What If Gravity is NOT Quantum? 

That bit with the equations ending up with the Schwarzschild radius is like the universe is doing stand-up comedy and going “thank you, I’m here all week”

My favorite channel where I don't understand 90% of what's going on but still continue to watch lol

Considering how much crazy stuff gravity is responsible for I wouldn't be surprised if it was even more fundamental than everything else we know

I will never be not amazed by people who REALLY understand math. You people are gifted beyond believe.

The fact that the graviton detector model maths spat out the Schwarzchild radius and so would prevent any confirmation of the existence of the graviton made me laugh more than it should have. The irony is amazing. "We did it! We made a graviton detector!" "Awesome! Where is it?" "In that Planck-sized black hole in the cor..."

I love how in all this insanely complex science the one thing Matt chooses to clarify is "1khz - that's a thousand hertz". Thanks Matt, it all makes sense now!

Us: what's gravity really? Universe: Shhh don't worry about it

It’s crazy how literally the most recorded/observed universal force still remains a mystery.

This is what S tier quality content looks like RU-vid. Promote more of THIS and less REACTIONS/PUNDITS/CLIP CHANNELS to the world. Signed the Internet.

This always confused me. Doesn’t Einstein’s theory mean that gravity is NOT a force but the curvature of Spacetime? If so, why would it act the same as the other “forces” and have any need to be quantized?

The fact that from quantum definitions, such as the uncertainty principle and Planck's distance, one can derive a relativistic concept such as the Schwarzchild radius, seems to indicate to me that there might be something there that connect both.

The more i become convinced Emergent Gravity proponents are onto something... gravity doesn't act at all like the other forces, most likely becouse it's not a fundamental force but rather an emergent one, like pressure

11:51 - Thank you for clearing up what a kilohertz was. Everything else in this video is easily understood, but without knowing what this strange kilohertz thing was, none of it would have made any sense whatsoever. So, thank you again.

I always wondered: why would it be quantum if gravity is just curvature of space?

Matt I did the survey, just for you! To show my gratitude towards you. So kind of you for posting videos that helps me to increase my knowledge and intelligence time to time! I eagerly look forward to more videos from SpaceTime! ❤

"Your videos always leave me in awe and eager to learn more about the mysteries of the universe. Thank you for fueling my curiosity. "

The way the slowly zooming stars do a parallax scroll when Matt is moved around the frame is just perfect.

15:05 I literally got chills when that equation appeared. is that as weird/spooky as it feels?

You know one thing that Matt said in an earlier video that I'm going to keep with me for the rest of my life. There really aren't any singularities. A singularity points to a gap in our understanding. Solving the singularity leads to new knowledge.

I love the fact I just begin theorising and mindstroming as soon as we started the questioning!

I hope someday all of humanity understands how important these videos are. Incredible work by Matt and the rest of the team; truly a gift to humanity

I want to thank Matt and the rest of the staff. This is my favorite channel of all time.

Matt discusses the near insurmountable problems of detecting the graviton (if gravity is quantum). But the question was what if gravity was NOT quantum. What if it's entropic gravity as contemplated by Sakharov and Padmanabhan? That's the question I expected to be addressed. How could we test those theories.

I'm a big fan of Freeman Dyson's book "Origins of Life" as someone belonging to a field of science. I had no idea he also had such a big contribution in the field of physics. The man's a legend.

This gives me a good enough reason to live the next few trillion years. Who can be done with life when we still don’t know if gravity is quantized?

1:36 I think you've got the strong and weak nuclear forces switched up there.

This is so unbelievably fascinating. Thank you for making this. I really wanna see quantum gravity figured out before I die

I think Wolfram's physics model that is computationally irreducible is going to give us the answer for how to quantise gravity.

You know, some years ago when I first started watching these Space Time episodes I had assumed myself pretty knowledgeable on the topic for somebody without a formal education, only to find myself rewatching episodes again and again to grasp ideas... Nowadays, I acknowledge the fact I have absolutely no knowledge on the topic and yet somewhat bizarrely only require one playthrough to understand what is being taught. That is to say, Matt has a fantastic way with words that has managed to educate a Dunning-Kruger effected simpleton like myself.

That Schwarzschild radius equation is the biggest middle finger in physics since the vacuum catastrophe

Thx, pretty good explanation of one of the most important questions in Physics..

Space belt 13 is mine just as soon as i do quad physics in a basic divine way. CN-12=N4 is dash light, but Bio 12 is inhalated light by some time lift thats out there beyond the sights of Jupiter.

The best part is when, in the middle of this very complex subject, Matt says, "...one kHz, that's 1000 hertz." Thanks Matt 😂

Wow! That’s uncertainty principle calculation for gravity beings us right back to general relativity 😮

If gravitons indeed existed and were emitted by mass, wouldn't that mean that mass would have to be diminishing over time, equivalent to the energy emitted through gravitons?

Best channel on YT by far. Keep up the amazing work. You inspire humanity

I'd love to see you explore this question more - what would be the implications if gravity simply is not quantum?

General relativity and quantum mechanics will never be combined until we realize that they take place at different moments in time. Because causality has a speed limit (c) every point in space where you observe it from will be the closest to the present moment. When we look out into the universe, we see the past which is made of particles (GR). When we try to look at smaller and smaller sizes and distances, we are actually looking closer and closer to the present moment (QM). The wave property of particles appears when we start looking into the future of that particle. It is a probability wave because the future is probabilistic. Wave function collapse happens when we bring a particle into the present/past. GR is making measurements in the predictable past. QM is trying to make measurements of the probabilistic future.

Consider the curious link between the Schwarzschild limit, which defines the event horizons of black holes, and the Compton wavelength, crucial for understanding quantum mechanics. What if these seemingly disparate scales, a geometric limit and a dynamic one, are connected by a process known as dissipative entropy, as explored in Prigogine's work, along with non-local long-range correlations? Taking Rényi entropy, which was discussed in a previous PBS video, as a form of dissipative entropy, we might find intriguing possibilities. One hypothesis around Rényi entropy involves the processing of imaginary spacetime topologies within black holes and their connections to black hole simulacra. This concept could serve as a bridge between gravity and quantum mechanics, suggesting that the expansive scale of black holes and the minute scale of quantum particles are both products of the same cosmic processes that shape the diagrams of matter-spacetime. The synthesis of these concepts could lead to a novel understanding of quantum gravity, where the universe's behavior is governed by principles that seamlessly integrate quantum entanglement, gravitational fields, and entropic dynamics. This unified behavior would reflect a cosmos where quantum and gravitational phenomena are different expressions of a deeper, entropic-driven reality. This perspective could offer a bridge between gravity and quantum mechanics, suggesting that both the geometric scale (Schwarzschild limit) and the quantum scale (Compton wavelength) are products of a cosmic process that shapes matter-spacetime topologies. I wonder if someone, inspired by Weirstrass' understanding of limits, as not static by dynamic, i.e. generated, produced, so if someone could derive the Schwarzschild radius and the Compton ray from dissipative entropy. Dissipative entropy goes beyond mere chaos; it's about the self-organization of the universe, influencing quantum states and spacetime's curvature alike. Imagine long-range correlations, akin to those observed in non-equilibrium systems like black holes, functioning like quantum entanglement but on a cosmic scale. Such correlations could elucidate the profound connections across different scales, pushing us closer to a unified theory of quantum gravity where the behavior of particles at the microscale and the structure of spacetime at the macroscale are derived from the same entropic underpinnings. The 'extremes' represented by the Schwarzschild limit and the Compton wavelength might be more closely related than we think, potentially linked through cosmic processes similar to wormholes (that may be the epitome of long-range correlations). This perspective implies that gravitational phenomena, from the macroscopic to the quantum level, are emergent properties arising from the same entropic interactions within the fabric of spacetime, challenging the traditional view of gravity as a purely classical force. Central to this discussion is the concept of nonlocality, which lies at the heart of quantum entanglement. By extending nonlocality to include gravitational interactions, we propose a mechanism by which quantum characteristics can impact and be impacted by the broader cosmic structure. This suggests that the universe is governed by entropic dynamics that effortlessly integrate the quantum and gravitational domains, pointing to the intrinsic quantum nature of all forces, including gravity.

I always try to come up with one takeaway from these very complex, wonderful videos. For this one: "Nature prohibits us from figuring out if gravity is a function of discrete bits of information called gravitons by forcing the production of a black hole if we try to measure something at the miniscule size needed to do so. Therefore, the best way to observe gravitons would be indirectly, but we aren't there yet." My 7-year-old brain's takeaway is "We'd better get cracking on this because being able to turn gravity on and off would be dope. Also, let's make some tiny black holes!"

This is definitely going in my "Most Mind Bogg-Ling stuff I watched in 2023" list. Love it.

Survey completed 💯 Also - been watching since high school. Now a fourth year physics and math major. Thank you for inspiring me. Matt is the greatest.

I appreciate the more in depth analysis even tho I have no idea what you're talking about

I do believe the reason higher-dimensional mathematics is so complex is because we've rooted the definition of orthogonality in perpendicularity, with no regard to implicit reducibility of composite numbers.

It's always seemed a little strange to me. To use an analogy: Gravity is a measurement of the bending of space (space flowing around mass) like light around an object could be measured by the shadow being cast. But you'll never find the particles that make up a shadow.

Thank you PBS Spacetime and Matt for teaching us. ❤

It's strange that gravity is at the same time the "least quantisable" force of nature, but yet it's so fundamentally connected with light, whitch itself has been quantised. Great video!

Amazing video. The appearance of the schwarzschild radius blew my mind. The relationship between Heisenberg, planck length, and black-holes is crazy. Did we stumble on a kind of circular reasoning? Or are we facing a fundamental relationship?

I completely understood 1×10 to the -37th of this video.

Looks to me like the programmers who coded the gravity didn't really talk with the people who coded all the other forces.

The equation for gravity, includes the mass of two objects and the distance between them. These objects are normally moving over a period of time. One could say, gravity is just a function of the movement of an object over time through space. Or space time. Maybe gravity isn't a force at all, but just a measure of how space time is affected by masses moving through it.

That was a great episode! Always watch y’all

What are the alternative theories of everything that doesn't rely on gravity being quantum (if it is indeed not)? I feel like that was missing from this video.

It'd be cool a video about negative masses and why they are not considered. Thanks Matt for the cool video!

That is actually crazy. What an underrated video.

Every once in a while a presenter hits a superlative level of communication. I believe this video is an example of that. After doing so many videos, to achieve such excellence is amazing. Keep up the good work.

Amazing episode! Thank you, I've learnt so much about cosmology from your show 😀

Not Quantum Gravity, but Quantum Spacetime!! One theory is that Mass slows time, which in turn bends Spacetime, which gives us the gravitational effect. So if Spacetime is warped by the slowing of time, rather than directly by mass itself, could it be time that is the quantum property, and as gravity is a consequence (not a cause), gravity is quanta due to its dependency on time.

As previously posted recently on this site, the number of dimensionless quantum unit squares needed to reconcile the universal gravitational constant with the Coulomb constant is M/m, where M is the (mostly hidden) mass of the universe, an m is the rest mass of an electron.

so, we have officially reached the unknowable, but possibly manipulatable. how abstractly challenging! now that is food for thought! love it!!!

If gravity ends up not being quantum, then I fully believe quantization will end up just being a more concrete example of some abstract quality that gravity also follows, similarly to how particles are just a convenient understanding of the excitations in fields. That said, the argument that "a graviton detector will turn into a black hole" is extremely compelling to me for non-quantized gravity, but it depends on how well-done that thought experiment was and whether there are any subtle assumptions lying in there that we can pick apart. (Maybe it's just because you guys and Dr. Becky posted at the same time, but I can't help but think of the issue with finding dark matter as particles too. Though that line of thinking has been pretty ravaged by bad PopSci and regurgitations, I feel like, so it's hard to approach as a layman who wants to be rigorous.)

Question: What if we assume gravitons are real? Can you make a video(if you haven't already) about what would that mean for quantum mechanics? Can we use hypothetical scenarios with gravitons to explain the universe with precision? Also, I love your show. You're the best!

Error at 3:45. His name is Niels Bohr and not Neils Bohr....

I am always impressed with the massive effort and competence displayed in these S-T videos. This one above many others.

Fantastic explanations and visuals, as always!

So what happens when you consider the possibility of black hole analogues for the other force carrying particles at sizes beyond the planck length?

8:15 Forgive my confusion but which complex interactions are you referring to here? With electrodynamics I can see how the magnetic fields may complicate the results but I am unaware of any analogous gravitational phenomena. Can someone assist?

I love how intriguing reality can be.

Graviton laser? I always wondered if that's what a 'tractor beam' was.

What if the Graviton is indistinguishable from a Planck Length blackhole? The Graviton's whole deal is mass, right? A maximally small singularity sounds a lot like a quanta of gravity. Maybe I'm just delirious from having Space Time on in the background all week.

I really appreciate this channel in all it's in depth videos, but I must admit not being a physisist, it's mostly very hard to understand. I would love some more items that bridge the gap a little more for us mere mortals

Fantastic video as always! I am sorry to point it out and I only say it because I am Danish and super proud of Niels Bohrs achivements. You spelled his name as “Neils” but the correct way is “Niels”. Just thought I’d let you know. Fantastic video and Matt is fantastic at making it somewhat understandable.

Gravity pulled us all here

Thank you for covering this subject. I have thought gravity may not be quantified but was always told you’re wrong campbell

Sorry if these questions are very dumb, but I'm genuinely curious about this: 1) Is this the reason why scientists are looking for a "Theory of Everything" that can combine General Relativity and Quantum Mechanics? 2) Is there a chance that anti-matter particles could be connected/related to negative mass?

This is a great educational tool for everyone. Than you!

I want a followup on this episode about Jonathan Oppenheim's theory of stochastic gravity (idk if that's actually what he calls it, but that's what I remember about it). He shows how we could resolve the black hole information paradox by assuming that, beneath certain scales, gravity is fundamentally stochastic, but not quantum. I.e. it is probabilistic but for different reasons than quantum mechanics is, so quantum mechanical laws like "information cannot be destroyed" do not apply. This means that infalling information is destroyed in his theory

Why in the fragment with four forces you have strong force for decay and weak force for holding nucleus together? Shouldn't it be vice versa?

New study discovered it. My math exam completed C-6 awhile ago. Im Physics D-5 now.

maybe the motion of stars around supermassive black hole at center of galaxy can be used to measure gravity field / waves for gravitons?

I'm still confused: what is the answer to the question "what if gravity is not quantum?" In other words, what are the implications for physics if gravity really is non-quantum? Would that be in any way paradoxical? (btw, loved the episode as always -- it's amazing how much awesomeness you fit into one of these little videos)

Thanks for the explanation of Freeman Dyson's analysis. I wasn't aware of it before. Loved the idea that a 10^37 improvement in sensitivity was "challenging, but ... not impossible"!

I always wondered about this.

Ad 1:29: "Three of the four forces of nature" is technically incorrect, since gravity is not a force. Then, why do we expect it should be quantized?

If gravitons exist, then what do we need curved space for? Honest question. I see that other people have asked about the "gravity is not a force" that we thought we learned from Einstein, but I couldn't find any answers in the ensuing conversations. Near the end Matt seemed to be saying that the notion of gravitons is kind of an open question, subject to potential future experimental confirmation, but that's the first I've ever heard that. Everyone always just glibly says that scientists want to unify the four forces, as though it's already known that gravity is a force (sorry Einstein!). Please, someone explain it to me like I'm 5.

matt im a neuroscience researcher now, i started watching the channel when i was 13 or 14 when the other host was still there. I am super excited about your documentary and i think itll be the only documentary ill pay for directly... when is it coming out? (the documentary analyzing how our brain constructs reality and the biases that arise in our conception of physics as a result) thanks for pbs space time for being an S tier channel for years now... you guided my intellectual development in ways i probably will never be able to truly appreciate

I hope you can come back to this topic of measurement by contrasting the proposals that assume a quantized gravity with the recent papers from Oppenheim, where he considers measurements for stochastic or randomized gravity, where General Relativity remains classical and unquantized. My understanding is that in his perspective, the GR gravitational field is not a boson-mediated force at all (in the standard model sense of force), so there are no gravitons. So what is Oppenheim's proposal for a measurement that supports his equations vs. the proposed quantized equations from String Theory or Loop Quantum Gravity or something else?

Uncertainty is a bizarre point dynamic that isn't uniform as it flows through all it's possible shapes and forms, while the speed cannot be understood clearly since the object isn't quite uniform like a ball. Something like that right? And also we don't know how the electron orbits the nucleus, we just have shaped clouds that show the likely areas it can be at with minimal energy, unless photon gives a bump, or is it absorbed, what's the difinitive proof? What information can we gather when we collide particles in as many off center configurations as possible, would we get new unseen hadrons? Helps in seeing if subatomic particles have even more dimensions to their constructs and properties to manifest

I was reading an article by Ethan Seigel today where he was answering a question about if gravitational waves had wave and particle duality. He responded that we just don't know but that if we could figure out a way to do the double slit experiment with gravitational waves we could find out if gravity is quantum or not.

I love any GR + quantum episode from Spacetime!

Itll take me tome to discover more about the belt to move up, but i know when its time to level up.

It seems like in the E/M case, the fundamental uncertainties exist in the position / momenta of the particles, rather than the charge the particles possess, which allows us to reconcile fundamental QM assumpions from an electromagnetic perspective. Do we have a fundamental (and effectively impervious to heisenberg uncertainty) unit of mass? Is it possible that the field related to the mass has no uncertainty, but the mass does?

Hey there, love your videos! I usually watch with subtitles on and noticed a few transcription errors in this video, some of which might make some people grumpy. 1:40 Quant gravity -> quantum gravity 3:28 the mid 192 -> the mid 1920s 4:49 one AIS of -> one axis of 5:33 field so bour and rosenell -> field so Bohr and Rosenfeld 7:32 principle but war and Rosenfeld -> principle but Bohr and Rosenfeld 8:42 B and Rosenfeld -> Bohr and Rosenfeld 9:11 B and Rosenfeld -> Bohr and Rosenfeld 10:41 the two Lio facilities -> the two LIGO facilities 11:06 gravitational W of detector -> gravitational wave detector 11:30 a ping 5 m 630 NM red -> a piddling 5 mW 630 nm red 11:47 10^ of -1 Jew per cubic M -> 10^-11 joules per cubic meter 12:00 3x 10^-48 Jew per Cub M -> 3x 10^-48 joules per cubic meter 12:20 10 37 -> 10^37 12:29 10 ^ of 37 -> 10^37 15:16 for the SQA Shield radius -> for the Schwarzschild radius 15:27 black coal -> black hole 16:49 vein as the B Rosenfeld -> vein as the Bohr-Rosenfeld

Thank you for the video. I love this content so much. Some maths today and also a strong question. If we can't detect a single graviton, do we really need to struggle to extract that theory from our beloved space-time? That's kind of fundamental. What about a non-serious video about muon-mediated cold fusion :) ? Or a video about spin echoes, Larmor procession and NMR ? I'm now working with it. Going for the patreon !!

Hello there! I have a little tough question for you. Hope you will respond. Here's the question: Can we use photo-sensitive material to make double-slit plate? I think by doing this can help us to observe which slit the electron has chosen to pass through. Another advantage of this could be that the disturbances produced from using the detector can be reduced. Thanks

LIGO but at the earth legrange points, no clue how useful this would be but it came to mind while watching

I'd love to know what breaks down with Liouville quantum field theory, a two-dimensional model of quantum gravity, when it's moved to 3 dimensions

I don't understand why gravity is regarded as a fundamental force. If it's the shape of spacetime that causes matter to move, it seems to me it's a property of spacetime rather than a separate field/force like electromagnetism etc., and gravity is a wave in the medium of spacetime rather than a wave in some field; and even if spacetime is quantized, then gravity would be quantized by association, not because it is carried by a boson of some kind. Can someone explain this to me? I'm sure I'm missing something. Also, how were weak and strong forces shown to be quantum without the positive/negative trick?

Go out to Endorcand take the soace test at the Chyming Platter. Near North towards center by east facing but central planet.

I've noticed that the list of pairs of properties said to be simultaneously undefinable in Heisenberg's uncertainty principle (HUP) are similar to the symmetries and conservation laws in Noether's theorum. Can anyone tell me if/how these two ideas are linked? For example, position and momentum can not be simultaneously measured with arbitrary precision in HUP and translational symmetry (invariance under changes of position) gives rise to the conservation of momentum in Noether's theorum.