3 added 114 characters in body edited Sep 19 '16 at 6:00 user36790 In relativity there is an invariant called the proper time, $$\tau$$. It's an invariant in the sense that all observers will agree on it's value. In special relativity the proper time is defined as: $$d\tau^2 = ds^2 = c^2dt^2 - dx^2 - dy^2 - dz^2$$$$\mathrm d\tau^2 = \mathrm ds^2 = c^2~\mathrm dt^2 - \mathrm dx^2 - \mathrm dy^2 - \mathrm dz^2$$ or $$d\tau^2 = -ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2$$$$\mathrm d\tau^2 = -\mathrm ds^2 = -c^2~\mathrm dt^2 + \mathrm dx^2 + \mathrm dy^2 + \mathrm dz^2$$ You see both sign conventions and I've never been sure which is more generally accepted. Anyhow, you can write the equation for $$ds^2$$$$\mathrm ds^2$$ as a matrix equation using: $$ds^2 = g_{\alpha\beta}x^\alpha x^\beta$$$$\mathrm ds^2 = g_{\alpha\beta}x^\alpha x^\beta$$ where $$x$$ is the vector $$(t, x, y, z)$$ and $$g$$ is the matrix: $$\left( \begin{matrix} -1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{matrix}\right)$$ The matrix is called the metric tensor (or a representation of it) and the signature is the number of positive and negative values on the leading diagonal. In this case it's (1,3) or you often just add together the negative and positive numbers to give, in this case, just 2. Exactly the same equation is used in general relativity, but the matrix representing the metric tensor is more complicated and generally not diagonal, so you have to diagonalise it to calculate the signature. The Wikipedia article goes into this in more detail. The reason we're interested in the signature is that we expect spacetime to have one timelike co-ordinate and three spacelike co-ordinates, so we expect the signature to be always (1,3). In relativity there is an invariant called the proper time, $$\tau$$. It's an invariant in the sense that all observers will agree on it's value. In special relativity the proper time is defined as: $$d\tau^2 = ds^2 = c^2dt^2 - dx^2 - dy^2 - dz^2$$ or $$d\tau^2 = -ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2$$ You see both sign conventions and I've never been sure which is more generally accepted. Anyhow, you can write the equation for $$ds^2$$ as a matrix equation using: $$ds^2 = g_{\alpha\beta}x^\alpha x^\beta$$ where $$x$$ is the vector $$(t, x, y, z)$$ and $$g$$ is the matrix: $$\left( \begin{matrix} -1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{matrix}\right)$$ The matrix is called the metric tensor (or a representation of it) and the signature is the number of positive and negative values on the leading diagonal. In this case it's (1,3) or you often just add together the negative and positive numbers to give, in this case, just 2. Exactly the same equation is used in general relativity, but the matrix representing the metric tensor is more complicated and generally not diagonal, so you have to diagonalise it to calculate the signature. The Wikipedia article goes into this in more detail. The reason we're interested in the signature is that we expect spacetime to have one timelike co-ordinate and three spacelike co-ordinates, so we expect the signature to be always (1,3). In relativity there is an invariant called the proper time, $$\tau$$. It's an invariant in the sense that all observers will agree on it's value. In special relativity the proper time is defined as: $$\mathrm d\tau^2 = \mathrm ds^2 = c^2~\mathrm dt^2 - \mathrm dx^2 - \mathrm dy^2 - \mathrm dz^2$$ or $$\mathrm d\tau^2 = -\mathrm ds^2 = -c^2~\mathrm dt^2 + \mathrm dx^2 + \mathrm dy^2 + \mathrm dz^2$$ You see both sign conventions and I've never been sure which is more generally accepted. Anyhow, you can write the equation for $$\mathrm ds^2$$ as a matrix equation using: $$\mathrm ds^2 = g_{\alpha\beta}x^\alpha x^\beta$$ where $$x$$ is the vector $$(t, x, y, z)$$ and $$g$$ is the matrix: $$\left( \begin{matrix} -1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{matrix}\right)$$ The matrix is called the metric tensor (or a representation of it) and the signature is the number of positive and negative values on the leading diagonal. In this case it's (1,3) or you often just add together the negative and positive numbers to give, in this case, just 2. Exactly the same equation is used in general relativity, but the matrix representing the metric tensor is more complicated and generally not diagonal, so you have to diagonalise it to calculate the signature. The Wikipedia article goes into this in more detail. The reason we're interested in the signature is that we expect spacetime to have one timelike co-ordinate and three spacelike co-ordinates, so we expect the signature to be always (1,3). 2 added 2 characters in body edited Apr 28 '12 at 6:07 John Rennie 285k4545 gold badges580580 silver badges827827 bronze badges In relativity there is an invariant called the proper time, $$\tau$$. It's an invariant in the sense that all observers will agree on it's value. In special relativity the proper time is defined as: $$d\tau^2 = ds^2 = c^2t^2 - dx^2 - dy^2 - dz^2$$$$d\tau^2 = ds^2 = c^2dt^2 - dx^2 - dy^2 - dz^2$$ or $$d\tau^2 = -ds^2 = -c^2t^2 + dx^2 + dy^2 + dz^2$$$$d\tau^2 = -ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2$$ You see both sign conventions and I've never been sure which is more generally accepted. Anyhow, you can write the equation for $$ds^2$$ as a matrix equation using: $$ds^2 = g_{\alpha\beta}x^\alpha x^\beta$$ where $$x$$ is the vector $$(t, x, y, z)$$ and $$g$$ is the matrix: $$\left( \begin{matrix} -1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{matrix}\right)$$ The matrix is called the metric tensor (or a representation of it) and the signature is the number of positive and negative values on the leading diagonal. In this case it's (1,3) or you often just add together the negative and positive numbers to give, in this case, just 2. Exactly the same equation is used in general relativity, but the matrix representing the metric tensor is more complicated and generally not diagonal, so you have to diagonalise it to calculate the signature. The Wikipedia article goes into this in more detail. The reason we're interested in the signature is that we expect spacetime to have one timelike co-ordinate and three spacelike co-ordinates, so we expect the signature to be always (1,3). In relativity there is an invariant called the proper time, $$\tau$$. It's an invariant in the sense that all observers will agree on it's value. In special relativity the proper time is defined as: $$d\tau^2 = ds^2 = c^2t^2 - dx^2 - dy^2 - dz^2$$ or $$d\tau^2 = -ds^2 = -c^2t^2 + dx^2 + dy^2 + dz^2$$ You see both sign conventions and I've never been sure which is more generally accepted. Anyhow, you can write the equation for $$ds^2$$ as a matrix equation using: $$ds^2 = g_{\alpha\beta}x^\alpha x^\beta$$ where $$x$$ is the vector $$(t, x, y, z)$$ and $$g$$ is the matrix: $$\left( \begin{matrix} -1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{matrix}\right)$$ The matrix is called the metric tensor (or a representation of it) and the signature is the number of positive and negative values on the leading diagonal. In this case it's (1,3) or you often just add together the negative and positive numbers to give, in this case, just 2. Exactly the same equation is used in general relativity, but the matrix representing the metric tensor is more complicated and generally not diagonal, so you have to diagonalise it to calculate the signature. The Wikipedia article goes into this in more detail. The reason we're interested in the signature is that we expect spacetime to have one timelike co-ordinate and three spacelike co-ordinates, so we expect the signature to be always (1,3). In relativity there is an invariant called the proper time, $$\tau$$. It's an invariant in the sense that all observers will agree on it's value. In special relativity the proper time is defined as: $$d\tau^2 = ds^2 = c^2dt^2 - dx^2 - dy^2 - dz^2$$ or $$d\tau^2 = -ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2$$ You see both sign conventions and I've never been sure which is more generally accepted. Anyhow, you can write the equation for $$ds^2$$ as a matrix equation using: $$ds^2 = g_{\alpha\beta}x^\alpha x^\beta$$ where $$x$$ is the vector $$(t, x, y, z)$$ and $$g$$ is the matrix: $$\left( \begin{matrix} -1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{matrix}\right)$$ The matrix is called the metric tensor (or a representation of it) and the signature is the number of positive and negative values on the leading diagonal. In this case it's (1,3) or you often just add together the negative and positive numbers to give, in this case, just 2. Exactly the same equation is used in general relativity, but the matrix representing the metric tensor is more complicated and generally not diagonal, so you have to diagonalise it to calculate the signature. The Wikipedia article goes into this in more detail. The reason we're interested in the signature is that we expect spacetime to have one timelike co-ordinate and three spacelike co-ordinates, so we expect the signature to be always (1,3). 1 answered Apr 27 '12 at 15:29 John Rennie 285k4545 gold badges580580 silver badges827827 bronze badges In relativity there is an invariant called the proper time, $$\tau$$. It's an invariant in the sense that all observers will agree on it's value. In special relativity the proper time is defined as: $$d\tau^2 = ds^2 = c^2t^2 - dx^2 - dy^2 - dz^2$$ or $$d\tau^2 = -ds^2 = -c^2t^2 + dx^2 + dy^2 + dz^2$$ You see both sign conventions and I've never been sure which is more generally accepted. Anyhow, you can write the equation for $$ds^2$$ as a matrix equation using: $$ds^2 = g_{\alpha\beta}x^\alpha x^\beta$$ where $$x$$ is the vector $$(t, x, y, z)$$ and $$g$$ is the matrix: $$\left( \begin{matrix} -1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{matrix}\right)$$ The matrix is called the metric tensor (or a representation of it) and the signature is the number of positive and negative values on the leading diagonal. In this case it's (1,3) or you often just add together the negative and positive numbers to give, in this case, just 2. Exactly the same equation is used in general relativity, but the matrix representing the metric tensor is more complicated and generally not diagonal, so you have to diagonalise it to calculate the signature. The Wikipedia article goes into this in more detail. The reason we're interested in the signature is that we expect spacetime to have one timelike co-ordinate and three spacelike co-ordinates, so we expect the signature to be always (1,3).