit together and somehow gave contradictory results. And trying to resolve
these paradoxes was one of the major issues that was difficult for a theoretical
physicist for many decades and basically people didn't work on it because they
have no good idea and then suddenly out of the blue somebody suggests the string
theory sounds interesting just let's study it and someone said oh it sounds
like it has gravity in it and oh yeah it actually has quantum mechanics and
gravity together in a consistent way so it was accidentally discovered to be an
answer to the question that people were looking for.
So it's a kind of a step towards a quantum gravity theory of everything. Just a small thing like that.
Exactly. Just a random discovery, I would say. Whether we deserved it or not, I don't know.
But in some sense, it just happened that what people were studying ended up being an answer to quantum gravity.
Exploration is often underrated, don't you think?
Exactly.
In the physical process, as though there's really a hypothesis set forth before you take a step.
That's really not how it's always done.
They always say just do the exploration, have a good question,
and you might get an answer to something else.
And this happens quite often in the history of science,
and I think string theory is a great example of that discovery.
Now, before string theory,
we might have imagined that when we looked at more probing microscopic levels,
we'd find little tiny point particles.
And some of those point particles might even be gravitons,
which are responsible for negotiating gravity between masses.
What's different in the string theory description?
Those little points are replaced by little loops of string? Is it that simple? Yes, to some
extent it's that simple. So the loops that we call strings, of course, could be
so small that they look like points and therefore there's not much of a
difference when you look at it from far away. But when you zoom in and when you
go to very high energies, this thing can stretch and then it gives you features like an extended object like strings.
We actually have learned in studying string theory that it's not just a one dimensional thing like a string, sometimes higher dimensional objects like membranes enter the story as well.
And so we have learned this more than just about strings, but we still call this subject string theory. But we have learned that inevitably demanding the fundamental particles,
everything is just point-like particles, is insufficient to give you a
consistent picture of how gravity works at the quantum or microscopic level.
I've sometimes heard people refer to M theory, and then I hear various things
about the M stands for. is the M for membrane? Well, I don't know what M stands for, but I think the main statement here is
that we don't really understand what the theory is, to be honest, in its form. We know how
it works in examples. And so sometimes people say M might sound for mystery. So we don't
exactly know what is M stands for really, but we're still working on understanding what this theory really is about.
It's one of those exciting human adventures that I think is still continuing after many decades.
So are people imagining that the entire spectrum of particles that we used to think were fundamental,
like an electron or a quark or a graviton are just different harmonics played on the
same kind of fundamental string? That could very well be. In other words, the
idea that strings in different excitations and different harmonics and
different configurations lead to different particles is certainly one of
the amazingly beautiful predictions of string theory. It unifies all the object
into one thing, though it could be like a membrane as well. So we don't have an exact description like that, but we think, generally speaking, the fact
that particles can unify and be manifested by one object in some form is certainly a natural
possibility there. So that really does instinctively sound like a unification, right, which is the huge
ambition. Can I unify all the matter forces and gravity all into one?
What do you think are the strongest arguments that we have
that string theory will continue to be rewarding
as a unified theory of everything?
Because as you said, it's not complete yet.
Yes, it's not complete.
So one thing you just mentioned is the aesthetics, of course,
the unifying of particles and forces
and gravity and everything together in one package is already quite amazing. So that's
already enough motivation to continue studying it. However, there's a lot more. And we have
learned that even if you are not interested in quantum gravity per se, suppose you're
just saying, you know, I'm interested in just particles and the forces between them. You encounter questions in that context which have to do with very strong
interactions between these particles and we end up not knowing the answer to those questions
in general. However, it turns out that what we have learned from string theory sheds light
even on those questions. So questions that have nothing to do per se with gravity has been answered using string
theoretic ideas. And so therefore we feel that the truth of the string theory is far
beyond just the gravity, even though that's the main claim to fame of quantum gravity
and string theory together. But I think the fact that there are more motivations to
study string theory is already quite remarkable. And it's reinforcing the fact that there are more motivations to study string theory is already quite remarkable
and it's reinforcing the idea that this got to be true in some form or another.
It cannot be just randomly there and we just stumbled upon it.
There are also some pretty severe criticisms or critiques of string theory and I know that
people get very emotional about this.
I've sort of heard it described as an attempt at a theory of everything,
but is it turning to the direction that we now kind of have a theory with no predictions
because there are so many possibilities?
Actually, that's a very good question.
There's a misunderstanding that string theory gives a huge number of possibilities,
infinitely many possibilities.
And that's the point I'm trying to clarify here.
What we are basically pointing out is that string theory gives you a finite number of
possibilities and cutting the possibility from an infinite set to a
finite set is a remarkable reduction. Yes, you eliminated an infinite number of
things. So when people criticize it in the way, say, oh it doesn't make a prediction
about anything, they're putting a very high bar for what string theory should be. Namely, when we talk, for example,
about quantum chromodynamics, which is the theory describing how the quarks work and
what are the forces between them inside the nuclei and so on, there are infinitely many
possible theories for that too. We just choose one of them. And nobody explains to you why
that particular one is the one we use.
So if you want to choose that level of precision, then you have to say we don't understand quantum
chromodynamics.
No, that's not what we say.
We say, well, you just pick that one and just study it and that's the way the universe is
working.
Don't ask more questions, so to speak.
So in that context, people don't say that we don't have a good theory of strong interactions.
They'd say we do.
Now string theory is trying to give you a broad perspective of what all universes
could look like, not just our universe. And so that's a grand, you know,
ambitious kind of thing. It's much more than just one theory described to one
particular universe. Now, you might say, I don't care about these other
possibilities. I just want the answer to my universe. And then that's where it
becomes harder. And that's where people become critical. But if you look
at it from the broad perspective of what it's trying to do, which is trying to tell you
what is the overarching truth of all possibilities, that's surprising. And in that context, it
tells you, for example, if you want to have the theories of strong interactions, there
are only a finite number of possible ways. Whereas without gravity, without strength there, you would have thought
there are instantly many possibilities. So it cuts it down to a finite set. For example,
the number of colors, we say there are three of them. It could have been four, five, six,
say up to arbitrary integer. Strength there says, no, there's an upper bound. It cannot
go beyond certain numbers. That is a surprise. So it cuts the number down, not make it large.
Just for the audience, colors in this sense is an abstract property. It's a cute name
given to different kinds of quarks.
That's right. So we say that each quark comes with three colors and that's just three degrees
of freedom. We just call them colors.
Now to discuss this kind of landscape of possibilities, as it's often referred to,
what are the differences in these universes from one point in the landscape of possibilities
to another point in the landscape? What's different about the universe than this one?
So in each conceivable universe that we can think of, in which case you have a quantum
theater of gravity interacting with particles and forces. It appears in a particular dimension. You have to have a particular spacetime dimension,
which is what we call the macroscopic spacetime, the big space, so to speak.
There are some curled up directions, what we call the compactification scale of the manifolds.
So these are tiny things that we don't care about in terms of large distance observations
that we do currently.
So you have to decide how many big dimensions do you want.
For example, in our universe, there are three spatial dimensions, which are macroscopic
and one time.
So that's the three plus one dimensional one.
That's one particular choice you're making.
But different universes that we get in string theory can have different dimensionalities.
Again, bounded, it doesn't go too far up.
It doesn't go beyond 11. And also, in each case that you choose dimensions, you could ask, okay, is it going
to be like a flat space? Is going to be some curvature in it? And this feature is sometimes
related to the property, if you talk about the space-time curvature, in the context of
the cosmological constants, which is positive in our universe. In addition, you could say how many particles do I have? What kind of
forces do I have? So all these different kinds of choices that are consistent in string theory
give rise to different dimensions, different number of particles, and different kinds of
structure for the curvature of space-time.
This landscape of possibilities was disheartening to people
for all the reasons you've just said,
that suddenly where's our single theory of the universe?
And now you've given me an infinite number,
but then you have this Swampland initiative.
Can you tell us how this helps restrict us
to try to find a better description
that might be just our universe?
Yes, so let me just give the analogy.
I said there are only three colors in our universe and the strong forces,
but it could have had more degrees of freedom, what we call more colors,
could have had four, five, six, etc.
So if you take the four-dimensional theory and ask,
suppose I want to have the simplest four-dimensional theory I have with matter.
It turns out the simplest four dimensional theory with matter, which can be described with quantum gravity, is what we call in
technical terms n equal to four super gravity theories. Now n equal to four
super gravity refers to the statement that the theory has extra symmetries,
what's called supersymmetry. So if you take this simple class and ask in this
class, do you have an upper bound in the number of colors, then you can show that you have to have less than 24 colors.
So suddenly you cut the number to a small set.
Now, if you did not know about string theory, you would have said,
take any number of colors, it's perfectly fine.
Those options that could have been true, we could call potentially consistent theory,
but most of them now we are claiming do not exist.
More than 24 don't exist.
Those belong to the swampland.
The ones that can be constructed in a string theory belong to the landscape.
So the first 23 or so are belonging to the landscape.
The rest are the swampland.
So therefore, what swampland brings to the table, it suddenly makes a prediction now.
You cannot have too large a number of colors, for example.
So you're relegating to the swampland all of these inconsistent theories.
You're hedging in the landscape,
which is now going to be surrounded by the swampland of impossible universes.
So really the initiative is to restrict yourself more and more and more.
So what kind of criteria are you using to decide what's viable and stays in the landscape
of possibilities?
And what has to be relegated to the swampland?
We try to find criteria which distinguish whether you are in the swampland or in the
landscape.
If I give you a putative theory and you ask you whether this is good or bad,
whether it's in landscape or swampland, how do you judge it? What are the criteria? It
would have been natural to say, you know what, I'll just put A, B, C, D as criteria, whatever
they are, to say if they are good and then I will know once and for all I found the theory.
However, we think the number of theories which are good are very, very small. So if you think about an ocean, and suppose there are some little islands on it, and then
I tell you how could I find the islands, it's much harder to say pinpoint one of the islands
unless you really, really know where it is.
And it's much easier to say, you know what, if you draw this line on the ocean, there's
nothing on the left side of it.
I can guarantee that you can't find anything.
These criteria where you cannot find a good theory or what is in the swampland is much easier
than to say what is good. So in other words, the reason we have the swampland program
is not that we are looking for bad things, but it's much harder to find the good things.
The good things are much more difficult because they are very rare.
These islands of possibilities, just to clarify, they don't have to be our universe, right?
They just have to be a consistent universe, one that is mathematically sensible and consistent,
even if maybe galaxies don't form in that island universe.
Is that right?
Exactly.
So then if you wanted to quantify all the islands, it's going to be very difficult.
However, you know, we are lucky in the following way.
You could say, well, we know one island, namely our island, our universe exists, and you can make observations on our universe.
So you can say that in this universe, these properties hold.
And then you say, oh, if I knew that there is this property on the island, then I can rule out some other property.
In other words, we are using observations of our universe combined with the criteria we know has to be generally true.
And suddenly we make predictions.
And so that's how we have actually made predictions for our universe based on the Swanplan principles.
based on the Swampland principles. It's a little tricky as I was thinking that instead of saying what the landscape is,
the Swampland is everything around the landscape, it's all the negative space.
Like as a mathematician, the thought that occurs to me is maybe there are inequalities.
Maybe if a certain parameter is too small, then I know nothing like that could possibly be in there.
Inequality constraints would give you whole big open sets in the space of possible universes.
So you could lop off enormous regions maybe.
Is that the vision?
Yeah, I think that's part of the vision.
I'm sure you'll remember that Alan Turing used this when he was cracking the Enigma code
during the war.
One of his big insights was to begin
to prune away all of the encryptions that couldn't possibly be right, because they
had a very short list of identifiers that they anticipated from a correct cipher.
And so it allowed them to use a blunt tool, right, to kind of scoop away a whole range
of possibilities, leaving, as Vafa is
describing, an island of possibilities, which is much, much smaller, and thereby narrowing
the field that you have to scour for models that actually begin to look like our universe.
For instance, let me give you an example of what might be an attribute of the landscape.
It might be that it's a higher dimensional space, so it's not just three dimensions of
space in one time, but it has extra spatial dimensions, and they're wrapped up in this
very complex origami.
And most of those spaces we know very, very, very little about.
It's really hard to investigate them and to understand them and to understand what their
predictions are
and if they could yield, for instance, the spectrum of particles that we observe around
us because it influences things like this.
So they're just trying to use a blunter instrument to eliminate from this huge range of possible
geometries, for instance, a smaller number that might yield a universe that is compatible
with the one we observe.
And did you get the impression from talking to him that
there's some notches in his belt? Like have they eliminated anything so far?
Well, I do think they feel they've had really surprising success,
and maybe it was a surprise even to other people. But more importantly,
Vafa really feels this is experimentally testable, at least in principle.
And while they haven't been able to do that yet, that's really the direction he wants
to go in, to combat not just the attitude, but maybe the failing on the theoretical side
to take experiment more seriously.
So more on this after the break. Welcome back to The Joy of Why.
We've been speaking with Harvard theoretical physicist, Kumran Vafa, about the string landscape
and the swamp land.
Now string theory is often critiqued as not being concretely connected to experiment and you're
really saying otherwise that there is a way to actually dig into cosmological or particle
physics experiments and use one lure to kind of hitch a whole bunch of other criteria to.
Exactly. And that's the exciting thing going on in string theory now. I can in fact tell
you where the Swamp Land Program picked up speed. So we proposed this general program
in around 2005 and by and large it was just a very few number of papers coming on the
subject until when BICEP came up with the measurement of
what they thought was gravitational wave and the data that they were giving was in contradiction
with some of the swampland principles. And so we were dismayed that how could it be possible? We
thought we understood this and then later BICEP was proven wrong. So then suddenly there's a huge
amount of activity came to this direction
precisely because we now realize we can have concrete predictions that is relevant for
observations in our universe.
So you're actually using observations of the light left over from the Big Bang, of polarizations,
of gravitational waves, also dark energy observations.
Everything that we can see we can use and think about how this fits or doesn't fit or what aspects can be predictive when you combine it with the Swamp-Land Principles.
Now, famously string theory is also incredibly mathematical. Do you believe that we really have the right mathematical tools to keep progressing? I think we have more conceptual questions that we need to understand better. I don't
think math has become a hindrance, even though what you're saying is absolutely true. That
is, the subject is very mathematical and it has led to new areas of mathematics being
researched. So, in fact, we can generate our own math tools. That's not going to stop us.
More, I think, is the issue, even though we have
a deep interaction with mathematics. We actually have to go beyond that in the sense that we
need to find what is string theory or what's it trying to do and so forth. Those are harder
questions. And if you ask the same question, what is string theory 10 years ago, 20 years
ago, the answer changes. And even now, if you ask what is string theory today from string
theorists, you get different answers. They have different perspectives of what it is
about.
Do you feel confident that we will reach a realistic, kind of more complete description
of string theory in our lifetime?
Well, we certainly are getting more and more complete. I don't know if there's an end
to human understanding of nature, and I think the same might apply and more complete. I don't know if there's an end to human understanding
of nature, and I think the same might apply to string theory.
I wouldn't be as bold as saying, yes, we're
going to get to the end of the story, even
in anybody's lifetime.
Knowing that we are making progress, that for sure
is happening now.
That I can say.
That's what science is after all.
We should be looking for progress and more
clarification.
So we are certainly evolving in our understanding
of what string theory is and how it may connect to our universe. And we have enough clues
right now to be confident that we are on the right track. Let me just say by
simpler example. We live in three spatial dimension and one temporal dimension.
That already should be surprising. If I give you a number integer D, which is the
dimension of space and time from 1 integer D, which is the dimension
of space and time from one to infinity, what are the chances that's less than 10? I mean,
it's almost chance zero. It is zero chance if it was a random number. It's not. And the
point here is that, for example, the Swamp Land program, we say that dimension cannot
be more than 11. That already is an example of why the thing is limited. So that's a very
strong prediction. People don't think about this natural thing because everybody takes for granted.
Yes, we have three large dimensions and one time. So what? Well, that's a big deal. Why not much,
much more? I think that goes back to when Einstein first starts to realize that space and time are actual
Mutable properties of the universe you have to start to ask why three and why one time and I think he did ask that And just didn't know yes in fact during his time
Kalousa and Klein came up with one more dimension and Einstein embraced that as an interesting extension of his theory
So these ideas about what are the limits of the dimension of space-time? And how does it work is part of the bread-and-butter of this theory. So these ideas about what are the limits of the dimension of space-time
and how does it work is part of the bread and butter of string theory. I think already we have
enough clues in our universe that we feel confident that we're going in the direction
of realizing concrete predictions of string theory in our universe.
So I'm fascinated to know what goes wrong when the number of dimensions goes above 11. So one of the properties and symmetries that I mentioned was supersymmetry. And we know that
whenever you don't have the symmetry, you get instabilities. Instabilities means that you cannot
have a stationary situation like our universe looks like. Now, we know that our universe does
not enjoy the symmetry that I just told you about when you go look at large distances. But we think that even in our universe, if
you go short distances, you might recover such a symmetry. So in other words, the symmetry
which allows us to have a long lasting universe has a limit in terms of the number of dimensions
you can go and it turns out you can prove mathematically that it cannot go more than
11.
So how fascinating that the large number of dimensions which we think of as the universe on the biggest scales is determined because of microphysics. That's actually the most
beautiful connection between the string theory and more broadly quantum gravity and quantum field
theory. Quantum field theory you build up things from small
scale. You say what's going on at the shortest scale and then you can describe the larger
scale. And that has been the mindset of physicists. It ended up that if you study quantum gravity,
it doesn't work that way. Namely, what happens at large distances is related to what's happening
at short distances and they are not independent. In other words, the large distance physics can dictate
backwards what's going on at shorter scales, which is not like what usually
happens in quantum field theories.
And there's another sense in which we can think about the connection of the small
and the large dimensions that I'm hoping you can explain to us a little bit, and
that is the possibility that 95%
of what makes up the universe, which is in the form of dark energy predominantly and some dark
matter, could also be connected to these higher dimensional theories, these string theories with
these extra tiny wrapped up dimensions. How do you make that incredible connection? This is one of the beautiful questions and connections with the ideas that are emerging in string theory.
So as you mentioned, 95% of the energy budget of the universe is not made of things we know of.
It's not made of electrons or protons and this and that that we know of. It's made of something else.
In fact, of the order of 70% or so of it is what we
call the dark energy. And in addition, there's about 25% or so of the energy budget, which is
what we call the dark matter. They are weakly interacting with our particles, the electrons and
the photons and all that cannot interact very effectively with them. So they are there. And the
only reason we know they are there is because they interact with gravity and gravitational effects we can detect
So what does this have to do with string theory and quantum gravity efforts to try to understand this?
It turns out we have gotten super lucky
What we have learned using Swampland ideas is that whenever you have a small parameter in a physical theory
It comes with consequences.
When you have an electric charge or you have masses or things, if things are extremely fine-tuned
to a small value, then there's some other predictions we can make. That's one of the new things we have
learned in string theory. In particular, we have learned that whenever you're extreme fine-tuning,
one of two things must necessarily happen.
Either there's some extra dimensions which are beginning to be big,
or there are some very tiny light strings which are becoming so ever so light.
This is a consequence of having some extreme parameters in your theory. Now
you might say, wow, where did that come from? This very strong statement comes from a property
we call string dualities. So string theory, when people study that, they try to take these
extreme parameters to extreme regimes, and each time you take the parameters in your
physical theory to extreme regimes, you found a new description takes over.
And so when a new description takes over, you begin to find
that this new description involves like particles.
And these like particles tend end up being related to two possibilities,
either a light tower of strings or long wavelengths
of gravitons in some extra dimensions.
These are the only two possibilities.
So these dualities, we don't have a deep explanation of them,
but we know it's true in string theory.
Okay, so we take that as one of the Swamp-Land principles.
Namely, if you have fine-tuned small parameter in your theory, look for either large dimension or light strings.
Now by a large dimension, you still mean kind of small.
Larger than, I mean, our universe could be
effectively infinite, so we're talking about
the other curled up dimensions.
The curled up dimensions that could go up to 11
altogether space-time
So the possibilities of having the seven extra spatial dimensions one of them or two of them or three of them becoming big and
Big would be what like a micron big a millimeter. Is that big? I mean
Micron is huge millimeter is huge even atomic size in that sense is huge
the reason is that the fundamental scale in gravity is 10 to the minus 33 centimeters and that is about 25 orders of magnitude smaller than
atomic scale. It's tiny, tiny, tiny. So anything much bigger than that is called big for us. Now,
so when I say big, I mean big compared to that tiny scale.
Right.
Now, we are given this situation in our universe
where one of the parameters in our universe
is extremely small, and that is the dark energy.
The dark energy is one of those amazing parameters
in the theory which has an extremely exciting,
interesting history
dating back to Einstein. Originally when he wrote his theory he put this term
which he called the cosmological constant which we now also called dark energy to
non-zero value in order to get a static universe. And later he abandoned that
after we found out that actually the universe is expanding so he put the dark
energy to zero and it stayed zero in the minds of theoretical
physicists for many, many decades after Einstein did that until shocking
observations in late 1990s told us that it's not zero.
And the reason people were shocked was that if it was not zero, it was so, so,
so small, 0.000000, zero, zero, zero,
you have 120 zeros and then you put a one at the end. It's that small. Okay. So that's
a tiny dark energy. Now, as I just was telling you about, whenever there's a tiny number
or fine-tuned parameter in your theory, you should be asking, what's happening to these
extra dimensions? Are they getting large or is there a light string somewhere?
So already we are saying that having the dark energy which is so extreme must necessitate having new particles.
Where are they? On the other hand, we say there's dark matter.
So now we are saying that two facts, the fact that dark energy extreme and there's some extra light particles around
could naturally play the role of dark matter.
So that's the idea that already automatically comes from these Swampland principles.
Now you can say, can we make it more quantitative?
It turns out that the dark energy predicts actually a length scale
and it turns out that length scale is about a micron. A micron is one thousandth of a millimeter.
And it suggests that exactly one of the extra dimensions
is of roughly that size.
Now, you could ask then, what about the dark matter?
Well, the dark matter would be the graviton waves
which were created in this extra dimension,
what we call the dark dimension.
So we have three spatial dimensions
that we know which are huge.
One more dimension, which is this micron scale,
and then the rest of them we think are much, much smaller.
So therefore, this one micron dimension space
will potentially carry in it some long gravity waves,
which would play the role of dark matter.
So in this context we have a unification of dark energy and dark matter just from this
simple principle that when you have extreme values in your physical theory there are light
particles.
So you're essentially saying that by observing the as yet unknown dark energy and dark matter
this could be an observation already of dark dimensions.
So there's other ideas competing, so it's not a smoking gun,
but we could actually be observing string theory.
Exactly, but there's actually more.
Because this tower of particles I was telling you about,
which comes from these light particles,
has to be weakly interacting,
which is the smoking gun of dark matter.
It's weakly interacting not only with us, but even with themselves.
So that is a property.
It's a prediction, I would say.
So we are making a prediction that whenever you have this dark energy being so extreme,
you better look for light particles which are very weakly interacting, just like our
universe has it.
So this is a prediction for our universe and in fact makes another prediction. You cannot directly detect them because their interaction strength is gravitational.
So these direct dark matter detections will not succeed based on this idea. So we are
making very specific prediction. But actually you can make it even a stronger prediction.
If you have two objects, two masses at a distance r, Newton taught us that there's a gravitational
force between them which attracts them and this force falls off with the inverse square of the distance between
them. That is a property of three dimensional space and one time. If you increase the number
of dimension, each time you add one dimension, the power of the distance in the force law
increases by one. So instead of distance squared in
three spatial dimensions, if you have one extra dimension, it becomes distance cubed.
And if you have more, it becomes distance fourth and so on. So if we have one larger
dimension, it should have been distance cubed. So we are making a prediction that if you
bring these objects together and put them at a distance roughly of a micron or so, you
should find the stronger gravitational force between them. This experiment to detect this is actually being undertaken now to bring
it down from 30 micron perhaps to 10 micron below to try to see if the force law changes
as we are making a prediction. So that's a very concrete prediction that we are making
based on this link.
A lot of what we're describing is of of course, a description of the universe writ large.
And here we are on this little globe, all of us together, working in international communities
to figure this out.
And I'm wondering how that played a role in your own experience.
That's one of the aspects of science I cherish a lot.
The fact that it doesn't recognize any artificial borders.
It's universal.
It's a borderless and timeless adventure
that human mind is engaged in.
And it's something that gives you a way to connect
with people across the world.
It transcends cultures
and it transcends artificial political lines that are drawn.
And in fact, I think cultural aspects actually are helpful in formulating specific viewpoints
about scientific questions, which is interesting because you have to come up with some kind
of thinking like, for example, aesthetics of what is good, unification is nice or not,
symmetry is nice or not, and it's good to have different cultural perspectives. So bringing that to the table is in the best interest of advancement
of science. And so in this sense, I actually was born in Iran, and I think that many of
the cultural aspects that I bring with me on the table for the scientific discussion
is maybe not as much shared because unfortunately
there are not as many scientists from my country as there could be. That's my bringing what
I bring to the table. But then there are all these other cultures. And so put all these
together, I think it's great. You have all these different perspectives and we should
cherish it.
It's so beautiful. I love that description. I know that you've also been very interested
in some of the philosophical
lessons about what we can extract from physical laws that are bigger lessons than even physics.
How does that influence your thinking as well now?
Yes, I think that part of the reason many of us scientists study nature is deeper than
just figuring out the equations describing how things move or how things work,
is to understand the bigger picture
of the meaning of existence, the meaning of things,
which bears on philosophical questions.
So I think science and philosophy are not disconnected.
You cannot ignore it.
Perhaps maybe too often,
but here are colleagues look down on philosophy and
maybe I think in some sense every scientist is perhaps an amateur
philosopher and we have our own viewpoint that's not rooted in science
but we say oh this is aesthetically beautiful and where is that coming from
that is not a scientific statement but it drives us quite a lot that's
motivation of a lot of our work is based on things that we consider pretty or
beautiful or something which is not sometimes easy to quantify.
To me, a lot of things that we are learning in science point towards this quality of magic.
But unlike the usual magics that once you explain it becomes boring, explaining these
magics does not get rid of its charm. And that's
the amazing part of science. This magic always continues to exist. And that's some unbelievable
quality of the truth in our nature, and it's philosophical, I would say. So I cherish
the connection between philosophy and science.
Well, it's been a pleasure to talk to you. Thanks so much for being with us.
Thanks, Jan. It was a pleasure to talk to you. Thanks so much for being with us. Thanks, Jan. It was a pleasure to talk with you.
Wow. There's a lot to chew on there. Let's see. I would love to hear some of your thoughts
about magic, about philosophy, and also about the diversity of cultures can be a strength
in science.
Yeah. I thought this was intriguing. The tension between magic and the reveal.
Yeah.
I mean, how do you feel about that? We approach these problems because they're mysterious,
because we love the problem, right? We love the pursuit of something that seems magical.
But he says the reveal doesn't ruin it.
Yeah, I love that. That's an original idea. I hadn't heard that before because it is true that, of course,
that's the whole point that magicians don't tell you how they do their trick and they're mad at other magicians who do
because it does tend to ruin the trick and there's no analogous thing in science, right? If you learn, for instance,
how Kepler's laws work because you now know Newtonian
calculus and theory of gravity and that sort of explains it.
That's the reveal.
And yet it's not ruined because then Einstein gives you another reveal.
And that's not ruining it either because maybe string theory has its own reveal.
Well, I thought it was very interesting that he was suggesting that the kind of approach
that we have in physics that things are beautiful.
That is actually a very strong kind of scalpel we use to eliminate theories that we have in physics that things are beautiful. That is actually a very strong kind of scalpel we use
to eliminate theories that we think are probably not viable
because they're just like, they're really ugly
and that nobody believes them.
That one I feel is much dicier.
We could probably get an argument about that.
That seems to me a relatively recent concept.
Like the insistence that nature has to be beautiful
and that symmetry and
beauty can be scientific criteria for deciding for or against certain theories.
That has worked for a few hundred years.
Few hundred.
Well, I mean, Feynman has remarks like this that he claims he doesn't care if it's beautiful
or not.
He just wants to know the truth.
And I don't know that beauty and truth,
I mean, who says the world has to be beautiful?
Yeah, I mean, it could have been a failed program, right? So I would say to that, oh,
absolutely. It seemed reasonable to look for symmetries, which is part of the idea of beauty.
It's actually in a very concrete, explicit way, symmetry.
Yes.
And it could have failed. Absolutely.
But it feels to me like it's about 300 years of that.
But I wonder if we're hitting the limits of that.
I would say Galileo was into this too, for sure. It's hard to know.
I'd love to hear more about it. We need to have him back.
Well, Steve, always fun to talk to you. We'll catch up again soon.
I'm looking forward to it.
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The Joy of Why is a podcast from Quanta Magazine, an editorially independent publication supported
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