Vector Space Algebra of Minkowski Space

Click For Summary

Discussion Overview

The discussion centers on the vector space algebra of Minkowski space in four dimensions, specifically exploring how to define a fourth vector orthogonal to three given space-like orthonormal vectors. Participants are examining the appropriate vector product analogous to the cross product in three dimensions.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how to define a fourth vector orthogonal to three space-like orthonormal vectors in Minkowski space.
  • Another participant asserts that any vector orthogonal to three space-like vectors would necessarily be time-like, providing an example vector.
  • A different participant introduces the concept of the "alternating product" as a potential method for finding a vector orthogonal to three others in four dimensions, explaining the use of the Levi-Civita symbol.
  • Another contribution suggests a method to construct a fourth vector using an orthonormal set and ensuring linear independence, detailing a subtraction process to eliminate parallel components.
  • A participant shares a resource for further reading on spacetime algebra, indicating its usefulness for understanding the topic.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the fourth vector and the methods to derive it, indicating that multiple competing approaches exist without a consensus on a single solution.

Contextual Notes

Participants reference specific mathematical constructs and conventions, such as the metric signature and the Levi-Civita symbol, which may depend on the definitions and assumptions used in their arguments.

Who May Find This Useful

This discussion may be of interest to those studying advanced topics in physics, particularly in the areas of relativity and vector algebra in higher dimensions.

thermobum
Messages
2
Reaction score
0
Consider the Minkowski space of 4 dimensions with signature (- + + +). How does the vector space algebra work here? More specifically given 3 space like orthonormal vectors how do we define fourth vector orthogonal to these vectors? I am looking for an appropriate vector product like it is in the case of 3-dimesnsions: i ^ j = k etc.
 
Physics news on Phys.org
Three "space like" vectors? Obviously, any vector orthogonal to all three would be time like: <a, 0, 0, 0>.

The nearest you can come to the cross product in 4 dimensions is the "alternating product" [itex]z_i= \epsilon_{ijkl}u_jv_kw_l}[/itex] where [itex]\epsilon[/itex] is defined by [itex]\epsilon_{ijkl}= 1[/itex] if ijkl is an even permutation of 1234, [itex]\epsilon_{ijkl}= -1[/itex] if ijkl is an odd permutation of 1234, [itex]\epsilon_{ijkl}= 0[/itex] if ijkl is not a permutation of 1234 (i.e. at least two indices are the same). Notice that that involves the product of 3 vectors (which is what you want).
 
HallsofIvy's construction is essentially the computation of the [4-]volume determined by four vectors (as edges of a parallelepiped).

To complete your problem, you could do this...
given an orthonormal set [tex]x^a[/tex], [tex]y^a[/tex], and [tex]z^a[/tex],
choose any vector [tex]u^a[/tex] so that [tex]e_{abcd}u^a x^b y^c z^d <>0[/tex] (so that [tex]u^a[/tex] is linearly independent of the set you have).
With this [tex]u^a[/tex], subtract out all of the components parallel to the orthonormal set... [tex]t^a=u^a- (g_{bc}u^b \hat x^c) \hat x^a - (g_{bc}u^b \hat y^c) \hat y^a - (g_{bc}u^b \hat z^c) \hat z^a[/tex].

Check the signs with your metric signature conventions.
 
Spacetime Algebra

For an excellent and thorough formulation of the algebra, go to the link
http://modelingnts.la.asu.edu/html/STC.html
and click on the link at the very top of the page, Spacetime Calculus, to download a pdf file.
 
Last edited by a moderator:
pkleinod, the link you provided is really an excellent source and is proving to very useful to me. Thanks and a Happy New Year to all the blokes who responded to this thread !
 

Similar threads

  • · Replies 18 ·
Replies
18
Views
4K
  • · Replies 13 ·
Replies
13
Views
5K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 25 ·
Replies
25
Views
4K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 9 ·
Replies
9
Views
4K
  • · Replies 1 ·
Replies
1
Views
3K