21:10 His paper on Bayesian entropy: arxiv.org/abs/1508.02421 for those interested. 37:19 The paper on deriving the "no hair"theorem from thermodynamics: arxiv.org/abs/1703.09241 59:35 Paper on quantum information in an expanding universe: arxiv.org/abs/1702.06959
Wait, as a Kerbal I must insist that it's not gravity that makes dropping something seem asymmetric in time: drop anything and it's just in an arbitrary point on an orbit. A narrow ellipse to be sure, but still there's nothing about orbits that look funny if you record them and play them backwards. It's when you hit the ground that you get a sense of the dirction of time, and specifically when you hit the ground inelastically. Sound and heat rarely converge on an object and launch it into the air, unless you do a lot of work to build a rocket. Of course, if it gets hot enough that sort of thing does happen: it's the energy of the fall being much higher than the background energy that makes hitting the ground a one-way process.
The Boltzmann brain argument depends on your prior. If you use a maximum entropy prior (every way of "building the universe" is equally good), then it works. But if you use a simplicity prior ("simpler universes are more likely"), then it doesn't work. A simplicity prior might sound like it's begging the question here, but actually there are good theoretical reasons to use it, based in the field of algorithmic information theory. Long story short, if the number of possible hypotheses is infinite, then the maximum entropy prior actually becomes ill-defined (if you try to assign equal probability to infinity things you get zero probability to any specific thing, which isn't a valid prior), but the simplicity prior actually has some very nice properties (see: Solomonoff induction).
05:55 if we could contrive a system like a room with people in it to decrease its entropy over time, including all enclosed subsystems such as the people and their brains, then they would experience that low entropy condition as the past. The time of our magical re-arrangement would be an abrupt end of time, or it would be a non-event followed by an uncertain future. It would be very hard to contrive such a room because a very specific low-entropy condition has to be reachable - specific tables, chairs, people's brains etc. Contriving a vast universe so that a uniformly dense and hot past is reachable may be easier.
If you consider the Earth and it's atmosphere as the "system", couldn't its entropy decrease since the planet is getting a large inflow of energy from the sun? And in that case, why wouldn't the arrow of time run backwards on Earth?
The arrow of time is represented by the real numbers, mathematically. What if time is complex, how do we conceive complex time? Life is a QC function, like Maldacena's universe (conjecture). We will never know the algorithm of QC function, so we never understand how information or error correction is achieved, it simply is in the realm of divine design.
@@stephenbryant7873 of course! I'd take a collection of post-it notes if that was all that was available lol :) It was more of a shout out to Sean 🙂 Happy to see others interested and supporting this.
Does dark matter have entropy? Is dark matter moving away from Big Bang like the rest of the universe? Is there dark entropy? Is dark matter in motion at all?
As far as we know, dark matter moves only under the force of gravity, and doesn't have inelastic collisions, even with other dark matter, that would cause it to clump up like familiar matter. So you get very sparse galaxy-sized clouds. You could certainly measure the entropy of a cloud of dark matter, like a cloud of anything else, but it's not clear what would make its entropy increase going forward. Collecting into galaxies was an increase in entropy, but it seems stuck that way now.
Thank you for your fascinating and brilliant lectures on physics for regular people. These are invaluable. Just a comment - maybe not politically correct or you may not consider it scientific, but it is based in actual reality: How do your thoughts about entropy respond to the concept of an all knowing God, who does not lose or forget knowledge, but keeps it eternally? It seems that the interplay between chaos and order are integrally related to intelligence and that intelligence is required to reverse entropy by creating order. This goes against the Newtonian idea of a universe that runs objectively without the intervention of a higher power. But - it is true and can be proven, personally, by sincere prayer, because God is actively seeking to communicate with His children, but it must be on His terms, which are ultimately most advantageous for us all.
P(PH|brain) does not equal very low. This argument at the end of your video is unconvincing. The way probabilities work is not such. They actually work by comparing outcomes with assumptions like so: Let's imagine a multiverse and then let's look at 10 randomly selected universes that contain brains. How many of those ten will have had a PH with low entropy in the past? I think all of them. I just think it's incredibly unlikely that a brain is randomly popping into existance and then even more incredibly unlikely that it would persist long enough for me to observe it or for it to ask this existential q itself. Therefore, it is A GOOD assumption, and NOT a bad assumption, that if any given brain exists, it lives in a universe with PH with low entropy.
Thanks again Professor Sean, for another fascinating video. . . . Regarding the Past Hypothesis, I can’t help wondering about the relationship between Cosmic Inflation and the very low Entropy of the early Universe. There are some well-respected physicists, (such as Sir Roger Penrose), who dispute the Inflation Theory because, (amongst other things), it would seem to require a large increase in Entropy, meaning that the “Pre-Inflation” Universe must have had an even lower Entropy state. . . . Prof. Sean, what are your opinion(s) please, of Roger Penrose’s objection to Inflationary Cosmology ? Thanks.
A way to falsify the Past Hypothesis of the early universe having low entropy: - estimate entropy of the observable universe, including black holes, from estimation of observable mass in our universe and number of black holes. - estimate the Big Bang entropy as a black hole starting condition using mass of the observable universe. - Big Bang entropy > observable universe entropy. Given that mass beyond observable universe is arbitrarily larger (due to expanding universe beyond observable universe), you should see the early universe not to have low entropy. If you don't, you can arbitrarily add mass to the Big Bang black hole until you do.
Absolutely true. Although 1 problem is you probably should model the "big bang state" of the universe as a white hole (energy inside horizon must escape) instead of a black hole (energy inside horizon can never escape, except by very slow Hawking Radiation). Not sure how to calculate white hole entropy. 2nd problem is that a classical white hole is probably not a great description of the state of the universe in which quantum gravity (whatever that ends up looking like) is probably very important.
We cannot compare the Big Bang singularity to that of a white hole: White hole singularities are anisotropic ( the Weyl curvature blows up at the beginning, just like it happens inside black holes in the end). On the contrary, in the Big Bang singularity, only the Ricci curvature diverged, while the Weyl was finite ( and very low) so the initial singularity ( if existed) was isotropic. This isotropy of the early universe is confirmed by all observations. This is evidence ( as Penrose pointed out) that the "past hypothesis" is correct. Besides that, there's another important thing that cannot be ignored: The entropy associated with the Cosmological deSitter horizons that prof. Carroll mentioned.
This video set has been amazing, so many details and concepts that are touched on in other science videos are given more time and detail here. Watched most of these multiple times at this point.
Sorry to comment just to complain, but the audio is really off on this one - very echo-y. Recorded in a different room? ANyway, thanks as always for doing these Q&As! They're what make this series better than just a recorded lecture series. Really enjoy them.
Sean added a note in the video that there was a microphone connection problem. His audio gets better when he leans down to his iPad, so I guess he ended up using the iPad audio instead of his USB microphone. Fortunately, the content is excellent, as usual.
Hello. Are our buildings, society in this moment a higher entropy than this main city that i am living now, 40 years ago?! Help me. Is the arangement higher or lower entropy today than 40 years ago of the city ?
" the persistence of memory", Salvador Dali was a time traveller, and you Sir with all what surrounds you, you are leaking...You are drops of information or grains of sand going through tiny holes in the space time fabric.The more the fabric is tight, the less you leak.What makes the fabric of space time tight Sir is Your ignorance, your entropy increasing and increasing when sitting on a sunnu beach😊On the other hand, when the fabric of space time becomes loose, you leak fast, time accelerates and you know when that happens Sir? When your entropy decreases, and you become a genious, understanding everything, almost everything.Technology kills you.
I just discovered this video series a couple of weeks ago and two years late, but what a series. THANK YOU SEAN… If there was ever a Silver Lining to the Pandemic, this has to be it. This series is a treasure, a window into a world most of us would normally have access to. Thanks again. I have been binge-watching and wanting to comment on every episode (and should have, because I have forgotten, have to re-watch, 😂) ideas and questions, and finally I had to stop to write this. As you explained “Evolution vs Randomly” for the advent of a brain, I could not help but think that these two concepts are not “unrelated” not mutually exclusive. Evolution IS a consequence of random changes were certain traits find better adaptability in the competition for resources. Sort of a convolution of random changes with what works better. There were “brains” many millions of years ago and they lived on unchanged for hundreds of millions of years. These brains were not capable of inventing Rockets and Smartphones because they did not need them to survive. If the meteor that “allegedly” killed the dinosaurs had not hit, would dinosaurs have invented “Science”? It took a major perturbation of the environment for mammals to take over, and many other challenges for humans with larger and larger brains to take over the planet. And even today, competition for resources drives us to imagine more clever ways to use them. Maybe we wipe ourselves out and maybe we don’t, but I maintain that what drives “progress” if you want to call it that, are environmental and existential challenges that threaten our position as the “number 1” in the world. So the brain does not strictly evolve linearly or logically it needs (or feeds from) randomness to find the best approach. What works better endures and dominates and what doesn’t diminishes or extinguishes altogether.
Expansion of the universe and the Nobel Prize❗ The Nobel Prize in 2011 was given to scientists for their contribution to the discovery of the expansion of the universe, where scientists have confirmed that the universe is expanding rapidly and constantly The Glory is to Allah Who has pointed out this fact in the Qur’an 14 centuries ago, the Almighty says: ("And the heaven We constructed with strength, and indeed, We are [its] expander") Quran chapitre 51 الذاريات 'the winnowing winds' Verse 47
Hey! I have a question that I could never quite wrap my head around, not a physics expert, just an interested layman, so sorry if it’s obvious. Looking backward, the general idea is that back beyond 1 Planck time we can make no meaningful observations within the framework of classical gravitation. However, if we cannot ‘see’ beyond The Planck Time, how do we know how the length of that time period? How do we know it lasted for 10^-43 seconds, and not some other length of time? Thanks!
How can we be sure about entropy when we don't know what's on the other side of black holes? Also, black holes aside, if everything is spreading apart, doesn't everything become radiation at some point? Like, wouldn't everything become the same thing? Isn't the result of max entropy actually order?
I'm particularly affected by this topic because a neurological condition and cerebellar dysfunction called dyschronometria also related to my dyslexia. I will try to save some money to buy "From Eternity to Here" book before go to cinema to watch "Tenet" ;-) Thanks Sean for your effort to educate general public about the importance of science and critical thinking in this age.
The quantity of information you can get out of an object is limited by its surface area. That's how I would have explained the coarse graining thing. There's a fundamental limit there, no need to go on about scales and measurements.
There’s only one wave function so all particles are ultimately entangled? When you say they become entangled as space expands do you mean they enter Hilbert space? Can this box of unentangled particles run out? What happens then?
......So do you have any comment on Eric Verlinde's ideas on entropy and gravity, dark energy, dark matter etc etc??? Love your videos and your approach to communicating this important work. Thank you for all, -all do benefit from deeper understanding.
