Thank you for not mentioning Descartes too early in the video. Most professors and teachers have a habit of (all too often) putting Descartes before the course.

My favourite Newton fun fact is that he wss appointed Warden of the Royal Mint, which was apparently meant to be mostly ceremonial but he took it super seriously and went undercover in bars and taverns to seek out counterfeiting operations, which he then attempted to prosecute.

The best description of getting a degree I've ever heard is "Every year we lie to you a little less."

The "I'm a medical doctor, I took one physics class, I understand relativity" bit made me cry laughing. As a physicist working in a hospital, I feel seen.

I appreciate what you are saying here about teaching something wrong as a "teaching technique." I'm a retired Math & Physics teacher. There is nothing more frustrating than trying to "un-teach" something a student mis-learned years ago because it was "easier." I still had students telling me that you can't subtract a bigger number from a smaller number because that was what their elementary teacher taught them 10 years ago. People remember that first leaning episode more strongly than several later correct teachings. To those that teach, please, either teach concepts correctly the first time or just say "you'll learn about that later." I found when leaving it as a mystery to be learned in the future half of the students were curious enough to research it on there own; and those who didn't at least didn't have the wrong idea implanted. As a Physics undergrad I was also taught about rest mass and relativistic mass. That was totally unnecessary.

7:00 When Descartes talks about "extension," he means extension into the physical world. Essentially, when something has the property of "extension," that just means it takes up space. This is especially important to the philosophy of Descartes, as he was a mind-body Dualist. This means he believed our conscious, thinky-thinky bits are a separate thing that exists in a different manner from physical, material stuff, like matter, which has the property of extension. He also asserts that extension is the ONLY property possessed by matter. When we experience matter through our senses, we can detect smells, and flavors, and how it reflects light, and what it sounds or feels like, but these aspects are only things that we "presuppose [as] extension," which is to say they are not properties inherent to a "bodily substance" i.e. matter. Essentially, the ONLY property matter possesses is that it takes up space (it is EXTENDED into the material plane), and everything else we perceive about this matter is simply our interpretation of this extension. All attributes that could possibly be ascribed to matter are only ever different expressions of its extension. Its a philosophy that's still relatively relevant today, surprisingly. If we think about olfactory sensors, for example,,,,, smell isn't a THING. Smells don't exist. Smells are our thinky-thinky bits interfacing with and creating an interpretation of matter through the SHAPE of a molecule (the shape of matter amounts to its extension). Even with sight, we are interfacing not with a REAL sensation, but rather the "shape" of the light (amplitude, frequency, wavelength). At least that's how I understand it from my philosophy course, please lmk if i got something wrong ^^ anyway its also kinda funny bc Descartes was EXTRAORDINARILY devoutly christian but was constantly playing with ideas that got christians angry at him lmfao. look up "descartes evil demon"

Tangential to the off topic discussion at 38:45, I love the mention that we did not know how stars worked until relativity. My best friend and I each have half of a series of books called "The Books Of Knowledge" (which is just a great title). It is an encyclopedia published around 1912, but had significant content-altering revisions over time. I believe I have the 1940's edition. Point is, that while perusing the book, I came across a fascinating question in a Q/A section of the book which goes as follows: "When will the sun run out of oxygen," to which the answer was some flavor of ¯\_(ツ)_/¯. The question implies a belief that the sun is literally just a space bonfire, so there is evidence of the general public wondering about how tf the sun did its shenanigans even significantly after the 1920's.

I think what some teachers forget is that you can just be honest about lying. You're not lying to conceal the truth, you're lying to make things easier to understand. Just tell them that.

my favorite Newton trivia is that there's that quote about understanding and appreciating the pre-existing research and scientific record: "If I have seen further, it is by standing on the shoulders of Giants", except, that's really not at all like Newton to do that. he was writing to Robert Hooke, one of Newton's many arch-rivals. Hooke was also somewhat short. Newton was just bragging about being taller lol

You should seriously consider make instructional videos on KZfaq. You have a natural ability to keep the audience engaged even when you explain a subject like this.

You are a born public speaker. I'm very impressed with your presentations. Not many people can structure a talk with such focus and present it in such a relaxed way for 1h+ without ever stuttering.

My teachers in HS and basic undergrad chemistry mentioned probability clouds and mentioned that it's pretty complicated so we don't get into them and I learned so much about chemistry and other subjects just from teachers admitting that they're not giving the full background; I was that 3rd grader talking about Bose-Einstein condensate when my teacher talked about 3 states of matter (much to her chagrin) but knowing there was so much more out there really helped me get interested in science; I really hope more educators catch on to this because it made school so much more engaging for me.

You were very kind about Brain Green docs. I watched one of his on String Theory and the only knowledge I gained from it was that strings are little multi-coloured circles that make wind-chime noises and float around your head.

I still resent the Bohr model of the atom, after knowing about it for more than half a century. I feel that telling lies to children is a stop-gap, band-aid fix that covers a failure to understand the issues well enough to communicate them effectively. I feel that time dilation and length contraction are similarly poorly dealt with, leading to things like the Twins Paradox that really shouldn’t be pradoxical if the issues were properly laid out in the first place. Part of the problem is: of the people who are drawn to physics, only those who have a real affinity for mathematics at a high level of abstraction survive even fairly basic physics study and tend not to be great communicators to people who need more concrete examples, which turns out to be most people. Crude concrete examples are worse than abstraction, and I’m sure they kill the enthusiasm of a lot of children who might otherwise have brought more vitality-and better communication-to the subject. Yeah, this video hit a nerve. I’ve just recently discovered your channel, and so far I find it refreshing and enjoyable.

I’m a research physicist and once was talking with a physics teacher/instructor and he started talking about relativistic mass. Since he taught it a lot and I never thought about it the kids (we were at a cub scout event) nearby probably thought that he understood things better. I was just kind of stuttering saying, “uhh, nobody thinks about mass like that anymore…it’s the old way” and he smoothly stated “no, mass increases with velocity and that’s why the speed of light can’t be reached.” The pyramid thing really helps to explain the situation. Enjoyed the video!

Thank you so much for this video. I never imagined the day would come when I would get representation in the media on this particular topic, and it's definitely validating.

"You know what holds the quarks together? Gluons...get it?" 46:10 Perfect delivery, I couldnt help but laugh out loud. Excellent Video once again, I learned a lot.

10/10. I was lucky enough to have a professor in my sophomore modern physics class as an undergrad who really hammered this point home. Love hearing you talk about that physics life, I kinda miss it sometimes but then I remember how miserable I remember everyone in grad school being and how they really aren't that much better off now.

Your videos are amazing. I'm a philosophy student whose biggest disappointment in life is not being a physicist, and these videos are some of the best I've ever watched.

39:51 i literally commented on one of reviewbrah's videos "wolfgang pauli lookin-ass" back in like 2015 when he was blowing up, i've only been watching your channel for two days but i'm consistently impressed how heavily you're focused on the real issues

Thank you sooo much!! I am 70 years old and have no formal background in either mathematics or physics but I enjoy reading about them and trying to get a handle on some general concepts and there is NOTHING more confusing than the myriad number of ways and definitions that seem to exist for similar concepts, in particular "energy". At least half my time is spent tracking down actual physicists to talk with them and have them clarify all of my misunderstandings and bad assumptions which are a result of sloppy use of language and educators trying to "dumb down" concepts and as a result teaching them incorrectly. Thanks for doing it right!!!

Don't know how your channel came to my algorithm recommendations, but I've now watched three +1 hr videos in a row, so you've earned a subscription! I love the passion you put into your arguments, along with the clear frustration about the state of public understanding of modern physics! Looking forward to more!

My favorite thing about this channel is the way she takes some of the most complex concepts in science, hears "explain like I'm 5", and presents these topics with the kindness and patience a decent person would show to a literal 5 year old, instead of the sneering contempt most of the internet associates with that phrase. Thanks for explaining these complicated subjects like I'm actually five.

I appreciate these video's so immensely. It's like I'm at a tea time at a seminar and having a casual chat with a physicist. It's very real and personable.

Babe wake up, a new acollierastro video just dropped!

Fun fact: Sickle cell has been shown to be adaptive on a population level. Human with sickle cell trait (single recessive allele) are highly resistant to malaria. There is a high degree of correlation between environmental risk for malaria exposure, and prevalence of sickle cell in local populations.

This video was very illuminating, thank you ❤ Btw regarding sickle cell disease: it’s believed that it’s been selected for since its appearance, because having 1 or both of the alleles of that gene as the SCD makes the subject resist malaria. And apparently the gene for SCD is recessive, so one can have 1 copy of it, not get the disease, and still not get decimated by malaria. :D

As someone who has consumed a lot of popular physics and has concocted a lot of crackpot theories (which I have NOT emailed to physicists and never really thought were true, but just enjoyed coming up with them), it's both exciting and kind of crushing to be shown in such ab entertaining way that you actually have to do the math... with the right formula no less. Thanks for the videos. Subscribed

I know it's a month later but small semantic point about evolution. Genetic mutations are random, selection pressures and environments not so much. If you have an accurate picture of a given environment and the selection pressures found in it, and the strength of those selection pressures, you can accurately predict the traits you would expect to see in a given population within that environment

This is gold, I didnt expect almost an hour lecture over the history and concept made clearer like never before. You inspire me!

Love how reflective you are on these subjects. You stand out on a platform(youtube) that tends to promote more reflexive styles.

Found this channel yesterday. OMG, you are fantastic! Thank you for the high quality, in depth, content.

I feel that same way about how they taught fractions in grade school. They taught fractions were these special things with special rules, but then you get to Algebra and learn they are just division problems you haven't solved yet. Everything about them suddenly becomes crystal clear.

Yay and best wishes! A thousand subscribers here, a thousand subscribers there, and by this time next year you’ll be well beyond 100k Then again I don’t know how to math and you probably could (in your sleep) use the current rate of growth to do such extrapolations In any case you’re great and us fans all believe in you!

Every KZfaq video longer than 1 hour that I have completely watched is on your channel. Brilliant content.

so i watched your string theory video, and now i'm binging your channel. As someone who read a lot of pop science as a kid and studied physics up until about 2nd year university, i'm probably who you are talking about when you talk about the "science interested public" in that string theory video, i'm excited to find a channel like this that ties together things that i sortof (don't) know in an engaging way with just the right amount of math. you're a great science communicator! this channel reminded me that physics is interesting and fun

I really enjoyed how you walked through the history, including the asides with "yeah, this was ultimately found incorrect". Keep it up!

I love these videos. I just finished your newest String Theory video and then the Post Doc Exodus video and now I’m here. You’re extremely relatable and I’m happy to see someone in my age group discussing issues in modern academics and physics and is largely dominated by wealthy people in their 60’s+. Really looking forward to anything you talk about.

I'm glad this video came up when I searched for "what is mass physics". I really enjoyed your presentation style. You are very bright yet disarming. A good dash of humor and personality. And of course, a clear and accessible conveyance of information. You quickly found a new subscriber in me. I am looking forward to your Videos section growing in scope! Thanks :)

currently going going down a rabbit hole of all your videos

PS: most of the mass of plants comes from air rather than soil.

Thanks for all the great compelling and entertaining videos. You have a real gift of communication!

I just found your channel and i love it. I love the long meandering story/lessons you tell!!!

I have a piece of paper that says I studied a lot of this stuff (and passed those courses) a while ago -- in the 70's, like last century. Your presentations are superb -- especially weaving in the history of how these things came about. Applause!

I don't know what y'all were expecting, but this is NOT the Roman Catholic "mass" I grew up with. Very weird take on transubstantiation. (Sorry, just trying to give you more "comments Bingo" material. ;-)

The cool thing about these longitudinal vs transverse mass is that quasiparticles in condensed matter often have an apparent "mass" which can be broken down like this. It's weird, but it's great that it didn't just come out of nowhere.

Just wanted to say (as an avid physics enjoyer, but not a student), that this 34:40 was the best explanation/calculation of time dilation and space contraction I've seen. I love your videos!

I love you! Every video has me in stitches. The way you land all those nuanced punches on chosen ignorance is amazing. You're a Warrior. (epic battle cry) FOR SCIENCE!!!!

When I decided to make a career pivot to physics, I was recommended to read T M Helliwell to get a foundation in special relativity. For those interested, great book. All you need is trig (and some patience). Helliwell is very deliberate in always using "m" to mean mass (the only mass), and avoids mentioning "relativistic mass" until an appendix (F) of the same name, which in short, says its discussion is not preferred.

This was great. I sincerely appreciate the work you are doing, and I think it adds a lot of value to the world. At least for me personally, I have gotten a lot out of your videos. I really love physics, but haven't had very much opportunity to engage with it since I left university, except through videos like yours. You are *really* good at this - like one of the best physics content producers on KZfaq good. I'm on board.

This was a very fun video, thanks for adding a ton of the history behind the discovery of all that led up to general relativity - standing on the shoulders of giants indeed. Also, I really like your presentation style, you answered the question "what if Jenny Nicholson had an advanced physics degree and did math and science videos" which I now regret not having asked before, but appreciate having an answer to.

I listen to many lectures on various subjects on physics. I find your presentations the most enjoyable.

Absolutely love the presentation and stubborn determination to correct common misconceptions.

I really enjoy listening and watching your discussions about the academic career field. This video was great too. You have a knack for conveying the infornarion and knowledge, especially for someone like myself who has an interest in science but do not have a physics degree.

When you pronounced ether with a short initial e, I thought, "Why is she doing that?" I have have always heard it pronounced with a long e. I love your show😊! I love your humour😂! I love your willingness to explain details for laypersons like me who want to do want to be more technically precise. And you always make the video easy to watch. Best wishes!

These videos are amazing, I giggled at your enthusiasm for the various equations

I found your channel today off of reddit and I really like how data driven your thinking is. Will be looking forward to your next video, keep up the good work. I really liked your glass video, the mass video, the flouride video and the spider video; I learned a lot. As a hopeful first gen academic I learnt a lot binge watching your channel today. I also enjoyed(?) your videos on SA and women in science. I sympathise and I thought the videos were illuminating on a bunch of figures that I had no idea about. Any who, good luck with the channel, will be looking forward to the next video.

I imagine this kind of dumbing down plagues every field of science (mathematics, sociology, economics, whatever). really cool to see things like that disproven!

Thank you! Took your advice & taking some remedial math & physics to take them at a higher level. Grateful I can always return to these videos.

BEST. EXPLANATION. I'VE. EVER. SEEN. This is awesomeeee!! I loved this video and I know that when I first learned of the idea of "rest mass" was extremely confusing to me and I never really took to it. This was well said thank you

Your explanation about the experience of time and length contraction depending on reference frame really helped the concepts seem less confusing. Most explanations fail to include that detail which makes them more confusing.

great video!! you will be pleased to know my ib physics teacher in high school was sure to teach us that things do Not get more massive at higher velocities As I recall though he didn't mention the E naught equation, though its been awhile

"[N]ot an argument of syntax. This is an argument of semantics." What a precise way to frame the discussion.

I just found your channel. This and your other videos are great, I've got some catching up to do. Old engineer, not a physicist, but I still recall catching a glimpse of the wonder in quantum mechanics classes in grad school. Loved the shout-out to Reviewbrah at 39:52

Fun fact: heterozygotes for the sickle cell allele are less susceptible to malaria, and the allele is enriched in malaria stricken regions. Certainly a steep evolutionary tradeoff, the immune system (including the immune system in a broader evolutionary context) is brutal. Neutral evolution and persistence of almost neutral deleterious alleles, especially lifetime traits that become harmful post reproductive age and experience damped selective pressure, is definitely super common. Interesting though that in this case, it is somewhat of a Red Queen situation, where population-level variation can maintain the species better than homogeneity. And while yes we certainly aren't evolving to get any smarter, parasites do tend to evolve in a specific way. If a pathogen is able to infect more people, or have an increased rate of transmission, then that strain will become more common. If a pathogen becomes too virulent, however, it loses some ability to spread from host to host (host dies or host can't move). Therefore, it will be outcompeted by strains that cause less host damage (at least, in the short term). Yes, rare genetic events occur by chance all the time that can do something completely unpredictable, such as recombination in a double infected cell (this is how we get pandemic-causing strains of influenza every century or so). But it is still true that pathogens tend to become attenuated to their host over time - especially following a shift in host range (like zoonotic leaps). Evolution is a certainly not a guided process, it's probability and statistics. But we understand broader trends, and are very successful at applying that understanding. For example, live attenuated vaccines are engineered via repeated passage. That is, you take a pathogen, infect a cell line with that pathogen, let it replicate, then collect it and do that again and again and again. You might passage it through a variety of cell lines, maybe even at a different temperature so it evolves to be more efficient at Not the human body temperature, depends on the pathogen. The live attenuated polio vaccine is still used in some places. Things don't evolve to be better, but when random variation in a population makes a thing more successful at making more of itself, more of that version of the thing will exist. Sorry i had to microbio info dump for a second, I love your content!!!

Your videos are so good! Informative, inspiring, and also fun. Keep it up :)

This is hands down the best video I've stumbled upon this year. Can't agree more on the teaching mentality.

Excellent video, really enjoy your relaxed pace and straightforward language.

I'm loving your content and your energy. Please keep it up. :)

This reminds me a lot of how calculus is taught. I can clearly remember how the prof started with a pretty intuitive description of what calculus is going to do, from getting slope from two points with ∆y/∆x or area by filling a curve with approximating rectangles and summing their areas, and leading us right up to "but what if we could make the delta so small... infinitesimally small..." and it was like oh cool, yeah calculus, what a natural extension to what we already know, and then hold on there, now we have to learn limits for a while because actually you just cannot understand what calculus *really* is until you learn limits (which challenge intuition a lot more than infinitesimals), and why? because the real meaning of calculus is written with epsilon and delta and the language of limits. Except it doesn't have to be that way. It's another case of historical baggage, because at one time, the definition of calculus using infinitesimals, while very appealing to the intuition, was not mathematically rigorous. And making sure math was rigorous was the big new thing. Using infinitesimals in your equations was like using a 🌭 in your equations, you could get away with shuffling it around using some subset of algebra for a little while but ultimately it's just a picture of a hotdog and we're not too clear on what happens if you ilke, take its square root or something. So we get the fundamental calc equations written in limit language using epsilon and delta, and infinitesimals get to be a fun little way to introduce calculus that for some reason perfectly explains what it is you're doing but isn't the right way because math. Since then infinitesimals were shown to be rigorous and we got surreals and nonstandard analysis and all that. And so everybody started teaching calculus with infinitesimals like they should and everyone was happy and nobody felt like it was even more correcter to use the limits definition and teach it that way to new calculus students for eternity just because that was the way you had to do it in 1960. Maybe someday, I know there's some amount of push for it, I had a copy of a calculus textbook that taught with infiitesimals and it was much clearer. I found an article* just now talking about this and the benefits to teaching this way, including "making dy/dx an actual quotient rather than code language for limℎ→0𝑓(𝑥+ℎ)−𝑓(𝑥)ℎ" ... yes, the mysterious dy and dx, no no, don't try to math those, they're not variables, they're just like... notation. Like the integral sign. They tell you you're doing calculus. (So they're 🌭. Got it. Well done guys.) * link.springer.com/article/10.1007/s11858-020-01194-2

your channel is pure win. so much fun. thank you.

okay enamored with this video. will be going through the rest of your channel now!

Thanks for taking the time to clarify all this. As someone with an undergraduate in physics and who has just started teaching high school science, I can say there is a huge gap between understanding physics and understanding physics pedagogy. A lot of these 'lies' don't need to be told if students are given the tools to help understand the real complexity of reality. If you need to introduce misconceptions to 'simplify' the model, then you need to rethink how you are teaching that concept.

Just started watching your videos and I'm really enjoying them! Though I want to note that there is an evolutionary advantage for sickle cell anemia, in that it provides some protection against malaria. Definitely a trade-off but apparently malaria is more deadly. Always thought that was an interesting fact.

Man, I am loving these... Keep it up, it's so good.

Great video! I love learning things like this, even if I don't have the background to grasp every bit and bob of math.

Thanks for explaining what a "hadron" is. I had assumed Hadron was a famous scientist I'd never heard of (like maybe James Webb). And I guess I assumed there must be a small Hadron collider somewhere, maybe in Australia. I probably didn't even think that much about it though, tbh. (Hmmm.... what else am I stupefyingly wrong about? Never mind, we'd be here centuries.)

Fun fact: newtonain dynamics is just a special case where the speed of light is infinity so all “relativistic” forces are canceled out… or I heard this at least

Thank you for these videos. You're making me remember my love of science and its beauty. That was sucked out of me throughout my master's studies. I'm a biologist and you're making me want to learn math!

As a science educator trying to take this idea on in my PhD, thank you for this video! So often things are over simplified for seemingly no good reason and creates long lasting misconceptions. Cheers.

This unearthed a lot of buried physics knowledge I forgot about from almost 10 years ago. Maybe you'll find it reassuring that one of the things I remembered is that my undergraduate (lower division, even) physics class that covered special relativity made sure we knew that E=mc^2, as written, only applies to non-relativistic, massive particles. I think we then talked about the more rigorous E_0 definition, but that part I don't quite remember.

Newtonian gravity is taught so badly in schools, I remember my physics teacher telling us to multiply mass by 9.8 to get the weight on Earth, and I said "9.8 what?" and he said "Don't worry about it." Well I worried about it, and it made me think physics was dumb, until I started learning physics for fun later in life and realised actually it's kind of awesome & I probably should have been a physicist, d'oh. (Instead I did chemistry, found I was fascinated by particle physics but excruciatingly bored by hydrocarbons and dropped out) Perhaps there's an alternate universe me doing interesting research because that teacher had just said "It's the rate of acceleration in metres per second per second, but you don't need to know that for the exam" or something. Bit of a microcosm of the focus on passing tests over actual understanding that put off me education in general tbh. It's interesting to learn of the issues with relativity in higher education, there seems to be this pattern in science education of teaching kids wrong, revealing a slightly less wrong version with a smarmy flourish that they have to learn all over again a couple of years later, and repeat until grad school, rather than building a proper foundation.

I’m studied linguistics and this was very entertaining and easy to understand even without the necessary background, great job!

You are so smart it’s a beautiful thing to take in. I’ve very much enjoyed this video and learned a lot

Hi! This is a really fun video just by the format of how you do things alone (and it's on something I always wanted to know if there was a more rigid definition!), so I just wanted to say it's awesome what you're doing for youtube science. Came from the one video on string theory and isaac (not Newton), so I thought I'd check it out. I'm still watching it, but I'm pausing to make a comment because the argument you showed between Descartes and Newton really caught me by surprise since I read about it before, and as far as I remember it goes something like this: First, the thing about putting one's hand into space was not really Descartes, but in fact Archytas! It's a common misconception since the wikipedia is kinda badly worded, but Descartes' conception is not about a specific scenario, in an edge case, but about the property of extension in practice: He says that the fundamental property of bodies, of matter, of actual things (not inside the mind) is extension, which is basically: (1) it has to occupy some space, (2) other bodies cannot occupy the same space. By a negative proof, if there is an entity that doesn't occupy any space at all (~1) it doesn't really exist for us, since it can't interact with anything, make itself shown/known. This also goes for the other property, since if there were bodies that could occupy the same space (~2), they would be phasing through each other, and again, not interacting: for example, if you hit a billiard ball, and it phases right through another, this other one doesn't really exist. With definitions (1) and (2), he can connect matter to mind: A body that doesn't obey "the law of extension" would interact with nothing, be it in touch, sight, sound (for a more modern analogy, photons would go right through it, not bouncing back - there would be no difference between it and a vacuum), and for Descartes, it would essentially not exist. And so, (3) bodies must be perceptible to the mind. This is the kind of thinking behind his famous phrase, "I think, therefore I am." If there is a given entity which for us presents no difference between it existing or not, we can discard it, and assume it doesn't exist. Beyond matter, and into thought, Descartes would go on to discard every knowledge whose existence could be faked by some form of illusion (optical or otherwise), only arriving at the single certainty that, whether what he perceives is an illusion or not, it's a certainty that he is perceiving, and therefore that he exists. Bodies can't be proven to exist, but if they do, they need to have the property of extension, of being able to be grasped by the senses, and so by thought. Mind can be proven to exist, but only as an entirely formal truth (I am), without any content to it (all can be doubted as illusory), and thus bodyless, unextended. There are many reproaches to Descartes' thought at its most radical on the basis of Atheism. Since he doubted the existence of everything not given to find a deeper truth, it would also be necessary to think, "what if God is an illusion?" at some point. Also, even assuming matter does exist, bodies must, can only really exist if they had extension, if they were susceptible to being perceived in some way, which therefore casts doubt onto an imperceptible God. Finally, (and this is the argument I think Newton is really trying to make) there is the fact that Descartes' philosophy is inevitably subject-centric. It's not hard to classify his philosophical branch as the one that you go for if you answer that if a tree falls in the forest and no one is around to hear it, it doesn't make a sound: argument (3) says bodies have to be perceptible to the mind, and if you equate that to extension, you essentially admit that things might stop existing the moment you stop looking at them, and begin existing again when you return to looking... which was easy to link up to Atheism when at the time it was considered a narcissistic position, whereas the religious one evoked images of community, of things existing independently of perception thanks to God's will, etc. Except, of course, this argument is a misrepresentation of Descartes! Although in his thought it's true that (3) bodies have to be perceptible to the mind, that's just an emergent property and not at all the definition of extension: again, it is only that (1) it has to occupy some space, and (2) other bodies cannot occupy the same space. Basically, it exists if you can interact with it; but it does not at all exist ONLY while it is currently being interacted with. Descartes is both less and more radical than that: less, because he admits the possible existence of matter outside, and completely independent of the mind; more, because in the question of the forest, he would say that even if he did hear or see a tree fall, what's to say it wasn't just an illusion? We're talking about entirely different philosophical levels here, of knowledge (epistemology) and existence (ontology), which Newton confuses severely.

This was a very interesting video. I'm personally really struggling with the concept of mass and how it relates to rest frames, especially in the context of quantum field theory. You give Hawking's definition as "resistance to acceleration", which i think we have to take to mean acceleration from a given force via F=ma. But this is _exactly_ the thing that changes with velocity. A moving object will experience less acceleration from a constant force than a stationary one because of relativistic momentum increase. And that is exactly what all these "it's great for teaching!" folk are on about. Yay, we get to keep using f=ma and p=mv! I think this should read "resistance to acceleration _from rest_ ". And this ties mass directly to a rest frame automatically. It also immediately tells you that you can't have massive objects moving at the speed of light _by definition_ . But how the Higgs mechanism ties a field to a rest frame is a complete mystery to me at this point. So far my conclusion is the following definition of mass: "the term in the equation that makes it act like we expect from a classical mass"...

This is part of the reason when I am talking about E=mc² I give the caviat that it is a shortened version of E²=(mc²)²+p²c². Now, normally, it comes up for me when describing the mass deficit in nuclear physics so I then show how the reference frame changes so little. Therefore, the momentum is negligible in the calculations. Then when writing it out to show that you can use the energy release to determine the mass deficit, I will write it as E=(m1-m2)c² but in this case, there is an intrinsic change mass.

Here to say I was the perfect audience for your talk. MIT in the late 1960s required every undergraduate to take four semesters of physics 8.01 8.02 8.03 8.04 (many freshman who took AP Physics in high school got credit for 8.01). One of the three courses was closely based on Feynman volume 1. Soon after graduating I turned to teaching general science and high school physics (which provided not just a paycheck but also a Vietnam era draft exemption). So in the space of a few years I not only learned that mass increases with velocity, I taught it too!

Thank you for this lecture! I learned something!

Thank you. This is absolutely a misconception I had and I'm now irritated that I was knowingly taught the wrong information because I turned around and unknowingly spread that misinformation to other people.

Watched the video, thought I should read up on this stuff to get back into mental shape, be annoyed that I have to look up the name and paper... and it's there, linked in the description. My savior!

Oh! Angela! I read that book when I was in fourth grade and I have never been able to remember what it was: Faraday’s book on the candle. Thanks so much for reminding me. I’m going to get it from the library and read it again. (I am 74 years old now)

GREAT video! I felt like it was specifically for me because I definitely fell into the camp of sorta-kinda-maybe thought relativistic mass was a thing until I watched that exact Don Lincoln video you referenced and then was still sorta confused after that. And I completely agree with you about the lies we tell to children. It doesn't make teaching much more complicated to caveat the things we say in a way that doesn't make it a lie. But the fact that you DID include a caveat makes it so that some day in the future when the kid is presented with more in-depth information that contradicts the lie, it's not a betrayal. It's just expanding on that caveat. Like I still remember how BETRAYED I felt in GenChem when my professor told us that ionic bonds and covalent bonds are not actually different phenomena. We've just divided a single phenomena into two regions based on electronegativity difference simply because compounds with electronegativity differences in excess of where we draw the line tend to have certain properties while compounds with electronegativity differences below that arbitrary line tend to have other properties. But compounds that are kind of close to that line can be blurry as to which side they belong on. Like that's why we say silicon dioxide (literally the most common compound in the Earth) makes covalent bonds (because the electronegativity difference between silicon and oxygen is not high enough to cross the arbitrary line we drew that divides ionic and covalent bonds), but it's NOT a molecular solid as we generally expect covalently bonded compounds to make. It makes crystals like ionic compounds. And just the other day I was subbing in an AP chemistry class in a local high school, and they were doing a practice AP test that asked them which would have a higher melting point: CO2 or SiO2, and a lot of them were confused because the answer didn't say anything about molar mass. Instead it mentioned that because SiO2 formed a matrix solid, it has a much higher melting point than CO2. And I wasn't sure what to do! Do I double down on the lie and just say something like, "you just need to know that SiO2 is a matrix solid because it's the most common substance on Earth." Or do I potentially damage their trust in their teacher and tell them she's been lying to them all along? I ended up deciding to explain the lie but with emphasis that the lie is useful to believe because it makes it very easy to predict behaviors and properties in lots of situations. It's just that in some few situations, it is helpful to know that it is a lie. But even knowing it is a lie doesn't mean you are going to know SiO2 is a matrix compound. That's still just something you have to know. But WHY it's a matrix compound instead of a molecular one is easier to understand because it's close to the line we've drawn to divide ionic from covalent bonds, so having a mix of covalent and ionic properties isn't all that surprising. But man, I had SO MUCH anxiety the whole time while I was explaining that thinking, "You're going to ruin their trust in their teacher forever!" But it just wouldn't be a problem if we just included the proper caveats in the first place. That same chemistry professor that first told me that ionic and covalent bonds aren't actually different did a good job of caveating other stuff like the Bohr model and some of the other quantum stuff you mentioned in this video that is relevant to chemistry (binding energies, etc). And I felt like knowing those caveats were there made it easier when I took Modern Physics to incorporate the spooky weirdness that that class got into.

Nice video! But it left me confused on one point. Why does the thermal energy (which I think is just a disordered form of kinetic energy) count towards an object's mass, while the overall kinetic energy of the object does not. Does this have to do with frame of reference? Like, do fast moving things appear heavier from some static "center of mass" frame? If so, how is this different from relativistic mass?

Arguments over semantics. Those are some of my favorite kinds of discussions!

your videos are the only reason I would ever care about this stuff thank you so much

I really enjoyed the video, thank you for your work. I am not sure but I believe that sickle cell does have an evolutionary benefit. I remember reading that people with sickle cell are less likely to die from malaria.

If you haven't tried going through Purcell's Electricity and Magnetism, it's worth a read as it develops magnetism and gauss's laws from electrostatics and special relativity. I hear they even have a version that uses SI rather than cgs now. I'm not sure I'd recommend it as a first text unless suffering is what lets you know you're alive, but it does give one a different perspective.

Brilliant explanation, and love the explanation of the early 20c eeether physics. I had a lot of questions about e = mc2, and Einstein’s place in the debate that you wrapped up nicely. Thanks

Thank you, this was a great lesson on education as well as historically contextual development of physics. "It's good for teaching" or "we don't want to discourage or scare students" are terrible reasons to teach bad semantics. Analogies need explanations at very least. And this is why curated shared curriculum is so important.