# Light Cone Analogue

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1. Jan 4, 2016

in Minkowksi, the set of all possible null rays from a point defines a cone (light cone).

Now imagine I change the signature of Minkowski from (-,+,+,+) to (-,-,+,+) i.e. a space with two timelike directions and a metric $ds^2=-dx_1^2-dx_2^2+dx_3^2+dx_4^2$. What kind of surface would the set of null rays form? Is it still a cone? Or is it something else?

Thanks

2. Jan 4, 2016

### Simon Bridge

How would you go about showing that the set of possible null-rays forms a cone in the regular metric?

3. Jan 5, 2016

Probably by noting that possible null trajectories have $ds^2=0$ and by differentiating with respect to the affine parameter, we see this corresponds to $x_1^2=x_2^2+x_3^2+x_4^2$ (working with Minkowski at the moment). This can be recognised as the equation of a cone (really a 4d hypercone I suppose).

Now for the case at hand we'd arrive at something like $x_1^2+x_2^2=x_3^2+x_4^2$ and I'm not sure how to interpret this? Would there be an apex in two of the directions?

4. Jan 5, 2016

### Staff: Mentor

Writing it $x_1^2 = - x_2^2 + x_3^2+x_4^2$ may help.

5. Jan 5, 2016

so it's like a "hyper-hyperboloid"? it looks like each choice of $x_1$ gives a different shaped hyperboloid.
I'm not quite sure for my imagination of 4-d objects isn't very well. But to illustrate the light cone you already contracted two space dimensions to one, i.e. the light cone is actually the shape of emerging circles $x_1^2 =x_2^2+x_3^2$ of radius $x_1 ∈ [0,∞[$. Applying the same here would give us $x_1^2 = - x_2^2 + x_3^2$ with $x_1 ∈ [0,∞[$, a hyperboloid. At least this is my understanding of the situation.