These are awesome. I’ve listened to probably over a 100 vids on physics, but these go a level deeper, with the actual physicists debating and explaining their ideas in a highly technical yet still understandable frame. And there’s host interjecting at appropriate moments to summarize and clarify a point when it gets too condensed. Kudos!

If patience was a field, Gross would be a particle

Regarding the Higgs particle in Loop Quantum Gravity (at around 1:05:00) I've done two articles regarding that (and also my thesis). The short answer is that not only you can define spontaneous symmetry breaking in loop quantum gravity, but you can arrive to a theory which does not require a Higgs particle at all to describe massive vector fields. Since youtube dows not like links in the comments I will provide the titles of both my articles and my thesis. If you google them you will probably end up with very few results. The Proca-field in Loop Quantum Gravity Spontaneous symmetry breaking in Loop Quantum Gravity Massive vector fields in loop quantum gravity

1) First I want thank skydevil for publishing this video. 2) I wish that David would let Carlo speak from time to time. I want hear what he has to say. David seems to be a bad listener, a big shame. If he did not want to dialogue and only give a conference, just organize a conference for David. This is very disappointing. David speaks for Carlo. He does let not Carlo speak. That is certainly bad manners. David does not let Carlo finish. It looks like a political show, "vote for me" kind of game, and very little interest on what Carlo has to say.

I was somewhat disappointed that David Gross did not allow Carlo to fully explain his full points before diving in. I think this should have been done by Phil, allowing each person to present and counter separately and then finally debate. Carlo was impeccably polite and complimentary as ever.

It's humbling to see that a discussion between two of my favourite minds in physics can resemble my Dad and I talking about politics at Christmas.

Gross is defensive of string theory because it forms part of his ego. It's hard to be objective when your identity is so highly entangled with a given framework.

I never knew there is this much tension between loops and strings, discussion was hard to moderate seemingly. My sympathy goes to Rovelli, who at least tries to keep discussion open, whence Gross is more on the established convention and declarative side of authorian science philosophy. That feels so last century in the current potentiality of debates we could have. But, I enjoyd and decided to listen a few more times and share to my friends interested in topics.

Good job Phil. I feel we need more such civilized debates of polarized issues in society. These guys definitely need to talk, and LISTEN to each other, more often. My current appreciation of both sides is that they are elegant storytelling. It will be interesting to see how it all unfolds in the future…

This was amazing. Phil was really good at managing the time. I loved David’s expressions he got so frustrated at times. Carlo was so polite.

If Prof Gross could let Prof Rovelli finish a whole sentence that would be nice

Gross surely is a giant of physics but he should learn how to have a fair discussion letting the interlocutor speak

Thank you Phil for bringing this debate to us. I enjoyed it a lot but as a physicist i would have preferred you to let them go more into detail! Cheers

Carlo: calm, not fidgeting, listening more than interrupting, clear about the limitations of the theory he works on. David: irritable, moving unhappily, constantly talking over panelists, admitting almost NO criticism of StrnThr. One looks to me an open minded scientist, the other closer to “scientific arrogance”.

I loved the comically barbed compliments of each other at the end: Carlo compares David to Einstein who was famously productive when younger but lost his way a bit in later years, David compliments Carlo on his endeavours in physical theory but says he is more impressive in his philosophical travails. A wonderful podcast indeed.

Great job as an interviewer and mediator. But it would be nice if you allowed them to dive deep into the technical stuff so that we the layman can research further and learn. Thanks

Wow great to see Gross & Rovelli go at it, thank you to all involved. It's interesting to see 2 very intelligent individuals have such different creative ideas/opinions about physics. I think such a huge & complex subject is hard to squeeze into 1 video. Perhaps a more structured format would of been better with it split over a few videos but i really enjoyed it thank you.

Wow you are getting some amazing guests lately! Love the debates! If I may attempt to constructively critique: you often asked the guests to “explain X bc people won’t know what that means”. 1) this wastes valuable time. It results in less info and less depth. We want to know what these titans think about deep things; we don’t need them to spend their time giving definitions off the top of their head when people can look up the precise definitions online. I imagine others are like me and are used to pausing to look things up and we want the nitty gritty technical conversation. 2) if someone really doesn’t know what a topic is, will a 10 second definition really be helpful for them to properly follow along? I think the majority of the time doing this is disruptive to the flow of the conversation and pointless in a time when we can pause YT and have Wikipedia access to get a general idea of any topic we are unfamiliar with. And for every person you are helping with these “explain X” stops, I wonder for how many of your viewers you are wasting their time.

Very cool format. It was great to see them squirm in their seats trying to let each other talk! Love the tension. Its a wonderful change from the expert panels where everyone agrees.

I came to this discussion since I follow Rovelli on social, so I’m biased towards his position. This was thoroughly run in the spirit of inquiry, IMHO., and thank you for always attempting to bring it to layman’s terms Thank you so much for organizing this stellar discussion- even of it did get Saturnian in parts, it’s been wholly illuminating 💡

A very fundamental problem in quantum gravity is that the action in GR depends only on the energy/matter/momentum distribution, so when you have to consider all fields and physics going on to solve an isolated problem like quantum gravity in absence of a precise description of the fields and stuff in space you run into problems pretty fast, there is no way to separate what happens at high energy with gravity from what happens at high energy in qed or qcd for example, they are attracted at the hip in a tricky way.

Thanks for this, I’m hoping to get into quantum mechanics later in life (I’m only 17 right now) and it’s great to see there’s plenty of discussion to be had if I get there. Also I think it’s great that you try to rephrase things in a way that the public can understand. I get that as a physicist it’s probably easy go down a rabbit hole of complex jargon but it doesn’t always make for good viewing. I hope you keep it up in the future :)

So nice to hear them both talk about their path into physics and what woke their passion.

Despite the at times unruly disagreements the honesty of the arguments is unquestionable and shines through brightly, which made this an amazingly helpful discussion. Very well done for promoting such an interesting and valuable debate (Thank you from Portugal).

Professor Rovelli is such a gentleman.

Thank you for taking the time to put this debate together, may be a follow up with Carlo Rovelli would be interesting. English is not his first language which allowed David Gross an advantage. David should learn to turn off his email notification chime. It seemed a little intrusive.

David just didn't understand that Carlo says that LQG is trying just to quantize gravity and doesn't turn off gauge bosons or matter and so on because it's just like Su(3) that's not unified to the electroweak part of the standard model. As a result LQG is consistent at this level. The other point is that there are arguments , very strong arguments, that LQG reproduces the classical limit of General relativity. So I agree totally with Carlo that string theory it's just useless. Wonderful mathematics but isn't supported by evidence in a concrete way: cosmological constant is positive and not negative, there's no supersymmetry that has been proven and a lot of problems with compactified dimensions. Calabi-Yau manifolds gives 3 complex dimensions, i.e. 6 more real dimensions, to add to the usual 4 dimensions of our space time that haven't been detected ever and probably will not be.

Interesting points. I wish you prevented David Gross from interrupting Carlo so often though.

Excellent. For once a proper debate on string theory and to a lesser extent a good discussion on strengths and weaknesses of loop quantum gravity.

I am surprised about the 80-year-old man David Gross never misses a trick on these points. Excellent discussion! Tnx

Great discussion! Kudos to Phil for his knowledge and moderation skills!

Thank you Prof Gross and Prof Rovelli for this exchange.

Great debate! Not as high quality as the Guth/Penrose one: as many comments noted, David was monopolizing the time, but that was to be expected from him; he probably does not realize that this actually does a disservice to his point of view. I also support the comments that it was not necessary to get them to explain "for laypersons" some of the terms they used; this just took up unnecessary time. In any event, looking forward to more debates of this kind.

I can't claim to even come close to understanding this, but enjoyed listening to these scientists debate their ideas

It was one of the most enjoyable discussions in physics I have ever listened to. Really awesome idea and I would only hope that it will be continued in the near future

Thanks for hosting these discussions. I love the channel. It must be hard trying to moderate the conversation with such big egos involved!

Thankyou Phil for bringing together two great explorers in this talk:)you are working really hard to bring us these kind of talks, thankyou:)

Thank you for these videos from actual physicist much more clarity than popular KZfaq videos trying to explain the issues within both

I have an amateurish question: If there is a smallest distance and space is "pixelated", is there a force that binds those pixels together? Like the strong force with quarks?

Very valuable. A bit too heated at moments; I don't like interruptions, even interruptions of interruptions; maybe a chess clock would help. Apart from that, good job - a good topic, a good choice of speakers and good interventions like requests to outline technical terms (e.g. holography, AdS, CFT, renormalization etc).

It was a nice debate. Thanx for this incredible video, appreciate it a lot!

In my opinion the main advantage of Quantum Loop Theory over String Theory is that QLT ist (gravitational) background independent and String Theory is not !

My favorite subjects outside of making music...awesome vid skydive!

This could have been great, but David did not allow any of us to really hear more about QFT. Carlo had to keep defending it, which did not allow me to learn what it was.

Note that the Cosmological Constant "problem" is only a problem from a QFT perspective. In General Relativity its value is simply not surprising and it is not particularly. small. (The reason for that is that it is a _constant_ in GR and so in an expanding universe, where other densities shrink with time, its role will grow until it starts to dominate.)

Wow, that will be great! I'm eager to watch it.

Love this channel for the amazing content! Thank you! Onward, ever onward!

I am watching the discussion after the fact (not live). As I listen to Prof. Gross say that string theory is like QFT, I am confused because my understanding is that string theory quantizes one dimensional objects (strings; position as function of parameter along the string and time) whereas QFT quantizes fields (field value as a function of space and time). In QFT a particle is a "wave" on top of the vaccum state of the field. On the other hand, I haven't seen strings referred to as excitations on top of some field. Also, in QFT there are virtual particles. Are there virtual strings?

Despite the session moving forth on a bumpy road, the conversation was very insightful to both sides for a complete layman like me.

If this were a poker game, then I would be convinced that Gross is bluffing. If they made a huge bet against me at the poker table, then, based on body language “tells” alone, I would call Gross, but fold to Rovelli. (Meaning, I'm more inclined to believe that Rovelli's bet is honest, at least in his own mind.)

David Gross ruined what could have been a great discussion with his pontificating and filibustering.

I found myself in a quantum loop of looking for knowledge finding Carlo’s book The Order of Time, and now this interview.

Its amazing we can hear Nobel laureates like this. Mind blowing.

Around -1:00 Prof. Gross says that AdS allows one to study "things in a box." But isn't the Minkowski background also homogeneous and isotropic? Of course I don't know the details so I can't tell what is so special about AdS that allows quantum gravity within it to be equivalent to a CFT in one less spatial dimension. In any case, conformal means "no length scale", right? But as far we know, there is a minimal length scale, the Planck scale and even in holography (the entropy of a black hole) the Planck area emerges.

Even if one gave them an own youtube channel I do not think they would have soon come to an end; great moderating skill! Nevertheless, one or two follow-ups seem to be required.

Great to have Skydivephil back. Debates are nice, but I prefer your pithier and more straightforward didactic pieces. I hope you get them up and running again soon. The graphics help along with the narrative

Constructing the standard model in an LQG background is just a nightmare. Part of the reason is how LQG treats spacetime on quantization. Putting a quantum field on such a spacetime then requires additional non-local corrections to restore Lorentz invariance. While perhaps one can still force their way through but I believe inconsistencies won't really stop popping up. The construction itself does not feel natural. Strings on the other hand is surprisingly tightly constrained and yet works without putting anything by hand into the theory. It just feels as natural as QFT in its respective regime. Moreover string theory actually replicates features of standard model in surprising detail even without heavy analysis. Yes there's the landscape, but it can actually be shown to contain models with a cosmological constant as small as ours using Flux compactification. For those who don't know, compared to the Planckian units the observed cosmological constant is smaller by 10^-123. That's an insane small number and honestly has every right to just not be present had the number of string vacuas been small in the landscape given that string units are closer to Planckian units. But because of the large number of vacuas it does exist. Counting the microstates of blackholes in string theory actually gives the correct (in fact exact) value of entropy to zeroth order as was shown by Hawking and Bekenstein. Granted this has quantitatively been shown only for BPS blackholes but that is by no means a small feat. The same can't be said for LQG. The original calculation of blackhole entropy had to be done by fixing the Immirzi parameter and in fact by cutting out the entire interior, which sort of beats the point for me. I haven't checked the new calculations so can't comment on that. Out of all this I feel the biggest success of string theory has to be the deep understanding of dualities (AdS/CFT and holography being just one derived example). Via compactifications of a newer more powerful formulation of type IIB strings, called F-Theory, we can now study dualities between different types of quantum field theories, one of which the Montonen Olive duality, was predicted independent of string theory in 1977. Of course there's more application of dualities, specifically as David said in the holographic formulation of QCD. Another field which is at its infancy but I believe might have something of substance is a proposal of trying to understand deep learning using AdS/CFT. All of this would have been impossible without string theory. Finally supersymmetry isn't something string theorists (or even particle phenomenologists with realistic expectations) expect to observe at TeV scales of the LHC. String scale as I said is close to Planck scale, so the supersymmetry of strings lives in the Planck regime. I can keep on writing but I guess one really needs to study it for themselves to truly understand the essence of all this.

As a layperson all I know is the jargon. String theory does not begin with Einstein's equations but yet produces a spin 2 massless particle, the graviton. However string theory's description of gravity is perturbative (around the Minkowski spacetime background) while LQG is "background independent." String theory incorporates all particles and interactions of particle physics, while LQG is about gravity/spacetime alone (though matter fields can be incorporated in the QFT way). String theory invokes supersymmetry (susy) which is under pressure due to lack of finding it at the energies of the LHC. However, susy may appear at higher energies or longer-lived decays. In some aspects strings and LQG seem to converge: the AdS-CFT correspondence and the Ryu-Takayanagi conjecture which pertain to a non-perturbative special case in string theory (??).

It was a very nice video. Thanks a lot.

its 23.30 here in India... let the fun begin..

A very illuminating discussion. Thanks for putting this together.

Super conversation, thanks.

This was a fascinating discussion to me, and I liked the occasional explanations (I have basic scientific background but not to this level ^^). BTW, Carlo mentioned the time from the beginning of "strings" to now, but I think he could also talk about the time x number of people involved. It seems string theory has been pretty prolific in those terms (at least a lot of money has been assigned to researching it, hasn t it ?)

Foam is made of bubbles and bubbles are spheres. If you imagine spheres being inflated at the speed of light, and overlapping, then you can use such a model to describe spacetime.

Quantum Entangled Twisted Tubules: When we draw a sine wave on a blackboard, we are representing spatial curvature. Does a photon transfer spatial curvature from one location to another? Wrap a piece of wire around a pencil and it can produce a 3D coil of wire, much like a spring. When viewed from the side it can look like a two-dimensional sine wave. You could coil the wire with either a right-hand twist, or with a left-hand twist. Could Planck's Constant be proportional to the twist cycles. A photon with a higher frequency has more energy. (More spatial curvature). What if gluons are actually made up of these twisted tubes which become entangled with other tubes to produce quarks. (In the same way twisted electrical extension cords can become entangled.) Therefore, the gluons are actually a part of the quarks. Mesons are made up of two entangled tubes (Quarks/Gluons), while protons and neutrons would be made up of three entangled tubes. (Quarks/Gluons) The "Color Force" would be related to the XYZ coordinates (orientation) of entanglement. "Asymptotic Freedom", and "flux tubes" make sense based on this concept. Neutrinos would be made up of a twisted torus (like a twisted donut) within this model. Gravity is a result of a very small curvature imbalance within atoms. (This is why the force of gravity is so small.) Instead of attempting to explain matter as "particles", this concept attempts to explain matter more in the manner of our current understanding of the space-time curvature of gravity. If an electron has qualities of both a particle and a wave, it cannot be either one. It must be something else. Therefore, a "particle" is actually a structure which stores spatial curvature. Can an electron-positron pair (which are made up of opposite directions of twist) annihilate each other by unwinding into each other producing Gamma Ray photons. Alpha decay occurs when the two protons and two neutrons (which are bound together by entangled tubes), become un-entangled from the rest of the nucleons. Beta decay occurs when the tube of a down quark/gluon in a neutron becomes overtwisted and breaks producing a twisted torus (neutrino) and an up quark, and the ejected electron. Gamma photons are produced when a tube unwinds producing electromagnetic waves.

Should I be under the impression that a lot of Gross' criticism is outdated? Like the low-energy limit, fermions etc, which Rovelli said now have solutions in LQG, but sadly that idea wasn't expanded as well as stringy ideas such as the holographic principle for example. This didn't offer too satisfactory of an explanation of why they're against LQG since Rovelli's response to some of the most important objectives was "no, we now have a solution" or something like that. Okay, any further criticism on that solution? I wish they went into spinfoam theory at least a bit too, since apparently that's crucial to understand why LQG makes sense or potentially why it doesn't.

A funny thing to note is the fact that Rovelli's approach is that of a physicist who builds theories starting from solid foundations, the methodology used to build the mathematics that describes the LQG stems from a canonical quantization. The solution of the loops is the solution of the wheeler dewitt equation which gives a quantum interpretation to the GR. David Gross says instead that string theory arises from a mathematical formalism that solves the problems of infinity that you have working on a small scale, turns a mathematical tool that solves a calculation problem into the model of a theory, the funny thing is that Gross speaks of a classical derivation of his theory, but this is not true. The funny thing is that he doesn't believe it either, in fact he repeats it over and over again like a mantra. Congratulations to the moderator who directed the discussion flawlessly.

great conversation but damn David seems like such an opp. like he was just laughing and shaking his head whenever carlo was talking, not even open to what he was saying. i guess it's difficult for both of them to consider the merits of another theory when you've devoted decades to yours but idk david's attitude in particular was annoying guess im biased cuz im reading carlo's book Reality is Not What it Seems right now and that shit is fire. LQG for the win! let's see some evidence of black holes exploding babyyyy

Fascinating discussion. I have to say that Mr. Halper did a yeoman's job attempting to keep the debate on point, particularly with Mr. Gross's constant interruptions. His body language, in fact, was horrible. Mr. Roveli, by contrast, was far more gracious, and eventually did become frustrated. I think decorum would have gone a long way to make the debate even more informative and applaud Mr. Halper for his efforts to do just that.

Brilliant conversation thank you

Great discussion! It shows how much the communication between these communities is needed. I would like to see an episode devoted to noncommutative geometry, yet another very promising idea which extends the notion of the space to encorporate forces from Standard Model

Well handled, Phil! With regards to criticism, I felt that David was more equipped with established mathematical results of string theory and Carlo responded to criticism with just ideas that are maybe in research but not in the literature.

Carlo so feisty! Bit tense sometimes 😅 but sort of a little universe of science, intellectualism, passion, career, ego, culture, legacy (and oh yes physics and unified field theory) contained in one video. Great match up producing a seminal discussion that delineates the issues, conflicts and even confluences of these two, er, loops/strands of theorists.

amazing debate... enjoyed a lot

I am really looking forward to this discussion. Nonsense type K12 on one side of the discussion and nonsense type B7 on the other side.

I think that saying frameworks are trivial is kind of a silly point to argue, its a bit like saying an instrument is trivial to make but tuning it is the difficult part, and yeah as long as you never tune the instrument it doesn’t matter if the sound is shit, but you need the right instrument to play the themes and sounds that go into a piece of music, building a framework is a bit like building an instrument before you know how to tune it properly, and both string theory and loop quantum gravity tries to do that very same exercise, all frameworks work like this initially and maybe some of them are flawed or can’t be tunes just right by simple interventions, but the exercise itself is necessary either way.

Darn, I know its hard to moderate but I just wished you would have given David more time to make his points.

Excellent moderation

Hi, Phil! :) Can I ask you a little favor? I'm planing to make a high-quality video (2n part) defending Vilenkin's model of tunneling from nothing against critiques by apologists and others, and there is a paper criticizing his model which unfortunately wasn't mentioned in your video (Before BB 9), and this paper makes some strong points which need to be addressed if the defense is going to be complete. A long time ago I asked Vilenkin, but he did not reply (probably because he doesn't know me). But you know him, so would you mind briefly asking him what are some of his thoughts on the paper? I would appreciate immensely! The paper is _Comments on A Universe from Nothing_ by Ikjyot Singh Kohli.

Finally got through it! =P Good grief! Like slogging through waste deep mud. Need to go through another time to try to get the meat of it, but extremely difficult to get it off the bone, to separate the wheat from the chaff...very convoluted. Highly recommend Jim Baggott's book, Quantum Space to shed light on both LQG & String theory, the hstory & evolution of both, and who's who. Very well written, engaging, a great read. 👍😜👍

I've learnt that who speaks too much has not really important to say ... BRAVO ROVELLI 👏👏👏

There was lots of good stuff in here, but it would have been better if david would have let carlo get a word in edgeways. i'm not entirely convinced that this was particularly approachable for people who don't know about a lot of the advanced topics in modern physics. there was a *lot* of terminology. even a couple of things i'm going to have to go and look up before i rewatch it.

Wish Carlo's patience to speak was more reciprocated .

More, please!

"The loopy guys" always crack me up

Gravity in absence of matter is just a lens, how do you quantize a theory that acts upon nothing? You don’t but even gravity without matter will give you a form of matter just by virtue of light interacting with light, giving the lensing effects, but beyond that its hopeless to go on because the matter is already interacting in the context of a fully fleshed out universe and most of the important details of a gravitational theory outside weak lensing in a classical are dependent on the details of the matter as it exists couplings and all.

Phil, thanks for this, great discussion

Very difficult to understand Carlo Rovelli...his English is fine, but the acoustics muddle his words. A headset microphone may help next time. But I must say, he rambles quite a bit, and needs to be more susinct. I.e. list the criticisms briefly, then explain each if necessary. BTW, I'm pro-LQG.

Great Conversation. With some grit.

Why did David not work on unifying QED and QCD?

Power vs seeking truth, - felt like a modern researcher talking to a powerful Pharisee. Sad really. Why do these two areas argue like this? Thank you Phil for moderating in a way that made this somewhat watchable. The discomfort of one of them was all ego, and saving face. Just angry power is not science, and neither can a prize protect, gatekeeping.

Great to see two giants brought together and debate. Thanks for organising this as I don't think I've seen something like this on other channels besides promoting a particular theory from an expert. Science needs debates to iron out conflicts to move forward and gain concensus. However, this interaction was more a cussing game rather than each side taking away something new. For example, David made constant allegations that LQG doesn't have matter (which I've heard many times from String Theorists), yet Carlo corrected him that he was mistaken. This correction wasn't elaborated, nor acknowledged from what I coukd see from David, and so came across to not be willing to change his views. At least this debate was a start, and hopefully more to follow that helps collaboration between cutting edge physics.

I wish Gross would stop interrupting Carlo.

I think neither David nor Carlo didn't like this conversation very much. It was unequal, uncomfortable. But very much valuable for non professional physicist to see the "conflict" in the physics.

One of the main arguments in favour of string theory that we're listening all the time is that " it has a nice classical limit: General Relativity", yet, in the case of the infamous firewalls many people from the string theory community were willing to abandon completely the equivalence principle that General Relativity is based on... The same also happens with the proponents of " fuzzballs". Isn't there an obvious contradiction? Its worth noting, also, that the violation of GR / QFT in the above cases is supposed to happen even in spacetime regions with arbitrarily low curvature! Also, their proposed solution to the black hole information problem massively violates relativistic causality/ locality, again, in regions that nothing similar is expected... There is not even the slightest observational indication that such violations are happening, or that will ever happen in our universe.

David Gross always interrupt Carlo Rovelli. He was not able to have enough chance to reply some criticism. He might have been annoyed but because of his respect to David Gross he did not show his reaction. By the way if you are making a discussion meeting for such a complex subject please expect the followers know enough terminology at least to follow the discussion. No need to explain terminology every time by interrupting the talks.

Hi from Portugal, amazing minds. ....

I think both of them did a good job in this conversation. I just happen to lean towards thinking David Gross has bet on the better horse here. For good reasons. But let’s all keep on working on the problems, following our noses and we’ll see where we end up.

I always thought that listening directly to the scientific discussions is the best way to learn things, but this time I really felt the need of someone to condense and dumb down the topic. Because in order to truly enjoy this debate we'd need about four hours of the actual debate and about 5 years of proper education on the subject matter. Also from purely mundane point it was difficult to listen to both speakers (for different reasons)

Excellent stand-off interview. One immediate comment: David Gross is “loopy” when he starts spouting “precise number of micro states of a black hole” and “the holographic framework is perfectly valid in describing the laws of physics” which are nonsense-speak. We remain without a valid quantum theory of gravity, and neither side has the upper ground.