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You still don’t need geometric algebra to combine the magnetic and electric fields, if you view them as differential forms on 4-dimensional spacetime. From what

You still don’t need geometric algebra to combine the magnetic and electric fields, if you view them as differential forms on 4-dimensional spacetime. From what I can tell, this approach is basically equivalent to what this article does. You could directly translate everything into the language of differential forms, because the only geometric products here are in fact just exterior products.

yiyus 6y ago
This "competition" between geometric algebra and differential forms makes me uncomfortable. As far as I see (and I'm not an expert), they are just different ways to express very similar concepts. They are still not exactly the same, since the geometric product is not defined in DFs, and there is no hodge star operator in GA, for example, but everything you can do using one formalism in practice can also be easily done using the other one. What am I missing?
- DreamScatter 6y ago
  You're mistaken, GA does have a Hodge star, as I've explained many times before https://grassmann.crucialflow.com/dev/algebra https://grassmann.crucialflow.com/dev/algebra The exterior product can be derived from the geometric product, so differential forms occur in geometric algebra.
  - yiyus 6y ago
    You can easily define it, that's what I meant saying that you can do the same things in practice, but it's not usually defined (at least in the books and articles I've read), and certainly it is not so ubiquitous as in DFs texts. And, of course, the exterior product is contained in the geometric product. I guess that, in the same way, you could define a geometric product operator when using a DFs formulation. Would you then say that geometric algebra occurs in differential forms? In any case, you did not attempt to answer my original question. Are GA and DFs just different ways to define "equivalent" concepts or is there some more fundamental difference that I am missing?
    - DreamScatter 6y ago
      No, I would say differential forms occur in geometric algebra, not the other way around.
      - yiyus 6y ago
        Fair enough. I have seen some comments (not in this thread, it was some time ago) that suggested that DFs allow the same as GA in practice, and everything GA does is adding an unnecessary geometric product, but exterior products should be enough (not my opinion, I can try to find the original comment if you want). I do not know enough to have an own opinion. You obviously know more than me about this, so I will ask you a slightly different question: if I learn GA well enough and totally ignore differential forms, what will I miss?
        chobytes 6y ago
        Differential forms aren't exactly comparable to GA... I would instead look at the relation between exterior algebra and GA. Differentials are a concept that the comes from doing calculus on manifolds, and exterior products of differentials are just used for tracking information about oriented volumes. To answer your question (switching differentials forms for for exterior algebras), you wont miss anything, as the wedge product is part of a GA.
    - deleted 6y ago
      [deleted]
- creata 6y ago
  If I remember correctly, the Hodge star much more closely belongs to geometric algebras than it does to exterior algebras, since you need a nondegenerate bilinear form to define the Hodge star, from which you can just as easily define the geometric product, and from the geometric product the Hodge star. To make an analogy, it sounds a bit like you're asking whether inner product spaces and vector spaces are equivalent. Every geometric algebra gives rise to a Hodge star, and an exterior algebra, and so on, but exterior algebras are a much more general concept, so they're less powerful until you tack that extra structure on.
  - DreamScatter 6y ago
    You are mistaken, the Hodge star does not belong to geometric algebra more than exterior algebra, that's the wrong way to look at it. Exterior algebra is just only a sub algebra of geometric algebra, they both have the same Hodge star. Saying the Hodge star belongs more in one than the other is a bit silly.
    - creata 6y ago
      Look, you already need a bilinear form to get the Hodge star. My point is that with that same bilinear form you also get an entire Clifford algebra, and a much more natural definition of the Hodge star. That's all I'm saying. Is that mistaken?
  - yiyus 6y ago
    To be honest, my question is much more practical, and much naiver too. I work with people who use Euler angles to express rotations, and it's a horrible world. I learned quaternions in my day, and there are some obvious advantages. When I discovered GA some years ago, it was really eye opening. It makes quaternions an easy to explain concept, even intuitive, and I've used it since then, not only in my own work but also to teach other people. Then, I learned about differential forms, and it's also very interesting, I think I could base my "intuitive explanations" in this other paradigm, but I'm not sure I should. I do not think it makes a big difference in my particular case, but as I said I find this "competition" a bit frustrating, and am trying to understand it better. I cannot discuss with a mathematician if the Hodge star is a GA or a DFs concept, but I have found it all over the place when reading about DFs, and not so in GA related material (though I have a vague idea about how the Hodge star operator can easily be defined in GAs using the pseudoscalar). But is this really my choice? I have listened opinions about which one is more general, but not really convincing arguments (at least not arguments that are obvious to me). Thanks for your explanation. I think I need to have a deeper look at this stuff. I like to get lost in these rabbit holes, but sometimes it goes a bit over my head.
- orbots 6y ago
  There is a fairly nuanced difference that doesn't really matter much unless you are a mathematician. Essentially in GA you'd do your differential geometry assuming a sort of ambient background space. In regular differential geometry the space of forms and vectors are abstracted and don't require a shared geometric embedding. I'm not a mathematician, so this is a very non-precise explanation, but that's how I understand it.

yiyus 6y ago

This "competition" between geometric algebra and differential forms makes me uncomfortable. As far as I see (and I'm not an expert), they are just different ways to express very similar concepts. They are still not exactly the same, since the geometric product is not defined in DFs, and there is no hodge star operator in GA, for example, but everything you can do using one formalism in practice can also be easily done using the other one. What am I missing?

DreamScatter 6y ago
You're mistaken, GA does have a Hodge star, as I've explained many times before https://grassmann.crucialflow.com/dev/algebra https://grassmann.crucialflow.com/dev/algebra The exterior product can be derived from the geometric product, so differential forms occur in geometric algebra.
- yiyus 6y ago
  You can easily define it, that's what I meant saying that you can do the same things in practice, but it's not usually defined (at least in the books and articles I've read), and certainly it is not so ubiquitous as in DFs texts. And, of course, the exterior product is contained in the geometric product. I guess that, in the same way, you could define a geometric product operator when using a DFs formulation. Would you then say that geometric algebra occurs in differential forms? In any case, you did not attempt to answer my original question. Are GA and DFs just different ways to define "equivalent" concepts or is there some more fundamental difference that I am missing?
  - DreamScatter 6y ago
    No, I would say differential forms occur in geometric algebra, not the other way around.
    - yiyus 6y ago
      Fair enough. I have seen some comments (not in this thread, it was some time ago) that suggested that DFs allow the same as GA in practice, and everything GA does is adding an unnecessary geometric product, but exterior products should be enough (not my opinion, I can try to find the original comment if you want). I do not know enough to have an own opinion. You obviously know more than me about this, so I will ask you a slightly different question: if I learn GA well enough and totally ignore differential forms, what will I miss?
      - chobytes 6y ago
        Differential forms aren't exactly comparable to GA... I would instead look at the relation between exterior algebra and GA. Differentials are a concept that the comes from doing calculus on manifolds, and exterior products of differentials are just used for tracking information about oriented volumes. To answer your question (switching differentials forms for for exterior algebras), you wont miss anything, as the wedge product is part of a GA.
  - deleted 6y ago
    [deleted]
creata 6y ago
If I remember correctly, the Hodge star much more closely belongs to geometric algebras than it does to exterior algebras, since you need a nondegenerate bilinear form to define the Hodge star, from which you can just as easily define the geometric product, and from the geometric product the Hodge star. To make an analogy, it sounds a bit like you're asking whether inner product spaces and vector spaces are equivalent. Every geometric algebra gives rise to a Hodge star, and an exterior algebra, and so on, but exterior algebras are a much more general concept, so they're less powerful until you tack that extra structure on.
- DreamScatter 6y ago
  You are mistaken, the Hodge star does not belong to geometric algebra more than exterior algebra, that's the wrong way to look at it. Exterior algebra is just only a sub algebra of geometric algebra, they both have the same Hodge star. Saying the Hodge star belongs more in one than the other is a bit silly.
  - creata 6y ago
    Look, you already need a bilinear form to get the Hodge star. My point is that with that same bilinear form you also get an entire Clifford algebra, and a much more natural definition of the Hodge star. That's all I'm saying. Is that mistaken?
- yiyus 6y ago
  To be honest, my question is much more practical, and much naiver too. I work with people who use Euler angles to express rotations, and it's a horrible world. I learned quaternions in my day, and there are some obvious advantages. When I discovered GA some years ago, it was really eye opening. It makes quaternions an easy to explain concept, even intuitive, and I've used it since then, not only in my own work but also to teach other people. Then, I learned about differential forms, and it's also very interesting, I think I could base my "intuitive explanations" in this other paradigm, but I'm not sure I should. I do not think it makes a big difference in my particular case, but as I said I find this "competition" a bit frustrating, and am trying to understand it better. I cannot discuss with a mathematician if the Hodge star is a GA or a DFs concept, but I have found it all over the place when reading about DFs, and not so in GA related material (though I have a vague idea about how the Hodge star operator can easily be defined in GAs using the pseudoscalar). But is this really my choice? I have listened opinions about which one is more general, but not really convincing arguments (at least not arguments that are obvious to me). Thanks for your explanation. I think I need to have a deeper look at this stuff. I like to get lost in these rabbit holes, but sometimes it goes a bit over my head.
orbots 6y ago
There is a fairly nuanced difference that doesn't really matter much unless you are a mathematician. Essentially in GA you'd do your differential geometry assuming a sort of ambient background space. In regular differential geometry the space of forms and vectors are abstracted and don't require a shared geometric embedding. I'm not a mathematician, so this is a very non-precise explanation, but that's how I understand it.