Hamish Todd
Hamish Todd
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Stream #3: Different approaches/notation for Projective Geometric Algebra from Eric Lengyel and Jon
Google "Projective Geometric Algebra" and you see that there are two different notations people are using. This video tries to clarify things around that!
Predictably, I made a few mistakes! I did a stream clarifying these but the connection let me down. So, here are those mistakes:
1. I say in this video that you can measure the angle between anything and anything, without dualization. That's not true! There is an interesting exception, which is that you can't measure the angle between points or lines at infinity.
2. I keep saying "polar space" here, but that's muddling the terminology. Ordinary space is "euclidean space", the other space (the tetris window full of lines) is actually called "dual euclidean", and then the term "polar" is meant to be used for the two of them added together. So "polar tetris" is the right name for the game, since it has both, but the window full of lines alone is "dual euclidean tetris", not "polar tetris"
3. I didn't use the word "antispace" here at all, which is an oversight. So I'll say it here: "anti space", which is Eric's term, is another term for the tetris window, whereas the tetris window full of lines is what he calls "space". Of the things that start with "anti-" in PGA (anti geometric product, anti reverse), all of them are specific to Eric's approach, with two exceptions: the antiwedge, which others call the join or regressive product, and the antinorm, which others call the ideal norm. -- Watch live at www.twitch.tv/hamish_todd
zhlédnutí: 277

Video

The Periodic Table of Geometric Algebras - CL(3,0,1) does all 3D game math, so what does CL(p,q,r) d
zhlédnutí 361Před 5 měsíci
Almost all games take place in 3D euclidean geometry, which for those of us who like geometric algebra/clifford algebra, means we work with a "signature"/"quadratic form" Cl(3,1,0). But what about other signatures? Here we go all the way from (1,0,1), which is 1D euclidean space, up to (4,2,0), Roger Penrose's "Twistor theory" BIG omission: I didn't mention that (0,0,N) is "exterior algebra", w...
Stream #0: Why all video game programmers should learn geometric algebra
zhlédnutí 331Před 7 měsíci
Streaming weekly at www.twitch.tv/hamish_todd Watch live at www.twitch.tv/hamish_todd
Visiting the grave of William Clifford, discoverer of Geometric Algebra
zhlédnutí 433Před rokem
William Kingdon Clifford (4 May 1845 - 3 March 1879) was an English mathematician and philosopher. He discovered Clifford/Geometric Algebra; he wrote about moral philosophy and how a lack of rigour can be as bad as being evil; he suggested (long before Einstein) that gravity could be the curvature of space; and he discovered a multitude of other things, before sadly dying at the age of 33. Alon...
Intelligence Signalling: why we do it, and why it matters
zhlédnutí 1,2KPřed rokem
We spend a lot of time showing off how intelligent we are, or worrying about how intelligent we aren't, or praising others for their intelligence. And we are not the only species that behaves this way. This behaviour of ours is a big part of our success as a species. But why do we do it? This presentation tries to answer that question, and explores some ways we could do it better 0:00 Agenda 1:...
An explanation of Quaternions, from the bridge in Dublin where they were discovered!
zhlédnutí 1,6KPřed rokem
Took a trip to Broome Bridge in Dublin, to the place where William Rowan Hamilton figured out the fundamental formula for quaternion rotation! Some may recognize the similarity this has to the "belt trick", aka the "Dirac scissors" or "Balinese candle dance". Turns out there are three versions of it! Huge thanks to Lauren for filming this!
Geometric programming language update #5 - conformal geometric algebra visualization / VR widget
zhlédnutí 1,2KPřed 2 lety
I was flip-flopping between projective geometric algebra and conformal geometric algebra for a while, this shows both. Probably gonna have PGA in general and CGA for little bits!
Geometric programming language update #4: visualizing quantum computing with hyperbolic geometry
zhlédnutí 656Před 2 lety
I spent 6 months trying to visualize two qubits, eg entanglement, with geometric algebra! It didn't go brilliantly, but I'll return to this at some point. At least I learned something! Twitter: hamish_todd/ www.patreon.com/hamish_todd
Geometric programming language update #3 - "meh" to pictogrammatic programming?
zhlédnutí 735Před 2 lety
I was working on this branch of the project until February 2021! Then I got distracted making a game about prediction markets. And then by the Unity version of this. Then by quantum computing (that's the next video!). Well, better late than never. twitter: hamish_todd/ patreon: www.patreon.com/hamish_todd If you want to play around with this codebase, here's the github github.com/ha...
Murder She Bet - frequently asked questions and rule clarifications
zhlédnutí 232Před 3 lety
murdershebet.com Murder She Bet is a game about betting on Murder Mysteries. It is also about how to "put a number on" your uncertainty, and updating your uncertainty with new information. 00:00 - Hello! 00:05 - When do you decide whether characters are guilty? When do you halt trading? 01:22 - Why does the game ask for access to my camera? 02:19 - How do I allow access to my camera? 02:37 - Wh...
How to set up and play a game of Murder She Bet
zhlédnutí 369Před 3 lety
murdershebet.com Murder She Bet is a game about betting on Murder Mysteries. It is also a game about educated guesses, and about putting your money where you'd usually only put your mouth.
Geometric Algebra as a tool in technical communication
zhlédnutí 4,6KPřed 3 lety
This presentation assumes familiarity with geometric algebra. hamishtodd1.github.io 0:00 Introduction 1:09 A course for helping young physicists learn Geometric Algebra 24:04 Geometry Programming as a path to fundamental understanding for young computer scientists 41:09 Thoughts on a Geometric Algebra based editor for young people 57:22 Project roadmap
Geometric programming language update #1
zhlédnutí 917Před 3 lety
Geometric programming language update #1
Geometric programming language update #2
zhlédnutí 581Před 3 lety
Previous videos: Update 0: project outline czcams.com/video/hR-MQm3c13Q/video.html Update 1: czcams.com/video/1_arAnsx_Zc/video.html
What does it look like to rotate things in 4 dimensions?
zhlédnutí 21KPřed 4 lety
In this video, we're not talking about time as the 4th dimension, but instead 4 dimensions of space. Or, if you believe time has to "the fourth dimension", then in this video we are rotating in 5 dimensions :) The code for this presentation platform can be found here github.com/hamishtodd1/hamishtodd1.github.io/tree/master/mixedReality -look in units/twoSphereExploration.js and units/threeSpher...
CootVR basics
zhlédnutí 1,3KPřed 5 lety
CootVR basics
The Royal Alcazar of Seville panoramic tour
zhlédnutí 241Před 5 lety
The Royal Alcazar of Seville panoramic tour
A Class of Spherical Penrose-like Tilings with Applications to Virology
zhlédnutí 791Před 5 lety
A Class of Spherical Penrose-like Tilings with Applications to Virology
Braid's importance for communication, 10 years on
zhlédnutí 845Před 5 lety
Braid's importance for communication, 10 years on
4 Things You Should Know About Maryam Mirzakhani
zhlédnutí 270KPřed 5 lety
4 Things You Should Know About Maryam Mirzakhani
How should scientists communicate about simulations?
zhlédnutí 402Před 6 lety
How should scientists communicate about simulations?
Unintuitive action at a distance in game mechanics and Explorable Explanations
zhlédnutí 709Před 6 lety
Unintuitive action at a distance in game mechanics and Explorable Explanations
What do viruses have in common with buildings? Virus, The Beauty of the Beast Chapter 2
zhlédnutí 420Před 6 lety
What do viruses have in common with buildings? Virus, The Beauty of the Beast Chapter 2
What does Zika virus have in common with Islamic art? Virus, The Beauty of the Beast chapter 3
zhlédnutí 1,5KPřed 6 lety
What does Zika virus have in common with Islamic art? Virus, The Beauty of the Beast chapter 3
What do golf balls have in common with viruses? Virus, The Beauty of the Beast chapter 1
zhlédnutí 3,6KPřed 6 lety
What do golf balls have in common with viruses? Virus, The Beauty of the Beast chapter 1
Lecture inside virtual reality: 6-dimensional patterns and crystals
zhlédnutí 1KPřed 6 lety
Lecture inside virtual reality: 6-dimensional patterns and crystals
How to cure an ear infection using knot theory
zhlédnutí 547Před 7 lety
How to cure an ear infection using knot theory
Wordy tutorials need to end. Why Game Designers should take a "Vow of Silence"
zhlédnutí 8KPřed 7 lety
Wordy tutorials need to end. Why Game Designers should take a "Vow of Silence"
What do origamists have in common with viruses?
zhlédnutí 675Před 7 lety
What do origamists have in common with viruses?
Internalized sexism at its most toxic - a presentation on "Spatial skills"
zhlédnutí 688Před 7 lety
Internalized sexism at its most toxic - a presentation on "Spatial skills"

Komentáře

  • @iam_sarah7333
    @iam_sarah7333 Před 2 dny

    she is the only woman in the world who achieved that medal you can do a research on internet about it

  • @ThomasGirdwood-ej1fx

    Have you read Neil Levy's work on this topic?

  • @AndrewBrownK
    @AndrewBrownK Před 3 dny

    really great talk again, great history. I'm still pretty new to this and biased because I learned from Eric's material first, however... There's lots of stuff in history, "arbitrary choices" as you would put it, that ossify and set in stone as convention unless they are quickly overturned. And so even if there is a small but compelling reason to arbitrate differently, it's worth considering, especially in a field so (dare I say) young. For some examples... qwerty keyboards. Maybe back in the day they couldn't have the foresight to predict dvorak or other formats would have some marginal advantages. You can very well argue "we'd all be better off if the better convention took root". But how much better off? only a marginal bit. And people today can still customize their keyboards as individuals if they prefer. And yes that customized keyboard won't be easier for others to immediately pick up. But it is what it is. Then there's driving on the left vs right side of the road. Why does it have to be different? idk. But it's not the end of the world. Just have to know which territory you're in and which rules apply. Would it be nicer if everywhere followed the same convention? sure. Anyway all that to say.... Personally, I like Eric's arbitration. I think the grade of the object matching the shape itself makes a lot of sense. If taking the wedge increases grade, and performing a join adds dimensionality of the objects, it's only reasonable that joins should be the wedge (and meets should be anti-wedge). You say it was completely arbitrary. And to some extent, you're right. It's all opinion and preference at the end of the day. But it's exactly in that way that I say my opinion and preference is Eric's approach. So which convention would be better for everyone to adopt? or rather what should we teach to new people? For every point of conflict Eric's approach has with other dialects of PGA, I think it gains just as many points for staying closer to Vanilla GA. Increasing grade = Increasing dimensionality. In the bottom left corner of the video I see the e1 e2 e3 as planes without orientation arrows and e31 e23 e12 as lines with spinny arrows. It feels wrong. Aesthetics is one of the appeals of GA. Everything changed when I realized rotations happen in planes (bivectors), not around axes (vectors (univectors, sorry lol)). Not staying true to that feels like a step backward, or as Eric might put it, a hack. Anyway. Again, there's no reason we can't all get along and translate between conventions with a little extra work. But if there is a conversation or hypothetical wishful thinking about "if only we had one convention instead", then my vote is in Eric's formulation. Apologies if I made any mistakes or miscomprehensions. No grief intended. These things just take time to digest.

  • @AndrewBrownK
    @AndrewBrownK Před 4 dny

    this is an amazing atlas thank you so much

  • @AndrewBrownK
    @AndrewBrownK Před 4 dny

    Fantastic talk with lots of lore. Great history for people born into GA like me instead of a conventional LA/matrix background

  • @AndrewBrownK
    @AndrewBrownK Před 4 dny

    31:45 absolutely love your description of cross products. Like programming in a language without types - "it's so easy to learn!" but the truth is you are not escaping types, you are just exclusively using the "Any" or "Object" type and not doing your future self many favors

  • @hamish_todd
    @hamish_todd Před 16 dny

    Here's the game that takes place in elliptic space! store.steampowered.com/app/485680/sphereFACE/

  • @Slapbattler666
    @Slapbattler666 Před měsícem

    Lol

  • @Slapbattler666
    @Slapbattler666 Před měsícem

    I can just imagine 4 dimensional beings just looking down on 3 dimensional beings talkign about them like fish.

  • @Slapbattler666
    @Slapbattler666 Před měsícem

    I fidn the mask distracing

  • @hamzeh-zp9tc
    @hamzeh-zp9tc Před měsícem

    روحت شاد مریم بانو خیلی حیف شدی🥀

  • @golden220student7
    @golden220student7 Před měsícem

    Thank you

  • @juha-petrityrkko3771
    @juha-petrityrkko3771 Před měsícem

    In this representation it could be worth mentioning that 4 dimensions enable two simultaneous but independent rotations, one along one 2D coordinate plane and the other along the plane of the remaining 2 dimensions. Thus there are no "axes" of rotation but rather planes. Considering the 3D surface of the 4D sphere, longitudes and latitudes could also work better if we visualize them as a grid of cells made of intersecting surfaces rather than lines. The system (or one of the possible systems) would have two orthogonal "equators" lined with prism-shaped cells while all the other cells are distorted cubes.

    • @angeldude101
      @angeldude101 Před 18 dny

      4D rotations don't correspond to linear coordinate axes, but you can argue that you still have "axes of rotation" that just happen to be planes rather than lines. This kind of thinking helps when not limiting yourself to rotations around the origin and also allows for more complex "rotations", like a 4D rotation-like transformation that's actually "around" a sphere.

  • @sergiomanuel2206
    @sergiomanuel2206 Před 2 měsíci

    Rotations in n dims = n C 2 (n choose 2)

  • @deepdockproletarianarchive4539

    great video, however it was infuriating hearing how you pronounce geodesic lmao

  • @lundsweden
    @lundsweden Před 2 měsíci

    Is there any evidence of higher spacial dimensions in our universe?

  • @consumeentertainment9310
    @consumeentertainment9310 Před 3 měsíci

    I am inspired . Thank you for making this video. ☺☺

  • @nellsea8086
    @nellsea8086 Před 3 měsíci

    Why mention anything about her religion most Iranians don’t even practice the religion they just have to role with it cuz of the government hence the headscarf when she was living in Iran. Would’ve preferred if you just focused on her great talent.

  • @incoprea2
    @incoprea2 Před 3 měsíci

    This is the best explanation I've seen!

  • @remomagalhaes4707
    @remomagalhaes4707 Před 3 měsíci

    By coincidence i have just taken i look of a picture of Clifford's grave in wikipedia

  • @remomagalhaes4707
    @remomagalhaes4707 Před 3 měsíci

    I agree with you that Clifford's algebra is revolutionizing the world

  • @remomagalhaes4707
    @remomagalhaes4707 Před 3 měsíci

    I guess The number of rotational degrees of freedom would be n*(n-1)/2 which is equal to the sum of k with k varying from 1 to n, with step 1.

    • @remomagalhaes4707
      @remomagalhaes4707 Před 3 měsíci

      The reason is simple. We just have to count the number of "planes" in a given space. Suppose we are in 4d space, then, there are 4 axis, and each axis can be combined with the other 3 remaining axis, which would result in 4x3=12 possibilities. But we have to divide the result by 2 because of the repeated permutation.

  • @etebmw2020
    @etebmw2020 Před 4 měsíci

    She was not Muslim not all people in Iran are Muslim. The Muslim and not Muslim were forced to bare hejab. But as you evidence seen the last 2 years iranain women ones again find thier own positions in this crazy terorist cuntry of Islamic repoblic of terorist and shown they are against terorist and all also not fan on Islam regardless of any kind of 76 kind of Islam. So you don't need to call he Muslim

  • @drdca8263
    @drdca8263 Před 4 měsíci

    The “number degrees of freedom” of rotation, is referring to the dimensionality of the rotation group, right? So, in 2D, the group is SO(2) (the circle group), and in 3D it is SO(3), and in n-D, SO(n) an element of SO(n) is specified by giving an ordered orthonormal basis of n-D space (such that the determinant you get is 1 rather than -1, but that requirement won’t change the dimensionality) specifying the first vector, there’s an n-1 dimensional choice there (the minus one is because the vector has to be of a specified length, so we remove a factor that is from scaling), and then the what remains is to pick an orthonormal basis for the subspace orthogonal to the vector chosen first. So, f(n) = (n-1) + f(n-1) And f(0)=0 so, f(1) = 0, f(2)=1, f(3)=3, f(4)=6, Yeah, these are the triangular numbers, so, the dimensionality of SO(n) is n(n-1)/2 . So, in particular, the dimensionality of SO(5) is 10.

  • @hyunjaecho1378
    @hyunjaecho1378 Před 4 měsíci

    enjoyed so much watching your stream! where can I watch your lectures? I want to learn more about this Geometric Algebra, it sounds awesome! And is there any resource that I can read or listen to ?

  • @superdom4
    @superdom4 Před 4 měsíci

    This was interesting, thanks for uploading

  • @josdurkstraful
    @josdurkstraful Před 5 měsíci

    1, 3, 6, 10, 15, 21, 28, 36..........?

  • @noegowego
    @noegowego Před 6 měsíci

    Awesome video. Very valuable information! Just wondering. To which extend have you explored Buddhism? The core of Buddhism, as a praxeology - not a religion, is that of a handbook on meditation. And depending on the branch, you might find different interpretations of the virtues (patience, compassion etc.) to go along with the meditation. But surely, you're not saying that meditation is virtue signalling? I mean, it could be, "Look at me, I can meditate for 2 hours. What's your record?" But for the most part it's a private process - one of the few things that we DON'T project on to the whole world. So where do you draw the line between useful things coupled with a genuine desire to share them with other people - and pointless bragging that might occasionally lead to scientific breakthroughs?

    • @hamish_todd
      @hamish_todd Před 5 měsíci

      Many, many people ask variations of this question about things other than meditation, and is a good question. At some point I want to have an entire lecture dedicated to answering it. There's a lot in the book "The Elephant In the Brain" about it too. I'd say meditation is intelligence and virtue signalling - remembering that, when I say those things, I do not mean it as an insult, because as I say in this video, I think signalling of various kinds has made the world a better place. As you say, it's a private process and not bragged about much. But that's also true of philosophical and scientific inquiry. Sometimes people brag about them, but there again, some people have bragged to me about meditation. Many people who meditate (most, for all I know) may never bring it up; but that's also true of many people who entertain philosophical thoughts in their down time. The thing is that in order for something to be a really convincing signal, it hurts you to brag about it or consciously do it with the goal of signalling. This may sound small but it is REALLY IMPORTANT; it is the subtlest part of the model. Again, The Elephant In The Brain is the number 1 text on this. > So where do you draw the line between useful things coupled with a genuine desire to share them with other people - and pointless bragging that might occasionally lead to scientific breakthroughs? I see no line, not even a blurry one. When people claim that they can describe one, I try to stop myself from listening to them, at least until they go back to saying more interesting things. To "share" is to brag, and to brag is to share. "Useful" scientific breakthroughs almost always start with someone working on things others would consider pointless. Of course, I myself sometimes look at a thing someone is doing and consider it pointless; and I sometimes look a person bragging and think less of them; and I sometimes spend time "sharing things that I consider useful" (like this video...), and then become aware that it came across as bragging about pointless, and then I get sensetive/self conscious about it. All of these things are parts of being human.

  • @SheppardXR
    @SheppardXR Před 6 měsíci

    Super elegant explanation!

  • @encapsulatio
    @encapsulatio Před 7 měsíci

    I suck at even remembering most of the math in high school. What are the minimum concepts i need to relearn to be able to properly learn geometric algebra? Thank you.

    • @hamish_todd
      @hamish_todd Před 5 měsíci

      1. When you add any two objects together, you get an object "between" them 2. Your basic objects are three orthogonal planes through the origin called e1, e2, and e3. There is a "plane at infinity" called e0. 3. The "geometric product" is transform composition. 4. Planes, lines, and points exist and are simultaneously both objects and transformations. e1 is a plane, but is also a planar reflection. 5. If you have a thing B and want to transform it using A, you use the formula ABA⁻¹ That's the minimum amount to remember! Here are some examples -Adding two points gets the point between them -Adding two rotations gets the rotation between them -e1*e2 = e12. e1 and e2 do planar reflections, so e12 is a 180 degree rotation, and it looks like a line, the axis - it happens to be the line where the e1 and e2 planes meet. Its inverse is -e12. -e013 is a "point at infinity". If you want to rotate it using e12, you take e12 * e013 * -e12 = -e013, which is the point at infinity in the opposite direction

  • @galoomba5559
    @galoomba5559 Před 7 měsíci

    The number of degrees of freedom is n choose 2 because rotation happens in 2 out of the n dimensions.

  • @jondo7680
    @jondo7680 Před 7 měsíci

    Nice, but that's not what I was looking for. It's said that a 4d creature can mirror 3d objects, so I'm looking for a computer animation that shows how that would look to us.

    • @hamish_todd
      @hamish_todd Před 5 měsíci

      You can work it out by thinking about what a 2D flatlander would see if you were to "mirror" one of their friends by rotating them in the plane they live in. First, you'd choose a line in their plane that you were going to mirror them around. Second, you'd rotate them 180 around that line. Their friend would see the line part of them stay in place - so, if the flatlander had their liver and spleen on that line, as you rotate them, their friend would see a 1D cross section of their liver and spleen appear to stay where it was while they were rotated. Before you start the rotation, their friend would see them; during, they'd see only the cross section; and at the end, they'd see their friend reflected, with that 1D cross section the only part that has stayed in place. So what's this like for 3D creatures rotated by a 4D creature? You work it out. Clue: in 4D, you do not rotate around lines, you rotate "around" planes.

  • @ahuramazda9037
    @ahuramazda9037 Před 7 měsíci

    Maryam...🇮🇷🇮🇷🇮🇷🇦🇿🇦🇿🇦🇿👏👏👏

  • @seyedalimortazavi5321
    @seyedalimortazavi5321 Před 8 měsíci

    Thank you hamish

  • @Garfield_Minecraft
    @Garfield_Minecraft Před 8 měsíci

    I love the fish so cute

  • @ot8479
    @ot8479 Před 9 měsíci

    Let's et one thing straight. There are strings of women in history who deserved way more than the field freaking medal. bUt of course, as always it is an old white creepy guy game only. Cause Katherine Johnson would have deserved two in a row.

  • @SaraAbyar
    @SaraAbyar Před 9 měsíci

    Gosh no she was not a muslim. 😤We Iranian woman are forced to wear hijab and be muslims we never chose it. If an Iranian woman leaves Iran and still has their hijab on it means they are probably a believer in Islam or they are so used to it they need more time to decide. If they don’t wear it anymore it means they never wanted to and they were forced to do so their entire lives. Man I’m so sick of religion oppression it breaks my heart to see that not even injustice about Maryam is acknowledged who’s going to acknowledge our’s?!

  • @sargoldart
    @sargoldart Před 9 měsíci

    Bad explanation, so many bloops. Also maryam was the first and only woman to win the filds and no where is mentioned she is muslim. Not just beacuse she is from iran, she is muslim. There are so many people from iran who are NOT muslim.

  • @JuliLa222
    @JuliLa222 Před 9 měsíci

    Woo yeah 👏

  • @financew8824
    @financew8824 Před 9 měsíci

    Fugitive brains from the Nazis of the Islamic regime....Maryam Mirza Khani, your soul is happy

  • @Tshego2000
    @Tshego2000 Před 10 měsíci

    The rule is (n)(n+1)/2

  • @michaeljackson3739
    @michaeljackson3739 Před 10 měsíci

    Maryam Mirzakhani was not a Muslim, she was an atheist.

  • @VulpinetideCuteTimes0w0
    @VulpinetideCuteTimes0w0 Před 10 měsíci

    It's interesting that the shadow of a 4 Dimensional Sphere, looks very much like the magnetic field.

  • @farahnazjalali1975
    @farahnazjalali1975 Před 10 měsíci

    Thinky

  • @HeisenbergHK
    @HeisenbergHK Před 11 měsíci

    So boring...

  • @stellazonoozi2829
    @stellazonoozi2829 Před 11 měsíci

    She was not muslim😡 We Iranians are Forced to be Muslim 😡

  • @hermes_logios
    @hermes_logios Před 11 měsíci

    Time is our broken perception of the 4th dimension. We here (living in a 3D experience) can’t perceive the entirety of 4D reality, so we break it down into 3D phenomena that change over time. So, time literally is the 4th spatial dimension, but spread out as a temporal experience. Time is the point where 3D intersects with a 4D object.

  • @hermes_logios
    @hermes_logios Před 11 měsíci

    2:20 The projection of the sphere’s shadow onto the 2D plane is identical to the Steinmetz diagram of electromagnetism. Apollonian circles, each pair of which meets at 90 degrees to one another, ie like being inside a torus. This is also what the Yin Yang symbol means. It’s a symbolic representation of a 4D sphere, or a 3D torus, as a 2D image.

    • @drdca8263
      @drdca8263 Před 4 měsíci

      A 4D sphere and a 3D torus aren’t the same? Also, do you mean a 3-sphere which lives in 4D space, or do you mean a 4-sphere? And, by a 3D torus, do you mean a solid torus (a three dimensional manifold with boundary isomorphic to S^1 x S^1 ) or do you mean S^1 x S^1 x S^1 ? If you glue two solid torii together at their boundary in a way that exchanges which direction around the boundary is contractable, then I believe you get the 3-sphere. Maybe that’s what you were getting at? (If you glue it the other way, uh, I think you get the product S^1 x S^2 ?)

    • @hermes_logios
      @hermes_logios Před 4 měsíci

      @@drdca8263 A 3D torus is a projection of a 4D sphere. In toplogical terms, I mean a 3-sphere, and I do not mean solid (which would be more appropriately called a "ball").

    • @drdca8263
      @drdca8263 Před 4 měsíci

      @@hermes_logios pi_1(a 3D torus) is non-trivial, but pi_1(S^3) is trivial. Like, there are non-contractible loops in the 3D torus (regardless of whether you mean D^2 cross S^1, or S^1 cross S^1 cross S^1, or S^1 cross S^1 ) , but no non-contractible loops in S^3 . But, any loop in the image of a projection should be the image of some loop in the domain of the projection, like, if you have some path in the image of the projection, and take a point in the preimage of some point in that path, there should be a path in the domain of the projection which goes through that point, and when composed with the projection, gives the path in the image of the projection (assuming that it is a linear projection, as in this video, not just a more general continuous surjection which is a one-sided inverse of some inclusion) and also assuming, that the space that is being projected, is convex... Oh wait, the 3-sphere isn’t convex, hold on.... Wait I have to go do something, I’ll say more later... But I think the argument I have here will work with some modification Edit: ok, meeting was canceled. Ok, so, while the 3-sphere as embedded in R^4 isn’t convex, for any two points on it, if they aren’t antipodal, there is a unique shortest path between them. If we take a loop in the image of S^3 under the projection, then... the preimage of that loop will have at least one point in the 3-sphere above each point in the curve in the image of the 3-sphere, and, ah, specifically each point in the loop should either have 1 or 2 points in its preimage, and, yes, we can indeed locally trace the path, though perhaps going around the loop could produce a path from one preimage of the base point to the other... But then, going around again would lead to the first one again, so, any loop in the projection, *when gone around twice*, is the image of some loop in S^3, And any loop in S^3 is contractible, and the image of the contraction of the loop will be a contraction of the image of the loop, and therefore, for any loop in the projection of S^3 (or any sphere) under a like, linear projection onto a “screen” one dimension down, like in this video, will have the repeating of that loop twice, be contractible. But! In the torus, there are loops which cannot be contracted, even after you repeat them any number of times. Therefore, the image of S^3 under a projection of the kind shown, is not the torus.

  • @phillipcoetzer8186
    @phillipcoetzer8186 Před 11 měsíci

    Reason I came here is to understand this better... when a hyper cube is rotated it looks like it's propagating ... so I got to thinking what does a sphere look like spinning.. does it also look like it propergates like a hypercube the I remembered the two ways you can rotate a hypercube and realised its not two ways but the combination of both or it would would not be rotation. I'm beginning to think that light lies on the boundary of two and 3 dimensional space and black holes are onthe boundary of 3 and 4th dimensional space and the rotation of an atom is 4th dimentional