This says that $(\Delta s)^2 = -c^2(t_2-t_1)^2 + (\vec{x_2}-\vec{x_1})^2=0$.

What I'm trying to imagine here (as per my understanding of the statement) is that two events are in the same light cone. Which means that two events may be separated by time and space. How does $(\Delta s)^2$ become zero then?

Can someone please explain what this means conceptually and show how $(\Delta s)^2=0$ mathematically? Would really appreciate that.

  • $\begingroup$ I see in a previous question that you are still struggling to understand the place that interval holds in special relativity and its importance. Lunging forward to the general theory with that uncertainty intact is going to be difficult. $\endgroup$ – dmckee --- ex-moderator kitten Jan 8 '17 at 19:58
  • $\begingroup$ I think I cleared the previous question for myself. $\endgroup$ – sequence Jan 8 '17 at 19:59

"Connected by propagation of a ray of light" means that the two events have a separation in spacetime such that if a ray of light were emitted from the location of event 1 at the same time as the occurrence of event 1, then that ray of light would reach the location of event 2 at the same time that event 2 occurs. If event 1 forms the vertex of the light cone, then in this scenario, event 2 lies on the surface of the light cone, not inside of it. The math you've already written shows how it equals zero - since light travels at speed $c$, $\left| \Delta \vec{x}\right| = \left|\vec{x}_2 - \vec{x}_1\right| = ct_2 - ct_1 = c\Delta t$. This exactly cancels the first term.

If event 2 were inside the light cone, then the interval between the two events would be $\left(\Delta s\right)^2 < 0$, since the new $\left|\Delta \vec{x}\right|$ would be smaller than the distance that light would travel, $c\Delta t$. Such events are called time-like and can be linked causally, since messages between the events need not exceed the speed of light, which forms the surface of the light cone.

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  • $\begingroup$ Can you please clarify what you mean by "since messages between the events need not exceed the speed of light"? Need not to exceed the speed of light in order for/(to do) what? $\endgroup$ – sequence Jan 8 '17 at 20:38
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    $\begingroup$ This means that some non-light means of communication could be used - for instance, a sound wave carrying instructions could pass from the location of event 1 to the location of event 2 in the allotted time. Thus, event 1 could in principal cause some effect at event 2. If event 2 were outside the cone, not even light could reach event 2 in that time, so event 1 could never affect event 2. $\endgroup$ – user138962 Jan 8 '17 at 20:44
  • $\begingroup$ So if event 2 is on the surface of the light cone of event 1, does this mean that event 1 is still causally connected to event 2? Say, a laser beam (event 1) may generate event 2 (explosion) with the speed of light. $\endgroup$ – sequence Jan 8 '17 at 20:53
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    $\begingroup$ Yes, it could be. The light cone represents the limit of this possibility, where light is the causal agent. $\endgroup$ – user138962 Jan 8 '17 at 21:00

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