The Breakthrough in Physics We’ve Been Waiting For

Summary

In an exhilarating discussion, Curt Jaimungal talks to Professor Ivette Fuentes about groundbreaking approaches to quantum gravity. They delve into the vital nexus of physics with quantum mechanics and general relativity. Fuentes elaborates on novel experiment proposals using quantum optics and ultra-cold atoms to test theories of space-time and gravity, inspiring a deeper look into long-standing theoretical disputes. Despite the challenges, her work represents a frontier for understanding the universe, challenging researchers to look beyond conventional theories and tech paradigms.

Highlights

• Galileo's story highlights the need to explore new paths in physics 🚀
• Professor Fuentes is passionate about merging quantum mechanics with general relativity using experiments 🌟
• She's exploring quantum optics with real-life experiments to test space-time theories 🎯
• Discovery awaits in quantum entanglement from Earth to satellite levels 🛰️
• Quantum clocks and Bose-Einstein condensates are pivotal in these groundbreaking experiments ⏰

Key Takeaways

• Galileo's telescope story teaches us to look beyond popular theories and explore new ideas 🔭
• Ultra-cold atoms and quantum optics could uncover the quantum nature of space-time ❄️
• Questioning string theory can lead to fresh insights and breakthroughs 🌌
• Advanced quantum technology is making groundbreaking experiments possible in space and Earth 🚀
• Entanglement experiments show quantum mechanics' vast applicability beyond small scales 🤯

Overview

In this lively discussion, Curt Jaimungal invites Professor Ivette Fuentes to explore her revolutionary journey in the physics world. Guided by the curiosity sparked by string theory, Fuentes navigated through various fields, eventually focusing on bridging quantum mechanics and general relativity using experimental physics.

Professor Fuentes shares intriguing insights from her work on quantum metrology and experimental proposals aimed at understanding space-time at quantum levels. Her work challenges the traditional perception of physics, encouraging a focus on experimental backing rather than mere theoretical frameworks.

Curt and Fuentes delve into the potential of ultra-cold atoms and quantum clocks in revealing new aspects of quantum gravity. Fuentes champions the exploration of uncharted territories, from space-savvy satellites to the core of atomic clocks, showcasing the evolving fabric of our universe.

Chapters

• 00:00 - 00:30: Introduction and Galileo's Telescope This chapter explores the historical resistance to new scientific tools and theories, starting with Galileo's invention of the telescope. It draws a parallel to modern science, where there is resistance to considering certain theories. A major focus is on the quest to reconcile quantum theory with gravity, suggesting that instead of more complex mathematics, the solution may lie in innovative experiments. The chapter mentions Professor Ivette Fuentes in this context.
• 00:30 - 01:30: Professor Ivette Fuentes's Proposal Professor Ivette Fuentes, a noted collaborator of Roger Penrose, is proposing innovative experiments with ultra-cold atoms and quantum technologies aimed at investigating the potentially quantum properties of space-time. However, these pioneering ideas are met with reluctance in the scientific community. This hesitancy may be attributed to the appeal of engaging in purely theoretical research, the momentum of existing research agendas, or possibly due to other underlying factors.
• 01:30 - 03:00: String Theory Critique In this chapter, the focus is on Professor Ivette Fuentes' innovative methods for testing quantum gravity, which are largely being ignored by the broader physics community. The discussion revolves around the challenges of moving away from prevailing trends and how these inventive approaches could impact the current understanding of quantum physics. The chapter sets the stage for an engaging discourse, highlighting the excitement of having Professor Fuentes bring her ideas to light.
• 03:00 - 04:30: Quantum Gravity and Experiments The chapter titled 'Quantum Gravity and Experiments' begins with the speaker expressing their appreciation for the host's podcast, noting that they are a frequent listener and viewer. This creates a sense of familiarity when meeting the host, akin to knowing them for a long time. The conversation captures the speaker's pleasant surprise and warmth upon realizing they are finally meeting the person behind the familiar voice and presence of the podcast.
• 04:30 - 06:00: Background and Career Path of Professor Fuentes Professor Fuentes is recognized for her in-depth discussions on advanced topics such as string theory. Her career reflects a keen ability to navigate complex physics concepts, much like the iterative problem-solving game 'whack-a-mole,' where solving one issue often leads to the emergence of another. She emphasizes the iterative nature of scientific inquiry, highlighting Einstein's struggle to align theories of acceleration and gravity with scalar fields. This chapter sets the foundation for understanding her academic journey, marked by problem-solving and theoretical integration.
• 06:00 - 07:30: Instrumental Role in Scientific Discoveries This chapter delves into the complex evolution of scientific theories, focusing on the developments and challenges in theoretical physics. Initially, the possibility of a variable speed of light is considered, which prompts a shift from scalars to tensors in mathematical modeling. The discussion transitions into string theory, initially requiring 26 dimensions, and highlights the introduction of supersymmetry as a solution to existing issues. Despite progress, challenges persist due to the existence of numerous types of string theory and complex dimensional requirements. The chapter concludes with the integration of heterotic strings as a partial solution, though gauge anomalies and other problems remain to be addressed. This narrative underscores the instrumental role of evolving mathematical and physical concepts in driving scientific breakthroughs.
• 07:30 - 10:00: Challenges in Unifying Quantum Mechanics and General Relativity The chapter discusses the challenges faced in unifying quantum mechanics and general relativity. It touches upon the social pressure within the scientific community to engage in this field, as it is often considered the 'only game in town.' The speaker mentions their active work on this unification problem, indicating it as a recent focus in their research, although they have yet to find a complete solution.
• 10:00 - 13:00: Pursuing Experiments in Quantum Physics The chapter titled 'Pursuing Experiments in Quantum Physics' discusses the ongoing results of certain quantum physics experiments. Although the full results are not yet available, the chapter highlights significant progress that has been achieved, which is described as an interesting step forward. The author acknowledges coming from a different background and suggests that their unique perspective adds intriguing value to the story. The chapter also mentions evenings spent reviewing related research, indicating a deep engagement with the topic.
• 13:00 - 15:00: Long-range Quantum Entanglement Experiments The chapter titled 'Long-range Quantum Entanglement Experiments' begins with a personal reflection from the speaker, who expresses excitement about their experience and introduces the audience to their work. The speaker invites the attendees to familiarize themselves with recent results in the field. The narrative delves into the speaker's academic background, recounting a formative experience with a quantum mechanics instructor named Luis de la Peña, who inspired the speaker's interest in the foundations of physics.
• 15:00 - 18:00: Time Dilation and Quantum Clocks The chapter discusses the speaker's interest in quantum mechanics, highlighting a significant encounter with a mentor who advised them against pursuing the field due to its challenges. The speaker was encouraged to focus on more mainstream topics, with the option to revisit quantum mechanics later.
• 18:00 - 21:00: Mass Superposition and Roger Penrose's Ideas The chapter discusses a conversation about taking different academic paths and the challenges associated with it. It highlights how a person, possibly an academic mentor, advised against following the same difficult path they took, indicating this as a gesture of generosity rather than discouragement. It also touches upon the speaker's aspiration to go to Fermilab.
• 21:00 - 25:00: Bose-Einstein Condensates and Quantum Experiments The chapter titled 'Bose-Einstein Condensates and Quantum Experiments' begins with a personal anecdote involving a student's ambition to partake in a summer competition. The student recounts their interaction with a professor when they requested a reference letter for the opportunity. Despite having excellent grades, the professor was initially reluctant, noting that many students achieve high marks. This opened up a broader conversation, reflecting on the student's journey and motivation as they approach the completion of their degree.
• 25:00 - 30:00: Proposing New Quantum Sensors The chapter discusses a conversation where the speaker shares their initial fascination with string theory, a branch of theoretical physics that treats particles as point-like systems, suggesting its beauty and initial captivating appeal.
• 30:00 - 31:30: Concluding Remarks and Future Directions The concluding chapter discusses the notion of multiple dimensions and string theory. The speaker expresses initial admiration for the unification of different particles through these concepts, highlighting their beauty. However, as they delved deeper, they encountered challenges, particularly with the concept of many dimensions. The speaker likens this to epicycles, suggesting a complex and potentially flawed explanation akin to historical astronomical models. This reflection forms the basis for discussing future directions and unresolved questions in the field.
• 31:30 - 33:30: Outro and Additional Information In this chapter, the discussion revolves around historical perspectives on planetary motion, emphasizing the ancient belief that heavenly bodies followed circular paths because circles were deemed the perfect shape. This belief illustrates how rigid adherence to certain ideas can hinder scientific progress. The chapter concludes by inviting further reflection on how such entrenched concepts can be reevaluated to enable advancement.

The Breakthrough in Physics We’ve Been Waiting For Transcription

• 00:00 - 00:30 When Galileo invented the telescope, people  didn't want to look through it, and that also   makes me think about a lot of the stuff happening  in science where people refuse to look at certain   theories. For decades, reconciling quantum  theory with gravity has been the holy grail of   theoretical physics. But what if the path forward  isn't through ever more convoluted mathematics,   but rather through ingenious experiments we could  perform right now? Professor Ivette Fuentes, the
• 00:30 - 01:00 close collaborator of Roger Penrose, is proposing  just that—groundbreaking tests using ultra-cold   atoms and quantum technologies that could probe  the ostensibly quantum nature of space-time. Yet,   despite its potential, many researchers in the  field are hesitant to pursue these ideas. Is it   the allure of purely theoretical work, the inertia  of established research programs, or simply
• 01:00 - 01:30 the challenge of breaking away from fashionable  thinking? In this episode, we'll explore Professor   Fuentes' inventive approaches to testing quantum  gravity and why they're being overlooked by much   of the physics community. Professor Ivette  Fuentes, it's a long time coming. I'm super   excited to have you on here. The audience, it's  going to be a treat for them. They don't realize   it right now, maybe, but the audience is in for a  great treat. So thank you and the floor is yours.
• 01:30 - 02:00 Thank you very much. I was just telling you just  now that I love your podcast and I listen to it,   see it very often, let's say. So it was very nice  meeting you just now because I felt that I've met   you forever. So this feeling of after seeing you  often in the evenings and then it's like, oh,   wow, that's you there. So that was really very  nice. And I was also telling you just now that
• 02:00 - 02:30 I saw some of the podcasts that you were talking  about string theory. Physics is like whack-a-mole.   Einstein said, I have this idea, acceleration and  gravity are the same. Problem, how do I make this   work with a scalar field? It's like a little mole  that comes up. He whacks it down. He says, okay,   maybe it was a mistake to unite space and time,  but then problem crops up. You have to introduce a
• 02:30 - 03:00 variable speed of light. So then he's like, okay,  let me knock that down. Forget about scalars. Let   me introduce tensors, a different mathematical  object. You knock that down. In order for string   theory to work, it needs to be 26 dimensional.  And they only had bosons at this point in the   story. Okay, why don't we add something called  supersymmetry? So we knock it down. Okay,   cool. Problem. There are so many types of  string theory. And now there's 10 dimensional,   not four dimensional, but it's some progress.  Okay, solution. You combine some heterotic   strings. Okay, problem. We still have five and we  have gauge anomalies. And how when one works in
• 03:00 - 03:30 foundations of physics and in unification, there's  like that, I think you mentioned that people say   it's like the only game in town and how there is  this sort of social pressure to work in the field.   So I am working at the moment in the unification  of quantum mechanics and general relativity. This   is kind of really focusing on that problem. It's  a more recent thing. I still don't have actually
• 03:30 - 04:00 my results available, but they're coming up. Of  course, not the full thing, but I think a nice,   interesting step. I think we managed to achieve,  but I come from a very different sort of a place.   And I thought that maybe the story of that could  be interesting in the light of the things you've   been discussing. Yes. And I've been looking at  your research in the evenings as well. And so
• 04:00 - 04:30 this is a wonderful experience for myself. And I  would love for the audience to get familiar with   you. So please go over your recent results.  Great. Yes. So I became interested in the   foundations of physics as a student at university.  I had a teacher, Luis de la Peña, who I enjoyed.   He was teaching me quantum mechanics. And I  particularly liked his class because he talked
• 04:30 - 05:00 a lot about interpretations of quantum mechanics.  So I was really fascinated with that. He was very   generous because at some point I approached  him and I said, you know, I want to work with   you on this topic. And he said, you know what?  It's been very difficult for me. It's been a   really difficult path and I don't want that for  you. So I would suggest work on something more   sort of mainstream. And if you're still interested  when you're grown up, let's say, you can come
• 05:00 - 05:30 back. And I think that is somehow related to what  you're saying, that he took a different path and   he found it extremely difficult and he wanted to  spare me of that. I think he could have just said,   yes, great, you know, a good student, come and  work with me. And instead of that, I think he was   very generous by saying that. So I approached  him also because I wanted to go to Fermilab.
• 05:30 - 06:00 There was like the possibility, a competition  to go and spend the summer there. And I asked   him for a reference letter and he said, why would  I give you a reference letter? And I said, well,   because you know, I got like A's in all your  classes. He said, well, many students do that.   And then from there we just went on talking. And  what I told him was that I was finishing my degree
• 06:00 - 06:30 and starting to see what I wanted to do. And I  mentioned to him that all of my classmates that   were interested in theory were actually going into  string theory. And that I actually, when I learned   about string theory, like all of my classmates,  I was absolutely fascinated with it. Now this,   I think this is a beautiful idea that we treat  particles like point, point like systems. And
• 06:30 - 07:00 then the idea that there is another dimension  or more dimensions, there are strings and how   you could unify the notion of different particles  in this way is beautiful and I loved it. But then   once I got more into it and, you know, sort of  issues started to pop out, especially the many   dimensions, then I thought, this reminds  me of the epicycles. And let me explain,   you know, what I meant. So I'm sure that most of  people in the audience are familiar with this,
• 07:00 - 07:30 but back in the time, people wanted to describe  the trajectories of planets. But back then people   used to think that they had to follow, nature had  to follow circles because circles is the perfect   figure. It's a shape. It's really interesting  how we get into these ideas and we get so stuck   in them, right? And those are the ones that don't  let us make progress. So we can come back to that
• 07:30 - 08:00 maybe later because we've been trying to unify  quantum mechanics and general relativity for   more than a hundred years. And we're probably  stuck with something equivalent to the perfect   figure and we're unwilling to let go of that. And  maybe we can talk later about that because there's   like some ideas. Actually, I think that maybe even  consciousness could be sort of something missing
• 08:00 - 08:30 in the equation, let's say. LRW Oh, yeah. HG So  if you try to describe the trajectory of a planet   using circles, well, first one was not possible. I  remember I also heard you talk about how you get a   problem and boom, you bang it and you fix it and  then another one comes out and you bang it, no?   LRW Right, right, the whack-a-mole. HG Exactly.  Yeah, so you do that with the circles and okay,
• 08:30 - 09:00 you add another circle and that doesn't work  that well, so you add another one. And well,   back then people used to need something like 600  circles to more or less describe the trajectory of   a planet. And then came Kepler and says, they're  not circles, they're ellipses. And boom, no more   necessary to hit things with a hammer anymore, it  just falls in everything beautifully, right? So
• 09:00 - 09:30 when I looked into string theory with a bit more  detail, not much, I very soon felt this reminds   me of the epicycles, it can't be right. And I  told that to my teacher, to Luis de la Peña, and   he smiled and he said, I'm going to give you the  reference letter where you want to go. HG That's   interesting. What was it specifically about what  you said that changed his mind? LRW Well, I think
• 09:30 - 10:00 he liked that I was so in a way critical of string  theory and that I was not going where everybody   else was going, because I just had a feeling  this is not right. HG Now, didn't he mean to go   into string theory when he said that you should go  into physics in a more mainstream manner? LRW No,   then let me tell you what that is, because that  was another important point. When I was finishing   my degree, I think also like many students, I  was also sort of in love with astronomy. Now
• 10:00 - 10:30 that's always like many students go also in that  direction, because it's just so attractive. And I   did an undergraduate thesis on Seyfert galaxies,  and I enjoyed that very much. But after my work,   and even my first paper is on Seyfert galaxies, I  thought, well, I could spend the rest of my life   studying these beautiful objects, but something  is missing. I didn't have it sort of very,   I was so aware of it like I'm now, but what was  missing for me was that studying these beautiful
• 10:30 - 11:00 objects, I felt were not really bringing me to the  point of asking questions like, what is the fabric   of reality? HG Ah, okay, okay. So something was  missing, not with the galactic data, but with what   even the most ideal answer to any astronomical  question could provide to the foundational   aspects of the questions at your heart. Okay.  HG Yes, yes. Somehow I felt like studying,
• 11:00 - 11:30 I could spend my whole life studying Seyfert  galaxies, and that would probably be a lot of fun,   and I would get some, you know, I already had  like a really good paper, it was a letter and   everything. But I felt like I'm not going to be  able, if I go in that direction, to really focus   on the questions that I'm really interested in.  And I guess, you know, like I was more interested   in understanding sort of more foundational  questions like deeper, what is reality about?
• 11:30 - 12:00 So I was in the cafeteria at the university, and  suddenly a colleague of mine, I'm still working   with him, Pablo Barberis, ran into the cafeteria  when I was doing my homework, and he said,   they demonstrated quantum teleportation. And I was  like, what? That was Anton Zeilinger's experiment.   But then it would play a role in my life as  well, because I ended up being a professor,
• 12:00 - 12:30 a visiting professor in Vienna within that group  for three years. But well, back then it was like,   oh, some people in Europe demonstrated quantum  teleportation. And Pablo also told me, you know   what, they also managed to trap single particles,  single atoms, in an ion trap and in a cavity. And   I remember my teacher, Luis de la Peña, used to  say, quantum mechanics is a theory that doesn't
• 12:30 - 13:00 apply to single particles. In experiments, we're  always doing experiments with an ensemble and many   particles and stuff like that. So then when I  heard that, I said, okay, that area is going to   get super interesting, because if now they can do  experiments with single atoms, we will be able to   address some of these fundamental questions. And  that's where I thought, okay, that's the right   thing to do. So I had already a little bit talked  to Luis about that. And when I told him, what
• 13:00 - 13:30 about quantum optics? He said, that's excellent.  So I went to Imperial College to work in the group   of Peter Knight, with the idea of doing quantum  optics. And when I arrived there, everybody was   working on quantum information and entanglement  measures and so on. So I ended up doing a PhD in,   let's say, in the interface of quantum optics  and quantum information. That went really well.
• 13:30 - 14:00 I did very well. And then from there, I went to  do a postdoc at the Perimeter Institute. Yes,   my neighbor. Yes, exactly. You're in Toronto,  right? When I arrived there, it was really   exciting, because I think I was the first or the  second postdoc to arrive there to work in quantum   information. The Institute wasn't established as  it is now yet. The building was like the old post   office in Waterloo. It was so cool. We had sofas  and a bar and a blackboard, and we sit at home. It
• 14:00 - 14:30 was really a fantastic experience. But when I got  there, there was one group in quantum information,   very small one. And then there was string  theory and quantum gravity and foundations   of physics. So we were such a small group. I  started to attend the seminars in gravity and
• 14:30 - 15:00 quantum field theory and curved space. I got very  jealous. I felt very jealous. I thought, oh gosh,   I'm missing out on something. Because you were in  the quantum information section? Yeah, and people   in quantum information were talking about quantum  cryptography. The idea of quantum cryptography is   beautiful, but if you work on that, then it's  again like, oh, how do you make a hack and how
• 15:00 - 15:30 do you fix it? And again, you get lost in those  things. And I was thinking, no, no, no, this is   really not for me. So I thought maybe I change and  I work on general relativity. But I had already   made a few jumps. No, I went from astrophysics, a  paper there, to quantum optics, and then I have a   paper on quantum computing. What am I going to do?  And I thought, well, maybe it's not a very good   idea. And I started, without knowing this was kind  of a new thing, I started like the, in a sense, as
• 15:30 - 16:00 a young researcher, I started to mix them. So I  wrote a paper that's called Alice Falls into a   Black Hole, Entanglement in Non-Inertial Frames.  Okay. That really, that's been, it's my most   cited paper. Wow. And it really sort of opened a  door for me, let's say, in the scientific world,   because you were also talking about how difficult  it is and how competitive it is to get you know,
• 16:00 - 16:30 a name and known and a position and so on. So  what I did is that I applied what I had learned   in Imperial College about measures of entanglement  to quantum field theory in curved spacetime and to   eternal black holes and so on. And it was a very  new thing to do. Now there's like a field more or   less established in that direction that people  call it relativistic quantum information. So
• 16:30 - 17:00 I was having a great time working on that. But  it was all very academic, you know, it's like,   oh, entanglement in black holes and things like  that. I still felt that I'm now getting lost in   maths and getting lost in maths. When you say  it was too academic, you mean too theoretical,   removed from experimental underpinning? Yes,  exactly. Yes. And then I got into this idea that,   you know what, I want to like, bring this stuff  to a point where I can do an experiment. I mean,
• 17:00 - 17:30 of course, not me, I'm a theoretician, but propose  an experiment. So I hired a postdoc. His name is   Carlos Sabin, who was someone doing theory, but  very close for experiments. And he was working   with superconducting circuits and stuff like that.  And it was really funny, because I just thought   I didn't even have a clear idea of how we would  get there. I just said, this is where I want to   go. And we together started to work with quantum  metrology, applying it to quantum field theory in
• 17:30 - 18:00 curved spacetime. So now that's going to go into  my slides. Sorry for the super long introduction.   But I thought it was relevant to what you were  talking about recently. And I actually managed   to start proposing experiments. Some of them,  at least partially, have already been tested,   you know, and the experiments have been done.  And that became sort of my path, studying
• 18:00 - 18:30 quantum and relativity, but really proposing  experiments. I was not working in unification,   because I was working with quantum field theory  in curved spacetime. So I'm going to tell you a   little bit more about that in a moment. And then  by doing that, that finally brought me to an idea   that is my own, inspired by the work of Roger  Penrose, who I talk to him very often. And then
• 18:30 - 19:00 I managed to kind of come up with a theory that  can be tested in the experiment, and we're going   to do that very soon. So that's kind of the story  of why my talk, which is also about unification,   comes from a very different perspective. It comes  from someone whose background is in quantum optics   and quantum information, and looking at  experiments and sort of trying to see
• 19:00 - 19:30 what we can learn from these theories and their  interplay, and try to make theories informed by   the experiment. Wonderful. Thank you for that  introduction. I have two quick questions. Maybe   they're addressed in the talk itself. So you  were in, firstly, astrophysics, then you went   to quantum information, then you saw some talks on  general relativity, and you thought maybe you want
• 19:30 - 20:00 to go into that field. But you said it would be  too much of a jump, of you jumping back and forth.   But would it be? Because astrophysics, does it not  already use general relativity? So how much of a   jump would that be? It would be like jumping  backward rather than jumping to the side, no?   Well, it could have been a bit like jumping back,  but the work that I was doing in astrophysics   was not really related to general relativity  directly. I was doing statistical studies on
• 20:00 - 20:30 how companions of sefer galaxies could trigger the  material of the galaxy to go into the black hole,   and so on. So yeah, I guess because of the type  of analysis that I was doing, it would have been   like another jump. Okay, okay, cool. And then now  you also mentioned that you work on quantum field   theory and curved spacetime. Now some people would  see that as unification because general relativity
• 20:30 - 21:00 has something to do with curved spacetime.  So can you please delineate those two? Yes,   actually I'm going to do that in my slides. Great.  But very quickly, that's the beauty. The beauty of   quantum field theory and curved spacetime  is that it allows you to study some,   let's say quantum effects and relativistic reflex  theory interplay in some scales, but it's not   the full theory. It doesn't resolve actually  what I think is the most interesting question.   So yeah, let me get started and I'm going to get  there very, very soon. Wonderful. Take it away.
• 21:00 - 21:30 The floor is yours. Okay, thanks. So yeah, there  are many fundamental questions that are unanswered   and very interesting ones. And I wrote in this  slide just a few that I find fascinating and maybe   some of them I work on as well. So for example,  what is the nature of dark matter? That is a big   one. So yeah, well questions like, is dark energy  driving the accelerated expansion of the universe?
• 21:30 - 22:00 What's the physics of the very early times and  cosmology? Does the equivalence principle hold for   quantum systems? And so on. There's many, many  interesting questions and fundamental physics   that don't have answers. And underpinning our  difficulties to find answers to these questions is   our difficulty actually to unify quantum mechanics  and general relativity. I went to a conference a
• 22:00 - 22:30 few years ago and it was all about how can we use  quantum experiments for fundamental physics and   many of these fundamental questions came up. And  then someone in the audience, a colleague of mine,   got up and said, well, but nobody is addressing  kind of the elephant in the room. What's the   elephant in the room? So that, you know, we for  more than a hundred years have tried to unify
• 22:30 - 23:00 quantum physics and general relativity and they're  incompatible. So how does that affect all these   other interesting questions? And then that's where  I think I felt, yes, that is a real interesting   question to answer. So this is like the very  typical cube that one sees in theoretical physics,   but I just, you know, a bit designed in  a different way where you have relativity   on one axis. So that would be C, the velocity of  light, then gravitation on another dimension that
• 23:00 - 23:30 would be the gravitational constant G and then  quantum physics would be H bar. So this cube here,   I'm trying to show that, well, we have some pieces  of the puzzle. So there are parts of the theories   that kind of answer some of the questions or work  well in some scales. And we have a lot of work
• 23:30 - 24:00 done in the last years in different pieces of this  puzzle, but we don't have the whole picture yet.   So quantum field theory and curved spacetime  I would say is like one of these big pieces   of a puzzle, but it doesn't do the whole thing  yet. So I'll go into why not in a moment. Okay,   so this actually title of this slide, should we  quantize gravity or gravitize quantum theory comes
• 24:00 - 24:30 from Roger Penrose. And what he means by that is,  should we keep the principles of quantum theory   and modify general relativity? That's what we  understand more by quantum gravity. Or should   we do the contrary, keep the principles of general  relativity and modify quantum theory? So I guess,   you know, like most people working on unification,  maybe follow the first line of quantizing gravity.
• 24:30 - 25:00 But Roger thinks differently, thinks that  quantum theory has a problem anyways, which   is the measurement problem. So he supports more,  let's say, the root of keeping the principles of   general relativity, and then trying to modify  quantum theory to bring them together. Now we
• 25:00 - 25:30 both agree that it's more likely you have to  modify both of them. But let's say Roger would   always give more priority to general relativity in  that sense. So I was writing here in this slide,   like a few things about both theories. So let's go  first to quantum theory, like same as in classical   physics, time is absolute in quantum theory. So  clocks tick at the same rate for any observer
• 25:30 - 26:00 independent of its state of motion. And this  comes from the theory of being invariant under   Galilean transformations. So the underpinning  transformations are Galilean transformations, just   as in classical physics. So in the same, know that  inherits that space and time are very different   notions. The Schrodinger equation treats space and  time completely different. It has one derivative
• 26:00 - 26:30 in time and two in x. It treats time like a  parameter, and then positions can be quantized,   and you use operators which are completely  different mathematical structures. So then already   from there, they would be incompatible with the  relativity. And just for some clarification,   quantum theory means quantum mechanics and not  quantum field theory. Yes, yes. I guess because   of my background, I use that more when I say  I like to use actually more quantum physics,
• 26:30 - 27:00 but that I'm just talking about Schrodinger  equation, fields is like a step more, you know,   yes. Well, then in quantum theory, we have the  superposition principle. So particles can be in a   superposition of two distinguishable locations at  a time. And then, well, this is what Roger calls,
• 27:00 - 27:30 well, many people call the measurement problem,  but in quantum theory, the outcome of measurements   is probabilistic, fundamentally probabilistic.  And then when we want to measure, let's say,   space or time, we have an uncertainty principle  that tells us that if you measure positions very   precisely, then you cannot simultaneously measure  momentum and so on. Also, it's kind of a bit of a
• 27:30 - 28:00 summary of some of these, let's say, fundamental  principles of physics. Then on the other hand,   in which way they're different and why are  they incompatible? Well, in relativity,   time and length are not absolute, are observer  dependent. So the underlying transformations in   relativity are Lorentz transformations. And if  you look at that, they mix space and time. So
• 28:00 - 28:30 let's say the more radical thing I think that we  learned from Einstein is that space and time are   not different in the way that we understand them  in classical physics and also in our experience,   right? If you tell anyone space and time are a  bit of the same thing, people would be shocked   with that. But that's what Einstein showed us,  that they actually belong together in a higher   dimensional object, which is space-time. And  they're both dependent on the state of the
• 28:30 - 29:00 observer. And then you have in relativity, if you  have gravity, for example, it curves space-time.   And then if you look at two different points in  space, you can see that time flows at different   rates at different points. So already there  you can see that in relativity, you have to
• 29:00 - 29:30 treat space and time on an equal footing. So let's  say equations, if you have that, you're having a   second derivative in space, you should also have  a second derivative in time. So you can already   see how that is already incompatible with quantum  theory. And so, a little bit also the question of   time is at the heart of our difficulties to unify  the theory. And then you could think about things,
• 29:30 - 30:00 how would you see if a mass is in a superposition  of two different locations, and then time flowing   at different rates? I mean, the Schrodinger  equation has only one derivative in time, it's   one time. You cannot think about such questions  yet with the theories that we have currently,   no? Another thing, well, just to finish with  the slides, in relativity, we don't have this
• 30:00 - 30:30 thing about the outcome of measurements being  probabilistic, but it's a deterministic theory   in that sense, and we can measure space and time  as precise as we want. But well, in my opinion,   the most interesting question that we have  to answer is what happens when we have a   massive superposition, where the mass is in a  superposition of two different locations in space.
• 30:30 - 31:00 And this is something that you cannot answer  with quantum field theory in curved spacetime,   because, well, I'm going to go more into that  later, but the theory assumes that you have a   fixed background, so a fixed spacetime metric,  which is a solution of Einstein's equations,   but the fields themselves, or the mass itself,  doesn't curve it. So you couldn't answer this
• 31:00 - 31:30 question. I think this is really an interesting  and important question, because we know,   for example, from the experiments, that you can  have the electromagnetic field in a superposition.   So you can take an electron and put the electron  in a superposition, and then you can see that   the quantum fields generated are in quantum  states. So we were talking about quantum optics,
• 31:30 - 32:00 and quantum optics has been a theory that has  been tested in many, many experiments. And we   know that the electromagnetic field can be in  quantum states. And now the big question is,   can gravity also be in a quantum state in this  sense? And well, if the mass is very small,   well, yes, because the moment that we have,  let's say, an atom in a superposition, in a way,
• 32:00 - 32:30 the gravitational field produced by the atom  is also in a superposition. But I think the big   question is more if that's a stable situation or  not. And that's where Roger, and I'm going to go   more into detail of that, comes in and says, well,  you can, but that is a very unstable situation,   and gravity collapses the wave function, which  would then resolve the measurement problem.
• 32:30 - 33:00 And that would explain more like the transition  between the classical world and the quantum world,   that would explain why we don't see, let's say,  this cup in a superposition of here and there,   and so on. I'm going to talk more about that in a  moment. But I guess my point here is that I think   this is the most interesting question to answer.  And there are good reasons to believe that gravity
• 33:00 - 33:30 could act different to the other forces. And  that is because gravity is the only one that   has an equivalence principle. So there is not an  equivalence principle for the others. So in the   equivalence principle, if you're in a lift and you  don't have any way to look at what's happening,   so in a box outside, you could not distinguish  when you feel an acceleration if that is because   you're in the presence of a gravitational field  or just because the box is being accelerated.
• 33:30 - 34:00 And that is something that is specific from  gravity, and that could distinguish gravity   from the other forces. So that is something also  that Roger argues that might hint at gravity   being fundamentally different. Okay, so I mean,  obviously the question is very important per se,   but also, as I said, it underpins other very  interesting fundamental questions in physics. I
• 34:00 - 34:30 found this picture, the one with the stars and so  on, online. It's a very famous one. Actually, you   know, one of the things I lost, because I lost my  talk just a few moments ago, were all the credits   to the images. So I'm sorry I had done that  detail and so on. But when I saw this picture,   I liked it very much. And it made me think about  how was it when we were trying to make sense of,
• 34:30 - 35:00 let's say if you want cosmology, of where are we?  What's this, let's say, world that we're seeing?   What are those points in the sky that appear at  night? In a way, what's the universe? And so on,   without instruments, you know? So I can imagine,  I like to have a romantic image of that, of, you   know, people sitting around the fireplace looking  at the sky and trying to make sense of where are
• 35:00 - 35:30 we. Without the telescope, you can imagine how  hard that would be and what sort of theories   humanity came up with when the only possibility  was to use our own instrument, that are our eyes,   and look at the sky. Then Galileo invented the  telescope. It's very interesting that as well,   that when Galileo invented the telescope,  many people didn't want to look through it.   And that also makes me think about a lot of the  stuff happening in science where people sort of
• 35:30 - 36:00 refuse to look at certain theories. That reminds  me, I also heard you talk about that, and you were   talking about, well, I mean, if you're working  in string theory or in quantum gravity, don't you   have sort of the moral responsibility of looking  at what other options are there? Yes. Right? And   that, I think it's like refusing to pay attention  to competitive theories or other ideas. I think
• 36:00 - 36:30 it's a little bit equivalent, like refusing to  look through the telescope. Interesting. Now,   somebody else comes with a new invention and  says, look, look at what's happening. You say,   no, I don't want to even look. But that  happened. Now, since then, telescopes have   developed incredibly. We have amazing, like the  latest pictures that you see are just like amazing   what they can do. But well, now with very good  instruments, we could look at the sky. We can
• 36:30 - 37:00 look really into the past of our universe and then  see that, oh, wow, it looks like the universe is   in expansion and so on. And we can come up with  more meaningful theories, with better theories,   thanks to those observations. Same if you think  about the microscopic world. So the Greek came   with the idea of the atoms. But again, it's not  until you build a microscope and you can look
• 37:00 - 37:30 into the microscopic world that you can do better  atomic physics. So I'm trying to make the point   here about how important have instruments been in  us making better theories and understanding things   better. Right? So when it comes to these scales  where quantum mechanics and general relativity   interplay, we're blind. We don't even have  our instrument. We don't even have our eyes.
• 37:30 - 38:00 We don't have anything. So how do you go about,  right, when you do that? So I think I understand   string theory and loop quantum gravity and many of  these very mathematical approaches in that sense   is that you do what you can when what you have at  hand. And what we're able to do is super powerful   studies with mathematics because our mathematics  is very developed. And you were also talking about
• 38:00 - 38:30 that, how actually string theory has allowed  mathematics to develop so much. And so much   we've learned about mathematics thanks to those  theories. But when you come up with theories and   mathematics, well, there's many possibilities.  You can make many theories, almost as many as   you can think about. But which one is the right  one? You know, I can make a theory, but then I
• 38:30 - 39:00 need to see if actually nature behaves like my  theory predicts. Right. And then I can have a   competing theory, a different one. And which one  is the right? Maybe even contradicting the two   theories in principle and their predictions. How  do you know which one is the right one? You need   to go to the experiment. You need to go to those  instruments. And we, well, I'm going to argue that   we sort of have them already. And we need to  start looking through them for resolving these
• 39:00 - 39:30 questions of unification. All right. What I think  we want to do is to get into this cycle in which,   let's say, you come up with an idea. So this would  be philosophy and creativity. So going back to the   example of the atoms, right? So the Greek came  up with using philosophy and creativity and so
• 39:30 - 40:00 on with the idea that there must be something in  matter that you cannot keep dividing. So there   must be this unit and the idea of it cannot be  divided anymore. So the idea of an atom. Then,   well, if you want to observe an atom,  well, that's like a really long way around,   right? But you have to do some theory about what  is an atom. So, well, a very, very long time after
• 40:00 - 40:30 people started to develop better theories of  the atom, or for example, I don't know, the   pancake theory where you had some electrons, like  raisins in a pancake, or even better, Bohr's model   where you have the nuclei and the electrons going  around like if they were planets around the sun,   right? So you need to create some theory so that  you can build an apparatus and then observe this
• 40:30 - 41:00 idea that you had that there's atoms. Because you  cannot build a machine or propose an experiment or   develop a new sensor without some sort of theory.  Your theory might be wrong, but at least it gives   you a starting point to say, okay, now I'm going  to build this machine. Then you build, let's say,   the microscope, and you look through it, and then  you get some sort of signals, and at some point,
• 41:00 - 41:30 like detectors click or something like  that, and you say, oh, there's my atom,   right? And then you might then find out that your  theory was actually not very good, but then you   can improve it and modify your apparatus, and then  you get into this really good cycle where you can   start making better theories all the time and  verify them into the experiment. So this is what
• 41:30 - 42:00 happened with quantum optics. It seems like this  is what happens with the general theory. So if I'm   understanding you correctly, it sounds like what  you're saying is you're initially on your couch   or in your shower, an idea comes to you, it's  an intuition, you then formulate it with words,   natural language, you then have to formulate it  into mathematical language, and then you have to   check that against quote-unquote reality with an  experiment. Yes. So you propose an experiment,   and the experimental proposal, that's what  I work a lot on in experimental proposals,
• 42:00 - 42:30 is also mathematical, right? I have to write down  my theoretical proposal. This is your Hamiltonian,   and these are your measurements, and this is the  precision, and I claim that you should be able   to build this device, and I'm going to show you  one of my works in that talk about, of course,   of my proposals to do that, and then you need to  build it and then check. Okay. And you were giving
• 42:30 - 43:00 a specific example in quantum optics? Please  continue. Well, with quantum optics, there's this   very healthy cycle, and I think that's why there's  been so much progress in quantum technologies,   is because this happens all the time. People  come up with an idea for a sensor, and they   write papers about it, they make a proposal,  then an experimental group gets a hold of it,   they work together, and boom, they show that, and  there comes again the cycle, and it's a wonderful   field. And I think I was used to that, so when I  started to work on Alice falls into a black hole
• 43:00 - 43:30 and entanglement in black holes, I was like, oh,  gosh, I can't check if what I propose is correct,   because there is no way to make a measurement in  a black hole. And that's how I started to say,   no, no, I want to do theory that it can actually  still work at the interplay of quantum mechanics   and general relativity, but that I can test in the  lab. So that's my group, and most of the last, I
• 43:30 - 44:00 don't know, maybe 15 years, that's what I've been  working on, on trying to propose experiments or   develop new sensors that will reach these scales  where quantum mechanics and general relativity   interplay, so that we can then get into this  cycle. And what is FP? GR, quantum theory, oh,   I forgot, what did I put here for, it's an old  slide, so quantum theory for sure, GR, and oh,
• 44:00 - 44:30 fundamental physics, it's fundamental physics,  yeah, maybe that's a really funny figure. Okay,   so when I was at university, and I learned about  quantum mechanics and general relativity back in   the day, well, for example, Luiz de la Peña  would say, quantum mechanics only applies to   a few particles at very small scales, so where  electrons and atoms live. And general relativity
• 44:30 - 45:00 applies to the large scales, starting with  actually, from GPS, to get the precisions we have,   we need to make corrections to general relativity,  so the proper time on Earth is different from the   proper time in a satellite, and you need to  make corrections to have the precisions that   we have in GPS. So it would start, let's say,  from those kind of scales onwards, I mean, we
• 45:00 - 45:30 know that general relativity doesn't really apply  to all these scales, because the rotating curves   of galaxies, the observations there contradict the  predictions of general relativity, and from there,   the whole idea of dark matter comes about, so it  doesn't really apply. But let's say, generally,   you're a student, and you're told quantum physics  applies to the very small, and general relativity   to the very big. Now, because of this circle that  I was telling you about, now, the experiments in
• 45:30 - 46:00 quantum technologies, they developed amazingly,  and now it completely challenged this picture,   and I want to tell you a lot about that. So  I'm going to talk about three things. One is   long-range quantum entanglement, so what are  the longest distances at which we can prepare   superposition states, or entangled states, and  so on, and how can we study such situations,
• 46:00 - 46:30 and what can we learn about the interplay  of quantum mechanics and general relativity   through long-range quantum experiments. Then  high sensitivity, actually, when I started   to work on using quantum theory, I wanted to  measure some relativistic effects. Some of my   colleagues in general relativity were laughing  at me, because they were saying, well, you know,
• 46:30 - 47:00 at small scales, forget it, space-time is a bit  flat. It's completely flat, sorry. You won't see   anything. I'll show you that that's not true. And  these are already experiments that have reached   relativistic effects, we're just not looking  through the telescope right yet, because, well,   I'll tell you more when I get there. The one that  hasn't gotten to scales where gravity kicks in,
• 47:00 - 47:30 in an important manner, is large mass quantum  experiments. So I also want to tell you about   the progress in that direction, and how far  we are from being able to see, for example,   if gravity indeed collapses quantum superpositions  and so on. I have a quick question, if you don't   mind. So with GPSs, they're using an atomic clock,  I presume, which is something that's a quantum   phenomenon, and then they have to correct because  of general relativity. So do people see that as an
• 47:30 - 48:00 interplay between general relativity and QFT, or  quantum mechanics there? I don't want to actually   go into the details of the question that you just  asked me, I have a slide on that, so it's like a   really question right to the point. But the short  answer now is that people brush the questions,   in a way, out. They find solutions, which I don't  think are solutions, but they're like, let's say,
• 48:00 - 48:30 well, they maybe approximately work, but actually  are not the right thing to do if you want to be,   let's say, rigorous with what you're doing. And  actually, that gives you the opportunity to answer   the these questions. So I'll go, I have a slide  on that, exactly, the question that you're asking   me. AG Great, we think alike. HL. Yes, I noticed  that before you from the podcast, in many ways,
• 48:30 - 49:00 actually. AG Cool. HL. Okay, so let's talk about  the long-range experiments. When I was a student,   you know, when Pablo came, my colleague, into  the cafeteria and told me they demonstrated   quantum teleportation in the lab that  was in Vienna, that was Anton Zeilinger,   it was in a tabletop experiment. So you have like  a table that could fit in in this room, let's say,   with mirrors and lasers and so on. And that's how  experiments look like in those times. Then Anton,
• 49:00 - 49:30 some years later, wanted to see how big can the  distances, can the experiment grow such that you   still have entanglement. So this is entanglement  between photons. Okay. And he was able to   demonstrate entanglement across two different  buildings in Vienna. So well, that was very   promising. So he said, well, let's keep going. And  then in 2011, he was already doing the experiment
• 49:30 - 50:00 across 144 kilometers in the Canary Islands. AG  Oh, so they're not physically connected tubes   that connected the two buildings, nor in this  1,000 kilometer case? HL. Well, there are many   experiments that are connected by a waveguide.  People do experiments like that. But no, these   are like free space experiments. AG Interesting.  HL. Yeah, they're beautiful. They're very, very
• 50:00 - 50:30 interesting. So Anton had a student from China  who then moved back to China. And then, you know,   he's made a lot of progress there. And together,  they launched a satellite, which is called Micius,   which is completely purposed to study quantum  entanglement and teleportation and cryptography
• 50:30 - 51:00 and so on. So this was, they launched it in  2016. And then they've demonstrated entanglement   across thousands of kilometers. So that's very  interesting, no? Because this whole notion of   quantum mechanics applies to very small scales.  Now we see that that's not the case. Well, of   course, photons are, you know, they're not massive  systems or anything like that. But already,   I think this starts showing that this division  of what are the scales where quantum applies and
• 51:00 - 51:30 where it's different, maybe in some senses, as we  first thought it would be, no? But well, I mean,   what's very interesting is like, as I mentioned  before, at the scales where satellites operate,   relativity kicks in. Again, the proper time of  clocks measured on Earth is different to the   clocks that you set that are in a satellite.  So you have to take into account at least a
• 51:30 - 52:00 gravitational redshift. So this is like a special  relativistic effect, but more than that. So that   is something that I've been very interested in.  I have a whole series of papers that use quantum   field theory in curved space-time to describe  the space-time of the Earth using, for example,   the Schwarzschild metric, which can be applied to  this case. And then you describe the photons and
• 52:00 - 52:30 the quantum states that travel from Earth to a  satellite or between links and between different   satellites. As using quantum field theory in  curved space-time, you can solve the equations   and then construct wave packets and study how the,  let's say, if you send a wave packet from Earth   to a satellite, how would this be modified due to  the curvature of the space-time on their light?
• 52:30 - 53:00 So this is no longer just special relativity using  gravitational redshift. That was what people were   using. We showed that if you use quantum field  theory in curved space-time, you could actually   go beyond that and really see how the curvature  of space-time affects the, for example, we wrote   some of these papers and we said, this is what  the curvature, how would it affect, for example,
• 53:00 - 53:30 quantum teleportation or quantum cryptography.  And then you could turn things around and use the   fact that these states are modified to actually  estimate the space-time parameters of the Earth   using quantum metrology. LWR Okay, cool. HL. So  that's an area of interest, and I've written a   series of papers in that direction, more or less,  trying to answer these sort of questions. But you   see, these are experiments that already are  taking place. And actually, there was a group
• 53:30 - 54:00 working in Germany that once the whole group came  to visit mine, because they had some results they   were not understanding, and they, using just  the gravitational redshift, and they wanted to   see if there was more to be understood from our  work. So this is, yes, an instance where you do   see that some interplay between quantum states  and the space-time of the Earth, the experiments
• 54:00 - 54:30 reach those scales, but there's very little apart  from our work. I don't see that there's many more   things, or the experiments actually, they take  into account the gravitational redshift, but they   still have to test these sorts of things. Now,  quantum field theory in curved space-time has not
• 54:30 - 55:00 been demonstrated in the experiment. Quantum field  theory, yes, I mean, so many times that's what   CERN and Fermilab and all of these experiments  are about. But when you have gravity included,   it still needs to be demonstrated. So some  of these predictions that we make could start   giving you some hints that quantum field theory in  curved space-time, let's say, it's a good theory   for these scales. It would be very nice to check  that. So for the audience member who's thinking,
• 55:00 - 55:30 how does this work logistically? Do you have to  petition for time from this satellite, or do you   have to ask the people who are in charge of the  satellite to perform an experiment? How does it   work? Well, I actually belong to a group that was  sort of a consortium in which they worked together   with the theoreticians, with the experimentalists,  and the group sort of discussed about which would
• 55:30 - 56:00 be things that would be interesting to  study. So the theoreticians would say,   well, we would like to test this theory.  Let's say I had a colleague, Tim Ralph,   who came up with a new theory that used quantum  field theory in curved space-time, but went beyond   that and took into account closed timelike curves.  And then he proposed an experiment. And then the
• 56:00 - 56:30 group found this interesting from a theoretical  point of view, but the important thing there was   that the experimentalists found it feasible to  do the experiment, and the experiment was done,   and the experiment didn't find evidence of this  sort of new theory. But you see, that is the sort   of thing, that's great, that's the sort of thing  you want to be doing, that people are creative,   come up with new ideas, again the circle, cast  it in language first, then in the language
• 56:30 - 57:00 of theoretical physics, which is mathematics,  make predictions, the experimentalists go test,   and they say, well, yes or no, and then you go  on. So I think that there are groups like that,   and usually also what we do is that we get  together, theoreticians with experimentalists,   and make a proposal that might or not get  funded. Of course, with space-based experiments,
• 57:00 - 57:30 it's more complicated. I have actually been  approached by NASA a few times, and they asked me,   do you have an experiment that you think we could  do? But the things I've been working on lately are   more things that you could also test on Earth. And  then you need to justify the expense. But well,   I did point out to these papers, and I said,  well, I think it would be great if you could test   some of these. But I haven't heard like, oh yes,  we're doing it, or anything like that yet. Okay,
• 57:30 - 58:00 so now we go to the clocks question that you were  asking me, and the very small scales. So yes,   like you were saying, quantum clocks are the most  precise clocks that we have, and actually, that is   what we use to distribute clocks in the planet.  You need to synchronize very well computers, and
• 58:00 - 58:30 airplanes, and all sorts of things that we need  a very, you know, for our instruments, we need   very precise ways of measuring time, and these are  done by atomic clocks. So, you know, very roughly,   how would an atomic clock work is that you  have many atoms here, for example, strontium,   trapped in an electromagnetic potential. So  the sample could be like atoms that are cold,   so that means they move very little, and they're  within some sort of volume, so typically like a
• 58:30 - 59:00 millimeter, and so on. So the energy levels, the  internal energy levels of atoms are very sharp.   So let's say between the ground state and the  excited state, the energy is very precise. So   you can use this as a frequency standard that  gives you like the ticks of the clock very   precisely. So you shine a laser, and you excite  the atoms, and so on, and well, that's more or   less what you use. So there was this beautiful  experiment done many years ago by Dave Wineland,
• 59:00 - 59:30 who got the Nobel Prize for trapping irons  in an ion trap. He did this experiment after,   in which he would take an atomic clock and then  sort of put another one, or just move his clock   upwards. I'm not sure actually what he did, but  he demonstrated time dilation at 33 centimeters.
• 59:30 - 60:00 So before we know, okay, we can see time dilation  if we're in the Earth and then in a satellite, we   know that. But now he said, look at these scales  of 33 centimeters, you can see time dilation   already. And that time dilation is just due to  the gravitational potential difference? Yes, due   to the Earth, just from the gravitational field  of the Earth. So basically, you're demonstrating   that the space-time is curved. So that's really  amazing. I mean, these clocks are super precise.
• 60:00 - 60:30 They have like a systematic uncertainty of they  can reach 10 to the minus 18. That means that the   error is one part in 10 to the 18. So that would  be more or less like in years. I used to have it   here because I forget, but the clock would lose  precision, would lose one second in something like
• 60:30 - 61:00 13 billion years. I had the number here exactly,  but I now lost it. But more or less, that precise   they are. And that's what I was telling my  colleagues in general relativity that found   it funny that I wanted to measure this curvature  of things. I said, no, look, I mean, these things   are so precise that it's not unthinkable that we  can actually measure general relativistic effects   at very small scales. So I was talking to Patrick  Gill. He's a colleague of mine who works at the
• 61:00 - 61:30 National Physics Laboratory. So that is like the  institution in the UK where they do all these,   with the Metrology Institute, where they do all  these standards of frequency and the different   units and so on. So he's working with the quantum  clocks with Helen Margolis and so on. And I was   telling them, you know what, soon you're going  to have a problem because you're going to get
• 61:30 - 62:00 that the proper time at the bottom of your sample  with the proper time at the top is going to be   different. And he was saying like, yeah, but we're  not too worried about this now. And so six months   later, exactly that happened. Two papers came out  showing that they could see time dilation. Well,   first there was like this one centimeter and then  even in one millimeter. So now if you think about
• 62:00 - 62:30 the quantum clocks, the clock in the atoms in the  bottom see a proper time different from the atoms   in the top. Yes. But okay, still, you know, people  working in clocks might not be that worried. When   did this result come out? That must have been a  couple of years ago. Okay. So fairly recently,   2020s? Oh, yeah. Yeah. Maybe this is actually...  Look, this is from 2020. This paper I put here   is one of the papers and it says published in  2022. I think it might have been submitted in
• 62:30 - 63:00 2020 or 2021, but it was published very recently.  Sure. It's still cutting edge. I see. So, okay.   So it's not a problem as long as the atoms are  independent, because then what you can do, which   is what we do with time dilation with GPS, is like  we know how that changes so we can theoretically   correct for it. And then you just take that  into account and you don't have a problem.
• 63:00 - 63:30 Okay. But now people want to make these clocks  more precise and beat this one 10 to the minus   18 uncertainty by entangling the atoms. Because  we've showed in quantum metrology that if you have   entangled atoms, you get, you know, a precision  instead of going like 1 over square root of n,   it's 1 over n. It's called the Heisenberg limit.  And this makes things much more precise. Okay. So
• 63:30 - 64:00 if you do that, then you have a problem. Then you  bang your head with quantum mechanics and general   relativity being incompatible. Why? Because what  time are you going to use? The proper time is   going to be different in different heights. And  the Schrodinger equation on the left-hand side   is like d and dt, an absolute time. So here you  have a relative time different at each height.
• 64:00 - 64:30 So which time you want to use? Okay. So again,  the experimentalists say, oh, we're not worried   about it at all event because we just use the time  at the center of the trap. That doesn't work that   well. And it's like a, it's a patch, but forget  about it. Let's say maybe for what they want to   do, it's good enough. I don't know. But from a  theoretical point of view, this is not the right   thing to do. But you're actually losing on the  possibility of learning what we should be doing,
• 64:30 - 65:00 because this is really a very good example where  you are at these stages where quantum mechanics   and general relativity interplay, but we don't  have a theory to describe that experiment. So what   I was telling, I recently went and visited the  group at NPL, at the National Physics Laboratory,   and I was having a little like discussion  about this. And I was telling them, you know,
• 65:00 - 65:30 that we don't have experiments to address these  questions. And now you're having an experiment   that actually is getting there. So let's use  this experiment to try. So you have a theory.   That's good. The theory is that you're using  is that you say, well, I can more or less do   with taking the proper time at the center of  my sample. Well, if you want to be rigorous,
• 65:30 - 66:00 really what you have is that you lost your notion  of clock time. And you need to come up with a   new thing. But that is what opens the opportunity  of, you know, you came up with a theory, which is   not very good, I think, which is measuring at the  center. Well, you mentioned theoretical problems,   but it sounds like what you're describing  is more akin to missed opportunities for   probing the interaction of general relativity with  quantum theory. Yes, both in a way, right? I mean,
• 66:00 - 66:30 what I was trying to explain to them is that as  a theoretician, we don't have a proper theory to   explain your experiment. Now, your experiment is  an experiment, and experiment is the experiment,   right? It's like in that sense, it's not wrong.  What is wrong is the theory that we're using to   describe your experiment. But you need to start  somewhere. Again, the little circle that we talked   about. So I start with a theory that's not very  good. Then you do the experiment. We look at the   experimental results. And then I come up with a  way of modifying my theory. So right now, I have
• 66:30 - 67:00 a PhD student working on this problem that I like  very much. And we've made some progress before,   not with atoms, but with light. I want to  show you more or less what we did before.   Please. So Einstein came up with this idea of the  Einstein light clock. So basically, he used this   idea of a clock to argue things for relativity  and so on. So he considered two mirrors and then
• 67:00 - 67:30 a photon bouncing back and forth. And that gave  you the tickings of the clock. And then he talked   about what happens if you move this clock, and so  on. But now, we can use quantum field theory and   quantum optics to quantize the idea of Einstein's  clock. So I've done that. I wrote another   series of papers in that direction. It's to say,  okay, now I have two mirrors, but I have a quantum
• 67:30 - 68:00 electromagnetic field inside. So when you do  that, you get the field that you can write down   as an infinite sum of different modes. So those  are states that are sharp in frequency, but the   photons are completely delocalized in your box.  But you can use quantum field theory to describe   that. So that was also a long journey, because  when I started to work with that, you could only
• 68:00 - 68:30 do this in flat space. And the only motion that  people could describe was a sinusoidal motion of   the walls. And this was like the dynamical Casimir  effect. But I wanted to do more than that. I   wanted to consider curved space from the Earth to  a planet and send a little box up to study how the   curvature, the underlying curvature of the Earth  would affect the quantum clock, or how would an
• 68:30 - 69:00 interplay of quantum states with time dilation  would look like, and all that sort of thing.   And gosh, that was really, really hard, because  solving those equations was very complicated.   And what allowed me to make progress was working  with George Malouko, a colleague in Nottingham,   where I used to work, who is an expert in quantum  field theory and curved space time. And then,   well, we managed to come up with a new methodology  where we could now start, let's say, solving those
• 69:00 - 69:30 sort of problems in a more general way. And then  I had... A student and a postdoc that helped me   generalize this to curved space and so on. And  then, so we've been now, we have a clock model,   which is basically Einstein's light clock,  but with a quantum field. We fix a frequency,   and the oscillations of the quantum states of this  frequency mode give you like the ticking of the   clock. But now we can move that in curved space  and ask questions about the interplay of time
• 69:30 - 70:00 dilation with quantum things. And we found some  interesting things, like when you move the clock,   due to things like called the dynamical Casimir  effect, you create particles, like photons inside   the clock, and these affect time dilation. LWR  Interesting. HL. So it's kind of fun doing that.   Again, I don't think you go to the  very fundamental questions by doing it,
• 70:00 - 70:30 but you start learning certain things. And one  of the things I was interested in is that, well,   if you use these clocks to measure space and  time or time dilation and so on, because the   state of the field is a quantum field, then you  start getting into these uncertainty principles,   things that you can actually not measure space and  time with infinite precision. Like if you measure
• 70:30 - 71:00 time very precisely, then space is not, and this  sort of thing. So I wanted to explore more the   constraints that you get in measuring space-time  by using a quantum system. LWR Usually when people   speak about Heisenberg's uncertainty, they're  talking about position and momentum, and you're   talking about space and time. HL. Yeah, they don't  go together. So you have energy and time, and then
• 71:00 - 71:30 momentum and position. LWR Yes. HL. But in these  clocks, you have an interplay of things. You have   states that obey minimum uncertainty in space  and momentum, so they're called Gaussian states,   coherent states. And then we move these in space,  and then you have constraints that come also from
• 71:30 - 72:00 the energy and the time. So I didn't kind of go  into much detail. I wasn't very precise when I   said that. But you start getting the role of the  different uncertainty principles that you get from   quantum theory, you know, playing a role in how  well your clock works and things like that, which   is very interesting. LWR Cool. This work goes  back to 2014. HL. Yeah. LWR So I'll leave a link   to all of the articles that have been mentioned in  this talk, either visually or just audibly, in the
• 72:00 - 72:30 description. So people, if you're interested,  you can read more. HL. Yes. This is how we got   started. So this first paper was in flat space,  but now I think this is the latest paper that was   published about clocks that was published in 2023.  There we can now, since we managed to, let's say,   generalize our techniques to include curved space  time. I mean, it sounds simple, but literally
• 72:30 - 73:00 it took us more than 15 years to be able to do  that. And yeah, we're using quantum field theory   in curved space time. So then we finally had some  theoretical methodologies that allow us to address   that question. And what we did is we looked at  a clock, a light clock, but we now were able   to describe the clock in the space-time of the  Earth, treating the space-time of the Earth with a   Schwarzschild metric, and come up with a model of  a clock and discuss how the clock ticks and, you
• 73:00 - 73:30 know, talk about the Schwarzschild radius of the  Earth and how does this show up in the face of the   clock and so on. So that was like, we then came  up with a notion of clock time. In this clock,   at each slice within the clock, the proper time  is different. And we said, okay, but you could   still build a clock by looking at the collective  oscillations. And that gave me an idea that, okay,
• 73:30 - 74:00 now maybe we can go back to the atoms and redefine  the notion of the clock time using the collective   oscillations and so on. But a student of mine  is working on that and we're just starting. We   don't really have much to say about the atomic  case yet. Okay, so now the last thing I want to
• 74:00 - 74:30 talk about with respect to these experiments  is math. So yeah, I was telling you how Roger   proposed many years ago that if you have a massive  superposition, this is unstable. And he argued   that by showing that there was a conflict between  the superposition principle and the equivalence   principle. So he said, yes, you could have a  superposition of a massive system that, for him,
• 74:30 - 75:00 this would already be quantum gravity because  you have a gravitational field in a superposition   of two different configurations. But these are  unstable and they decay very quickly and that is   why we don't see superposition in the classical  world. So what kind of masses would you need   in order to see if the predictions of Roger are  correct or not? Lyle Troxell Do you mind briefly
• 75:00 - 75:30 outlining why is it that the superposition  contradicts the equivalence principle or the   strong equivalence principle? Yes, so he starts  by describing a mass that is in a superposition   that is falling. And then he says, okay, if you  describe the situation from a Newtonian point of   view, and he writes the wave function, and now  from an Einsteinian point of view, and he writes
• 75:30 - 76:00 a different equation. So he says, these wave  functions have to be the same up to a phase. No,   because in quantum theory, states, wave functions  are equivalent up to a phase. But you see,   his whole argument, I actually, I'm going to show  you a paper that I wrote with Roger in the next   slide. And in that paper, we write an introduction  where we go through Roger's arguments. But they're
• 76:00 - 76:30 not necessarily simple. And one of the reasons  why is because we don't have a theory for that. So   Roger makes arguments that are good arguments,  well-informed, but without actually having a   theory. So sometimes the arguments are talking  about quantum field theory and curved spacetime,   and then he might make a Newtonian approximation  and so on. But he shows that the Einsteinian point
• 76:30 - 77:00 of view is different from the Newtonian point of  view, and that there is a contradiction there. And   that then, because of that, he argues that these  superpositions should be short-lived. And he goes   beyond that, because he gives you a formula that  measures sort of the error, and this gives you   an energy uncertainty, and it's related to the  gravitational self-energy of the difference in
• 77:00 - 77:30 the superposition. So you take some, maybe that is  maybe going more technical, but we can if you want   to, because I know that your followers are quite  well-educated in physics. So let me jump and then   I come back a little bit here. This is the paper  that I mentioned that I wrote with Roger, and what
• 77:30 - 78:00 we did in this paper is that we calculated how  massive would these superpositions have to be if   we used the Bose-Einstein condensate, I'm going  to come back to that. But we found that you need   at least something like 10 to the 9 atoms in a  superposition. And let me tell you where the field   is now. So well, people started to put electrons  in a superposition of two different locations
• 78:00 - 78:30 using like a double-slit experiment, I don't know,  maybe 90 years ago, I don't remember when was the   first experiment with electrons. And from there  they said, okay, it works for electrons, amazing,   let's do it now with atoms, and you know,  then it's like how bigger can the states,   the system get, and the record is held by Markus  Arndt's group in the University of Vienna as well,
• 78:30 - 79:00 where he has been able to put big molecules  in a superposition, and by big I mean the   molecules have around 2,000 atoms. Wow. But you  know, for gravity to act, you need at least 10   to the 9. Actually for molecules you need even  more. So you can see we're very far from that.   What do you mean for gravity to act? I thought  the assumption is that gravity acts as long as you   have mass, don't these have masses? Yes, but these  are stable superpositions. Oh, according to the
• 79:00 - 79:30 calculation from Penrose? Yes, this is what Markus  showed, that you can have these superpositions,   and I think they lasted milliseconds, I don't  exactly remember how long he had them for. So they   are stable for that long in the lab. So gravity  is not causing the collapse of superpositions at   those scales. I see, I see. But now the question  is, is Roger right? Because if Roger is right,
• 79:30 - 80:00 then that explains why we don't see superpositions  in the macroscopic world. And what would be super   interesting is to see that, now that is a big  open question in fundamental quantum mechanics,   is to understand what takes you from quantum  states being in superpositions to the classical   world where we don't see quantum superpositions.  It's a very interesting question. Markus and many
• 80:00 - 80:30 other people are trying to address this question  in an experimental point of view, by trying to   put more mass into the superpositions. There are  many different experiments going on at the moment,   and they use, for example, nanoparticles,  nanobeads made of silicon or silica, diamonds,   little mirrors, rods, even membranes. There's  many, many experiments going on. And also a
• 80:30 - 81:00 record has been held by Markus Aspelmeyer, also  in Vienna. I spent three years in Vienna because   of these amazing people and experiments there. I  was very lucky to get a visiting professorship for   that long and be in the same environment where  these amazing scientists are. Markus was able   to bring one of these nanobeads to the quantum  regime by cooling it down to a lower vibrational
• 81:00 - 81:30 state. So they're already in the quantum, let's  say, scales, but with 10 to the 8 atomic masses,   so quite a big bead. But he cannot put them  yet into a superposition of two different   locations. That has not been possible. Also,  one of my colleagues, and I'm in Southampton,   so one of my colleagues there, Hendrik Ulbricht,  also has a very recent, amazing paper where he
• 81:30 - 82:00 takes these little beads and he manages to measure  gravity. But this is all classical. But anyway,   I mean, at those scales where quantum starts to  kick in, well, what he wants to do is push these   experiments so that maybe he sees some quantum  gravity still far from that, but right, let's say,   approaching. But this is where things are at with  respect to the experiments with big mass. So what
• 82:00 - 82:30 I did with Roger is that when he started to tell  me about his proposal and the experiments that   people were doing, I noticed that all of these  experiments were using solids, no? Mirrors,   beads, and so on. And it's very difficult  to cool a solid to very cold temperatures
• 82:30 - 83:00 where you have little noise. So they haven't been  able to make more progress because of the noise,   because you can't cool them enough. Now, a  Bose-Einstein condensate is a really beautiful   system. I think it's my favorite system because  you can reach half a nano-Kelvin. The coldest   things that we can do. And you can get up to 10  to the atoms. I mean, that's not very common,   but there's been an experiment. Using hydrogen,  in which they cool 10 atoms into a condensate. So
• 83:00 - 83:30 let me tell you a little bit what a condensate  is. So you have a, let's say, when you learn   quantum mechanics, you learn that if you put a  particle in a potential, well, the particle is   there moving in the potential, but if you cool  it to the ground state, it will, let's say,   if you manage to the ground state, the atom will  be completely delocalized within the potential.   So you don't know what the position of the atom  is in that whole thing, no? That's really, I don't
• 83:30 - 84:00 know, when I did that in quantum mechanics, I  loved it. Now think about having 10 to the eight,   10 to the 10 atoms, all cooled down. But atoms are  bosons, so they can all occupy the same quantum   state. So you can cool them all down to the ground  state. And that is what is called a Bose-Einstein   condensate. So you have the biggest system that  behaves in a quantum mechanical way. And like I   said, in the experiment, people have been able to  cool these systems to half a nanokelvin. Right.
• 84:00 - 84:30 So I was wondering if then this would be a good  system to test Roger's predictions, and that's   what we did together. We said, okay, how would  it go with a Bose-Einstein condensate? And well,   also super complicated, because you would have  to create a superposition of all the atoms on   the left with all the atoms on the right. And  although the temperatures are that low, people
• 84:30 - 85:00 have not been able to create these superpositions.  They're called noon states because you have N,   zero, zero, N. Yes, cool. And you know what, the  record is by one of my colleagues called Chris   Westbrook, and he's been able to do two atoms. And  like, so you can have many, like you can have up   to 10 to the, like you can have many atoms in  quantum states in a Bose-Einstein condensate,   but not many atoms in a spatial superposition  of two different space locations. That's where
• 85:00 - 85:30 gravity acts. So this is what I now have been  working on in the last two years. And well,   it's not related to, it's inspired by this work  with Roger, but it's a complete new thing. I   hope I can talk about it at a later time with  you. But in that previous paper with Roger,   you know, we studied things like, Roger  had given formulas for uniform spheres,
• 85:30 - 86:00 and in a BC you could have pancakes or elongated  BCs with different distributions of the density.   And we studied if these would enhance the effects  predicted by Roger. And then, well, you have a lot   of losses and we studied the losses and so on. And  that's how we came up with this. Well, with a BC,   you need at least 10 to the nine particles, maybe  even 10 to the 10, in order to start being able
• 86:00 - 86:30 to actually verify that the energy uncertainty  of gravitational origin that Roger predicts has   an effect. So now I'm gonna finish this part with  the slides, just telling you of an example of the   work that I've done, where I brought together  quantum field theory and curved spacetime to,   let's say, propose a new sensor. And it was quite  bold, because I came up with a proposal that you
• 86:30 - 87:00 could use a Bose-Einstein condensate. The sample  itself can be 100 micrometers, 50 micrometers, the   cloud of atoms. And the experiment is, again, a  tabletop experiment. We could put it in this room,   no? Cool. And I claim that you could use the BC,  because you see an atom, we saw how precise they   are. And a BC, you might want to see it as 10 to  the eight atoms cooled down to the ground state.   So this is a very precise, it's a system that  is very sensitive to spacetime distortions. And
• 87:00 - 87:30 I made a proposal on how could you use the system  to detect gravitational waves. Wow. And it's quite   crazy, because gravitational waves are detected  in LIGO, where the apparatus measures each arm   three kilometers. So this is very bold. And I've  been really kind of, when I met Roger, that was in
• 87:30 - 88:00 2017, I was really invested in that, and trying  to convince the community that you need to do   this experiment, because it really opens up a new  direction. And Roger was trying to convince me to   work on the collapse of the wave function due to  gravity. I was very reluctant, because I thought,   I want to put my time and my energy into this. And  well, after the years, Roger managed to pull me
• 88:00 - 88:30 more into what he's doing. But yeah. So when you  talk about using atoms to measure gravity, what   we usually do in quantum technologies is an atom  interferometer. So let's say you have a atom, and   you hit it with a laser, with a photon, and you  make the atom, you put it in a superposition of
• 88:30 - 89:00 two different positions, but they're free-falling.  So they follow different trajectories, and then   you recombine them with lasers. And they recombine  at a point. But because they went through   different trajectories, they pick information in  a phase that depends on the local gravitational   field. And this is what a quantum gravimeter is.  Interesting. And I put here a single particle   detector, because although they throw maybe 10  to the six atoms at once into the interferometer,   all the atoms are independent. And each atom goes  through this superposition of trajectories. And
• 89:00 - 89:30 then they interfere at a point. So I put here the  interference is local, because it's at the point   where they recombine. And then this is limited  by the time of flight. And the equation is very   simple. It's just this equation that's here.  Basically, it depends with the time of flight   squared, which means the bigger the detector, the  more precise it is. That's why LIGO is so big.   And they're thinking because they want to go to,  well, LIGO is with light, but the principle is the
• 89:30 - 90:00 same. They now want to make a bigger detector in  space called LISA to have more precision. So a lot   in physics, the tendency is to go very big, big  experiments, of course, are very expensive. And I,   my, my husband says that I'm a rebel because I  like, you know, if everybody's doing one thing, I   always want to do something different that applies  to everything in my life. Yeah, that's another   aspect that unifies us. Yeah, really? Yeah. No,  it's like I'm a contrarian at heart. Yeah, yeah,
• 90:00 - 90:30 yeah, yeah, exactly. So if everybody wants to make  big detectors, I want to make them very small and   so on. But it has paid off for me in science. So  maybe sometimes in life can make me like a Grinch   in Christmas and things like that, because I was  like, oh, I don't want to do what everybody does.   So socially, I don't want to go to the movie that  everybody's watching. But in science, it's been,   it's been good, you know. So, so, well, here I  also write that this is compatible with Newtonian
• 90:30 - 91:00 gravity, because this is an experiment that is  described with the Schrodinger equation. If you   treat the local gravitational field by Newtonian  gravity, everything works very nicely. And like I   said, these are already commercial. My colleague,  Philippe Bouyer, has founded a company that he now   sold called MuQuans. And there are other like  Mark Kasevich does that as well, in which,   you know, they built these interferometers, these  gravimeters. And they sell them, they're like,
• 91:00 - 91:30 like a meter big, I think, and so on. And that's  like, you cannot make them smaller than that,   because then you lose precision. So if you  wanted to get atom interferometers to apply   them to fundamental physics, to learn about the  equivalence principle, or to measure anything,   respect to gravity, so you want to make them more  precise, you have to make them bigger. So Philippe
• 91:30 - 92:00 Bouyer did this amazing experiment in which he  put his atom interferometer in a plane. So he   flew the plane as well, and let it free fall for  a bit to get the long baselines. He also has an   amazing experiment underground called, oh gosh,  is it? I forgot the name of it now. But it is like   the arms of the atom interferometer are 300 meters  long. So this is huge. You can see here in sort of
• 92:00 - 92:30 the tunnels and so on. And in Germany, you have a  drop tower. That is like, what is it like this? A   drop tower. Yeah. So they put up here. Oh, okay.  Oh, a drop tower. They put up there like an atom   interferometer, and then they let it drop to get  these long interferometer arms and be able to be   more precise. Some other people also look at these  atom interferometries and put lasers and slow
• 92:30 - 93:00 down the atoms so that they get, so for example,  this paper by Guglielmo Tino is really beautiful,   trying to miniaturize the detectors. So what  I came up with this idea was, well, if you're   trying to do interferometry in using these sort  of, call it spatial interferometry, because the   atom goes through two different positions. Yeah.  The precision is going to be limited by how big it
• 93:00 - 93:30 is. So you're going to have to make them bigger  to be more precise. But if instead of that,   we do interferometry, not in space, but in  frequency, then what is going to limit your   precision is time. Uh-huh. So the sensor can be  very small, but you're going to have to produce   quantum states that live longer in time. So with  this idea that I called frequency interferometry,
• 93:30 - 94:00 I came up with a number of sensors, including the  gravitational wave detector. And then I applied   it to searches for dark energy, searches for  dark matter. I also patent an idea on how to use   these states to measure the local gravitational  field. So this might have commercial applications   in the future. And I like that because I  like more fundamental questions. Actually,
• 94:00 - 94:30 my favorite question is like, what's the nature of  reality? What are we doing here? Where am I? Oh,   yeah. It's a dangerous question, huh? Yeah.  Very. All of these things. But when you're   doing that and you find some interesting things,  why not also come up with something that can be   patented and commercialized and so on? And yeah,  then when I met Roger, I was really invested in   this. And I'm still working on it. I have some  recent results. It was in the old slides. It
• 94:30 - 95:00 doesn't matter. But I think I managed to give you  a flavor of what you can do by bringing together   quantum technologies and apply them to fundamental  questions of where things are at. I think I want   to finish by saying that this last proposal is  an example where we used not quantum mechanics,
• 95:00 - 95:30 but let's say a more fundamental theory  because it takes into account relativity,   which is quantum field theory in curved spacetime.  And although it's not the finished theory because   it cannot address the question of superpositions  of mass, you can apply it without problem to   specific cases like the propagation of spacetime  of packages in the spacetime of the Earth and many   other interesting instances. This allows you to  come up with, let's say, new sensors. And the
• 95:30 - 96:00 theoretical predictions that we've made is that  these sensors are so, in principle, they still   have to test them, so precise that you might be  able to detect a gravitational wave with a tiny   system. And these are for high frequencies, by the  way. They don't really compete with LIGO because   LIGO works in a different frequency regime. This  would be for frequencies higher than the ones that   LIGO detects. But, you know, let's say using these  patches of the theory that incorporate relativity,
• 96:00 - 96:30 I think already show you that you can, in  principle, make sensors that allow you to go   closer to these scales where I was talking about  that we don't have the guide to unify. You know,   when people were trying to detect gravitational  waves, the first apparatus that were built in   Maryland, you can still see them, there are these  Weber bars. So Weber predicted that the phonons,
• 96:30 - 97:00 so the vibrational modes of these big metallic  bars would resonate with gravitational waves.   And then he claimed that he actually had this.  He detected one, and then this got sort of   controversial and then eventually disproved. But  actually, the proposal that we made in which you   can implement it by using a BEC and using the  vibrational modes, like the phonon modes of the   BEC, but because you can cool the BEC to half a  nanokelvin, that's 10 orders of magnitude cooler
• 97:00 - 97:30 than the Weber bars were cooled initially, then  you can prepare the phonons in a highly quantum   state, which you cannot do unless you go to those  cold temperatures. And then you can exploit all   the sensitivities that we were talking about,  quantum technologies, to see changes in the   space-time. And that's how we came up with that  proposal. You know, I think I can talk forever,
• 97:30 - 98:00 so maybe it's good to leave it here. I think let's  see if I had some kind of concluding. Well, yes,   my concluding slide was to say that I've managed  to raise funding to build a new experiment. So I'm   working with Philippe Bouyer and Chris Westbrook,  who are going to test some of my predictions in   a new proposal that I have for unifying quantum  theory and gravity. And well, we're still working
• 98:00 - 98:30 very closely with Reuters. So I'm very excited,  because it's like a new era for me, now being able   to work this close with the experiments. And yeah,  I'll leave it here. Professor, thank you so much.   You've given far more than just a flavor. I lost  count of how many pioneering ideas there are here,   with actual practical consequences in the near  term. Near term being within a couple of years. I
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