 Imagine that at some point \(\mathbf{O}\) of the spacetime an idealized point-sized flashbulb flashes for (literally) an instant. It follows from the invariance of the speed of light in all directions that Alice, passing through \(\mathbf{O}\) as above, will find herself at the center of an expanding sphere of photons. The radius of the sphere expands with speed \(c\). (It follows that Bob, also passing through \(\mathbf{O}\) but moving with some constant velocity with respect to Alice, must find himself also at the center of such a sphere, even though he and Alice are moving away from each other. Such is relativistic life!) If we try to diagram this situation, it is helpful to suppress one spatial dimension, as we have in all the figures above, and so the two-dimensional cut through the expanding sphere looks like an expanding circle, which becomes a cone when that growth is plotted vertically up the diagram (and so is called the light cone.) More precisely, this figure is just half the light cone. If two photons (restricting ourselves to two dimensions now) converged on point \(\mathbf{O}\) from opposite directions, the lines indicating their histories would mark the other half, the past lobe, of the light  cone.[9]