Calculating Cross Product in 3D and 7D

In summary, the ability to take the cross product in only 3 and 7 dimensions is related to the existence of anti-symmetric linear pairings and vector fields on the sphere. This is explained by a theorem in differential geometry involving determinants of matrices. The determinants can have different outcomes depending on the size of the matrix, which can explain why the cross product is possible in some dimensions but not others.
  • #1
theperthvan
184
0
Why is it possible to take the cross product in only 3 and 7 dimensions?
 
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  • #2
Er, could you clarify the question?


Although I don't know precisely what you're asking, I suspect the answer has to do with the Quaternions and Octonions.
 
  • #3
If you mean 'why do their exist anti-symmetric linear pairings x/\y : R^nxR^n-->R^n for some n, and not others', then Hurkyl is getting there. There is a theorem in differential geometry that explains this, though I don't know what it is saying (i.e. I can't encapsulate it into a nice bite sized slogan for the lay person).
 
  • #4
existence of pairings produces vector fields on the sphere, and these exist only in a few cases. maybe this is related.
 
  • #5
Well, the idea is that a matrix is created. The determinent can do very funny things. Just try to find the determinents of 3x3, 4x4, 5x5, 6x6, 7x7. You may figure out why...
 
  • #6
OK. Thanks
 
  • #7
prasannapakkiam said:
Well, the idea is that a matrix is created. The determinent can do very funny things. Just try to find the determinents of 3x3, 4x4, 5x5, 6x6, 7x7. You may figure out why...

PLease could you elaborate on why determinants of matrices can explain the (non-) existence of smooth (no-where zero, I imagine) vector fields on S^n?
 

1. What is a cross product?

A cross product is a mathematical operation that takes two vectors as input and produces a third vector that is perpendicular to both of the input vectors. It is commonly used in 3D and 7D geometry to calculate the direction of a surface normal or to find a vector that is perpendicular to two given vectors.

2. How do you calculate a cross product in 3D?

To calculate a cross product in 3D, you can use the formula:
A x B = (AyBz - AzBy, AzBx - AxBz, AxBy - AyBx)
Where A and B are the two input vectors and x represents the cross product operator. This formula is also known as the "right-hand rule" and can be used to determine the direction of the resulting vector.

3. Can you calculate a cross product in 7D?

Yes, a cross product can be calculated in 7D using a similar formula as in 3D. The only difference is that instead of three components, the resulting vector will have seven components. The formula can be generalized as:
A x B = (A2B3 - A3B2, A3B1 - A1B3, A1B2 - A2B1, A4B5 - A5B4, A5B6 - A6B5, A6B7 - A7B6, A7B1 - A1B7)
Where A and B are the two input vectors and x represents the cross product operator.

4. Are there any real-world applications of calculating cross products in 3D and 7D?

Yes, calculating cross products is used in many fields such as physics, engineering, and computer graphics. In physics, cross products are used to calculate the torque and angular momentum of an object. In engineering, they are used to determine the force exerted on an object by a magnetic field. In computer graphics, they are used to create 3D models and animations.

5. Is there a difference between a cross product and a dot product?

Yes, a cross product results in a vector while a dot product results in a scalar. In other words, a cross product gives us information about the direction of two vectors while a dot product gives us information about their magnitude. Additionally, a dot product is commutative (A · B = B · A) while a cross product is anti-commutative (A x B = -B x A).

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