You have finally crossed the boundary marking the limits of what I can currently understand.

There's the physicists Oppenheim and Oppenheimer, now all we need is an Oppenheimest.

Matt O'Dowd is unquestionably a fantastic host, as was his predecessor, but I wanna give a shoutout to those workin behind the scenes, especially the graphics team. Trying to visualize quantum phenomena is an actual exercise in madness, and you guys always make it professional and, at least as far as my tiny brain can understand, pretty good visualizations. The music is chill, mildly spooky, but with a scifi twist, which is spot on for this kind of video. The title cards are always fantastic, and rarely do I see an upload where I think "Iunno, that doesn't sound very interesting." Everyone on your team does a great job, EVEN YOU SUSAN, so keep it up! We appreciate every unit of planck time and body superfluids you guys put into these videos!

13:35 "... although the derivation is a bit much for this episode." Noooo! I need more! Entropic gravity is one of the most interesting subjects Spacetime has covered in recent times! Please cover more!

Having the Newtonian gravity equation just 'drop out' of entopy math really feels like a thread worth pulling

This wins 1st place for the number of stops, rewinds and pauses. A thrilling journey - Thank you

The algebra connecting entropic force to newtons universal law of gravitation is an absolute masterpiece. I never realized how beautiful math could be

This is the absolute pinnacle of space and time series on UKposts. Although I have to watch it several times to comprehend it. I thoroughly enjoy every episode. Thank you!

Starting from thermodynamics and going to gravity is a really cool idea.

'if you allow that entropy to hold you in your seat for a bit' 'No' *floats away*

The animations/visuals of this channel have always been really good, but at this point I would be utterly lost without them. Thank you to the writers and illustrators. Your combined talent almost lets me understand the most bizarre and abstract ideas.

Man, my favorite youtube channel that i don't actually understand.

Watching the animated algebra as a way of summarizing the path between two versions of the equation is AMAZING. Just a good bit of explanatory visualization, that.

I really like how you guys don't shy away from hard-to-wrap-your-head-around topics. Ya'll did a great job of breaking this down into consumable pieces, thanks!

I love the direction this series is heading! Please don't stop.

I just had a nutty thought. Wouldn't it be weird if the universe is recording information on it's surface depending on the events within it, and that's why it's growing? Like a hard drive that's constantly saving all of the operations of a computer, past and present, and in doing so must grow in size? Weird.

Fascinating, I didn't understand any of it. But gravity as an emergent force has a certain elegance to it.

You know what was also emergent? Plant walls! They messed up alpha glucose and made beta glucose which was just a flip on one side’s connection, so it became water insoluble and they didn’t have the enzyme to break it down, so their cells pushed these fibers to the edge of their walls. This mutation was just left there.

I've been thinking this was the case since my high school physics class. I even wrote a paper on it, and I got a C- on it, for it being "science fiction."

The animation is just perfect, describes exactly what Matt says 10/10

Please tell Leonardo Scholz that their animations were/are sheer perfection

What if gravity was just the friends we met along the way.

it still could be a self circulatory derivation of the end equation. By introducing of Beckenstein-Hawkings corelation for enthropy and gravity one "injects" inderectly the Newtons law of gravity in to the thermodynamical equation.

the outro music when he doesn't answer questions is so spooky and threatening

@PBS please make him the next host of Cosmos v3.. I think Carl would approve.

That may be the most satisfying physics derivation I've ever seen. You guys are doing awesome stuff!

One little problem: If Bekenstein-Hawking formula uses grav theory and thermodynamics to derive the calculation for entropy at the surface (13:09), this means Verlinde is also using gravity (+Thermodynamics, +Q-theory, +Relativiity) to derive ... Gravity ? Voilà

I have been working on a novel hypothesis for the last 2 years. I call it the time force hypothesis. It started out as a quest to understand gravity but ended up being much more. Not only is the idea a solid and plausible explanation of gravity, but it also explains the enigmatic phenomena of time as a mechanical action caused by the expansion of our universe. The basic premise of the idea is speculative at this point but is absolutely intuitive from a logical perspective. Essentially, it assumes that time is the 4th spatial dimension, and we, as threedimensional entities, reside on a membrane well-described by quantum field theory. This membrane is the fabric of our observable universe and is suspended in 4-dimensional space. A major event occurred in the 4th dimension, causing the energetic pressure on one side of the membrane to be higher than the surrounding 4D space. We know this event as the Big Bang. Since that event, our lower-dimensional 3D fabric has been thrust through the 4th "time" dimension. Our experience of this acceleration is both what we call time and gravity. In other words, we experience irreversible time because our 3D fabric is moving at an incredible velocity through 4D space. This is why matter has such incredible amounts of energy associated with it (E=mcA2), making the famous equation a calculation of momentum held within matter as we hurl through a higher dimension. The above description of time was derived from my attempt to define what gravity actually "is." Most people don't realize that the greatest minds in modern physics have no idea what gravity "is." So, I began researching the issue as a full-time job about 2 years ago. After thousands of hours and nearly 1 00 physics books, this is my idea. The Big Bang event was not the beginning of everything in 4-dimensional space but rather an energetic disturbance. It was the beginning of our 3D universe. The hot, dense moment created an energy gradient in 4D space, and this energetic pressure wave is the driving force of our 3dimensional fabric. Through Einstein's equivalence principle, we understand that this acceleration would affect our universal reference frame with the "equivalence" of gravity, hence the name "equivalence principle." I postulate that the perfect amount of acceleration since the Big Bang is what causes/produces Newton's Big G. Since this acceleration has been applied to our fabric, we have traveled an incredible distance. Variations in density in the early universe caused the fabric to deform due to varying inertial values, creating a surface topography described by Einstein's theory of General Relativity. If we adopt this perspective, unifying quantum mechanics and gravity becomes easy by removing it from the picture altogether. Quantum phenomena like waveparticle duality and superposition can be explained by the interaction of quantum particles in a stable 4D loop. Quantum entanglement and the measurement problem can be explained as intersections between particles along their paths in 4D space. Matter's natural frequency/vibration could be linked to stable 4D loops as energy packets race around their track through our 3D fabric's quantum field stack. On the other side of the size scale, we might speculate on what Dark Matter is. It could be nothing more than the residual warping of our 3D fabric caused by mass that progressed through the region at some point. This might be why dark matter exists in large amounts around galaxies and other large-scale objects. This hypothesis also explains Dark Energy, the 4th-dimensionaI energetic pressure that makes up 69% of the universe, creating a giant growing hypersphere that we reside on the surface of. The Time force is the remnants from the first cause that send us on this journey through time. In short, I propose a speculative but intriguing explanation for time, gravity, dark energy, dark matter, quantum entanglement, superposition, and waveparticle duality with one postulate (the 4th "time dimension is spatial) and one action based on the second law of thermal dynamics, all in the length of a UKposts comment. If you actually read this far and would like to read drafts of my actual scientific paper I have been working on for the last 2 years, email me at timeforcehypothesis@gmail.com. It's a far more formal read, and I get into a bit of math, but I'm always looking for feedback.

I usually understand some of what he's saying but today...I got nothing. I'm going to have to study up and watch that again

The audio quality this episode is way better than it has been lately! Matt sounds like a real person again and not an AI!

haven't watched pbs space time in a while, the old intro has such nostalgia. I'm gonna miss it

I see whoever made the visuals on this visited the same DMT space as me. Right on buddy!

Ever since I learned that gravity is curvature of space (and more or less understand it) I've a hard time to see gravity as a true force, let alone a fundamental force. The entropy approach is new to me, certainly an interesting idea. Would love to see a video about the link with dark matter.

I have no idea what is going on but I watched it all.

Have you guys covered the research that was done that breaks the concept of relying on Dark matter? It showed the universe might be much older than we once thought. I won't explain the whole thing. It's just fascinating to see a concept that gets around the road block physics has been stuck on for yrs. It's worth looking into it

Please do the next episode in this series next! It's a challenging viewing experience to have a miniseries like this interrupted by unrelated episodes, because the concepts are complex and it's not always easy to track down the earlier episodes later

Reminds me of an emergent property of mass. Like water is wet only when you have enough.

Honestly, the more I think about it after watching this, the more the idea makes sense. My academic background is in cognitive science, and the idea of consciousness as an emergent property resulting from the complexity of the brain has stuck with me ever since I encountered it in my first year of university thanks to the work of Daniel Dennett and others. If gravity is something similar, that explains why it disappears at the quantum level; it just doesn't exist at such scales, and can only be perceived at the macro level, just as it ceases to make sense to talk about minds and thoughts at the level of a single neuron or synapse.

Thanks for covering this! I've been following this idea for the past decade and happy to see a video on it. Cheers!

Since it made sense to me that gravity is sort of the "opposite" of entropy this video makes so much sense!

Good to see that you have slowed down…I can understand much better now

This a hood classic

This is your best episode yet. I’ve been following this channel for many years. Look forward to more on this topic

The force keeping me in my seat is generally called laziness. 😊

I have a hunch that, if the universe is holographic, then the emergent dimension is time, the center is the present, and the boundary is the infinite future. In which case, entropic gravity would be maximizing the final entropy of the end-state universe... and the boundary would have laws explaining correlations, but no actual dynamics.

Putting the bekenstein-hawking formula on a mug and smashing it was a very nice touch!

Well boys looks like we're going to be rewatching this one a gazillion times

This idea is elegant, but honestly, these last couple of videos have gone over my head a bit. 😅 I hope I'm alive to see our understanding of gravity get a little more concrete one day. Thank you guys for yet another excellent video! God be with you out there, everybody. ✝️ :)

I enjoyed the trippy Amiga music at the end

FINALLY! I've been wanting more mathematical background and I got it. Simply lovely!

12:07 - This sequence is SO satisfying

Gravity is probably just a property of time, we are looking at a 3 dimensional outcome of a 4th dimensional feature. If you were able to flow time backwards, then in theory gravity would be negative as a push force. Therefore gravity isn’t a force, just a marker of the property of time.

Oppenheim's stochastic gravity has long been my favorite theory that attempts to reconcile QM/GR... it seems almost TOO obvious to be true, which makes it super attractive to me--because we can test it and rule it out so easily. Someone seriously needs to try one of the experiments he proposes in his presentations!

Relevant XKCD: 2904 At this point, I firmly believe (and fear) that thermodynamics will turn out to be the unifying theory of everything.

Look at you teasing us with merch we can never get again. I put my It's Never Aliens and The Devs Will Hear shirts in the drawer for posterity.

Damn, that was a sleeper "SpaceTime" drop at the end. Quick and clean.

I want to thank you for giving Erik some attention. I love his idea of gravity. But it takes a while for understanding it.

This seems to be another suggestion that our universe exists inside a huge black hole.

Im a Junior in college for theoretical physics and this video made 100% sense and was amazing. Can’t say I would have understood if I wasn’t here but glad to be able to

Fascinating! And really compelling for an amateur. Your delivery and exposition of the material was tremendously helpful. Thank you.

When an episode references another episode, you should link it in the description! Every time! Because it can be hard or impossible to find!

Just what I needed when my brain is switching to its most entropic state, i.e. sleeping. Now it’s going to be fighting that entropy for at least 30 more minutes.

I can’t get enough of these videos!! There’s nothing that puts me to sleep like Dr. Matt’s soothing voice and physics I can’t understand.

This is really brilliant (and mind-blowing) stuff! Very nicely explained and visualized!

Most real opinion I’ve seen in the last 4 hours

I heard of entropic gravity 10 years ago, but heard of Erik's vesion only a few years ago. The idea that gravity is caused by temperature is interesting it makes me remember the argument about how a blackhole blocks certain frequencies and cassimir effect it makes me think that matter blocks wavelengths caused by the temperature of the surface of the borders of the universe and thus matter will drift towards each other's because they shade each other's from this radiation. one prediction would be that gravity constant will change with the age of the universe. and maybe the dark energy is just this incompatibility between gravity measured now and the gravity in the distant past (edge) the universe.

The reason for last weeks episode makes much more sense now. Great video.

This is by far one of the most fascinating episodes you’ve ever done!

Some of this is on the right track but the theory is putting the cart before the horse. Gravity is not an entropic reaction via the holographic boundary, it is entropic because time itself is a spatial metric pursuant to which mass creates "defects" that form additional units of space that radiate outward. The radiation of space due to the presence of mass explains the expansion of the universe and dark energy. As the space radiates it creates inward pressure on mass that is perceived as a gravitational force in 3 dimensions and this radiative pressure on cosmic scales explains dark matter. And of course this inverse relation between mass attraction and spatial expansion is why gravity can also magically drop out of the thermodynamics as explained in this video. The very reason we have an observable universe is because the gravitational and cosmological constants are closely balanced and therefore it will take a very very long time for entropy to reach its maximum.

It might be better to think of Entropy in terms of "stability" instead of order and disorder. This could proved more insight in understanding "energy states" in the observable Universe. There are many "Islands of Stability" at every size and location, and we see that everywhere in our lives and surroundings.

You had me at "If you let that entropy hold you in your seat for a bit, I'll tell you all about it"

This is excellent work. The difficulty at this level of physics, as you suggested, is that even if Verlinde's model makes all the same predictions as other models, until it can make predictions that the other models can't, there's no real way to test it. Otherwise we run the risk that it's just a complex way of expressing the same math without noticing that this is what we're doing.

Gravity as an entropic vacuum created by the inherent deficit of cumulative information. Attractive theory, but to me at first glance it feels more like a thought experiment than a functional state. I will ponder it further, thanks!

Very honestly: WTF did you just tell me? Holy moley. What a ride... I will have to have a deeper think about that. Implications are crazy...

This is by far the best channel on youtube. Lovely, lovely stuff.

The closed caption at 7:51 says "If all the gives you a headache then you're not alone" even though it's not said in the episode

Actually, even if you drop gravity (lol) out of the list, there's still four fundamental forces: electromagnetism, strong and weak nuclear, and *_stupidity._* Yes, stupidity is a fundamental force of the universe; quantum uncertainty is the force of stupidity acting on a subatomic scale, while the entire population of Florida is an example of its macroscale effect. "There are only two truly infinite things, the universe and human stupidity. And I'm not entirely sure about the universe."

My problem with the whole derivation is that it uses the results from BH thermodynamics as a basis of its calculations It feels a bit like circular logic

Ok so I'm not alone in thinking this video was the hardest concept to grasp that PBS has ever done lol

I don't think I've ever consumed and assimilated as much complex information as I have in the past 16 hours, studying as much as I can about QFT, GR and AdS/CFT. Proud to say I understood pretty much everything on the second watch. Can't wait for next week!

Interesting, but my biggest question about Verlinde's Entropic Gravity wasn't explored. At least in his original formulation, it seemed to require a preferred frame of reference. Did he get around that problem? How?

I mean if I'm understanding right this also works for the big bang. If the boundaries of space are ever increasing there is a restrictive force, gravity, pulling back. This restrictive force can be and is related to entropy. And therefore gravity = entropy. Probably don't see it as entropy because it's working on a scale so much larger than we can even fathom. So we've been calling it gravity this whole time. I'm sure there's holes, but not a bad thought. I'll have to watch this again later to confirm some things.

to be honest you explained this so well. i have had more difficulty with some of your other episodes than in the combination of this and the previous holographic one. great stuff explaining his theory.

Thank you so much for this explanation of Verlinde's theory!!! One of the best videos of PBS Spacetime!!!

I hope that the followup episode that "picks this apart" will mention the work of Zhi-Wei Wang and Samuel Braunstein, which established that general spacetime surfaces that are not spherically symmetric and do not exhibit horizon-like behavior fail to obey an analog to the first law of thermodynamics. That undermines a major assumption of entropic gravity.

10:43 : wait, if N is the number of possible microstates, then the entropy should be log(N), not N. Surely the number of possible *configurations* is not proportional to the surface area, but rather the surface area is proportional to the *log of the number of possible configurations*, right? Like, as if each Plank length area portion of the surface had an independent (qu)bit? So, at 10:52 , should this N be “number of microstates”, or “log of number of microstates”? 10:58 : uh, ok.. (I guess this gives us a way to assign a K < (1/4) to any sphere surface in the bulk? 11:26 : why does it gain the same amount of entropy? I guess because the additional information it needs to describe is the same? Hm. Also, I’m not getting where this formula is coming from. If a black hole has entropy proportional to surface area, and the surface area is proportional to square of Schwartzchild radius, and the Schwartchild radius is proportional to the mass, then increasing the mass from M to M+m should increase the entropy from a M^2 to a (M + m)^2, an increase of a*(2 M m + m^2) (for some constant a) Oh, is this something about the de Broglie wavelength of the particle crossing the boundary? uhh, what’s the formula for that again..? For photons, E = h f , and lambda = c/f , So lambda = c/(E/h) = h c/E , ok that doesn’t appear to be what’s going on.. Edit: ah, I suppose the *Compton wavelength* (not de Broglie wavelength) is the one to use, with lambda = h/(mc) So, the 2pi(m c/hbar) depicted is, well, hbar=h/(2pi), so (mc/hbar) = (mc/(h/(2pi))=(mc/h) * (2pi) , hm, ok, so what they have written there is (2pi)^2 k_B ((delta x)/lambda) for lambda the Compton wavelength. Ok. Hm. So, if a particle with wavelength lambda travels a distance delta x, into a black hole? Why does the entropy increase by that amount?? 11:45 : oh, if I had kept watching he was just about to say it was the Compton wavelength. I’m unclear on why (2pi)^2 k_B would be “the minimum entropy” to add. It seems to make some sense that ((delta x)/lambda) could be like, “what fraction of the way through the one wavelength distance, had it traveled” I think, maybe. 12:50 : wait, but for the black hole entropy, K was (1/4), and in general it should be less, so, it can’t be 1! What?

That is really satisfyingly elegant.

For the historical record, both fire and water were considered 'elementary', we now know the former is a form of plasma and the latter to be a molecule. Some day in the future I am sure people will say "remember when we used to think gravity was a fundamental force?"

Is this not circular reasoning? Given that the Bekenstein-Hawking limit was calculated with a theory of gravity already. Although we could really change the perspective a bit, and assume that the Bekenstein bound is the more fundamental law, if we are to assume that spacetime is "informational" in nature.

Omg, finally something that makes intuitive sense to me. I struggle with understanding a lot of the recent previous videos you've put out. But all this one had me do was super impose a curved 2D world into a 3D one, and I've already been doing that for a while now. The math is about one or two levels above me still, but the concept is so very intuitively explained that I am inclined to believe it now

Love it! This rabbit hole paid dividends & nothing will ever be too complex to cover; I think the maths in this video were laid out gorgeously & am looking forward to the next one after the (brief?) hiatus/break!

I moaned when I saw new spacetime video notification

Maybe gravity isn't a force but accumulation of all tractive forces

A good step forward to the understanding. The other contradictory step to understanding is how the same entropy is the same as the expansion of the universe.

Thank you for all that work that you and your friends at PBS put to these videos, they are really well

General relativity and quantum mechanics will never be combined until we realize that we are observing them 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 is what we perceive as the present moment and is what divides the past from the future. GR is making measurements in the observed past and therefore, predictable. QM is attempting to make measurements of the unobserved future and therefore, unpredictable.

My favorite kind of spacetime episode, the one which I can barely follow. Ah, I missed those, they were once more common! It wouldn't be interesting otherwise! Gotta use those neurons! 😂 Had a great laugh at 13:37, the derivation of General Relativity from the Enthropic Force at the Boundary of an Holographic Universe could be probably be the more mind melting episode of Space Time ever! 🤯🤯🤯

I've personally believed something similar for over twenty years, didn't know it had a name though. An experimental setup would be to measure the miniscule gravity of a near perfect vacuum, then compare that measurement as various energies and matter traverse through that vacuum.

Weird comment, but I love the lighting/color balance in this episode. The visual edit feels so sharp.

Before I watch, I'd like to say this: I have been giving this thought and I have 3 ideas regarding quantization of gravity: equivalence principle (seeing how acceleration of motion behaves at the quantum level and extrapolating from that), time (looking from the perspective of time dilation gradients causing what's perceived as gravity and extrapolating from there), and tying it all in with the higgs field (looking at how interaction with the higgs field affects motion at the quantum level and tying it with the previous two), and then statistically modeling it all to see how gravitation emerges at the macro level. Perhaps gravity is just a statistical phenomenon of quantum events, similar to how the probablistic nature of quantum physics becomes deterministic once the particle count becomes huge enough to where it becomes a matter of statistics. Many people don't understand that quantum mechanics doesn't "break down" at the macro level, but rather that at the macro scale, we can only see the statistical behavior that emerges when huge amounts of particles are at play. A ball can quantum tunnel through a wall, but only if almost all of its constituents happen to quantum tunnel simultaneously and end up in positions that would perfectly replicate the original ball, which is so unlikely that you'd need an eternity for that to start happening. This example is meant to be analogue to my idea of gravity behaving differently at the quantum level. So perhaps just like there are no quantum basketballs, there is also no quantum gravity, but rather an emergent phenomenon that only behaves like general relativity's gravity at the macro scale, all caused by the statistical outcome of quantum particles' motion being affected by the higgs field, time dilation, and the physics of acceleration. If you look at gravity from this perspective, it ends up being more of an imaginary or pseudoforce, manifesting due to the behavior of interacting quantum fields (including the higgs field) at different velocities and rates of time. I have seen an example that shows that the time dilation gradient caused by a warped spacetime field causes an object's particles' motion to be affected along the gradient, which ultimately causes the object to change direction and fall (Science Asylum channel), and this led me to forming the aforementioned idea. Being that I am not a physicist but rather a slightly autistic fan of physics, I lack the skillset to express these ideas mathematically. But what I described is what I saw in my mental physics simulator, and hopefully, this provides enough insight for you to understand what I'm trying to express.