36.2: Minkowski Geometry
- Page ID
- 70225
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Consider the following pseudo inner-product which is used to model special relativity in \(R^4\):
\[\langle X,Y \rangle = -x_1y_1 - x_2y_2 -x_3y_3 + x_4y_4 \nonumber \]
It has the following norms and distances:
\[\lVert X \rVert = \sqrt{|\langle X,X \rangle|} \nonumber \]
\[ d(X,Y) = \lVert X - Y \rVert = \lVert ( x_1 - y_1, x_2-y_2, x_3 - y_3, x_4 - y_4) \rVert \nonumber \]
\[ = \sqrt{|-(x_1 - y_1)^2 - (x_2-y_2)^2 - (x_3 - y_3)^2 + (x_4 - y_4)^2|} \nonumber \]
The Minkowski Geometry is called pseudo inner product because it violates one of the inner product axioms. Discuss the axioms in your group and decide which one it violates.
The Physical Interpretation of Minkowski Geometry
The distance between two points on the path of an observer in Minkowski geometry corresponds to the time recorded by that observer in traveling between the two points.
We assume that Alpha Centauri lies in the \(x_1\) direction from the Earch. The twin on Earth advances in time \(x_4\). There is no motion in either the \(x_2\) or \(x_3\) directions. Twin 2 on board the rocket advances in time and moves toward Alpha Centauri and back to the Earth.
Let \(P=(0,0,0,0)\), \(R=(4,0,0,5)\), and \(Q=(0,0,0,10)\).
- \(d(P,Q)=10\) means that Twin 1 ages 10 years from \(P\) to \(Q\). Because \(x_1\) does not change and only the time \(x_4\) changes. Twin 1 does not travel and stay on Earth for 10 years.
- \(d(P,R)=3\) means that Twin 2 ages 3 years in traveling from \(P\) to \(R\). When Twin 2 arrives at the \(R\), the time on the earth has passed 55 years, though the recored time by Twin 2 is only 33 years.
- \(d(R,Q)=3\) means taht Twin 2 ages 3 years in traveling from \(R\) to \(Q\). When Twin 2 travels back to the Earth \(P\), it records 3 years but the time at the Earch has passed 5 years.
- The time from \(P \rightarrow R \rightarrow Q\) is shorter than \(P \rightarrow Q\).
The star cluster Pleiades in the constellation Taurus is 410 light years from Earth. A generational spaceship to the cluster traveling at constant speed ages 850 years on a round trip. By the time the spaceship returns to Earth, how many centuries will have passed on Earth?
How fast was the spaceship going relative to earth?