# Homework Help: Lightlike Separations & Events

1. May 3, 2012

### Tobbin92

1. The problem statement, all variables and given/known data

One event is seperated from another by a lightlike interval, can the first event have caused the other.

2. Relevant equations

The equations are not important.

3. The attempt at a solution

It all seems to boil down to the definition of an event, can you call the light arriving at that point in space an event? Then you could say that the first event caused the other. However if the photon is supposed to "cause" an event that implies that time has to go on before the event occurs, even if an infinitessimaly small amount of time floats by, the first event could not have caused the other since it then would be a timelike interval instead.

2. May 3, 2012

### lj612

I would say in general that an emission event can be said to end at the moment a photon leaves its source, and an absorption event can be said to start the moment the photon reaches its destination. This supports your argument, and I would be inclined to agree with your answer, just be sure you have all your definitions of what is meant by "event" cleared up with your teacher/text.

3. May 4, 2012

### Tobbin92

I found the solution after a lot of searching in my coursebook. Apparently it falls out from the definition of an event, as event is defined (At least in my literature "Spacetime Physics - An introduction to special relativity") the initiation of an event is an event in itself.

Therefore the only thing that can initiate such initiating events are gravitons, neutrinos and of course photons because these are the only things that travel at the speed of light. This leads to the conclusion that an event separated by a lightlike interval from a second event could have been caused by the second event.