# B Riddle of the bomb in the train

1. Sep 8, 2016

### entropy1

I saw in an other topic the following thought experiment:

In the middle of a train there is a bom with two lightdetectors. At both ends of the train there are two switchable lightsources. The two lightdetectors measure the respective light from both sources.

We switch on both switches simulateously as seen from within the train, and the lightsources start sending a photon in the direction of the bomb. Now, if the bomb detects both photons simultaneously, the bomb explodes.

Suppose that seen from within the train, the photons arrive simulaneously at the bomb and the bomb sets off.

Now, the obvious question is, how does this look from the platform? At relativistic speed, in this particular event, the photons don't arrive simultaneously! However, the bomb still explodes.

So, how is this not contradictory? The mechanism in the bomb requires simultanuity. Nevertheless, the bomb sets off... when? When the first photon seems to arrive? When the second photon seems to arrive? It is a mystery to me. Anyone an answer? I hope I was clear.

2. Sep 8, 2016

### Borg

I think that you're confusing the view from the platform. I believe that they would see them arrive at the central detectors at the same time. However, unlike a person on the train, they would not see them leave the sources at the same time.

3. Sep 8, 2016

### PeroK

The light reaches the bomb simultaneously from both sources in all reference frames. This is because the two events take place at the same location, so all reference frames will have a single time and location for this pair of events. And for the third event of the bomb exploding.

But, the two events of the light leaving the sources at either end of the train are not at the same location. These events, therefore, will only be simultaneous in the train's reference frame. In any other reference frames, they will not be simultaneous.

4. Sep 8, 2016

### entropy1

Ok. Thanks for the clear answers!

5. Sep 8, 2016

### Staff: Mentor

No, they do arrive simultaneously. They don't depart simultaneously.

Edit: oh, I see I am relatively late to respond.