The most beautiful idea in physics - Noether's Theorem
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- Опубликовано: 22 сен 2015
- Homework:
-What do you think of this idea? Have you heard of it before?
Maybe you’ve heard about things like super symmetry in physics try find out how that’s related.
-If you know some calculus and classical physics, try and find a proof of this theorem.
Try come up with strange systems with strange symmetries then see if you can figure out what’s conserved.
The proof and maths of Noether's theorem:
There are two ways to approach Noether's theorem that I know of. The most common is through Lagrangian mechanics- where the proof is surprisingly simple but unfortunately quite opaque (see math.ucr.edu/home/baez/noether...) . The other way, and the way I allude to in this video, is using hamiltonian mechanics. I find this way a bit easier to understand and it involves the generators of the transforms more. A great resource for this is the last lecture in this course: www.physics.usu.edu/torre/6010...
but it requires multivariable calculus and a little knowledge of Hamiltonian mechanics.
Its all fun and games, until your exam is tomorow....
LockenJohny101 lol, my exam is tomorrow and this is not even in my course
same
God said "let there be a bang" and there was a bang. A big one. And the only one we have proof of. I argue that if even one atom were misplaced, nothing would have happened.
There is no proof whatsoever of the ridiculosly naîve big bang. The universe is moving alright, but not outward since there is nothing, no spatial dimensions "outside", it continuously moves whitin itself, known as the Donut Theory. From this movement of the spatial dimensions themselves emanates the red shift.
Even the big bangers are now saying there was no "bang". Nowadays, it's just a simple expansion of of space-time, optionally with inflation and even more optionally with effervescence.
"writes most beautiful bit of physics in spare time
100% not jealous" xDD
power of math, so beautiful
One thing I absolutely love about Noether's theorem (you know, besides the whole "she literally figured out _why_ conservations occur") is the fact that symmetry in position implies momentum conservation, and symmetry in time implies energy conservation. Meaning that the relationship between space and momentum is very similar to the relationship between time and energy - something that Einstein *_also_* figured out through a _completely_ different route. And the fact that 2 different people could arrive at such a fundamental truth about the universe through such different means is mindblowing.
Oh, one more thing: Angular momentum is conserved because of rotational symmetry - yet elliptical planetary orbits also have a conserved angular momentum. How exactly do the two fit in? Does it mean "same energy even though the orbit shifted slightly"?
_"Angular momentum is conserved because of rotational symmetry - yet elliptical planetary orbits also have a conserved angular momentum. How exactly do the two fit in? Does it mean "same energy even though the orbit shifted slightly"?"_
No, it means "same angular momentum". The position changes, and the velocity changes, but their (cross) product stays the same. This is true with either circular or elliptical orbits (or with no orbit at all). Remember that position and velocity are vectors, with both magnitude (distance and speed) and direction. In circular orbits, the magnitudes are constant, and only the directions change; in elliptical orbits both magnitudes and directions change. But in either case, the cross product is constant.
I think you can also see the elliptical orbit as a combination of rotation and translation (it pops right out if you use polar coordinates) so you could think of it as a combination of rotational symmetry (conservation of angular momentum) and translational symmetry (conservation of momentum). I'm not 100% sure about this it's just an idea that popped into my head and l would love for someone with more knowledge about this topic to correct or approve this idea
In the video she talks mainly about potential energy. In the case of an elliptical orbit the potential energy is not concerved, but kinetic + potential energy is conserved, so the TOTAL energy of the system is still conserved. If the planet is closer to the center the potential energy is lower but kinetic energy is higher, and vice versa. So now indeed your question arises what the symmetry is - it's not a circle! What you need to remember here is that not the shape of the path has to be symmetric, but 'the way the planet acts' (in Lagrangian physics terms: the action). If the planet would be rotated to another point, the path would also be rotated with it, so the planet would still act the same, but it is rotated a bit, including the path it takes. So the symmetry doesn't have to mean it takes the exact path as before, but that it follows the same path, with that path just being rotated over the same angle as the object was. Does that help?
Newton also sort of figured it out.
F*t= change in momentum
F*d= energy
@@michaelsommers2356 how do I bookmark this comment?
I love your videos. I always hear a smile in your voice.
Yea, you can kinda tell she can barely contain herself. so kewl.
@@RSolimov 'kewl'?
@@fukpoeslaw3613 cool but with a cooler spelling.
@@SteveGouldinSpain tewk me half a year, but finally I know!
@@fukpoeslaw3613 kewl. 😹
I'm so excited you got around to this video. Were you just using conservation of energy as an intuitive reference? Because I think (traditionally in Noether's theorem) it's treated as a complete independent thing.
+The Science Asylum Traditionally its symmetry of the lagrangian, but there's an equivalent Hamiltonian formulation that I got excited about. Way more intuitive to me!
Looking Glass Universe
I think I should sit down with the math then. Lagrangians are weird. Anyway, I'm glad you got excited about it. It showed :-)
:D
Hi crazies!
@@LookingGlassUniverse Nah, AdS/CFT is way more beautiful
After 2 or 3 years later and also constantly learning new things, i finally feel like I understand what’s being said said in this video. Truly feels amazing finally able to grasp it. Thank you 😊
The coolest thing about this video is that you explained the intuition, motivation, and significance of Noether's theorem without invoking the Lagrangian at all. I watched this video when it came out (in high school for me) and thought it was an amazing concept. I'm commenting now after taking classical dynamics for my physics degree, and this intuition was really useful for understanding the lagrangian, even though usually, its the other way around. Thanks again! Love the new "self-teaching" physics video too btw!
Thank you for this great explanation of Noether's Theorem. I've been reading a biography of Emmy Noether, and I have a physics background, but I didn't really understand her work at all. Now I understand it a bit more. Thank you. I subscribed to your channel and will definitely be checking out some more of your work. Thanks again!
P.S. Emmy Noether is really a tragically unknown figure in physics. I hope more people learn about her. That's why I decided to read up on her, because she's one of the most influential women in math/science (one of the most influential of men and women, in fact) but she is still totally unknown to most. It's a shame. But your video surely has reached new audiences and for that you deserve praise.
This is the best introductory discussion of Noether's Theorem I have seen. Thank you for your excellent work!
Here is a suggestion. It seems to me that your examples of systems whose energy has not changed after a particular transformation might be easier to understand for viewers (for me anyway) if you considered a small, continuous change rather than a large jump change. A system whose energy did not change after a jump (for example, an instantaneous rotation through 60 degrees) might turn out to have periodic energy dependence with period, in this example, 30 or 60 degrees. While a jump change is not ruled out, I do not think a discontinuous change would best exemplify the type of conservation law you are aiming to illustrate in this video. The real-world conservation laws that I am aware of (in classical physics with or without relativity) are all based on continuous quantities.
You don't have to call it an infinitesimal change. You might just say the change is gradual, or something like that.
P.S. I might be wrong about this. It was just a thought.
No I love the suggestion. If I could go back, that's a change I'd want to make. Hopefully I will give Noether's theorem another shot in a while- I wasn't too happy with that one. I really appriciate the suggestions for improving it!
+Jack McMillan So each symmetry to be characterized requires its own Lie algebra to be set up based on the relevant degrees of freedom in the neighborhood of a given point in the problem space -- something like that?
Hi there. I've got to ask...
"
So...a symmetry is when you change a system and some number computed
from the initial condition doesn't change. And,
A conservation law says that when a system changes, there's a number
that characterized the system in the first instance that's the same in
the second instance.
This theorem seems...tautological...
"
I first posted this to the original video (just now), but I realize that's probably not going to get a good answer. Can you help me sort my confusion?
Edit...perhaps I am under-appreciating the development of ideas that it takes to come to such conclusions...particularly when I am now speaking from the shoulders of all the others who had to toil so!
I am not qualified to assist you with that. Sorry. Perhaps the following might be of help:
physics.stackexchange.com/questions/4959/can-noethers-theorem-be-understood-intuitively
Hi there Legionary42.
Maybe I can help you a little bit. Symmetry means something very specific in physics. Symmetry refers to a certain transformation of a system's dynamical information (which consists of positions, velocities and perhaps even time) which leaves the Lagrangian unchanged. These transformations are physically interpreted as a change of coordinates.
Note something very VERY important though. The only kinds of symmetries Emmy Noether discusses in her groundbreaking paper are so called continuous symmetries, where each symmetry transformation can be built up by a large number of tiny infinitesimal symmetry transformations, such (to be more precise, the group of symmetries forms a Lie group). An example is rotational symmetries.
A conservation law refers to a physical scalar quantity that doesn't change with time. Notice, a priori these 2 concepts seem to have no no connection whatsoever but Noether's theorem shows otherwise. There is no intuitive argument I can think of to explain this relationship and in fact I'm not even sure there exists one. A Lie group refers to something very very specific (namely a differentiable manifold, equipped with a group operation) & the fact that every "continuous symmetry" (i.e. what we intuitively think of as being a continuous symmetry operation) can be given enough structure for it to form a Lie Group is a highly unintuitive fact. Symmetry, the way we intuitively think of it, is just a map on the phase space which leaves the Lagrangian unchanged. This is not enough to derive a conservation law. So although we have intuitive notions of what we mean by symmetry and conservation laws, those intuitive ideas are not enough to establish Noether's theorem. We have to introduce a lot more structure to our model to establish the connection. Unfortunately this breaks down the intuition.
Moreover I think I should point out that although in classical mechanics, you get conservation laws only from continuous symmetries, In Quantum Field Theory, even discrete symmetries have a conservation law. For instance think of the harmonic oscillator modelled by the potential (1/2) kx^2. You can check that the Lagrangian doesn't change if you swap x for (-x). There is no conservation law for classical mechanics for such a symmetry. There is one in QFT.
I love how happy your voice is when you are talking about this subject :)
I am currently a physics major in college. I absolutely love your work! It is so refreshing and clarifying yet introduces fascinating questions to ponder :)
Great video, I loved it. This channel is really one of the best I know on RUclips and you do a great job explaining science phenomena.
Thank you
You know something madam, I’ve gorged videos and articles looking for an explanation to make this concept clear, and yours was the only one that did! Good job. Very good job indeed.
Thank you, ma'am. I watched this at least 5 times now (I'll probably watch it again, when my head stops spinning), and I feel like I understand much more than I did before, but also like I've only scratched the surface of the ideas presented here.
Beautiful presentation of a beautiful theorem. Thank you!
Thank you! I have read about Noether's theorem and I know Einstein really like her work. I think I have a good understandings of the basics. Well done.
I knew that therr was something in Physics that had to do with symmetry implying a something, but I was not sure. I went to RUclips to find a video explaining it. And yours was bu far the best. Thank you. I have subscribed to your channel, and I am working my way threw all of your videos now. I love them. Keep up the great work!
Wow, this is AWESOME! I finally UNDERSTAND why there are these random 'laws.' And on your roller coaster example, I'VE HAD THE EXACT SAME THOUGHT!!! Yeah, I solved the problem, but I'm just using some law, so to really explain anything I must explain where the law comes from! This is so cool! Great video. I cannot express how long I've been thinking this and how annoyed I've been with 'It's just the law' (I really feel like this should have been explained to me years ago, maybe in high school or something) But, it's never to late to learn, so thanks!
yes this was definately not in HSC AUstralian higher physics 2004 though clearly shouldve been. I think its because symmetry has buddhist connotations, and Australia has a Christian hangover.
I feel 100% the same! :)
To be honest even if you don't know about Noether's theorem, conservation laws are just mathematical derivations from basic Dynamics (Newton's laws, Schrodinger Equation, etc.) They are not "random" or artificial, just magic numbers given rise to by mathematics. Of course if you are still not satisfied and want to go deeper you can always fall back to the Principle of Least Action.
I always watch your vids while doing my stats homework. Keeps me motivated!
Never heard it before,my friend suggested me to look for noether theorem ,I searched it and I'm glad I found it 💙💙💙. You explained it so well ,thank you 😇🙏
I've seen a lot of videos and read a lot of books explaining Noether's theorem. Considering the balance between completeness and simplicity, this video is by far the best.
I agree, I also find it extremely beautiful that conversation rules are connected with symmetries of time and space. When I've read about Emma Noether's theorem, it blew my mind!
I haven't heard any of this before, and now it's another theorem for me to explore! Thanks you for the great explanation.
Really interesting. I had encountered symmetries and conservation laws before, of course, but it had never occurred to me that they'd be related like this. It makes sense, and I think the rotational example is probably the best one in order to understand it. If an object behaves the same no matter where in its orbit it is, then logically it'll always come back to where it started, and angular momentum is conserved. It had never occurred to me though. Thanks for explaining it!
On the second question, I don't really understand supersymmetry beyond that my physicist brother thinks it's hilarious that we haven't found any supersymmetric particles yet. I have some grasp of what supersymmetry is for, but not much at all about what it is. Looking it up, it seems to be about symmetry between bosons and fermions. But would energy be equivalent in that? If these supersymmetric particles exist they'd have to be of higher mass than particle accelerators can produce, so the boson version of a down quark, for instance, would have to have a much higher mass than it. And if it has a higher mass it has more energy, so then how could it be symmetric? Researching this has left me more confused than when I started... Maybe I'll ask my brother when he wakes up.
On examples of symmetries, my music training is kicking in and I keep thinking of things like diminished seventh chords, augmented triads, and whole-tone or chromatic scales. those are translationally symmetric, at least if you don't count octave. I have no idea what, physically, is conserved there though. On the diminished seventh chords at least, harmonic function is conserved, which is actually an incredibly cool feature of those. (Seriously, diminished seventh modulations are my all-time favorite piece of music theory.) But that's not a physical property, it's an observational effect. If you're talking physically, you do go up in pitch as you go, and I believe higher frequency is higher energy, so not really symmetric. If you rotate it around so you're actually playing the same register of notes though I suppose you're conserving something. I don't know if that's valid though, since it requires accepting the octave as a fundamental unit. I may be cheating, but if I am I still don't have any useful answers so at least I'm not a very good cheater.
I've started commenting on videos with my 12tone account instead of my personal account (this one) to keep them a little more separate. I don't know if you recognize me anyway, but I've been watching and commenting for a while and I didn't want you to think I'd just disappeared!
+razorborne bro that some really nice text. :)
saleem khatib Thanks!
This is an interesting question (I know I'm 6 years late to respond)! I don't have a full answer, but I will say the behavior of pitch classes in music works a lot like a cyclic group in abstract algebra! If you accept the octave as a fundamental unit, you can "quotient" out the octave and just focus on pitch classes, which is similar to if you took all the integers and divided them by 12, concentrating only on the remainder after the quotient. Physically, I think you could interpret this symmetry as a dilational symmetry in frequency (since moving everything a major third up for example is equivalent to multiplying the frequency by the third root of two). However, I can't think of a way you could apply Noether's theorem to this situation because Noether's theorem requires a *continuous* symmetry. For example, if you move forwards 10 seconds, 2 seconds, 0.01 seconds, 0.00001 seconds, etc., it doesn't matter: energy is still conserved in Newtonian mechanics. However, with the chords that you mention, the symmetry is *discrete* in that it only works for specific values of movement. If you move an augmented triad up a major third or an augmented fifth, it stays invariant, but if you move it up a whole tone, a semitone, a quarter-tone, etc., the symmetry breaks, meaning the conditions of Noether's theorem aren't met.
Also, I'm not sure if there's an analogous principle of least action (which Noether's theorem depends on) in this situation, but I'm sure there's some way to draw a physical interpretation of this, maybe in studying the frequency spectrum of sound decay over time? This is just wild hypothesizing since it's 2 AM, but it would be interesting to see if there's a spectral equivalent of Noether's theorem since all the examples given (translation, rotation, time translation) are symmetries in space-time while your example is a movement in frequency space. I suspect that if you could formulate an appropriate Lagrangian, there might be some way to apply Noether's theorem, although it would probably have nothing to do with music. It's still an interesting thought experiment though!
For an elaboration on the abstract algebra though, check out this cool stack exchange post about Messiaen's modes of limited transposition and group theory :) math.stackexchange.com/questions/4045800/music-and-maths-modes-of-limited-transposition
Also, I love your video on diminished seventh modulations
As a theoretical physicist, I'm very happy that this video exists. It certainly is the most beautiful theorem in physics and i wish i could explain it that well in simple terms.
In 2020, your videos are great presentations, drawings and humor in addition to the science. Thanks
This video is extremely descriptive and very easy and understandable from layman point of view. Really good work
I've seen a few other of your videos, but this one earned the sub. Math and physics with a solid backing in both? Absolutely. As a CS/Math person (who spends most of his time where the two look like the ssme thing), I would almost venture to say that I'd prefer a mechanics built from conservation laws implied by symmetries, because symmetries are the building blocks of the algebra we use to solve the problems in the first place.
I think I'll spend the afternoon poking at Noether's theorem with respect to multivectors and geometric algebra. That angular momentum conservation due to rotational symmetry is begging to be described in terms of bivectors, heck, kepler's laws probably drop out if the orbit's spinor varies in time...
Starting third year astrophysics. Noether's Theorem came up. Wanted it explained concisely. This is what I was looking for!
This is a really good video demonstrating physical laws. For me, when trying to look this theorem up, I found wikipedia no help, being dropped into an infinite cycle of looking up unfamiliar terms. This video really showed the essence.
Thanks for this video! I've heard of this before but I just noticed an interesting thing watching this. I realize that in a lot of formulas, you see position and moment together, and energy and time together. For instance, I've seen that the Heisenberg uncertainty principle can be expressed in terms of position and momentum, but can also be expressed in terms of time and energy. We also know that the momentum of a light wave is related to its wavenumber (cycles per distance), and the energy of a light wave is related to its frequency (cycles per time). Perhaps this is what you mean when you say that more appears in quantum mechanics. Very cool!
New subscriber and I have been loving your channel. Looking forward to hearing more about your research!
+Maria Ciara Lalata Thank you!! Yeah, I should make a video about that :P
Beautiful presentation, as usual. Thx
Great video! My QM prof mentioned Noether's theorem in passing so I figured I'd check it out.
Please start making videos again. These are great
You have one of the greatest videos! Really amazing.
Hey. What a great explanation of the magnificent Noether's Theorem. It's applications in QM are huge and the connection to supersymmetry gives us String Theory. Thanks so much for these videos, they are unreal. I'd love to talk further, if you're down :)
4:20 "I used to not like using conservation laws because they can make it seem too easy."
Love it, I feel exactly the same.
Well i learned the theorem in a more complicated way. I think this is the best way to think of it.Its really helpful.Thank you
Her voice is so soothing and lovely, with a hint of a smile in it. It tingles my brain.
Best explanation I've heard yet. I almost get it. Thanks :)
Thank you for helping me understand what the theorems about. Although I barely understand what you were saying at all. I strangely get what you are trying to say. Then again, I am just a person who is trying to do a book report about Emmy Noether. Not helpful that I don't even take a physics class. But, then again, thank you very much!
this the best video about noether's theorem ngl;
would love to know how do u mathematically derive it.
Thanks for the wonderful video! I'm not a physicist but I've heard about Noether's theorem while reading about the "Theory of Everything". Maybe you can follow up by talking about Gauge Symmetries?
This reminds me of the uncertainty principle, in that everything comes in pairs that multiply to give angular momentum.
Momentum * Distance = Angular momentum * Angle (dimensionless in radians) = Energy * Time
More precisely, the pairs multiply to give "action" (which only happens to have the same unit as angular momentum). Planck's constant is a tiny quantum of action.
Thanks. Studying momentum generating functions in Finance/Economics and its nice to have the real physics concepts salient :D
In statistics that's a different stuff. It's a function used to compute mean values of things snd measures of deviation from a certain value.
Excellent video! Thank you
I heard about this from Feynman's lecture on symmetry. "Watch out!” - lol. Best physics punchline ever?
Wonderful explanation.
Amazing channel, congrats!
this is a great channel, thanks
Utterly Captivating!! Keep up the good work!! ;-)
" The most profound and far-reaching idea" is quite ambitious even thoug Thank you for this explanation, such a brilliant carrier that Emmy had by the way.
It's beautiful and thought provoking.
Why haven't there been any new videos? This was one of my favorite channels.
Thanks for putting this up. I believe there is a more basic version of this that predates Noether's theorem by many centuries, because you can easily make an argument by contradiction, where you suppose that a certain quantity is _not_ conserved, under a certain transformation, and then it follows that by measuring that quantity, you could tell where you are on the particular dimension on which the quantity varies. If there is no way for you to tell where you are on that dimension, by measuring that quantity, that means that the quantity is conserved under transformations on that dimension. If you could tell, by measuring a certain quantity, that means there has to be at least some type of difference between one place and another place on that dimension, in other words, an asymmetry.
For example, the temperature outside is not conserved, from one day to the next, and consequently, you can look at a thermometer for 30 days in a row, and (in case you had no other information) you could ascertain the position of the earth in its solar orbit. That tells you that the position of the earth in the summer can not be exactly the same as in winter, and in fact, the earth's solar orbit does not have rotational symmetry, at all, and in fact, the weather is the first way that people knew that (and the positions of the stars, of course).
If you applied the same reasoning to linear momentum, which is conserved, on the other hand, you could ask, if you rode a train from one town to another town, whether you could tell how far you had gone by measuring how fast the train was moving. If you could tell where you were by how fast it went, that would mean that our path did not have a symmetry under a linear translation, precisely because you could tell the difference between one place and the next. (I am implying an idealized train, but you see what I mean.) If it were possible on such a train to observe, for example, "We're going faster, so we must be almost there," that would mean that there was a difference between your momentum in the first town and your momentum in the second town, in other words, an asymmetry under linear translation.
I believe what Noether's theorem does is merely formalize and generalize this fact, which people had already been applying for many centuries.
Thank you for this great lesson
Fantastic explanations! Thanks a lot
Very nice, my understanding of conservation laws come from Chemistry, so this explanation really rounded out my understanding. In Chemistry, the conservation of matter means that no new matter can form, nor can it disappear. While this is workable and provable, we are guessing that it is not true based on the big bang theory. It is my guess that being that E = MC2 that saying matter and energy are the same thing, that you have proven that it only conserves if there is symmetry, that the size of space itself creates potential energy and that is the source of all matter.
Thanks for the great video, but can this explain other conservation quantities?
Take charge conservation for example. Take an electron as the object (or say, the system) moving toward a proton (just like the translation example about 6:30 in the video), its energy is changed, but the charge still conserved.
I love your voice and the content of your videos!
Hi, could you please update the links in description?
Great video btw :)
Thank you for this video!
What a great video! Subbed.
I think you can take it a step further and say that symmetries can also imply potential energy. For example, an object going up will have the same speed when it falls back down to the same height; therefore gravitational potential energy exists. Or an object moving at a spring will have the same speed when it gets repelled back to the same distance, thus elastic potential energy exists. I’m not sure what to call that symmetry though.
Thank you really nicely made video.
wow! I really added the la-la-la myself when the particle flew by... and then you added it.. that's cool...
I love the name of your channel-it makes me smile every time I see it-and I really liked this video, however I have a question regarding the roller coaster problem. At 4:15, where does the factor (g/R) come from? In other words, how does ω2 = g/R?
Is it because arad = ω2r?
Furthermore, since the initial gravitational potential energy at h has to equal the gravitational potential energy at 2R plus the kinetic energy, or rotational energy as you’ve called it, then
mgh = mg2R + ½ mv2, or mgh = mg2R + ½ I ω2, where I =mr2, and v=Rω.
And as long as v > 0, then the roller coaster will make the loop (or am I wrong in this assumption?); therefore, if we let v=0, we see that h = 2R, so we ensure that h > 2R so
that v will be greater than zero.
Thank you for clarifying, and I will continue to watch, smile, and learn from your videos.
Great video! Puts my grad professors explanation to shame! Earned a sub from me!
Well from Classical mechanics, related to what you said in descriptions, Lagrangian and Hamiltonian essentially is the same as Newtonian. But I think these 3 approaches have different perspective so useful in different situations. I think since the theorem can be directly related to the action of the system, it is natural to think of it in Lagrangian framework. But I believe Noether's theorem is more general in mathematical sense as a great contribution to abstract algebra, as you indicated in the video. Sorry I did not really say much actually but there is a LOT to say about it if one chooses to focus in such route.
this is very interesting, never thought that there was any reason for conservation laws, much less that they were connected to symmetries. from what i found about super symmetry it appears to be a type of transformation involving particle's spin value, which would imply that fermions and bosons are simply a transformation of this value. however because there are no superpartners that have been discovered, the symmetry is said to be spontaneously broken.
Hello, and thanks for yours very nices videos.
I have somes questions about this video :
- At 3:06, you say that a particule that is closer to a nearby planet has less potential energy so this transformation is not symmetric, but the loss potential energy is expected to be transformed in cinetic energy, and so the total energy is the same and the transformation is then symmetric .. so what's wrong with my explanation ?
- at 7:35, you define the time translation symmetry by the fact that the energy of the system is the same some time later, and then say that for this type of symmetry, it is the energy wich is conserved. But that seems logic because the consequense is the hypothesis used. What's the utility for this ?
Thank you in advance
Grégory
Oh, I was looking at this some time ago... I remember that I found out that dilatation symmetry implies the tracelessness of the stress-energy tensor, which is definitely a peculiar way of describing a 'conserved quantity'...
Funnily enough I had been thinking of different generators and the symmetries they imply, but I can write about that a bit later
A theorem of great scope yet a simple idea, sounds exactly like something a mathematician would come up with. It should definitely be mentioned in the classrooms in some form.
I've heard of super symmetry and CPT symmetry, stuff to look up.
So a system like a falling apple doesn't have translational symmetry but it can still have time translation symmetry since the total energy is conserved.
Fun stuff, very nicely done. Can you provide links to proofs? The ones provided are not working.
Mindblow at 7:57 when I realise that these line up perfectly with the quantum operators on the wave function!
We work out the momentum of a particle by the rate of change of the wave function with respect to position (or translation!)
Similarly, we work out the energy using the rate of change of the wave function with respect to time.
I can only then imagine that the spin of a particle is calculated using the rate of change of the wave function with respect to its rotation in some sense? Also, please return to your videos! I am only discovering them now and seeing that you seem to be on some quite long hiatus. :-(
A clearer explanation of what I was just saying: ruclips.net/video/LZie2QC5Jbc/видео.html
Wow that's so cool! I didn't realize that yet.
I began watching this video thinking that Noether was a pun joining the words No Ether, but it is actually a real name! and really apt for the theorem associated with it too! coincidence or what!
+Xavier Sebastien I know! The first time I saw this theorem that's exactly what I thought!!
+Xavier Sebastien I thought that too! But, more it reminded me of the noyes (pronounced noise) game. So No ether=> Noether and No Yes to Noyes.
+Scott Mc Logic en.wikipedia.org/wiki/Emmy_Noether
NeedsEvidence Nice
Actually Noether was German and we'd usually write it like "Nöther", but due due migration over history, Umlaute in names got replaced with the respective vocal + e (ä=ae, ö=oe, ü=ue).
Just want to mention it because I find it funny that English speakers would tear apart the name between exactly the two letters which once belonged together :D
Halliday once said that we use physics to make it more easier to understand the nature around us. And to help us, we use tools like symmetries. I aways agreed with him on that, and now that i watched your video i can totally see where and how those symmetries help us. Also, i think we humans use symmetries because of our power to recognize patterns.
Thanks very much for adding this
It looks very good !
Beautifully done. You obviously have had quite a bit of formal training in Physics/Maths. to produce something of such quality.
Hi!! Been enjoying your videos alot! So happy that I found this channel :) One question I had was in 8:00 you comment, "Turn any symmetry into a conservation law, and vice versa." I thought Noether showed that the converse(conservation => symmetry) is not necessary true? Thank you!
Thank you so much for your explanation, I heard about this from Jack Kruse, a renowned brain surgeon who is very interested in quantum biology. Best to you.
This was a lot easier to understand than my professor's messy lecture notes.
Excellent bit of toe-dipping, as usual. I'm amazed I only discovered this channel a week ago. All the vids are SO thought-provoking & SO entertaining - a wonderful combo - and the homework questions an added bonus. The link in the description here is broken: those lectures (based on Goldstein) are currently at www.physics.usu.edu/torre/6010_Fall_2016/Lectures.html. Lecture 4 (the 5th in the list) describes Noether's theorem & Lecture 12a discusses generators.
such a cool video!!
Hey these are the things that are "linked" together with the uncertainty principle - the plot thickens!
Ding ding ding! Well spotted. I'm writing those videos now.
come back! your videos are awesome!
Really soon! Promise :)
+Looking Glass Universe woo! I'm excited!
If you can, could you (or someone else) summarize Dirac/bra-ket notation briefly, or perhaps just provide links to a webpage that you think summarizes it in layman's terms. As a British High-school student watching your (really fantastic) videos out of simple curiosity, i think it would help immensely with my understanding of the quantum mechanics ones in particular. Thanks so much :)
I have heard of invariants in Mathematics in combinatorics and game theory. The idea is to look for some mathematical quantity that is invariant under an operation. This helps a lot. For example, in the game Nim - the invariant is the XOR of all the number of stones in each pile !
Why do i imagine that you are speaking right before you are about to laugh?
+Abrar Soudagar yes, it's really annoying! I think she's doing it with good intent like for leaving a good impression, but it doesn't really work :)
+Alhun Aydin - It does work. It expresses joy in and excitement over the ideas.
It sounds a bit like dialect to me. Australians often sound a bit cheery in English to me, like Norwegians also do by default, to my Swedish ears.
It's an Aussie-Indian accent. It's upbeat usually unless the person is depressed.
Abrar Soudagar - It's hard to focus on what she's saying cause all I can picture is her talking with this big shit eating grin on her face! LOL! Seems like she's about to burst into laughter. I'm like: what's so funny???
Sorry, apparently I get distracted easy.
Suppose we put the 3 objects you showed (the apple and two balls) in a box and let them collide as they did on the plane of the screen (let's call it x-y plane), with one varying condition: The objects collide in the box while accelerating towards the earth with PE = mgh. Would the momentum of the system simply be in the sum of the momenta in the x-y plane plus the one in the z direction, since p=mv and the vectors add?
Great video.
Awesome content..
Good job!
Was just about to apply this idea of conservation and symmetries to some logical systems that I have been examining; but then you asked it in the homework. But say we take (in first order logic) the logical expression X Y. Now we can see by the truth table that we could switch X and Y (X on the right and Y on the left) and we get the same truth table. However, if we tried that with X => Y, this is not equal to Y => X (If X not equal to Y of course). So an interesting question might be, what is conserved? Possibly Logic? Well I am going to look in to it, but before I do, I must thank you for bringing these ideas to my attention - your videos are amazing, and while I am sure you hear that all the time, I suppose it doesn't hurt to add to the pile of compliments ;).
How did you subtitute ω squared with g/R in the energy example? Shouldn't that give you ω^2=(v/R)^2=v^2/R^2 instead?
Hey "Looking Glass Universe"! How can one contact you with a question!? Through YT comments?
I found your channel today while procrastinating boring exam revision and now I am watching vid after vid after vid non-stop. I am in my third year of engineering and watching these videos makes me wish I had gone with my gut and studied physics instead. Thank you for helping me rediscover my love for physics and maths (looking up Hamiltonian mechanics atm so I can better understand Noether's theorem).
Regarding this vid specifically: we often rely on "conservation of mass" in engineering. After watching this vid I'm wondering, what kind of symmetry would lead to conservation of mass?
Is conservation of mass even a real law like conservation of energy? Or is it more of an assumption rather than a law?
+Shazrazmatazisaspaz Thank you for the really sweet comment :)! And it's not at all too late for studying physics, esp when you have an engineering background!
Also, great question! I think that conservation of mass is similar to conservation of charge- and there is a version of noether's theorem that applies to that. It comes from 'gauge symmetry' which you may have heard of in electromagnetism? But, conservation of mass is infact a special case of conservation of energy, since energy and mass are the same thing. Kind of cool right?
+Looking Glass Universe Yeeahh after asking that question I did some more research and found out that mass and energy are the same thing. HEAPS COOL
Right??
Defs. I just read that if c were expressed in natural units then the distance and time units are related so that light travels 1 distance unit in 1 time unit (this is kinda cool because it does away with pesky metres and seconds). And so in natural units c=1, and then E=mc^2 reduces to just E=m. Which really highlights how samesame-but-different energy and mass are!
Is there any difference with curie's principle ?
A minor point but at around 0:34 you say that "Symmetries imply conservations" but what you have in the image is an equvalence and during the rest of the video it sounds like you're talking about an equivalence rather than an implication. Other than that this was a very good video for introducing the concept of symmetries and the relation to conservations, of which I didn't know; but you learn something new every day and I say thank you for that! :)
This is so freaking awesome
I didn't know that relation, you've oppened doors for new reflexions, thank's a lot ... it's as strong as the Heisenberg uncertainty, the symetry of Quantum properties and mass-energy-speed-time relation, it must be linked somehow … One thing I'm sure : your video is awesome, they all are, you manage to explain complex concepts in a very pleasant, elegant and graspable way and with nice little real life examples and illustrations.