John Donoghue: We Have Already Quantized Gravity (And It Works)

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I think the popular phrasing is totally wrong, quantum physics and gravity go perfectly

well as well as any other theory we know about.

One of the biases of the field is that it's huge.

And we don't really have any evidence for that.

I'm actually a champion of a crazier theory.

You heard it before, quantum theory and general relativity are fundamentally incompatible.

But is that actually true or is it something we just say so often we start to believe it?

Professor John Donahue thinks this entire framing is misleading.

gravity is a field, the metric, so you quantize it like QCD.

In fact, Feynman and Duitt did exactly that several decades ago.

So what's the actual problem?

Donahue argues it's hidden assumptions.

Perhaps something like causality, supersymmetry or grand unification, even so-called naturalist

could be a human bias rather than an objective law.

Today we delve into quadratic gravity theory and another more speculative theory called random

dynamics.

On this channel, my name's Kurt Jaimungal and I interview researchers about the theories

of reality most often in physics.

And today's a particularly technical talk, so be prepared.

I'm excited because you'll see why John Donahue is a legend in the subject of gravity

and its quantization.

We'll delve into effective field theory and learn why this professor's judicious restraint

unnerves his colleagues.

Mr. Donahue, you're known in a sense for being radical for not being radical.

Explain that.

Well, I am in a way quite conservative because I grew up as a phenomenologist where I learned

to listen to what nature is telling us and nature has told us gradually over time that

the fundamental interactions are gauge series and the gauge series are composed in particular

ways.

And we've learned to understand quantum mechanics and the fundamental interactions through

the interactions with experiment.

And so I tend not to deviate from that very much.

You mentioned that quantum mechanics may fail at some point and when we're thinking of quantum

gravity, we're assuming that's something about quantum mechanics, whether it's the

direct von Neumann axioms or something is held sacred.

And then gravity has to bow in a sense to those.

But you said when we were speaking off air that you don't think that's necessarily true,

and I'd like you to comment more about that.

Okay.

So the point is that all our theories have limits.

We've tested them in some range of energies and conditions.

And we're used to think over the various interactions as heavy limits like the standard model

we expect to be supplanted by other interactions.

But the same should hold true.

We should hold the same standard to quantum mechanics.

Standard assumption is that quantum mechanics is valid at all energies, all scales, all everything.

Of course, as present, we don't really need deviations.

But nevertheless, there could be deviations both at high energies or on macroscopic scales.

And so there are experiments testing quantum mechanics on macroscopic scales.

And I think they're very interesting because that's a frontier for the limits of quantum

mechanics.

And perhaps the theory as we know now will be changed at some scale.

Macroscopic scale.

The connection to gravity is that gravity may be the best place to test this because you

can get macroscopic bodies out of gravity, whereas it's hard to get very macroscopic charges,

for example, if you're trying to get larger amounts of charges, hard, larger amounts of

mass is easy.

So when popularizers talk about quantum mechanics and GR are incompatible, are they getting

something wrong?

And also, I just said quantum mechanics and not quantum theory.

People talk about quantum mechanics, quantum theory and quantum field theory.

I don't know if you see those as different when speaking about general relativity and

the combination of those two.

Yes, actually, I think the popular phrasing is totally wrong that quantum physics and gravity

go perfectly well as well as any other theory that we know about.

The quantum gravity involves the field, which is the metric, that field is quantized, it

was done by Feynman and DeWitt in the exactly the same way we do QCD.

There's no difference at all in the framing of it.

So if you just take standard quantum theory and standard gravity, you have a quantum theory.

That quantum theory is a little different than the standard model in that it's explicitly

an effective field theory, which I can explain what it is.

But it's a quantum theory and it can make quantum predictions perfectly well.

That's one of the lessons of the last 30 years.

Now is anyone of your colleagues thinking that gravity as an EFT for low energies is wrong

in some way?

It's my understanding that no one would disagree with that.

Yeah, I don't think any would disagree with it at this stage.

It took a while for that the effective field theory viewpoint to percolate through the

field because many of us grew up learning about renormalizable field theories as the only

valid quantum field theories.

But that's not true.

We have other theories that are effective field theories that we could make perfectly

good for quantum predictions with and generatively false in that same category.

Okay, so then are there any problems in quantum gravity that are considered to be fundamental

issues like you mentioned non-renormalizability that you think this is a pseudo problem is not

even a problem?

Well, it's a problem in the log run.

If you want a totally consistent theory at all scales, do you want me to explain effective

field theory a little bit?

Please.

So effective field theory is really just quantum theory with thinking about the scales done

right and the important ingredients really the uncertainty principle.

So in many cases, even in the standard model, there is unknown physics at high energies.

We just don't know what about because we've never tested it.

But the uncertainty principle tells us that the effect of that is local because delta

e is big, delta x is small then and so it appears effectively local.

And so all the unknown effects of very high energy appear as local terms in an effect

of Lagrangian, in a Lagrangian.

And they're just constants that we don't know.

There's like the mass of the electron is one of those constants, the charge of the electron

is another, the gravitational constant.

These are terms that are local.

But nevertheless, the quantum effects are involved dynamics at the energies you're working

at and those can be done with the degrees of freedom and the interactions that you have

at those energies.

They're not local in that sense.

And so effect of field theory basically just separates out the unknown effects from

high energy from the known effects that the energy you're working at and makes predictions.

And in the end, we think of all over theories like effect of field theories.

Now you didn't mention the name Ken Wilson, but I think it's probably useful too.

So can you talk about how Wilson changed the understanding of what a non-renormalizable

theory is and also feel free to expand on what a non-renormalizable theory is as well

and what the view for Wilson for supposed Wilson was?

Right.

So Wilson was clearly important in this change in the viewpoint that his point of view

that he espoused was that there are various local terms in the Lagrangian doesn't need

to be restricted.

So we derive our physics from an action from a Lagrangian.

The Lagrangian for a normalizable theory has just a few terms, but in principle there

could be terms of higher dimensions, which I need to explain dimensions, I guess.

The Lagrangian that we write out could have many terms.

So for example, you could have fields with two powers of the fields, four powers of

the fields, six powers of the fields, et cetera.

And those are set to be increasing dimensions.

And normally we limit ourselves to the lowest dimensions.

Wilson told us that quantum effects generate the other ones also.

And then we can include them in the fundamental theory, in the Lagrangian equally well.

We just have to have a way of determining them either by track calculation or by measurements.

I want to just clarify that the dimensions here, they're not spacetime dimensions, they're

just powers of the field.

Other powers of fields.

That's a great thank you very much.

So powers of the fields.

And so the usual theories that we do, the renormalizable theories like the standard model,

are very limited in the powers of the fields that they use.

The effective field theories are more generous, say they're more general.

In 1994 or so, you calculated the correction to the Newtonian potential.

Yes.

Look me through that.

So the interesting insight there is that the long distance corrections to the Newtonian

potential, the ones that fall like a power, since they're not local in the sense, because

their long distance are then determined not by physics from very high energies, but only

by physics from low energies.

And we know that physics.

So we know that it's general relativity, we know the interactions of general relativity,

we know the Feynman rules, because Feynman and Dewitt told us those.

And so the long distance corrections to the Newtonian potential are rigorously calculable,

we know what we know at the present energies.

And so that was a very explicit example of how effective field theories work.

By knowing the low energy degrees of freedom, we can make calculations that are valid at

low energies without needing to know the high energy theory.

There's something that I was thinking that might be interesting to the listeners of

just a perspective on why historically, it was difficult to think of a general relativity

being a quantum theory.

And this is because of the effect in this of the classical geometric picture.

When you learn general relativity, one of the interesting features that you first learn

is that general relativity doesn't look like a force, it's straight line motion in a

curve space time.

And so you think of a manifold or space times structure as being fundamental.

And there's this sort of mantra that I think probably goes back to Wheeler.

The theory is not a forces geometry.

And if you come into the game thinking about geometry and not about forces, then gravity

does look different than other theories.

Other theories have forces and particles mediating the forces.

But that's really a feature of the classical theory.

And it's not really fundamental.

You could equally well forbid a general relativity as a theory with exchange particles with waves

on space time, like gravitational waves, and we now detect in LIGO with exchange particles.

And if you formulate it like a field theory, then the quantum theory looks very similar

to the quantum theories, other quantum theories.

If you're formulating in the geometric picture, it looks quite different.

And so I think the historically some of the trouble has come just because the classical

theory has been so successful using classical methods that it was hard to switch to quantum

methods.

Is there a reason that when I watch your lectures that you start from the path, you only

use the path integral approach, as far as I've seen, and not something else, like the

canonical?

That's certainly true.

I mean, it's certainly true.

Also, if you think of X commuted with P being IH bar and all that, it looks much more

complicated when X suddenly is a general relativistic coordinate.

But all our fundamental theories are now only formulated in path integrals.

You can't formulate the standard model in a Hamiltonian theory, and you do it use any

effective, any reasonably useful formulation.

So all our theories are formulated that way.

And general relativity makes the most sense that way.

And this is a lesson that goes back to the wit in a way when a Feynman, this is a bit

technical.

But when Feynman showed that you needed ghost fields to get the correct degrees of freedom

in general relativity, the wit formulated in the path integral framework.

And then the QCD is also formulated that way.

So basically, you have to use the same formulation that you use for QCD.

You would never do anything else for QCD except path integrals.

I'm sure you've taught quantum field theory.

Yes.

So do you teach canonical quantization at all?

I do.

You know, I think the place where in the framework where you're progressing from quantum

mechanics to quantum field theory, you need to go through a phase where you define

quantum, basically, you're the original quantum of photons.

And so doing second quantization for photons is a necessary step to introducing the idea

of quantum.

But then at some stage, and so yes, you do that, you define the Fox space, you define

one photon, two photon states.

At some stage, to do theories like QCD, the weak interactions, you have to get into path

integrals and show that they have the same content.

And if you start with path integrals to begin with, it's actually there is a place where

it becomes, there's a logical disconnect on what is the one quantum state.

So there's, I mean, there's two very good books that do this.

The one is Tony Z's quantum field theory book and the other is Pyramones.

They start straight from path integrals, but there's a stage in there where there's this

logical disconnect of how you define one quantum.

And so yes, I tend to go through the canonical and then show that the same results come out

of path integrals.

Earlier you were saying that most people or most students have heard the geometric interpretation

of general relativity and that that can stime me then when thinking about making that

compatible with quantum theory.

So I'm wondering, are there any other myths or whatever you want to call them that you

have to dispel to students?

Well, I think that's the bait, right?

I think if you start with general relativity as a, as a field theory, a classical field theory,

then you could merge into the quantum theory quite easily.

The trouble is it does, it does take a while to get there because it takes a while even

to get to the quantization of QCD.

It's not, it's not the day two on a quantum course.

It's a little further on and you have to have that background.

And so getting to path integrals has been the least the way I teach it, the best way

to get there.

I asked earlier about the tension between quantum mechanics and gravity.

Now should I have said quantum field theory and general relativity or do you even say

allowed quantum mechanics and general relativity?

I tend to say quantum field theory.

I probably have not said quantum mechanics, I've said quantum theory, but quantum theory

in any relativistic system has become quantum field theory.

And so we're talking about quantum field theory really when we want to do this.

So has there been any trouble making general relativity consonant with quantum mechanics

as such or as soon as you, well, if you're talking about GR, you're talking about special

relativity as a special case and as soon as you combine that with quantum mechanics

you get Q of T.

So you do have to get to the quantum field theory because just because for any relativistic

field theory, any relativistic system, you need quantum field theory rather than quantum

mechanics.

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So how do you see then the tension between quantum theory and gravity?

I don't see any tension at the...

It's at low energies, sure, but the world is not just low energy.

So at the moment, we know low energies, and so I wouldn't say there's any tension between

the standard model and quantum mechanics.

There are problems if you try to take the standard model to high energies.

For example, if you try to take the Higgs potential at very high energies, there's an instability.

So at high energies, the standard model doesn't work very well.

We expect it to be super-supplement with something else.

The same thing is true with general relativity.

We expect a more complete theory at high energies.

That more complete theory could have various characteristics, but when we think of an effective

field theory, we always think of what the limits of the theory are.

And those... the effective field theory tells us that there are limits to where we can

apply the effective field theory in the at high energies.

So the strength theorists are concerned with UV completion, and they believe that you

would require some extended object.

So do you disagree with them, or is your disagreement more empirical or philosophical

or what?

I don't think there's a disagreement between the strength theory approach and the effect

of field theory approach.

Strength theory is, as one possible, UV completion.

And it's interesting because it's one where we have the most calculational control.

So it is useful to see what a complete quantum theory of gravity could look like.

And as far as I can see, it should match onto the effective field theory in all the regions

where we expect them to both overlap.

And so I don't see any conflict there.

There's something called the swampland conjectures, I'm sure you've heard of.

So does EFT gravity or quadratic gravity sit in the landscape or in the swampland?

So EFT has a set of parameters which are undefined by the EFT.

And there is a portion of those parameters that live in the swampland and a portion

that live in the non-swampland.

So the swampland program basically divides the EFT parameter space into two regions, swampland

and not.

We can do quadratic gravity if you want.

Sure.

I would love to hear about that.

The Einstein theory follows from an action that, if I let me neglect the cosmological

constant in all the follows, just follows from a very simple Lagrangian, which is just

this scalar curvature.

So if you take the scalar curvature, it has to be a general covariant and the Rensen variant

object, there is one simple one, which is just the scalar curvature by itself that gives

you Einstein gravity.

However, consistent with all the things that I've just said, you can also take the scalar

curvature squared.

That also works.

So that's what, when we say quadratic, that means curvature squared and it turns out

there's two terms.

One is the scalar curvature squared and the other is the richi curvature squared or the

vial tensor squared.

There's two possible terms at the square, I'll show the Lagrangian on screen.

Okay.

Okay.

Then, the beauty of that is if you do that, the theory becomes renormalizable.

And so it fits in and with all the other theories that we don't, as renormalizable quantum

field theories.

And that was shown by Stella back in the 70s.

So it's, it's singled out as a particularly nice theory, a very conservative theory where

the metric is still the, the main degrees of freedom, the theory is renormalizable.

And it's, it's essentially unique in that.

So this is, this is the theory that were, that I have done some work on in the past

10 years or so with Gabriel Menon as my collaborator and others in the field that are, are

also exploring this.

There's a butt here.

There's a butt.

Okay.

And the butt is that the reason this works is that you, the curvature is, it involves

two derivatives of the metric.

The curvature squared then involves four derivatives of the metric.

And when you do go from two derivatives, the four derivatives, things happen that are

part of the usual axioms of quantum field theory, the unusual axioms, the requirement of

analyticity restricts you to two derivatives.

When you go to four, then something falls apart.

And there's been a lot of debate on what it is, it could be unitarity, it could be causality,

it could be stability.

My sense is that at least in path endocratization, the theory is unitary, it's stable, but it

does give up causality at high energies.

And the reason for this is that analyticity, this assumption of analyticity is also tied

into the usual requirements for analyticity.

Wait, analyticity is required for analyticity.

You mean for causality?

Antelasticity is required for causality, I'm just saying.

Okay.

So are you referring to ostracratsky or something else when you say that there is stability

itself?

So the stability is ostracratsky.

So the there's a classical theory if you put higher derivatives in exhibits and stability,

which was shown by ostracratsky almost 200 years ago.

However, there's a reasonable number of ways that we feel that that is not true in

the quantum theory.

Interesting.

So I think that ostracrats, the instability is not an issue.

There seems to be a stable theory.

One indication is that these higher derivative theories can be simulated on a lattice without

showing its abilities.

As far as I can tell, the theory seems to be stable against decays to be stable, to

lower energy states.

It does require a particular sign for a coupling constant.

So there are three coupling constants in the theory.

They have to take particular signs.

And one of the signs is particularly interesting.

And the reason they have to take signs is partially the stability issue.

There are new states in the theory.

And if you choose the sign wrong, you get tachyonic states.

And those ordinary matter will just decay into infinite number of tachyons, and you'll

be unstable.

So there is a sign choice involved.

Why don't you give the viewer an understanding and intuitive understanding of quadratic gravity?

Because when you have just the richi scalar and you vary it, you get the Einstein equations.

And that has some intuitive picture that people like to espouse of geometry and geodesics

and so forth.

So what changes when you add these quadratic terms to the intuitive picture people have of

what general relativity is?

Well, so low energy should be the same picture.

These quadratic terms are negligible at low energies.

So everything that we've tested experimentally would be the same.

The trick only happens when the curvatures or the energies are very large.

And then the fact that the curvature involves two derivatives, curvature squares at val 4,

derivatives turn into energies, meaning that if you have high curvature or high energy,

the other terms dominate.

You would still have a geometric theory, but it would be satisfied different equations.

There would be fourth-order differential equations.

For the quantum theory, the important part of that is that normally with two derivatives,

propagators for particles go like one over the energy squared, four derivatives, it goes

like one over m, two to the fourth.

And so what changes is that all the high energy stuff becomes better behaved because the

one over energy of the fourth kills off high energy divergences and makes things renormalizable.

And so the fact that there's four derivatives is crucial to making the high energy theory

well-behaved.

So in contrast to general relativity, where the high energy theory has bad behavior, here

the high energy theory has good behavior, just like QCD, for example.

What about singularities?

It doesn't give any insight into singularities, or black holes, or the big bang, or what

have you?

It's a good question.

I don't think the answer is completely known at this stage.

It's not even clear that the question is well posed because at high energies, it's

not clear that you should be using a simple classical picture, and the singularity theorems

are all classical theorems.

There are work by Bob Holdenman, Sally, and collaborators that show that there are non-singular

black hole solutions that come out of the quadratic gravity case.

So there's not a clear understanding of which one should be selected, but certainly there

are non-singular solutions to quadratic gravity.

Interesting.

Earlier when you said that general relativity can be formulated as a force theory, not

just as a geometric theory, like Wheeler suggested, are you referring to it classically

or are you referring to it quantumly?

Both.

How can it make sense as a particle theory classically if it's a sealed theory?

So for example, there's now a whole body of a whole subfield where they treat black hole

mergers using quantum field theory techniques.

And basically, there's a classical limit that you can take of doing climate diagrams,

and you could pick out the quantum effects of the classical effects, I'm sorry, of

black hole mergers.

It started with work by Goldberg and Rothstein, and has now become a much larger field,

including some of the people that use standard relativity techniques for a long time.

And very quickly, you caught up to the other techniques using differential equations.

Equivalently, just as you said, that GR can be formulated as a force theory versus curvature

theory.

I'm sure you've heard that you can add torsion and then formulate general relativity

as a torsion theory, it's like carton theory, or you can turn on non-metricity and with

metric of fire.

So there's all these different little tweaks that you can do to GR to get an equivalent

theory.

Is there anything about quadratic gravity that would rule out torsion or non-metricity?

No, I don't think so.

I haven't explored the extensions that much, but the extensions using torsion and non-metricity

do exist and people have done some explorations.

I think it's a great exercise.

For me, quadratic gravity is the simplest version, and it's in some ways the most conservative

because it doesn't do anything beyond what we know we need.

We already know that we would expect a curvature squared terms to be in the action.

And so we're just using them in a minimal way.

I'd like to talk about causality.

Okay.

So you have a phrase that they're dueling arrows of causality.

That could be yours or your collaborator, Menennaz.

What did you mean by that?

That they're dueling arrows of causality?

So this is what I referred to earlier in that, that you're, when you do this theory with

four derivatives, any theory with four derivatives, you give up some portion of the usual axiomatic

field theories.

And it seems that at least with path in our quantization followed the usual pathway,

what you get is you're giving up some aspects of causality.

And that aspect is the following.

There is an arrow of causality that's built into our theories.

And that that's contained in the factors of lie in the quantization procedure.

If you do path integrals, it's e to the i s, e to the i types, the action.

If you chose e to the minus i s, you would actually have the exact same theory, but just

the different arrow of causality, things in positive time would influence things the

later negative time.

Basically, time reversal changes i to minus i, it's time reversal's anti-unitary.

And so if you take e to the i s and do time reversal on it, you get a time reverse theory

that's exactly the same.

And the nature of the theories with higher derivatives is that you get a mass listed in

case of gravity, a light particle with the usual arrow of causality and a very heavy

particle with the opposite arrow of causality.

So if you produce this guy, if you come in with positive energy, it propagates not forward

in time like normally does, but it propagates backwards in time for a short period of time.

It decays it, you then get normal particles out, but you've had a short period of time

where this thing propagated with a different direction in time, which is a bit, takes a

while to get wrap your head around, but at least I've come to peace with it.

Why does it take some time to wrap your head around?

Well, so we're used to the idea that particles can propagate to the left particles, can

propagate to the right when they're carrying positive energy.

However, the standard theory is if you have a positive energy particle that only propagates

in one direction in time, propagating in the backwards direction in time doesn't normally

occur.

And that is because we use E to the IS as the, as the, in the path integral.

It's equivalently if you do commentators with IH bar as opposed to minus IH bar.

And so we're not used to seeing, in this case, when you decompose the interactions, you

get some, the light particle with E to the IS and the massless, the heavy particle, the

ghost particle with E to the minus IH.

And let me make a caveat just to be, to be honest, the, the theory that has this, seems

to work, we've been able to do one loop calculations in it.

I think it's still not clear how what does higher loop calculations.

And we don't know for sure that it works at all orders, which is one reason for exploring

lattice calculations.

And so I, I shouldn't say that it's known to be without flaws, but it's known at, at some

order in perturbation theory.

Does the S matrix assume causality?

So the S matrix defines a transition from states that, stable states in the past, the

stable states in the future.

Right.

Well, if light cones are somehow fuzzy, then that's what I'm wondering is what would become,

what would be considered in versus out how do you define the S matrix?

So in this case, all these heavy particles that have the funny propagation are unstable.

And so they don't live in the distant past or the distant future when you do define

the S matrix.

They just live in its intermediate states.

So you go through a region, so you imagine during a scattering, you take usual particles

in, you scatter them, some, some have residences that go to heavy particles that are normal

particles.

Some have these funny ones that propagate backwards for a short period of time.

But then you end up with all your normal particles in the future.

So the proof that I view to tell you that Gabriel and I did, it makes a different, an important

feature that only normal particles live in the distant past and the distant future for

the S matrix.

Does this mean there is something non-local occurring at the big bang?

Um, I don't know how to answer that.

I mean, I don't know the answer.

We are trying to look at some of the implications for the early universe, but it's not really

clear how one defines the start of the universe in any theory.

We could describe it accurately after a certain phase where the physics that we know kicks

in, but I don't know what starts the universe.

We told Quantum Magazine that you're not convinced that quadratic gravity is the final

theory.

Right.

So, but that's much like the standard model in the sense that I also don't, the standard

model I don't expect to be the final theory.

In the case of quadratic gravity, it feels to me like the standard model.

It feels like it's, it's a renormalizable quantum theory that would kick in at some scales,

which may be the scale of inflation.

And that's for those scales, it would be a perfectly fine theory.

But at that level, we have the standard modeling, which has three separate theories involved.

We have a renormalizable theory of gravity.

That doesn't feel like the final theory to me.

It feels like something else is going to explain the origin of all of these theories.

And so I expect, or I would naturally expect that there's some deeper theory that explains

the standard model and gravity both.

And three, three theories of possibility, that's perfectly fine, but it could be something

else.

I'm actually a champion of a, of a crazy theory, which is Holger Nielsen.

Holger Nielsen had this crazy idea that, that at very high energies, everything happens.

So basically you can imagine everything you can imagine happening.

It's sort of, he calls it random dynamics, and that the reason why at low energies, you

only see certain theories is that some theories are protected and are able to have massless

fermions, massless gauge bosons.

So gauge symmetry and chirality protect, feels from generating masses.

So if you gauge bosons, it's probably well known chirality is probably less well-known

that for fermions, if you have chiral fermions, you have a left field by itself, it can't

have a mass.

And so his, his argument was that the reason we see at low energies, the standard model

with chiral fermions engaged to, it's the only ones that are protected at low energies

from generating large masses.

And therefore you get, you get, you won as she, two as she, three.

And by the same logic, you could, you could get a gravity theory that has, is protected

by a general covariance from having larger masses.

And so the, this is sort of the anti-unification, instead of unifying, you have everything

happening at high energies, but only some things living to low energies.

I just like it as an idea.

Okay, this is super interesting, because you're so conservative in terms of, you don't

want to posit more than necessary, but then over here you, you're extremely attracted

to something that is quite zany.

Well, this, this is a, a bit, yeah, it's a bit contrary and I have to say it's, it's,

we, we, if you search yourself for biases, one of the biases of the field is that,

that, it takes, you to fire.

And we don't really have any evidence for that.

There's no evidence for gauge unification at all.

Um, people normally talk about unifying electricity and magnetism, but that's not really

two gauge theories.

What you've done is you've identified the single gauge theory that describes both those

phenomena, you won.

And the electric weak unification isn't really unification.

You started out with ENM and a weak theory and you, you've explained it by having two

theories that mix as you two cross you one.

They're not unified.

They're just mixing.

Uh-huh.

So that we've, we've never really seen unification.

So the idea of, of unification could just totally be a bias.

It could be that the theory as we go up in energies is you one, S, you two, S, you three,

S, you four, S, you five, S, you six, S, you seven, or something else as you go up.

Um, it doesn't have to ever come together into a unified theory, a unified gauge theory.

Yes.

Okay.

So, so this, this, I'd like the, the random dynamics I did mainly just because it's,

it's an example of how our, our biases may be making us miss.

What is the ultimate I energy theory?

Most of my best ideas don't happen during interviews.

They come spontaneously.

Most of the time in the shower actually or while I'm walking until I had flawed, I would

frequently lose them because by the time I write down half of it, it's gone.

I tried voice capture before like Google home and it just cuts me off in the middle.

It's so frustrating.

Most of my ideas aren't these 10 seconds sound bites.

They're ponderous.

They're long winded and I wind around.

They're discursive.

They're five minutes long.

Apple notes, even Google keep the transcription.

There's horrible, but plot lets me talk for as long as I want and there's no interruptions.

It's accurate captured, organizes everything into clear summaries, key takeaways, action

items.

I can even come back later and say, hey, what was that thread I was talking about regarding

consciousness and information?

In fact, this episode itself has a plot summary below and I'm using it right now over here.

My personal workflow is that I have their auto flow feature enabled so it sends me an email

anytime I take a note.

Look the fact that I can just press it and it turns on instantly like right now it's

starting to record without a delay is extremely underrated.

This by the way is the note pro and then this is the note pin.

I have both.

Over 1.5 million people use plot around the world.

If your work depends on conversations or the ideas that come after them, it's worth checking

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Okay.

So at first when you were saying that there hasn't been unification, you qualified that

with gauge unification.

Yes.

So because the standard folklore that people hear is, well, there was the heavens and

the earth and Newton unified that with gravity.

Then there was space and time and then Einstein unified that with space time.

And then there's electromagnetism or electricity and magnets and they're unified and so

forth.

Yeah.

But those are just identifying the appropriate interactions.

It's not a unification of any interactions.

E and M, electricity and magnetism look like different phenomena, but we've actually

backed all you find out that it's really the same underlying theory, just one theory.

So if you want to use the word unify, it's okay, but it's not unification in the sense

that the particle physics field assumes is going to happen at high energies.

We assume that we're going to merge into some S u 5 or S u 10 or E a cross E 8 or some

unified theory with board degrees of freedom, but which has a higher symmetry than what we

have at the moment.

That's a beautiful, beautiful idea.

It's a great idea.

I'm not saying it's wrong.

I'm just wondering if that's a preconception that keeps us from seeing other options.

So you're not saying there's something inherently wrong with guts, with granny and a fight

theory.

You're saying that there are alternatives or maybe you are saying there's something wrong

with guts.

You know, one of the lessons of the LHC, maybe, that there is something wrong with guts,

because the reason that we, as a field, were so convinced that something was going to happen

out at the LHC, basically went back to a conflict with the idea of granny and a fight theory.

In granny and a fight theory, you have this very heavy scale and the symmetries broken.

The need is to have the electro-week theory happen at a lower scale.

But in a granny and a fight theory, that's a very unnatural thing to have happened because

you have the naturalness argument says that the electro-week scale would naturally be

very close to the grand-unified scale, because radiative corrections generate large effects

and would normally bring it back up.

So the naturalness argument led us to believe that there was some mechanism that prevented

that's protected the electro-week scale and kept it at low energies.

And this, for example, was supersymmetry.

The argument for low energy supersymmetry basically came from trying to protect grand-unified

theories from large radiative corrections and allow the electro-week scale to be very

much less than the grand-unified scale.

However, we've learned from the LHC that that doesn't happen.

There is no evidence of supersymmetry at the LHC.

And so if we really believed our arguments about naturalness, the conclusion should be

that there shouldn't be any high-energy theory that has a naturalness problem.

And so there shouldn't be grand-unified theories.

I'm just shocked because I entered into this conversation with you and probably you the

same thinking that this would be one of the most, not ordinary, but uncontroversial podcasts

that I've had, or podcasts guests.

In fact, I even started it by saying that you're known in a sense for the radicalness

of you if there is any radicalness would be that you are not radical in a place where

you can see your colleagues as engaging in some speculation where it may be unwarranted

in your frame of mind.

Okay.

And now we're getting to something that philosophers called Enthonyms, I don't know if you've

heard this term, Enthonyms.

Not particularly, but.

An Enthonyms is an argument with an unstated premise.

So it sounds to me like you're attacking some of these unstated or hidden, there are

these assumptions that are so widely held, like that grand unification is going to occur

that we have supersymmetry and that unification, even that, the fact that you say that unification

also may be something to be questioned, but you're talking about a particular kind of

unification.

A particular kind.

That's super interesting.

I want to know.

And this is something that we should be doing.

And in a way, it actually relates to our starting topic, which is one of the unstated

assumptions for decades was the general to the quantum mechanics were incompatible.

I, you know, one of the things everyone should be doing at every step of the way is, is

questioning these assumptions and seeing if they're based, in fact, or are something

that's keeping us from, from progress.

And in the case of the general relativity, the effective field theory techniques told

us that that assumption was, was in fact incorrect.

And so we're just continuing to do that, you know, the quadratic gravity is questioning

whether an assumption of analyticity and causality is valid at all scales.

So I'm doing it again with, with unification.

What if a string theory says, look, string theory uniquely gives a finite two to two

graviton amplitude at all orders?

Is that not evidence that string theory is on the correct track?

Um, well, it does that open sub-assultions.

Um, I, you know, I think if it's, if it really was proven to be unique with absolutely

no exceptions, then, then it, that's a powerful statement.

I think that's unlikely to happen.

I mean, the one of the assumptions of the string theory program is causality holds at

all scales.

And so if I give up causality, I could probably, hmm, get a perfectly fine amplitude at all

energies.

And again, some of the assumptions that we've made at high energies is the, are built

into the theories that we're constructing.

And they may be correct.

I'm not saying that it's incorrect and it's, but it's also possible that there are alternate

ways of, of completing a high energy theory that I would expect is not unique to string

theory.

What do you have advice that you consistently give your students?

Um, most, the biggest advice I'd like to give by students was to read widely and think

about things outside of their own research areas and practice what I say is through, you

know, when you're reading papers, read the, the introductions and the abstracts, um, so

that you get a sense of why people feel things are important problems and what, what approaches

they use.

And then just to keep thinking about things outside of your own research area, I'm basically,

you're not going to be doing your, your thesis topic the rest of your life.

You know, the, you, everyone is going to move on from the thesis topic and they have

to find the richest pathway to, to having a, a long career, you know, so I started working

on the cork models and did up on quantum gravity, so it's quite a distinction.

Hmm.

So when you say read outside your research area, you don't mean to say that if your physicist

start looking into biology, well, if that interests you, I think you could, um, uh, if

you, there are many people that have made interesting transitions from, from physics,

theoretical physics to biological physics.

If that's a, interest of yours, I think that's a wonderful thing to do.

I, but I do tend to mean if, uh, if you're working on electroweak phase transitions that

you should also read up on, um, grand notification and string theory, you know, get at least

a, a basic understanding of the whole field as a whole, your particular research topic,

you're required as a student to become an expert on a very particular topic and you really

need to do that, but you also for future success need to look outside of that.

And I think many successful theorists have, do that early in their career too.

One of the things that I've always done when I want, on search committee's looking

for faculty members is to see if they've done anything that's not exactly their thesis

work.

So, you know, you, they have lost them are said to be one of the top people on subfield

x, y or z.

But then many, many people also have also written papers or had little collaborations that

were outside of that.

And that's always a positive sign because it shows that they're interested and able

to contribute to more than one, one area, uh-huh.

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Professor, are there any lessons that you wish you'd learned sooner in life?

In some ways, I guess it would have been nice to put my own advice into action a little

sooner in that working on some of the other areas earlier in my career, I think, could

have been fun.

But then again, I don't regret anything particular, so, you know, I probably could have done

the effective field theory six or seven years earlier.

Now, I've gravitated six or seven years earlier.

I basically knew the general principles at that stage from doing effective field theory

work.

I just, to do the correction to the Tony Potential, I knew was going to be a very long

calculation with a result that not able to be measured, and I was pretty much a phenomenologist

at the time.

And so I'd like to do measurable things, so I put off doing it until one stage, I decided

I just needed to do it.

But I could have done that six, six years earlier might have been fun.

What idea of yours has faced the most resistance?

Well, I was on the Sandtropic bandwagon for a while, and the Sandtropic stuff has tremendous

resistance by large numbers of people.

But you are no longer?

I know you had a 1998 paper on it.

No, I don't, I just don't see anything that I could usefully could do at the moment.

I sort of read out of useful things to do.

I still think it's a one of the pathways to a fundamental theory that makes a good deal

of sense.

I get it.

So it's part of this questioning the hidden assumptions that we're doing is that we

grew up thinking that there is a unique, a unique theory, a grand unified theory in particular

that was going to explain everything, give us all the masses and a couple of constants

of the theory of that we see in nature.

And the anthropic ideas make that actually seem very unlikely, just such a small portion

of parameter space that is consistent with existence of stars and atoms, that it seems

to be almost a very unlikely possibility.

Speaking of hidden assumptions, are there any hidden assumptions that your colleagues

or perhaps even your students have questioned that you said that's a bit too far?

I wouldn't question that.

My attitude on that is I would probably never say it, but I ask if they can do anything

useful with it by questioning it, if they would like to give up four dimensions, fight,

do something with it.

So I guess I wouldn't ever say, not out loud, they don't go there, but I'd say whether

it's a profitable thing to do to suggest.

Sure, it's been an honor to speak with you.

Thank you.

Very good.

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