"just quickly, let's review all of quantum mechanics" LOL

"Let's review all of quantum mechanics." "Actually, quantum mechanics forbids this."

I feel like a dog watching humans talk.

9:07 - For everyone wanting to learn more about the topic - "Quantum Gravity" I suggest reading these two books - "Three Roads to Quantum Gravity" by Lee Smolin and "Reality is not what it seems" by Carlo Rovelli. They are masterpieces especially the one written by Carlo Rovelli really gives you a great insight into this topic.

"Loop quantum gravity tries to quantize general relativity, with no strings attached." I see what you did there!

"So quickly, let's review all of quantum mechanics" Sounds like the last math class before an exam...

"A seriously loopy space-time, with no strings attached."

I find it fascinating how deep we're going. It's going to get to a point where it's meaningless to talk about spacetime, mass, charge and other familiar concepts to us, and it's going to be purely abstract exchange and evolution of data. It's like Sims finding out they're made out of bytes.

I'm going to need this explained again... much slower... with finger puppets.

*Best. Host. Ever.*

"Bröther may I have some quantum lööps?" -Schrödinger's cat, shortly before death (maybe)

"With no *strings* attached.". Hands down one of the best puns ever.

If space is actually a 'pixel grid' of loops, and space is expanding, does this mean that the loops themselves are expanding? Or are the loops 1 dimensional and therefore cannot expand. I would assume if they have constant size like the planke length, there are more loops being created as expansion occurs. I know common sense doesnt work at this scale so any help would be appreciated! Ty!

Me: Oh boy, I am tired. Today was really exhausting. PBS Space Time: Loop Quantum Gravity Explained Me: Yes I definitely have the energy for this. EDIT: Okay, I actually did get a bit of a hold on this concept. I definitely need to start watching more of your videos to truly be caught up, but I cannot express how grateful I am for you guys to be making all these ideas approachable for people like me. It’s a gift.

Masterpiece of physics presentation. For any budding physics geeks 16 year old, this is a treasure.

It must be weird being that guys kid, "son, it's time we had a talk about loop quantum gravity"

“bröther, can I has quantum lööps

Me: oh look a new PBS Space time video, and the title sounds so interesting! 2 minutes into the video: I have no idea what is being discussed here and I think my brain is oozing out of my ear

This is beautifully explained - I'm a physics student, finishing up my fourth year at uni, and even with having studied Quantum Field Theory and General Relativity, I find string theory and LQG almost unapproachable to understand. Most explanations I've found tend to get buried in the maths without every really explaining the idea of the theory itself . This, however, succinctly explained both the motivation and idea behind the theory in a simple manner. Thank you very much!

If anyone is interested, a really great book on this is "Reality Is Not What It Seems: the journey to quantum gravity" by Carlo Rovelli. He makes the topic super approachable while also going into enough detail that you feel like you actually learned something. I would totally recommend it! (But maybe skip the first couple of chapters because its just kinda superfluous history 😅)

"How to talk to your kids about loop quantum gravity"

Whoever does your graphics, give them a high-five for me. They are well thought out, and awesome.

'Not true, damn..." Very nice. Took me a second. It was so good that if it hadn't have been for the preceding puns, I would have definitely missed it.

I would say this is probably one of the best videos, educational monologues, that I have ever had the pleasure of listening to.

In today's episode of "holy hell that title's already making my brain hurt" Followed about twenty minutes by "brain hurts a little less but this makes more sense I think?" Followed an hour later by "I still don't get it :("

I understand literally every other video you've done, but this space of loops sends me for a loop in space.

im very thankful for your videos. I have been following since the start of my qft journey. You guys give the big picture of what all this maths is for

So you come home, and there he is, Matt O'Dowd, sitting in your kitchen: "It's time we talked about Loop Quantum Gravity."

"So just quickly, let's just review all of quantum mechanics." Good one, Matt.

I see you've broken down holonomy and spinor bundles into somewhat digestible somewhat normal words, and I optimistically challenge you to do the same for topos theory and quantum logic.

"So quickly let's just review all of Quantum Mechanics," best throw away line so far.

" I like my gravity loopy, not stringy"

I think I stroked out after he said "So just quickly let's review all of quantum mechanics"

I'll never open a can of Spaghetti-Os again without thinking of loop quantum gravity.

"Which is a concept too abstract even for this episode." Hahahaha!

One of the most comprehensive videos I’ve seen on this subject yet.

I've been watching (and rewatching) Dr Leonard Susskind's lectures on General Relativity. So I think I actually understood most of what you presented here.

This is one of the best PBS Space Time episodes I have seen yet. Good shot!

I think this theory has more potential than string theory because it is much less complicated and it might predict something verifiable

I'm so amazed i understand this, thanks for the awesome explanations every time

How do we think about cosmic inflation and expansion in the context of LQG? Is it new loops being added to the fabric of space? Or just a reconfiguring of the connections between them? Or both? What does a black hole look like in LQG?

Bröther, may I have some *Lööp* gravity?

I only understand about 20% of the content in these videos but I can't stop watching them.

Really nice explanation of LQG. Great fun! Thanks to your supporters.

woo ive been waiting for this episode for so long! as a fan request, could you do one on quantum darwinism or conformal cyclic cosmology (ik the latter you gave a quick reference in a video a bit ago), i feel like especially with the quality of animation you guys have both of those topics would be super lovely to watch a video on

Universe: Okay little humans, what is this picture? Humanity: Uhm...it's a ball! Universe: Are you sure? Humanity: Uhm...no, it's a string! Universe: Are you sure? Humanity: I dunno. Oooh, wait, it's a loopy thingy! Universe: Sure about that?...

Lovely video , THANK YOU. Thank you for not watering this down and great job with reduction.

"So let's just quickly review all of quantum mechanics". Delivered completely straight and dry. I love it.

I remember having commented on LQG in another episode, esp. on its prediction of slowed-down gamma photons - an effect that clearly violates Lorentz invariance. Indeed, LQG, as designed by Lee and Smolin, treats time as an extra variable different from space. I would be really glad to see LQG translated to a manifest Lorentz invariant form...

Very interesting. I wonder why exactly they started with an abstract space of metrics on 3D slices instead of an abstract space of 4D metrics in the first place? I'd love to learn more about this problem of time.

It seems to me that the big issue in fundamental theories of everything is in the limits of human conceptualization: To understand a theory, we need to base it on something, which takes the form of "holding" one concept in place and defining all other concepts by their relationship to the held concept. Relativity "holds" concepts of matter and energy in place to talk about space and time, while quantum mechanics "holds" the concepts of space and time in place to talk about matter and energy. The problem arises because a theory of everything, by its whole point, CAN'T "hold" any concepts in place, because then it wouldn't be describing that concept at a fundamental level; but a theory without any concepts held in place is missing the basis needed to relate concepts together. This is why String Theory and Loop Quantum Gravity both end up inventing new concepts (gravity loops, strings); they need some concept to be the basis on which to build everything else... but that just means there's a new concept that needs to be explained, and the theory isn't quite fundamental. There may be an argument to be made that an actual theory of everything, at least in a form comprehensible to us, is actually impossible.

Awesome episode! I'm going to have to watch this a couple more times.

13:01 Thank you for mentioning that. That was such an amazing observation. As attractive as loops are to me when compared to strings, they don't seem to agree with the data. As Sagan put it, our preferences don't count.

I'm a simple man. I see a new PBS Space-time video and I press like.

Interesting as always. In a perfect vacuum, modeling the most particle-sparse regions of the universe, is there an expansive effect?

You know, I love that this show exists and I have been a patreon supporter for quite a while... but some of these episodes... melt my brain. I am just a poor social scientist! I won't stop watching... but man this channel is intense sometimes.

Thank you so much for reminding me how little I know, friend. I'm gonna go eat some crayons now.

Awesome, I've been waiting for this!!

Since loop quantum gravity defines the smallest possible time as well as the smallest possible measure of area and volume, could it be possible the appearance of wave functions/particles duality arises from the topographical uncertainty of the quantized "space" said particle information has to pass through?

16:10 - Man! I want to hear more about naked singularities! Looking forward to the episode!

To those who are interested in loop quantum gravity and perhaps are quite confused about it, I would highly suggest reading the book "Reality Is Not What It Seems" by Carlo Rovelli. It is so beautifully written, and it gives an entirely different perspective on the nature of reality where time does not exist.

One of the biggest things I've always had trouble wrapping my head around about LQG, is why spin is the quantum operator that is used to mathematically represent the fabric of space. In what way does adding quanta of spin - as opposed to any other quantum operator - to the connections used to describe space, make the space of metrics appear just like a space of quantum fields, thus enabling gravity to be quantized, in a way that it could not be with connections built from regular ol' vectors?

Loop quantum (and sub-quantum) gravity can be easily explained. The newly researched electro-static (ES) and electro-gravitic (EG) model solves and fully explains the gravity question ... and the answer for boson force carriers. Aether gravitons - and + are the composites of Aether electrinos and positrinos, that are composites of smaller Aether electrons and positrons, ... that are the composites of conventional electrons and positrons, that make up the atomic proton and neutron, ... and the higher muon electron, muon positron, tau electron, and tau positron. There are additional multiple Aether particulates and matter particles. The electro-static (ES) and electro-gravitic (EG) model shows that the quark space-time fabric has its associated gluons acting as attractive force objects. Gluons are bosons that also display mass (say gravity), but they are also FORCE CARRIERS (i.e. tensor bosons of electrical and gravitational FORCE) showing that space contains a gravity and tension particle, that displays its gravitational force, gravity tension field, and gravity wave. These gluon bosons associate with their quark particle, and they also create compositions of string and ring theory, leading to space-time fabric theory. As such, there are 6 gluon gravity constants ... and their space-time fabric and tension Up quark, up gluon boson, ... down quark, down gluon boson ... Up/down fabric Charm quark, charm gluon boson, ... strange quark, strange gluon boson ... Charm/strange fabric Top quark, top gluon boson, ... bottom quark, bottom gluon boson ... Top/bottom fabric While the quark space-time fabrics contains gluon boson force carrierss, ... in the Aether and matter realms, there are Aether bosinos and matter bosons, and their respective Aether and matter space fabrics as tensor bosinos/bosons and force carriers. Tau electron tensor boson force carrier Muon electron tensor boson force carrier Large Electron tensor boson force carrier Smaller electron tensor bosino force carrier Electrino tensor bosino force carrier Graviton tnesor bosino force carrier These same-levels also have their neutrinos/neutrons and photinos/photons, which are said bosino/bosons, thus they also are force carriers and display their own manifestation as a force carrier. So quark space-time fabrics DO HAVE gravity, FORCE, and fields, ... even so matter space fabrics have gravity, FORCE, and fields. Bosons are force carriers (i.e Hubble tension, cosmic tension, with their particulate and particle tensions). When you have these Aether particulates and matter particles, you have: Matter realm: Tau electron, Tau positron, Tau electron neutron boson, Tau electron photon boson, and Tau tensor boson force carrier Muon electron, Muon positron, Muon electron neutron boson, Muon electron photon boson, and Muon tensor boson force carrier Large Electron, positron, electron neutron boson, electron photon boson, and electron tensor boson force carrier Aether realm: Small Electron, positron, electron neutrino bosino, electron photino bosino, and electron tensor bosino force carrier Electrino, positrino, electrino neutrino bosino, electrino photino bosino, and electrino tensor bosino force carrier Graviton -, graviton +, graviton neutrino bosino, graviton photino bosino, and graviton tensor bosino force carrier So gravity is a FORCE, that the actual objects of such gravity are now known and able to be understood in simple terms. Quantum quark space-time fabrics and matter space fabrics are part of the gravity answer Sub-quantum Aether matter space fabrics are the rest of the gravity answer. As such, gravity is both quantum and sub-quantum. One can have a pure Down-Up-Down-Up ... string, .. or the higher Strange-Charm-Strange, ... Bottom-Top-Bottom strings but these electro-static (ES) string ... can also have their higher gravity loops (ring theory) when their strings are distributed with their quark gluon tenson boson force carriers. Up-g-down-g-up-g-down .... As string theory develops into fabric theory and loop (ring) theory of gravity fields, gravity force, and gravity waves ... even so the same matter fabrics can have: (Electro-static) Electron - positron - electron - positron ... => electron string => electron fabric or electron loop (ring) gravity Inserting an equivalent graviton energy object into this string makes the electro-gravitic (EG) string, space fabrics, and loop (ring) gravity. E - g - P - E - g - P - g - E ..... The electro-static (ES) and electro-gravitic (EG) model fully explains over 31+ common separate physics conundrums and anomalies that they can't explain, or admit that the Aether exists, or "what" object is gravity, or "what" is gravity, etc., etc..

Wow... It's going to take a few viewings to understand this, even though you did it an amazingly non-mathematical way. Something tells me this is the most accessible description of Loop Quantum Gravity anywhere.

Can you make a separate video on the experimental predictions of both string theory and loop quantum gravity ?

09:20 I would have called this theory "Chainmail Spacetime Gravity": CSG [the letters themselves even look like what it is described on each aditional dimension].

this was such an amazing watch! This actually jogged my mind back to a notion i was contemplating trying to understand gauge theory, for some reason in my head it related to governing of limiting degrees of freedom for subatomic interactions but governance with the shape of extremely small extra dimensional space, when this stuff connects in this conceptual way.... it gives me goose bumps.

I learned about LQG after I had an insight into Turing machines, the stopping problem, and the discreet nature of the Universe. A puddle of mud is Turing complete depending on how its inputs and outputs are considered. This Turing completion happens in nature at every scale both quantum and GR scales. This sort of Turing-completeness is almost axiomatic. Its a property of the universe itself and everything from computers, human brains, to bottles of water contain some form of virtualization of the outer universe they are contained in and can be used to simulate the variables of any other phenomenon in the nature inside or outside of that system albeit at different "speeds". As an amateur I can only vaguely express my thoughts, but I have this bet that the nature of the universe is in fact discreet and that by looking into the nature of "pure virtualization" is key. it is very logical to assume the universe itself another one of these virtualizations. A "pure virtualization" is a concept I came up with for a virtual machine that is described in a 100% machine-independent turing complete code. So for example writing an emulator for an X86 PC in ANSI C with absolutely zero reference to the original machine in a way that the emulated machine could in turn emulate itself using the exact same code thereby defining its own architecture from either a physical description (from without) or an abstract description (from within). Either way, it's impossible to tell which one is the "real" way to describe the virtualization. Both are correct because both are actually the same description relative to any other abstraction that lies above them in the hierarchy of virtualizations. So far my money is on a Universe that is a virtual machine with two fundamental operations "Instantiate" and "Null". One spawns itself and the other is nothing (or a hole if you will) and only through the two of them forming a progressing single-dimensional fractal graph pattern does time, space and information exist. The 'connections' talked about in LQG and other theories to me seem like these graph nodes, and waveform collapse is just the way our human VMs interpret the progression of this information. The progression of information on a graph being what we know as 'time'. The idea of gravitational pull comes from asking the question from our experiences of being pulled toward instead of understanding the universe as 'progression of information from the big bang'. Gravity is the result of regions of higher entropy in the mass category of abstractions. Electromagnetism is equally relativistic and follows the same principal. Example: Just because there's a shorter distance mass-connection wise from the anode to the cathode of a battery cell across the air, doesn't mean relative to an "electron" (I'm simplifying a quantum informational concept here, but you get the idea if you're following along) the metal wire is a shorter path - at least when we frame relativity in terms of 'relative to the big bang'. The kicker is that mass, electromagnetisms, and even the concept of > 1 dimension are actually abstractions defined by US as creatures of Earth/physical domain and are meaningless to the quantum soup that is the universe, but AS mentioned above it's still a valid description regardless of being defined from within us or from without the universe. I think a lot about this stuff and should have probably gone to school. Am I insane or am I a genius, I don't know. Someone give me an honorary degree or commit me please. I am fairly certain I am sane but that too just seem so relativistic when you've seen the things I've seen! I think I know what Turing and Von Neuman saw when they looked at this stuff and it's incredible.

I feel like every time there's some weird shit going on in physics and cosmology, Carlo Rovelli is always involved.

So your explaining something I have never heard of, this should be interesting.

I have absolutely no idea on mostly what you are talking about but I keep coming back because it's interesting and I want to learn

This makes so much sense on a macro level. GR tells us the background or stage (space) is dynamic, therefor QM must take that into account. Einstein tries this in the 1950’s but it was a mess. In 1965 the Wheeler-DeWitt equation attempts this, but is unsolvable... until you utilize the information in the connections of space. Those ideas and that path seems seem like a very logical way to reconcile GR and QM. This is currently my level of understanding and I get lost in the math beyond that, but it seems like the idea for the structure and form of loops comes from the connections utilized in the math. Going from mathematics and abstract understanding to a solid picture is always the hard part. Moving from analogy towards reality. Love this stuff. Graphics guy or team is amazing. Best in the biz.

Just noticed that you're saying "Ashketar" instead of Ashtekar. Easy to do, I guess. :)

I would like to take a moment to appreciate how unfathomably smart these people are.

I waited for a video on loop quantum gravity... And here it is... ♥️♥️♥️😎 Thank you for making Awsome videos♥️😍

For those interested in gravity, I can whole heartedly recommend a book, Three Roads to Quantum Gravity, excellently written, and somehow makes complex topics understandable without losing (edited from "loosing") the depth of the material.

Oh finally I've been waiting for this one for so long 😁

I would be quite interested to know how the properties of the gravitational field change depending on the temperature of the body. When energy is added to the motion gravity should increase. The same thing should work with energy supplied in other forms.

This is actually really intuitive to follow

That fractal loop spiral you showed...where can I look at a good 10 minute video of that. That was so awesome. It went 3D.

I've looked practically everywhere for the idea of representing quantum gravity field carriers with ultra-stable Planck-scale bubbles of true vacuum, where each bubble is treated as having a negative energy core enveloped by positive energy whereby the opposing potential values allow gravity effects to be Planck-scale small but at the same time extremely weak with very low net energy, in notable contrast to field particles completely dominated by positive energy such as photons. String theory loops suggested for gravity quanta have some things half in common with this idea if the bubble's negative energy core is displaced slightly to make a dipole, or a little more in common if the core is extended along one axis to create an opposed dipole pairing effect. Consider the string theory loop to be a toroidal flow-surface and the effect experienced face-on is reminiscent of being peripherally dragged-in while passing through a short hole, as opposed to getting the head-on direct pull of a negative energy effect. Such true-vacuum bubbles conceivably can have zero rest-mass, possibly plus or minus a basic quantum of rotational inertia setting it into ultra-slow rotation so it evolves along its radiated path. Pretty good at searching for new ideas that may or may not actually be new, did it for a couple of decades as a primary patent examiner, but I can't find this stable bubble idea, rotating or nonrotating, anywhere. I guess it's all mine unless everyone who matters decides it's correct.

Been waiting for this video, thanks.

Could you do a Video about the big bounce theory and how it works without singularities? I've always wondered why the classic Theorys predict singularities. Couldn't there just be a point where the collapse stops, for example at the Planck length?

The loops in loop quantum gravity don’t sound that much different from looped strings. I guess the math probably makes it more distinct by not needing multiple dimensions but can you expand on the differences of these fundamental pieces, please?

I like Matt's presentations. He is easy to understand, though he is delivering some seriously dense content. He doesn't talk like an auctioneer, nor like a teacher talking to a 7 year-old child. That's a terrific balance to strike. I don't claim to fully understand LQG after watching this, but I am quite surprised at how much I learned. I feel I have a basic understanding, enough for an amateur to be getting on with.

A deeper video on spin networks would be pretty cool

My loop gravity is always attracting me to this video all the time

The most promising idea nowadays in physics (in my opinion) is that gravity is an emergent property of quantum entanglement. Search keys are: 1) Raamsdonk, gravity, entanglement; 2) Susskind, ER=EPR.

"Notre-Dam" *snaps* Too soon. Too soon.

Read all Carlo Rovelli books, even bought his Covariant Loop Quantum Gravity !! Your video is such a good explanation of Ashtekar, Smolin and Rovelli's work... thank you very much for that ! A french fan of the only real quantom gravity theory that could work indeed xDD

I rarely understand what he's talking about but enjoy hearing him say it.

The wave function described as, a "fuzzy space of possibilities", is excellent. It really helps me broadly grasp what this is all about.

We’re so close to finally finding a cure for loopis!

Almost like a there was causality universe collided with the singularity and both are intertwined. Definitely awesome channel.

Great episode as always

Universe *slaps tummy* "It can fit many löps"

"Moving through 3D slices of 4D space-time" My first impression is that the movement would be a topological homeomorphism. Am I perceiving this correctly? If not, what is it really?

That's a good explanation, thanks, just what I wanted to know.

Find my mistake, please! Energy is detected as various forms of motion (vibration, speed, etc.). Motion only makes sense in terms of spacetime, because is the variarion of something in regards to spacetime coordinates Gravity warps spacetime, giving particles gradients of "energy cost", in the sense that, depending on direction they'll need more or less energy to move. Were we to understand all movement as consequence of spacetime warping, energy would be the amount by which you bend a particular amount of spacetime. The limit cases for motion that we are aware of are light speed and blackholes. If light speed is a speed limit for particles, then following our assuption above, it would be the maximum elasticity of spacetime for a foton of space. No particle can travel faster because there isn't a way to bend spacetime in that scale by more than that. On a large scale though, certain spacetime warping wavelenghts could have constructive interference and merge into bigger waves, eventually generating a wave so big that it is larger than the warping of light, hence the blackhole. Different particles would be different wavelenghts of spacetime warping, and that's why they'd be uncertain for us to measure: they're spacetime warped. Different properties perceived bu us like color or weight are dissonant spacetime waves traped in a larger one, like higher pitched sounds being transported by a lower pitched one. Thanks for reading, sorry for bad english and crazy thoughts :P

what he said: It's too abstract to visualize what I heard: space is a knit tube, got it.